Deep foundation pit supporting system in soft clay / silt layer environment, and construction method

Abstract

Disclosed in the invention are a deep foundation pit supporting system in a soft clay / silt layer environment, and a construction method, comprising the following steps: construction preparation; construction scheme design and feasibility evaluation: formulating a construction scheme, and evaluating whether the stability of the deep foundation pit supporting system meets requirements; support and reinforcement design and construction for soft clay / silt: designing a structure for supporting and reinforcing soft clay / silt, and constructing a steel sheet pile retaining wall, an out-pit cement mixed pile retaining wall, and an in-pit cement mixed pile retaining wall; earth excavation and internal support construction in a foundation pit: carrying out earth excavation and using internal steel pipe supports for supporting; and in-pit construction and supporting system conversion: pouring bottom-sealing concrete at the bottom of the deep foundation pit, pouring a bearing platform on the bottom-sealing concrete, and removing the internal steel pipe supports during the pouring process. The method of the present invention can effectively enhance the stiffness of the deep foundation pit supporting system, prevent groundwater infiltration, and increase the weight of a foundation pit bottom-sealing member so as to prevent heave and water inrush of a soft clay / silt body at the bottom of the foundation pit.

Description

Deep foundation pit support system and construction method in silty and soft soil environment Technical Field

[0001] This invention relates to the field of bridge abutment foundation pit construction technology, and in particular, to a deep foundation pit support system and construction method in a silty soft soil environment. Background Technology

[0002] In the construction of underground structure foundation pits, to ensure the safety of the underground structure construction and the surrounding environment, foundation pit support refers to the measures taken to support, reinforce, and protect the sidewalls of the foundation pit and the surrounding environment, ensuring the smooth implementation of foundation pit excavation and main structure construction. Common foundation pit support methods include: pile support, pile bracing, pile anchors, pile cantilever, diaphragm wall support, diaphragm wall + bracing, cement retaining wall, soil nailing wall (sprayed anchor support), inverted arch wall, undisturbed soil slope, pile and wall with bracing system, simple horizontal bracing, and reinforced concrete piles, etc.

[0003] In a bridge project in the Guangdong Delta Plain, the No. 7 main pier foundation traverses layers of silty clay, medium sand, silty clay, silty sand, and silty clay, and sits atop a layer of silty clay. The natural moisture content of the soil layers at the construction site of the No. 7 main pier foundation is 28.7–49.5%, water saturation is 88.3–99.0%, void ratio is 0.790–1.319, bearing capacity is 50–220 kPa, cohesion under direct shear is 3.8–14.8 kPa, and internal friction angle is 2.3°–13.7°. The soil exhibits high moisture content, large void ratio, low bearing capacity, and low shear strength, with poor physical and mechanical properties and geological conditions. This results in weak lateral support for the cofferdam of the No. 7 main pier foundation pit, making it highly susceptible to slippage, collapse, and instability. Therefore, unconventional foundation pit support methods are required.

[0004] Given that the No. 7 main pier foundation of the bridge is located approximately 3 meters away from the Meixi River, a secondary water source, and the foundation pit is a 7.5-meter deep pit excavated directly above water, in order to prevent the foundation pit from easily slipping, collapsing, and becoming unstable due to the low strength and weak lateral support of the silt and soft soil layer after excavation, and to prevent the silt and soft soil layer at the bottom of the foundation pit from heaving and water inrush accidents.

[0005] Therefore, in the construction of the No. 7 main pier foundation pit, how to develop a deep foundation pit support system with high rigidity, high anti-heave safety factor of the silt and soft soil layer at the bottom of the foundation pit, and good water-stopping performance of the silt and soft soil layer is an urgent problem to be solved for the safe construction of the No. 7 main pier foundation pit of this bridge. Summary of the Invention

[0006] This invention provides a deep foundation pit support system and construction method in a silty soft soil layer environment to solve the technical problems of low stiffness of the deep foundation pit support system, low heave resistance safety factor of the silty soft soil at the bottom of the foundation pit, and poor water-stopping performance of the silty soft soil layer in a silty soft soil layer environment.

[0007] The technical solution adopted in this invention is as follows:

[0008] A deep foundation pit support system and construction method in a silty soft soil environment includes the following steps: Construction preparation: Obtaining the stratum distribution at the site of the foundation to be constructed and constructing cast-in-place piles at the base of the foundation; Construction scheme design and feasibility assessment: Developing a construction scheme and using soil mechanics formulas and computer software for modeling, analysis, and verification to assess whether the stability of the deep foundation pit support system meets the specifications; Silt and soft soil support and reinforcement design and construction: Designing and constructing the structures in the deep foundation pit support system used to support and reinforce the silty soft soil, and constructing the structures extending along the perimeter of the deep foundation pit. The steel sheet pile retaining wall is used to support the silt and soft soil. An external cement mixing pile retaining wall is constructed close to the outer perimeter of the steel sheet pile retaining wall, and an internal cement mixing pile retaining wall is located below the deep foundation pit. These elements work together to reinforce the silt and soft soil. The excavation and internal support construction within the foundation pit include: excavation within the steel sheet pile retaining wall, and the excavated foundation pit is supported by internal steel pipe supports; construction within the pit and conversion of the support system: bottom sealing concrete is poured at the bottom of the excavated deep foundation pit, and a foundation is poured on top of the bottom sealing concrete, with the internal steel pipe supports being removed during the pouring process.

[0009] Furthermore, the step "Construction Scheme Design and Feasibility Assessment" specifically includes: construction scheme preparation; construction scheme verification: using soil mechanics formulas and finite element analysis software to model and calculate or verify the load, stability and strength of the deep foundation pit support system in the construction scheme; expert review and approval of the construction scheme.

[0010] Furthermore, the step "construction scheme verification" specifically includes: design conditions; deep foundation pit support structure design; load calculation; construction content and adverse working condition analysis; component strength verification; steel pipe internal support stability calculation; and verification of the foundation of the cement mixing pile retaining wall in the pit.

[0011] Furthermore, the step "Design and construction of silt and soft soil support reinforcement" specifically includes: design and construction of steel sheet pile retaining wall structure; and design and construction of cement mixing pile retaining wall structure.

[0012] Furthermore, the step "Steel Sheet Pile Retaining Wall Structure Design and Construction" specifically includes: steel sheet pile retaining wall structure design and quality requirements; steel sheet pile retaining wall construction: including construction preparation, site leveling and excavation of guide trenches, and steel sheet pile driving construction.

[0013] Furthermore, the step "structural design and construction of cement mixing pile retaining wall" specifically includes: structural design of cement mixing pile retaining wall and analysis of the principle of silt and soft soil reinforcement; construction of cement mixing pile retaining wall.

[0014] Furthermore, the step "construction of cement mixing pile retaining wall" specifically includes: design of cement mixing pile retaining wall construction; construction of cement mixing pile retaining wall outside the foundation pit; construction of rectangular cement mixing pile retaining wall inside the foundation pit; and construction of straight-supported cement mixing pile retaining wall inside the foundation pit.

[0015] Furthermore, after completing the step "Design and construction of silt and soft soil support reinforcement", and before proceeding to the step "excavation of earthwork and construction of internal support in the foundation pit", the following steps are also included: monitoring and measurement: setting up monitoring points and early warning prompts on the top of the cement mixing pile retaining wall 301 outside the pit and on the ground.

[0016] Furthermore, the step "excavation and internal support construction within the foundation pit" specifically includes: excavation of the foundation pit; and construction of internal steel pipe supports within the foundation pit.

[0017] Furthermore, the step "construction within the pit and conversion of the support system" specifically includes: bottom sealing concrete construction; foundation construction and conversion of the support system.

[0018] The present invention has the following beneficial effects:

[0019] In the construction method of this invention, during the step "Construction Scheme Design and Feasibility Assessment," soil pressure is calculated using soil mechanics formulas, and finite element analysis software is used for modeling, analysis, and verification to assess whether the stability of the deep foundation pit support system meets the specifications, providing scientific theoretical and technical support for bridge abutment foundation pit construction. In the construction method of this invention, after the bottom sealing concrete is poured in the step "Construction within the Pit and Conversion of the Support System," the support system within the deep foundation pit is shown in Figures 3 and 8-9, including: bottom sealing concrete, an in-pit cement mixing pile retaining wall on the inner side of the foundation pit, an out-of-pit cement mixing pile retaining wall on the outer side of the foundation pit, a sheet pile retaining wall, and steel pipe internal supports. Compared with traditional foundation pit support methods, the support system of this invention can effectively enhance the stiffness of the deep foundation pit support system and resist soil slippage, collapse, and instability around the foundation pit. In the construction method of this invention, after the bottom sealing concrete is poured in the step "Construction within the Pit and Conversion of the Support System," as shown in Figures 3 and 8-9, the support system within the deep foundation pit includes: bottom sealing concrete, an in-pit cement mixing pile retaining wall on the inner side of the foundation pit, an out-of-pit cement mixing pile retaining wall on the outer side of the foundation pit, a sheet pile retaining wall, and steel pipe internal supports. 3. As shown in Figures 8-9, a rectangular "cup"-shaped waterproof structure is formed by the cement-mixing pile retaining wall inside the pit, the sealing concrete at the bottom of the pit, the sheet pile retaining wall, and the cement-mixing pile retaining wall outside the pit. Compared with traditional pit support methods, this invention can effectively prevent groundwater from seeping into the pit or construction wastewater from seeping out of the pit into the nearby river, thus polluting the river water quality and improving the water-stopping performance of the silt and soft soil layer. In the construction method of this invention, after the bottom sealing concrete is poured in the step "construction inside the pit and conversion of the support system", as shown in Figures 3 and 8-9, a rectangular "frame"-shaped rigid anti-buoyancy structure is also formed by the cement-mixing pile retaining wall inside the pit and the sealing concrete at the bottom of the pit. Compared with traditional pit support methods, this invention can effectively increase the weight of the bottom sealing components of the pit, preventing the silt and soft soil at the bottom of the pit from heaving and water inrush, thereby improving the heaving resistance safety factor of the silt and soft soil at the bottom of the pit.

[0020] In addition to the objectives, features, and advantages described above, the present invention has other objectives, features, and advantages. The invention will now be described in further detail with reference to the figures. Attached Figure Description

[0021] The accompanying drawings, which form part of this application, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings:

[0022] Figure 1 is a construction flowchart of the deep foundation pit support system and construction method in a silty soft soil layer environment according to a preferred embodiment of the present invention.

[0023] Figure 2 is a schematic diagram of the layout of the deep foundation pit support system;

[0024] Figure 3 is a schematic diagram of the layout of the deep foundation pit support system;

[0025] Figure 4 shows the calculation model for strength verification of components in the deep foundation pit support system;

[0026] Figure 5 is the calculation model for the foundation verification of the cement mixing pile retaining wall in the pit.

[0027] Figure 6 is a diagram of steel sheet piles and their connection structure;

[0028] Figure 7 is a plan layout diagram of the silt and soft soil reinforcement project;

[0029] Figure 8 is a cross-sectional view of 1-1 in Figure 7;

[0030] Figure 9 is a cross-sectional view of 2-2 in Figure 7.

[0031] Legend: 1. Silt and soft soil layer; 2. Cast-in-place piles; 3. Cement mixing pile retaining wall; 301. Cement mixing pile retaining wall outside the pit; 302. Cement mixing pile retaining wall inside the pit; 3021. Rectangular cement mixing pile retaining wall; 30211. Outer rectangular cement mixing pile retaining wall; 30212. Inner rectangular cement mixing pile retaining wall; 30213. Irregularly shaped cement mixing pile retaining wall; 302131. H-shaped cement mixing pile retaining wall; 302132. Full-area cement mixing pile retaining wall; 302133. Straight cement mixing pile retaining wall; 3022. Straight-braced cement mixing pile retaining wall; 4. Steel sheet pile retaining wall; 5. Steel pipe internal bracing; 501. Straight steel pipe internal bracing; 5011. Steel corbel; 5012. Steel waler; 5013. Steel pipe support rod; 502. 6. Inclined steel pipe internal support; 7. Bottom sealing concrete. Detailed Implementation

[0032] The embodiments of the present invention will be described in detail below with reference to the accompanying drawings. However, the present invention can be implemented in many different ways as defined and covered below.

