Deep foundation pit supporting system in silt soft soil layer environment and construction method

Abstract

The invention discloses a deep foundation pit supporting system in a silt soft soil layer environment and a construction method. The construction method comprises the following steps that construction preparation is conducted; construction scheme design and feasibility evaluation: compiling a construction scheme, and evaluating whether the stability of the deep foundation pit supporting system meets the requirement or not; design and construction of silt soft soil support reinforcement: designing a structure for supporting and reinforcing silt soft soil, and constructing a steel sheet pile enclosure wall, an out-pit cement mixing pile enclosure wall and an in-pit cement mixing pile enclosure wall; earth excavation and inner support construction in the foundation pit are conducted, specifically, earth excavation is conducted, and a steel pipe inner support is adopted for supporting; and in-pit construction and supporting system conversion, bottom sealing concrete is poured at the pit bottom of the deep foundation pit, bearing platform pouring is carried out on the bottom sealing concrete, and in the pouring process, a steel pipe inner support is dismantled. According to the method, the rigidity of a deep foundation pit supporting system can be effectively enhanced, underground water permeation is prevented, and the weight of a foundation pit bottom sealing component is increased so as to prevent the foundation pit bottom silt soft soil body from upheaval and water burst accidents.

Description

Technical Field

[0001] The invention relates to the technical field of bridge foundation pit construction, and in particular to a deep foundation pit support system and a construction method in a silt soft soil environment. Background Art

[0002] In the construction of underground structure foundation pit projects, in order to ensure the safety of underground structure construction and the surrounding environment of the foundation pit, foundation pit support is a support, reinforcement and protection measure adopted for the side walls of the foundation pit and the surrounding environment, ensuring the smooth implementation of foundation pit excavation and main structure construction. Common foundation pit support includes: pile support, pile support, pile anchor, pile cantilever, underground continuous wall support, ground-connected wall + support, cement retaining wall, soil nail wall (spray anchor support), reverse arch wall, original soil slope, pile, wall plus support system, simple horizontal support, reinforced concrete pile row and other forms.

[0003] In a bridge project in the Guangdong Delta Plain, the bridge 7# main pier cap passes through the silty clay layer, medium sand layer, silty silty clay layer, silty silt sand layer, and silty silty clay layer, and is located on the silty silty clay layer. The natural moisture content of the soil layer in the construction site of the bridge 7# main pier cap is 28.7~49.5%, the water saturation is 88.3~99.0%, the porosity is 0.790~1.319, the foundation bearing capacity is 50~220kPa, the direct quick shear cohesion is 3.8~14.8kPa, and the internal friction angle is 2.3°~13.7°. It has high moisture content, large porosity, low bearing capacity, low shear strength, poor physical and mechanical properties of the soil, poor geological conditions, and small lateral support for the cofferdam of the foundation pit of the bridge 7# main pier cap, which is very easy to slip, collapse and instability, so unconventional foundation pit support is required.

[0004] The bridge's 7# main pier cap is located about 3m away from the secondary water source, Meixi River. The cap foundation pit is a 7.5m deep pit excavated near the water. In order to prevent the cap foundation pit from slipping, collapsing and becoming unstable due to the low strength and lateral support of the silt soft soil layer after excavation, the silt soft soil layer at the bottom of the foundation pit may bulge and cause water gushing.

[0005] Therefore, in the construction of the foundation pit of the 7# main pier cap, how to develop a deep foundation pit support system with high rigidity, high safety factor against uplift of the silt soft soil layer at the bottom of the foundation pit, and good water-stopping performance of the silt soft soil layer under the silt soft soil environment is an urgent problem to be solved in the safe construction of the foundation pit of the 7# main pier cap of the bridge. Summary of the invention

[0006] The present invention provides a deep foundation pit support system and a construction method in a silt soft soil environment, so as to solve the technical problems that the deep foundation pit support system has low rigidity, the silt soft soil body at the bottom of the foundation pit has low anti-uplift safety factor, and the silt soft soil layer has poor water-stopping performance.

[0007] The technical solution adopted by the present invention is as follows:

[0008] A deep foundation pit support system and construction method in a silt soft soil layer environment, comprising the following steps: construction preparation: obtaining the distribution of the strata at the foundation to be constructed and constructing the cast-in-place piles at the bottom of the foundation; construction scheme design and feasibility evaluation: preparing a construction scheme, and using soil mechanics formulas and computer software to model, analyze and verify, so as to evaluate whether the stability of the deep foundation pit support system meets the requirements of the specification; silt soft soil support reinforcement design and construction: designing the structure used to support and reinforce the silt soft soil in the deep foundation pit support system, and constructing the structure extending along the perimeter of the deep foundation pit. The steel sheet pile retaining wall is used to support the silt soft soil, and the cement mixing pile retaining wall outside the pit that is set close to the outer periphery of the steel sheet pile retaining wall and the cement mixing pile retaining wall inside the pit located below the deep foundation pit cooperate to reinforce the silt soft soil; earth excavation and internal support construction in the foundation pit: earth excavation is carried out in the steel sheet pile retaining wall and the excavated foundation pit is supported by steel pipe internal support; construction in the pit and conversion of the support system: bottom concrete is poured at the bottom of the excavated deep foundation pit, and the pedestal is poured on the bottom concrete and the steel pipe internal support is removed during the pouring process.

[0009] Furthermore, the step "construction plan design and feasibility assessment" specifically includes: preparation of construction plan; verification of construction plan: using soil mechanics formulas and finite element analysis software modeling to calculate or verify the load, stability and strength of the deep foundation pit support system in the construction plan; expert demonstration and approval of the construction plan.

[0010] Furthermore, the step "Construction plan 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 cement mixing pile retaining wall foundation verification in the pit.

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

[0012] Furthermore, the step "Structural design and construction of steel sheet pile retaining wall" specifically includes: structural design and quality requirements of steel sheet pile retaining wall; construction of steel sheet pile retaining wall: including construction preparation, site leveling and excavation of guide trenches, and steel sheet pile driving construction.

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

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

[0015] Furthermore, after completing the step of "silt soft soil support reinforcement design and construction" and before carrying out the step of "excavation of earthwork in the foundation pit and internal support construction", it also includes the steps of: monitoring and measurement: setting up monitoring 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 of "excavation of earth in the foundation pit and construction of internal supports" specifically includes: excavation of earth in the foundation pit; construction of internal supports of steel pipes in the foundation pit.

[0017] Furthermore, the step of "in-pit construction and support system conversion" specifically includes: bottom concrete construction; pedestal construction and support system conversion.

