Method for eliminating negative friction of pile foundation in deep and thick soft soil layer

Abstract

The invention discloses a method for eliminating negative friction of a pile foundation in a deep and thick soft soil layer, which relates to the technical field of foundation construction and comprises the following steps of: cleaning the surface of a site and replacing and filling the site to the designed elevation of the site level of a station; overload prepressing is conducted; sand wells are arranged in a treatment area in the reservoir, and dewatering wells are arranged on the periphery; setting a deformation monitoring point, and speculating the unloading time of overload filling; after pre-pressing treatment, the shear strength of the soil layer is obtained through an in-situ test, and whether the pre-pressed soil body reaches the required consolidation degree or not is determined. According to the method, the modes of overload prepressing and rainfall consolidation are adopted, the overload prepressing method is used for enabling a normally consolidated soil layer to be in an over-consolidated state, deformation under the using load can be greatly reduced, then the rainfall consolidation method is matched, the drainage efficiency can be improved, the duration needed by drainage consolidation is shortened, and the construction period is shortened; and the risk that the bearing capacity of the pile foundation is reduced in the later period is avoided while the engineering cost is reduced.

Description

Technical Field

[0001] The present invention relates to the technical field of foundation construction, and particularly relates to a method for eliminating negative skin friction of pile foundations in deep soft soil layers. Background Art

[0002] At present, during the construction of pile foundations in soft soil areas, due to the high water content of silty clay and the difficulty of water drainage, the soft soil layer will settle under the action of structural loads in the later stage. The settlement of the stratum will cause negative skin friction on the pile body around the pile foundation, thus generating a downward pull load, which reduces the bearing capacity of the pile foundation and affects the stability and safety of pile foundation construction.

[0003] In the prior art, in view of the problem that the settlement of the stratum causes negative skin friction around the pile foundation, resulting in a reduction in the bearing capacity of the pile foundation, currently, the main method is to increase the pile length to maintain the bearing capacity of the pile foundation. However, this method will lead to a significant increase in construction costs and is not conducive to the efficient development of enterprises. Therefore, a method for eliminating negative skin friction of pile foundations in deep soft soil layers is proposed. Summary of the Invention

[0004] The purpose of the present invention is to provide a method for eliminating negative skin friction of pile foundations in deep soft soil layers to solve the problems mentioned in the above background art.

[0005] To achieve the above purpose, the present invention provides the following technical solution: A method for eliminating negative skin friction of pile foundations in deep soft soil layers, comprising the following steps:

[0006] Clear the surface of the site and replace the soil according to the site requirements to reach the design elevation of the station yard apron.

[0007] Determine the thickness of surcharge fill according to the structure base and conduct surcharge preloading.

[0008] Arrange sand drains in the treatment area inside the depot and precipitation wells on the periphery to jointly form a drainage channel for drainage.

[0009] Set deformation monitoring points to estimate the unloading time of surcharge fill.

[0010] After preloading treatment, obtain the shear strength of the soil layer through in-situ tests, and determine whether the soil after preloading reaches the required degree of consolidation based on the soil strength.

[0011] Preferably, the sand drains arranged within the site treatment range include ordinary sand drains and precipitation sand drains. The ordinary sand drains and precipitation sand drains are both arranged in a plum blossom pattern, and the distance between adjacent ordinary sand drains and precipitation sand drains is 3 - 5 m.

[0012] Preferably, the ordinary sand drains are formed by punching holes in the treatment range with a punching device and filling medium coarse sand into the holes, so that the medium coarse sand in the wells can become a drainage channel for water in the silty clay layer, and the depth of the ordinary sand drains is such that the bottom of the well reaches the bottom of the silty clay aquifer.

[0013] Preferably, the well diameters of both the precipitation sand wells and the ordinary sand wells are 400 - 600 mm. The precipitation sand well consists of an ordinary sand well and a PVC filter pipe well arranged at the top of the ordinary sand well. The PVC filter pipe well in the precipitation sand well is located at the top of the silty clay layer and above, and the ordinary sand well in the precipitation sand well is located in the silty clay layer.

