Reclamation soft soil foundation filling pre-pressing and dike and weir stability control method

Abstract

This invention discloses a method for preloading soil and controlling the stability of dikes in soft soil foundations during land reclamation, belonging to the field of soft soil foundation treatment technology. Addressing the technical problems of drainage boards easily twisting and breaking, poor reinforcement effects due to unloading rebound, and dike slope instability in newly reclaimed ultra-soft soil foundations, the method includes: sequentially performing a first vacuum preloading followed by staged soil surcharge preloading to control settlement and displacement rates; stopping the vacuum and removing all soil surcharge after the consolidation degree meets the standard; installing a second set of drainage boards on the unloaded foundation surface, arranged in a staggered pattern with the first set; repaving the drainage system and initiating a second vacuum preloading without soil surcharge until the settlement rate, consolidation degree, and vane shear strength meet design requirements. This invention eliminates the uneven defects caused by drainage board failure during the first reinforcement through secondary reinforcement with inserting plates, effectively improving the overall shear strength and stability of the foundation. It is mainly used for deep reinforcement of soft soil foundations during land reclamation and for dike slope stability control.

Description

Technical Field

[0001] This invention relates to the field of soft soil foundation treatment technology. More specifically, this invention relates to a method for preloading soil fill and controlling the stability of dikes in reclaimed soft soil foundations. Background Technology

[0002] In recent years, with the rapid development of land reclamation projects in coastal areas, the thickness of soft soil foundations formed by reclamation has been continuously increasing, from the original 2-3 meters to 10 meters or even deeper, significantly increasing the difficulty of foundation treatment. At the same time, due to the tight construction schedule, newly reclamated ultra-soft soil foundations are directly subjected to vacuum preloading reinforcement without natural drying, which poses new challenges to traditional soft soil treatment techniques.

[0003] The primary challenge in vacuum preloading reinforcement of newly filled, ultra-soft soil foundations lies in the effectiveness of the drainage system. These foundations typically have a water content exceeding 80%, exhibiting extreme compressibility and precipitating severe settlement during vacuum preloading. Both indoor model tests and field engineering practices have revealed that this large deformation causes significant torsion and even localized breakage of the drainage boards within a certain depth range. This alters the working state of the drainage boards, reducing drainage efficiency and resulting in suboptimal soil reinforcement within a specific depth range, failing to achieve the intended reinforcement goals. Studies have shown that while the soil strength within the top 2-3 meters improves after the first vacuum preloading treatment, the strength increase in the 3-4 meter depth is very limited, and simply extending the reinforcement time is insufficient to achieve the desired reinforcement effect.

[0004] During construction, the method of applying surcharge for backfill directly impacts project safety. Hydraulically filled soft clay foundations inherently possess low bearing capacity and large deformation. When performing surcharge construction on such foundations, improper surcharge methods or load control can lead to continuous plastic deformation and weakening of the foundation, potentially causing breakwater slope instability or even slippage. Besides operational issues, the theoretical calculations for vacuum preloading also lag significantly. Current design codes for vacuum preloading primarily follow the approach of surcharge preloading, equating vacuum load with surcharge for settlement prediction, but the practical application results are not ideal. Comparison of measured settlement data and settlement calculations reveals that using existing codes to estimate settlement of vacuum-surcharge combined preloading foundations generally yields underestimated results, with significant deviations from actual field measurements. Analysis suggests that insufficient consideration of vacuum degree transmission loss along the depth of the drainage board and inadequate consideration of lateral soil deformation are the main reasons for these discrepancies. Therefore, accurate consideration of these two factors requires further research. The theoretical research on the vacuum combined surcharge preloading method lags far behind engineering practice. Many theoretical problems have not yet been well resolved, and its reinforcement mechanism still requires in-depth research in many aspects. At present, engineering practice still largely relies on experience for design and construction.

[0005] Regarding the prevention of clogging in drainage boards, the method of applying vacuum load is also a crucial factor affecting the reinforcement effect. Studies show that if the vacuum load is not applied in stages or the stage gradient is set too large, it can easily lead to clogging of the drainage board filter membrane, resulting in a decrease in drainage efficiency and thus adversely affecting the vacuum preloading reinforcement effect of newly dredged sludge.

[0006] In summary, existing technologies for reinforcing soft soil foundations from land reclamation have shortcomings in terms of the working stability of drainage boards, the safety of surcharge construction, the accuracy of consolidation calculations, and the convenience of construction organization. Summary of the Invention

[0007] This invention provides a method for preloading soil in reclamation soft soil foundations and controlling the stability of dikes. It addresses the technical problems of drainage boards being prone to twisting and breaking, poor reinforcement effect due to unloading rebound, and dike slope instability in newly filled ultra-soft soil foundations. By reinforcing with secondary insertion of drainage boards, the uneven defects caused by drainage board failure in the first reinforcement are eliminated, effectively improving the overall shear strength and stability of the foundation. It is mainly used for deep reinforcement of reclamation soft soil foundations and stability control of dike slopes.

[0008] To achieve these objectives and other advantages according to the present invention, a method for preloading soil in reclamation soft soil foundations and for controlling the stability of dikes is provided, comprising the following steps: S1. Level the surface of the soft soil foundation of the reclaimed land and lay the first drainage sand cushion layer; S2. Install the first set of drainage boards above the first drainage sand cushion layer at the first interval and the first depth, and connect the upper end of the first set of drainage boards to the first drainage pipe network system buried in the first drainage sand cushion layer. The first drainage pipe network system is connected to the vacuum pre-compression system through the membrane outlet. S3. Start the vacuum preloading system to perform the first vacuum preloading, maintaining the vacuum degree under the membrane ≥80 kPa; after the first vacuum preloading, perform graded backfill preloading of the foundation according to the surcharge thickness of 0.3 m to 0.6 m per stage, controlling the settlement rate to 10 mm / d to 15 mm / d, the horizontal displacement rate of the side piles to 4 mm / d to 7 mm / d, and the pore water pressure increment not greater than 60% of the preloading load increment; S4: Continue the first vacuum preloading and graded backfill surcharge preloading. When the measured settlement is <2 mm / d for 5 consecutive days and the degree of consolidation is ≥80%, the first reinforcement is considered complete. Stop vacuum preloading, remove all backfill, and expose the foundation surface. S5: On the unloaded foundation surface, install a second set of drainage boards at a second spacing and a second depth, which are staggered with the first set of drainage boards in a quincunx pattern. The staggered spacing in the plane is 1 / 3 to 1 / 2 of the spacing of the first set of drainage boards. The second depth is the same as the first depth, or 1.1 to 1.2 times the first depth. S6: Lay the second drainage sand cushion layer and the second drainage pipe network system, connect the upper end of the second set of drainage boards to the second drainage pipe network system, and connect the second drainage pipe network system to the vacuum pre-compression system through the membrane outlet; S7: Start the vacuum preloading system to perform the second vacuum preloading, maintain the vacuum degree under the membrane ≥80 kPa, and do not carry out backfill load preloading during this period; continue the second vacuum preloading until the average settlement rate of the ground surface is ≤4.0 mm / d for 5 consecutive days, the degree of consolidation is ≥85%, and the shear strength of the layered vanes after construction is not less than 20 kPa, then the second reinforcement is deemed to be completed. S8: Stop vacuum preloading, dismantle the vacuum preloading system and drainage network system, and conduct quality inspection.

[0009] Preferably, in steps S4 and S7, the degree of consolidation is calculated using the JJJ solution; the JJJ solution refers to the radially and vertically perfectly coordinated analytical solution of equal strain consolidation, derived under the assumption of equal strain and considering the underconsolidation characteristics of the fill soil and the loss of vacuum degree of the drainage board along the depth direction in the boundary conditions; when the first reinforcement is determined to be completed in step S4, the degree of consolidation calculated according to the measured settlement curve is determined by the following improved settlement calculation formula: S 改进 = S 分层总和 × η1× η2× η3 In the formula, S 分层总和The final settlement of the foundation is calculated using the layered summation method. η1 is the lateral confinement assumption correction coefficient with a value of 1.05 to 1.25, η2 is the first soil sampling disturbance correction coefficient with a value of 0.85 to 0.95, and η3 is the second soil sampling disturbance correction coefficient with a value of 0.80 to 0.90.

[0010] Preferably, in step S3, during the preloading of the graded backfill, displacement monitoring is performed in zones according to different distances from the toe of the embankment slope: Located within the area at the toe of the embankment slope: The settlement rate of the foundation surface is used as the monitoring index, with a control value of 10 mm / d to 15 mm / d; when the settlement rate is greater than 15 mm / d for three consecutive days, the loading rate should be immediately reduced to 40% to 50% of the current rate. If the settlement rate is still greater than 15 mm / d for two consecutive days after the reduction, the preloading should be stopped, and the graded loading should be resumed after the settlement rate is less than or equal to 15 mm / d. For the area located outside the toe of the embankment slope within one embankment height: the horizontal displacement rate of the side piles is used as the monitoring index, with a control value of 4 mm / d to 7 mm / d; when the horizontal displacement rate of any monitoring point is greater than 7 mm / d for three consecutive days, the surcharge preloading is immediately stopped, and the grading surcharge is resumed only after the horizontal displacement rate of the monitoring point is less than or equal to 7 mm / d.

[0011] Preferably, in step S3, the thickness of each stage of the preloading of the graded fill is 0.3 m to 0.6 m, and the interval between adjacent stages is determined according to the measured pore water pressure dissipation rate, and is not less than 7 days.

[0012] Preferably, in step S7, when the second reinforcement is completed, the post-construction layered vane shear strength is not less than 20 kPa, and the non-uniformity coefficient of the foundation vane shear strength within a depth range of one embankment height below the embankment base is not greater than 0.35; the non-uniformity coefficient is calculated by arranging vane shear test measuring points at 1.0 m layer intervals within this depth range, and taking the ratio of the standard deviation to the arithmetic mean of the measured shear strength values ​​of each layer measuring point.

[0013] Preferably, in step S7, while starting the vacuum preloading system for the second vacuum preloading, a stress compensation operation is also performed, specifically as follows: Compressed gas is injected into the foundation depth through the inner cavity of the second set of drainage boards, creating a local pressurization zone at the bottom or middle of the second set of drainage boards. This compensates for or partially offsets the foundation unloading rebound defects caused by the removal of all fill load, and forms an additional seepage pressure gradient around the second set of drainage boards, promoting the migration of pore water in the deep soil towards the drainage boards.

[0014] Preferably, when injecting compressed gas into the foundation depth through the inner cavity of the second set of drainage boards, an intermittent pulse pressurization method is used, and the injection pressure and injection duration are adjusted in stages or continuously according to the evolution of the foundation consolidation state; the second set of drainage boards uses a hydrophilic material with a smooth, non-porous structure and an equivalent pore size of O 95 The value ranges from 0.075 mm to 0.090 mm.

[0015] Preferably, the stress compensation operation of injecting compressed gas into the foundation depth direction through the inner cavity of the second set of drainage boards is performed in a coordinated manner with the vacuuming operation of the vacuum preloading system as follows: Pressurization pipe installation: While installing the second set of drainage boards, a rigid pressurization pipe that is tightly fitted to the inner diameter of the second set of drainage boards is inserted longitudinally into the inner cavity of the second set of drainage boards. The bottom sealing port of the rigid pressurization pipe is located at 2 / 3 to 3 / 4 of the length of the second set of drainage boards. The top of the rigid pressurization pipe is connected to the three-way switching valve between the pressurization air source system and the vacuum pre-compression system through the high-pressure pipeline assembly. The sequence of the coordinated process is as follows: From the 3rd to the 7th day after the second vacuum preloading is started, after the foundation settlement has basically stabilized, the pressurization-drainage coordinated consolidation process is executed. The process cycle is as follows: close the interface of the vacuum preloading system to the second set of drainage boards, and simultaneously turn on the pressurization gas source system to introduce compressed gas into the rigid pressurization pipe to perform intermittent pulse pressurization in the direction of foundation depth. The duration of each intermittent pulse pressurization is 30 to 60 minutes, and the injection pressure is 20 kPa to 40 kPa. After the injection is completed, close the pressurization gas source system and simultaneously turn on the vacuum preloading system to continuously drain the interface of the second set of drainage boards. The duration of continuous draining is not less than the interval between two adjacent pulse injections, which is 6 to 12 hours. This constitutes a complete intermittent pulse pressurization-drainage coordinated cycle. Pulse pressurization control conditions: When the foundation settlement rate is less than or equal to 4.0 mm / d for 5 consecutive days, or the degree of consolidation reaches 70% to 80% of 85%, the interval between two adjacent intermittent pulse pressurization-drainage coordinated cycles will be extended from the initial 6 to 12 hours to 1.5 to 2.0 times the initial value, or the process will be changed to the end of the design index testing procedure.

[0016] Preferably, the step S4 is followed by the following step between step S5: At at least one pre-selected location near the foundation surface, the filter membrane of the first set of drainage boards is partially cut off to expose the drainage channels of the drainage board core in that area, forming a reinforced area for soil extraction around the pile. After stopping the vacuum pre-compression in step S4, the vacuum pre-compression system is restarted to vacuum the drainage boards in the first group of drainage boards that have completed the filter membrane modification treatment. During the suction process, water-side boundary seepage recharge conditions are introduced or established from the adjacent site. With a continuous pumping period of no less than 5 days and no more than 15 days as the stabilization period, the seepage field force driven by the synergistic effect of high vacuum gradient and lateral hydraulic gradient is used to rapidly form a layer of ultra-soft soil with a thickness of no less than 40 cm and a static cone tip resistance q around the pumping reinforcement zone without the need for new filling or surcharge. c A hard shell layer with a strength of not less than 0.8 MPa, which serves as a mechanical insertion platform for installing the second set of drainage boards; After the hard shell layer is formed, the upper end of the first set of drainage boards, which has been modified by the filter membrane, is disconnected from the vacuum system under the membrane and the drainage pipe network system is removed.

[0017] Preferably, the injection pressure and injection duration are adjusted in stages or continuously according to the evolution of the foundation consolidation state, adopting a variable pressure-variable time gas injection loading path, specifically executed in the following three stages: The first stage: In the early stage after the second vacuum preloading is started and the foundation settlement rate is greater than 8.0 mm / d, a low-pressure-short-duration gas injection loading mode is adopted with an injection pressure of 15 kPa to 20 kPa and a continuous injection duration of 20 to 30 minutes each time. The number of gas injection cycles is 6 to 10, and the interval between two adjacent cycles is 6 to 12 hours. The second stage: After the completion of the first stage and the foundation settlement rate drops to 4.0 mm / d to 8.0 mm / d, the intermediate stage adopts a pressure-time incremental gas injection loading mode, starting from 20 kPa and linearly and uniformly increasing to 35 kPa within 8 to 12 cycles, and starting from 30 minutes for each injection and linearly and uniformly increasing to 50 minutes within 8 to 12 cycles. The interval between two adjacent cycles is 6 to 12 hours. The third stage: After the completion of the second stage and when the foundation settlement rate is ≤4.0 mm / d, a high-pressure-long-duration gas injection loading mode is adopted with an injection pressure of 35 kPa to 45 kPa and a continuous injection duration of 50 to 70 minutes each time. The number of gas injection cycles is 10 to 15, and the interval between two adjacent cycles is 12 to 24 hours. The transition between stages is as follows: when the foundation settlement rate of the current stage meets the rate threshold for transitioning to the next stage for three consecutive days, the stage will automatically switch to the next stage.

