Self-decompression dry bottom sealing structure suitable for water-rich soft clay foundation pit and design method

Abstract

The invention relates to the technical field of building construction, and particularly discloses a self-decompression dry bottom sealing structure suitable for a water-rich soft clay foundation pit and a design method. The self-decompression dry bottom sealing structure comprises a water seepage assembly arranged in the foundation pit and located at the bottom of the foundation pit, and a plurality of water collecting wells arranged at the corners of the foundation pit and communicating with the water seepage assembly. The water seepage assembly comprises an isolation layer, a lower hydrophobic layer, a water permeable layer, an upper hydrophobic layer and a cushion layer which are sequentially arranged from bottom to top; and the permeable layer is connected with the water collecting well. Osmosis pressure can be effectively released, the anti-floating requirement of bottom sealing concrete is eliminated or reduced, the bottom sealing thickness can be greatly reduced or even replaced by a leveling thin cushion layer, meanwhile, the excavation depth of a foundation pit is reduced, and the overall stability of the foundation pit is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of building construction, and more particularly to a self-decompression dry bottom sealing structure suitable for a water-rich soft clay foundation pit and a design method thereof. Background Art

[0002] In the existing technology, there are two major difficulties in the design and construction of foundation pits in water-rich soft clay:

[0003] First, the soil is soft, with poor shear parameters, poor anti-uplift and anti-deformation capabilities; second, the water content is high and the groundwater level is high. Although it is clay soil, it is difficult to dry-seal the bottom without taking effective measures and the anti-floating requirements are high. The main reasons are:

[0004] (1) Soft clay has a high water content, even reaching saturation. For example, the silty clay in layers (3)1 and (4) of Shanghai has a water content of more than 40%, and water will accumulate when excavated.

[0005] (2) Although the permeability of clay soil is weak, hydraulic connection and pressure difference still exist, which slowly penetrates, and the bottom seal is still resistant to floating. Moreover, the bottom seal is mostly plain concrete, which has poor bending resistance and high thickness requirements;

[0006] (3) Some soft clays are partially intercalated with thin layers of silt sand, and the comprehensive permeability coefficient is greater than 10- 5 cm / s, excavation also seeps water;

[0007] (4) Water-rich soft clay is often under-consolidated. Active precipitation over a large area is not only ineffective, but also easily leads to surrounding subsidence, endangering existing buildings and facilities.

[0008] In response to the above unfavorable conditions, traditional groundwater treatment methods have their own advantages and disadvantages. For example, water-stop curtains and high-pressure rotary jet grouting are expensive and take a long time to build; open drainage by pumping has poor safety and cannot be operated after the bottom is sealed; and wellpoint dewatering is not suitable for water-rich soft clay. Summary of the invention

[0009] The technical problem to be solved by the present invention is to provide a self-decompression dry bottom sealing structure and design method suitable for water-rich soft clay foundation pits, which can effectively release the osmotic pressure, cancel or reduce the anti-floating requirements of the bottom sealing concrete, and the bottom sealing thickness can be greatly reduced or even replaced with a thin leveling cushion layer. At the same time, the excavation depth of the foundation pit is reduced, and the overall stability of the foundation pit is improved:

[0010] The solution adopted by the present invention to solve the technical problem is:

[0011] A self-decompression dry bottom sealing structure suitable for a water-rich soft clay foundation pit, comprising a water seepage component arranged in the foundation pit and located at the bottom of the foundation pit, and a plurality of water collection wells arranged at the corners of the foundation pit and connected to the water seepage component;

[0012] The water seepage component comprises an isolation layer, a lower hydrophobic layer, a permeable layer, an upper hydrophobic layer and a cushion layer which are arranged in sequence from bottom to top; the permeable layer is connected to a water collection well.

[0013] In some possible implementations, the water-permeable layer includes a plurality of groups of seepage pipes installed on the lower hydrophobic layer and connected to the water collection well, and a geotextile wrapped around the outside of the seepage pipes;

[0014] Each group of the seepage pipes is provided with a plurality of groups of seepage holes communicated with the interior thereof.