[0014] Preferably, submersible pumps are arranged in both the precipitation sand wells and the precipitation wells. The water in the silty clay layer is collected into the upper soil layer with good permeability through the drainage channels formed in the ordinary sand wells. The water in the soil layer is pumped out by the PVC pipe wells in the precipitation sand wells through submersible pumps, and at the same time, the surrounding precipitation wells pump water synchronously through submersible pumps to achieve the drainage purpose.

[0015] Preferably, a nylon net is wrapped around the outer wall of the PVC filter pipe in the precipitation sand well. The precipitation well is a non - sand concrete pipe, and a nylon cloth is wound around the outer wall of the pipe, and 2 - 4 mm filter material is filled outside the pipe.

[0016] Preferably, the deformation monitoring points include overall settlement monitoring points, layered settlement monitoring points, pore water pressure monitoring points, ground surface horizontal displacement monitoring points, and inclinometer monitoring points. The deformation data monitored by each monitoring point of the deformation monitoring points can be used to obtain the relationship curve between the vertical deformation of settlement and time and the relationship curve between pore water pressure and time. The unloading time of the surcharge fill can be inferred based on the relationship curves.

[0017] Preferably, the layout scheme of the precipitation wells is specifically as follows:

[0018] Outside the gravel ballast area of the reservoir, precipitation wells are used for closed surrounding dewatering, and inside, dewatering wells are used;

[0019] The well diameter of the precipitation well is 600 mm, and the well diameter of the dewatering well is 400 mm. The well depth of both is 23 m, which is determined based on the ground elevation of 5.50 m. Actually, it is controlled by the bottom elevation of - 17.50 m. Among them, the spacing of the precipitation wells is 15 m, and the spacing of the dewatering wells is 25 m;

[0020] The precipitation wells are backfilled with graded sand and gravel, with a particle size of 0.5 - 3.2 cm, and cement slurry with a water - cement ratio of 0.5 - 0.6 is injected, and the slurry is replenished 1 - 2 times. The backfill reaches 2.0 m above the wellhead. For the upper 2.0 m, depending on the original site conditions, it is either grouted once to the ground or backfilled with C15 plain concrete, and the ground is restored.

[0021] Preferably, the specific steps for the pile foundation construction in the soft soil layer are as follows:

[0022] S1. Considering the natural ground elevation of 5.200 m, fill to the designed yard elevation of 6.780 m according to the filling requirements of the station yard specialty;

[0023] S2. Construct ordinary sand drains and dewatering wells, and conduct pumping and surrounding dewatering after the construction is completed;

[0024] S3. Conduct surcharge filling to an elevation of 7.330 m;

[0025] S4. Construct dewatering sand drains and pile foundations of the bearing platform, and conduct surcharge loading to 2.500 m;

[0026] S5. Unload the surcharge soil, end the surrounding dewatering, and conduct construction inspection pits and pile foundations under the integral ballast bed.

[0027] Preferably, the loading rate of the surcharge preloading should meet the requirements that the maximum vertical deformation of the shaft foundation is less than 15 mm / d and the horizontal displacement at the edge of the surcharge preloading is less than 5 mm / d.

[0028] Compared with the prior art, the technical effects of the present invention:

[0029] By adopting the methods of surcharge preloading and dewatering consolidation, the present invention makes the normally consolidated soil layer in an over-consolidated state by using the surcharge preloading method, which can greatly reduce the deformation under the service load. Coupled with the dewatering consolidation method, it can improve the drainage efficiency, reduce the duration required for dewatering consolidation, shorten the construction period, realize the consolidation settlement of the soft soil layer in advance, avoid the generation of negative skin friction on the pile body in the later stage, and at the same time optimize the pile length, reduce the project cost and avoid the risk of reduced bearing capacity of the pile foundation in the later stage. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 It is a plan view of the surcharge preloading of the present invention.

[0031] Figure 2 It is one of the sectional views of the surcharge preloading of the present invention.

[0032] Figure 3 It is another sectional view of the surcharge preloading of the present invention.