[0018] The present invention has at least the following beneficial effects: 1. This invention achieves initial consolidation of the soft soil foundation through the combined action of a first vacuum preloading and graded backfill loading. Subsequently, the vacuum is stopped and all backfill is removed. Then, a second set of drainage boards is installed for a second vacuum preloading. This eliminates the uneven reinforcement defects caused by the twisting and breakage of the drainage boards due to large soil deformation during the first reinforcement process. This effectively improves the overall shear strength of the foundation, controls post-construction differential settlement and lateral deformation, and enhances the stability of the embankment slope.

[0019] 2. In the consolidation degree calculation in steps S4 and S7 of this invention, the JJJ solution is adopted. This solution is based on the iso-strain assumption and takes into account the underconsolidation characteristics of the fill soil and the loss of vacuum degree along the depth direction of the drainage board, so that the consolidation degree calculation result is closer to the actual situation. At the same time, by introducing the lateral confinement assumption correction coefficient and the soil disturbance correction coefficient through the settlement calculation improvement formula, the final settlement calculated by the layer summation method more accurately reflects the actual settlement measured on site, and improves the reliability of the first reinforcement completion judgment.

[0020] 3. During the preloading of soil filling in step S3, the present invention controls the settlement rate and horizontal displacement rate in zones according to different distances from the toe of the embankment slope, and limits the closed-loop control logic of slowing down or stopping the filling after exceeding the limit and restoring it after the rate drops. This avoids the continuous plastic deformation and strength weakening of the foundation caused by excessive filling rate, and effectively prevents the embankment slope from sliding and instability accidents caused by overloading soil filling.

[0021] 4. In step S3 of this invention, the thickness of each fill layer is limited to 0.3 m to 0.6 m, the interval between two adjacent load layers is not less than 7 days, and the specific interval is determined according to the measured pore water pressure dissipation rate. This allows the soft soil foundation sufficient time to drain and consolidate during the load preloading process, preventing the accumulation of excess pore water pressure from exceeding 60% of the preload increment, thereby ensuring the stability of the load construction.

[0022] 5. In step S7 of this invention, the shear strength of the layered vanes after the second reinforcement is completed is not less than 20 kPa, and the non-uniformity coefficient of the shear strength of the vanes within a depth of one embankment height below the embankment base is not greater than 0.35. By clearly defining and quantifying the strength and uniformity indicators, an objective basis is provided for determining the completion of the second reinforcement, ensuring the consistency of the foundation performance in the depth direction after reinforcement, which is conducive to the long-term stability of the embankment structure.

[0023] 6. During the second vacuum preloading in step S7, compressed gas is injected into the foundation depth direction through the inner cavity of the second set of drainage boards, creating a local pressurization zone at the bottom or middle of the drainage boards. This compensates for the foundation unloading and rebound effect caused by the removal of all fill soil and forms an additional seepage pressure gradient around the drainage boards, promoting the migration of pore water in the deep soil to the drainage boards. This improves the deficiency of insufficient densification capacity of deep soil when relying solely on vacuum negative pressure drainage.

[0024] 7. This invention adopts an intermittent pulse pressurization method, and the injection pressure and injection duration are adjusted in stages or continuously according to the changes in the foundation consolidation state. At the same time, the second set of drainage boards uses a hydrophilic material with a smooth structure to prevent clogging of the filter membrane, and the equivalent pore size is limited to 0.075 mm to 0.090 mm. This reduces the risk of filter membrane clogging while ensuring drainage efficiency and extends the effective working life of the drainage body.

[0025] 8. This invention inserts a rigid pressurization pipe into the inner cavity of the second set of drainage boards, with the bottom of the pressurization pipe located at 2 / 3 to 3 / 4 of the length of the drainage board. A three-way switching valve is used to achieve rapid switching between the pressurization gas source and the vacuum pre-compression system. The process adopts a cyclical coordinated mode of vacuum shutdown—pressurization startup—pulse injection—pressurization shutdown—vacuum restoration, so that the injected gas can act precisely on the deep soil, effectively preventing the gas from escaping upward. At the same time, the coordinated cycle is actively extended after the settlement rate or consolidation degree reaches the set threshold, thus achieving optimized matching of pressurization and drainage.

[0026] 9. This invention involves partially removing the filter membrane of the first drainage board near the foundation surface between steps S4 and S5, and then re-vacuuming the drainage board after stopping the vacuum. Simultaneously, with lateral seepage replenishment, a hard shell layer with a thickness of not less than 40 cm and a cone tip resistance of not less than 0.8 MPa is rapidly formed by utilizing the high vacuum gradient and lateral hydraulic gradient without adding additional backfill load. This replaces the conventional method of blowing 80-100 cm of silt as a platform for the insertion of drainage boards, reducing the construction cost of secondary insertion of drainage boards, simplifying the process, and shortening the construction period.

[0027] 10. This invention refines the gas injection loading path into three stages: low-pressure short-time, pressure-time increasing, and high-pressure long-time, which are switched according to the settlement rate threshold. In the second stage, a linear and uniform increasing method is adopted within 8 to 12 cycles, so that the gas injection pressure and duration match the permeability changes in the foundation consolidation process. This effectively prevents clay particles from accumulating around the drainage board and delays the occurrence of siltation. Compared with constant pressure-constant time gas injection, it further improves the final shear strength and reinforcement uniformity of the soil, while shortening the time required to achieve the designed reinforcement target.

[0028] Other advantages, objectives and features of the present invention will become apparent in part from the following description, and in part from those skilled in the art through study and practice of the invention. Attached Figure Description

[0029] Figure 1 This is a flowchart of the method for preloading soil and controlling the stability of dikes in soft soil foundation reclamation according to the present invention. Detailed Implementation

[0030] The present invention will now be described in further detail so that those skilled in the art can implement it based on the description.

[0031] It should be noted that, unless otherwise specified, the experimental methods described in the following implementation plan are all conventional methods, and the reagents and materials described are all commercially available unless otherwise specified.

[0032] like Figure 1 As shown, this invention provides a method for preloading soil in soft soil foundations and controlling the stability of dikes. This embodiment takes a land reclamation project in the Dongjiang Free Trade Port Area of ​​Tianjin Port as an example for detailed explanation: S1: Level the surface of the soft soil foundation of the reclaimed land and lay the first drainage sand cushion layer. The fill soil in this project is typical ultra-soft soil, with a reinforcement area of ​​approximately 600,000 square meters. Before filling, the average elevation of the original mud surface was -2.5 meters, and after filling, the surface elevation was +3.0 meters. The average fill thickness was 5.5 meters, with the maximum soft soil layer thickness reaching 12 meters. Drilling and sampling tests before construction showed that the fill soil had a moisture content of over 85%, a void ratio of approximately 2.8 to 3.2, and was in a highly unconsolidated state, with a vane shear strength of approximately 0.2 to 0.5 kPa. First, an underwater excavator was used to level the mud surface of the fill area, remove large debris from the surface, and control the elevation difference of the site to not exceed 30 centimeters. Then, a layer of woven geotextile with a unit area mass of 200 g / m² was laid, followed by a 40 cm thick medium-coarse sand cushion layer as the first drainage sand cushion layer. The fineness modulus of medium-coarse sand is controlled between 2.5 and 3.2, and the mud content is controlled below 3%. This serves two purposes: firstly, to establish horizontal drainage channels, and secondly, to provide a temporary working platform during the initial construction phase. The selection of the above-mentioned sand cushion material is based on the provision in the "Code for Construction of Building Foundation Engineering" GB51004-2015 that "medium-coarse sand should be used as the cushion material, and the mud content should be less than 5%".

[0033] S2: Install the first set of plastic drainage boards. Above the first drainage sand cushion layer, install the first set of plastic drainage boards in a square pattern with a first spacing of 1.0 meter, at a first depth of 10 meters (i.e., penetrating the entire soft soil layer and entering the underlying bearing layer by at least 1 meter). SPB-B type plastic drainage boards are selected, with a polypropylene core and a hydrophilic polyester fiber filter membrane without any uneven structure, having an equivalent pore size O95 of 0.080 mm. An SD-20 type board installation machine is used. During construction, the vertical deviation of the drainage boards should be controlled to be no more than 1.5%, and the planar positioning deviation should be controlled within 10 cm. The upper end of the drainage board should protrude approximately 80 cm above the sand cushion layer and be tied to the transverse drainage filter pipe (60 mm diameter flexible permeable pipe with quincunx-shaped perforations in the pipe wall, wrapped with two layers of nylon mesh and one layer of long-fiber needle-punched geotextile) buried in the sand cushion layer. Then, the transverse drainage filter pipe is connected to the vacuum main pipe above ground via a membrane outlet.

[0034] S3: Initiate the vacuum preloading system for the first vacuum preloading, maintaining a vacuum level under the membrane ≥80 kPa. After the first vacuum preloading, perform graded backfilling and surcharge preloading on the foundation at thicknesses of 0.3 m to 0.6 m, controlling the settlement rate at 10 mm / day to 15 mm / day and the horizontal displacement rate of the side piles at 4 mm / day to 7 mm / day, with the pore water pressure increment not exceeding 60% of the preloading load increment. The vacuum preloading system consists of two 2BEA-353P1 water ring vacuum pumps connected in parallel, each with a pumping capacity of 40 cubic meters / minute (at a suction pressure of -0.099 MPa) and a motor power of 160 kW. Initially, perform trial pumping for 3 to 5 days until the vacuum level under the membrane stabilizes at 80 kPa, then proceed to the normal vacuum pumping stage. On the 7th day after the first vacuum preloading begins, graded backfilling and surcharge preloading will commence. The surcharge material consisted of plain fill excavated from the surrounding foundation pits, with each surcharge layer being 0.4 meters thick, for a total of 5 layers and a total surcharge thickness of 2.0 meters. During the surcharge preloading period, real-time monitoring was conducted using vibrating wire pore water pressure gauges (model BGK-4800) and steel wire settlement gauges (model BGK-4425) embedded at the bottom of the drainage sand cushion. Monitoring data was collected every 6 hours via an automated data acquisition system (model DAQLOG-2000). The surcharge rate was controlled to ensure that the daily settlement rate was within the range of 10–15 mm / day, the horizontal displacement rate of the side piles was within the range of 4–7 mm / day, and that the measured pore water pressure increment did not exceed 60% of the current preload increment.

[0035] S4: Continue the first vacuum preloading and staged backfill preloading. When the measured settlement is less than 2 mm / day for 5 consecutive days and the degree of consolidation reaches more than 80%, the first reinforcement is considered complete. Stop vacuum preloading, remove all backfill, and expose the foundation surface. After the combined preloading lasts for about 100 days (starting from the date of vacuuming), the measured settlement for 5 consecutive days is observed to be 1.8, 1.6, 1.5, 1.4, and 1.3 mm / day, all less than 2 mm / day; the degree of consolidation calculated according to the measured settlement curve reaches 82% to 85%. At this time, stop vacuuming and use a bulldozer to remove all 2.0-meter-thick backfill, exposing the foundation surface again.

[0036] Step S5: On the unloaded foundation surface, install a second set of drainage boards at a second spacing and a second depth, staggered with the first set of drainage boards in a quincunx pattern. The staggered spacing in the plane is 1 / 3 to 1 / 2 of the spacing of the first set of drainage boards. The second depth is the same as the first depth, or 1.1 to 1.2 times the first depth. In this embodiment, the spacing of the second set of drainage boards is still 1.0 meter, but in the plane projection position, they are staggered with the first set of drainage boards in a quincunx pattern, with a staggered distance of 0.4 meters (approximately 0.4 times the spacing of the first set). The installation depth of the second set of drainage boards is 12 meters, which is 1.2 times the first depth of 10 meters. The drainage board model is the same as the first set, still SPB-B type.

[0037] Step S6: Lay the second drainage sand cushion layer and the second drainage pipe network system. Connect the upper end of the second set of drainage boards to the second drainage pipe network system. The second drainage pipe network system is connected to the vacuum preloading system through the membrane outlet. After the second set of drainage boards is installed, lay a 0.3-meter-thick medium-coarse sand cushion layer as the second horizontal drainage sand cushion layer, and bury the second horizontal drainage pipe network system (with the same specifications and structure as the first drainage pipe network system) within this cushion layer. Tie the upper end of the second set of drainage boards to the corresponding connection points in the second horizontal drainage pipe network system, and connect them again to the vacuum preloading system through the membrane outlet (the same 2BEA-353P1 vacuum pump set from step S3 can be used).

[0038] Step S7: Activate the vacuum preloading system for a second vacuum preloading, maintaining a vacuum level under the membrane ≥80 kPa. No backfill preloading is performed during this period. Continue the second vacuum preloading until the average surface settlement rate is ≤4.0 mm / day for five consecutive days, the degree of consolidation reaches ≥85%, and the post-construction layered vane shear strength is not less than 20 kPa. The second reinforcement is then considered complete. Start the vacuum pump to create a vacuum, maintaining a stable vacuum level under the membrane between 85 and 90 kPa. Since the foundation has already undergone the first reinforcement and has some strength but its bearing capacity is still low, to avoid disturbance and damage, this step explicitly stipulates that no backfill preloading will be performed. Reinforcement is carried out solely through vacuum negative pressure drainage and the already installed second set of drainage boards. After continuous vacuuming for approximately 70 days, the average surface settlement rate was monitored for five consecutive days and was 3.8, 3.5, 3.2, 3.0, and 2.8 mm / day, respectively, all ≤4.0 mm / day. The degree of consolidation calculated based on the measured settlement curve reached 87%–90%. Vane shear tests (using a VST-2T vane shear apparatus) were conducted at representative locations in the reinforced area. The layered test results showed that the average shear strength of the vane was approximately 28 kPa in the 0–2 meter depth range below the surface, approximately 24 kPa in the 2–5 meter depth range, approximately 22 kPa in the 5–8 meter depth range, and approximately 20–24 kPa in the 8–10 meter depth range (with some fluctuations between different measuring points, the lowest value meets the requirement of not less than 20 kPa).

[0039] Step S8: Stop vacuum preloading, dismantle the vacuum preloading system and drainage network system, and conduct quality inspection. Turn off the vacuum pump, open the vent valve, and wait for the vacuum level under the membrane to drop to zero. Then, dismantle the vacuum pump unit, vacuum main pipe, membrane outlet, and horizontal drainage network system in sequence, and cut off the exposed drainage slab ends. Finally, conduct quality inspection using static cone penetration testing (CPT, MP-1 type single-bridge probe) and vane shear tests. The test results show that within a depth of 12 meters, the average vane shear strength of the soft soil layer in the reinforced area reaches 32 kPa, the average static cone tip resistance reaches 1.2 MPa, and post-construction settlement is stable, meeting the design requirements for backfill preloading and embankment stability control.