[0033] Referring to Figures 1-9, a preferred embodiment of the present invention provides a deep foundation pit support system and construction method in a silty soft soil environment, including the following steps:

[0034] Construction preparation: Obtain the geological distribution at the site of the pier cap to be constructed and construct 2 cast-in-place piles under the pier cap;

[0035] Construction scheme design and feasibility assessment: Prepare the construction scheme and use soil mechanics formulas and computer software to model, analyze and verify it in order to assess whether the stability of the deep foundation pit support system meets the specifications.

[0036] Design and construction of silt and soft soil support and reinforcement: Design the structure in the deep foundation pit support system for supporting and reinforcing silt and soft soil, and construct a steel sheet pile retaining wall 4 extending along the perimeter of the deep foundation pit to support the silt and soft soil. Also construct an external cement mixing pile retaining wall 301 set close to the outer perimeter of the steel sheet pile retaining wall 4 and an internal cement mixing pile retaining wall 302 located below the deep foundation pit to reinforce the silt and soft soil.

[0037] Excavation and internal support construction within the foundation pit: Excavation is carried out within the steel sheet pile retaining wall 4, and the excavated foundation pit is supported by steel pipe internal supports 5.

[0038] Construction inside the pit and conversion of the support system: The bottom sealing concrete 6 is poured at the bottom of the excavated deep foundation pit, and the foundation is poured on the bottom sealing concrete 6. During the pouring process, the steel pipe internal support 5 is removed.

[0039] In the construction method of this invention, during the step "Construction Scheme Design and Feasibility Assessment," soil pressure is calculated using soil mechanics formulas, and finite element analysis software is used for modeling, analysis, and verification to assess whether the stability of the deep foundation pit support system meets the specifications, providing scientific theoretical and technical support for bridge abutment foundation pit construction. In the construction method of this invention, after the pouring of the bottom sealing concrete 6 in the step "Construction within the Pit and Conversion of the Support System," the support system within the deep foundation pit is shown in Figures 3 and 8-9, including: bottom sealing concrete 6, an in-pit cement mixing pile retaining wall 302 on the inner side of the foundation pit, an out-of-pit cement mixing pile retaining wall 301 on the outer side of the foundation pit, a steel sheet pile retaining wall 4, and a steel pipe internal support 5. Compared with traditional foundation pit support methods, the support system of this invention can effectively enhance the stiffness of the deep foundation pit support system and resist the slippage, collapse, and instability of the soil on the surrounding side walls of the foundation pit. In the construction method of this invention, after the pouring of the bottom sealing concrete 6 in the step "Construction within the Pit and Conversion of the Support System," as shown in Figure 3... As shown in Figures 8-9, a rectangular "water cup" waterproof structure is formed by the cement-mixing pile retaining wall 302 inside the foundation pit, the bottom sealing concrete 6, the sheet pile retaining wall 4, and the cement-mixing pile retaining wall 301 outside the foundation pit. Compared with traditional foundation pit support methods, the structure of this invention can effectively prevent groundwater from seeping into the foundation pit or construction wastewater from seeping out of the foundation pit into the nearby river and polluting the river water quality, thereby improving the water-stopping performance of the silt and soft soil layer 1. In the construction method of this invention, complete After the bottom sealing concrete 6 is poured in the "construction inside the pit and conversion of the support system" step, as shown in Figures 3 and 8-9, a rectangular "frame" type rigid anti-buoyancy structure is formed by the cement mixing pile retaining wall 302 inside the pit and the bottom sealing concrete 6 inside the pit. Compared with the traditional foundation pit support method, the structure of the present invention can effectively increase the weight of the bottom sealing components of the foundation pit, prevent the silt and soft soil at the bottom of the foundation pit from heaving and water inrush, thereby improving the heaving resistance safety factor of the silt and soft soil at the bottom of the foundation pit.

[0040] Optionally, step "S1: Construction Preparation" specifically includes:

[0041] 1. Geological Distribution of Pier Abutment: According to the Engineering Geological Survey Report of this bridge and the strata revealed by the detailed drilling borehole (GSZK27) at the location of the No. 7 main pier pier abutment: the strata distribution from the ground surface down to -21.59m is as follows: silty clay layer, medium sand layer, silty clay layer, silty sand layer, silty clay layer, etc., silty soft soil layer 1; the original ground elevation of the No. 7 main pier pier abutment is 4.2m, the area of ​​the pier abutment pit is 1531.2㎡ (66×23.2m), the perimeter is 178.4m, the bottom elevation of the pit is -3.3m, the excavation depth of the pit is 7.5m, and the pier abutment is located on the silty clay layer.

[0042] 2. Construction of the second cast-in-place pile under the pier cap: According to the design of the pile foundation construction drawings of the No. 7 main pier of the bridge, 30 cast-in-place piles with a length of 128m, a diameter of 2.2m, and C35 concrete were constructed. The construction of the No. 7 main pier cap can only be carried out after the construction quality of the cast-in-place piles 2 has been inspected and accepted.

[0043] Optionally, step "S2: Construction Scheme Design and Feasibility Assessment" includes the preparation, verification, expert review, and approval of the construction scheme. By using soil mechanics formulas to calculate earth pressure and employing MIDAS / CIVIL finite element analysis software to model, analyze, and verify the strength of components, the stability of the 5013 steel pipe support rod, and the 302 foundation of the cement mixing pile retaining wall within the pit, it can be determined whether the stability of the deep foundation pit support system meets the specifications. The feasibility and implementability of the construction scheme will be reviewed and verified by senior experts, providing scientific theoretical and technical support for the construction of the bridge abutment foundation pit project. Specifically, this includes the following:

[0044] S201: Construction plan preparation: Based on the characteristics of the project and the actual conditions of the construction site, prepare a construction plan for the deep foundation pit support system. The construction content includes: design and construction of silt and soft soil support reinforcement, excavation of the foundation pit and construction of internal support, construction inside the pit and conversion of the support system.

[0045] S202: Construction scheme verification: The load, stability and strength of the deep foundation pit support system in the construction scheme are calculated or verified using soil mechanics formulas and MIDAS / CIVIL finite element analysis software.

[0046] S203: Expert Review and Approval of the Construction Plan: By calculating earth pressure using soil mechanics formulas and employing MIDAS / CIVIL finite element analysis software to model, analyze, and verify the strength of components, the stability of the 5013 steel pipe support rod, and the 302 foundation of the cement mixing pile retaining wall within the pit, it can be concluded that the stability of the deep foundation pit support system meets the specifications. This "Construction Plan for Deep Foundation Pit Support System" has undergone centralized review and review by senior experts, and the plan is deemed feasible and implementable. The "Construction Plan for Deep Foundation Pit Support System," which has been reviewed and approved by experts, will be submitted to the construction unit, supervision unit, design unit, and relevant departments for approval.

[0047] Specifically, step "S202: Construction Scheme Verification" includes:

[0048] 1. Design conditions, specifically including:

[0049] (1) Obtaining relevant technical parameters of each soil layer: According to the "Engineering Geological Survey Report" and construction drawing design of this bridge, the following data can be obtained: the normal water level elevation of the No. 7 main pier cap is 2.5m, the ground elevation is 4.2m, the bottom elevation of the cap is -2.5m, and the bottom elevation of the foundation pit is -3.3m; according to the detailed geological survey data of borehole QSZK27, the following data can be obtained: the bottom elevation, layer thickness, cohesion c, and internal friction angle of the silty clay layer of stratum No. 2, the medium sand layer of stratum No. 3-5, the silty clay layer of stratum No. 4, the silty sand layer of stratum No. 4-13, and the silty clay layer of stratum No. 4-1. The parameters of natural heavy γ are detailed in Table 1.

[0050] Table 1 Technical parameters of soil layers at each borehole location QSZK27

[0051] (2) Material parameters: Q235 and Q355 steel are used, and their quality meets the requirements of the current national standards "Carbon Structural Steel" (GB / T700) and "Low Alloy High Strength Structural Steel" (GB / T1591), respectively.

[0052] Table 2 Design values ​​for steel strength

[0053] 2. Design of deep foundation pit support system, specifically including:

[0054] (1) Deep foundation pit support system: As shown in Figures 2 and 3, the deep foundation pit support system consists of cement mixing pile retaining wall 3 + steel sheet pile retaining wall 4 + steel pipe internal support 5 + bottom sealing concrete 6. Among them: the cement mixing pile retaining wall 3 adopts a 28-day unconfined compressive strength of not less than 1.0MPa; the steel sheet pile retaining wall 4 adopts PU600×210×18mm steel sheet piles with Q355P material and a length of 18m; the steel pipe internal support 5 includes straight steel pipe internal support 501 and inclined steel pipe internal support 502; the bottom sealing concrete 6 is laid on the bottom surface of the foundation pit, with a thickness of 0.8m and a strength grade of C30.

[0055] (2) Steel pipe internal support 5: As shown in Figures 2 and 3, each layer of steel pipe internal support 5 includes multiple straight steel pipe internal supports 501 and multiple inclined steel pipe internal supports 502. Each steel pipe internal support 5 is composed of steel brackets 5011, steel walers 5012 and steel pipe support rods 5013. Steel bracket 5011 is tightly attached to the inner side of the sheet pile retaining wall 4 and welded to the inner wall of the sheet pile retaining wall 4 to form a triangular support frame; steel waler 5012 is tightly attached to the inner side of the sheet pile retaining wall 4 on all four sides of the foundation pit; straight steel pipe inner support 501 is set between the steel walers 5012 on both sides of the foundation pit at opposite lengths, and the movable ends and fixed ends of the steel pipe support rod 5013 at both ends are connected to the steel walers 5012 on both sides; inclined steel pipe inner support 502 is set between the steel walers 502 on adjacent length and width sides of the foundation pit, and the movable ends and fixed ends of the steel pipe support rod 5013 at both ends are connected to the steel walers 502 on adjacent length and width sides at a 45° angle. Wherein:

[0056] 1) In the vertical direction within the foundation pit, the center of the first layer of steel pipe inner support 5 is 1700mm away from the top of the steel sheet pile retaining wall 4, the center of the second layer of steel pipe inner support 5 is 4000mm away from the center of the first layer of steel pipe inner support 5, and the center of the second layer of steel pipe inner support 5 is 2300mm away from the bottom of the foundation pit.

[0057] 2) The first layer of steel walers 5012 uses Q235, 3Ⅰ56a I-beams. The steel pipes used for the straight steel pipe internal support 501 are φ630*10mm steel pipes, and the steel pipes used for the inclined steel pipe internal support 502 are φ480*8mm steel pipes. The second layer of steel walers 5012 uses Q235, 3HN700*300 H-beams. The steel pipes used for the straight steel pipe internal support 501 are φ1000*12mm steel pipes, and the steel pipes used for the inclined steel pipe internal support 502 are φ480*8mm steel pipes. The steel bracket 5011 uses I25a I-beams; the steel pipe support rod 5013 consists of two parts: a support rod system and auxiliary components. The support rod system includes: the main steel pipe rod, the movable end, and the fixed end. The auxiliary components include: flanges, high-strength bolts, and steel wedges, etc. The main steel pipe rod is assembled from multiple steel pipe sections, which are connected by flanges and high-strength bolts. The center elevations of the first and second layer steel pipe internal supports 5, from top to bottom, are 3.0m and -1.0m, respectively.

[0058] 3) At the midpoint of the length side of the foundation pit, the center-to-center horizontal distance of the three straight steel pipe internal supports 501 is 5250mm, and the center-to-center horizontal distance of the remaining three straight steel pipe internal supports 501 on both sides is 5500mm. The center of the outermost straight steel pipe internal support 501 is 11250mm from the inner side of the sheet pile retaining wall 4 on the width side of the foundation pit, and two inclined steel pipe internal supports 502 are set at the four corners of this area.