[0033] Figure 4 It is a sectional view of the ordinary sand drain of the present invention.

[0034] Figure 5 It is a sectional view of the dewatering sand drain of the present invention.

[0035] Figure 6 It is a sectional view of the dewatering well of the present invention.

[0036] Figure 7 It is a plan view of the deformation monitoring of the preloaded foundation of the present invention.

[0037] Figure 8 It is a sectional view of the overall settlement monitoring of the present invention.

[0038] Figure 9This is a schematic cross-sectional view of the layered settlement monitoring of the present invention.

[0039] Figure 10 This is a schematic cross-sectional view of the horizontal displacement monitoring of the present invention.

[0040] Figure 11 This is a schematic cross-sectional view of the pore water pressure monitoring of the present invention.

[0041] Figure 12 This is a schematic plan view of the in-situ test of the present invention. Specific implementation manners

[0042] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative efforts shall fall within the protection scope of the present invention.

[0043] The present invention provides a method for eliminating the negative skin friction of pile foundations in deep soft soil layers as Figures 1 - 12 shown, including the following steps:

[0044] Overloading preloading: Level the site and replace the soil according to the site requirements to the design elevation of the station yard apron. Specifically, remove all surface obstacles on the construction site, excavate the original soil layer, and replace it with fill soil (such as sand, gravel, etc.) that meets the engineering requirements. Considering the natural ground elevation of 5.200 m, the construction site is filled with professional fill to the design apron elevation of 6.780 m; then, determine the overloading fill thickness according to the structure base for overloading preloading. Specifically, continue to fill the soil on the station yard apron to the elevation of 9.830 m for overloading preloading. The preloading time shall not be less than 6 months. The actual time is determined by the construction party through further analyzing the relationship curves of vertical deformation and time, as well as pore water pressure and time for monitoring items such as vertical displacement, layered settlement, and pore water pressure on site. When the settlement change within 2 consecutive weeks is within 0.01 m / d through the relationship curve, it can be determined that the foundation reaches the corresponding degree of consolidation and meets the convergence and unloading conditions; by means of overloading preloading, the normally consolidated soil layer is in a state of overconsolidation, so that the deformation under the service load is greatly reduced, avoiding the risk of reduced bearing capacity of the pile foundation in the later stage;

[0045] Drainage and consolidation: sand wells are arranged in the treatment area within the reservoir, and precipitation wells are arranged on the periphery to form a drainage channel for drainage; the sand wells are specifically composed of ordinary sand wells and precipitation sand wells, both of which have a diameter of 500mm and a well depth that reaches the bottom of the silt aquifer. The ordinary sand wells and precipitation sand wells are arranged in a plum blossom shape, and the well spacing between two adjacent ordinary sand wells and two adjacent precipitation sand wells is 8m, so that the well spacing between adjacent ordinary sand wells and precipitation sand wells is 4m. The well depth of the ordinary sand well is that the well bottom reaches the bottom of the silt aquifer, and the well depth is generally 44m. The ordinary sand well is formed by drilling a hole on the foundation (treatment area within the reservoir) with a drilling equipment and filling the hole with medium and coarse sand (medium sand and coarse sand). The medium and coarse sand in the well can become a drainage channel for water in the silt soil layer, thereby solving the problem that the silt soil has a small permeability coefficient and water is difficult to discharge, which helps to speed up drainage efficiency and shorten construction period.

[0046] In summary, in this embodiment, the silty soil has a high water content and water is not easy to drain out. Under the action of the later structural load, the soft soil layer will produce settlement. The settlement of the stratum will cause the pile body to bear friction resistance around the pile foundation, thereby generating a downward load and causing the bearing capacity of the pile foundation to decrease. The "overload preloading + drainage consolidation" method is adopted. Through overload preloading, the normally consolidated soil layer is put into an over-consolidated state, so that the deformation under the use load is greatly reduced. Combined with precipitation consolidation to accelerate drainage, the consolidation settlement of the soft soil layer can be achieved in advance, avoiding the generation of pile body friction resistance in the later period, shortening the construction period, saving engineering costs, and avoiding the risk of reduced pile foundation bearing capacity in the later period.