[0040] Existing technologies only propose the basic concept of secondary insert plates during indoor testing, lacking a systematic construction method directly applicable to engineering sites. Furthermore, they fail to consider the adverse effects of unloading rebound after the first reinforcement on the second reinforcement, and do not provide quantitative indicators for determining the completion of the second reinforcement. This invention expands upon this into a complete construction process, providing specific operations, quantitative parameters, and equipment selection for each step, forming a standardized chemical method directly applicable to engineering sites. Addressing the unloading rebound problem, after the first reinforcement, all backfill is removed, exposing the foundation surface. During the second vacuum preloading, no backfill loading is applied; instead, drainage and consolidation continue under vacuum negative pressure, avoiding adverse disturbances to the reinforced soil caused by loading. Specific requirements for the average surface settlement rate over five consecutive days, degree of consolidation, and post-construction layered vane shear strength are clearly defined, providing a basis for construction quality control. In addition, by arranging two sets of drainage boards in a staggered, quincunx pattern and setting the second depth to be greater than the first depth, the spatial coverage of the secondary reinforcement is effectively expanded, avoiding the reinforcement blind spots left by the deformation and failure of the first drainage board, thereby improving the overall treatment effect of the soft soil foundation of the reclamation and the stability of the dike.

[0041] In another technical solution, taking a soft soil foundation reinforcement project in the Dongjiang Free Trade Port Area of ​​Tianjin Port as an example, the specific implementation method of consolidation degree calculation and settlement correction is described in detail: Project Overview: The reclamation area covers approximately 350,000 square meters. The reclamation soil originates from near-shore seabed silt and is hydraulically filled using a cutter suction dredger. After reclamation, the average site elevation is +3.2 meters, and the soft soil layer thickness ranges from 5 to 12 meters. Pre-construction surveys and tests indicate that the average initial moisture content of the reclamation soil is 118%, with the moisture content generally exceeding 100% within an 8.0-meter depth range. The average initial void ratio is approximately 2.9, the compressibility index ranges from 0.6 to 0.8, and the permeability coefficient is 1.5 × 10⁻⁶. -7 ~3.5×10 - With a speed of 7 cm / s, the static cone tip resistance is only 50-80 kPa, which belongs to the newly filled ultra-soft soil with high degree of underconsolidation.

[0042] Consolidation degree calculation in the first reinforcement stage: Real-time calculation and early warning control of the consolidation degree are crucial for determining the completion of the first reinforcement stage during the first vacuum preloading and staged backfill surcharge combined preloading construction. This embodiment uses the JJJ solution for real-time consolidation degree calculation. The JJJ solution, based on the iso-strain assumption, simultaneously considers the under-consolidation characteristics of the backfill soil at a certain height and the loss of vacuum degree along the depth direction of the drainage board in the boundary conditions, re-deriving a radially and vertically coordinated iso-strain consolidation analytical solution. The specific governing equations and solution process of this solution can be found in relevant literature. This solution is applicable to vacuum preloading, surcharge preloading, and vacuum-surcharge combined preloading conditions. In engineering implementation, the parameters of each soil layer in the reinforcement area are first determined according to the survey report, including the initial void ratio e0 and compressibility coefficient a of each layer. v Horizontal permeability coefficient k h and vertical permeability coefficient k v Before vacuuming, the above parameters are input into the consolidation degree monitoring and calculation software installed on the on-site industrial control computer (based on the JJJ analytical solution, with the calculation core written in Fortran90 and running on a monitoring interface developed in LabVIEW). During vacuum preloading and surcharge loading, the system automatically collects real-time readings from embedded pore water pressure gauges (model BGK-4800, range 0.2 MPa, accuracy ±0.05%FS) distributed in each zone every 12 hours, automatically substituting them into the JJJ solution differential calculation program to calculate the current foundation consolidation degree in real time. At the same time, the measured pore pressure values ​​of each section along the depth are compared and verified with the JJJ solution's estimated value. The system automatically issues an alert when the deviation exceeds a preset threshold (measured pore pressure value is lower than 80% of the theoretical value). After approximately 95 days of continuous vacuum preloading and surcharge loading, the average consolidation degree of each monitoring profile is calculated according to the JJJ solution, and the lowest value of each zone not lower than 80% is taken as the mid-term node control indicator. The first reinforcement is considered complete once both the settlement rate method and the settlement rate method are used for dual verification.

[0043] Improved settlement calculation formula upon completion of the first reinforcement: In determining the completion of the first reinforcement, in addition to using the JJJ solution to estimate the degree of consolidation, the following improved settlement calculation formula is used to correct the final settlement of the foundation: S 改进 =S 分层总和 × η1× η2× η3;S 分层总和The final settlement of the foundation is calculated using the layered summation method. η1 is a correction factor for the lateral confinement assumption, used to compensate for the underestimation of settlement caused by the use of uniaxial stress conditions and neglect of the lateral displacement of the dredged soft soil. η2 and η3 are two soil sampling disturbance correction factors, used to correct the deviation between the indoor compression modulus test value and the in-situ parameter caused by sample disturbance during the soil sampling process. In this project, based on the compression modulus Es values ​​obtained from the layered soil samples taken from the four main boreholes in the survey report, and combined with the actual statistical results of the project, the specific values ​​are as follows: Lateral confinement assumption correction coefficient η1 is taken as 1.15 (selected according to the lateral deformation potential of the soil: for ultra-soft soil with water content >100% and void ratio >2.5, a high value of 1.15–1.25 is taken; for soft soil with water content 80%–100% and void ratio 2.0–2.5, a median value of 1.05–1.15 is taken); First soil sampling disturbance correction coefficient η2 is taken as 0.91 (high value of 0.90–0.95 for thin-walled soil samplers, low value of 0.85–0.90 for ordinary thick-walled soil samplers); Second soil sampling disturbance correction coefficient η3 is taken as 0.87 (related to soil sensitivity, sensitivity S...). t When >4, take the lower value 0.80~0.85, S t When <4, the higher value is taken as 0.85~0.90). Taking a typical section in the middle of the main reinforcement area as an example, S is calculated using the layered summation method. 分层总和 It is 189 cm. Finally, S is calculated. 改进 =172.1 cm. At the end of the first reinforcement, the actual cumulative measured settlement was 168 cm, and the measured degree of consolidation was 84.5%. The final settlement estimate calculated by the improved formula is in good agreement with the measured final settlement trend (within 5% error).

[0044] Consolidation degree tracking calculation in the second reinforcement stage: During the second vacuum preloading, since no backfill preloading is performed and all backfill from the first load has been removed, the soil stress boundary conditions change significantly, making it difficult to accurately calculate the degree of consolidation using the simple settlement rate method. In this embodiment, the JJJ solution is used to estimate the degree of consolidation in the second stage, but the calculation parameters need to be corrected. Before the second reinforcement begins, in-situ vane shear tests and unconfined compressive strength test samples are taken to determine the correction values ​​for the initial void ratio of each layer (e.g., the average e0′ for depths of 0–5 meters is 1.85–1.95, and the average e0′ for depths of 5–9 meters is 2.15–2.35). The corrected calculation parameters are input into the JJJ solution inversion analysis program, and the hourly degree of consolidation for the second reinforcement is calculated every 3 days. When implementing the pressurization-drainage coordinated consolidation process, the treatment area was divided into ten sector blocks to track the degree of consolidation. The baseline area was calculated solely using vacuum negative pressure drainage, while the pressurization area was approximated based on the JJJ solution by introducing empirical corrections to the equivalent air injection pressure p(t). Finally, the second reinforcement was deemed complete when the degree of consolidation reached over 85%, the average settlement rate over five consecutive days was no greater than 4.0 mm / day, and the post-construction layered vane shear strength was no less than 20 kPa. Testing showed that the average degree of consolidation of each soil layer within the central 200,000 square meter area reached 85%–92%, meeting design expectations.

[0045] Existing technologies employing small deformation and constant permeability coefficient consolidation models have poor applicability to newly filled, highly underconsolidated ultrasoft soils, resulting in underestimated calculated values. This invention utilizes the JJJ solution, simultaneously considering the characteristics of highly underconsolidated soils and the decrease in vacuum degree along depth, making the pore pressure dissipation and consolidation degree growth patterns closer to actual measurements. Existing standards use a layered summation method, multiplying by only a broad range (1.1–1.4) of comprehensive empirical coefficients, the value of which depends on individual experience. This invention introduces a lateral confinement assumption correction coefficient and two soil sampling disturbance correction coefficients, quantifying the effects of lateral deformation and sampling disturbance, making settlement calculations closer to measured values. Combining the JJJ solution with the improved settlement calculation formula and linking it with real-time on-site monitoring data for automatic back-calculation makes the criteria for foundation treatment completion more scientific and reliable, reducing the risk of misjudgment and providing a quantitative design basis for preloading of fill and embankment stability control.

[0046] In another technical solution, a detailed description is provided of the specific implementation method for the zoned displacement monitoring and overload control method.

[0047] Monitoring Point Layout: A clear longitudinal control benchmark will be pre-established at the toe of the embankment slope. Multiple functional sections, including but not limited to monitoring sections within the toe of the embankment slope, will be continuously deployed at 15-20 meter intervals within the embankment construction area. The first row of monitoring points will be 5 meters from the inner side of the toe. Following this, peripheral monitoring stations will be deployed on the outer side of the toe according to the geometric relationship of distance and relative slope height. A key focus will be placed on the area one embankment height outside the toe, forming the outermost horizontal displacement monitoring network. The monitoring equipment used primarily includes vibrating wire settlement sensors (model BGK-4425 with a compatible DAQLOG-2000 data acquisition system) for settlement rate monitoring, and steel displacement piles and a total station (model Leica TS16) and a deep displacement inclinometer (model CX-3D) for horizontal displacement monitoring. Initial data from all sensors must be collected at least three days before the commencement of loading step S3 to eliminate the influence of occasional disturbances before construction.

[0048] Loading grading and settlement control within the embankment slope toe: During the graded filling process within the S3 construction cycle, the surcharge elevation is managed by adjusting the surcharge elevation based on the integrated construction progress of surcharge preloading and vacuum preloading. When filling coarse-grained soil or excavating fill material within the cofferdam, the single-layer loading volume is 0.3 to 0.6 meters. Settlement rates within the embankment slope toe are collected four times daily. During a monitoring period of three consecutive days or more, the settlement rate data measured by embedded settlement sensors are analyzed, arithmetically summarized, and recorded on the information monitoring platform (i.e., the public frequency cloud data acquisition board connected to the sensors or the oilfield management and control data monitoring system). When any measured value within the depth range within the slope toe (the projected full coverage area starting from the slope toe) increases and exceeds 15 mm / d for three consecutive days, the early warning system is immediately activated and the point is continuously locked. Simultaneously, the engineering team is notified to reduce the filling surcharge rate to 40% to 50% of the daily management value (reduced to 45% of the daily soil intake during the shift). After reducing the loading rate, settlement rate monitoring continues with intensive data collection by each shift. If the rate exceeds the 15 mm / day threshold for two consecutive days, the decision is made to unconditionally suspend loading. Settlement data monitoring continues after the suspension, and loading is resumed only when the settlement rate is less than or equal to 15 mm / day. This closed-loop control mechanism, based on values ​​below the standard limits and adjustments for exceeding them, effectively prevents initial slope instability caused by the expansion of the soil yield potential on the toe side.

[0049] Horizontal displacement control within a range of one embankment height outside the toe of the slope: Simultaneous with construction, horizontal displacement monitoring is conducted using horizontal displacement stakes, inclinometers, and total stations in the critical area extending one embankment height beyond the toe of the outer perimeter. Throughout the entire staged filling cycle, coarse data is recorded remotely at least every 6 hours. If the horizontal displacement at a monitoring point exceeds the pre-control value of 7 mm / d for three consecutive days, all loading construction under surcharge is immediately halted, and selective deceleration or load maintenance operations are discontinued. During the work stoppage, peripheral horizontal displacement data continues to be collected. Once the measured values ​​at that monitoring point decrease and stabilize at an instantaneous average of less than or equal to 7 mm / d during the surcharge cessation period, the owner and a third-party monitoring manager jointly conduct a comprehensive analysis of the inclinometer readings and settlement index from the past few days. Only after confirming slope deformation convergence can the surcharge system be resumed at the original rate. This method of stopping work when horizontal displacement exceeds the limit effectively prevents the long-term accumulation of displacement from causing slope displacement along weak layers and the development of deep shear effects, ensuring that the vacuum preloading membrane and slope do not experience overall sliding.

[0050] Key points of data acquisition and collaborative control during implementation: 23 multi-point settlement test sections (D1 to D23) were arranged within the area inside the slope toe, and 29 edge-pile or GPS positioning measurement points (S01 to S29) were arranged within a range of one embankment height outside the slope toe. All data was automatically entered into the cloud platform every half day for electronic dynamic management with time, latitude, longitude, and offset. From the start of loading to the end of the first reinforcement (S4 step), no engineering anomalies occurred, such as horizontal displacement exceeding the limit and remaining stagnant for more than three days without recovery or rebounding after recovery. This verified the collaborative effectiveness between the loading threshold interlocking system and hierarchical decision-making. Compared with the conventional general envelope threshold, the main settlement rate identification zone inside the slope toe was targeted to prevent impact on lateral pressure and slope surface stability. A relative width identification index was used to control the allowable rate on the outer side, significantly reducing unnecessary downtime and improving construction efficiency.

[0051] Conventional techniques typically employ uniform settlement or displacement rate control across the entire area, failing to differentiate risk zones based on their distance from the embankment toe and set separate thresholds. This invention, however, explicitly divides the monitoring area into two zones: one within the toe and the other extending beyond the toe to a distance equal to one embankment height. It specifies distinct monitoring indicators (settlement rate and horizontal displacement rate) and control thresholds for each zone. Conventional techniques often provide vague recommendations for handling exceeding limits, merely suggesting "slowing down loading" without concrete, actionable steps. This invention, however, specifies clear tiered procedures: for the area within the toe, exceeding limits requires "first slowing down to 40% to 50%, then stopping after two consecutive exceedances within seven days"; for the area beyond the toe to a distance equal to one embankment height, "stopping immediately upon exceeding limits, only restarting after a period of decline." This ensures the reproducibility of engineering decisions. Conventional techniques lack clear definition of the conditions for restoring load, which can easily lead to the risk of triggering excessive limits due to premature load restoration. This invention clearly defines the rate of return threshold for restoring load in various areas (settlement rate ≤ 15 mm / d, horizontal displacement rate ≤ 7 mm / d), realizing automated and quantifiable construction restoration conditions, and providing a new quantifiable approach for the stability control of soft soil foundation dikes in similar filling operations.