[0015] In some possible embodiments, the seepage pipe includes a seepage pipe 1 for connecting two adjacent groups of water collection wells to form a ring structure, and a seepage pipe 2 located in the ring structure and connected to the seepage pipe 1 and having the same structure as the seepage pipe 1; the seepage pipe 1 and the seepage pipe 2 are PVC pipes and are both provided with seepage holes.

[0016] In some possible implementations, the second water seepage pipe is a single-layer structure, and all the second water seepage pipes of the single-layer structure are arranged in parallel.

[0017] In some possible implementations, the seepage pipe 2 is a double-layer structure, and all the seepage pipes 2 in each layer are arranged in parallel; the seepage pipes 2 in the double-layer structure are arranged in a cross shape.

[0018] In some possible implementations, the porosity of the seepage holes is 2%-3% and arranged in a plum blossom shape; the density of the anti-seepage geotextile is not less than 400g / m 2 ; The isolation layer is made of bamboo fences; the cushion layer is made of concrete and has a thickness of 10-20cm.

[0019] In some possible implementations, the lower hydrophobic layer is made of crushed stone 1, the crushed stone 1 is 16-25 mm, the thickness of the lower hydrophobic layer is a, the outer diameter of the seepage pipe is b, and a=1.75-2.5b;

[0020] The upper hydrophobic layer is made of crushed stone 2, the crushed stone 2 is 10-16 mm, the thickness of the lower hydrophobic layer and the permeable layer is c, the outer diameter of the seepage pipe is b, c≥2b.

[0021] A design method for a self-decompression dry bottom sealing structure suitable for a water-rich soft clay foundation pit according to the above is characterized in that it specifically comprises the following steps:

[0022] Step S1: calculation of foundation pit seepage volume;

[0023] Step S2: according to the obtained foundation pit seepage volume, the minimum inner diameter of the seepage pipe is obtained and the seepage pipe is selected;

[0024] Step S3: According to the calculation of foundation pit seepage volume and the selection of seepage pipes, numerical analysis software is used to build a model, calculate the seepage volume value, set the pressure head distribution after self-decompression bottom sealing, and calculate the thickness of the cushion layer after self-decompression.

[0025] In some possible implementations, the step S1 specifically refers to calculating the seepage volume of the foundation pit according to Darcy's law:

[0026] Q=kAi (1);

[0027] i=Δh / L (2);

[0028]

[0029] Where:

[0030] Q is the amount of water seepage in the foundation pit;

[0031] K is the equivalent permeability coefficient of the bottom soil layer of the pit;

[0032] A is the area of ​​the pit bottom excavation surface;

[0033] i is the infiltration hydraulic gradient;

[0034] Δh is the total seepage head difference;

[0035] L is the seepage path, the distance from the excavation surface to the top surface of the micro-permeable layer or impermeable layer below the bottom of the foundation pit;

[0036] β is the equivalent permeability adjustment coefficient;

[0037] H i is the thickness of the i-th soil layer;

[0038] k i is the permeability coefficient of the i-th soil layer.

[0039] In some possible implementations, the step S2 specifically refers to: substituting the solved foundation pit seepage volume Q into formula (4) to solve the minimum inner diameter of the seepage pipe;

[0040]

[0041] Where:

[0042] D is the inner diameter of the seepage pipe;

[0043] γ is the coefficient of variation;

[0044] Q is the water seepage at the bottom of the foundation pit;

[0045] T is the drainage cycle of the water collection well;

[0046] δ is the weight coefficient of the flow rate per meter in the seepage pipe;

[0047] μ is the functional coefficient of the seepage pipe.

[0048] Compared with the prior art, the present invention has the following beneficial effects:

[0049] The present invention can effectively release the osmotic pressure, cancel or reduce the anti-floating requirements of the bottom seal concrete, and the bottom seal thickness can be greatly reduced or even replaced by a thin leveling cushion layer. At the same time, the excavation depth of the foundation pit is reduced, and the overall stability of the foundation pit is improved.