[0047] It should be noted that the bottom of the precipitation sand well is an ordinary sand well, and the top is a pipe well composed of PVC filter pipes. The PVC filter pipes are Φ219 / 4 PVC pipes, connected by threaded connections, and covered with a layer of 80-mesh nylon mesh. The ordinary sand wells in the precipitation sand wells are located in the silty soil layer, while the PVC pipe wells in the precipitation sand wells are located at and above the top of the silty soil layer. The depth of the precipitation sand wells is generally 47m (PVC pipe wells are above 25m). The precipitation sand wells and ordinary sand wells together constitute a sand well structure, and together form a drainage system for the silty silty clay layer. The water in the silty soil layer passes through the drainage channel formed by the ordinary sand wells filled with medium-coarse sand, and the water in the silty soil layer is collected into the upper permeable soil layer. At the same time, a submersible pump is installed at the PVC pipe well position of the precipitation sand well to pump out the water collected in the ordinary sand well above it to the PVC pipe well position, so as to achieve the drainage purpose, speed up the drainage efficiency, and help to further shorten the construction period.

[0048] Furthermore, a circle of dewatering wells is arranged around the construction site (the crushed stone ballast bed area outside the warehouse). Non-sand concrete pipe wells are used. Nylon cloth is wound around the outer wall of the pipe. The pipe is filled with 2 - 4 mm filter material. The well spacing is 15 m, the well depth is 23 m, and the well diameter is 600 mm. Determined by the ground elevation of 5.50 m, actually controlled by the bottom elevation of -17.50 m. Submersible pumps are installed in the dewatering wells. The initial pump capacity of the submersible pump is recommended to be 25 m 3 / h, the head ≥ 25 m. The pump capacity is adjusted according to the actual water level in the follow-up, and dewatering is carried out as needed. The advance pumping time is recommended to be no less than 15 d. By setting up dewatering wells around the construction area to pump water, the groundwater level is lowered, forming a closed dewatering area to prevent external water sources (such as rainfall or surface water) from flowing into the construction area, which helps to ensure the dryness in the construction area; and after the dewatering wells complete their usage purposes, the power supply can be cut off, the downhole pumps, cables, and pump pipes can be pulled out and recycled, and then backfilled with graded sand and gravel with a particle size of 0.5 - 3.2 cm, and cement slurry with a water-cement ratio of 0.5 - 0.6 is injected, and the slurry is replenished 1 - 2 times, backfilled to 2.0 m from the wellhead, and the upper 2.0 m is backfilled to the ground by one-time grouting or with C15 plain concrete according to the original site conditions, and the ground is restored; among them, sand wells (ordinary sand wells and dewatering sand wells) can be entrusted to professional construction teams to drive steel pipes using pile driving machines to ensure that the hole forming depth meets the design requirements. After hole forming, medium and coarse sand is poured into the hole to form sand piles. The sand content greater than 0.5 mm should account for more than 50% of the total amount, and the mud content should not be greater than 3%;

[0049] While using dewatering wells for closed dewatering around the crushed stone ballast bed area outside the warehouse, a dewatering well scheme is adopted inside. The dewatering well diameter is 400 mm, the well spacing is 25 m, and the others are the same as the dewatering wells. Dewatering wells are set inside the construction area to further lower the groundwater level through natural drainage or pumping, improve the drainage of the soil body, reduce the pore water pressure, help improve the bearing capacity of the soil body, reduce the settlement risk, and create good conditions for the subsequent structural construction;

[0050] In this embodiment, the construction unit needs to strengthen the construction quality of ordinary sand wells, dewatering sand wells, and dewatering wells. During the construction process, the monitoring equipment needs to be protected from being damaged, and the monitoring data should be collected in a timely manner and fed back to the design party to optimize the design and achieve information-based design and construction. At the same time, the construction of the wellbore needs to meet the following technical requirements:

[0051] When laying out the well positions, in order to avoid pipelines and underground obstacles, the well spacing can be adjusted locally, but the total number of dewatering wells remains unchanged. The well diameter error is ±20 mm; the verticality error ≤ 1%;

[0052] The filter material in the aquifer section should have a certain roundness. The mud content of the filter material should be ≤ 3%, and the particle size should be 2 - 4 mm. It is necessary to avoid the deviation of the filter pipe and the bridging phenomenon of the filter material in the hole caused by too fast or uneven filling speed. After well washing, the filter material should be replenished in time when it sinks. The actual filling volume is required to be not less than 95% of the theoretical calculated value;

[0053] The requirement for well washing is to achieve "clear water and clean sand". Well washing should be carried out immediately after the pipe installation and filling. The time interval from well completion to well washing should not exceed 8 hours. When the well washing effect is not good, high-pressure well washing with an air compressor can be used. The well washing control pressure can be 0.3 - 0.7 MPa. During well washing, from top to bottom, wash once every 5 m;

[0054] Control of the sand content in pumping. To prevent ground settlement caused by the fine particulate matter in the formation being carried out by pumping groundwater, the sand content in the pumped and drained water should meet the requirements that the sand content within half an hour of pumping from the tube well is less than 1 / 10000, and the sand content during the normal operation of the tube well is less than 1 / 50000.

[0055] In addition, in this embodiment, it is also necessary to monitor the overall settlement, layered settlement, lateral displacement, inclinometer, and pore water pressure of the foundation at the preloaded foundation, and conduct in-situ tests on the foundation after overloading, which shall be carried out in accordance with the "Technical Code for Building Foundation Treatment" (JGJ79 - 2012). As Figures 7 - 12 shown, specifically, by setting deformation monitoring points, the unloading time of the overloaded fill can be inferred; among them, the deformation monitoring points include overall settlement monitoring points, layered settlement monitoring points, pore water pressure monitoring points, surface horizontal displacement monitoring points, and inclinometer monitoring points. Through these monitoring points, data of monitoring items such as vertical displacement, layered settlement, and pore water pressure at the construction site can be obtained. Through the analysis of these data, the relationship curves between the vertical deformation of settlement and time and the relationship curve between pore water pressure and time can be obtained. By observing the two relationship curves, when the settlement change is within 0.01 m / d for two consecutive weeks, it can be considered that the convergence and unloading conditions are met;

[0056] As Figure 7 and Figure 8 shown, the settlement monitoring is carried out by setting elevation points in the overloaded preloading area, and using settlement plates to regularly measure the height change of this point and record the monitoring data, which can monitor the overall settlement situation, evaluate the settlement trend and value under the overloaded load of the foundation, and judge the bearing capacity and stability of the foundation;

[0057] As Figure 7 and Figure 9 shown, the layered settlement monitoring is carried out by embedding induction devices. The rod body with an induction ring in the induction device is inserted into a position more than 1 m below the stable bottom layer to monitor the change of interlayer settlement and record the data, which can evaluate the consolidation situation and settlement characteristics of different layers of soil, identify the bearing capacity and settlement distribution of each layer, and help optimize the subsequent fill design;

[0058] like Figure 7 and Figure 10 As shown, horizontal displacement monitoring is to set up inclinometers in areas where lateral displacement may occur, regularly record the data of the inclinometers, monitor the horizontal displacement of the foundation, and evaluate whether the soil will slide or tilt after overloading, so as to ensure the stability of the construction area;

[0059] like Figure 7 and Figure 11 As shown in the figure, pore water pressure monitoring is to set pore water pressure gauges at different depths of the foundation soil, regularly record the changes in pore water pressure through the pressure gauges, analyze the drainage and consolidation conditions of the soil, and help determine the impact of overload on the soil and the effective stress state of the soil.