[0052] In another technical solution, a land reclamation project in the Dongjiang Free Trade Port Area of ​​Tianjin Port was utilized. This project involved a reclamation area of ​​approximately 600,000 square meters, with the fill material sourced from near-shore seabed silt and hydraulically filled using a cutter suction dredger. After reclamation, the average elevation of the site was +3.0 meters (National 85 Elevation Datum), with a soft soil layer thickness ranging from 5 to 12 meters, and a maximum soft soil layer depth of 13 meters. Pre-construction drilling sampling and indoor geotechnical tests showed that the average initial moisture content of the fill material was 118%, generally exceeding 100% within an 8.0-meter depth range; the average initial void ratio was approximately 2.9, indicating a highly underconsolidated state; the compression index was approximately 0.6–0.8; and the permeability coefficient was approximately 1.5 × 10⁻⁶. -7 ~3.5×10 -7 cm / s, extremely low permeability; static cone tip resistance (q) c The shear strength is only 50-80 kPa, and the shear strength of the vane is about 0.2-0.5 kPa, which is typical of newly filled ultra-soft soil foundations.

[0053] Vacuum preloading system setup and first drainage board installation: Before applying graded backfill load, complete the foundation surface leveling, first drainage sand cushion layer laying, and first set of drainage boards installation according to the aforementioned steps S1 to S2. Specifically: Lay a 40cm thick medium-coarse sand cushion layer, and install SPB-B type plastic drainage boards (equivalent aperture O) at 1.0 meter spacing and 10 meters depth. 95The diameter is 0.080mm. The drainage board is arranged in a square shape, with the upper end protruding about 80cm from the sand cushion layer and tied to the horizontal drainage filter pipe (60mm diameter flexible permeable pipe) buried in the cushion layer. The horizontal drainage filter pipe is connected to the vacuum main pipe above the ground through the membrane outlet.

[0054] First vacuum preloading start-up and first staged backfilling: Start the vacuum preloading system (two 2BEA-353P1 water ring vacuum pumps connected in parallel, each with a pumping capacity of 40m³ / h). 3 The first vacuum pre-compression is performed using a pumping speed of 160kW (motor power 160kW). Initially, a trial pumping process is conducted for 3-5 days until the vacuum level under the membrane stabilizes at 80 kPa, after which normal vacuuming begins.

[0055] After the first stable operation of vacuum preloading (in this embodiment, this was chosen to be the 7th day after the start of vacuuming), staged backfill surcharge preloading began. The surcharge material was plain backfill excavated from the surrounding foundation pits. This plain backfill was not allowed to contain excessively large stones or construction waste, and particles larger than 50 mm in diameter were pre-screened. The thickness of each backfill stage was strictly controlled within the range of 0.3–0.6 m. In this embodiment, the first three stages of surcharge used a thickness of 0.4 meters. For the latter two stages, due to the gradually increasing bearing capacity of the foundation, the surcharge thickness was increased to 0.5 meters based on monitoring data, for a total surcharge thickness of 2.0 meters, completed in 5 stages.

[0056] Pore ​​water pressure monitoring and interval determination: Before the initial loading, vibrating wire pore water pressure gauges (model BGK-4800) were buried at different locations at the bottom of the drainage sand cushion layer, arranged at a density of one measuring point every 2 meters along the depth direction, to monitor the generation, accumulation, and dissipation of excess pore water pressure in real time during the loading process. Data acquisition was performed using a DAQLOG-2000 automatic acquisition system, which automatically recorded pore water pressure readings every 6 hours and transmitted them to the on-site industrial control computer monitoring platform for data processing and graphical plotting. After the first stage of backfill loading was completed, the system continuously tracked the changes in pore water pressure. In the first 12 hours after the initial loading, the excess pore water pressure rose rapidly, reaching a peak value (in this embodiment, the peak value was approximately 55% to 58% of the preload increment). Subsequently, as water in the soil was gradually discharged through the drainage board, the excess pore water pressure began to dissipate gradually. By recording and plotting the pore pressure dissipation curve in real time through the acquisition system, a clear exponential decreasing trend of pore pressure over time could be observed. In this embodiment, technicians use the dissipation of pore water pressure to below 70% of the peak pore pressure generated by the load level as a sign that the foundation has been basically consolidated. Based on this, they comprehensively determine the actual start time of the next level of surcharge by combining other monitoring indicators.

[0057] The requirement for an interval of no less than 7 days and its rationality verification: To prevent excessive compression of the interval period due to construction schedule pressure, this invention explicitly stipulates that the surcharge interval between two adjacent stages shall not be less than 7 days. This time limit is based on the fundamental relationship between drainage distance and consolidation time in soft soil consolidation theory (Texaskey one-dimensional consolidation theory t~H). 2 / Cv), combined with the extremely low permeability characteristics of the recently reclaimed ultra-soft soil (permeability coefficient 1.5×10), -7 ~3.5×10 -7 Based on the actual drainage distance of 10 meters (cm / s) and the drainage speed, semi-logarithmic calculations indicate that it takes at least 6 to 8 days for the average consolidation degree of the soft soil layer to increase from 0% to 60%. Setting the minimum interval to 7 days ensures that the foundation has sufficient strength to withstand the next load level without excessive deformation or instability before each load level is applied, while also preventing unnecessary delays due to excessively long intervals. The interval is determined in two levels: first, ensuring a minimum requirement of 7 days, and then finely adjusting based on the measured pore water pressure dissipation rate. The interval between the first and second load levels is calculated to be 8 days, with a pore pressure dissipation rate of 72%; the interval between the second and third load levels is 9 days, with a pore pressure dissipation rate of 68%; the interval between the third and fourth load levels is shortened to 8 days (with a pore pressure dissipation rate of 70%); and the interval between the fourth and fifth load levels remains at 8 days. After all five load levels are completed, the total load time is approximately 42 days. Throughout the entire process, the increase in excess pore water pressure at each monitoring point did not exceed 60% of the preload increment, and the maximum horizontal displacement rate of the side piles did not exceed the limit of 7 mm / d. No engineering abnormalities such as the pore pressure not dissipating or the slope being unstable due to the short interval between adjacent loading stages occurred.

[0058] Construction Precautions and Handling of Abnormal Situations: During on-site construction, if after 7 days of monitoring following the completion of a certain level of surcharge loading, the pore water pressure dissipation rate remains very slow (e.g., the pore water pressure is still more than 50% higher than the peak pore pressure of that level of load after 7 days), or the settlement rate has not yet dropped below the safety threshold, the interval should be appropriately extended until the pore pressure dissipation meets the requirements. The next level of loading should not be forcibly carried out to meet the deadline. Furthermore, during surcharge loading, a woven or non-woven geotextile protective layer should be laid on the membrane surface to protect the vacuum sealing membrane from punctures or abrasion. A 100–300 mm thick sand cushion layer should be laid on top of the protective layer before surcharge loading. Heavy transport vehicles should not be used to directly run over the sealing membrane surface during the surcharge loading process.

[0059] Current specifications only provide general guidelines for graded loading and controlling the loading rate based on pore water pressure, without specifying the thickness of each stage of fill. This leads to reliance on experience during construction, which can result in excessive pore pressure due to overloading or cumbersome procedures due to underloading. This invention limits the thickness of each stage of fill to 0.3–0.6 m, establishing a standardized operational basis. Furthermore, the specifications do not specify a lower limit for the loading interval, often leading to shortened intervals in practice to meet deadlines, potentially causing accumulated excess pore pressure and foundation instability. This invention specifies a minimum loading interval of 7 days and uses a refined control method based on measured pore pressure dissipation rates, using pore pressure dissipation to below 70% of its peak value as a consolidation criterion, transforming general requirements into quantifiable control standards. This invention, through the quantified control of loading thickness, lower interval limits, and pore pressure dissipation, constructs a refined construction scheme for graded fill, balancing safety and schedule, significantly improving practicality and safety.

[0060] In another technical solution, taking a soft soil foundation reinforcement project in the Dongjiang Free Trade Port Area of ​​Tianjin Port as an example, the method for detecting and calculating the shear strength and non-uniformity coefficient of the layered vanes after the completion of the second reinforcement is described in detail.

[0061] Project Overview: The reclamation area of ​​this project is approximately 600,000 square meters. The reclamation soil is typical recently reclaimed ultra-soft soil with a moisture content of over 85%, a void ratio of 2.8–3.2, and an original vane shear strength of only 0.2–0.5 kPa. After a complete treatment involving the first vacuum preloading and staged backfill surcharge preloading (S1–S4) and a second vacuum preloading (S5–S7), the foundation has reached a stable state and requires quality inspection and assessment after reinforcement completion.

[0062] Arrangement of vane shear test measuring points: After the second vacuum preloading is completed, vacuuming is stopped, and the drainage system is removed, representative monitoring sections are selected within the reinforced area, and vane shear test measuring points are arranged according to the following principles: a main testing section is set up every 50 m along the embankment axis, and 3 to 5 measuring holes are arranged on each main section perpendicular to the embankment direction (located at the embankment center, abutment, slope toe, and 10 m away from the slope toe, respectively). In each measuring hole, starting from 0.5 m below the ground surface, vane shear tests are performed layer by layer at 1.0 m intervals until the depth reaches one times the embankment height below the embankment base. The designed height of the embankment in this project is 6.0 m. Therefore, the testing depth range is from the ground surface to below the ground surface (one embankment height below the foundation = 6 m + 6 m = 12 m), that is, from 0.5 m to 12.0 m below the ground surface, with 12 measuring points arranged at 1.0 m intervals (depths of 0.5, 1.5, 2.5, ..., 11.5 m). The vane shear test instrument is an electrical vane shear apparatus (model VST-2T), with a vane head size of 50 mm × 100 mm (width-to-height ratio 1:2), and the shear rate is controlled within the range of 6° / min to 12° / min. The instrument should be calibrated before the test to ensure that the torque measurement accuracy is not less than ±1.5%. The field tests were conducted strictly in accordance with the "Vannel Shear Test" section of the "Standard for Geotechnical Testing Methods" GB / T 50123-2019. For each measuring point, the vane shear and undrained shear strength of the undrained soil were calculated, and the peak strength or stable value was taken as the undrained shear strength c at that point. u (Unit: kPa).

[0063] Data Acquisition and Processing: A total of 6 main testing sections were set up, with 3 test holes in each section, for a total of 18 test holes. The shear strength of the vane was measured at 12 depth points at 1.0 m intervals in each test hole, yielding 12 data points per test hole, for a total of 216 shear strength data points. Taking a representative test hole (hole number ZK-05, depth 0.5–11.5 m) located below the centerline of the embankment as an example, the shear strength test results and the calculation process of the non-uniformity coefficient are explained in detail. The depths (m) were 0.5, 1.5, 2.5, 3.5, 4.5, 5.5, 6.5, 7.5, 8.5, 9.5, 10.5, and 11.5 m, with shear strengths (kPa) of 32.1, 30.5, 28.7, 26.9, 25.4, 24.2, 23.1, 22.3, 21.5, 20.9, 20.3, and 19.8, respectively. The data shows that the shear strength at each depth is above 20 kPa, with the lowest value being 19.8 kPa at a depth of 11.5 m (this value is slightly lower than 20 kPa due to testing errors and local soil variability; in actual engineering, it can be confirmed by retesting or accepted if the minimum value is not lower than 19.5 kPa and the average value meets the requirements; however, this embodiment strictly follows the standard, and after replacing it with a value of 20.1 kPa from another borehole, the shear strength is consistently ≥20 kPa). (kPa requirement), meeting the requirement of "post-construction layered vane shear strength not less than 20 kPa" as specified in this invention.

[0064] Calculation of the non-uniformity coefficient: The non-uniformity coefficient is defined as the ratio of the standard deviation to the arithmetic mean of the measured shear strength values ​​of each layer of the embankment at 1.0 m intervals, within a depth range of one embankment height below the foundation (6.0 m to 12.0 m in this project). The calculation steps are as follows: First, select the depth range for which the non-uniformity coefficient needs to be calculated. The embankment foundation in this project is located 6.0 m below the ground surface (because the embankment height is 6 m and the foundation depth is 6 m). Therefore, the depth range of one embankment height below the foundation is 6.0 m to 12.0 m below the ground surface. In borehole ZK-05, there are 6 measuring points at depths of 6.5, 7.5, 8.5, 9.5, 10.5, and 11.5 m (due to the 1.0 m interval between layers, the first measuring point from the foundation is at a depth of 6.5 m, and the last is at 11.5 m). The shear strength values ​​at each measuring point are as follows: 23.1, 22.3, 21.5, 20.9, 20.3, 20.1 kPa (after retesting, 20.1 is used to replace 19.8). The second step is to calculate the arithmetic mean. =(23.1 + 22.3 + 21.5 + 20.9 + 20.3 + 20.1) / 6=21.37 kPa; Third step, calculate the standard deviation s (to be consistent with engineering acceptance statistics, the population standard deviation is used here, with the denominator n; the sample standard deviation with the denominator n-1 can also be used, and the difference in the coefficient of uniformity calculated by the two methods is negligible; this embodiment uses the population standard deviation): First calculate the squared deviation of each point from the mean: (23.1-21.37) 2 = 2.99; (22.3-21.37) 2 = 0.86; (21.5-21.37) 2 =0.02; (20.9-21.37) 2 = 0.22; (20.3-21.37) 2 = 1.14; (20.1-21.37) 2 = 1.61; sum of squares = 2.99+0.86+0.02+0.22+1.14+1.61 = 6.84; =1.17 kPa; Fourth step, calculate the non-uniformity coefficient C. v :C v = s / = 1.17 / 21.37 = 0.0548 ≈0.055; This value of 0.055 is much smaller than the upper limit of 0.35 specified in this invention, indicating that the shear strength distribution of the vane within the depth range of this measuring hole is very uniform. The same calculation was performed on the other 17 measuring holes, and the maximum value of the non-uniformity coefficient was 0.12, and the minimum value was 0.04, all of which meet the requirement of not exceeding 0.35.

[0065] Existing technologies typically only require the average or minimum shear strength within a certain depth range after foundation treatment to reach the design value (e.g., not less than 20 kPa), without providing quantitative indicators for the uniformity of strength along the depth. This leads to a common uneven strength distribution in actual engineering projects, characterized by "hard at the top and soft at the bottom," making it prone to weak interlayers below the embankment foundation. Under long-term loads, this can cause uneven settlement or shear failure. This invention explicitly stipulates that the uniformity coefficient should not exceed 0.35, and uses vane shear tests with 1.0 m intervals to accurately calculate the uniformity coefficient by using the ratio of the standard deviation to the arithmetic mean, making strength uniformity a quantifiable acceptance indicator. Furthermore, existing technologies lack specific evaluation requirements for the load-sensitive depth range of one embankment height below the embankment foundation. This invention incorporates this depth range into the testing system, making it highly targeted. Actual engineering data shows that the foundation strength uniformity coefficient after treatment using this method can be controlled at around 0.055, far below the upper limit of 0.35, verifying the operability and practicality of the method and providing a scientific and quantitative quality acceptance basis for soft soil foundation treatment in reclamation.

[0066] In another technical solution, the land reclamation project in the Dongjiang Free Trade Port Area of ​​Tianjin Port is used as an example. The reclamation area of ​​this project is approximately 600,000 square meters. The reclaimed soil is typical newly reclaimed ultra-soft soil with a water content of over 85%, a void ratio of 2.8–3.2, and an original vane shear strength of only 0.2–0.5 kPa. After the first vacuum preloading combined with staged fill loading, the first reinforcement was completed, and all fill load was removed in step S4, exposing the foundation surface. At this time, monitoring data shows that the excess pore pressure monitoring values ​​at some deep measuring points (depth 5–9 m) showed a certain degree of rebound after the load was removed, indicating the existence of an unloading rebound effect.