[0050] Compared with the prior art, the invention uses simple materials, is easy to obtain on site, has low cost and high construction efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0051] Figure 1 It is a structural schematic diagram of the self-decompression dry bottom sealing structure, foundation pit and pile foundation in the present invention;

[0052] Figure 2 It is a schematic diagram of the first arrangement of the seepage pipe, the water collection well and the pile foundation in the present invention;

[0053] Figure 3 It is a schematic diagram of the second arrangement of the seepage pipe, the water collection well and the pile foundation in the present invention;

[0054] Figure 4 It is a schematic diagram of the third arrangement of the seepage pipe, the water collection well and the pile foundation in the present invention;

[0055] Figure 5 This is a comparison table of pressure heads in Schemes 1 to 6 in Example 1 of the present invention;

[0056] Among them: 1. Isolation layer; 2. Lower hydrophobic layer; 3. Seepage component; 31. Seepage pipe one; 32. Seepage pipe two; 4. Upper hydrophobic layer; 5. Cushion layer; 6. Collection well; 10. Foundation pit; 20. Pile foundation. DETAILED DESCRIPTION

[0057] In the present invention, unless otherwise clearly specified and limited, the terms "installed", "connected", "connected", "fixed" and the like should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral body; it can be directly connected, or indirectly connected through an intermediate medium, it can be the internal connection of two elements or the interaction relationship between two elements. The "first", "second" and similar words mentioned in this application do not indicate any order, quantity or importance, but are only used to distinguish different components. Similarly, "one" or "one" and other similar words do not indicate a quantity restriction, but indicate the existence of at least one. In the implementation of this application, "and / or" describes the association relationship of the associated objects, indicating that there can be three relationships, for example, A and / or B can represent: A exists alone, A and B exist at the same time, and B exists alone. In the description of the embodiments of the present application, unless otherwise specified, the meaning of "multiple" refers to two or more. For example, multiple positioning columns refer to two or more positioning columns. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0058] The present invention is described in detail below.

[0059] like Figure 1-Figure 4 As shown:

[0060] A self-decompression dry bottom sealing structure suitable for a water-rich soft clay foundation pit 10, comprising a water seepage component 3 arranged in the foundation pit 10 and located at the bottom of the foundation pit 10, and a plurality of water collection wells 6 arranged outside the foundation pit 10 and connected to the water seepage component 3;

[0061] The water seepage component 3 includes an isolation layer 1, a lower hydrophobic layer 2, a permeable layer, an upper hydrophobic layer 4 and a cushion layer 5 which are arranged in sequence from bottom to top; the permeable layer is connected to a water collection well 6.

[0062] Specifically, the isolation layer 1 is arranged at the bottom of the foundation pit 10, which can, on the one hand, isolate the water-rich soft soil and the hydrophobic layer on the excavation surface of the foundation pit 10, and on the other hand, utilize its strong structural rigidity to improve the vertical bearing capacity of the bottom of the foundation pit 10;

[0063] The upper hydrophobic layer 4 and the lower hydrophobic layer 2 are both used for drainage and hydrophobicity;

[0064] The permeable layer is used to guide the seepage water to the water collection well 6 and then drain it openly;

[0065] The cushion layer 5 is used for trimming and leveling after self-decompression, and serves as the base layer for subsequent foundation construction, and is not resistant to floating;

[0066] The present invention releases the osmotic pressure by itself through the seepage component 3 arranged at the bottom of the foundation pit 10, and drains the seepage water to the water collection wells 6 around the foundation pit 10 for open discharge, thereby eliminating or reducing the pressure head after the bottom of the foundation pit 10 is sealed, and realizing dry bottom sealing.

