Construction method for optimizing bearing capacity of sand cushion layer of open caisson

Abstract

The invention discloses an open caisson sand cushion bearing capacity optimization construction method, and relates to the technical field of open caisson sand cushion construction. The open caisson sand cushion bearing capacity optimization construction method comprises the steps that after an open caisson foundation pit is excavated to the designed elevation, a base is leveled and compacted through rolling, and a horizontal working face with the flatness not exceeding 2 cm is formed; fully paving dense meshes made of PP (Polypropylene) with the pore diameter of less than or equal to 2mm on the substrate; paving medium-coarse sand on the base; a plate compactor or a small road roller is used for vibrating in a striping mode in the longitudinal direction; and if the bearing capacity does not reach the standard, supplementing the vibration or adjusting the grading until the acceptance is qualified. By means of the systematic construction technology and the accurate quality control means, the problems that in the prior art, the isolation effect is poor, grading control is inaccurate, and construction quality is unstable are solved, and the bearing performance and construction quality of the open caisson sand cushion layer are remarkably improved.

Description

Technical Field

[0001] This invention relates to the field of caisson sand cushion construction technology, and in particular to a construction method for optimizing the bearing capacity of caisson sand cushion. Background Technology

[0002] With the continuous development of urban underground space, caisson structures are increasingly widely used in municipal engineering. As a key component of the caisson foundation, the construction quality of the sand cushion layer directly affects the stability of the first section of the caisson and is the foundation for the successful construction of the caisson.

[0003] Traditional construction methods have many technical bottlenecks: the isolation layer material is mostly ordinary geotextile or plastic film, which is easily damaged under the rolling of construction machinery and the impact of sand, resulting in insufficient stability of the interface between the base and the sand layer; the control of sand gradation lacks scientific standards, and unreasonable particle size distribution often causes local weak areas; the setting of vibration process parameters relies too much on experience, and it is difficult to ensure uniformity of vibration effect.

[0004] More importantly, when insufficient bearing capacity is detected, conventional methods such as supplementary compaction or thickening often only address the symptoms, failing to fundamentally solve the problems of poor gradation or localized defects. These issues are particularly pronounced in soft soil foundations or high groundwater levels, severely limiting the engineering application effectiveness of caisson structures. Summary of the Invention

[0005] The main objective of this invention is to propose a construction method for optimizing the bearing capacity of caisson sand cushion layers, aiming to solve the technical problems of low bearing performance and construction quality of caisson sand cushion layers in the prior art.

[0006] To achieve the above objectives, the present invention proposes a construction method for optimizing the bearing capacity of caisson sand cushion layers, the method comprising: After excavating the caisson foundation pit to the design elevation, level the base and compact it to form a horizontal working surface with a flatness of no more than 2cm; The base is fully covered with a dense mesh of PP material with a mesh size ≤2mm; wherein, the overlap width of two adjacent dense mesh sheets is ≥10cm, and they are anchored with U-shaped steel nails at a spacing of ≤50cm, with an anchoring depth of ≥20cm. Medium-coarse sand with a particle size of 0.25mm to 5mm is selected, and the proportion of 1mm to 3mm particles in the medium-coarse sand is controlled to be ≥60% and the mud content is ≤3%. Oversized impurities are removed by screening equipment. The medium-coarse sand is spread on the base layer in layers of 30cm thickness. After spreading, the layer thickness is checked with a level or total station, with an allowable deviation of ±5mm. Use a plate compactor or a small road roller to vibrate longitudinally in strips; each strip is 1.5m wide, the vibration frequency is 40Hz, and the compaction is performed 3 times with adjacent areas overlapping by 20cm; After each two layers of vibration compaction are completed, a static load testing machine is used to apply a load of 40 kPa / level, stabilize the pressure for 30 minutes, and test the bearing capacity to be ≥160 kPa. If the bearing capacity does not meet the standard, supplement the vibration or add 5% quartz sand with a particle size of 2mm to 4mm to adjust the gradation until it passes the acceptance test.

[0007] In one embodiment, the step of fully covering the substrate with a dense mesh of PP material with a aperture ≤2mm includes: The dense mesh is laid longitudinally along the foundation pit, with the edge of the dense mesh extending 50cm beyond the outer contour line of the caisson. Use 8mm diameter U-shaped steel nails to drive vertically into the soil or sand layer, with the top protruding ≤3cm above the mesh surface; A 5cm thick layer of fine sand with a particle size ≤1mm is covered on the dense mesh, and then compacted twice with a light roller.

[0008] In one embodiment, the step of using a plate compactor or a small roller to vibrate the strips longitudinally includes: Before compaction, water is sprayed evenly using a misting nozzle to bring the moisture content of the sand layer to 10±1%. Vibrate the surfaces in a grid pattern, first longitudinally and then laterally, with an interval of ≤10 minutes between adjacent areas. After compaction in each area, samples were taken using the water injection method; the sampling depth was 2 / 3 of the layer thickness, and the dry density was tested to be ≥1.75 g / cm³. 3 .