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

[0019] In the construction method of the present invention, when performing the step "construction scheme design and feasibility assessment", the soil pressure is calculated by 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 requirements of the specification, thereby providing scientific theory and technical support for the construction of the bridge foundation pit project; in the construction method of the present invention, after the bottom concrete pouring in the step "construction in the pit and conversion of the support system" is completed, the support system in the deep foundation pit is as follows: Figure 3 , Figure 8-9 As shown, it includes: bottom concrete, cement mixing pile retaining wall inside the foundation pit, cement mixing pile retaining wall outside the foundation pit, steel sheet pile retaining wall and steel pipe inner support. Compared with the traditional foundation pit support method, the support system of the present invention can effectively enhance the rigidity of the deep foundation pit support system and resist the sliding and collapse of the soil body on the side walls around the foundation pit. In the construction method of the present invention, after the bottom concrete pouring in the step "construction in the pit and conversion of the support system" is completed, as shown in the figure, Figure 3 , Figure 8-9 As shown, a rectangular "water cup" waterproof structure is formed by the cement mixing pile retaining wall inside the foundation pit, the bottom concrete at the bottom of the foundation pit, the steel sheet pile retaining wall and the cement mixing pile retaining wall outside the foundation pit. Compared with the traditional foundation pit support method, the structure of the present invention can effectively prevent groundwater from seeping into the foundation pit or construction sewage in the foundation pit from seeping into the nearby river outside the foundation pit and polluting the river water quality, thereby improving the water-stopping performance of the silt soft soil layer; in the construction method of the present invention, after the bottom concrete pouring in the step "construction in the pit and conversion of the support system" is completed, as shown in the figure Figure 3 , Figure 8-9As shown, a rectangular "frame" type rigid anti-floating structure is formed by the cement mixing pile retaining wall inside the foundation pit and the bottom sealing concrete at the bottom of the foundation pit. Compared with the traditional foundation pit support method, the structure of the present invention can effectively increase the weight of the bottom sealing component of the foundation pit, prevent the silt and soft soil at the bottom of the foundation pit from uplifting and water gushing accidents, thereby improving the anti-uplift safety factor of the silt and soft soil at the bottom of the foundation pit.

[0020] In addition to the above-described purposes, features and advantages, the present invention has other purposes, features and advantages. The present invention will be further described in detail with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The drawings constituting a part of this application are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation of the present invention. In the drawings:

[0022] Figure 1 It is a construction flow chart of a deep foundation pit support system and a construction method in a silt soft soil layer environment of a preferred embodiment of the present invention;

[0023] Figure 2 It is a plan view of the deep foundation pit support system layout;

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

[0025] Figure 4 It is a calculation model for component strength verification in deep foundation pit support system;

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

[0027] Figure 6 It is the structural diagram of steel sheet piles and their connections;

[0028] Figure 7 It is the plan layout of silt soft soil reinforcement;

[0029] Figure 8 yes Figure 7 1-1 section diagram in;

[0030] Fig. 9 yes Figure 7 Section 2-2 in .

[0031] Legend:

[0032] 1. Silt soft soil layer; 2. Cast-in-place piles;

[0033] 3. Cement mixing pile retaining wall; 301. Cement mixing pile retaining wall outside the pit;

[0034] 302. Cement mixing pile retaining wall in the pit; 3021. Quasi-rectangular cement mixing pile retaining wall; 30211. Outer rectangular cement mixing pile retaining wall; 30212. Inner rectangular cement mixing pile retaining wall; 30213. Special-shaped cement mixing pile retaining wall; 302131. H-shaped cement mixing pile retaining wall; 302132. Quasi-full-area cement mixing pile retaining wall; 302133. I-shaped cement mixing pile retaining wall;

[0035] 3022, straight support cement mixing pile retaining wall;

[0036] 4. Steel sheet pile retaining wall;

[0037] 5. Steel tube internal support; 501. Straight steel tube internal support; 5011. Steel corbel; 5012. Steel purlin; 5013. Steel tube support rod; 502. Inclined steel tube internal support;

[0038] 6. Bottom concrete. DETAILED DESCRIPTION

[0039] The embodiments of the present invention are 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.

[0040] Reference Figure 1-9 The preferred embodiment of the present invention provides a deep foundation pit support system and construction method in a silt soft soil environment, comprising the following steps:

[0041] Construction preparation: learn the ground distribution at the cap to be constructed and construct the cast-in-place piles 2 at the bottom of the cap;

[0042] Construction scheme design and feasibility assessment: prepare construction schemes and use soil mechanics formulas and computer software to model, analyze and verify to assess whether the stability of the deep foundation pit support system meets the regulatory requirements;

[0043] Design and construction of silt soft soil support and reinforcement: Design the structure used to support and reinforce silt soft soil in the deep foundation pit support system, construct a steel sheet pile retaining wall 4 extending along the perimeter of the deep foundation pit to support the silt soft soil, and construct an outer pit cement mixing pile retaining wall 301 arranged close to the outer periphery of the steel sheet pile retaining wall 4 and an inner pit cement mixing pile retaining wall 302 located below the deep foundation pit to cooperate to reinforce the silt soft soil;

[0044] Excavation and internal support construction in the foundation pit: Excavation is carried out in the steel sheet pile retaining wall 4 and the excavated foundation pit is supported by the steel pipe internal support 5;

[0045] Construction in the pit and conversion of the support system: pour the bottom concrete 6 at the bottom of the excavated deep foundation pit, and pour the pedestal on the bottom concrete 6 and remove the steel pipe inner support 5 during the pouring process.

[0046] In the construction method of the present invention, when performing the step "construction scheme design and feasibility assessment", the soil pressure is calculated by 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 requirements of the specification, thereby providing scientific theory and technical support for the construction of the bridge foundation pit project; in the construction method of the present invention, after the bottom concrete 6 is poured in the step "construction in the pit and conversion of the support system", the support system in the deep foundation pit is as follows: Figure 3 , Figure 8-9 As shown, it includes: bottom concrete 6, cement mixing pile retaining wall 302 inside the foundation pit, cement mixing pile retaining wall 301 outside the foundation pit, steel sheet pile retaining wall 4 and steel pipe inner support 5. Compared with the traditional foundation pit support method, the support system of the present invention can effectively enhance the rigidity of the deep foundation pit support system and resist the sliding and collapse of the soil body on the side walls around the foundation pit. In the construction method of the present invention, after the bottom concrete 6 is poured in the step "construction in the pit and conversion of the support system", as shown in FIG. Figure 3 , Figure 8-9 As shown, 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 at the bottom of the foundation pit, the steel sheet pile retaining wall 4 and the cement mixing pile retaining wall 301 outside the foundation pit. Compared with the traditional foundation pit support method, the structure of the present invention can effectively prevent groundwater from seeping into the foundation pit or construction sewage in the foundation pit from seeping into the nearby river outside the foundation pit and polluting the river water quality, thereby improving the water-stopping performance of the silt soft soil layer 1; in the construction method of the present invention, after the bottom sealing concrete 6 in the step "construction in the pit and conversion of the support system" is poured, as shown in the figure Figure 3 , Figure 8-9 As shown, a rectangular "frame" type rigid anti-floating structure is formed by the cement mixing pile retaining wall 302 on the inner side of the foundation pit and the bottom sealing concrete 6 at the bottom of the foundation pit. Compared with the traditional foundation pit support method, the structure of the present invention can effectively increase the weight of the bottom sealing component of the foundation pit, prevent the silt and soft soil at the bottom of the foundation pit from uplifting and water gushing accidents, thereby improving the anti-uplift safety factor of the silt and soft soil at the bottom of the foundation pit.