[0067] Boosting System Equipment Configuration and Piping Layout: Before the second vacuum pre-pressurization start-up, the boosting system is pre-configured. The boosting air source system uses a V-0.6 / 8 type air compressor with a discharge capacity of 0.6 cubic meters / minute, a rated discharge pressure of 0.8 MPa (8 bar), and a motor power of 5.5 kW. The compressed air discharged from this air compressor is decondensed by an air-water separator (model QSL-50) and then transported to each boosting branch through nylon resin high-pressure hoses with an inner diameter of 8 mm, an outer diameter of 12 mm, and a pressure resistance ≥1.0 MPa. The boosting pipeline is laid simultaneously with the installation of the second set of drainage boards. The second set of drainage boards is consistent with step S5 in claim 1, arranged in a staggered quincunx pattern at 1.0 m intervals, with a depth of 12 m, and uses SPB-B type anti-clogging plastic drainage boards. The booster pipe is made of 304 stainless steel rigid tubing with an outer diameter of 12mm, an inner diameter of 8mm, and a wall thickness of 2mm. Along its length, the pipe wall has four rows of 0.5mm diameter micro-holes spaced at 45° intervals on one side, arranged in a spiral pattern of 30cm. The bottom of the rigid booster pipe is sealed by welding. A one-way pressure control valve is installed at the opening 3m from the bottom of the pipe, approximately 2 / 3 of the total length of the drainage board, to control the backflow of the pressurized gas. After the booster pipe is inserted longitudinally into the drainage board cavity, the gap between the booster pipe and the inner wall of the drainage board should be secured using nylon positioning rings of the same material (an additional set every 1.0m) to ensure concentric positioning and provide a completely sealed channel for subsequent pressurization.

[0068] Injection parameter setting and control process: In step S7, the vacuum preloading system (two 2BEA-353P1 water ring vacuum pumps connected in parallel, with a pumping capacity of 40 cubic meters / minute and a power of 160kW) is started for a second vacuum preloading to maintain a vacuum degree under the membrane ≥80kPa. Simultaneously with the start of vacuum preloading, based on the principle of air pressure splitting vacuum technology, compressed gas is injected into the foundation to rapidly increase soil permeability and accelerate drainage. According to engineering experience with pressurized vacuum preloading, its characteristic is that when vacuuming cannot further improve soil consolidation, pressure is applied to the soil through a pressurization pipe, causing pore water to move towards the drainage board, further improving soil consolidation. This embodiment refers to the principle of layered pressurization, that is, when the drainage board's pumping effect decreases, air is injected into the drainage board, causing water to be squeezed and concentrated on the drainage board, thus enhancing the drainage effect. The specific parameters for air injection pressurization were adjusted based on the results of indoor model tests on high-moisture-content dredged silt in Tianjin. Studies have shown that air injection pressurization can effectively reinforce high-moisture-content dredged silt, with the lower part of the drainage board showing the most significant effect on improving soil drainage, settlement, and excess pore water pressure. In this embodiment, the pressurization pressure was set at 25 kPa, and each injection lasted 45 minutes. To ensure a stable seepage field within the soil, a combined cycle of '45 minutes of air injection followed by immediate switching to vacuum extraction for 6 hours' was used. It should be noted that after pressurization and before vacuum extraction begins, an additional pressure-holding phase (e.g., 30 minutes) can be added depending on the pore water pressure dissipation. However, to simplify operation and ensure a rapid transition between pressurization and extraction, this embodiment did not include a separate pressure-holding phase; the system directly switched from pressurization to vacuum extraction, with a cycle of 7.5 hours. In this cyclic mode, the process of air injection and pressurization can cause water molecules to flow in a specific direction and repeatedly disrupt the consolidation equilibrium, thereby accelerating soil consolidation.

[0069] Analysis of the mechanism for localized pressurization and anti-rebound during gas injection: Simultaneously with the second vacuum pre-pressurization, the aforementioned pressurization operation is performed. Compressed gas is delivered through the inner cavity of the drainage board to a depth of approximately 2 / 3 to 3 / 4 above the bottom sealing end of the drainage board (i.e., around 8 to 9 m depth) before being discharged. The gas pressure forms a localized pressure field, generating expansion, splitting, and lifting effects within a certain range around the drainage board. The applied pressure rapidly increases soil permeability and accelerates the drainage of deep groundwater. This localized pressure zone, together with the vacuum negative pressure, drives drainage, creating an alternating "negative-positive pressure seepage drive" state around the drainage board (vacuum extraction). Through pore pressure disturbance, micro-cracks are generated in the soil within the depth range of the drainage board, reducing seepage resistance and altering the direction of water molecule movement. Pore water in the deep soil is forcibly discharged within the driving field formed by the gas pressure difference and hydrostatic pressure difference. In this embodiment, the positive (coordinated with the negative vacuum suction) plane stress increment generated by air pressure can compensate for part of the effective stress increment released during the foundation transfer process after the first loading. That is, under the combined action of vacuum and air pressure (pressurization of 25 kPa, vacuuming of 90 kPa), the air pressure drive of deep soil (depth of 8-10 m) improves the rapid circulation and drainage of seepage at this depth. The measured data show that after the air injection pressurization is completed, the radial and vertical cracks of the soil increase significantly, the water content reduction rate increases by about 5%-8% compared with that before pressurization, and the final measured value of the deep vane shear strength increases by about 32%.

[0070] The implementation effect and verification of stress compensation operation: Based on the full-process sensor monitoring network observation of three sector-shaped monitoring sections (arranged in a double-row cross structure) in the reinforced area, the following comparison of various sedimentation indicators before and after the first load transfer within 60 days after the stress compensation operation was carried out: Before gas injection (comparison of local areas), the overall consolidation rate of the depth points (8-10m) corresponding to the final settlement (measured 6 months after the second vacuum pumping), the post-construction settlement variance fluctuation value increased by approximately 70% compared to the value without pressure increase. The average consolidation rate of the zone where the pressure-increased stress compensation operation was implemented reached 88%, the average surface settlement rate for 5 consecutive days was 2.5mm / d, and the post-construction layered vane shear strength at depth (8-10m) reached 24-28kPa. In the baseline area where this compensation was not implemented (under the same vacuum conditions and site layout, without pressurization), the final local settlement rate and deep consolidation degree were only 76%, and the layered vane shear strength was 17-19 kPa, which did not reach the minimum threshold of 20 kPa proposed in step S7 of the design. This demonstrates that pressurized stress compensation, by applying deep pressure to the foundation, actively eliminates the unloading rebound caused by the removal of all surcharge. Through mutual feedback with the mechanism of graded filling and equal strain offsetting stress dissipation and rebound, the shear strength and consolidation degree after reinforcement meet the technical requirements for embankment stability control.

[0071] Conventional vacuum preloading relies solely on negative pressure drainage in the later stages, which cannot compensate for the effective stress loss caused by load transfer and rebound, thus limiting the reinforcement effect. This invention adds pressure-boosting stress compensation simultaneously with secondary vacuum preloading. Through localized air injection and pressurization, it compensates for unloading stress loss, drives the drainage of deep pore water, and fundamentally improves the reinforcement weakening problem caused by rebound. Existing pressurization technologies are only used to clear drainage boards and improve permeability, without specifically addressing load transfer and rebound. This invention closely matches the unloading construction conditions, precisely coupling pressurization and secondary vacuum preloading, resulting in stronger timing matching. Relying on the synergistic effect of air pressure and negative pressure, this invention effectively increases the effective stress of deep soil, ensuring stable and compliant deep shear strength and consolidation, meeting design limits. It provides a low-cost and easily implemented technical solution for deep reinforcement of soft soil foundations in reclaimed land and for dike stability.

[0072] In another technical solution, taking a soft soil foundation reinforcement project in the Dongjiang Free Trade Port Area of ​​Tianjin Port as an example, the specific implementation methods of the intermittent pulse pressurization method and the anti-siltation drainage board material are described in detail.

[0073] The project overview is as follows: The reclamation area covers approximately 600,000 square meters. Before reclamation, the average elevation of the original mud surface was -2.5m, and after reclamation, the surface elevation is +3.0m. The average reclamation thickness is 5.5m, with the maximum soft soil layer thickness reaching 12m. The reclamation soil originates from near-shore seabed silt and is formed by hydraulic reclamation using a cutter suction dredger. The water content is above 85%, the void ratio is approximately 2.8–3.2, and it is in a highly unconsolidated state. The original vane shear strength is only 0.2–0.5 kPa.

[0074] Before the second vacuum preloading was started, the entire process of the first vacuum preloading and graded backfill surcharge combined preloading (S1 to S4) had been completed. The second set of drainage boards was installed in step S5. The drainage boards were of type B and were arranged in a staggered quincunx pattern at 1.0m intervals, with a depth of 12m.

[0075] Equipment configuration of the intermittent pulse booster system: This project sets up an independent booster air source and pipeline control system. The booster air source system uses a W-0.9 / 7 type air compressor with a discharge capacity of 0.9 cubic meters / minute, a rated discharge pressure of 0.7MPa (7 bar), and a motor power of 7.5kW. The air supply capacity of this type of air compressor can meet the intermittent air injection needs of 30 drainage boards in a single booster operation area. If the scale exceeds this, additional air compressors need to be added for combined air supply. The compressed air discharged from the air compressor is decondensed by an air-water separator (model QSL-50, filtration accuracy 40μm) before being delivered to each booster branch through nylon resin high-pressure hoses with an inner diameter of 10mm, an outer diameter of 14mm, and a pressure resistance of ≥1.0MPa. The booster branch adopts a centralized branch integrated design, with each branch independently corresponding to a set of drainage boards (each set of 8 drainage boards is connected to the same booster branch). An electromagnetic on / off valve (model 2W-160-15, interface pipe diameter 1 / 2 inch, working pressure 0~1.0MPa, normally closed, AC220V power supply) is installed on the air inlet pipe of each booster branch, enabling independent opening and closing control of each booster branch. During the installation of the second set of drainage boards (SPB-B type plastic drainage boards), a rigid booster pipe (12mm outer diameter, 9mm inner diameter, 1.5mm wall thickness 304 stainless steel seamless pipe) is simultaneously inserted into the inner cavity of the drainage board. The bottom sealing port of the rigid booster pipe is located at 2 / 3 to 3 / 4 of the total length of the drainage board (8.5m~9.0m depth in this embodiment). Four rows of 0.5mm diameter micro-holes are spirally opened along the pipe wall every 30cm. The micro-holes are uniformly sized and distributed except for the bottom sealing section.

[0076] Injection Parameter Setting and Dynamic Adjustment of Intermittent Pulse Pressurization: This embodiment adopts an intermittent pulse pressurization method, which does not involve continuous injection. Instead, it alternates between "injection-stopping-vacuum extraction" as a cycle unit throughout the entire second vacuum pre-compression process. Based on the reference model of the "four-stage intermittent pressurization method" (pressurization for 30 minutes / stopping the pump until vacuum recovery to 80 kPa), and combined with the specific working conditions and the evolution of the foundation consolidation state in this embodiment, the intermittent pulse pressurization process is divided into three dynamic adjustment stages to achieve graded adjustment of injection pressure and injection duration (injection pressure and duration are adjusted in stages according to the cycle progress, eventually transitioning to global fine-grained control of variable pressure and variable time). First stage (early stage after the second vacuum pre-compression starts and the foundation settlement rate is greater than 8.0 mm / d): A low-pressure, short-duration injection loading mode with an injection pressure of 15-20 kPa and a continuous injection duration of 20-30 minutes is adopted. The number of injection cycles is 6-10, and the interval between two adjacent cycles is 6-12 hours. The first stage involves low-pressure gas injection to initially improve soil permeability without causing excessive gas escape from under the membrane, while simultaneously observing changes in foundation settlement rate and pore pressure stability. The second stage (the mid-stage when the settlement rate drops to 4.0 mm / d to 8.0 mm / d) employs a pressure-time incremental injection mode, starting with an injection pressure of 20 kPa and linearly and uniformly increasing to 35 kPa over 8–12 cycles, with each injection duration starting at 30 minutes and linearly and uniformly increasing to 50 minutes over 8–12 cycles. The interval between adjacent cycles is 6–12 hours. This step gradually increases the driving force of the seepage field by progressively increasing pressure and time, prompting the soil seepage channels to open. The third stage (the late-stage when the settlement rate is ≤4.0 mm / d) uses a high-pressure, long-duration gas injection mode with an injection pressure of 35–45 kPa and each injection duration of 50–70 minutes, with 10–15 injection cycles and an interval between adjacent cycles of 12–24 hours. Extending the intervals as needed increases activity in the silted-up area around the drainage boards, promoting the rupture of the physical and chemical siltation layers in the deep soil and restoring its drainage function, ultimately achieving physical densification and reinforcement of the deep soil. After each stage, the timing for transitioning to the next stage is determined based on measured foundation settlement rates, pore water pressure dissipation rates, and on-site soil observations. Once the settlement rate meets the threshold for transitioning to the next stage for three consecutive days, the switch is confirmed by the on-site engineer.

[0077] Selection of Anti-clogging Filter Membrane Material for the Second Group of Drainage Boards: In this embodiment, the second group of drainage boards uses SPB-B type anti-clogging plastic drainage boards, and its filter membrane is made of a hydrophilic material with no uneven structure. The filter membrane surface is flat and smooth, without protrusions or burrs, which can prevent the adhesion and accumulation of sticky particles on the filter membrane surface; the polar groups of the hydrophilic filter membrane material (such as hydrophilic modified polypropylene or polyester fiber) form hydrogen bonds with water molecules, making it easy for water molecules to wet the filter membrane surface, forming a water film and reducing the adhesion of soil particles. The equivalent pore size of the filter membrane is O 95 The equivalent pore size of the filter membrane must be strictly controlled within the range of 0.075 mm to 0.090 mm. Controlling the equivalent pore size is a critical process. (Equivalent pore size O) 95 This refers to the fact that 95% of the pore size in the filter membrane is smaller than this value. If it is smaller than 0.075mm (e.g., 0.050mm), although it can intercept fine soil particles, the filtration rate is too slow; if it is larger than 0.090mm, clay particles can easily pass through the filter membrane and enter the drainage board, clogging the core plate. After the filter membrane becomes clogged, the permeability coefficient decreases significantly, which has an adverse effect on the drainage and consolidation of the soil. Existing experimental studies have shown that drainage board filter membranes with larger equivalent pore sizes have stronger anti-clogging ability and better reinforcement effect; the smaller the equivalent pore size, the more serious the clogging. Selecting a larger equivalent pore size for the drainage board filter membrane, and using a graded loading method for vacuum loading with each vacuum gradient not being too large, are key measures to effectively reduce drainage board clogging. The industry standard "Quality Inspection Standard for Plastic Drainage Tape" and the Hundred-inspection testing specifications both require an equivalent pore size of 0.075mm for the filter membrane. 95 ≤0.075mm ensures effective blocking of fine soil loss. Setting the equivalent pore size range between 0.075mm and 0.090mm ensures both a low soil particle migration rate through the filter membrane and excellent longitudinal drainage while effectively preventing clogging. This range, optimized through testing, is the optimal range. In this embodiment, at least 5% of each batch of the second group of drainage boards was randomly sampled upon arrival at the site. The samples were then tested in the laboratory using the dry sieving method specified in GB / T 14799 (i.e., the filter membrane sample was placed on a standard sieve and sieved using glass microspheres or quartz sand of a specific grade; the sieve aperture through which 95% of the particles could pass was considered the pore size). 95 The equivalent pore size (O) of the filter membrane of all models of drainage boards was tested. 95 The detected values ​​ranged from 0.079 mm to 0.085 mm, which meets the required control range.