[0009] In one embodiment, after the step of sampling using the ring cutter method after vibration of each area, the construction method for optimizing the bearing capacity of the caisson sand cushion layer further includes: If the difference in dry density among three consecutive samples is >0.05 g / cm³ 3 If the difference is ≤0.03g / cm, then the area should be vibrated again and resampled until the difference is ≤0.03g / cm. 3 .

[0010] In one embodiment, after each two layers of vibration compaction, a static load testing machine is used to apply a load of 40 kPa / level, stabilize the load for 30 minutes, and test the bearing capacity to be ≥160 kPa. This step includes: Circular steel plates with a diameter of 30cm are symmetrically arranged on the surface of the sand cushion layer, with the measuring points spaced at 1 / 4 of the diameter of the caisson. Increase the pressure gradually by 40 kPa per level, stabilize the pressure at each level for 30 minutes, and record the settlement. When the total settlement is ≥50mm or the load is ≥200kPa, stop loading, plot the PS curve and determine the bearing capacity.

[0011] When the total settlement is ≥50mm or the load is ≥200kPa, the loading is stopped, and the steps for plotting the PS curve and determining the bearing capacity include: If the load value corresponding to the inflection point of the PS curve is <160kPa, then additional vibration or 5% quartz sand with a particle size of 2mm to 4mm should be added to adjust the gradation. After additional vibration, the load should be measured again until the inflection point load is ≥160kPa.

[0012] In one embodiment, if the bearing capacity does not meet the standard, the steps of supplementing vibration or adding 5% quartz sand with a particle size of 2mm to 4mm to adjust the gradation until the acceptance is qualified include: Based on the test results, the required gradation range is calculated, and the quartz sand content is determined to be 5%–10%. The quartz sand and the medium-coarse sand are mixed using a forced mixer for a mixing time of ≥3 minutes. After the mixture is spread, three samples are randomly selected for sieving tests to ensure that the proportion of 1mm to 3mm particles is ≥65%.

[0013] In one embodiment, if the bearing capacity is not up to standard, the method for optimizing the bearing capacity of the caisson sand cushion layer further includes the following steps: supplementing with vibration or adding 5% quartz sand with a particle size of 2mm to 4mm to adjust the gradation until acceptance is passed. After wetting with water, compact it again.

[0014] The technical solution of this invention enhances the bonding force between the mesh and the soil layer by laying a PP material dense mesh with an aperture of ≤2mm on the substrate and using an overlap width of ≥10cm and U-shaped steel nails for anchoring. This effectively solves the problems of easy damage, weak anchoring, and sand loss in traditional isolation layers. Medium-coarse sand with a particle size of 0.25mm to 5mm is selected, and the proportion of 1mm to 3mm particles is strictly controlled to be ≥60% and the mud content ≤3%. Combined with screening equipment to remove oversized impurities, the sand gradation is optimized, effectively improving the interlocking effect of the sand cushion layer. This invention improves the density and compaction of sand cushion layers, overcoming the problems of uneven density and large fluctuations in bearing capacity in existing technologies. By accurately verifying the layer thickness with a level or total station and using a plate compactor or small roller with systematic vibration paths and standardized parameter control, it ensures thorough vibration coverage, achieving a uniform and dense sand layer. Combined with staged static load tests to detect bearing capacity and targeted supplementary vibration or the addition of quartz sand to adjust gradation, it accurately identifies and optimizes construction problems, avoiding the quality instability and project delays caused by single testing methods and blind rework in traditional construction. In summary, this invention significantly improves the bearing capacity and construction quality of caisson sand cushion layers through systematic processes and quality control methods, solving the technical problems of poor isolation effect, inaccurate gradation control, and imperfect construction quality control in existing technologies. Attached Figure Description

[0015] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the structures shown in these drawings without creative effort.

[0016] Figure 1 This is a schematic flowchart of an embodiment of the construction method for optimizing the bearing capacity of the caisson sand cushion layer provided by the present invention.

[0017] The realization of the objective, functional features and advantages of the present invention will be further explained in conjunction with the embodiments and with reference to the accompanying drawings. Detailed Implementation

[0018] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0019] It should be noted that if the embodiments of the present invention involve directional indicators (such as up, down, left, right, front, back, etc.), the directional indicators are only used to explain the relative positional relationship and movement of the components in a specific posture. If the specific posture changes, the directional indicators will also change accordingly.

[0020] Furthermore, if the embodiments of this invention involve descriptions such as "first" or "second," these descriptions are for descriptive purposes only and should not be construed as indicating or implying their relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined with "first" or "second" may explicitly or implicitly include at least one of those features. Additionally, the use of "and / or" or "and / or" throughout the text includes three parallel solutions. For example, "A and / or B" includes solution A, solution B, or a solution where both A and B are satisfied. Furthermore, the technical solutions of the various embodiments can be combined with each other, but this must be based on the ability of those skilled in the art to implement them. When the combination of technical solutions is contradictory or impossible to implement, it should be considered that such a combination of technical solutions does not exist and is not within the scope of protection claimed by this invention.