[0047] Optionally, step “S1: construction preparation” specifically includes:

[0048] 1. The strata distribution of the cap: According to the Engineering Geological Survey Report of the bridge and the strata exposed by the detailed exploration drilling position (GSZK27) of the 7# main pier cap: the strata distribution at the position of -21.59m below the ground surface are silty clay layer, medium sand layer, silty silty clay layer, silty silt sand layer, silty silty clay layer and other silty soft soil layers 1; the original ground elevation of the 7# main pier cap of the bridge is 4.2m, the cap foundation pit area is 1531.2㎡ (66×23.2m), the circumference is 178.4m, the foundation pit bottom elevation is -3.3m, the foundation pit excavation depth is 7.5m, and the cap is located on the silty silty clay layer.

[0049] 2. Construction of bored piles 2 at the bottom of the pedestal: According to the design drawing of the pile foundation of the 7# main pier of the bridge, 30 bored piles 2 with a length of 128m, a pile diameter of 2.2m and C35 concrete were constructed; the construction of the 7# main pier pedestal can be carried out only after the construction quality of the bored piles 2 has been inspected and accepted.

[0050] Optionally, step "S2: construction scheme design and feasibility assessment" includes construction scheme preparation, verification, expert demonstration and approval; by using soil mechanics formula to calculate soil pressure, using MIDAS / CIVIL finite element analysis software to establish a model, analyze and verify the strength of the component, the stability of the steel pipe support rod 5013 and the foundation of the cement mixing pile retaining wall 302 in the pit, it can be known whether the stability of the deep foundation pit support system meets the requirements of the specification; the feasibility and feasibility of the construction scheme will be reviewed and demonstrated by senior experts, providing scientific theoretical and technical support for the construction of the bridge foundation pit project, specifically including the following:

[0051] S201: Preparation of construction plan: According to the project characteristics and the actual situation of the construction site, prepare the construction plan of the deep foundation pit support system. The construction content includes: design and construction of silt soft soil support reinforcement, excavation and internal support construction in the foundation pit, construction in the pit and support system conversion.

[0052] S202: Construction plan verification: Use soil mechanics formulas and MIDAS / CIVIL finite element analysis software modeling to calculate or verify the load, stability and strength of the deep foundation pit support system in the construction plan;

[0053] S203: Expert demonstration and approval of the construction plan: By using soil mechanics formulas to calculate soil pressure, using MIDAS / CIVIL finite element analysis software to establish models, analyze and verify the strength of components, the stability of steel pipe support rods 5013, and the foundation of cement mixing pile retaining wall 302 in the pit, it can be found that the stability of the deep foundation pit support system meets the requirements of the specification. The "Deep Foundation Pit Support System Construction Plan" has been centrally reviewed and demonstrated by senior experts. This plan is feasible and can be implemented. The "Deep Foundation Pit Support System Construction Plan" that has been demonstrated by experts to be feasible and implementable will be submitted to the construction unit, supervision unit, design unit and relevant departments for approval.

[0054] The step “S202: construction plan verification” specifically includes:

[0055] 1. Design conditions, including:

[0056] (1) Obtain 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 7# main pier cap is 2.5m, the ground elevation is 4.2m, the cap bottom elevation is -2.5m, and the foundation pit bottom elevation is -3.3m; and according to the detailed geological survey data of QSZK27 hole position, 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 silty clay layer of stratum No. 4, the silty silt sand layer of stratum No. 4-13, and the silty clay layer of stratum No. 4-1 The parameters of natural heavy γ are shown in Table 1.

[0057] Table 1 Technical parameters of each soil layer at QSZK27 hole position

[0058]

[0059]

[0060] (2) Material parameters: The steel used is Q235 and Q355 steel, and its quality meets the requirements of the current national standards "Carbon Structural Steel" (GB / T700) and "Low Alloy High Strength Structural Steel" (GB / T1591) respectively.

[0061] Table 2 Design values ​​of steel strength

[0062]

[0063] 2. Design of deep foundation pit support system, including:

[0064] (1) Deep foundation pit support system: Figure 2 , Figure 3As shown in the figure, the deep foundation pit support system is composed of cement mixing pile retaining wall 3 + steel sheet pile retaining wall 4 + steel pipe inner support 5 + bottom sealing concrete 6. Among them: the cement mixing pile retaining wall 3 adopts 28d unconfined compressive strength not less than 1.0MPa; the steel sheet pile retaining wall 4 adopts PU600×210×18mm steel sheet piles with a material of Q355P and a length of 18m; the steel pipe inner support 5 includes a straight steel pipe inner support 501 and an inclined steel pipe inner support 502; the bottom sealing concrete 6 is arranged on the bottom surface of the foundation pit, and its thickness is 0.8m and the strength grade is C30.

[0065] (2) Steel pipe inner support 5: Figure 2 , Figure 3 As shown, each layer of steel tube inner supports 5 includes a plurality of straight steel tube inner supports 501 and a plurality of inclined steel tube inner supports 502 , and each steel tube inner support 5 is composed of a steel corbel 5011 , a steel purlin 5012 and a steel tube support rod 5013 . The steel corbel 5011 is close to the inner side of the steel sheet pile retaining wall 4 and welded to the inner wall of the steel sheet pile retaining wall 4 to form a triangular support frame; the steel purlin 5012 is close to the inner side of the steel sheet pile retaining wall 4 on the four sides of the foundation pit; the straight steel pipe inner support 501 is set between the steel purlins 5012 on both sides of the relative length in the foundation pit, and the flexible ends and fixed ends at both ends of the steel pipe support rod 5013 are connected to the steel purlins 5012 on both sides; the inclined steel pipe inner support 502 is set between the steel purlins 502 on the adjacent length side and width side in the foundation pit, and the flexible ends and fixed ends at both ends of the steel pipe support rod 5013 are connected to the steel purlins 502 on the adjacent length side and width side at a 45° angle. Among them:

[0066] 1) In the vertical direction of the foundation pit, the center of the first layer of steel tube 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 tube inner support 5 is 4000mm away from the center of the first layer of steel tube inner support 5, and the center of the second layer of steel tube inner support 5 is 2300mm away from the bottom of the foundation pit.

[0067] 2) The first-layer steel purlin 5012 adopts Q235, 3Ⅰ56a I-steel, the steel pipe used in the straight steel tube inner support 501 is φ630*10mm steel pipe, and the steel pipe used in the inclined steel tube inner support 502 is φ480*8mm steel pipe; the second-layer steel purlin 5012 adopts Q235, 3HN700*300 H-steel, the steel pipe used in the straight steel tube inner support 501 is φ1000*12mm steel pipe, and the steel pipe used in the inclined steel tube inner support 502 is φ480*8mm steel pipe. The steel bracket 5011 adopts I-beam of Ⅰ25a; the steel pipe support rod 5013 consists of two parts: the support rod system and the auxiliary components. The support rod system includes: the steel pipe main rod, the flexible end, and the fixed end. The auxiliary components include: flanges, high-strength bolts and steel wedges. The steel pipe main rod is assembled and connected by multiple sections of steel pipes, and the multiple sections of steel pipes are connected by flanges and high-strength bolts. The center elevations of the first and second layer steel pipe internal supports 5 are 3.0m and -1.0m from top to bottom.

