Soft soil foundation treatment method based on deep stirring and vacuum preloading

Abstract

The invention discloses a soft soil foundation treatment method based on deep stirring and vacuum preloading, which comprises the following steps: arranging deep stirring piles in the deep layer of a soft soil foundation, and mixing a curing agent with in-situ soil to form a pile body network; plastic drainage plates are arranged between the piles, and the ground surface is covered with an airtight film; the vacuum system applies negative pressure of-50 to-80 kPa to drive pore water to be discharged; a settlement meter, a pore water pressure meter and a vacuum sensor are arranged and wirelessly fed back to the central system; the negative pressure is automatically adjusted based on a PID algorithm or a fuzzy algorithm; and predicting the settlement trend by combining a regression model. According to the soft soil foundation treatment method based on deep stirring and vacuum preloading, rigidity is enhanced through the pile nets, real-time self-adaptive pressure control is achieved, uniform solidification is achieved, the construction period is remarkably shortened, the bearing capacity is improved, and the method is suitable for coastal deep soft soil.

Description

Technical Field

[0001] This invention belongs to the field of geotechnical engineering technology, specifically relating to a method for treating soft soil foundations based on deep mixing and vacuum preloading. Background Technology

[0002] Among existing soft soil foundation treatment technologies, the engineering community has developed several mature methods, mainly including surcharge preloading, vacuum preloading, deep mixing, and replacement layer methods. These methods have achieved significant application results under different geological conditions, but each has specific limitations and cannot fully meet the comprehensive requirements of short construction periods, large treatment depths, uniform settlement, and economic efficiency. Surcharge preloading applies additional stress by surcharged earth or gravel on the surface, forcing the pore water in the soft soil to drain and restructure, thus improving bearing capacity. It is simple to construct and low in cost, and is widely used in coastal highways. However, it has a long construction period, requires large sites, and is prone to inducing slope instability and uneven settlement. Vacuum preloading uses an airtight membrane covering and a vacuum pump to apply negative pressure, combined with plastic drainage boards to accelerate radial drainage. It achieves a consolidation degree of 85%-95%, uniform settlement, and is environmentally friendly. However, the negative pressure transmission depth is limited, drainage boards are prone to clogging, and the membrane damage rate is high, resulting in a failure rate of >20% for deep soft soil. Deep mixing utilizes specialized equipment to inject a curing agent and mix in situ to form high-strength piles, significantly improving bearing capacity and shortening post-construction settlement. However, it is costly, suffers from uneven soil consolidation between piles, and its quality depends heavily on the uniformity of mixing. The replacement layer method involves excavating surface soft soil and backfilling with a sand and gravel cushion to distribute the load, offering good immediate effects and a simple process. However, it is limited to shallow layers, involves large volumes of earthwork, and causes significant environmental disturbance. In summary, existing methods struggle to simultaneously achieve rapid consolidation, deep treatment, uniform settlement, and economic reliability. Although preliminary explorations have been made of combining vacuum preloading and deep mixing, the lack of system integration for enhancing pile network stiffness, optimizing radial drainage, and implementing real-time adaptive pressure control fails to effectively address the problems of low consolidation efficiency and high risk of uneven consolidation in deep soft soils. Summary of the Invention

[0003] The technical problem to be solved by the present invention is to provide a soft soil foundation treatment method based on deep mixing and vacuum preloading that enhances stiffness through pile network, achieves uniform consolidation through real-time adaptive pressure control, significantly shortens the construction period, improves bearing capacity, and is applicable to deep soft soil in coastal areas.

[0004] The technical solution of this invention is as follows: A method for treating soft soil foundations based on deep mixing and vacuum preloading includes the following steps: Step 1: Install multiple deep mixing piles in the deep layer of soft soil foundation, use solidifying agent to mix with in-situ soil, and form a pile network using special mechanical equipment; Step 2: Install plastic drainage boards between the deep mixing piles and cover the ground surface with an airtight membrane; Step 3: Apply negative pressure to the soil using a vacuum preloading system connected to the plastic drainage board; the negative pressure drives pore water out. Step 4: Set up settlement gauges, pore water pressure gauges and vacuum pressure sensors in the construction area to monitor the soil settlement, pore water pressure and negative pressure data in the drainage board in real time. Step 5: Based on the real-time collected settlement, pore water pressure and negative pressure data, and combined with PID control algorithm or fuzzy control algorithm, automatically adjust the application intensity and loading rate of vacuum negative pressure until the foundation settlement reaches the predetermined value, or the consolidation degree and strength of the soil meet the design requirements.