[0021] With the continuous development of urban underground space, caisson structures are increasingly widely used in municipal engineering. As a key component of the caisson foundation, the construction quality of the sand cushion layer directly affects the stability of the first section of the caisson and is the foundation for the successful construction of the caisson.

[0022] Traditional construction methods have many technical bottlenecks: the isolation layer material is mostly ordinary geotextile or plastic film, which is easily damaged under the rolling of construction machinery and the impact of sand, resulting in insufficient stability of the interface between the base and the sand layer; the control of sand gradation lacks scientific standards, and unreasonable particle size distribution often causes local weak areas; the setting of vibration process parameters relies too much on experience, and it is difficult to ensure uniformity of vibration effect.

[0023] More importantly, when insufficient bearing capacity is detected, conventional methods such as supplementary compaction or thickening often only address the symptoms, failing to fundamentally solve the problems of poor gradation or localized defects. These issues are particularly pronounced in soft soil foundations or high groundwater levels, severely limiting the engineering application effectiveness of caisson structures.

[0024] To address this technical problem, this invention proposes a construction method for optimizing the bearing capacity of caisson sand cushion layers.

[0025] Please see Figure 1 In one embodiment of the present invention, the construction method for optimizing the bearing capacity of the caisson sand cushion layer includes: S10, after excavating the caisson foundation pit to the design elevation, level the base and compact it to form a horizontal working surface with a flatness of no more than 2cm; S20, the base is fully covered with a dense mesh of PP material with an aperture of ≤2mm; wherein, the overlap width of two adjacent dense meshes is ≥10cm, and they are anchored with U-shaped steel nails at a spacing of ≤50cm, with an anchoring depth of ≥20cm. S30 uses medium-coarse sand with a particle size of 0.25mm to 5mm, and controls the proportion of 1mm to 3mm particles in the medium-coarse sand to be ≥60% and the mud content to be ≤3%, and removes oversized impurities through screening equipment. S40, spread the medium-coarse sand on the base layer in layers of 30cm thickness, and check the layer thickness with a level or total station after spreading, with an allowable deviation of ±5mm. S50, using a plate compactor or small roller to vibrate longitudinally in strips; each strip is 1.5m wide, the vibration frequency is 40Hz, and it is vibrated 3 times with adjacent areas overlapping by 20cm; For S60, after each two layers of vibration are completed, a static load tester is used to apply a load of 40 kPa / level and stabilize the pressure for 30 minutes to test the bearing capacity ≥160 kPa; For S70, if the bearing capacity does not meet the standard, supplement the vibration or add 5% quartz sand with a particle size of 2mm to 4mm to adjust the gradation until it passes the acceptance test.

[0026] In practical applications, close-mesh wire mesh can be understood as a grid-like structure with specific pore sizes and materials. Its main function is to prevent sand loss and enhance the bonding force with the soil layer. For example, other grid structures made of materials with similar pore sizes and strength characteristics can be selected, such as mesh made of nylon or other polymer composite materials, primarily to achieve effective isolation and stable support for the sand cushion layer. Furthermore, a design where the overlap width between two adjacent close-mesh wire mesh panels is ≥10cm can be achieved by increasing the density of fixing points in the overlap area, such as using double or multiple rows of fixing members for reinforcement, thereby ensuring the continuity and stability of the overall laying. Specifically, the U-shaped steel nail anchoring method can also be replaced by other forms of fixing devices, such as spiral ground nails or chemical anchors, primarily to achieve a firm fixation of the wire mesh.

[0027] Furthermore, the selection of medium-coarse sand can be achieved by using other aggregates with similar particle size distribution and physical properties, such as graded gravel or crushed stone mixtures, primarily to optimize the compaction and load-bearing capacity of the sand cushion layer. In addition, the use of screening equipment can be replaced by manual screening or other forms of grading devices, such as vibrating screens or water-washing screening equipment, primarily to effectively remove oversized impurities. Specifically, the calibration function of levels or total stations can also be achieved through other high-precision measuring tools, such as mechanical thickness measuring devices, primarily to achieve accurate control of the paving layer thickness.

[0028] The innovation of this application lies in solving the problems of unstable bearing capacity and imprecise construction quality control of caisson sand cushion layers through systematic construction techniques and quality control methods. Specifically, a complete construction system is formed through comprehensive optimization of foundation treatment, material selection, construction parameters, and testing methods. For example, dense mesh and accurate slope control are introduced in the foundation treatment stage; the proportion requirements of key particle size ranges are clearly defined in the material selection stage; and strip vibration and staged testing are adopted during construction. These measures together constitute a highly operable and stable construction method. As a result, accurate control of the bearing capacity of the sand cushion layer is achieved, avoiding problems such as poor isolation effect, inaccurate gradation control, and limited testing methods in traditional construction methods.