[0059] 4) Within the excavation pit, at a position 4950mm from the intersection of the length and width sides, install the first inclined steel pipe internal support 502; at a position 11100mm from the intersection of the length and width sides, install the second inclined steel pipe internal support 502. The two inclined steel pipe internal supports 502 intersect the adjacent length and width sides at a 45° angle, respectively. The center of the first inclined steel pipe internal support 502 is 4345mm from the center of the second inclined steel pipe internal support 502.

[0060] 3. Load Calculation

[0061] (1) Calculation of earth pressure acting on the supporting structure:

[0062] The standard values ​​of active earth pressure and passive earth pressure acting on the outer and inner sides of the supporting structure are calculated using two methods: combined water and soil calculation and separate water and soil calculation. The standard value P of the active earth pressure at each calculation point on the outer and inner sides of the supporting structure is then calculated. ak Standard value of passive earth pressure strength P pk The details are as follows:

[0063] 1) Water and soil combined calculation method:

[0064] Based on the cohesion c (kPa) and internal friction angle of each soil layer in Table 1 and σ calculated from formulas (9) and (10) ak σ pk The standard value of active earth pressure intensity P at the calculation point in each soil layer on the outer and inner sides of the supporting structure is calculated according to formulas (1), (2), (3) and (4). ak Standard value of passive earth pressure strength P pk .

[0065] In the formula:

[0066] P ak -Standard value of active earth pressure strength (kPa) at the calculation point in the i-th soil layer outside the supporting structure;

[0067] P pk -Standard value of passive earth pressure intensity (kPa) at the calculation point in the i-th soil layer inside the supporting structure;

[0068] σ ak σ pk - These are the standard values ​​(kPa) of vertical stress in the soil at the calculation points on the outer and inner sides of the supporting structure, respectively, which are calculated using formulas (9) and (10);

[0069] K a，i K p，i - These are the active earth pressure coefficient and passive earth pressure coefficient of the i-th soil layer, respectively.

[0070] C i , - The cohesion (kPa) and internal friction angle (°) of the i-th soil layer, according to the cohesion c (kPa) and internal friction angle of each soil layer in Table 1. Values.

[0071] 2) Soil and water separation method:

[0072] According to the internal friction angles of each soil layer in Table 1 Formulas (2) and (4) can be used to calculate the active earth pressure coefficient K. a，i Passive earth pressure coefficient K p，i And σ calculated from formulas (9) and (10) ak σ pk The standard value of active earth pressure intensity P at the calculation point in each soil layer on the outer and inner sides of the supporting structure is calculated according to formulas (5) and (6). ak Standard value of passive earth pressure strength P pk .

[0073] In the formula:

[0074] P ak -Standard value of active earth pressure strength (kPa) at the calculation point in the i-th soil layer outside the supporting structure;

[0075] P pk -Standard value of passive earth pressure intensity (kPa) at the calculation point in the i-th soil layer inside the supporting structure;

[0076] σ ak σ pk - These are the standard values ​​(kPa) of vertical stress in the soil at the calculation points on the outer and inner sides of the supporting structure, respectively, which are calculated using formulas (9) and (10);

[0077] K a，i K p，i - These are the active earth pressure coefficient and passive earth pressure coefficient of the i-th soil layer, respectively.

[0078] C i - The cohesion (kPa) of the i-th soil layer is taken from the cohesion c (kPa) of each soil layer in Table 1;

[0079] u a u p - These are the water pressures (kPa) at the calculation points on the outer and inner sides of the support structure, respectively, calculated using formulas (7) and (8).

[0080] (2) Calculation of water pressure in static groundwater:

[0081] Based on the bottom elevation and normal water level elevation in Table 1, the vertical distance h from the groundwater level outside the foundation pit to the active earth pressure intensity calculation point can be determined. wa The vertical distance h from the groundwater level inside the foundation pit to the passive earth pressure intensity calculation point wp and the specific gamma ions in groundwater w Take 10kN / m 3 The water pressure u at the calculation points on the outer and inner sides of the supporting structure is calculated according to formulas (7) and (8). a u p :

[0082] u a =γ w h wa (7)

[0083] u p =γ w h wp (8)

[0084] In the formula:

[0085] ua u p - These represent the water pressure (kPa) at the calculation points on the outer and inner sides of the supporting structure, respectively;

[0086] γ w -Specific gravity of groundwater (kN / m³) 3 Take 10 kN / m 3 ;

[0087] h wa - The vertical distance (m) from the groundwater level outside the foundation pit to the active earth pressure intensity calculation point is determined according to the bottom elevation and normal water level elevation in Table 1;

[0088] h wp - The vertical distance (m) from the groundwater level inside the foundation pit to the passive earth pressure intensity calculation point is determined based on the bottom elevation and normal water level elevation in Table 1.

[0089] (3) Calculation of standard value of vertical stress in soil:

[0090] Based on the natural unit weight γ of each soil layer in Table 1 and the depth of each soil layer calculated from the bottom elevation, the total vertical stress σ at the calculation points on the outer and inner sides of the supporting structure can be calculated. ac σ pc And the standard value of the additional vertical stress Δσ in the soil at the calculation point under the j-th additional load on the outside of the support structure is calculated by formula (11). k，j The standard values ​​of vertical stress σ in the soil at the calculation points on the outer and inner sides of the supporting structure are calculated according to formulas (9) and (10). ak σ pk :

[0091] σ ak =σ ac +∑Δ k，j (9)

[0092] σ pk =σ pc (10)

[0093] In the formula:

[0094] σ ak σ pk - These represent the standard values ​​(kPa) of vertical stress in the soil at the calculation points on the outer and inner sides of the supporting structure, respectively;

[0095] σ ac -Calculation point on the outside of the supporting structure, the total vertical stress (kPa) generated by the self-weight of the soil;

[0096] σ pc -Calculation point on the inner side of the supporting structure, the total vertical stress (kPa) generated by the self-weight of the soil;

[0097] Δσ k，j - The standard value (kPa) of the additional vertical stress in the soil at the calculation point under the j-th additional load on the outside of the supporting structure is obtained by formula (11).

[0098] (4) Calculation of the standard value of additional vertical stress in soil under uniformly distributed additional load:

[0099] According to the "Code for Design of Highway Bridge and Culvert Foundations" (JTG 3363), the standard uniformly distributed load for road bridges is generally 10 kN / m. 2 Up to 20kN / m 2 If the value of q0 is between 20kN / m, then q0 takes the value of 20kN / m. 2 The standard value of the additional vertical stress Δσ in the soil at the calculation point under the j-th additional load on the outside of the supporting structure is calculated according to formula (11). k，j .

[0100] Δσ k，j =q0 (11)

[0101] In the formula:

[0102] Δσ k，j - Standard value of additional vertical stress in the soil at the calculation point under the j-th additional load on the outer side of the supporting structure (kPa)

[0103] q0 - Standard value of uniformly distributed additional load (kPa), taken as 20kN / m 2 .

[0104] (5) Vehicle load: Based on the actual load, the effect on the foundation pit support structure is considered as a local additional load, which is reflected in the earth pressure calculation.

[0105] 4. Analysis of construction content and adverse working conditions: The analysis of construction content and adverse working conditions is shown in Table 3.

[0106] Table 3 Analysis of Construction Contents and Unfavorable Working Conditions

[0107] 5. Component strength verification, specifically including:

[0108] (1) Calculation model: Using MIDAS / CIVIL finite element analysis software, a single pile was used to establish a component strength verification calculation model: general beam elements were used to simulate the component, elastic supports were used to simulate the soil spring, and general supports were used to simulate the constraint at the bottom of the steel sheet pile retaining wall, as shown in Figure 4.

[0109] (2) Component strength verification

[0110] 1) Strength verification of components under unfavorable working condition 1:

[0111] Strength verification of the steel sheet pile retaining wall 4: The steel sheet pile wall thickness is 18mm, the material is Q355P, and the maximum combined stress is 34.3MPa < 295MPa, which meets the requirements.

[0112] 2) Strength verification of components under unfavorable working condition 2:

[0113] Strength calculations for the steel sheet pile retaining wall: 4. Steel sheet pile wall thickness 18mm, material Q355P, maximum combined stress 83MPa < 295MPa, meets requirements; 5012 Steel waler strength calculations: Waler uses 3Ⅰ56a I-beams, material Q235, maximum combined stress 85MPa < 205MPa, maximum shear stress 23MPa < 120MPa, meets requirements; 5013 φ630mm steel pipe support rod strength calculations: Steel pipe wall thickness 10mm, material Q235, maximum combined stress 82MPa < 215MPa, meets requirements; 5013 φ480mm steel pipe support rod strength calculations: Steel pipe wall thickness 8mm, material Q235, maximum combined stress 18MPa < 215MPa, meets requirements.

[0114] 3) Strength verification of components under unfavorable working condition 3:

[0115] Strength calculations for the steel sheet pile retaining wall: 4. Steel sheet pile wall thickness 18mm, material Q355P, maximum combined stress 209MPa < 295MPa, meets requirements; 3Ⅰ56a steel waler 5012 strength calculations: Waler uses 3Ⅰ56a I-beams, material Q235, maximum combined stress 55MPa < 205MPa, maximum shear stress 7MPa < 120MPa, meets requirements; 3HN700*300mm steel waler 5012 strength calculations: Waler uses 3HN700*300mm H-beams, material Q235, maximum combined stress 165MPa < 205MPa. The maximum shear stress is 91.1 MPa < 120 MPa, which meets the requirements; Strength verification of φ630mm steel pipe support rod 5013: steel pipe wall thickness 10mm, material Q235, maximum combined stress 76MPa < 215MPa, which meets the requirements; Strength verification of φ480mm steel pipe support rod 5013: steel pipe wall thickness 8mm, material Q235, maximum combined stress 24MPa < 215MPa, which meets the requirements; Strength verification of φ1000mm steel pipe support rod 5013: steel pipe wall thickness 12mm, material Q235, maximum combined stress 141MPa < 215MPa, which meets the requirements.

[0116] 4) Strength verification of components under unfavorable working condition 4:

[0117] Strength calculations for the steel sheet pile retaining wall: 4. Steel sheet pile wall thickness 18mm, material Q355PQ345, maximum combined stress 102MPa < 295MPa, meets requirements; 5012 Steel waler strength calculation: Waler uses 3Ⅰ56a I-beams, material Q235, maximum combined stress 68MPa < 205MPa, maximum shear stress 17MPa < 120MPa, meets requirements; 5013 φ630mm steel pipe support rod strength calculation: Steel pipe wall thickness 10mm, material Q235, maximum combined stress 89MPa < 215MPa, meets requirements; 5013 φ480mm steel pipe support rod strength calculation: Steel pipe wall thickness 8mm, material Q235, maximum combined stress 18MPa < 215MPa, meets requirements.

[0118] 6. Stability calculation of steel pipe support rod:

[0119] (1) Strength verification of φ1000*12mm steel pipe support rod 5013, calculated according to formula (12): meets the requirements.

[0120] In the formula:

[0121] N - Calculate the design value of axial pressure (N) within the component's range;

[0122] -Stability coefficient of axially compressed components;

[0123] A - Cross-sectional area (mm²);

[0124] f - Design strength of steel (MPa);

[0125] E - Elastic modulus of steel (MPa);

[0126] β - equivalent bending moment coefficient, which is taken as 1.0 according to the standard;

[0127] M - The maximum design bending moment (N·mm) within the calculated component area;

[0128] γ m - Cross-sectional plastic development coefficient, taken as 1.15;

[0129] W - Section modulus (mm) 3 );

[0130] N′ EX -Parameters, by Calculate (mm).

[0131] The values ​​of each parameter are shown in Table 4:

[0132] (2) Strength verification of φ630*10mm steel pipe support rod 5013, calculated according to formula (13): meets the requirements.