[0060] The following table shows the detection frequency of each monitoring point

[0061] Monitoring Items Before Trial Pumping When Starting to Pump Water One Month Before Completion of Preloading Before Unloading Vertical Settlement Once Every 5 Days Twice a Day Twice a Day Once Every 5 Days Layered Settlement Once a Day Twice a Day Once a Day Once Every 5 Days Pore Water Pressure Once a Day Once a Day Three Times a Day Once Every 5 Days Inclinometer Tube Once a Day Twice a Day Once a Day Once Every 5 Days Surface Horizontal Displacement Pile Once a Day Twice a Day Once Every 5 Days Once Every 5 Days

[0062] After preloading, the shear strength of the soil layer needs to be determined through in-situ tests, and the soil strength is used to determine whether the soil has reached the required degree of consolidation after preloading. Figure 12 As shown, the in-situ test can be carried out by cross-plate shear test or static penetration test. The inspection depth should not be less than the designed treatment depth, and the test should be carried out 3 to 5 days after unloading.

[0063] In addition, the construction sequence of the pile foundation in the soft soil layer in this embodiment is as follows:

[0064] S1. Based on the natural ground elevation of 5.200m, fill the site to the designed site elevation of 6.780m according to the professional filling requirements of the station. Before filling, it is necessary to remove debris, vegetation and other obstacles on the ground of the construction site to ensure that the ground of the construction site is flat. During the filling process, the filling method should be used;

[0065] S2. Carry out the construction of ordinary sand wells and precipitation wells. Install submersible pumps in the precipitation wells to pump water. The precipitation wells are spaced to form a closed precipitation area, which can prevent external water from flowing into the construction area and keep the construction environment dry, thereby achieving the purpose of pumping water and surrounding precipitation.

[0066] S3. Carry out overload filling to an elevation of 7.330m. By evenly filling the overload soil in layers in the construction area, additional load is applied to promote soil consolidation, reduce subsequent settlement, and help accelerate the discharge of pore water in the soil;

[0067] S4. Construction of dewatering sand wells and pile foundations, and soil compaction to a depth of 2.500m;

[0068] S5. Unload the piled soil. After the dewatering enclosure is completed, conduct construction inspection pits and pile foundations under the integral track bed. After monitoring the soil settlement and the stress of the pile foundations to determine the consolidation of the soil, the piled load can be unloaded layer by layer, the dewatering wells can be backfilled, and the overall quality of the pile foundations and the track bed can be inspected.

[0069] Among them, during the process of surcharge preloading, the requirements of foundation bearing capacity and stability control should be met, and the vertical deformation, horizontal displacement and pore water pressure of the foundation should be monitored. The loading rate of surcharge preloading should meet the requirements that the maximum vertical deformation of the shaft foundation is less than 15 mm / d and the horizontal displacement at the edge of surcharge preloading is less than 5 mm / d, so as to ensure the stability of the construction process.

[0070] Finally, it should be noted that the above are only the preferred embodiments of the present invention and are not used to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions recorded in the foregoing embodiments, or perform equivalent replacements on some of the technical features. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for eliminating the negative skin friction of pile foundations in deep soft soil layers, characterized in that, It includes the following steps: Clear the site surface and backfill it to the designed elevation of the station yard apron according to the site requirements; Determine the thickness of surcharge filling based on the structure base and conduct surcharge preloading; Arrange sand drains in the in - store treatment area and precipitation wells on the periphery to jointly form a drainage channel for drainage; Set deformation monitoring points to infer the unloading time of surcharge filling; After preloading treatment, obtain the shear strength of the soil layer through in - situ tests, and determine whether the soil reaches the required degree of consolidation based on the soil strength; 2. A method for eliminating the negative skin friction of pile foundations in deep soft soil layers according to claim 1, characterized in that, The sand drains arranged within the site treatment range include ordinary sand drains and precipitation sand drains. The ordinary sand drains and precipitation sand drains are both arranged in a plum - blossom pattern, and the spacing between adjacent ordinary sand drains and precipitation sand drains is 3 - 5m.

3. A method for eliminating the negative skin friction of pile foundations in deep soft soil layers according to claim 2, characterized in that, The ordinary sand drain is formed by punching holes within the treatment range with a punching device and filling medium - coarse sand into the holes, enabling the medium - coarse sand in the well to become a drainage channel for water in the silt - clay layer. The depth of the ordinary sand drain is such that the bottom of the well reaches the bottom of the silt - clay aquifer.