[0005] Furthermore, the vacuum pre-compression system applies a negative pressure range of -50 kPa to -80 kPa to the plastic drainage board.

[0006] Furthermore, the mixing ratio of the curing agent to the in-situ soil is as follows: for conventional soft soil layers, the mass ratio of the curing agent to the in-situ soil is 5%-8%; for high moisture content or extremely soft sections, the ratio of the curing agent to the in-situ soil is 8%-12%. The injection depth and dosage of the curing agent are adjusted according to changes in soil properties.

[0007] Furthermore, the drainage system consists of plastic drainage boards and airtight membranes laid between the piles, with the plastic drainage boards laid at intervals of 1.2m to 2m and at depths of 40cm to 70cm.

[0008] Furthermore, the sedimentation gauge, pore water pressure gauge, and vacuum pressure sensor transmit monitoring data to the central data processing center in real time via a wireless transmission system.

[0009] Furthermore, the expression for the PID control algorithm in step 5 is: , in, To control the output, i.e., the vacuum negative pressure value; The error between the target value and the actual value. This is a proportionality coefficient used for rapid response to settlement deviations; This is the integral coefficient, used to compensate for long-term pore water pressure residuals and prevent integral saturation. The differential coefficient is used to predict sudden changes in settlement rate and prevent over-compaction.

[0010] Furthermore, the deep mixing piles are used in conjunction with a vacuum preloading system to consolidate the soil as a whole through a pile network.

[0011] Furthermore, the negative pressure adjustment process of the vacuum pre-compression system adopts a real-time monitoring and data feedback mechanism. Based on the Terzaghi consolidation theory and the Boussinesq theoretical model, the negative pressure application strategy is dynamically adjusted. The consolidation process is predicted by the sedimentation and pore water pressure change data, and the negative pressure value is adjusted in real time.

[0012] Furthermore, step 5 also includes: predicting the soil settlement trend and pore water discharge efficiency based on real-time collected settlement, pore water pressure, and negative pressure data; predicting future settlement using a regression analysis model; and providing a basis for vacuum negative pressure adjustment; the expression of the regression model is: , Among them, among them, For time Settlement at time Pore ​​water pressure, It is a vacuum negative pressure. This is the pore water pressure regression coefficient, used to quantify the linear effect of pore water pressure on settlement; The regression coefficient for vacuum negative pressure is used to quantify the linear effect of vacuum negative pressure on sedimentation. This is a constant term used to compensate for the initial state of the soil or nonlinear residuals, ensuring prediction accuracy.

[0013] The beneficial effects of this invention; This invention discloses a soft soil foundation treatment method based on deep mixing and vacuum preloading, which utilizes "deep mixing piles to construct a stiffness framework + PVD to shorten the equivalent drainage path + controlled vacuum preloading + online monitoring and PID control". The "adaptive control" closed-loop construction method makes vacuum negative pressure and effective stress increment approximately equivalent and significantly improves the time factor. This allows for rapid dissipation of pore pressure, accelerated consolidation, and controllable shortening of the construction period according to a set trajectory while ensuring safe amplitude and slope limits. At the same time, pile-soil composite improves overall stiffness, and adaptive pressure distribution suppresses excessive local suction, resulting in more uniform settlement and significantly reduced differences. Early strength and bearing capacity are more easily aligned with the design, facilitating earlier commencement of superstructure construction. The combination of three-line monitoring (settlement-pore pressure-vacuum) and regression prediction allows for early identification, rapid handling, and full traceability of anomalies (air leakage, blockage, excessively rapid settlement), ensuring quality traceability. Compared with traditional solutions such as high-fill preloading or single deep mixing, the amount of surcharge and solidifying agent, excavated material, and transportation is smaller, resulting in more flexible overall costs. It is more friendly to narrow working surfaces, high water levels, and adjacent existing structures, with lower construction disturbance and environmental impact. Overall, it achieves comprehensive benefits such as faster consolidation, more uniform settlement, controllable process, manageable risks, and green and low-carbon development, while also reducing uncertainty in later maintenance. Attached Figure Description

[0014] Figure 1This is a flowchart illustrating the steps of a soft soil foundation treatment method based on deep mixing and vacuum preloading according to the present invention.