[0068] 3) The horizontal distance between the centers of the three straight steel tube internal supports 501 in the middle of the length side of the foundation pit is 5250mm, and the horizontal distance between the centers of the remaining three straight steel tube internal supports 501 on both sides is 5500mm. The center distance of the outermost straight steel tube internal support 501 to the inner side of the steel sheet pile retaining wall 4 on the width side of the foundation pit is 11250mm, and two inclined steel tube internal supports 502 are respectively set at the four corners in this area.

[0069] 4) On the adjacent length side and width side in the foundation pit, the first inclined steel tube inner support 502 is set at a position 4950mm away from the intersection of the length side and the width side; the second inclined steel tube inner support 502 is set at a position 11100mm away from the intersection of the length side and the width side. The two inclined steel tube inner supports 502 intersect with the adjacent length side and the width side at an angle of 45°, and the center of the first inclined steel tube inner support 502 is 4345mm away from the center of the second inclined steel tube inner support 502.

[0070] 3. Load calculation

[0071] (1) Calculation of earth pressure acting on the supporting structure:

[0072] The standard values ​​of active earth pressure strength and passive earth pressure strength acting on the outside and inside of the supporting structure are calculated by two methods: water-soil combined calculation and water-soil separate calculation. The standard values ​​of active earth pressure strength P of the calculation points in each soil layer outside and inside the supporting structure are calculated. ak , Standard value of passive earth pressure strength P pk , as follows:

[0073] 1) Water and soil combination algorithm:

[0074] According to the cohesion c (kPa), internal friction angle of each soil layer in Table 1 and σ calculated by formula (9) and formula (10) ak , σ pk According to formula (1), formula (2), formula (3) and formula (4), the standard value of active earth pressure strength P at the calculation point in each soil layer outside and inside the supporting structure is calculated. ak , Standard value of passive earth pressure strength P pk .

[0075]

[0076] Where:

[0077] P ak - Standard value of active earth pressure strength at the calculation point in the i-th soil layer outside the supporting structure (kPa);

[0078] P pk - Standard value of passive earth pressure strength at the calculation point in the i-th soil layer inside the supporting structure (kPa);

[0079] σ ak , σ pk - are the standard values ​​of vertical stress in the soil at the calculation points on the outside and inside of the supporting structure (kPa), calculated by formula (9) and formula (10);

[0080] K a，i , K p，i - are respectively the active earth pressure coefficient and passive earth pressure coefficient of the i-th soil layer;

[0081] C i , - The cohesion (kPa) and internal friction angle (°) of the i-th layer of soil are calculated according to the cohesion c (kPa) and internal friction angle of each soil layer in Table 1. Get the value.

[0082] 2) Water and soil classification method:

[0083] According to the internal friction angle of each soil layer in Table 1 Formula (2) and formula (4) can be used to calculate the active earth pressure coefficient K a，i , Passive earth pressure coefficient K p，i , and σ calculated by formula (9) and formula (10) ak , σ pk According to formula (5) and formula (6), the standard value of active earth pressure strength P of the calculation point in each soil layer outside and inside the supporting structure is calculated. ak , Standard value of passive earth pressure strength P pk .

[0084]

[0085] Where:

[0086] P ak - Standard value of active earth pressure strength at the calculation point in the i-th soil layer outside the supporting structure (kPa);

[0087] P pk - Standard value of passive earth pressure strength at the calculation point in the i-th soil layer inside the supporting structure (kPa);

[0088] σ ak , σ pk - are the standard values ​​of vertical stress in the soil at the calculation points on the outside and inside of the supporting structure (kPa), calculated by formula (9) and formula (10);

[0089] K a，i , K p，i - are respectively the active earth pressure coefficient and passive earth pressure coefficient of the i-th soil layer;

[0090] C i -The cohesion of the i-th soil layer (kPa), which is taken according to the cohesion c (kPa) of each soil layer in Table 1;

[0091] u a 、u p - are the water pressures (kPa) at the calculation points on the outside and inside of the supporting structure, respectively, calculated using formulas (7) and (8).

[0092] (2) Calculation of water pressure of static groundwater:

[0093] According to the layer 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 strength calculation point can be determined. wa , the vertical distance h from the groundwater level inside the foundation pit to the passive earth pressure strength calculation point wp , and the gravity of groundwater γ w Take 10kN / m3 and calculate the water pressure u at the calculation points on the outside and inside of the supporting structure according to formula (7) and formula (8): a 、u p :

[0094] u a =γ w h wa (7)

[0095] u p =γ w h wp (8)

[0096] Where:

[0097] u a 、u p - The water pressure at the calculation points on the outside and inside of the supporting structure respectively (kPa);

[0098] γ w -The density of groundwater (kN / m3), take 10kN / m3;

[0099] h wa - The vertical distance (m) from the groundwater level outside the foundation pit to the active earth pressure strength calculation point is determined according to the layer bottom elevation and normal water level elevation in Table 1;

[0100] h wp -The vertical distance (m) from the groundwater level inside the foundation pit to the passive earth pressure strength calculation point is determined according to the layer bottom elevation and normal water level elevation in Table 1.

[0101] (3) Calculation of standard value of vertical stress in soil:

[0102] According to the natural weight γ of each soil layer in Table 1 and the depth of each soil layer calculated from the bottom elevation of the layer, the vertical total stress σ at the calculation points outside and inside the supporting structure can be calculated: ac , σ pc , and the standard value of additional vertical stress Δσ in the soil at the calculation point under the jth additional load outside the supporting structure is calculated by formula (11): k，j According to formula (9) and formula (10), the standard value of vertical stress in the soil at the calculation points outside and inside the supporting structure is calculated: ak , σ pk :

[0103] σ ak =σ ac +∑Δσ k，j (9)

[0104] σ pk =σ pc (10)

[0105] Where:

[0106] σ ak , σ pk - are the standard values ​​of vertical stress in the soil at the calculation points on the outside and inside of the supporting structure (kPa);

[0107] σ ac - Total vertical stress (kPa) caused by the deadweight of the soil at the calculation point outside the supporting structure;

[0108] σ pc - Total vertical stress (kPa) caused by the deadweight of the soil at the calculation point inside the supporting structure;

[0109] Δσk，j -The standard value of additional vertical stress in the soil at the calculation point under the jth additional load outside the supporting structure (kPa), calculated by formula (11).