[0067] In some possible implementations, the water-permeable layer includes a plurality of groups of seepage pipes installed on the lower hydrophobic layer 2 and connected to the water collection well 6, and a geotextile wrapped around the outside of the seepage pipes;

[0068] Each group of the seepage pipes is provided with a plurality of groups of seepage holes communicating with the inside thereof;

[0069] Further, the seepage pipe includes a seepage pipe 1 31 for connecting two adjacent groups of water collection wells 6 and forming an annular structure, and a seepage pipe 2 32 located in the annular structure and connected to the seepage pipe 1 31 and having the same structure as the seepage pipe 1 31; the seepage pipe 1 31 and the seepage pipe 2 32 are PVC pipes and are both provided with seepage holes;

[0070] The seepage pipe 1 31 will be located outside the bottom of the foundation pit 10, and the seepage pipe 2 32 will not be installed directly above the pile foundation 20 and will be staggered with the pile foundation 20; both ends of each group of seepage pipes 2 32 are connected to the seepage pipe 1 31 respectively, so that the seepage water in the bottom of the foundation pit 10 and around the seepage pipe 1 31 can enter the seepage pipe 1 31 and the seepage pipe 2 32 through the seepage holes and be discharged through the water collection well 6;

[0071] Further, such as Figure 2 , Figure 3 As shown, for the arrangement of the second seepage pipe 32, a single layer can be adopted, wherein all the second seepage pipes 32 on the single layer are arranged in parallel;

[0072] Of course Figure 4 As shown, a double-layer cross-shaped layout can also be adopted, where all the second seepage pipes 32 on each layer are arranged in parallel, and both ends of each group of second seepage pipes 32 are respectively connected to the first seepage pipe 31.

[0073] In some possible implementations, in order to allow the seepage water to enter the seepage pipe through the seepage holes, the opening rate of the seepage holes is 2%-3% and arranged in a plum blossom shape; the density of the anti-seepage geotextile is not less than 400g / m 2 .

[0074] In some possible implementations, the isolation layer 1 is made of bamboo fence sheets.

[0075] The lower hydrophobic layer 2 is made of crushed stone 1, the crushed stone 1 is 16-25 mm, the thickness of the lower hydrophobic layer 2 is a, the outer diameter of the seepage pipe is b, a=1.75-2.5b; preferably, a=2b.

[0076] In some possible implementations, the upper hydrophobic layer 4 is made of crushed stone 2, the crushed stone 2 is 10-16 mm, the thickness of the lower hydrophobic layer 2 and the permeable layer is c, the outer diameter of the seepage pipe is b, and c≥2b.

[0077] With the above arrangement, the particle size of the gravel in the lower hydrophobic layer 2 is larger than that of the gravel in the upper hydrophobic layer 4, which facilitates the flow of seepage water.

[0078] In some possible implementations, the cushion layer 5 is made of concrete with a thickness of 10-20 cm. The concrete is plain concrete with a strength of C30.

[0079] When carrying out the above-mentioned construction method, the following construction steps are specifically included:

[0080] Excavation of foundation pit 10;

[0081] After excavation and leveling, the accumulated water is pumped out and an isolation layer 1 formed by bamboo rakes is laid, which is used to drain water on the one hand and to make up for the extremely low bearing capacity of water-rich soft clay on the other hand by using its certain bending stiffness;

[0082] Lay the lower hydrophobic layer, pre-bury the seepage pipe wrapped with geotextile, and connect the seepage pipe with the water collection well 6;

[0083] The upper hydrophobic layer is paved, the cushion layer 5 is constructed, and the self-decompression dry bottom sealing structure is completed; then the foundation construction can be carried out.

[0084] Embodiment 1:

[0085] The bottom sealing construction is carried out in the foundation pit 10 with a plane size of 10.7m×7.6m and an excavation depth of 9m. Among them, the main supporting components are Larsen-IV steel sheet piles, 2HM588×300mm purlins and supports; the upper surface contains fluid plastic-soft plastic (3)1, (4) layers of silty silty clay, which has the characteristics of high compressibility, high sensitivity, low strength, etc., which is the main special rock and soil in this area and has poor engineering properties. The upper groundwater in the bridge site area is mainly Quaternary pore water. During the survey, the stable groundwater level was measured to be 0-2.2m deep; the groundwater is mainly replenished by atmospheric rainfall and rivers, and discharged by leaking into surface water bodies, evaporation, artificial mining, etc. The water level changes slightly with the alternation of seasons; when calculating the foundation pit 10, the groundwater depth is considered to be 0.5m; the design method of the present invention is used for design, including the following steps:

[0086] (1) The seepage volume of foundation pit 10 is calculated as follows:

[0087] Q=kAi;

[0088] i = Δh / L;

[0089]

[0090] Where:

[0091] Q is the water seepage at the bottom of foundation pit 10 (m 3 / day);

[0092] K is the equivalent permeability coefficient of the bottom soil layer of the pit (cm / sec);

[0093] A is the area of ​​the pit bottom excavation surface (m 2 );

[0094] i is the infiltration hydraulic gradient;

[0095] Δh is the total seepage head difference (m);

[0096] L is the seepage path (m), the distance from the excavation surface to the top surface of the micro-permeable layer or impermeable layer below the bottom of the foundation pit 10;

[0097] β is the equivalent permeability adjustment coefficient, which is set to 1.0 by default and can be tested or corrected by numerical analysis;

[0098] Hi is the thickness of the i-th soil layer (m);

[0099] ki is the permeability coefficient of the i-th layer of soil (cm / sec);

[0100] The seepage volume is calculated, see the theoretical seepage volume calculation table of foundation pit 10 excavation surface:

[0101]

[0102] Theoretical seepage calculation table of foundation pit 10 excavation surface

[0103] (2) Seepage pipe selection:

[0104] The minimum inner diameter of the seepage pipe in the water-rich soft clay foundation pit 10 is calculated as follows:

[0105]

[0106] Where:

[0107] D is the inner diameter of the seepage pipe (mm);

[0108] γ is the coefficient of variation, which is 1.3 by default;

[0109] Q is the water seepage at the bottom of the foundation pit 10 (m 3 / day);

[0110] T is the drainage cycle of the water collection well (hour);

[0111] δ is the weight coefficient of the flow rate of the seepage pipe, 1 / S pipe, S pipe is the equivalent total length of the seepage pipe. When the inner diameters of the seepage pipes are the same, S pipe is the total length; when the inner diameters of the seepage pipes are the same, the calculation is weighted by area;

[0112] μ is the seepage pipe function coefficient, which takes into account the function reduction caused by the impurities inside the seepage pipe itself and the failure of local anti-filtration;

[0113] According to the above foundation pit 10 seepage volume Q, six selection schemes for seepage pipe layout are as follows:

[0114] Option 1: No self-decompression drainage is arranged;

[0115] Solution 2: Arrange water collection wells 6 at the corners around the foundation pit 10;

[0116] Solution 3: water collection wells 6 are arranged at the four corners, and a water seepage pipe 31 connected to the water collection wells 6 to form a ring structure;

[0117] Solution 4: water collection wells 6 are arranged at the four corners, a seepage pipe 1 31 connected to the water collection well 6 to form a ring structure, and a seepage pipe 2 32 is arranged longitudinally;

[0118] Solution 5: water collection wells 6 are arranged at the four corners, a seepage pipe 1 31 connected to the water collection well 6 to form a ring structure, and a seepage pipe 2 32 is arranged horizontally;

[0119] Solution 6: water collection wells 6 are arranged at the four corners, a seepage pipe 1 31 connected to the water collection well 6 to form a ring structure, a seepage pipe 2 32 is arranged longitudinally, and a seepage pipe 32 is arranged transversely;

[0120] According to the above formula, the inner diameter D of the seepage pipe of Scheme 3 to Scheme 6 (Scheme 1 and Scheme 2 are not included in the calculation because no seepage pipe is arranged), see the calculation table for the selection of seepage pipe for foundation pit 10:

[0121]

[0122] Calculation table for selection of seepage pipes for foundation pit 10

[0123] Specifically, the more uniform the layout of the seepage pipes is, the lower the inner diameter requirement is, and the cheaper the on-site layout and burial are; and the layout of the seepage pipes has a key influence on the pressure head distribution and the stress on the bottom seal after the bottom seal.