[0015] Figure 2 This is a schematic diagram of the layout of deep mixing piles and drainage system in a soft soil foundation treatment method based on deep mixing and vacuum preloading according to the present invention.

[0016] Figure 3 This is a flowchart illustrating the automated control and optimization adjustment steps of a soft soil foundation treatment method based on deep mixing and vacuum preloading according to the present invention. Detailed Implementation

[0017] like Figure 1-3 As shown, a soft soil foundation treatment method based on deep mixing and vacuum preloading is used to treat highly compressible foundations such as dredged silty soil and deep soft clay. The goal is to achieve an overall consolidation degree of 90% or more within a limited construction period, with settlement entering a stable range, bearing capacity and strength meeting design requirements, and differential settlement being minimized.

[0018] I. Preliminary Preparation and Testing: After arriving at the site, geotechnical and consolidation tests were conducted by sampling in layers to obtain the one-dimensional consolidation coefficient. (m) 2 / s), permeability coefficient (m / s), initial void ratio, etc.; construct a test section of not less than 20 m × 20 m at a representative location, and pump for 7–14 days according to the formal process to check the airtightness and data transmission quality, and correct the equivalent drainage path based on the actual measurements of the test section. (m) and initial values ​​of subsequent control parameters.

[0019] II. Deep Mixing: Deep mixing piles are installed within soft soil layers, with common pile diameters ranging from 50cm to 80cm. The specific selection depends on factors such as soil strength requirements and bearing capacity requirements. The pile spacing is typically 1.2m-2m, determined based on soil consolidation requirements and pile coverage. The pile depth needs to penetrate the soft soil layer and reach the underlying harder layer, with common pile lengths ranging from 5m to 15m, determined by the thickness of the soft soil layer and the hardness of the underlying layer. Generally, the piles should penetrate the soft soil layer and reach the underlying harder layer to ensure effective soil reinforcement. The mass ratio of the curing agent to the in-situ soil (based on dry weight) is 5%-8% for conventional soils and 8%-12% for soils with high moisture content. Dual / triaxial equipment is used for segmented grouting and re-mixing 2-3 times, with a lifting rate of 0.6-1.0m / min. This establishes a rigid framework for the foundation, controls lateral displacement and differential settlement, and provides stable conditions for subsequent vacuum pressure transmission.

[0020] III. Drainage System and Airtight Covering: Plastic drainage boards (PVD) are installed between piles at intervals of 1.2–2.0 m. If the end encounters a permeable layer, the board should stop 0.2–0.5 m above it; if it is an impermeable layer, the board should stop 0.3–0.5 m above it. A 0.4–0.7 m thick sand mat and drainage layer are laid on the surface, with perforated branch pipes connecting to the main pipe and then to the vacuum station. A ≥1.5 mm composite airtight membrane is then laid on top, with the edges embedded in L-shaped anchor grooves, secured with waterproof sealant and continuous sandbags, with an overlap width ≥150 mm, and air extraction and observation ports are provided. To confirm the hydraulic capacity of the pipeline, Darcy's law is used to check the flux. , in, It is the drainage flux per unit area (m / s). It is the permeability coefficient (m / s). It is the hydraulic gradient; if the actual pumping does not match the theory and the pore pressure drops slowly, prioritize checking for blockages or air leaks.

[0021] IV. Vacuum Pre-compression in Stages: The vacuum pre-compression system (such as Menard Vacuum / MVC) employs dual-pump redundancy with gas-water separation and check valve protection. Negative pressure is applied in three stages: first, maintaining pressure at 35–45 kPa for approximately 24 hours and checking for leaks; then, observing at 55–65 kPa for approximately 48 hours; finally, based on monitoring performance, the operating upper limit is raised to 70–80 kPa. To avoid disturbances, the rate of change is controlled. , Under the premise of airtight membrane system and unobstructed drainage, vacuum negative pressure and effective stress increment are approximately equivalent: , in, The effective stress increment (kPa). The setpoint for vacuum negative pressure (kPa) is used to predict settlement and construction period.