[0110] (4) Calculation of standard value of additional vertical stress in soil under uniform additional load:

[0111] According to the "Highway Bridge and Culvert Foundation Design Code" (JTG3363), the standard uniformly distributed load of road bridges is generally between 10kN / m2 and 20kN / m2, so q0 is taken as 20kN / m2. The standard value of the additional vertical stress Δσ in the soil at the calculation point under the action of the jth additional load outside the supporting structure is calculated according to formula (11): k，j .

[0112] Δσ k，j =q0 (11)

[0113] Where:

[0114] Δσ k，j -Standard value of additional vertical stress in the soil at the calculation point under the jth additional load outside the supporting structure (kPa)

[0115] q0-standard value of uniform additional load (kPa), take 20kN / m 2 .

[0116] (5) Vehicle load: Based on the actual load, the effect on the foundation pit support structure is considered as a local additional load and reflected in the soil pressure calculation.

[0117] 4. Analysis of construction content and adverse working conditions: Analysis of construction content and adverse working conditions is shown in Table 3.

[0118] Table 3 Analysis of construction content and adverse working conditions

[0119]

[0120] 5. Component strength verification, including:

[0121] (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 components, elastic supports were used to simulate soil springs, and general supports were used to simulate constraints at the bottom of the steel sheet pile retaining wall. Figure 4 shown.

[0122] (2) Component strength verification

[0123] 1) Component strength verification for unfavorable working condition 1:

[0124] Strength verification of steel sheet pile retaining wall 4: the wall thickness of the steel sheet pile is 18mm, the material is Q355P, the maximum combined stress is 34.3MPa<295MPa, which meets the requirements.

[0125] 2) Component strength verification for unfavorable working condition 2:

[0126] Strength calculation of steel sheet pile retaining wall 4: the wall thickness of the steel sheet pile is 18mm, the material is Q355P, the maximum combined stress is 83MPa<295MPa, which meets the requirements; strength calculation of steel purlin 5012: the purlin adopts 3Ⅰ56a I-steel, the material is Q235, the maximum combined stress is 85MPa<205MPa, the maximum shear stress is 23MPa<120MPa, which meets the requirements; strength calculation of φ630mm steel pipe support rod 5013: the wall thickness of the steel pipe is 10mm, the material is Q235, the maximum combined stress is 82MPa<215MPa, which meets the requirements; strength calculation of φ480mm steel pipe support rod 5013: the wall thickness of the steel pipe is 8mm, the material is Q235, the maximum combined stress is 18MPa<215MPa, which meets the requirements.

[0127] 3) Component strength verification for unfavorable working condition 3:

[0128] Strength calculation of steel sheet pile retaining wall 4: Steel sheet pile wall thickness 18mm, material Q355P, maximum combined stress 209MPa<295MPa, meet the requirements; 3Ⅰ56a steel purlin 5012 strength calculation: The purlin adopts 3Ⅰ56a I-steel, material Q235, maximum combined stress 55MPa<205MPa, maximum shear stress 7MPa<120MPa, meet the requirements; 3HN700*300mm steel purlin 5012 strength calculation: The purlin adopts 3HN700*300mm H-steel, material Q235, maximum combined stress 165MPa<205MPa, The maximum shear stress is 91.1MPa<120MPa, which meets the requirements; strength calculation of φ630mm steel pipe support rod 5013: steel pipe wall thickness is 10mm, material is Q235, maximum combined stress is 76MPa<215MPa, which meets the requirements; strength calculation of φ480mm steel pipe support rod 5013: steel pipe wall thickness is 8mm, material is Q235, maximum combined stress is 24MPa<215MPa, which meets the requirements; strength calculation of φ1000mm steel pipe support rod 5013: steel pipe wall thickness is 12mm, material is Q235, maximum combined stress is 141MPa<215MPa, which meets the requirements.

[0129] 4) Component strength verification for unfavorable working condition 4:

[0130] Strength calculation of steel sheet pile retaining wall 4: the wall thickness of the steel sheet pile is 18mm, the material is Q355PQ345, the maximum combined stress is 102MPa<295MPa, which meets the requirements; strength calculation of steel purlin 5012: the purlin adopts 3Ⅰ56a I-steel, the material is Q235, the maximum combined stress is 68MPa<205MPa, the maximum shear stress is 17MPa<120MPa, which meets the requirements; strength calculation of φ630mm steel pipe support rod 5013: the wall thickness of the steel pipe is 10mm, the material is Q235, the maximum combined stress is 89MPa<215MPa, which meets the requirements; strength calculation of φ480mm steel pipe support rod 5013: the wall thickness of the steel pipe is 8mm, the material is Q235, the maximum combined stress is 18MPa<215MPa, which meets the requirements.

[0131] 6. Calculation of steel pipe support rod stability:

[0132] (1) Strength verification of φ1000*12mm steel pipe support rod 5013, calculated according to formula (12): meets the requirements.

[0133]

[0134] Where:

[0135] N-design value of axial pressure within the calculation component range (N);

[0136] -Stability coefficient of axially compressed members;

[0137] A-cross-sectional area (mm);

[0138] f- steel design strength (MPa);

[0139] E-elastic modulus of steel (MPa);

[0140] β-equivalent bending moment coefficient, which is taken as 1.0 according to the specification;

[0141] M-the maximum design value of the bending moment within the calculation component range (N·mm);

[0142] γ m - Section plastic development coefficient, take 1.15;

[0143] W-section modulus (mm 3 );

[0144] N' EX -Parameters, press Calculation (mm).

[0145] The values ​​of each parameter are shown in Table 4:

[0146]

[0147] (2) Strength verification of φ630*10mm steel pipe support rod 5013, calculated according to formula (13): meets the requirements.

[0148]

[0149] Where:

[0150] N-design value of axial pressure within the calculation component range (N);

[0151] -Stability coefficient of axially compressed members;

[0152] A-cross-sectional area (mm);

[0153] f- steel design strength (MPa);

[0154] E-elastic modulus of steel (MPa);

[0155] β-equivalent bending moment coefficient, which is taken as 1.0 according to the specification;

[0156] M-the maximum design value of the bending moment within the calculation component range (N·mm);

[0157] γ m - Section plastic development coefficient, take 1.15;

[0158] W-section modulus (mm 3 )

[0159] N' EX -Parameters, press Calculation (mm).

[0160] The values ​​of each parameter are shown in Table 5:

[0161]

[0162] 7. Verification of the foundation of the retaining wall with cement mixing piles in the pit

[0163] Since the original soil at the location of the bridge's 7# main pier cap is relatively weak, cement mixing piles are used to reinforce the silt soft soil to form a cement mixing pile retaining wall 3 structure. From an economical perspective, in order to fully utilize the excess force of the supporting structure, the silt soft soil reinforcement structure is optimized and designed. The silt soft soil reinforcement structure includes: a cement mixing pile retaining wall 301 outside the pit and a cement mixing pile retaining wall 302 inside the pit with the steel sheet pile retaining wall 4 as the boundary.