[0133] In the formula:

[0134] N - Calculate the design value of axial pressure (N) within the component's range;

[0135] -Stability coefficient of axially compressed components;

[0136] A - Cross-sectional area (mm²);

[0137] f - Design strength of steel (MPa);

[0138] E - Elastic modulus of steel (MPa);

[0139] β - equivalent bending moment coefficient, which is taken as 1.0 according to the standard;

[0140] M - The maximum design bending moment (N·mm) within the calculated component area;

[0141] γ m - Cross-sectional plastic development coefficient, taken as 1.15;

[0142] W - Section modulus (mm) 3 )

[0143] N′ EX -Parameters, by Calculate (mm).

[0144] The values ​​of each parameter are shown in Table 5:

[0145] 7. Foundation calculation of cement mixing pile retaining wall in the pit

[0146] Because the original soil at the location of the No. 7 main pier abutment of the bridge is relatively weak, cement mixing piles were used to reinforce the silty soft soil, forming a cement mixing pile retaining wall 3 structure. From an economic perspective, to fully utilize the surplus load of the supporting structure, the silty soft soil reinforcement structure was optimized. The silty soft soil reinforcement structure includes: an external cement mixing pile retaining wall 301 and an internal cement mixing pile retaining wall 302, bounded by a steel sheet pile retaining wall 4.

[0147] Among them: the external cement mixing pile retaining wall 301 is a rectangular cement mixing pile retaining wall with a thickness of 0.6m that extends outward from the steel sheet pile retaining wall 4; the internal cement mixing pile retaining wall 302 is a rectangular cement mixing pile retaining wall 3021 with a certain thickness that extends inward from the steel sheet pile retaining wall 4; and five parallel cement mixing pile retaining walls 3022 with a width of 1.5m (three rows), a lateral spacing of 2.5m, and a pile length of 6.0m are arranged in the middle of the length side of the foundation pit, as shown in Figure 7.

[0148] The following uses solid modeling to analyze and verify the two most unfavorable working conditions of the cement mixing pile retaining wall 302 in the pit before and after the concrete bottom sealing.

[0149] (1) Computational Model

[0150] Using MIDAS / CIVIL finite element analysis software, a solid model for the foundation verification calculation of the cement mixing pile retaining wall was established: solid elements were used to simulate the structure of the cement mixing pile retaining wall 302 in the pit, elastic supports were used to simulate the soil spring, and general supports were used to simulate the constraint at the bottom of the cement mixing pile retaining wall 3, as shown in Figure 5.

[0151] (2) Unfavorable working conditions before concrete sealing

[0152] A first layer of steel pipe internal support 5 and a second layer of steel pipe internal support 5 are installed on the steel sheet pile retaining wall 4. When the foundation pit is excavated to the bottom of the foundation pit sealing concrete 6, the structure of the cement mixing pile retaining wall 302 in the pit is simulated as the third layer of concrete internal support structure. Corresponding active earth pressure is applied, and the structure of the cement mixing pile retaining wall 302 in the pit is calculated and analyzed, as well as the lateral force of the cement mixing pile retaining wall 302 in the pit is calculated.

[0153] Strength verification of the 302 cement mixing pile retaining wall in the pit: The maximum combined stress of the 302 cement mixing pile retaining wall in the pit is 1.05MPa, and the wall body strength of the 302 cement mixing pile retaining wall in the pit is not less than 1.2MPa.

[0154] (3) Unfavorable working conditions after concrete sealing

[0155] A bottom sealing concrete 6 is poured on the bottom surface of the foundation pit inside the steel sheet pile retaining wall 4. Under the combined action of the self-weight of the bottom sealing concrete 6 and the active lateral pressure, the structure of the cement mixing pile retaining wall 302 inside the pit is calculated and analyzed.

[0156] Strength verification of the 302 cement mixing pile retaining wall in the pit: The maximum combined stress of the 302 cement mixing pile retaining wall in the pit is 1.3MPa, and the wall strength of the 302 cement mixing pile retaining wall in the pit is not less than 1.5MPa.

[0157] (4) Through the calculation and analysis of the above two most unfavorable working conditions of the cement mixing pile retaining wall 302 in the pit before and after the concrete bottom sealing, it can be seen that the optimized structure of the cement mixing pile retaining wall 302 in the pit meets the requirements of the foundation pit support construction.

[0158] Optionally, the step "Design and Construction of Silt and Soft Soil Support Reinforcement" specifically includes:

[0159] S3: Structural design and construction of steel sheet pile retaining wall;

[0160] S4: Structural design and construction of cement mixing pile retaining wall 3.

[0161] In this optional scheme, step "S3: Structural Design and Construction of Steel Sheet Pile Retaining Wall" specifically includes:

[0162] S301: Structural Design and Quality Requirements for Steel Sheet Pile Retaining Walls; specifically, it includes:

[0163] 1. Steel Sheet Pile Retaining Wall 4 Structural Design: As shown in Figure 6, the steel sheet pile retaining wall 4 uses PU600×210×18mm steel sheet piles with a length of 18m made of Q355P material. The steel sheet pile consists of a web, flanges, and interlocking plates. One end of the flange is welded to the interlocking plate, and the other end is welded to any end of the web in a "U" shape. The interlocking plate is located away from the web, and the opening of the steel sheet pile faces the opposite direction to the opening of the interlocking plate. The reverse "U-shaped protrusion" design of the interlocking plates on both outer sides of each steel sheet pile can be used to interlock adjacent steel sheet piles to form an interlocking structure. When the steel sheet piles are interlocked, a watertight structure is formed, thereby increasing the strength of the steel sheet pile retaining wall 4 structure.

[0164] 2. Quality requirements: The quality of the sheet piles shall meet the requirements of the standard "Hot-rolled Steel Sheet Piles" (GB / T20933); any damaged or misaligned sheet piles shall be corrected to ensure correct dimensions and smooth, bend-free interlocks; the quality inspection standard for smooth interlocks is to use a 1m long sheet pile to fit into each group of sheet piles, and those that pass smoothly are considered qualified.

[0165] S302: Construction of Sheet Pile Retaining Wall 4: This includes construction preparation, site leveling and excavation of guide trenches, and sheet pile driving. Specifically, it includes:

[0166] 1. Construction preparation: Based on the construction drawings of the No. 7 main pier foundation of this bridge, the engineering technicians laid out and determined the construction position of the steel sheet pile retaining wall 4 at the designed location of the No. 7 main pier foundation pit on the construction site.

[0167] 2. Site leveling and excavation of guide trench: Use an excavator to level the site to ensure the working path and construction conditions of the sheet pile driver; drive positioning piles, and excavate a guide trench with a width of 1.0m and a depth of 0.5m along the layout line of the sheet pile retaining wall 4 of the foundation pit; the guide trench is excavated as it is driven during construction.

[0168] 3. Sheet pile driving construction: Using a sheet pile driving machine, a sheet pile is hoisted into the guide trench and then driven vertically downwards into the silty clay layer below the deep foundation pit as the starting point for sheet pile driving; then, along the layout line of the sheet pile retaining wall 4 of the foundation pit, four sets of sheet piles are driven in a straight line in sequence, with the ends connected. The ends of the sheet piles are locked together with the starting point sheet piles to form a rectangular sheet pile retaining wall 4 with a length of 66m and a width of 23.2m, which resists the slippage, collapse and instability of the soil on the side walls of the foundation pit. The sheet piles are arranged in a staggered manner with the ends connected, and they are interlocked with each other. The U-shaped opening of the sheet pile is opposite to that of the U-shaped opening of the previous sheet pile.

[0169] After the sheet piles are driven, the sheet pile retaining wall 4 is in a straight line shape to prevent it from forming an "S" shape, which would cause the sheet pile locks to not engage or disengage, resulting in water seepage into the sheet pile retaining wall 4 and collapse and instability of the foundation pit soil. This also prevents groundwater from seeping into the foundation pit or construction wastewater from seeping out of the foundation pit into the nearby river and polluting the river water quality.

[0170] The process involves the following steps: When driving in the next sheet pile, align the locking mechanism of the next sheet pile with the locking mechanism of the previous sheet pile, ensuring they are interlocked. Then, gently hammer the next sheet pile with a vibratory hammer until it penetrates 1-2 meters into the ground. After confirming that the verticality and horizontal position of the next sheet pile are within acceptable limits, continue hammering until the set depth is reached. During the driving process, continuously monitor the verticality and horizontal deviation of the sheet piles. Use two total stations (or plumb bobs) to control these deviations in two directions. After driving, the top elevation of the sheet pile retaining wall 4 should be 4.7 meters, and the bottom elevation should be -13.3 meters. The verticality of the sheet piles should be ≤1%, and the horizontal deviation ≤15 centimeters.

[0171] In this optional scheme, step "S4: Design and construction of cement mixing pile retaining wall structure" specifically includes:

[0172] S401: Structural Design of Cement Mixing Pile Retaining Wall and Analysis of the Principle of Silt and Soft Soil Reinforcement; and S402: Construction of Cement Mixing Pile Retaining Wall; among which, the step "S401: Structural Design of Cement Mixing Pile Retaining Wall and Analysis of the Principle of Silt and Soft Soil Reinforcement" specifically includes:

[0173] 1. Cement mixing pile retaining wall structure design: As shown in Figures 2, 3, 7, 8, and 9, cement mixing piles are used to reinforce the silty soft soil. After reinforcement, the internal friction angle of the silty soft soil can reach over 15° and the cohesion can reach over 20 kPa. The silty soft soil reinforcement structure adopts an interlocking cement mixing pile retaining wall 3 with a pile diameter of 0.60m, a pile spacing of 0.45m, and a pile overlap of 0.15m. This includes an external cement mixing pile retaining wall 301 on the outside of the pit, with the steel sheet pile retaining wall 4 as the boundary, and an internal cement mixing pile retaining wall 302 on the inside of the pit. The bottom elevation of 301 is 0.5m lower than the top elevation of the cement mixing pile retaining wall 302 inside the pit, and overlaps it vertically by 0.5m in the foundation pit, forming a rectangular "sleeve" type silt and soft soil reinforcement structure. This weakens the stress concentration of the steel sheet pile retaining wall 4 at the junction of the foundation pit bottom and the silt and soft soil layer, enhances the rigidity of the steel sheet pile retaining wall 4, thereby enhancing the rigidity of the deep foundation pit support system. Together with the steel sheet pile retaining wall 4 and the foundation pit bottom sealing concrete 6, it forms a rectangular "cup" type waterproof structure, preventing groundwater from seeping into the foundation pit or construction wastewater from seeping out of the foundation pit into the nearby river, thus polluting the river water quality and improving the water-stopping performance of the silt and soft soil layer 1. Among them:

[0174] (1) Cement mixing pile retaining wall 301 outside the pit: As shown in Figures 2, 3, 7, 8 and 9, a row of rectangular cement mixing pile retaining walls 301 with a width of 0.60m and a pile length of 8.0m is constructed on the top surface of the silt soft soil layer 1 outside the steel sheet pile retaining wall 4 on all four sides of the pit, facing outwards from the pit, and abuts against the outside of the steel sheet pile retaining wall 4; the top elevation of the cement mixing pile retaining wall 301 outside the pit is 4.2m and the bottom elevation is -3.8m; the 28d unconfined compressive strength of the cement mixing pile retaining wall 301 outside the pit is ≥1.0MPa, the vertical deviation is ≤+1%, and the planar position deviation is ≤50mm.