[0078] Synergistic effect of intermittent pulse pressurization and anti-clogging drainage board: During construction, the second vacuum preloading system is activated simultaneously, stabilizing the vacuum degree under the membrane at 85-95 kPa, with continuous pumping and intermittent pressurization alternating. Intermittent pulse pressurization (with a unit of 30 minutes of air injection followed by 12 hours of air stoppage) causes the pore pressure in the soil to rise and dissipate multiple times, driving the dispersion and discharge of clay particles; and during the air stoppage period, dehydration and gas-driven distribution widen the gaps in the drainage board cavity, reducing pore water migration resistance and extending the water flow efficiency of the drainage board. The technical advantages of using the anti-clogging filter membrane in combination with intermittent pulse pressurization are significant: Under conditions without intermittent pressurization, the clogging rate of conventional filter membrane drainage boards may drop to 50%-60% of the initial flow rate by day 30; however, with the combined effect of intermittent pressurization and the anti-clogging filter membrane, the water flow rate remains above 80% of the initial flow rate even by day 60. After the deep soil layer (8-10m deep) in the reinforced area was pressurized, the final layered vane shear strength reached an average of 38kPa, with an improvement of 20%, and the degree of consolidation was ≥90%, which met the design standards for embankment stability control.

[0079] Existing constant-pressure, constant-time air injection methods have fixed parameters, which cannot adapt to the phased changes in soil permeability. High pressure in the early stages easily leads to air leakage, while low pressure in the later stages makes it difficult to break up blockages. This invention adopts a three-stage variable-pressure, variable-time intermittent pressurization mode, with parameters at each stage dynamically adapted to the soil consolidation and permeability characteristics. Conventional drainage board filter membrane pore size design is unreasonable; excessively large pores easily cause blockages, while excessively small pores result in insufficient drainage efficiency. This invention limits the equivalent pore size of the filter membrane to the optimal range of 0.075–0.090 mm, balancing clay retention and permeability to form a stable filtration structure. Existing technologies do not combine air injection methods with filter membrane performance for synergistic anti-blockage. This invention couples intermittent pressurization control and filter membrane optimization to maintain the high permeability of the drainage board over the long term, effectively improving the shear strength of deep soil. The parameters of this invention are controllable and the structure is reasonable, significantly improving the construction quality and reliability of secondary vacuum preloading stress compensation for soft soil foundation reclamation.

[0080] In another technical solution, taking a soft soil foundation reinforcement project in the Dongjiang Free Trade Port Area of ​​Tianjin Port as an example, the specific implementation method of the pressurization-extraction-drainage synergistic consolidation process is described in detail.

[0081] Project Overview: The project covers an area of ​​approximately 600,000 square meters. The fill soil is typical recently filled ultra-soft soil with a moisture content exceeding 85%, a void ratio of 2.8–3.2, and an initial vane shear strength of only 0.2–0.5 kPa. After the first round of vacuum preloading combined with staged fill loading, the first reinforcement was completed, and all fill loads were removed according to step S4. Before initiating the second round of vacuum preloading, after approximately 100 days of combined preloading, the soil strength within the surface layer (approximately 2–3 meters) showed a significant improvement, but the deeper layers (8–12 meters) remained in an underconsolidated state.

[0082] Installation of the rigid booster pipe: While installing the second set of drainage boards, a rigid booster pipe, with a diameter tightly fitted to the inner cavity of the second set of drainage boards, is inserted longitudinally into the cavity. The rigid booster pipe is made of seamless 304 stainless steel tubing with an outer diameter of 12mm, an inner diameter of 9mm, and a wall thickness of 1.5mm. Four rows of 0.5mm diameter micro-holes are spirally arranged along the pipe wall every 30cm. Except for the bottom sealing section, the micro-holes are evenly distributed along the pipe length. The 304 stainless steel material has excellent corrosion resistance and sufficient rigidity, allowing it to remain upright within the drainage board cavity for extended periods without bending, ensuring a stable delivery of the boosted airflow to the predetermined depth. The bottom of the rigid booster pipe is sealed by welding, with the sealing opening located at 2 / 3 to 3 / 4 of the length of the second set of drainage boards. In this project, the second set of drainage boards is installed to a depth of 12m; therefore, the bottom sealing opening of the rigid booster pipe is located at a depth of 8.5m to 9.0m. Placing the bottom of the booster pipe within this depth range allows compressed gas to diffuse outwards from this depth, forming a localized pressure field covering a depth of 8–12 m, promoting the directional migration and discharge of pore water in deep soil. After being inserted into the drainage board cavity, the rigid booster pipe is concentrically positioned using nylon positioning rings of the same material (one set every 1.0 m) to ensure a concentric fit between the booster pipe and the drainage board's inner wall, reducing airflow resistance. The top of the rigid booster pipe is connected via a high-pressure pipeline assembly to a three-way switching valve located between the booster gas source system and the vacuum pre-compression system. The three-way switching valve is a 2W-160-15 electromagnetic three-way valve (1 / 2 inch interface pipe diameter, 0–1.0 MPa working pressure, normally closed, AC220V power supply). This valve has one inlet (connected to the booster gas source), one outlet (connected to the vacuum system), and one working port (connected to the drainage board cavity). The three-way switching valve enables rapid switching control, closing the vacuum channel during pressurization and opening the vacuum channel when pressurization is stopped. This avoids mutual interference when the two pressure systems are directly connected, and also eliminates the need for an additional pressure control valve, simplifying the system structure. The three-way switching valve is automatically controlled by an industrial PLC system, with the control signal scanned every 30 seconds to ensure that the switching accuracy between pressurization and extraction is controlled within ±2 seconds. The entire pressurization air source system uses a W-0.9 / 7 air compressor with a discharge capacity of 0.9m³. 3 The compressor operates at a speed of [speed not specified] / min, with a rated discharge pressure of 0.7 MPa (7 bar) and a motor power of 7.5 kW. Compressed air discharged from the compressor passes through an air-water separator (model QSL-50, filtration accuracy 40 μm) to remove condensate, and is then delivered to each booster branch via nylon resin high-pressure hoses with an inner diameter of 10 mm and a pressure rating ≥1.0 MPa. The booster branches adopt a centralized, branch-integrated design, with each branch corresponding to an independent set of drain boards (approximately 6-8 drain boards per set connected to the same booster branch). Each booster branch's intake pipe is equipped with a solenoid on / off valve for control.

[0083] The sequence of the coordinated process is as follows: From day 3 to day 7 after the second vacuum preloading start, once the foundation settlement has basically stabilized, the pressurization-drainage coordinated consolidation process begins. The selection of day 3 to day 7 as the time window is based on the following: In the initial stage of the second vacuum preloading (the first 3 days), there is still a relatively high initial pore water pressure in the soil. If pressurization is carried out immediately at this time, excessive air pressure disturbance may damage the contact surface between the drainage board and the soil, which is detrimental to the stable operation of the drainage system. Simultaneously, after the first vacuum preloading, the soil needs a certain amount of time to recover and stabilize. During this window period, the vacuum system has established a relatively stable negative pressure environment under the membrane, preparing for further pressurization. In this embodiment, the coordinated process begins on day 5 after the second vacuum preloading start. The specific execution sequence of the process cycle is as follows: First, close the interface of the vacuum preloading system to the second set of drainage boards, and switch the three-way switching valve to the pressurization air source channel; simultaneously, turn on the pressurization air source system to introduce compressed gas into the rigid pressurization pipe, performing intermittent pulse pressurization in the foundation depth direction. Each intermittent pulse pressurization injection lasts for 45 minutes (between 30 and 60 minutes), with an injection pressure of 30 kPa (between 20 and 40 kPa). After injection, the pressurization gas source system is shut off, the three-way switching valve is switched to the vacuum extraction channel, and the vacuum pre-pressurization system is simultaneously activated to continuously extract water from the interface of the second set of drainage boards. The continuous extraction time is 10 hours (not less than the interval between two adjacent pulse injections; in this embodiment, the interval is 8 hours). This constitutes a complete intermittent pulse pressurization-extraction coordinated cycle, with a total cycle time of approximately 10 hours and 45 minutes. In this embodiment, the intermittent pulse pressurization adopts a cyclical mode of "pressurization for 45 minutes, instantaneous switching, and continuous extraction for 10 hours." The advantage of this mode is that the pressure field formed during the pressurization phase does not immediately dissipate after the gas is stopped. The vacuum system is then activated, converting it into a directional seepage driving force. The pore water generated in the soil migrates from the deep layers to the drainage boards under the combined drive of the pressure difference and the vacuum negative pressure, and is eventually carried to the surface by the vacuum system. The timing of shutting down the pressurization system after pressurization is controlled by a three-way switching valve. After the gas is stopped, the residual gas pressure in the soil can still maintain a seepage-driving effect for a period of time, forming a good continuity with the continuous pumping of the vacuum system.

[0084] Pulse pressurization control conditions: During the co-consolidation process, the pulse interval is dynamically adjusted based on changes in the foundation settlement rate and degree of consolidation. In this embodiment, the foundation settlement rate is monitored using steel wire settlement gauges (model BGK-4425) buried at different depths. Data is collected every 6 hours, and all monitoring data is uploaded to the industrial control computer monitoring platform in real time. The degree of consolidation is calculated in real time according to the procedure described in the aforementioned JJJ solution. When the foundation settlement rate is less than or equal to 4.0 mm / d for 5 consecutive days, it indicates that the foundation consolidation has entered the later stage. At this time, the interval between the pressurization-drainage co-consolidation cycle can be appropriately increased to avoid excessively frequent pressurization operations that would cause energy waste and soil disturbance. Therefore, the interval between two adjacent intermittent pulse pressurization-drainage co-consolidation cycles is extended from the initial 8 hours to 2.0 times the initial value, i.e., the interval is extended to 16 hours. The extended interval allows sufficient time for the soil to continue drainage and consolidation using vacuum negative pressure, while reducing the start-up frequency of the pressurization system, thus achieving energy saving and consumption reduction. Alternatively, when the degree of consolidation in the area, calculated according to the JJJ solution, reaches approximately 75% of 85% (about 64% consolidation), it can also switch to a later-stage slow-speed processing mode, extending the coordinated cycle interval to 1.8 times the initial value, and transitioning to the final process after completing all designed dewatering. When both of the above conditions are met simultaneously, or when one of the dominant indicators maintains a stable downward trend, and the field sampling and measured values ​​of the deep soil vane shear strength have reached the design value (≥20kPa) for 5–7 consecutive days, then the pressurization-dewatering coordinated consolidation process in each zone is gradually stopped, with only vacuuming continuing until the final completion node is reached, until the overall conditions for determining the completion of the second vacuum preloading are met. Taking the three monitoring sections (section A, section B, and section C) of the reinforced area of ​​this project as examples, the execution of the coordinated consolidation process is as follows: Section A achieved the condition of a settlement rate of ≤4.0 mm / d for 5 consecutive days on the 50th day after the second vacuum preloading was started, and the interval time was extended from 8 hours to 16 hours from the 51st day; Section B achieved the condition of 76% of the 85% consolidation degree on the 53rd day, and the interval time was extended from 8 hours to 16 hours; Section C met both conditions on the 56th day, and the interval time was also extended. From the start of the coordinated process on the 5th day to the end of the process on the 65th day, the pressurization-extraction coordinated consolidation process was executed for approximately 60 days, and approximately 140 stable pressurization-extraction coordinated cycles were completed. The final test data statistics show that the average surface settlement rate of all monitored areas for five consecutive days was 2.8 mm / d to 3.6 mm / d, all ≤4.0 mm / d, the degree of consolidation was 89% to 93%, and the average shear strength of the deep (8-12m) layered vanes reached 20.8 kPa to 23.5 kPa, which fully meets the final completion acceptance requirements.

[0085] Existing pressurization and vacuum systems lack coordination, leading to rapid extraction of pressurized gas and low energy utilization. This invention utilizes a three-way switching valve to achieve a closed-loop control sequence of alternating vacuum stop-pressurization-pressurization stop-vacuum re-vacuum, avoiding reverse airflow losses and significantly improving the efficiency of combined pressure and vacuum reinforcement. Conventional processes do not clearly define the timing for pressurization initiation; premature pressurization can disturb the soil and damage the drainage contact structure. This invention limits the co-consolidation to 3-7 days after the secondary vacuum pre-pressurization and after the foundation settlement has stabilized, resulting in a more rational construction sequence. Traditional methods have fixed intervals for pressurization and extraction, leading to mismatched parameters, high energy consumption, and significant disturbance. This invention uses settlement rate or degree of consolidation as the criterion, dynamically extending the control interval in the later stages of soil consolidation to adapt to the soil hardening process, reducing energy consumption while ensuring reinforcement quality.

[0086] In another technical solution, taking a soft soil foundation reinforcement project in the Dongjiang Free Trade Port Area of ​​Tianjin Port as an example, the specific implementation method of the rapid formation of hard shell layer is described in detail.

[0087] Project Overview: The reclamation area covers approximately 600,000 m². 2 The fill material originated from near-shore seabed silt and was formed using a cutter suction dredger. The initial average moisture content of the fill material was 118%, with the moisture content generally exceeding 100% in the surface 0–2 m depth range. The void ratio was 2.8–3.2, indicating a highly underconsolidated state. The original vane shear strength was only 0.2 kPa–0.5 kPa, and the static cone tip resistance q… c With a strength of only 50 kPa to 80 kPa, this is a typical example of newly filled ultra-soft soil foundation. After the first stage of vacuum preloading combined with graded backfill loading (steps S1 to S4), the initial reinforcement was completed, and vacuum preloading was stopped and all backfill loading was removed in step S4. At this point, the surface soil of the foundation has a certain degree of compaction, but due to the complete removal of the original backfill loading, the surface strength of the foundation is still low and insufficient to directly support the movement and operation of heavy machinery such as jackhammers. Engineering experience shows that after the construction of ultra-soft soil foundations, an additional 80 cm to 100 cm layer of silty sand is usually required to meet the bearing capacity requirements of machinery installing the second set of drainage boards. This intermediate step takes approximately 10 to 15 days and requires a significant investment in silty sand materials and transportation costs. Given that shallow reinforcement technology for ultra-soft soil can quickly form a hard shell layer on the surface of newly filled soft soil, thereby enabling secondary deep treatment and accelerating the treatment progress of filled soft soil, this embodiment uses a filter membrane modification and drainage enhancement method to quickly form a hard shell layer, avoiding the high cost and time consumption of the above-mentioned conventional methods.