[0164] Among them: the cement mixing pile enclosure wall 301 outside the pit is a rectangular cement mixing pile enclosure wall with a thickness of 0.6m extending outwards from the steel sheet pile enclosure wall 4; the cement mixing pile enclosure wall 302 inside the pit is a quasi-rectangular cement mixing pile enclosure wall 3021 with a certain thickness extending inwards from the steel sheet pile enclosure wall 4, and 5 parallel straight support cement mixing pile enclosure walls 3022 with a width of 1.5m (three rows), a horizontal adjacent spacing of 2.5m, and a pile length of 6.0m are arranged in the middle position of the length side of the foundation pit. Figure 7 shown.

[0165] In the following, the entity modeling method is used 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 is sealed.

[0166] (1) Computational model

[0167] Using MIDAS / CIVIL finite element analysis software, a solid model is established to verify the foundation of the cement mixing pile retaining wall: the solid unit is used to simulate the structure of the cement mixing pile retaining wall 302 in the pit, the elastic support is used to simulate the soil spring, and the general support is used to simulate the constraint at the bottom of the cement mixing pile retaining wall 3, such as Figure 5 shown.

[0168] (2) Unfavorable working conditions before concrete bottom sealing

[0169] The first layer of steel pipe inner support 5 and the second layer of steel pipe inner 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 bottom concrete 6, the structure of the cement mixing pile retaining wall 302 in the pit is simulated as the third layer of concrete inner support structure, and the corresponding active earth pressure is applied to calculate and analyze the structure of the cement mixing pile retaining wall 302 in the pit, and the lateral force of the cement mixing pile retaining wall 302 in the pit is calculated.

[0170] Structural strength verification of the in-pit cement mixing pile retaining wall 302: The maximum combined stress of the in-pit cement mixing pile retaining wall 302 is 1.05MPa, and the wall strength of the in-pit cement mixing pile retaining wall 302 is not less than 1.2MPa.

[0171] (3) Unfavorable working conditions after concrete bottom sealing

[0172] The bottom sealing concrete 6 is poured on the bottom surface of the foundation pit in the steel sheet pile retaining wall 4. Under the combined effect of the deadweight of the bottom sealing concrete 6 and the active side pressure, the structure of the cement mixing pile retaining wall 302 in the pit is calculated and analyzed.

[0173] Structural strength verification of the in-pit cement mixing pile retaining wall 302: The maximum combined stress of the in-pit cement mixing pile retaining wall 302 is 1.3MPa, and the wall strength of the in-pit cement mixing pile retaining wall 302 is not less than 1.5MPa.

[0174] (4) Through the calculation and analysis of the two most unfavorable working conditions of the in-pit cement mixing pile retaining wall 302 before and after the concrete bottom is sealed, it can be seen that the optimized in-pit cement mixing pile retaining wall 302 structure meets the requirements of foundation pit support construction.

[0175] Optionally, the step “silt soft soil support reinforcement design and construction” specifically includes:

[0176] S3: Structural design and construction of steel sheet pile retaining wall 4;

[0177] S4: Structural design and construction of cement mixing pile retaining wall 3.

[0178] In this optional solution, step “S3: structural design and construction of steel sheet pile retaining wall 4” specifically includes:

[0179] S301: Structural design and quality requirements of steel sheet pile retaining wall 4; specifically, it includes:

[0180] 1. Steel sheet pile retaining wall 4 structure design: Figure 6 The steel sheet pile retaining wall 4 adopts PU600×210×18mm steel sheet piles with a material of Q355P and a length of 18m; the steel sheet piles are composed of a web, a wing plate and a lock, wherein: one end of the wing plate is welded to the lock, and the other end is welded to any end of the web to form a "U" shape, the lock end is arranged away from the web, and the opening direction of the steel sheet pile is opposite to the opening direction of the lock; the reverse "U-shaped protrusion" design of the locks on both sides of each steel sheet pile can be used to chain adjacent steel sheet piles to form an interlocking structure, and a watertight structure is formed when the steel sheet piles are interlocked, thereby increasing the strength of the steel sheet pile retaining wall 4 structure.

[0181] 2. Quality requirements: The quality of steel sheet piles shall comply with the requirements of the "Hot-rolled Steel Sheet Piles" (GB / T20933) standard. If there are damaged steel sheet piles or those with improper locks, they must be corrected to make the size correct and the lock mouth smooth without bends. The quality inspection standard for smooth lock mouth is to use a piece of steel sheet pile with a length of 1m to cover each group of steel sheet piles. Those that pass the inspection smoothly are considered qualified.

[0182] S302: Construction of steel sheet pile retaining wall 4: including construction preparation, site leveling and excavation of guide trenches, and steel sheet pile driving construction. Specifically, it includes:

[0183] 1. Construction preparation: According to the construction drawing design of the 7# main pier cap of this bridge, the engineering and technical personnel will mark out and determine the construction position of the steel sheet pile retaining wall 4 at the designed position of the foundation pit of the 7# main pier cap at the construction site.

[0184] 2. Site leveling and excavation of guide trenches: Use an excavator to level the site to ensure the walking path and construction conditions of the steel sheet pile driving machine; drive positioning piles, and use an excavator to excavate a guide trench with a width of 1.0m and a depth of 0.5m along the layout line of the steel sheet pile retaining wall 4 of the foundation pit; the guide trench is excavated as it is driven during construction.

[0185] 3. Steel sheet pile driving construction: Use a steel sheet pile driving machine to hoist a steel sheet pile into the guide groove, and then drive the steel sheet pile vertically downward into the silty clay layer below the deep foundation pit as the starting point for steel sheet pile driving; then, along the layout route of the foundation pit steel sheet pile retaining wall 4, complete four groups of several steel sheet piles arranged in a straight line in a head-to-tail manner and driven in sequence, and lock and connect them with the tail ends of the steel sheet piles at the driving starting point to form a rectangular steel sheet pile retaining wall 4 with a length of 66m and a width of 23.2m, to resist the sliding, collapse and instability of the soil on the side walls around the foundation pit, and make the rear steel sheet pile and the previous steel sheet pile staggered in relative direction and connected head-to-tail, lock and engage with each other, and make the "U"-shaped opening direction of the rear steel sheet pile opposite to the "U"-shaped opening direction of the previous steel sheet pile.

[0186] After the steel sheet piles are driven in, the steel sheet pile retaining wall 4 is in a straight line shape, which prevents the steel sheet pile retaining wall 4 from being in an "S" shape, causing the steel sheet pile lock to be unfastened or disengaged, resulting in water seepage in the steel sheet pile retaining wall 4 and collapse and instability of the foundation pit soil, preventing groundwater from seeping into the foundation pit or construction sewage in the foundation pit from seeping into the nearby river outside the foundation pit and polluting the river water quality.