[0029] The working principle of this application embodiment is as follows: Through systematic construction steps and accurate parameter control, the bearing capacity of the caisson sand cushion layer is optimized. First, after leveling and compaction, the base is formed into a horizontal working surface with a flatness of no more than 2cm, providing a uniform stress foundation for subsequent sand layer laying and avoiding uneven sand cushion layer thickness or local settlement caused by base undulation. Further, a dense mesh of PP material with a mesh size ≤2mm is fully laid on the base, with an overlap width of ≥10cm between adjacent meshes, and anchored with U-shaped steel nails at intervals ≤50cm and an anchoring depth ≥20cm. This enhances the mechanical interlocking between the mesh and the soil layer, preventing the problems of easy damage and weak anchoring of traditional isolation layers, and ensuring that the sand is not lost during construction.

[0030] Medium-coarse sand with a particle size of 0.25mm to 5mm is selected, and the proportion of particles with a size of 1mm to 3mm is strictly controlled to be ≥60% and the mud content ≤3%. Simultaneously, oversized impurities are removed using screening equipment, thereby optimizing the gradation distribution within the key particle size range and enhancing the interlocking effect between sand particles to achieve a stable and dense compaction. Specifically, medium-coarse sand is spread in 30cm thick layers on the base, and the layer thickness is checked using a level or total station, with an allowable deviation of ±5mm. This ensures accurate thickness control of the layered spreading, avoiding uneven layer thickness that could affect the compaction effect.

[0031] Using a plate compactor or small roller, vibration is applied in longitudinal strips, each 1.5m wide, at a frequency of 40Hz, three times, with adjacent areas overlapping by 20cm. This forms a systematic vibration path and energy input, ensuring thorough coverage and promoting sand grain rearrangement to achieve optimal compaction. After every two layers of vibration, a static load tester is used with a load of 40kPa / level, held for 30 minutes, to check if the bearing capacity meets the standard of ≥160kPa. This phased quality verification mechanism allows for the timely identification and handling of potential problems.

[0032] If the bearing capacity does not meet the standard, supplementary vibration or the addition of 5% quartz sand with a particle size of 2mm to 4mm to adjust the gradation will be adopted until the acceptance is qualified. This targeted measure can accurately locate the cause of the problem and effectively solve it, avoid blind rework, and ensure that the final sand cushion layer has a uniform and dense structure to support the stability of the caisson, thereby solving the problems of unstable bearing capacity of the caisson sand cushion layer and imprecise construction quality control.

[0033] In an embodiment of the present invention, the step of fully covering the substrate with a dense mesh of PP material with a aperture ≤2mm includes: S21, the dense mesh is laid along the longitudinal direction of the foundation pit, and the edge of the dense mesh extends 50cm beyond the outer contour line of the caisson; S22 uses 8mm diameter U-shaped steel nails to be driven vertically into the soil or sand layer, with the top protruding ≤3cm from the mesh surface; S23, cover the dense mesh with a 5cm thick layer of fine sand, the fine sand having a particle size ≤1mm, and compact it twice with a light roller.

[0034] Specifically, dense mesh refers to a mesh structure made of polypropylene material with evenly distributed small holes, which can be produced using weaving or extrusion molding processes. The design purpose of pore size ≤2mm is to effectively prevent sand loss while allowing moisture penetration. In practical applications, laying dense mesh along the longitudinal direction of the excavation pit ensures that the mesh is aligned with the pit's orientation, reducing the risk of wrinkling and displacement. Extending the edge of the dense mesh 50cm beyond the outer contour of the caisson provides an additional buffer zone, preventing sand loss at the edges and enhancing the overall coverage effect.

[0035] U-shaped steel nails refer to metal anchors with a U-shaped structure, which can be manufactured using cold bending or hot forging processes. The 8mm diameter specification is chosen to ensure sufficient anchoring strength while avoiding excessive damage to the mesh. The restriction of ≤3cm of the top protruding from the mesh surface aims to prevent the anchor from interfering with subsequent construction or causing mechanical damage. In practical applications, driving the anchor vertically into the soil ensures the maximum contact area between the anchor and the soil, thereby improving the fixing effect.

[0036] In addition, the fine sand layer refers to a sandy cover layer composed of fine particles with a particle size ≤1mm, which can be obtained by screening natural sand or artificial sand making. The design purpose of a 5cm thick cover is to form a protective layer to prevent direct damage to the dense mesh during construction. Compacting the cover layer twice with a light roller can enhance its density, improve the protective effect, and enhance surface smoothness.

[0037] Specifically, the above-mentioned technical solution addresses the problems of unstable mesh laying, unstable edges, and lack of protection through systematic laying, anchoring, and protection measures. Laying dense mesh along the longitudinal direction of the foundation pit ensures the directionality and stability of the mesh, while the edges extending beyond the design provide necessary safety margins. The proper placement of U-shaped steel nails achieves secure anchoring while controlling the exposed height to avoid construction interference. The covering and compaction of a fine sand layer forms an effective protective barrier, improving the overall construction quality.

[0038] The above technical solutions not only optimized the isolation effect and overall stability, but also significantly improved construction efficiency and quality control. The accurate laying method of the dense mesh, combined with scientific anchoring and protection measures, forms a complete anti-loss system, providing a reliable foundation for subsequent sand cushion layer construction.