[0175] (2) Cement mixing pile retaining wall 302 inside the pit: As shown in Figures 3, 7, 8, and 9, the cement mixing pile retaining wall 302 inside the pit includes a rectangular cement mixing pile retaining wall 3021 and a straight-supported cement mixing pile retaining wall 3022; on the top surface of the silt and soft soil layer 1 inside the sheet pile retaining wall 4 on all four sides of the pit, facing the center of the pit, a rectangular cement mixing pile retaining wall 3021 and a straight-supported cement mixing pile retaining wall 3022 with a pile length of 6.0m are constructed, abutting against the inner side of the sheet pile retaining wall 4; the top elevation of the cement mixing pile retaining wall 302 inside the pit is -3.3m, and the bottom elevation is -9.3m; the 28-day unconfined compressive strength of the cement mixing pile retaining wall 302 inside the pit is ≥1.0MPa, the vertical deviation is ≤+1%, and the planar position deviation is ≤50mm. Among them:

[0176] 1) Rectangular Cement Mixing Pile Retaining Wall 3021: As shown in Figure 7, a rectangular cement mixing pile retaining wall 3021 with a pile length of 6.0m is constructed on the top surface of the silt and soft soil layer 1 on the inner side of the sheet pile retaining wall 4 on all four sides of the foundation pit, facing the center of the foundation pit. The rectangular cement mixing pile retaining wall 3021 includes: an outer rectangular cement mixing pile retaining wall 30211, an inner rectangular cement mixing pile retaining wall 30212, and an irregular cement mixing pile retaining wall 30213. Among them, the irregular cement mixing pile retaining wall 30213 includes: an H-shaped cement mixing pile retaining wall 302131, a full-area cement mixing pile retaining wall 302132, and a straight cement mixing pile retaining wall 302133.

[0177] 2) Direct-supported cement mixing pile retaining wall 3022: As shown in Figure 7, on the top surface of the silt soft soil layer 1 inside the steel sheet pile retaining wall 4 at the middle position of the length side of the foundation pit, facing the center side of the foundation pit, five parallel, 1.5m wide (three rows), 2.5m lateral spacing between adjacent piles, and 6.0m long direct-supported cement mixing pile retaining wall 3022 are constructed. Their two ends abut against the outer rectangular cement mixing pile retaining wall 30211, the straight cement mixing pile retaining wall 302133 and the inner rectangular cement mixing pile retaining wall 30212 on the inner side of the foundation pit, respectively.

[0178] 2. Analysis of the principle of silt and soft soil reinforcement:

[0179] (1) Setting up an outer layer reinforcement structure for silt and soft soil: The outer layer reinforcement of silt and soft soil adopts the cement mixing pile reinforcement method. By constructing an outer cement mixing pile retaining wall 301 on the top surface of the silt and soft soil layer 1 outside the steel sheet pile retaining wall 4 on all four sides of the foundation pit, the top elevation of the outer cement mixing pile retaining wall 301 is 0.5m lower than the top elevation of the steel sheet pile retaining wall 4. This outer layer reinforcement structure prevents stones and other objects on the top of the outer cement mixing pile retaining wall 301 and the ground from falling into the foundation pit and injuring construction workers, and prevents surface water on the top of the outer cement mixing pile retaining wall 301 and the ground from flowing into the foundation pit. This outer layer reinforcement structure for silt and soft soil can make the internal friction angle of the silt and soft soil reach more than 15° and the cohesion reach more than 20kPa, thereby improving the shear strength, bearing capacity and lateral support force of the silt and soft soil, and resisting the slippage, collapse and instability of the soil on the side walls of the foundation pit.

[0180] (2) Install a rectangular "sleeve" type silt and soft soil reinforcement structure and a rectangular "water cup" type waterproof structure.

[0181] By constructing a rectangular cement-mixing pile retaining wall 3021 on the top surface of the silty soft soil layer 1 inside the sheet pile retaining wall 4 on all four sides of the foundation pit, facing towards the center of the foundation pit, the top elevation of the rectangular cement-mixing pile retaining wall 3021 is 0.5m higher than the bottom elevation of the cement-mixing pile retaining wall 301 outside the pit. Furthermore, the retaining wall 3021 is vertically overlapped by 0.5m in the foundation pit to form a rectangular "sleeve" type silty soft soil reinforcement structure. This weakens the stress concentration at the junction of the sheet pile retaining wall 4 at the bottom of the foundation pit and the silty soft soil layer, thus strengthening the sheet pile retaining wall 4. The rigidity is increased, thereby enhancing the rigidity of the deep foundation pit support system and resisting the slippage, collapse and instability of the soil on the side walls of the foundation pit; the rectangular cement mixing pile retaining wall 3021, together with the straight-supported cement mixing pile retaining wall 3022, the foundation pit bottom sealing concrete 6, the steel sheet pile retaining wall 4 and the cement mixing pile retaining wall 301 outside the pit, form a rectangular "water cup" waterproof structure, preventing groundwater from seeping into the foundation pit or construction sewage from seeping out of the foundation pit into the nearby river and polluting the river water quality, thereby improving the water-stopping performance of the silt soft soil layer 1.

[0182] (3) Set up a rectangular "frame" type rigid anti-buoyancy structure

[0183] Five parallel, 1.5m wide (three rows), 2.5m laterally spaced, and 6.0m long straight-supported cement mixing pile retaining walls 3022 are constructed on the top surface of the silt soft soil layer 1 on the inner side of the steel sheet pile retaining wall 4 at the middle position of the long side of the foundation pit, facing the center of the foundation pit. The two ends of the retaining walls abut against the outer rectangular cement mixing pile retaining wall 30211, the straight cement mixing pile retaining wall 302133 and the inner rectangular cement mixing pile retaining wall 30212 on the inner side of the foundation pit, respectively. The straight-supported cement mixing pile retaining wall 3022, together with the rectangular cement mixing pile retaining wall 3021 and the bottom sealing concrete 6 inside the foundation pit, form a rectangular "frame" type rigid anti-buoyancy structure, which effectively increases the weight of the bottom sealing components of the foundation pit and prevents the silt and soft soil at the bottom of the foundation pit from heaving or water inrush, thereby improving the heaving safety factor of the silt and soft soil at the bottom of the foundation pit; and together with the rectangular cement mixing pile retaining wall 3021, the bottom sealing concrete 6 inside the foundation pit, the cement mixing pile retaining wall 301 outside the pit, the steel sheet pile retaining wall 4, the first layer of steel pipe internal support 5, and the second layer of steel pipe internal support 5, they form a steel pipe concrete support system, which enhances the rigidity of the deep foundation pit support system and resists the slippage, collapse and instability of the soil on the surrounding side walls of the foundation pit.

[0184] In this optional scheme, step "S402: Construction of cement mixing pile retaining wall 3" specifically includes:

[0185] 1. Construction design of cement mixing pile retaining wall, as shown in Figures 2, 3, and 7-9, specifically includes:

[0186] (1) Cement mixing pile retaining wall 301 outside the pit: On the top surface of the silt and soft soil layer 1 outside the steel sheet pile retaining wall 4 on all four sides of the pit, facing the outside of the pit, a row of rectangular cement mixing pile retaining wall 301 with a width of 0.60m and a pile length of 8.0m is constructed to abut against the outside of the steel sheet pile retaining wall 4.

[0187] (2) Cement mixing pile retaining wall 302 inside the pit: On the top surface of the silt and soft soil layer 1 inside the sheet pile retaining wall 4 on the four sides of the pit, facing the center of the pit, a cement mixing pile retaining wall 302 with a pile length of 6.0m is constructed inside the pit, abutting the inner side of the sheet pile retaining wall 4; The construction of the cement mixing pile retaining wall 302 inside the pit includes: rectangular cement mixing pile retaining wall 3021 and straight-supported cement mixing pile retaining wall 3022; The construction of the rectangular cement mixing pile retaining wall 3021 includes: outer rectangular cement mixing pile retaining wall 30211, inner rectangular cement mixing pile retaining wall 30212 and irregular cement mixing pile retaining wall 30213; Irregular cement mixing pile retaining wall 30213 inside the pit includes: H-shaped cement mixing pile retaining wall 302131, full-area cement mixing pile retaining wall 302132 and straight cement mixing pile retaining wall 302133.

[0188] (3) Cement mixing pile retaining wall 3: Cement mixing pile retaining wall 3 adopts interlocking cement mixing piles with a pile diameter of 0.60m, a pile spacing of 0.45m, and a pile overlap of 0.15m; near the construction site of the No. 7 main pier abutment of the bridge, a cement slurry mixing station and storage tank are set up. After the cement slurry is mixed, it is pumped to the grouting pipe of the cement mixing pile drilling machine by the grouting hose; two cement mixing pile drilling machines are equipped at the construction site. Cement slurry composed of cement, silica fume, nano silica, talc powder, early strength water reducing agent, and mixing water with a water-cement ratio of 0.60 is prepared at the construction site. Cement mixing pile retaining wall 3 is constructed to reinforce the silt and soft soil with a volume ratio of 1:0.30 between the silt and soft soil.

[0189] The function of setting up high-performance cement mixing pile retaining wall 3:

[0190] In the silty soft soil layer 1, a cement grout prepared from cement, silica fume, nano silica, talc, early-strength water-reducing agent, and mixing water is injected by spraying. The cement grout is mixed, squeezed, and filled between the pores of the silty soft soil particles by a cement mixing pile drilling machine at a volume ratio of 1:0.30, and then quickly solidifies to form a high-performance cement mixing pile retaining wall 3 with high shear strength, early strength, high bearing capacity, and good water-stopping performance. This solves the technical problems of low bearing capacity, low shear strength, small lateral support force, poor water-stopping performance, and poor foundation pit stability of the silty soft soil layer 1, improves the stiffness of the deep foundation pit support system, the safety factor of the silty soft soil body at the bottom of the foundation pit against heave, and the water-stopping performance of the silty soft soil layer 1, thereby improving the stability of the foundation pit.

[0191] (4) Construction method of cement mixing pile retaining wall 3: As shown in Figures 2, 3, and 7-9, the cement mixing piles are constructed using a "skip-one-build" method, completing the cement mixing piles on the outside and inside of the foundation pit one by one. Then, many cement mixing piles on the outside and inside of the foundation pit are connected side by side to form the cement mixing pile retaining wall 301 outside the pit and the cement mixing pile retaining wall 302 inside the pit. The specific construction method is as follows:

[0192] 1) Measurement and layout: Within a 0.60m wide area outside the four sides of the steel sheet pile retaining wall of the foundation pit, the engineering technicians measure and lay out the area to determine the drilling position of each cement mixing pile.

[0193] 2) Drilling rig positioning: Start the winch to move the cement mixing pile drilling rig to the designated drilling position. Use positioning clips to ensure that the centering error of the cement mixing pile drilling position is ≤50mm. The guide and mixing shaft must be perpendicular to the ground, with a verticality deviation of ≤1.0%.

[0194] 3) Grouting and Sinking: Connect the grout delivery hose to the storage tank, grout pump, and cement mixing pile machine. Turn on the motor of the cement mixing pile machine. The mixer blades rotate in opposite directions. Using the weight of the cement mixing pile machine, the drill rod sinks vertically to the designed reinforcement depth at a uniform speed of 0.3-0.5 m / min. Simultaneously, grout is sprayed at a uniform speed of 0.3-0.5 m / min. When the drill rod reaches 0.5 m from the top surface of the silt-soft soil layer 1, turn on the grout pump. At a grouting pressure of 0.40 MPa-0.60 MPa, continuously pressurize the cement grout from the central pipe of the cement mixing pile machine into the silt-soft soil layer 1. The mixer blades mix the cement grout with the silt-soft soil layer 1. Grouting continues while mixing until the designed reinforcement depth is reached. During grouting, the cement grout is continuously stirred. A grout flow meter is used to control the grout delivery speed during grouting construction. The outlet pressure of the grouting pump is maintained at 0.40 MPa-0.60 MPa, and the grout delivery speed must be kept constant. During shotcreting, an automatic shotcreting recorder must be installed to record: shotcreting volume, drilling speed, and drilled pile length.

[0195] 4) Spraying and Lifting: When the cement mixing pile machine sinks to the designed reinforcement depth, after spraying grout at the end of the cement mixing pile for 30 seconds, it is lifted and mixed at a uniform speed of 0.3 to 0.5 m / min. Cement grout is sprayed at a grouting pressure of 0.40 MPa to 0.60 MPa, and the cement grout is continuously pressed into the silt and soft soil layer 1 from the central pipe of the cement mixing pile machine. The mixing blades of the mixer mix the cement grout with the silt and soft soil layer 1. Spraying is carried out while mixing until the distance from the top surface of the silt and soft soil layer 1 is 0.5 m, and then the spraying is stopped, thus completing one mixing process.