[0187] Among them: when inserting the last steel sheet pile, align the lock of the last steel sheet pile with the lock of the previous steel sheet pile, and after they are engaged with each other, turn on the vibrating hammer to gently hammer the last steel sheet pile first, so that the last steel sheet pile is 1 to 2 meters into the soil, and then observe the vertical verticality and horizontal position of the last steel sheet pile. After passing, continue to hammer the next steel sheet pile into the soil until the set depth. During the insertion process, observe the vertical verticality and plane position deviation of the steel sheet pile while inserting, and use two total stations (or line hammers) to control the vertical verticality and plane position deviation of the steel sheet pile in two directions. After insertion, the top elevation of the steel sheet pile retaining wall 4 is 4.7m, and the bottom elevation of the steel sheet pile retaining wall 4 is -13.3m. The vertical verticality of the steel sheet pile is ≤1%, and the plane position deviation is ≤15cm.

[0188] In this optional solution, step “S4: design and construction of cement mixing pile retaining wall structure” specifically includes:

[0189] S401: Structural design of cement mixing pile retaining wall and analysis of the principle of silt soft soil reinforcement, and S402: Construction of cement mixing pile retaining wall; wherein, step “S401: Structural design of cement mixing pile retaining wall and analysis of the principle of silt soft soil reinforcement” specifically includes:

[0190] 1. Cement mixing pile retaining wall structure design: Figure 2 , Figure 3 , Figure 7 , Figure 8 , Fig. 9 As shown, the silt soft soil is reinforced by cement mixing piles. After reinforcement, the internal friction angle of the silt soft soil can reach more than 15° and the cohesion can reach more than 20kPa. The silt soft soil reinforcement structure adopts an interlocking cement mixing pile retaining wall 3 with a pile diameter of 0.60m, a pile center spacing of 0.45m, and a pile overlap of 0.15m, including an outer pit cement mixing pile retaining wall 301 outside the foundation pit with the steel sheet pile retaining wall 4 as the boundary and an inner pit cement mixing pile retaining wall 302 inside the foundation pit. The bottom elevation of the outer pit cement mixing pile retaining wall 301 is lower than that of the inner pit cement mixing pile retaining wall. The top elevation of the retaining wall 302 is 0.5m, and it overlaps 0.5m in the vertical direction of the foundation pit, forming a rectangular "sleeve" type silt soft soil reinforcement structure, which weakens the stress concentration of the steel sheet pile retaining wall 4 at the junction of the foundation pit bottom and the silt 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, and forms a rectangular "water cup" type waterproof structure with the steel sheet pile retaining wall 4 and the bottom sealing concrete 6 of the foundation pit bottom, preventing groundwater from seeping into the foundation pit or construction sewage in the foundation pit from seeping into the nearby river outside the foundation pit and polluting the river water quality, thereby improving the water stopping performance of the silt soft soil layer 1. Among them:

[0191] (1) Cement mixing pile retaining wall outside the pit 301: Figure 2 , Figure 3 , Figure 7 , Figure 8 , Fig. 9 As shown, on the top surface of the silt soft soil layer 1 outside the steel sheet pile retaining wall 4 on the four sides of the foundation pit, facing the outside of the foundation pit, a row of rectangular cement mixing pile retaining walls 301 (width 0.60m) and 8.0m in length are constructed outside the pit, abutting 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 plane position deviation is ≤50mm.

[0192] (2) Cement mixing pile retaining wall 302 in the pit: Figure 3 , Figure 7 , Figure 8 , Fig. 9As shown, the cement mixing pile enclosure wall 302 in the pit includes a quasi-rectangular cement mixing pile enclosure wall 3021 and a straight-support cement mixing pile enclosure wall 3022; on the top surface of the silt soft soil layer 1 inside the steel sheet pile enclosure wall 4 on the four sides of the foundation pit, facing the center of the foundation pit, a quasi-rectangular cement mixing pile enclosure wall 3021 and a straight-support cement mixing pile enclosure wall 3022 with a pile length of 6.0m are constructed, abutting the inner side of the steel sheet pile enclosure wall 4; the top elevation of the cement mixing pile enclosure wall 302 in the pit is -3.3m, and the bottom elevation is -9.3m; the 28d unconfined compressive strength of the cement mixing pile enclosure wall 302 in the pit is ≥1.0MPa, the vertical deviation is ≤+1%, and the plane position deviation is ≤50mm. Among them:

[0193] 1) Rectangular cement mixing pile retaining wall 3021: Figure 7 As shown, on the top surface of the silt soft soil layer 1 inside the steel sheet pile retaining walls 4 on the four sides of the foundation pit, facing the center side of the foundation pit, a quasi-rectangular cement mixing pile retaining wall 3021 with a pile length of 6.0m is constructed; the quasi-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 a special-shaped cement mixing pile retaining wall 30213; among which: the special-shaped cement mixing pile retaining wall 30213 includes: an H-shaped cement mixing pile retaining wall 302131, a quasi-full-area cement mixing pile retaining wall 302132 and a straight-line cement mixing pile retaining wall 302133.

[0194] 2) Straight support cement mixing pile retaining wall 3022: Figure 7 As shown, on the top surface of the silt soft soil layer 1 inside the steel sheet pile retaining wall 4 at the middle position on the length side of the foundation pit, facing the center side of the foundation pit, five parallel straight-supported cement mixing pile retaining walls 3022 with a width of 1.5m (three rows), a lateral adjacent spacing of 2.5m, and a pile length of 6.0m are constructed, and their two ends respectively abut the outer rectangular cement mixing pile retaining wall 30211, the I-shaped cement mixing pile retaining wall 302133 and the inner rectangular cement mixing pile retaining wall 30212 on the inner side of the foundation pit.

[0195] 2. Analysis of the reinforcement principle of silt soft soil:

[0196] (1) Setting up a silt soft soil outer layer reinforcement structure: The silt soft soil outer layer reinforcement adopts a cement mixing pile reinforcement method. By constructing an outer cement mixing pile retaining wall 301 on the top surface of the silt soft soil layer 1 outside the steel sheet pile retaining wall 4 on the four sides of the foundation pit, the top elevation of the outer cement mixing pile retaining wall 301 is made 0.5m lower than the top elevation of the steel sheet pile retaining wall 4. This prevents objects such as stones on the top of the outer cement mixing pile retaining wall 301 and the ground from falling into the foundation pit and injuring construction operators, 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. The silt soft soil outer layer reinforcement structure can make the internal friction angle of the silt soft soil body 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 soft soil body, and resisting the sliding, collapse and instability of the soil body on the side walls around the foundation pit.