[0039] In embodiments of the present invention, the step of using a plate compactor or a small road roller to vibrate longitudinally in strips includes: S51, before vibration, water is sprayed evenly using a misting nozzle to make the moisture content of the sand layer reach 10±1%; S52, vibrate in sections according to the path of the grid distribution, first longitudinally and then transversely, with an interval of ≤10 minutes between adjacent sections; S53, after vibration in each area, samples were taken using the water injection method; the sampling depth was 2 / 3 of the layer thickness, and the dry density was tested to be ≥1.75 g / cm³. 3 .

[0040] Specifically, an atomizing nozzle is a device that can disperse liquid water into fine water mist particles, which can be achieved using an ultrasonic atomizer or a pressure sprayer. By controlling the spray volume and spraying time, the moisture content of the sand layer is kept stable at 10±1%, thereby optimizing the vibratory compaction of the sand layer and avoiding problems such as local loosening or over-compaction caused by uneven moisture distribution.

[0041] The grid-like distribution of paths refers to dividing construction areas in a crisscross pattern, which can be accurately located using a total station combined with grid-based point placement technology. This path planning method ensures comprehensive and complete vibration coverage, while limiting the interval between adjacent areas to no more than 10 minutes to prevent differential settlement between areas due to time differences.

[0042] In practical applications, the water-filling sampling method is a field testing method for determining the density of sand layers. The principle is to dig a test pit with a diameter of 15-20 cm and a depth of 20 cm (at 2 / 3 of the layer thickness) on the surface of the compacted sand cushion layer, collect and weigh the collected wet sand sample, then lay an impermeable plastic membrane in the test pit and fill it with water until it is level with the surface. The volume of water poured in is recorded as the volume of the test pit. By measuring the moisture content of the sand sample, the mass of dry sand is calculated, and then the dry density is obtained by dividing the dry sand mass by the volume of the test pit. The criterion for determining the dry density is a dry density ≥ 1.75 g / cm³. 3 If it is not up to standard, additional vibration is required.

[0043] In this scheme, the method is applied to the quality inspection after each 1.5m wide vibratory strip is completed, forming a closed-loop control in conjunction with claim 4: if the difference in dry density between three consecutive sampling points exceeds 0.05g / cm³... 3 This indicates uneven compaction and requires further vibration until the difference is ≤0.03g / cm³. 3 Compared to the traditional ring cutter method, the water injection method is more suitable for coarse sand materials and deep sampling. It can provide real-time feedback on the vibration effect and ensure the compaction quality of each sand cushion layer through quantitative indicators. Ultimately, it ensures that the overall bearing capacity meets the design requirement of ≥160kPa. It is a key quality control link in this patented technical solution.

[0044] Specifically, the above-mentioned scheme achieves uniformity and reliability of the sand cushion layer's density through systematic control of process parameters. First, by accurately controlling the moisture content, the physical properties of the sand layer are optimized, making it easier to compact. Second, a grid-like vibration path is used for zoned compaction, and the vibration sequence and time intervals are strictly controlled to ensure uniform stress throughout the sand cushion layer and avoid localized settlement differences. Finally, by timely sampling and testing of dry density, a real-time quality monitoring mechanism is established, facilitating adjustments to construction parameters based on the test results.

[0045] Based on the aforementioned substrate treatment and sand gradation control, the above-mentioned vibration compaction process is organically integrated with the previous procedures. By laying a fine-mesh mesh on the leveled substrate and strictly controlling the sand gradation, combined with a systematic vibration compaction process, the problem of unstable quality caused by inaccurate parameter control in traditional construction is effectively solved. In particular, when a large difference in dry density is found during testing, targeted treatment can be carried out through supplementary vibration and other measures, thereby significantly improving construction efficiency and quality stability.

[0046] In an embodiment of the present invention, after the step of sampling using the ring cutter method after vibration of each area, the method for optimizing the bearing capacity of the caisson sand cushion layer further includes: S54, if the difference in dry density among three consecutive samples is >0.05 g / cm³ 3 If the difference is ≤0.03g / cm, then the area should be vibrated again and resampled until the difference is ≤0.03g / cm. 3 .

[0047] Specifically, the difference in dry density among three consecutive samples refers to the maximum difference in dry density values ​​between three adjacent samples collected at certain intervals within the vibrating area. This difference can be determined using rapid on-site testing equipment or standard laboratory methods. A second pass of vibration can be understood as performing a complete vibration operation on the area again, aiming to eliminate localized uneven compaction. Resampling refers to collecting samples again at the same location or in a nearby area after the second pass of vibration for testing, in order to verify the effectiveness of the remedial measures.

[0048] In detail, this scheme involves setting a dry density difference of more than 0.05 g / cm³ between three consecutive samples. 3 The established criteria effectively identify localized density fluctuations. When a dry density difference exceeds a set threshold, additional vibration is immediately applied to that area. This timely dynamic adjustment mechanism prevents the expansion of localized defects. Simultaneously, resampling verifies the remedial effect, and the final dry density difference is controlled within 0.03 g / cm³, ensuring the overall uniformity of the sand cushion layer's density. This process, combined with the aforementioned layered paving and mechanical vibration steps, forms a complete quality control system from initial compaction to quality inspection and defect repair, significantly improving the overall performance and construction efficiency of the sand cushion layer.