[0022] V. Online Monitoring and Data Transmission: At least eight settlement points will be set up at the center, midpoint of the radius, and edge of the site; pore pressure gauges will be installed at typical depths of the soft layer (e.g., 2 / 5 / 8 m); vacuum pressure sensors will be installed at the beginning and end of the main pipe and the ends of key branch pipes. During the trial operation, sampling will be conducted every 5–15 minutes; after stabilization, sampling will be conducted every 30–60 minutes. Data will be transmitted back to the central platform via LoRa / 4G, and a unified clock will be used to plot the settlement. pore water pressure and measured values ​​of vacuum negative pressure Curve, calculated simultaneously and As a basis for control and diagnosis (e.g.) Superdomain or A slowdown will trigger a pressure maintenance / de-pressure check or anomaly investigation.

[0023] VI. Adaptive Pressure Control: Based on the target settlement trajectory Or target consolidation trajectory For reference, let the error be... The new vacuum setpoint is calculated using a PID control law: , in, To control the output, i.e., the vacuum negative pressure value; The error between the target value and the actual value. This is a proportionality coefficient used for rapid response to settlement deviations; This is the integral coefficient, used to compensate for long-term pore water pressure residuals and prevent integral saturation. These are differential coefficients used to predict sudden changes in settlement rate and prevent over-compaction; The entire process is Apply amplitude limiting of 50–80 kPa and slope limiting. To control the construction period and pace, Terzaghi one-dimensional consolidation was incorporated: , in, This is a time factor used to measure the standard time of the consolidation process; This is a one-dimensional consolidation coefficient, obtained from consolidation tests; For duration, For equivalent drainage paths, The volumetric degree of consolidation (0–1) represents the ratio of pore pressure dissipation to volumetric consolidation completion, used to indicate the achievement of the target degree of consolidation (e.g., ...). The theoretical time required is also the target trajectory of the PID controller. Provide timeline reference; When deep pore pressure dissipation is found to be lagging, the additional deep stress is estimated using the axial solution of a uniformly distributed load: , in, For depth Additional stress on the axis, The equivalent loading radius can be taken as the characteristic radius of the pressure control zone; For depth, This is the measured value of vacuum negative pressure; based on this, the pressure increase slope can be slowed down or the pressure holding time extended to make the response of deep and shallow layers more consistent; Meanwhile, linear regression is used: , Online estimation of pore water pressure regression coefficients (Used to quantify the linear effect of pore water pressure on sedimentation) and vacuum negative pressure regression coefficient (Used to quantify the linear effect of vacuum negative pressure on settlement), to quantitatively predict short-term settlement trends, and to serve as a reference for the next pressure setting; where, For time Settlement at time Pore ​​water pressure, It is a vacuum negative pressure. This is a constant term used to compensate for the initial state of the soil or nonlinear residuals, ensuring prediction accuracy. If the measured-prediction deviation is amplified and the vacuum variance increases simultaneously, priority should be given to checking for leaks at the membrane edge. If the deviation is concentrated in a few places and the pore pressure decreases more slowly, it is mostly due to local plate blockage or poor contact.

[0024] VII. Compliance Judgment and Unloading / Sealing: When the degree of consolidation reaches... ,and If the pressure approaches 0 and remains stable for 48–72 hours, and the cumulative settlement increase does not exceed 2–3 mm within any 72–120 hour observation window, it can be determined that the consolidation and stability requirements are met. Subsequently, the pressure is gradually reduced to 0 at a rate of about 5 kPa every 6–12 hours. After the pump is stopped, monitoring continues for 24–48 hours. Once the curve is confirmed to be stable, the membrane layer and surface drainage are removed. Finally, the strength and bearing capacity are verified to meet the standards by vane plate, pore pressure static cone penetration test, or load plate test, and all process data are archived.

[0025] 8. Common Abnormalities and Corresponding Measures: When it is difficult to maintain vacuum, the fluctuation variance increases, and the pore pressure decreases more slowly, it is mostly due to membrane edge leakage; Corresponding measures: repair the edge seal, add pressure sandbags, re-do the overlap, and temporarily reduce the control pressure. Tighten ; When the pressure at certain measuring points does not drop or the settlement is delayed, it is mostly due to PVD blockage. The corresponding measures are: to add a plate or open a bypass in the abnormal area, temporarily increase the upper limit by 5-10 kPa and maintain the pressure for a limited time, and then return to normal. When settlement is too rapid or the edge-center difference is amplified, the corresponding measures are: automatic pressure reduction according to PID, and if necessary, stop pumping and temporarily unload in different areas, while reviewing the pile network and monitoring layout; In the event of a pump set or power supply failure, the following measures should be taken: immediately switch to the backup pump to maintain the minimum pressure; if the failure cannot be restored within 2 hours, slowly restore the pressure according to the procedure to prevent backflow.