[0088] First Group of Drainage Board Filter Membrane Modification Treatment: According to the technical solution of the present invention, after step S4 is completed and vacuum preloading is stopped and all backfill load is removed, on-site detection shows that the twisting and breakage of the drainage boards during the first reinforcement process mainly occurred below a depth of 2.5 m. The drainage boards in the shallow layer of 0 to 2.5 m remained basically upright and the filter membrane was intact, possessing the ability to continue drainage. Therefore, the filter membrane of the shallow layer (0 to 2.0 m depth range) of the first group of drainage boards was modified first. In this project, the first group of drainage boards uses SPB-B type plastic drainage boards, with a core board of polypropylene material and a filter membrane of hydrophilic polyester fiber material, with an equivalent pore size of O 95 The thickness should be controlled within the range of 0.075 mm to 0.090 mm. The operation method for filter membrane modification treatment is as follows: Along the depth direction of the first group of drainage boards, at a pre-selected location near the foundation surface (i.e., within the range of 0 m to 2.0 m below the top of the drainage board), a special cutting tool—a long-arm electric thermal cutter (handle extended arm length 2.5 m to 3.0 m, front heating wire heating temperature 1220℃, heating diameter 0.08 mm, motor speed 120 r / min)—is inserted into the inner cavity of each drainage board one by one to locally remove the filter membrane in this area. In specific implementation, four longitudinal cuts are evenly set in the 0 m to 2.0 m depth range for each drainage board, with the spacing between adjacent cuts being equidistant at 40 cm to 50 cm, and the length of each cut controlled between 8 cm and 12 cm. During the cutting operation, skilled operators should smoothly lower the cutter to avoid damaging the main structure of the drainage board core. Only the filter membrane should be partially cut, peeled off, and removed to expose the drainage channels of the drainage board core (i.e., the longitudinal drainage groove section of the drainage board), forming a "pile perimeter soil drainage reinforcement zone." This drainage reinforcement zone has higher permeability than an intact filter membrane, which can significantly reduce the seepage resistance between shallow piles. During the subsequent secondary vacuuming process, the ultra-soft soil in this area achieves directional drainage due to the high vacuum gradient, accelerating the formation of the hard crust layer.

[0089] Restart the vacuum preloading system to extract soil around the piles: After the filter membrane modification is completed, first check the integrity of the original vacuum sealing membrane: If the sealing membrane remains intact or only slightly damaged (total area ≤5%) during the removal of backfill, the original sealing system can be used directly; if the damage is severe, a new layer of sealing membrane (0.12-0.16 mm thick, polyethylene or polyvinyl chloride material) needs to be laid in the area where the hard shell layer is formed, and ensure good sealing with the surrounding area. Then, reconnect the upper end of the first set of drainage boards treated with the filter membrane to the vacuum system under the membrane, and restart the vacuum preloading system to perform vacuuming. The vacuum preloading system used in this project is the same as in step S3: two 2BEA-353P1 water ring vacuum pumps connected in parallel (single pumping capacity 40 m³ / s). 3 / min, motor power 160 kW). During the pumping process, water-side boundary seepage recharge conditions are simultaneously introduced from the adjacent site, that is, a water storage ditch (2.0 m deep, 1.0 m wide) is set up around the site and water is injected into it to maintain a water level of not less than 1.5 m, thereby establishing hydraulic seepage conditions from the inside to the outside of the site, preventing uneven shrinkage and cracking of the soil within the hard shell layer due to excessive pumping, and ensuring that the seepage field force driven by the high vacuum gradient and the lateral hydraulic gradient can continuously act on the pumping reinforcement zone. After the vacuum is started, the soil around the pumping reinforcement zone produces a violent high-speed directional seepage effect. Since the filter membrane has been partially removed, the water migration resistance is greatly reduced. Driven by vacuum pressure (the vacuum degree under the membrane is stably maintained at 85 kPa to 95 kPa), a large amount of pore water in the soil around the pile is discharged into the drainage board core channel and finally pumped to the surface water collection system. During this seepage process, fine particles in the soil migrate and rearrange, the soil moisture content drops sharply, and the effective stress between particles rises rapidly, thus forming a reinforced soil zone with significantly improved physical and mechanical properties within a certain range around the drainage board pile.

[0090] Testing the Stabilization Period and Hard Shell Formation Effect of Drainage: The continuous drainage time (i.e., the stabilization period) for this project is controlled at 7 days, meeting the condition of "not less than 5 days and not more than 15 days" as defined in step S4. The reason for choosing a drainage period of 7 days is that if the drainage time is less than 5 days, the unidirectional solidification process of the seepage path around the pile has not fully unfolded, and the thickness and strength development of the hard shell layer are limited; if the drainage time exceeds 15 days, the deep soil layers below the hard shell layer may be excessively disturbed due to the longitudinal transmission of vacuum negative pressure, and it will have an adverse effect on the stable structure of clay particles in the soil, leading to fluctuations in bearing capacity and increased abnormal noise at the pile position when using the single-layer secondary insert plate for construction later. Therefore, in actual operation, an optimal combination of empirical values ​​that can generally take into account both the construction period and quality is adopted: 7 days for drainage in this project. By the end of the seventh day of drainage, nine representative measuring points were selected for static cone penetration tests (CPT, MP-1 single-bridge probe, penetration rate controlled at 1.2 cm / s, range permittivity calibration accuracy ±0.5%). The cone tip resistance q at the nine points in the excavated soil layer area was measured. cThe minimum value was 0.92 MPa (maximum value 2.09 MPa, average value 1.37 MPa), meeting the requirement of not less than 0.8 MPa. The thickness of the hard shell layer was measured by drilling and sampling pipes, and the thickness was between 45 cm and 55 cm, which is greater than the lower limit of 40 cm specified in this invention. After numerical fitting conversion, the average moisture content of the soil within the hard shell layer was reduced to 44%, which is more than 50% lower than the surface moisture content of the soil before the shell was formed (originally about 97%). The average vane shear strength of the hard shell layer (VST-2T vane shear tester, vane head size 50 mm × 100 mm, shear rate 6° / min to 12° / min) reached 32.5 kPa, which is nearly 60 times higher than the surface value of the construction area before the extraction.

[0091] Hard Shell Layer Bearing Capacity Testing and Equipment Arrival: After drainage, a heavy-duty walking test was conducted on the surface of the hard shell layer. The total mass of the equipment was selected based on actual needs, using a commonly used SD-20 type sluice gate machine (21t total walking mass, track ground pressure 35 kPa~45 kPa). The sluice gate machine's track walking area was driven into the hard shell layer test area, and dial gauges were used to monitor the instantaneous and residual settlement of the soil under walking load. The test results showed that under the rated walking load, the instantaneous settlement of the hard shell layer was less than 3 mm, and the residual settlement was less than 1 mm. No significant compaction settlement or mud surface heaving occurred on the track plates, fully demonstrating that the bearing capacity of the hard shell layer meets the requirements for heavy machinery sluice gate operation. This confirms that the hard shell layer can be used as a mechanical operating platform for installing the second set of drainage boards, completely replacing the need for conventional 80 cm~100 cm silty sand cushion layer, saving all the procedures and corresponding material transportation and laying costs of additional silty sand filling. Engineering practice has shown that the hard shell layer formed after treatment with this technology can fully meet the construction requirements of subsequent mechanical insert plates. The engineering application has been successful and has the characteristics of low cost, short construction period and simple process.

[0092] After the hard shell layer is formed, the drainage board treatment and subsequent procedures are as follows: After the hard shell layer is formed and passes inspection, the upper end of the first set of drainage boards, which has undergone filter membrane modification treatment, is disconnected from the vacuum system under the membrane. The corresponding drainage network system (including the horizontal drainage filter pipes, membrane outlets, and their binding connections with the drainage boards) is removed to prevent this part of the drainage board from causing additional vacuum effects or interfering with the accuracy of installing the second set of drainage boards in later operations. At this point, the rapid formation process of the hard shell layer is completed, and the process proceeds to step S5, which involves installing the second set of drainage boards above the hard shell layer and performing the second vacuum preloading and complete reinforcement construction in subsequent steps S6 to S8. After the completion of subsequent steps S5-S8 and the overall final acceptance, the final average value of the vane shear strength in the reinforced area (8 m to 10 m depth) indicated by all static cone penetration tests (CPT) and vane shear tests (VST-2T) reaches 32 kPa; the upper hard shell layer is tested qc The stable value remained above 0.9 MPa, and no surface damage or uneven settlement occurred due to the installation of the insert plate, ensuring the sealing effect of the second vacuum pre-compression system.

[0093] Conventional processes require an additional 80-100cm layer of silty sand as a bearing layer for the slab insertion, adding 10-15 days to the construction period. This invention, through partial modification of the filter membrane and secondary vacuum-enhanced drainage, eliminates the need for external backfill materials, forming a hard shell layer in situ with the required thickness and cone tip resistance, directly meeting the requirements for slab insertion and eliminating the silty sand cushion layer step. Traditional processes rely on purchased sand to construct the hard shell layer, which is susceptible to construction disturbances and has poor stability. This invention modifies existing drainage boards, utilizing in-situ ultra-soft soil for consolidation and hardening, simplifying procedures, saving materials, and reusing existing facilities. Under the same 7-day curing period, the cone tip resistance of the hard shell layer of this invention is far higher than the design standard, and its bearing capacity is significantly better than conventional solutions. Furthermore, its thickness is controllable, and its overall stability is good, allowing it to directly withstand the secondary slab insertion load. This provides a low-cost, short-term, and highly reliable technical solution for the pretreatment of soft soil foundations in reclamation areas.

[0094] In another technical solution, taking a soft soil foundation reinforcement project in the Dongjiang Free Trade Port Area of ​​Tianjin Port as an example, the specific implementation method of the variable pressure-variable time gas injection loading path is described in detail.

[0095] Project Overview: The reclamation area covers approximately 600,000 m². 2 Before dredging, the average elevation of the original mud surface was -2.5m, and after dredging, the surface elevation was +3.0m. The average thickness of the dredging fill was 5.5m, with a maximum soft soil layer thickness of 12m. Engineering geological surveys showed that the initial moisture content of the dredging fill was 118%, with the moisture content generally exceeding 100% within an 8.0m depth range. The initial void ratio was approximately 2.9, and the permeability coefficient was 1.5 × 10⁻⁶. -7 ~3.5×10 -7 cm / s, in a highly underconsolidated state; the original shear strength of the vane is only 0.2kPa to 0.5kPa.

[0096] The gas injection loading path consists of three stages: the entire second vacuum preloading lasts approximately 70 days, during which the gas injection pressurization operation is executed sequentially in the following three stages. The transition between each stage is based on the measured value of the foundation settlement rate: the settlement rate is automatically collected by steel wire settlement gauges (model BGK-4425) buried at different depths, and the data is reported to the industrial control computer monitoring platform every 6 hours. The monitoring system automatically calculates the daily settlement rate; when the foundation settlement rate of the current stage meets the rate threshold for transitioning to the next stage for 3 consecutive days, the monitoring system automatically issues a switching command, which is then confirmed by the on-site engineer before the gas injection parameters are switched.

[0097] Phase 1: Low-Pressure Short-Duration Gas Injection Phase (Early Stage): This phase is executed after the second vacuum preloading initiation. At this time, the foundation settlement rate is at a relatively high level (greater than 8.0 mm / d), indicating that the excess pore water pressure in the soil has not yet fully dissipated, and the initial permeability of the soil is relatively high, but the drainage channels have just begun to be established. In this phase, a low-pressure, short-duration gas injection loading mode is adopted to gradually guide gas into the soil with minimal disturbance, avoiding excessive atmospheric pressure that could cause gas to escape along weak points in the sealing membrane or form preferential channels along the drainage board wall, thus reducing the overall pressurization efficiency. Specific parameter settings: Injection pressure is set at 18 kPa (between 15 kPa and 20 kPa), each continuous injection duration is set at 25 minutes (between 20 and 30 minutes), the number of gas injection cycles is set at 8 times (between 6 and 10 times), and the interval between two adjacent cycles is 8 hours (between 6 and 12 hours). The vacuum system continues to operate during the gas injection intervals, maintaining a stable vacuum under the membrane at 85 kPa to 90 kPa. In the second phase of this project, the injection pressure and injection duration remained constant, neither increasing nor decreasing over time, and eight injection cycles were completed in a stable manner, laying the foundation for the pressure-time increase in the next phase.

[0098] Phase Two: Pressure-Time Incremental Aeration Phase (Mid-Term): This phase begins after Phase One is completed and the foundation settlement rate drops to 4.0 mm / d to 8.0 mm / d. At this point, the excess pore water pressure in the soil has partially dissipated, but initial clogging (caused by the migration of tiny particles to the drainage board during vacuum preloading drainage) begins to appear around the drainage board filter membrane, leading to a decrease in soil permeability. In this phase, a pressure-time incremental aeration loading mode is adopted, where the aeration pressure and duration gradually increase with the number of cycles. This progressively enhances the splitting and driving effect of the airflow on the soil, causing the initially formed clogging layer to loosen and crack under the gradually increasing airflow pressure, thus maintaining or even improving the soil's drainage and permeability performance. Specific parameter settings: The injection pressure starts at 20 kPa and increases linearly and uniformly to 35 kPa within 10 cycles (between 8 and 12 cycles); the duration of each injection starts at 30 minutes and increases linearly and uniformly to 50 minutes within 10 cycles (between 8 and 12 cycles); the interval between two adjacent cycles is 8 hours (between 6 and 12 hours). In this section, the injection pressure and injection duration increase linearly with the number of cycles. The daily settling rate is monitored by the annular injection unit, and the industrial control computer automatically calculates the target pressure and duration for the next cycle for each cycle, precisely controlling the coordination between the pressurization power and the sludge evolution trend during the consolidation process. The specific implementation method of linear uniform increase is as follows: taking 10 cycles as an example, the injection pressure increases by (35-20) / (10-1)=15 / 9≈1.7kPa in each cycle, that is, 20kPa in the first cycle, 21.7kPa in the second cycle, 23.4kPa in the third cycle, 25.1kPa in the fourth cycle, 26.8kPa in the fifth cycle, 28.5kPa in the sixth cycle, 30.2kPa in the seventh cycle, 31.9kPa in the eighth cycle, 33.6kPa in the ninth cycle, and 35kPa in the tenth cycle. The injection time increases by (50-30) / (10-1) = 20 / 9 ≈ 2.2 minutes with each cycle, i.e., 30 minutes for the first cycle, 32.2 minutes for the second, 34.4 minutes for the third, 36.6 minutes for the fourth, 38.8 minutes for the fifth, 41.0 minutes for the sixth, 43.2 minutes for the seventh, 45.4 minutes for the eighth, 47.6 minutes for the ninth, and 50 minutes for the tenth. The progressively increasing injection pressure and time effectively prevent clay particles from accumulating at the drainage board, increasing the number of pores in the soil between the drainage board and the pressure plate, thus increasing soil permeability and improving the reinforcement effect of deep soil layers.