[0049] The above technical solutions not only solved the problem of uneven compaction of the sand cushion layer, but also avoided repeated large-scale rework, thus improving construction efficiency and quality control accuracy.

[0050] In an embodiment of the present invention, after each two layers of vibration compaction, a static load testing machine is used to apply a load of 40 kPa / level, stabilize the load for 30 minutes, and test the bearing capacity to be ≥160 kPa. The steps include: S61 uses circular steel plates with a diameter of 30cm, symmetrically arranged on the surface of the sand cushion layer, with the measuring point spacing being 1 / 4 of the diameter of the caisson; S62, pressurize in increments of 40 kPa, stabilize the pressure for 30 minutes after each increment, and record the settlement. S63, stop loading when the total settlement is ≥50mm or the load is ≥200kPa, plot the PS curve and determine the bearing capacity.

[0051] Specifically, the circular steel plate refers to the rigid component used to apply pressure in the static load test. It can be made of steel plate with uniform thickness to ensure the consistency of the loading area. In practical applications, the measuring point spacing refers to the distance between adjacent test points, which can be adjusted according to the diameter of the caisson to ensure the rationality of the test point distribution. Gradual pressurization refers to the process of gradually increasing the load according to a fixed pressure increment, which can be achieved through the automatic control system of the static load testing machine to simulate the actual load action process. Furthermore, the P-S curve is the relationship curve between pressure and settlement, which can be plotted in real time through a data acquisition system to intuitively reflect the bearing capacity characteristics.

[0052] In detail, the above-mentioned optimized construction method for the bearing capacity of the caisson sand cushion layer first ensures the consistency of loading conditions among different measuring points through the standardized setting of 30cm diameter circular steel plates. Based on this, a symmetrical arrangement and a scientific spacing based on 1 / 4 of the caisson diameter ensure that the testing points fully cover the key areas of the sand cushion layer, effectively avoiding the impact of local deviations on the overall evaluation results. Furthermore, a fixed loading rate of 40kPa / level, coupled with a stabilization time of 30 minutes per level, prevents instantaneous settlement errors caused by excessively rapid loading while ensuring sufficient load application. Simultaneously, by setting dual stopping conditions of total settlement ≥50mm or load ≥200kPa, a clear test termination standard is established, ensuring the safety and integrity of the testing process. Finally, using the P-S curve analysis method, the bearing capacity inflection point can be accurately identified, thereby achieving an accurate evaluation of the bearing performance of the sand cushion layer.

[0053] The above technical solutions not only solve the standardization problems in the testing process, but also significantly improve the accuracy and reliability of bearing capacity assessment, providing a strong guarantee for the quality control of caisson sand cushion construction.

[0054] In an embodiment of the present invention, the steps of stopping loading when the total settlement is ≥50mm or the load is ≥200kPa, plotting the PS curve, and determining the bearing capacity include: S631. If the load value corresponding to the inflection point of the PS curve is <160kPa, then add additional vibration or add 5% quartz sand with a particle size of 2mm to 4mm to adjust the gradation. After adding additional vibration, retest until the inflection point load is ≥160kPa.

[0055] Specifically, the P-S curve refers to the load-settlement curve, which is obtained by recording the load and corresponding settlement data during the loading process and plotting this data as a curve. In practice, computer software can be used to automatically plot this curve to more accurately identify the inflection point. The inflection point load value is a key indicator reflecting the bearing capacity of the sand cushion layer; when it is below 160 kPa, it indicates that the bearing capacity is insufficient. Supplementary vibration is an effective means to improve the compaction of the sand cushion layer, which can be achieved by increasing the number of vibrations or adjusting the vibration parameters. Adding quartz sand of a specific particle size is an important measure to optimize the gradation of sand. The dosage and particle size range of the quartz sand need to be strictly controlled to ensure the gradation adjustment effect.

[0056] In detail, during the construction of the caisson sand cushion layer, a loading test is first conducted according to the established plan. Loading is terminated when the total settlement reaches 50mm or the load reaches 200kPa. Then, a P-S curve is plotted, and the bearing capacity is determined by analyzing the curve characteristics. If the inflection point corresponds to a load value less than 160kPa, targeted measures are immediately taken. At this time, depending on the actual situation, additional vibration or gradation adjustment can be selected, with gradation adjustment using the addition of 5% quartz sand with a particle size of 2mm to 4mm. After treatment, retesting is required until the bearing capacity requirements are met. This treatment method effectively solves the problem of lacking specific measures when the bearing capacity is insufficient, avoiding the waste of resources caused by blind rework. Simultaneously, this scheme is closely integrated with the aforementioned sand cushion layer construction technology, ensuring the stability and reliability of construction quality through a systematic processing procedure.