[0026] The above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A method for treating soft soil foundations based on deep mixing and vacuum preloading, characterized in that, Includes the following steps: Step 1: Install multiple deep mixing piles in the deep layer of soft soil foundation, use solidifying agent to mix with in-situ soil, and form a pile network using special mechanical equipment; Step 2: Install plastic drainage boards between the deep mixing piles and cover the ground surface with an airtight membrane; Step 3: Apply negative pressure to the soil using a vacuum preloading system connected to the plastic drainage board; the negative pressure drives pore water out. Step 4: Set up settlement gauges, pore water pressure gauges and vacuum pressure sensors in the construction area to monitor the soil settlement, pore water pressure and negative pressure data in the drainage board in real time. Step 5: Based on the real-time collected settlement, pore water pressure and negative pressure data, and combined with PID control algorithm or fuzzy control algorithm, automatically adjust the application intensity and loading rate of vacuum negative pressure until the foundation settlement reaches the predetermined value, or the consolidation degree and strength of the soil meet the design requirements.

2. The soft soil foundation treatment method based on deep mixing and vacuum preloading according to claim 1, characterized in that, The vacuum preloading system applies a negative pressure ranging from -50 kPa to -80 kPa to the plastic drainage board.

3. The soft soil foundation treatment method based on deep mixing and vacuum preloading according to claim 1, characterized in that, The mixing ratio of the curing agent to the in-situ soil is as follows: For conventional soft soil layers: the mass ratio of curing agent to in-situ soil is 5%-8%; for high moisture content or extremely soft sections: the ratio of curing agent to in-situ soil is 8%-12%. The injection depth and dosage of the curing agent are adjusted according to changes in soil properties.

4. The soft soil foundation treatment method based on deep mixing and vacuum preloading according to claim 1, characterized in that, The drainage system consists of plastic drainage boards and airtight membranes laid between the piles. The spacing between the plastic drainage boards is 1.2m to 2m, and the laying depth is 40cm to 70cm.

5. A method for treating soft soil foundations based on deep mixing and vacuum preloading according to claim 1, characterized in that, The sedimentation gauge, pore water pressure gauge, and vacuum pressure sensor transmit monitoring data to the central data processing center in real time via a wireless transmission system.

6. The soft soil foundation treatment method based on deep mixing and vacuum preloading according to claim 1, characterized in that, The expression for the PID control algorithm in step 5 is: ， in, To control the output, i.e., the vacuum negative pressure value; The error between the target value and the actual value. This is a proportionality coefficient used for rapid response to settlement deviations; This is the integral coefficient, used to compensate for long-term pore water pressure residuals and prevent integral saturation. The differential coefficient is used to predict sudden changes in settlement rate and prevent over-compaction.

7. The soft soil foundation treatment method based on deep mixing and vacuum preloading according to claim 1, characterized in that, The deep mixing piles are used in conjunction with a vacuum preloading system to consolidate the soil as a whole through a pile network.

8. A method for treating soft soil foundations based on deep mixing and vacuum preloading according to claim 1, characterized in that, The negative pressure adjustment process of the vacuum preloading system adopts a real-time monitoring and data feedback mechanism. Based on the Terzaghi consolidation theory and the Boussinesq theoretical model, the negative pressure application strategy is dynamically adjusted. The consolidation process is predicted by the sedimentation and pore water pressure change data, and the negative pressure value is adjusted in real time.

9. A method for treating soft soil foundations based on deep mixing and vacuum preloading according to claim 1, characterized in that, Step 5 further includes: predicting soil settlement trends and pore water discharge efficiency based on real-time collected settlement, pore water pressure, and negative pressure data; predicting future settlement using a regression analysis model; and providing a basis for vacuum negative pressure adjustment. The expression for the regression model is: ， Among them, among them, For time Settlement at time Pore ​​water pressure, It is a vacuum negative pressure. This is the pore water pressure regression coefficient, used to quantify the linear effect of pore water pressure on settlement; The regression coefficient for vacuum negative pressure is used to quantify the linear effect of vacuum negative pressure on sedimentation. This is a constant term used to compensate for the initial state of the soil or nonlinear residuals, ensuring prediction accuracy.

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