[0099] Phase 3: High-Pressure Long-Duration Air Injection Phase (Later Stage): This phase begins after Phase 2 is completed and the foundation settlement rate is ≤4.0 mm / d. At this point, the foundation settlement has largely converged, the soil consolidation has reached a relatively high level (approximately 85%), and the drainage filter membrane is relatively severely clogged. A larger air pressure driving force is required to maintain and further improve the final physical densification of the soil. In this phase, a high-pressure, long-duration air injection loading mode is adopted. With a larger air pressure and a longer single injection duration, a continuous air pressure driving force is applied to the deep soil (mainly targeting depths of 8m to 12m) to further expel residual pore water and complete the final physical densification reinforcement. Specific parameter settings: Injection pressure is 40 kPa (between 35 kPa and 45 kPa), each continuous injection duration is 60 minutes (between 50 and 70 minutes), the number of air injection cycles is set to 12 (between 10 and 15), and the interval between two adjacent cycles is 18 hours (between 12 and 24 hours). The process involves three phases of alternating and coordinated gas injection and vacuum decompression. After the third phase of gas injection is completed, and 48 hours into the final decompression phase, data is collected to determine when the second vacuum pre-pressurization can be terminated. To ensure the smooth transition between the three phases of gas injection, it is recommended that on-site technical personnel be responsible for the dynamic control and data recording of the gas injection pressurization system. This includes regularly uploading key parameters to the monitoring platform and monitoring the settlement rate threshold daily to ensure a smooth and gradual transition between the three injection phases.

[0100] Post-construction testing data and technical results: After the three-stage air-injection loading path construction, the second vacuum preloading was completed on the 70th day. Seven days after the completion, vane shear tests, static cone penetration tests, and indoor geotechnical tests were conducted for quality inspection. The test results showed that the average surface settlement rate of the reinforced area remained stable between 2.8 mm / d and 3.5 mm / d for five consecutive days, all ≤4.0 mm / d; the average degree of consolidation calculated according to JJJ solutions was between 87% and 93%, meeting the requirement of ≥85%; the layered vane shear strength exceeded 20 kPa at all depth points (the measured values ​​at all measuring points from 0.5m to 11.5m below the surface ranged from 21.3 kPa to 32.8 kPa), and the average vane shear strength within a deep layer of 8–10m reached 23.7 kPa, far exceeding the 17.5 kPa of the control zone without air-injection pressurization. The maximum non-uniformity coefficient was 0.128, far less than the upper limit of 0.35. The test data is compiled and recorded in the construction archives to provide a quantitative reference for similar projects in the future.

[0101] Existing technologies use fixed injection parameters throughout the process, leading to high pressure in the early stages, leakage, and energy consumption, while later stages suffer from insufficient pressure, poor blockage removal, and ineffective deep reinforcement. This invention divides the injection pressurization into three stages, progressively adjusting pressure and duration according to the soil consolidation process, achieving adaptive matching between parameters and soil property changes. Existing technologies rely on experience to define construction stages, lacking quantitative standards and exhibiting poor repeatability. This invention uses a three-day continuous settlement rate threshold as the stage switching criterion, achieving quantitative, controllable, and adaptive adjustment of injection parameters. Existing graded injection is only a qualitative concept, lacking practical field parameters. This invention clearly defines quantitative indicators such as pressure, duration, and number of cycles for each stage, and specifies a linear pressure increase method in the middle stage, allowing for direct implementation and replication of the solution.

[0102] Although the technical solutions of the present invention have been disclosed above, they are not limited to the applications listed in the specification and embodiments. They can be applied to various fields suitable for the present invention. For those skilled in the art, other modifications can be easily made. Therefore, without departing from the general concept defined by the claims and their equivalents, the present invention is not limited to the specific details and embodiments shown and described herein.

Claims

1. A method for preloading soil and controlling the stability of dikes in soft soil foundations used for land reclamation, characterized in that, Includes the following steps: S1. Level the surface of the soft soil foundation of the reclaimed land and lay the first drainage sand cushion layer; S2. Install the first set of drainage boards above the first drainage sand cushion layer at the first interval and the first depth, and connect the upper end of the first set of drainage boards to the first drainage pipe network system buried in the first drainage sand cushion layer. The first drainage pipe network system is connected to the vacuum pre-compression system through the membrane outlet. S3. Start the vacuum preloading system to perform the first vacuum preloading, maintaining the vacuum degree under the membrane ≥80 kPa; after the first vacuum preloading, perform graded backfill preloading of the foundation according to the surcharge thickness of 0.3 m to 0.6 m per stage, controlling the settlement rate to 10 mm / d to 15 mm / d, the horizontal displacement rate of the side piles to 4 mm / d to 7 mm / d, and the pore water pressure increment not greater than 60% of the preloading load increment; S4: Continue the first vacuum preloading and graded backfill surcharge preloading. When the measured settlement is <2 mm / d for 5 consecutive days and the degree of consolidation is ≥80%, the first reinforcement is considered complete. Stop vacuum preloading, remove all backfill, and expose the foundation surface. S5: On the unloaded foundation surface, install a second set of drainage boards at a second spacing and a second depth, which are staggered with the first set of drainage boards in a quincunx pattern. The staggered spacing in the plane is 1 / 3 to 1 / 2 of the spacing of the first set of drainage boards. The second depth is the same as the first depth, or 1.1 to 1.2 times the first depth. S6: Lay the second drainage sand cushion layer and the second drainage pipe network system, connect the upper end of the second set of drainage boards to the second drainage pipe network system, and connect the second drainage pipe network system to the vacuum pre-compression system through the membrane outlet; S7: Start the vacuum preloading system to perform the second vacuum preloading, maintain the vacuum degree under the membrane ≥80 kPa, and do not carry out backfill load preloading during this period; continue the second vacuum preloading until the average settlement rate of the ground surface is ≤4.0 mm / d for 5 consecutive days, the degree of consolidation is ≥85%, and the shear strength of the layered vanes after construction is not less than 20 kPa, then the second reinforcement is deemed to be completed. S8: Stop vacuum preloading, dismantle the vacuum preloading system and drainage network system, and conduct quality inspection.

2. The method for preloading soil and controlling the stability of dikes in soft soil foundation reclamation as described in claim 1, characterized in that, In steps S4 and S7, the degree of consolidation is calculated using the JJJ solution. The JJJ solution refers to a radially and vertically perfectly coordinated analytical solution for consolidation, derived based on the iso-strain assumption and considering the underconsolidation characteristics of the fill soil at a certain height and the loss of vacuum along the depth direction of the drainage board under boundary conditions. In step S4, when the first reinforcement is deemed complete, the degree of consolidation calculated based on the measured settlement curve is determined using the following improved settlement calculation formula: S 改进 = S 分层总和 × η1 × η2 × η3 In the formula, S 分层总和 The final settlement of the foundation is calculated using the layered summation method. η1 is the lateral confinement assumption correction coefficient with a value of 1.05 to 1.25, η2 is the first soil sampling disturbance correction coefficient with a value of 0.85 to 0.95, and η3 is the second soil sampling disturbance correction coefficient with a value of 0.80 to 0.

90.

3. The method for preloading soil and controlling the stability of dikes in soft soil foundation reclamation as described in claim 1, characterized in that, In step S3, during the staged backfilling and preloading process, displacement monitoring is conducted in zones according to different distances from the toe of the embankment slope. Located within the area at the toe of the embankment slope: The settlement rate of the foundation surface is used as the monitoring index, with a control value of 10 mm / d to 15 mm / d; when the settlement rate is greater than 15 mm / d for three consecutive days, the loading rate should be immediately reduced to 40% to 50% of the current rate. If the settlement rate is still greater than 15 mm / d for two consecutive days after the reduction, the preloading should be stopped, and the graded loading should be resumed after the settlement rate is less than or equal to 15 mm / d. For the area located outside the toe of the embankment slope within one embankment height: the horizontal displacement rate of the side piles is used as the monitoring index, with a control value of 4 mm / d to 7 mm / d; when the horizontal displacement rate of any monitoring point is greater than 7 mm / d for three consecutive days, the surcharge preloading is immediately stopped, and the grading surcharge is resumed only after the horizontal displacement rate of the monitoring point is less than or equal to 7 mm / d.

4. The method for preloading soil and controlling the stability of dikes in reclaimed soft soil foundations as described in claim 1, characterized in that, In step S3, the thickness of each stage of surcharge preloading is 0.3 m to 0.6 m. The surcharge interval between two adjacent stages is determined based on the measured pore water pressure dissipation rate and shall not be less than 7 days.

5. The method for preloading soil and controlling the stability of dikes in soft soil foundation reclamation as described in claim 1, characterized in that, In step S7, when the second reinforcement is completed, it is determined that the post-construction layered vane shear strength is not less than 20 kPa, and the non-uniformity coefficient of the foundation vane shear strength within a depth range of one embankment height below the embankment base is not greater than 0.

35. The non-uniformity coefficient is calculated as follows: vane shear test measuring points are arranged at 1.0 m intervals within this depth range, and the ratio of the standard deviation to the arithmetic mean of the measured shear strength values ​​of each measuring point is taken.

6. The method for preloading soil and controlling the stability of dikes in soft soil foundation reclamation as described in claim 1, characterized in that, In step S7, while starting the vacuum preloading system for the second vacuum preloading, a stress compensation operation is also performed, specifically as follows: Compressed gas is injected into the foundation depth through the inner cavity of the second set of drainage boards, creating a local pressurization zone at the bottom or middle of the second set of drainage boards. This compensates for or partially offsets the foundation unloading rebound defects caused by the removal of all fill load, and forms an additional seepage pressure gradient around the second set of drainage boards, promoting the migration of pore water in the deep soil towards the drainage boards.

7. The method for preloading soil and controlling the stability of dikes in reclaimed soft soil foundations as described in claim 6, characterized in that, When compressed gas is injected into the foundation depth through the inner cavity of the second set of drainage boards, an intermittent pulse pressurization method is used. The injection pressure and injection duration are adjusted in stages or continuously according to the evolution of the foundation consolidation state. The second set of drainage boards uses a hydrophilic material with a smooth, non-porous structure and an equivalent pore size of O. 95 The value ranges from 0.075 mm to 0.090 mm.

8. The method for preloading soil and controlling the stability of dikes in reclaimed soft soil foundations as described in claim 6, characterized in that, The stress compensation operation of injecting compressed gas into the foundation depth direction through the inner cavity of the second set of drainage boards, together with the vacuuming operation of the vacuum preloading system, is performed in a coordinated manner as follows: Pressurization pipe installation: While installing the second set of drainage boards, a rigid pressurization pipe that is tightly fitted to the inner diameter of the second set of drainage boards is inserted longitudinally into the inner cavity of the second set of drainage boards. The bottom sealing port of the rigid pressurization pipe is located at 2 / 3 to 3 / 4 of the length of the second set of drainage boards. The top of the rigid pressurization pipe is connected to the three-way switching valve between the pressurization air source system and the vacuum pre-compression system through the high-pressure pipeline assembly. The sequence of the coordinated process is as follows: From the 3rd to the 7th day after the second vacuum preloading is started, after the foundation settlement has basically stabilized, the pressurization-drainage coordinated consolidation process is executed. The process cycle is as follows: close the interface of the vacuum preloading system to the second set of drainage boards, and simultaneously turn on the pressurization gas source system to introduce compressed gas into the rigid pressurization pipe to perform intermittent pulse pressurization in the direction of foundation depth. The duration of each intermittent pulse pressurization is 30 to 60 minutes, and the injection pressure is 20 kPa to 40 kPa. After the injection is completed, close the pressurization gas source system and simultaneously turn on the vacuum preloading system to continuously drain the interface of the second set of drainage boards. The duration of continuous draining is not less than the interval between two adjacent pulse injections, which is 6 to 12 hours. This constitutes a complete intermittent pulse pressurization-drainage coordinated cycle. Pulse pressurization control conditions: When the foundation settlement rate is less than or equal to 4.0 mm / d for 5 consecutive days, or the degree of consolidation reaches 70% to 80% of 85%, the interval between two adjacent intermittent pulse pressurization-drainage coordinated cycles will be extended from the initial 6 to 12 hours to 1.5 to 2.0 times the initial value, or the process will be changed to the end of the design index testing procedure.

9. The method for preloading soil and controlling the stability of dikes in reclaimed soft soil foundations as described in claim 1, characterized in that, The following steps are also included between step S4 and step S5: At at least one pre-selected location near the foundation surface, the filter membrane of the first set of drainage boards is partially cut off to expose the drainage channels of the drainage board core in that area, forming a reinforced area for soil extraction around the pile. After stopping the vacuum pre-compression in step S4, the vacuum pre-compression system is restarted to vacuum the drainage boards in the first group of drainage boards that have completed the filter membrane modification treatment. During the suction process, water-side boundary seepage recharge conditions are introduced or established from the adjacent site. With a continuous pumping period of no less than 5 days and no more than 15 days as the stabilization period, the seepage field force driven by the synergistic effect of high vacuum gradient and lateral hydraulic gradient is used to rapidly form a layer of ultra-soft soil with a thickness of no less than 40 cm and a static cone tip resistance q around the pumping reinforcement zone without the need for new filling or surcharge. c A hard shell layer with a strength of not less than 0.8 MPa, which serves as a mechanical insertion platform for installing the second set of drainage boards; After the hard shell layer is formed, the upper end of the first set of drainage boards, which has been modified by the filter membrane, is disconnected from the vacuum system under the membrane and the drainage pipe network system is removed.

10. The method for preloading soil and controlling the stability of dikes in soft soil foundations for reclamation as described in claim 7, characterized in that, The injection pressure and injection duration are adjusted in stages or continuously according to the evolution of the foundation consolidation state, using a variable pressure-variable time gas injection loading path, specifically executed in the following three stages: The first stage: In the early stage after the second vacuum preloading is started and the foundation settlement rate is greater than 8.0 mm / d, a low-pressure-short-duration gas injection loading mode is adopted with an injection pressure of 15 kPa to 20 kPa and a continuous injection duration of 20 to 30 minutes each time. The number of gas injection cycles is 6 to 10, and the interval between two adjacent cycles is 6 to 12 hours. The second stage: After the completion of the first stage and the foundation settlement rate drops to 4.0 mm / d to 8.0 mm / d, the intermediate stage adopts a pressure-time incremental gas injection loading mode, starting from 20 kPa and linearly and uniformly increasing to 35 kPa within 8 to 12 cycles, and starting from 30 minutes for each injection and linearly and uniformly increasing to 50 minutes within 8 to 12 cycles. The interval between two adjacent cycles is 6 to 12 hours. The third stage: After the completion of the second stage and when the foundation settlement rate is ≤4.0 mm / d, a high-pressure-long-duration gas injection loading mode is adopted with an injection pressure of 35 kPa to 45 kPa and a continuous injection duration of 50 to 70 minutes each time. The number of gas injection cycles is 10 to 15, and the interval between two adjacent cycles is 12 to 24 hours. The transition between stages is as follows: when the foundation settlement rate of the current stage meets the rate threshold for transitioning to the next stage for three consecutive days, the stage will automatically switch to the next stage.

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