4. A method for eliminating the negative skin friction of pile foundations in deep soft soil layers according to claim 3, characterized in that, The well diameters of both the precipitation sand drain and the ordinary sand drain are 400 - 600mm. The precipitation sand drain consists of an ordinary sand drain and a PVC filter pipe well arranged at the top of the ordinary sand drain. The PVC filter pipe well in the precipitation sand drain is located at the top of the silt - clay layer and above, and the ordinary sand drain in the precipitation sand drain is located within the silt - clay layer.

5. A method for eliminating the negative skin friction of pile foundations in deep soft soil layers, characterized in that, Submersible pumps are arranged in both the precipitation sand drain and the precipitation well. The water in the silt - clay layer is collected into the upper soil layer with good permeability through the drainage channel formed within the ordinary sand drain. The PVC pipe well part in the precipitation sand drain pumps out the water in the soil layer through the submersible pump, and at the same time, the surrounding precipitation wells pump water synchronously through the submersible pump to achieve the purpose of drainage.

6. A method for eliminating the negative skin friction of pile foundations in deep soft soil layers according to claim 5, characterized in that, The outer wall of the PVC filter pipe in the precipitation sand drain is wrapped with nylon mesh. The precipitation well is a non - sand concrete pipe, and the outer wall of the pipe is wound with nylon cloth, and the outside of the pipe is filled with 2 - 4mm filter material.

7. A method for eliminating the negative skin friction of pile foundations in deep soft soil layers according to claim 2, characterized in that, The deformation monitoring points include overall settlement monitoring points, layer - by - layer settlement monitoring points, pore water pressure monitoring points, surface horizontal displacement monitoring points, and inclinometer monitoring points. The deformation data monitored by each monitoring point of the deformation monitoring points can obtain the relationship curve between settlement vertical deformation and time and the relationship curve between pore water pressure and time, and infer the unloading time of surcharge filling based on the relationship curve.

8. A method for eliminating negative skin friction of pile foundations in deep soft soil layers, characterized in that, The specific layout plan of the precipitation well is as follows: Outside the gravel ballast area outside the storage, precipitation wells are used for closed surrounding dewatering, and inside, dewatering wells are used; The well diameter of the precipitation well is 600mm, and the well diameter of the dewatering well is 400mm. The well depths of both are 23m, which are determined based on the ground elevation of 5.50m, and actually controlled by the bottom elevation of - 17.50m. Among them, the spacing of the precipitation wells is 15m, and the spacing of the dewatering wells is 25m; The precipitation well is backfilled with graded sand and gravel with a particle size of 0.5 - 3.2cm, and cement slurry with a water - cement ratio of 0.5 - 0.6 is injected, and the slurry is replenished 1 - 2 times, backfilled to 2.0m above the wellhead. For the upper 2.0m, depending on the original site conditions, it is either grouted to the ground at one time or backfilled with C15 plain concrete, and the ground is restored.

9. A method for eliminating the negative skin friction of pile foundations in thick soft soil layers, characterized in that, The specific steps for the pile foundation construction in the soft soil layer are as follows: S1. Considering the natural ground elevation of 5.200m, fill it to the designed yard apron elevation of 6.780m according to the filling requirements of the station yard specialty; S2. Construct ordinary sand drains and dewatering wells. After the construction is completed, carry out pumping and surrounding dewatering; S3. Carry out overloading filling to an elevation of 7.330 m; S4. Construct dewatering sand drains and pile foundations of the pile caps, and carry out surcharge loading with soil to 2.500 m; S5. Unload the surcharge soil, end the surrounding dewatering, and construct inspection pits and pile foundations under the integral track bed.

10. A method for eliminating negative skin friction of pile foundations in deep soft soil layers, characterized in that, The loading rate of the surcharge preloading shall meet the requirements that the maximum vertical deformation of the shaft foundation is less than 15 mm / d and the horizontal displacement at the edge of the surcharge preloading is less than 5 mm / d.

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