[0196] 5) Re-mixing: During implementation, each cement mixing pile must undergo a re-mixing process of four mixing and four spraying operations, repeating the previous operation. The initial sinking and lifting mixing should be performed at a speed of 0.3 or 0.4 m / min; during the re-mixing, the sinking mixing should be performed at a speed of 0.4 or 0.5 m / min, and the lifting mixing at a speed of 0.3 or 0.4 m / min, with a brief pause between the two mixing operations. The normal forming time for each cement mixing pile should be no less than 40 minutes, and the spraying pressure should be no less than 0.4 MPa. The re-mixing and spraying method helps to compensate for insufficient spraying volume in a single operation and accelerates the slurry reaction process.

[0197] 6) Relocation: After one cement mixing pile is completed, the next cement mixing pile must be constructed using the same method. Repeat the above steps. After each day's reinforcement is completed, use clean water to clean the storage tank, mortar pump, cement mixing pile machine, and related pipelines for reuse.

[0198] 2. Construction of the external cement mixing pile retaining wall 301 on the outside of the foundation pit; specifically, as shown in Figure 7, a row of rectangular cement mixing pile retaining walls 301 (0.60m wide, 8.0m long) are constructed on the top surface of the silt soft soil layer 1 on the outside of the steel sheet pile retaining wall 4 on all four sides of the foundation pit, facing outwards from the foundation pit, and abutting against the outside of the steel sheet pile retaining wall 4; after construction, the external cement mixing pile retaining wall 301 and the steel sheet pile retaining wall 4 are firmly bonded; the construction quality requirements are: the top elevation of the external cement mixing pile retaining wall 301 is 4.2m, the bottom elevation is -3.8m, the 28-day unconfined compressive strength is ≥1.0MPa, the vertical deviation is ≤+1%, and the planar position deviation is ≤50mm.

[0199] 3. Construction of the rectangular cement mixing pile retaining wall 3021 inside the foundation pit; as shown in Figure 7-9, the construction of the rectangular cement mixing pile retaining wall 3021 includes: outer rectangular cement mixing pile retaining wall 30211, inner rectangular cement mixing pile retaining wall 30212, and irregular cement mixing pile retaining wall 30213; the irregular cement mixing pile retaining wall 30213 inside the foundation pit includes: H-shaped cement mixing pile retaining wall 302131, near-full-area cement mixing pile retaining wall 302132, and straight cement mixing pile retaining wall 302133; its construction quality requirements are: the top elevation of the rectangular cement mixing pile retaining wall 3021 is -3.3m, the bottom elevation is -9.3m, the 28-day unconfined compressive strength is ≥1.0MPa, the vertical deviation is ≤+1%, and the planar position deviation is ≤50mm.

[0200] (1) Construction of the outer rectangular cement mixing pile retaining wall 30211 inside the foundation pit: As shown in Figure 7-9, three rows of cement mixing piles (1.5m wide and 6.0m long) are constructed on the top surface of the silt soft soil layer 1 inside the steel sheet pile retaining wall 4 on all four sides of the foundation pit, facing the center of the foundation pit. Many cement mixing piles are connected to each other in the longitudinal and transverse directions to form the outer rectangular cement mixing pile retaining wall 30211 inside the foundation pit, which abuts against the inner side of the steel sheet pile retaining wall 4. After construction, the outer rectangular cement mixing pile retaining wall 30211 inside the foundation pit is firmly bonded to the steel sheet pile retaining wall 4.

[0201] (2) Construction of the inner rectangular cement mixing pile retaining wall 30212 on the inner side of the foundation pit: As shown in Figure 7-9, on the top surface of the silt soft soil layer 1 at a distance of 3.75m from the inner edge of the outer rectangular cement mixing pile retaining wall 30211 on the inner side of the foundation pit, two rows (width 1.05m) of cement mixing piles with a pile length of 6.0m are constructed in the direction towards the center of the foundation pit. Many cement mixing pile retaining walls are connected to each other in the longitudinal and transverse directions to form the inner rectangular cement mixing pile retaining wall 30212 on the inner side of the foundation pit.

[0202] (3) Construction of the irregular cement mixing pile retaining wall 30213 inside the foundation pit: On the top surface of the silt and soft soil layer 1 within the range between the outer rectangular cement mixing pile retaining wall 30211 and the inner rectangular cement mixing pile retaining wall 30212 inside the foundation pit, construct H-shaped cement mixing pile retaining wall 302131, full-area cement mixing pile retaining wall 302132 and straight cement mixing pile retaining wall 302133 to form the irregular cement mixing pile retaining wall 30213 inside the foundation pit, as detailed below:

[0203] 1) Construction of H-shaped cement mixing pile retaining wall 302131: As shown in Figure 7-9, on the top surface of the silt and soft soil layer 1 between the three adjacent cast-in-place piles 2 on the left half of the foundation pit along the bridge direction, double rows (1.05m wide) and 6.0m long "H"-shaped cement mixing piles are constructed. Many cement mixing piles are connected to each other longitudinally and laterally to form an H-shaped cement mixing pile retaining wall 302131. Its two ends respectively abut against the outer rectangular cement mixing pile retaining wall 30211 and the inner rectangular cement mixing pile retaining wall 30212 on the inner side of the foundation pit. Some of them abut against the adjacent cast-in-place pile 2; on the top surface of the silt and soft soil layer 1 between the three adjacent cast-in-place piles 2 on the right half of the foundation pit along the bridge direction, two rows (1.05m wide) and 6.0m long "H"-shaped cement mixing piles are constructed respectively. Many cement mixing piles are connected to each other in the longitudinal and transverse directions to form an H-shaped cement mixing pile retaining wall 302131. Its two ends abut against the outer rectangular cement mixing pile retaining wall 30211 and the inner rectangular cement mixing pile retaining wall 30212 on the inner side of the foundation pit respectively, some of which abut against the adjacent cast-in-place pile 2.

[0204] 2) Construction of the full-area cement mixing pile retaining wall 302132: As shown in Figures 7-8, on the top surface of the silt and soft soil layer 1, located at the upper left and lower left corners of the outer rectangular cement mixing pile retaining wall 30211 on the left half of the foundation pit along the bridge direction, several rows of cement mixing piles with a pile length of 6.0m are constructed. These cement mixing piles are connected longitudinally and laterally to form the full-area cement mixing pile retaining wall 302132, which abuts against the outer rectangular rectangular cement mixing pile retaining wall 30211 on the inner side of the foundation pit and the inner layer of the foundation pit. Rectangular cement mixing pile retaining wall 30212: On the top surface of the silty soft soil layer 1, at the upper right and lower right corners of the outer rectangular cement mixing pile retaining wall 30211 on the right half of the foundation pit along the bridge direction, several rows of cement mixing piles with a pile length of 6.0m are constructed. The cement mixing piles are connected to each other longitudinally and laterally to form a quasi-full-area cement mixing pile retaining wall 302132, which abuts against the outer rectangular cement mixing pile retaining wall 30211 and the inner rectangular cement mixing pile retaining wall 30212 on the inner side of the foundation pit, respectively.

[0205] 3) Construction of the straight cement mixing pile retaining wall 302133: As shown in Figure 7-8, on the top surface of the silt and soft soil layer 1 between the outer rectangular cement mixing pile retaining wall 30211 and the inner rectangular cement mixing pile retaining wall 30212 on both sides of the foundation pit, double rows (1.05m wide) and 6.0m long straight cement mixing piles are constructed. Many cement mixing piles are connected to each other longitudinally and laterally to form a straight cement mixing pile retaining wall 302133, which abuts against the two opposite cast-in-place piles 2 in the middle of the foundation pit.

[0206] 4. Construction of the straight-supported cement mixing pile retaining wall 3022 inside the foundation pit; As shown in Figure 7-9, on the top surface of the silt soft soil layer 1 inside the steel sheet pile retaining wall 4 at the middle position of the length side inside the foundation pit, facing the center side of the foundation pit, construct 5 parallel cement mixing piles with a width of 1.5m (three rows), a horizontal adjacent spacing of 2.5m, and a pile length of 6.0m. Connect the cement mixing piles longitudinally and laterally to form the straight-supported cement mixing pile retaining wall 3022. Its two ends respectively abut against the outer rectangular cement mixing pile retaining wall 30211, the straight cement mixing pile retaining wall 302133 and the inner rectangular cement mixing pile retaining wall 30212 inside the foundation pit.

[0207] Preferably, after completing the step "Design and construction of silt and soft soil support reinforcement", and before proceeding to the step "Excavation and internal support construction in the foundation pit", the following step is also included:

[0208] Optionally, step "S5: Monitoring and Measurement" specifically includes: setting up monitoring and early warning points on the top of the cement mixing pile retaining wall 301 outside the pit and on the ground; specifically including:

[0209] 1. Monitoring Content: After the silt and soft soil reinforcement construction is completed and before the excavation of the foundation pit, monitoring points will be set up on the top of the cement mixing pile retaining wall 301 outside the steel sheet pile retaining wall 4 on all four sides of the foundation pit and on the ground. According to the characteristics of this project, the monitoring content includes: horizontal displacement of the top of the cement mixing pile retaining wall (hereinafter referred to as the retaining wall), vertical displacement of the top of the retaining wall, axial force of the support, water level inside and outside the retaining wall, groundwater level monitoring, horizontal displacement of deep soil, and daily inspection.

[0210] 2. Monitoring point layout: (1) Horizontal displacement monitoring points at the top of the retaining wall are laid out around the perimeter of the foundation pit. According to Article 5.2.1 of the "Technical Standard for Monitoring of Foundation Pit Engineering" (GB50497), the horizontal spacing of the monitoring points shall not exceed 20m. The monitoring points shall be laid out with no less than 3 on the length side, no less than 1 on the width side, and 1 at each of the four corners of the foundation pit; (2) Vertical displacement monitoring points at the top of the retaining wall share the same location as the horizontal displacement monitoring points at the top of the retaining wall; (3) Support axial force monitoring points are laid out with 5 axial force monitoring points inside the first layer of steel pipes, 501 inside the middle straight steel pipe on the length side of the foundation pit, and 501 inside the first inclined steel pipe at each of the four corners of the foundation pit. One support axial force gauge is installed on each of the supports 502. The support axial force gauge is installed vertically from the support 501 axial force monitoring point in the first layer of steel pipe to the corresponding support 501 axial force monitoring point in the second layer of steel pipe. (4) Water level monitoring points inside and outside the retaining wall: Water level monitoring points are set on the outer wall of the cement mixing pile retaining wall 301 outside the pit, and the readings can be read directly. A scale is set on the inner side of the steel sheet pile retaining wall 4 to set up water level monitoring points on the inner side of the retaining wall. (5) Groundwater level monitoring points: One water level measuring hole is set on each side of the center line of the length of the foundation pit. (6) Deep soil horizontal displacement monitoring points: One soil displacement monitoring point is set on each side of the center line of the length and width of the foundation pit.

[0211] 3. Testing methods: (1) Horizontal displacement monitoring of the top of the retaining wall is carried out using the polar coordinate method; (2) Vertical displacement monitoring of the top of the retaining wall is carried out using the geometric leveling method, with a digital level instrument and an Invar barcode ruler for observation; (3) Support axial force monitoring is carried out using a comprehensive testing instrument; (4) Water level monitoring inside and outside the retaining wall is carried out using direct observation; (5) Groundwater level monitoring is carried out using direct observation; (6) Horizontal displacement monitoring of deep soil is carried out using an inclinometer.

[0212] 4. Monitoring cycle, frequency, and warning values:

[0213] (1) Monitoring period: The specific period is based on the on-site construction time, starting from the start of the foundation pit excavation until the end of the foundation pit backfilling.