[0057] The above technical solutions not only established clear standards for determining bearing capacity, but also provided specific treatment measures, significantly improving construction efficiency and quality control.

[0058] In an embodiment of the present invention, if the bearing capacity does not meet the standard, the steps of supplementing vibration or adding 5% quartz sand with a particle size of 2mm to 4mm to adjust the gradation until the acceptance is qualified include: S71. Based on the test results, calculate the required gradation range and determine the quartz sand content to be 5%–10%. S72, The quartz sand and the medium-coarse sand are mixed using a forced mixer for a mixing time of ≥3 minutes; S73, after the mixture is spread, three samples are randomly selected for sieving tests to ensure that the proportion of 1mm to 3mm particles is ≥65%.

[0059] Specifically, the gradation range refers to the distribution range of the proportion of particles of different sizes in sand. It can be determined by analyzing test data and combining design requirements with actual construction conditions. In practical applications, computer software can be used for calculation, or empirical formulas can be used for estimation. The purpose is to accurately locate the particle size range that needs to be adjusted, thereby avoiding blind operation. A forced mixer is a device with a forced mixing function. It can adopt various structural forms such as twin-shaft and planetary mixers to ensure that quartz sand and medium-coarse sand can be fully and uniformly mixed. Sieve analysis refers to the process of analyzing the particle size of sand samples through standard sieves. It can be carried out by mechanical vibrating sieving or manual sieving, and its purpose is to verify whether the adjusted gradation meets the requirements.

[0060] In detail, the above-mentioned scheme achieves accurate control of the bearing capacity optimization process through a series of orderly and closely related steps. First, based on the test results, the required gradation range is calculated and the quartz sand content is determined. This step provides clear data support for subsequent operations, effectively avoiding the uncertainty caused by relying on experience-based estimations in traditional methods. Then, a forced mixer is used for mixing, with the mixing time strictly controlled to be no less than 3 minutes. This standardized operation ensures the uniformity of the mixture, fundamentally solving the problem of local unevenness. Finally, random sampling and sieving tests are conducted on the mixture, focusing on whether the proportion of 1mm to 3mm particles reaches more than 65%. This verification process covers representativeness of different areas and can comprehensively reflect the gradation adjustment effect. The entire process forms a complete closed loop from problem diagnosis to solution implementation to effect verification, significantly improving the efficiency and reliability of bearing capacity adjustment. Simultaneously, this scheme, organically combined with the aforementioned caisson sand cushion construction method, provides a more refined and scientific approach to gradation optimization, effectively solving the technical problem of insufficient targeted treatment measures when bearing capacity is substandard.

[0061] In an embodiment of the present invention, if the bearing capacity is not up to standard, the method for optimizing the bearing capacity of the caisson sand cushion layer further includes the following steps: after supplementing vibration or adding 5% quartz sand with a particle size of 2mm to 4mm to adjust the gradation until the acceptance is qualified. S701, after wetting with water, compact again.

[0062] It should be noted that this step is a necessary procedure after remedial measures (supplementary vibration or addition of 5% quartz sand to adjust the gradation) are taken when the bearing capacity is insufficient. When quartz sand with a particle size of 2mm to 4mm is added and mixed with the original medium and coarse sand, the new mixture is in a loose state and lacks effective interlocking force between particles. At this time, it is necessary to moisten the sand layer by sprinkling water to bring the moisture content of the sand layer to the optimal moisture content of 10±1%. The water can form a capillary water film between the particles, which plays a dual role of lubrication and bonding, making it easier for the sand particles to move and fill the pores during the subsequent compaction process, thereby achieving a higher density.

[0063] The second compaction is performed using the same plate compactor or small roller as described above, following the standard of longitudinal strip compaction, each strip 1.5m wide, vibration frequency 40Hz, 3 passes, and 20cm overlap between adjacent areas. After compaction, samples should be taken using the water injection method to test the dry density, which should be ≥1.75g / cm³. 3 The bearing capacity was verified to be ≥160kPa through static load testing. This step is the final stage of the remedial measures, ensuring that the sand cushion layer after the gradation adjustment meets the same compaction standard as the original design. This prevents the bearing capacity from still not meeting the standard or uneven settlement from occurring later due to insufficient compaction after the admixture. It is a key component of the "rectification-re-inspection" mechanism in the quality control closed loop.

[0064] The above description is merely an exemplary embodiment of the present invention and does not limit the scope of protection of the present invention. Any equivalent structural transformations made based on the technical concept of the present invention and the contents of the specification and drawings of the present invention, or direct / indirect applications in other related technical fields, are included within the scope of protection of the present invention.