[0124] (3) Calculation of pressure head after self-decompression bottom sealing:

[0125] Since the self-decompression seepage pipes are arranged discontinuously, the pressure head height at the bottom of the pit will be distributed in a continuous manner, which is determined by finite element analysis and calculation;

[0126] The soil layer around foundation pit 10 is simulated by solid unit, the steel sheet pile is simulated by plate unit, the surrounding purlin and inner support beam unit are simulated, and the interface unit is used to simulate the water-stop curtain effect of the steel sheet pile; the soil layer parameters of foundation pit 10 are shown in the soil layer parameter table of foundation pit 10:

[0127] The soil layer adopts the modified Mohr-Coulomb constitutive method; the total seepage head boundary is added to the sides of the model, and the total seepage head boundary of the excavation surface is set according to the excavation depth;

[0128]

[0129] Foundation pit 10 soil layer parameter table

[0130] After calculation, the excavation face flow and pressure head of Scheme 1 to Scheme 6 are obtained respectively, see the flow calculation table of Scheme 1 to Scheme 6 of Foundation Pit 10:

[0131]

[0132] Flow calculation table for scheme 1 to scheme 6

[0133] It can be seen from this that the combination Figure 5 As shown, arranging only four groups of water collection wells 6 cannot reduce pressure and drain water. At least a ring of seepage pipes must be arranged around them. That is, Schemes 2 to 6 can basically achieve drainage and pressure reduction, and converge from Scheme 4 to basically reach the envisioned state.

[0134] When the pressure-reducing seepage pipe is not installed in Scheme 1, the excavation surface at the bottom of foundation pit 10 has a seepage pressure of 8.5m after the bottom is sealed;

[0135] When only the water collection wells 6 are arranged in the four corners of the scheme 2, the seepage pressure still exists and is 5.6m, and the anti-floating thickness of the bottom cover is still relatively high;

[0136] When the seepage pipes were arranged in a ring around the scheme 3, the seepage pressure was greatly improved, and only 1.2m remained;

[0137] Convergence is achieved from Scheme 4, with essentially no pressure, and the bottom cover can be optimized to leveling cushion 5.

[0138] In summary, Scheme 6 is more applicable in terms of the selection of seepage pipe diameter, pressure reduction and drainage effect, and pressure head of the pit bottom excavation surface.

[0139] Based on Scheme 6, on-site construction was carried out and flow monitoring was carried out after the construction was completed; the monitored seepage volume was stable at 1.39m 3 / day, which is 16% less than the theoretical value. The main reason is the comprehensive influence of stratum uniformity, geological survey parameters and the function of water seepage channel. According to the on-site survey, before the foundation is poured, after the self-decompression bottom sealing structure of the present invention, the concrete surface of the cushion layer 5 is dry without cracks and water leakage, indicating that the expected goal of self-decompression dry bottom sealing is finally achieved.

[0140] The present invention is not limited to the above-mentioned specific embodiments, but extends to any new features or any new combination disclosed in this specification, as well as any new method or process steps or any new combination disclosed.

Claims

1. A self-decompression dry bottom sealing structure suitable for water-rich soft clay foundation pit, characterized in that: It includes a water seepage component arranged in the foundation pit and located at the bottom of the foundation pit, and a plurality of water collection wells arranged at the corners of the foundation pit and connected to the water seepage component; The water seepage component comprises an isolation layer, a lower hydrophobic layer, a permeable layer, an upper hydrophobic layer and a cushion layer which are arranged in sequence from bottom to top; the permeable layer is connected to a water collection well.

2. The self-decompression dry bottom sealing structure suitable for water-rich soft clay foundation pit according to claim 1 is characterized in that: The permeable layer includes a plurality of groups of seepage pipes installed on the lower hydrophobic layer and connected to the water collection well, and a geotextile wrapped around the outside of the seepage pipes; Each group of the seepage pipes is provided with a plurality of groups of seepage holes communicated with the interior thereof.