[0214] (2) Monitoring frequency and early warning values ​​are shown in Table 6:

[0215] Table 6 Monitoring Frequency and Early Warning Values

[0216] The monitoring frequency should be increased when any of the following conditions occur:

[0217] ① The monitoring data changes significantly or the rate of change accelerates;

[0218] ②The surrounding ground suddenly experiences significant subsidence or severe cracking;

[0219] ③ Nearby buildings experience sudden large settlement, uneven settlement, or severe cracking;

[0220] ④ Reorganize construction after an accident occurs;

[0221] ⑤ Other abnormal situations that affect construction safety occur.

[0222] (3) During the construction process, observe the condition of the foundation pit in real time. If there are any abnormal noises or sudden increases in deformation, construction should be stopped immediately, personnel should be evacuated, and construction should continue only after the cause has been identified and effective measures have been taken.

[0223] 5. Warning Values ​​and Control Values: Early warnings for the safety and risk status of the project during construction are divided into three categories: monitoring early warning, inspection early warning, and comprehensive early warning, as detailed below:

[0224] (1) Monitoring and early warning: Based on the monitoring and control indicators proposed in the design data, the early warning status of the monitoring points during the construction process is divided into three levels according to the severity: yellow monitoring and early warning, orange monitoring and early warning and red monitoring and early warning.

[0225] 1) Yellow monitoring warning: When both "dual control" indicators (cumulative change and rate) reach 70% to 85% of the monitoring control value (including 85% of the monitoring control value), or when one of the dual control indicators reaches 85% to the control value (excluding the monitoring control value), while the other indicator does not reach 85% of the monitoring control value;

[0226] 2) Orange monitoring and early warning: When both "dual control" indicators reach 85% to 100% of the monitoring and control value (inclusive), or when one of the dual control indicators reaches or exceeds the monitoring and control value;

[0227] 3) Red monitoring and early warning: When both "dual control" indicators reach or exceed the monitoring control values, and the measured rate of change shows a sharp increase.

[0228] (2) Inspection and early warning: Early warning is issued when safety hazards or unsafe conditions are discovered during inspections during the construction process. It is divided into three levels according to the severity, from least to most serious: yellow inspection and early warning, orange inspection and early warning and red inspection and early warning.

[0229] (3) Comprehensive Early Warning: During the construction process, based on on-site monitoring and inspection information, and through on-site verification, comprehensive analysis, consultation among participating parties, and expert demonstration, early warnings are issued in a timely and comprehensive manner to determine the unsafe state of the project risks. The comprehensive early warning is classified into four levels according to the severity, from least to most severe: blue comprehensive early warning, yellow comprehensive early warning, orange comprehensive early warning, and red comprehensive early warning.

[0230] Optionally, step "S6: Excavation and internal support construction within the foundation pit" specifically includes:

[0231] S601: Excavation of earthwork within the foundation pit; specifically including:

[0232] 1. Setting up water collection wells: On the foundation pit plane, according to the earthwork excavation sequence and excavation zones, set up interconnected drainage ditches around the foundation pit in zones, and leave several water collection wells in the areas with large seepage in each zone. Install submersible pumps in the water collection wells to drain water and achieve the purpose of drainage and dewatering. The water collection wells are set on the inner side of the steel sheet pile retaining wall 4, with dimensions of 800×800×400mm.

[0233] 2. Excavation of the foundation pit: As shown in Figure 3, excavation of the foundation pit can only begin after the conditions for excavation have been inspected and approved. The excavation of the foundation pit shall be carried out in three steps from top to bottom. During the excavation process, a sump must be set up in a timely manner for drainage to ensure the smooth progress of the excavation. The specific construction steps are as follows:

[0234] Step 1: When the first layer of earthwork is excavated to 1.0m below the center line of the first layer of steel pipe inner support 5, cut the steel casing and break the over-poured pile heads in time, clean the soil on the steel sheet pile retaining wall 4 and the cast-in-place pile 2, and install the first layer of steel corbel 5011, steel waler 5012 and steel pipe support rod 5013.

[0235] Step 2: When the second layer of earthwork is excavated to 1.0m below the center line of the second layer steel pipe inner support 5, cut the steel casing and break the over-poured pile heads in time, clean the soil on the steel sheet pile retaining wall 4 and the cast-in-place pile 2, and install the steel corbel 5011, steel waler 5012 and steel pipe support rod 5013 of the second layer.

[0236] Step 3: When the third layer of earthwork is excavated to the design elevation of -3.3m at the bottom of the foundation pit, the steel casing is cut and the over-poured pile head is broken down to the design elevation of -2.3m at the top of the cast-in-place pile 2. The soil on the steel sheet pile retaining wall 4 and the cast-in-place pile 2 is then cleared.

[0237] 3. Earthwork excavation principles: Earthwork excavation in the foundation pit shall strictly follow the "space-time effect" theory and be carried out in accordance with the principles of "overall segmentation, longitudinal segmentation, vertical layering, synchronous excavation operation of excavators, support as excavation is carried out, and time-limited balance".

[0238] 4. Overall excavation method for foundation pit: The first layer of soil is excavated inside the foundation pit using an excavator, and the second and third layers of soil are excavated outside the foundation pit using a long-arm excavator. In order to ensure the safety of the machinery during construction, the soil excavation is carried out in a layered step method, with a step width of ≥4m. Loading is prohibited within 2m outside the foundation pit.

[0239] S602: Construction of internal support for the foundation pit; as shown in Figures 2 and 3, the construction of internal support for the foundation pit includes: installation of steel corbels 5011, installation of steel walers 5012, installation of steel pipe support rods 5013, and construction of bottom sealing concrete 6, as detailed below:

[0240] 1. Construction process flow, specifically including:

[0241] (1) Construction sequence: surveying and setting out → installation of steel bracket 5011 → installation of steel waler 5012 → installation of steel pipe support rod 5013 → construction of bottom sealing concrete 6.

[0242] (2) Installation of steel bracket 5011: Steel bracket 5011 is welded manually.

[0243] (3) Installation of steel waler 5012: The steel waler 5012 is installed in place by manual labor and crawler crane. The connection between the steel waler 5012 segments is made by butt welding, and steel plates are used to reinforce the weld. The gap between the steel waler 5012 and the steel sheet pile retaining wall 4 is supported by I-beam welded horizontally.

[0244] (4) Installation of steel pipe support rod 5013: Several sections of steel pipe are spliced ​​together on the ground to form a section of steel pipe main rod, and then spliced ​​together with the steel pipe main rod with flexible end and the steel pipe main rod with fixed end to form a whole steel pipe support rod 5013. The whole rod is hoisted into place by manual labor and crawler crane.

[0245] (5) Construction of bottom sealing concrete 6: On the bottom surface of the foundation pit, pour a bottom sealing concrete 6 with a thickness of 0.8m and a strength grade of C30.

[0246] (6) Removal sequence: Steel pipe support rod 5013 is removed → Steel waler 5012 is removed → Steel bracket 5011 is removed.

[0247] 2. Materials and processing of steel brackets, steel walers, and steel pipes, specifically including:

[0248] In this optional scheme, the steel bracket 5011, steel waler 5012, and steel pipe support rod 5013 are all assembled and welded on-site from purchased semi-finished products. The steel bracket 5011 uses I25a I-beams and is welded to the inner wall of the steel sheet pile retaining wall 4 to form a triangular support frame. The steel pipe support rod 5013 consists of a support rod system and auxiliary components. The support rod system includes: the main steel pipe rod, the movable end, and the fixed end. The auxiliary components include: flanges, high-strength bolts, and steel wedges. The main steel pipe rod is assembled from multiple sections of steel pipe, and the sections are connected using flanges and high-strength bolts. The first layer of steel waler 5012 uses Q235, 3I56a I-beams. The steel pipes used for the straight steel pipe inner support 501 are φ630*10mm steel pipes, and the inclined steel pipe inner support 501... The steel pipes used in section 02 are φ480*8mm steel pipes; the second layer of steel walers 5012 uses Q235, 3HN700*300 H-beams; the steel pipes used for the straight steel pipe internal support 501 are φ1000*12mm steel pipes; the steel pipes used for the inclined steel pipe internal support 502 are φ480*8mm steel pipes; before processing, a cutting list is prepared according to the usage location of the steel walers 5012, and the materials are cut according to the cutting list and stacked neatly by category; all the end faces of the directly load-bearing steel plates of the steel walers 5012 are pre-milled flat; the connection between the sections of the steel walers 5012 must be butt-welded and extended using the equal strength connection method when assembling the steel walers 5012.

[0249] 3. Installation of steel brackets, steel walers, and steel pipe support rods, specifically including:

[0250] (1) Installation of Steel Bracket 5011: When the first layer of earthwork is excavated to 1.0m below the center line of the first layer of steel pipe internal support 5, the engineering technicians measure and set out the position of the first layer of steel pipe internal support 5, promptly clean the soil on the steel sheet pile retaining wall 4 and the cast-in-place pile 2, and weld the first layer of steel bracket 5011. When the second layer of earthwork is excavated to 1.0m below the center line of the second layer of steel pipe internal support 5, the engineering technicians measure and set out the position of the second layer of steel pipe internal support 5, promptly clean the soil on the steel sheet pile retaining wall 4 and the cast-in-place pile 2, and weld the second layer of steel bracket 5011. The steel bracket 5011 is triangular in shape.

[0251] (2) Installation of steel walers 5012: After the first layer of steel brackets 5011 is installed, the first layer of steel walers 5012 is hoisted and installed; after the second layer of steel brackets 5011 is installed, the second layer of steel walers 5012 is hoisted and installed. The processed steel walers 5012 are installed in sections on the steel bracket 5011 supports using manual labor and a 50t crawler crane. They are leveled and positioned, and small steel plates are used to support any uneven areas on the steel bracket 5011 supports. The specific installation is as follows:

[0252] The steel waler 5012 segments are connected by butt welding of equal strength to form a continuous steel beam, ensuring no cantilever structure and guaranteeing the overall stability of the steel waler 5012. Steel plates are used for reinforcement at the weld joints. The welded joints of the steel waler 5012 are positioned away from the center of the inner support 5 of the steel pipe, within 1 / 6 of the spacing of the inner support 5 on either side of its centerline. The gap between the steel waler 5012 and the steel sheet pile retaining wall 4 is supported horizontally by welded I-beams to ensure uniform stress distribution on the steel waler 5012. Six and three fall protection devices for the steel waler 5012 are installed on the length and width sides of the excavation pit, respectively. 16mm thick perforated steel plates are welded onto the steel waler 5012 and the steel sheet pile retaining wall 4 respectively. 16mm diameter steel wire ropes and buckles are used to connect the steel waler 5012 and the steel sheet pile retaining wall 4 to prevent the steel waler 5012 from falling.

[0253] (3) Installation of steel pipe support rod 5013. Before installing the steel pipe support rod 5013, calculate or measure the actual length of the steel pipe support rod 5013 at each installation location. The total length of the spliced ​​steel pipe support rod 5013 is slightly less than the actual length. The gap between the steel pipe support rod 5013 and the steel waler 5012 is locked by using specially shaped steel wedges after the support axial force is applied.

[0254] 1) After the first layer of steel walers 5012 is installed, the center position of the straight steel pipe inner support 501 is determined on the steel walers 5012 on both sides of the foundation pit. The cross line method is used to accurately position the support. On the side of the steel walers 5012 facing the center of the foundation pit, a triangular steel plate support with the end face perpendicular to the axis of the steel pipe support rod 5013 is welded. The steel pipe support rod 5013 is then installed. During installation, a crawler crane is used for overall hoisting. The two ends of the steel pipe support rod 5013 are placed on the triangular steel plate supports on the steel walers 5012 on both sides of the foundation pit. The center positions of the inclined steel pipe internal supports 502 are determined by laying out the steel walers 5012 on the adjacent length and width sides of the four corners of the foundation pit. Accurate positioning is achieved using the cross-line method. On the side of the steel walers 5012 facing the center of the foundation pit, wedge-shaped triangular steel plate supports with end faces perpendicular to the axis of the steel pipe support rod 5013 are welded. The steel pipe support rod 5013 is then installed using a crawler crane for overall hoisting, placing both ends of the steel pipe support rod 5013 on the wedge-shaped triangular steel plate supports on the adjacent length and width sides of the steel walers 5012 at the four corners of the foundation pit. After the second layer of steel walers 5012 is installed, the second layer of steel pipe support rods 5013 is installed using the same method.