Claims

1. A construction method for optimizing the bearing capacity of a caisson sand cushion layer, characterized in that, The construction method for optimizing the bearing capacity of the caisson sand cushion layer includes: After excavating the caisson foundation pit to the design elevation, level the base and compact it to form a horizontal working surface with a flatness of no more than 2cm; The base is fully covered with a dense mesh of PP material with a mesh size ≤2mm; wherein, the overlap width of two adjacent dense mesh sheets is ≥10cm, and they are anchored with U-shaped steel nails at a spacing of ≤50cm, with an anchoring depth of ≥20cm. Medium-coarse sand with a particle size of 0.25mm to 5mm is selected, and the proportion of 1mm to 3mm particles in the medium-coarse sand is controlled to be ≥60% and the mud content is ≤3%. Oversized impurities are removed by screening equipment. The medium-coarse sand is spread on the base layer in layers of 30cm thickness. After spreading, the layer thickness is checked with a level or total station, with an allowable deviation of ±5mm. Use a plate compactor or a small road roller to vibrate longitudinally in strips; each strip is 1.5m wide, the vibration frequency is 40Hz, and the compaction is performed 3 times with adjacent areas overlapping by 20cm; After each two layers of vibration compaction are completed, a static load testing machine is used to apply a load of 40 kPa / level, stabilize the pressure for 30 minutes, and test the bearing capacity to be ≥160 kPa. If the bearing capacity does not meet the standard, supplement the vibration or add 5% quartz sand with a particle size of 2mm to 4mm to adjust the gradation until it passes the acceptance test.

2. The construction method for optimizing the bearing capacity of the caisson sand cushion layer as described in claim 1, characterized in that, The step of fully covering the substrate with a dense mesh PP material with a aperture ≤2mm includes: The dense mesh is laid longitudinally along the foundation pit, with the edge of the dense mesh extending 50cm beyond the outer contour line of the caisson. Use 8mm diameter U-shaped steel nails to drive vertically into the soil or sand layer, with the top protruding ≤3cm above the mesh surface; A 5cm thick layer of fine sand with a particle size ≤1mm is covered on the dense mesh, and then compacted twice with a light roller.

3. The construction method for optimizing the bearing capacity of the caisson sand cushion layer as described in claim 1, characterized in that, The steps for longitudinal slitting vibration using a plate compactor or small roller include: Before compaction, water is sprayed evenly using a misting nozzle to bring the moisture content of the sand layer to 10±1%. Vibrate the surfaces in a grid pattern, first longitudinally and then laterally, with an interval of ≤10 minutes between adjacent areas. After compaction in each area, samples were taken using the water injection method; the sampling depth was 2 / 3 of the layer thickness, and the dry density was tested to be ≥1.75 g / cm³. 3 .

4. The construction method for optimizing the bearing capacity of the caisson sand cushion layer as described in claim 3, characterized in that, Following the step of sampling using the ring cutter method after vibration of each area, the construction method for optimizing the bearing capacity of the caisson sand cushion layer also includes: If the difference in dry density among three consecutive samples is >0.05 g / cm³ 3 If the difference is ≤0.03g / cm, then the area should be vibrated again and resampled until the difference is ≤0.03g / cm. 3 .

5. The construction method for optimizing the bearing capacity of the caisson sand cushion layer as described in claim 1, characterized in that, After each two layers of vibration compaction are completed, a static load testing machine is used to apply a load of 40 kPa / level, stabilize the load for 30 minutes, and test the bearing capacity to ≥160 kPa. The steps include: Circular steel plates with a diameter of 30cm are symmetrically arranged on the surface of the sand cushion layer, with the measuring points spaced at 1 / 4 of the diameter of the caisson. Increase the pressure gradually by 40 kPa per level, stabilize the pressure at each level for 30 minutes, and record the settlement. When the total settlement is ≥50mm or the load is ≥200kPa, stop loading, plot the PS curve and determine the bearing capacity.

6. The construction method for optimizing the bearing capacity of the caisson sand cushion layer as described in claim 5, characterized in that, When the total settlement is ≥50mm or the load is ≥200kPa, the loading is stopped, and the steps for plotting the PS curve and determining the bearing capacity include: If the load value corresponding to the inflection point of the PS curve is <160kPa, then additional vibration or 5% quartz sand with a particle size of 2mm to 4mm should be added to adjust the gradation. After additional vibration, the load should be measured again until the inflection point load is ≥160kPa.

7. The construction method for optimizing the bearing capacity of the caisson sand cushion layer as described in claim 1, characterized in that, If the bearing capacity is insufficient, the steps to supplement compaction or add 5% quartz sand with a particle size of 2mm to 4mm to adjust the gradation until acceptance is achieved include: Based on the test results, the required gradation range is calculated, and the quartz sand content is determined to be 5%–10%. The quartz sand and the medium-coarse sand are mixed using a forced mixer for a mixing time of ≥3 minutes. After the mixture is spread, three samples are randomly selected for sieving tests to ensure that the proportion of 1mm to 3mm particles is ≥65%.

8. The construction method for optimizing the bearing capacity of the caisson sand cushion layer as described in claim 1, characterized in that, If the bearing capacity is not up to standard, supplementary vibration or addition of 5% quartz sand with a particle size of 2mm to 4mm to adjust the gradation until acceptance is passed. After this step, the construction method for optimizing the bearing capacity of the caisson sand cushion layer also includes: After wetting with water, compact it again.

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