3. The self-decompression dry bottom sealing structure suitable for water-rich soft clay foundation pit according to claim 2 is characterized in that: The seepage pipe includes a seepage pipe 1 for connecting two adjacent groups of water collection wells to form a ring structure, and a seepage pipe 2 located in the ring structure and connected to the seepage pipe 1 and having the same structure as the seepage pipe 1; the seepage pipe 1 and the seepage pipe 2 are PVC pipes and are both provided with seepage holes.

4. The self-decompression dry bottom sealing structure suitable for water-rich soft clay foundation pit according to claim 3 is characterized in that: The second seepage pipe is a single-layer structure, and all the second seepage pipes of the single-layer structure are arranged in parallel.

5. The self-decompression dry bottom sealing structure suitable for water-rich soft clay foundation pit according to claim 3, characterized in that: The seepage pipes 2 are of a double-layer structure, and all the seepage pipes 2 in each layer are arranged in parallel; the seepage pipes 2 in the double-layer structure are arranged in a cross shape.

6. The self-decompression dry bottom sealing structure suitable for water-rich soft clay foundation pit according to claim 2, characterized in that: The opening rate of the seepage holes is 2%-3% and arranged in a plum blossom shape; the density of the anti-seepage geotextile is not less than 400g / m 2 The isolation layer is made of bamboo fences; the cushion layer is made of concrete with a thickness of 10-20cm.

7. The self-decompression dry bottom sealing structure suitable for water-rich soft clay foundation pit according to claim 2, characterized in that: The lower hydrophobic layer is made of crushed stone 1, the crushed stone 1 is 16-25 mm, the thickness of the lower hydrophobic layer is a, the outer diameter of the seepage pipe is b, and a=1.75-2.5b; The upper hydrophobic layer is made of crushed stone 2, the crushed stone 2 is 10-16 mm, the thickness of the lower hydrophobic layer and the permeable layer is c, the outer diameter of the seepage pipe is b, c≥2b.

8. A design method for a self-decompression dry bottom sealing structure suitable for a water-rich soft clay foundation pit according to any one of claims 1 to 7, characterized in that: The specific steps include: Step S1: calculation of foundation pit seepage volume; Step S2: according to the obtained foundation pit seepage volume, the minimum inner diameter of the seepage pipe is obtained and the seepage pipe is selected; Step S3: According to the calculation of foundation pit seepage volume and the selection of seepage pipes, numerical analysis software is used to build a model, calculate the seepage volume value, set the pressure head distribution after self-decompression bottom sealing, and calculate the thickness of the cushion layer after self-decompression.

9. The self-decompression dry bottom sealing structure and design method suitable for water-rich soft clay foundation pit according to claim 8, characterized in that: The step S1 specifically refers to calculating the seepage volume of the foundation pit according to Darcy's law: Q=kAi (1); i=Δh / L (2); Where: Q is the amount of water seepage in the foundation pit; K is the equivalent permeability coefficient of the bottom soil layer of the pit; A is the area of ​​the pit bottom excavation surface; i is the infiltration hydraulic gradient; Δh is the total seepage head difference; L is the seepage path, the distance from the excavation surface to the top surface of the micro-permeable layer or impermeable layer below the bottom of the foundation pit; β is the equivalent permeability adjustment coefficient; H i is the thickness of the i-th soil layer; k i is the permeability coefficient of the i-th soil layer.

10. A self-decompression dry bottom sealing structure and design method suitable for water-rich soft clay foundation pit according to claim 9, characterized in that: The step S2 specifically refers to: substituting the solved foundation pit seepage volume Q into formula (4) to solve the minimum inner diameter of the seepage pipe; Where: D is the inner diameter of the seepage pipe; γ is the coefficient of variation; Q is the water seepage at the bottom of the foundation pit; T is the drainage cycle of the water collection well; δ is the weight coefficient of the flow rate per meter in the seepage pipe; μ is the functional coefficient of the seepage pipe.

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