[0255] 2) Weld 16mm thick perforated steel plates to the steel pipe support rod 5013 and the steel sheet pile retaining wall 4 respectively, and use 16mm diameter steel wire rope and buckle lock to connect the steel pipe support rod 5013 and the steel sheet pile retaining wall 4 to prevent the steel pipe support rod 5013 from falling.

[0256] 3) When hoisting the steel pipe support rod 5013, a two-point hoisting method is adopted, with the hoisting points located at a distance of 0.2 times the total length of the steel pipe support rod 5013 from its end. During the application of axial force to the steel pipe support rod 5013, the crawler crane must not lower the steel pipe support rod 5013; the crawler crane can only be removed after the steel pipe support rod 5013 is locked. The first layer of steel pipe support rods 5013 has a maximum weight of approximately 3.4t and a hoisting distance of 16m, using a 50t crawler crane; the second layer of steel pipe support rods 5013 has a maximum weight of approximately 6.4t and a hoisting distance of 16m, using a 75t crawler crane.

[0257] 4) Weld and install a support axial force gauge on the movable end face of the steel pipe support rod 5013 where axial force monitoring points need to be installed. Apply axial force prestress to the steel pipe support rod 5013 using two 500t hydraulic jacks at the movable end. After the axial force prestress of the steel pipe support rod 5013 stabilizes at the design value, insert a steel wedge at the movable end for limiting, completing the application of axial force prestress to the steel pipe support rod 5013. The axial force of the first layer of steel pipe support rods 5013 is 1116kN, and the axial force of the second layer of steel pipe support rods 5013 is 5010kN.

[0258] 4. Installation quality requirements

[0259] The installation deviations of steel pipe support rod 5013 must be within the allowable range specified in the standard: the difference between the center elevation of steel pipe support rod 5013 and the top surface elevation of steel pipe support rod 5013 on the same floor is ±30mm; the difference in elevation between the two ends of steel pipe support rod 5013 is ≤20mm and the smaller of 1 / 600 of the length of steel pipe support rod 5013; the deflection of steel pipe support rod 5013 is ≤1 / 1000 of the length of steel pipe support rod 5013; the horizontal axis deviation of steel pipe support rod 5013 is ≤30mm.

[0260] Optionally, step "S7: Construction in the pit and conversion of the support system" specifically includes:

[0261] S701: Construction of bottom sealing concrete 6, specifically including:

[0262] The bottom sealing concrete 6 inside the excavation pit, together with the rectangular cement mixing pile retaining wall 3021 and the directly supported cement mixing pile retaining wall 3022 on the inner side of the excavation pit, forms a rectangular "frame" type rigid anti-buoyancy structure, effectively increasing the weight of the bottom sealing components of the excavation pit and preventing the heave and water inrush of the silt and soft soil at the bottom of the excavation pit, thereby improving the heave resistance safety factor of the silt and soft soil at the bottom of the excavation pit; and together with the cement mixing pile retaining wall 301 outside the pit, the steel sheet pile retaining wall 4, the rectangular cement mixing pile retaining wall 3021 and the directly supported cement mixing pile retaining wall 3022 on the inner side of the excavation pit, and the first The first layer of steel pipe internal support 5 and the second layer of steel pipe internal support 5 constitute a steel pipe concrete support system, which effectively enhances the rigidity of the deep foundation pit support system and resists the slippage, collapse and instability of the soil on the side walls of the foundation pit. It also forms a rectangular "water cup" waterproof structure with the rectangular cement mixing pile retaining wall 3021, the straight support cement mixing pile retaining wall 3022, the steel sheet pile retaining wall 4 and the cement mixing pile retaining wall 301 outside the pit, which prevents groundwater from seeping into the foundation pit or construction sewage from seeping out of the foundation pit into the nearby river and polluting the river water quality, thereby improving the water-stopping performance of the silt soft soil layer 1.

[0263] 1. When the third layer of earthwork is excavated to the design elevation of -3.3m at the bottom of the foundation pit, the steel casing should be cut in time and the pile heads of the over-cast piles should be broken down to the design elevation of -2.3m at the top of the cast-in-place pile 2. The soil on the steel sheet pile retaining wall 4 and the cast-in-place pile 2 should be cleaned in time.

[0264] 2. Level the bottom surface of the foundation pit and inspect its planar position, geometric dimensions, and bottom elevation. After acceptance, pour a 0.8m thick layer of C30 bottom sealing concrete 6 on the bottom surface of the foundation pit. The pouring sequence of the bottom sealing concrete 6 is as follows: pour from both ends of the width side of the foundation pit towards the middle of the foundation pit.

[0265] 3. After the bottom concrete reaches its design strength, install the foundation reinforcement.

[0266] S702: Construction of foundation and conversion of support system, specifically including:

[0267] 1. Removal of the second layer of steel pipe support rod 5013: After the bottom sealing concrete 6 reaches the design requirements, the second layer of steel pipe support rod 5013, steel waler 5012, and steel corbel 5011 can be removed, and the first conversion of the deep foundation pit support system can be gradually completed.

[0268] 2. First pier cap construction: After the second layer of steel pipe support rods 5013, steel walers 5012, and steel corbels 5011 are removed, the first 3m high reinforcement, embedded parts, formwork installation, and C40 high-performance large-volume concrete of the pier cap are completed, up to a position 2.5m away from the bottom surface of the first layer of steel pipe support rods 5013.

[0269] 3. First backfilling of the cavity: After the first pile cap concrete reaches 75% of the design strength, cohesive soil is used to backfill the cavity between the pile cap concrete and the steel sheet pile retaining wall 4 in layers. The thickness of each backfill layer is 300mm and the compaction degree is ≥90%.

[0270] 4. Removal of the first layer of steel pipe support rod 5013: The first layer of steel pipe support rod 5013, steel waler 5012, and steel corbel 5011 can only be removed after the backfill soil in the cavity reaches the top surface of the first foundation. The already constructed first foundation structure and the backfill soil in the cavity are used as internal supports for the foundation pit, completing the second conversion of the deep foundation pit support system.

[0271] 5. Second pier cap construction: After the first layer of steel pipe support rods 5013, steel walers 5012, and steel corbels 5011 are removed, continue to complete the second pier cap construction with a height of 2m, including reinforcement, embedded parts, formwork installation, and C40 high-performance large-volume concrete, up to the top surface of the pier cap.

[0272] 6. Second Backfilling of the Cavity: After the second foundation concrete reaches 75% of its design strength, cohesive soil is used to backfill the cavity between the foundation concrete and the sheet pile retaining wall 4 in layers. Each layer of backfill is 300mm thick and has a compaction degree of ≥90%. The existing second foundation structure and the backfill soil in the cavity are used as internal supports for the foundation pit, completing the third conversion of the deep foundation pit support system.

[0273] 7. Stop foundation pit monitoring and measurement, and remove steel sheet pile retaining wall 4: After the first, second and third conversions of the above-mentioned deep foundation pit support system, foundation pit monitoring and measurement can be stopped, steel sheet pile retaining wall 4 can be removed, and the construction of the No. 7 main pier cap of this bridge can be successfully completed.

[0274] Optionally, in addition to the bridge abutment foundation pit construction field described in the above embodiments, the construction method of the present invention can also be applied to the foundation pit construction of underground structures such as urban rail transit underground stations, underground utility tunnels, underground passages, underground culverts, and basements of industrial and civil buildings.

[0275] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A deep foundation pit support system and construction method in a silty soft soil environment, characterized in that, Includes the following steps: Construction preparation: Obtain the geological distribution at the site of the pier to be constructed and construct the cast-in-place piles under the pier (2); Construction scheme design and feasibility assessment: Prepare the construction scheme and use soil mechanics formulas and computer software to model, analyze and verify it in order to assess whether the stability of the deep foundation pit support system meets the specifications. Design and construction of silt and soft soil support reinforcement: Design the structure in the deep foundation pit support system for supporting and reinforcing silt and soft soil, and construct a steel sheet pile retaining wall (4) extending along the perimeter of the deep foundation pit to support the silt and soft soil. Also construct an external cement mixing pile retaining wall (301) closely attached to the outer perimeter of the steel sheet pile retaining wall (4) and an internal cement mixing pile retaining wall (302) located below the deep foundation pit to reinforce the silt and soft soil. Excavation and internal support construction in the foundation pit: Excavation is carried out inside the steel sheet pile retaining wall (4), and the excavated foundation pit is supported by steel pipe internal support (5); Construction inside the pit and conversion of the support system: The bottom sealing concrete (6) is poured at the bottom of the excavated deep foundation pit, and the foundation is poured on the bottom sealing concrete (6) and the steel pipe internal support (5) is removed during the pouring process.

2. The deep foundation pit support system and construction method in a silty soft soil environment according to claim 1, characterized in that, The step "Construction Scheme Design and Feasibility Assessment" specifically includes: Construction plan preparation; Construction scheme verification: The load, stability and strength of the deep foundation pit support system in the construction scheme are calculated or verified using soil mechanics formulas and MIDAS / CIVIL finite element analysis software. Expert review and approval of the construction plan.

3. The deep foundation pit support system and construction method in a silty soft soil layer environment according to claim 2, characterized in that, The step "Construction Plan Verification" specifically includes: Design requirements; Design of support structure for deep foundation pits; Load calculation; Analysis of construction content and adverse working conditions; Component strength verification; Stability calculation of internal steel pipe support (5); Foundation verification of cement mixing pile retaining wall (302) in pit.

4. The deep foundation pit support system and construction method in a silty soft soil environment according to claim 1, characterized in that, The specific steps of "Design and Construction of Silt and Soft Soil Support and Reinforcement" include: Steel sheet pile retaining wall (4) Structural design and construction; Cement mixing pile retaining wall (3) Structural design and construction.

5. The deep foundation pit support system and construction method in a silty soft soil environment according to claim 4, characterized in that, The specific steps of "Structural Design and Construction of Steel Sheet Pile Retaining Wall (4)" include: Steel sheet pile retaining wall (4) Structural design and quality requirements; Steel sheet pile retaining wall (4) construction: including construction preparation, site leveling and excavation of guide trenches, and steel sheet pile driving construction.

6. The deep foundation pit support system and construction method in a silty soft soil layer environment according to claim 4, characterized in that, The specific steps of "Design and Construction of Cement Mixing Pile Retaining Wall Structure" include: Structural design and silt and soft soil reinforcement principle analysis of cement mixing pile retaining wall (3); Construction of cement mixing pile retaining wall (3).

7. The deep foundation pit support system and construction method in a silty soft soil layer environment according to claim 6, characterized in that, The specific steps of "construction of cement mixing pile retaining wall" include: Construction design of cement mixing pile retaining wall (3); Construction of the cement mixing pile retaining wall (301) outside the foundation pit; Construction of the rectangular cement mixing pile retaining wall (3021) inside the foundation pit; Construction of the straight-supported cement mixing pile retaining wall (3022) inside the foundation pit.

8. The deep foundation pit support system and construction method in a silty soft soil environment according to claim 1, characterized in that, After completing the step "Design and Construction of Silt and Soft Soil Support Reinforcement", and before proceeding to the step "Excavation and Internal Support Construction in the Foundation Pit", the following steps are also included: Monitoring and measurement: Monitoring points and early warning prompts are set up on the top of the cement mixing pile retaining wall (301) outside the pit and on the ground.

9. The deep foundation pit support system and construction method in a silty soft soil environment according to claim 1, characterized in that, The step "excavation and internal support construction within the foundation pit" specifically includes: Excavation of soil within the foundation pit; Construction of steel pipe internal support in the foundation pit (5).

10. The deep foundation pit support system and construction method in a silty soft soil environment according to claim 1, characterized in that, The specific steps of "construction within the pit and conversion of the support system" include: Construction of bottom sealing concrete (6); Foundation construction and support system conversion.

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