[0197] (2) Setting up a rectangular "sleeve" type silt soft soil reinforcement structure and a rectangular "water cup" type waterproof structure

[0198] By constructing a quasi-rectangular cement mixing pile retaining wall 3021 on the top surface of the silt soft soil layer 1 inside the steel sheet pile retaining wall 4 on the four sides of the foundation pit toward the center of the foundation pit, the top elevation of the quasi-rectangular cement mixing pile retaining wall 3021 is made 0.5m higher than the bottom elevation of the cement mixing pile retaining wall 301 outside the pit, and the quasi-rectangular cement mixing pile retaining wall 3021 is overlapped by 0.5m in the vertical direction of the foundation pit to form a rectangular "sleeve" type silt soft soil reinforcement structure, so as to weaken the stress concentration of the steel sheet pile retaining wall 4 at the junction of the foundation pit bottom and the silt soft soil layer, and strengthen the steel sheet pile retaining wall 4 Stiffness, thereby enhancing the stiffness of the deep foundation pit support system and resisting the sliding, collapse and instability of the soil on the side walls around the foundation pit; the rectangular cement mixing pile retaining wall 3021 and the straight support cement mixing pile retaining wall 3022, the bottom sealing concrete 6 of the foundation pit, the steel sheet pile retaining wall 4 and the cement mixing pile retaining wall 301 outside the pit form a rectangular "water cup" type waterproof structure, which prevents groundwater from seeping into the foundation pit or construction sewage in the foundation pit from seeping into the nearby river outside the foundation pit and polluting the river water quality, thereby improving the water-stopping performance of the silt soft soil layer 1.

[0199] (3) Setting up a rectangular "frame" type rigid anti-floating structure

[0200] Five 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 on the length side of the foundation pit, facing the center side of the foundation pit. The walls are 1.5 m wide (three rows), 2.5 m apart in the horizontal direction, and 6.0 m long. The two ends of the walls are respectively abutted against the outer rectangular cement mixing pile retaining wall 30211, the I-shaped cement mixing pile retaining wall 302133 and the inner rectangular cement mixing pile retaining wall 30212 on the inner side of the foundation pit. The straight support cement mixing pile retaining wall 3022, the quasi-rectangular cement mixing pile retaining wall 3021, and the bottom sealing concrete 6 in the foundation pit form a rectangular "frame" type rigid anti-floating structure, which effectively increases the weight of the bottom sealing components of the foundation pit, prevents the uplift and water gushing accidents of the silt and soft soil at the bottom of the foundation pit, thereby improving the anti-uplift safety factor of the silt and soft soil at the bottom of the foundation pit; and then forms a steel tube concrete support system with the quasi-rectangular cement mixing pile retaining wall 3021, the bottom sealing concrete 6 in 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 inner support 5, and the second layer of steel pipe inner support 5, thereby enhancing the rigidity of the deep foundation pit support system and resisting the sliding, collapse and instability of the soil on the side walls around the foundation pit.

[0201] In this optional solution, step "S402: construction of cement mixing pile retaining wall 3" specifically includes:

[0202] 1. Construction design of cement mixing pile retaining wall, such as Figure 2 , Figure 3 , Figure 7-9 As shown, specifically including:

[0203] (1) Cement mixing pile retaining wall 301 outside the pit: On the top surface of the silt soft soil layer 1 outside the steel sheet pile retaining wall 4 on the four sides of the foundation pit, 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 towards the outside of the foundation pit, abutting the outside of the steel sheet pile retaining wall 4.

[0204] (2) In-pit cement mixing pile retaining wall 302: On the top surface of the silt soft soil layer 1 inside the steel sheet pile retaining wall 4 on the four sides of the foundation pit, facing the center of the foundation pit, an in-pit cement mixing pile retaining wall 302 with a pile length of 6.0 m is constructed, abutting the inner side of the steel sheet pile retaining wall 4; the construction of the in-pit cement mixing pile retaining wall 302 includes: a quasi-rectangular cement mixing pile retaining wall 3021 and a straight-supported cement mixing pile retaining wall 3022; the construction of the quasi-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 a special-shaped cement mixing pile retaining wall 30213; the special-shaped cement mixing pile retaining wall 30213 inside the foundation pit includes: an H-shaped cement mixing pile retaining wall 302131, a quasi-full-area cement mixing pile retaining wall 302132 and a straight-shaped cement mixing pile retaining wall 302133.

[0205] (3) Cement mixing pile retaining wall 3: The cement mixing pile retaining wall 3 uses interlocking cement mixing piles with a pile diameter of 0.60 m, a pile center spacing of 0.45 m, and a pile overlap of 0.15 m. A cement slurry mixing station and a storage tank are set up near the construction site of the bridge's 7# main pier cap. After the cement slurry is mixed, it is pumped into the grouting pipe of the cement mixing pile drilling rig by a slurry hose. Two cement mixing pile drilling rigs are equipped at the construction site. Cement slurry composed of cement, silica fume, nano-silicon dioxide, talcum powder, early strength water reducing agent, and mixing water with a water-cement ratio of 0.60 is prepared on site. The cement mixing pile retaining wall 3 is constructed with silt soft soil reinforced at a volume ratio of silt soft soil to cement slurry of 1:0.30.

[0206] The purpose of setting up high-performance cement mixing pile retaining wall 3:

[0207] The cement slurry prepared by cement, silica fume, nano silicon dioxide, talcum powder, early strength water reducing agent and mixing water is sprayed and pressed into the silt soft soil layer 1; the cement slurry is mixed, squeezed and filled with pores between silt soft soil particles by a cement mixing pile drilling rig according to the volume ratio of silt soft soil to cement slurry of 1:0.30, and 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; 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 silt soft soil layer 1 are solved, the rigidity of the deep foundation pit support system, the anti-uplift safety factor of the silt soft soil body at the bottom of the foundation pit and the water-stopping performance of the silt soft soil layer 1 are improved, and the stability of the foundation pit of the cap is improved.

[0208] (4) Construction method of cement mixing pile retaining wall 3: Figure 2 , Figure 3 , Figure 7-9 As shown, the cement mixing piles adopt a "jump one hit one" jumping pile construction method to complete the cement mixing piles outside the foundation pit and the cement mixing piles inside the foundation pit one by one, and then connect the cement mixing piles outside the foundation pit and inside the foundation pit 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:

[0209] 1) Measurement and layout: In the 0.60m width area outside the steel sheet pile retaining wall 4 on the four sides of the foundation pit, the engineering and technical personnel measured and laid out to determine the drilling position of each cement mixing pile.

[0210] 2) Drilling rig in place: Start the winch to move the cement mixing pile drilling rig to the designated drilling position, use the positioning card to ensure that the cement mixing pile drilling position centering error is ≤50mm, the guide and mixing shaft must be perpendicular to the ground, and the verticality deviation is ≤1.0%.

[0211] 3) Spraying and sinking: Connect the slurry hose to the storage tank, mortar pump and cement mixing pile machine, turn on the cement mixing pile machine motor, and the mixer blades turn in opposite directions. With the help of the deadweight of the cement mixing pile machine equipment, the drill rod sinks vertically at a uniform speed of 0.3-0.5m / min to the designed reinforcement depth. While sinking, spray and stir at a uniform speed of 0.3-0.5m / min. When the drill rod enters 0.5m from the top surface of the silt soft soil layer 1, turn on the mortar pump, and press the cement slurry from the center pipe of the cement mixing pile machine into the silt soft soil layer 1 at a grouting pressure of 0.40MPa-0.60MPa. The cement slurry and the silt soft soil layer 1 are mixed by the mixer blades, and the slurry is sprayed while mixing until the designed reinforcement depth. During the spraying process, the cement slurry is continuously stirred. During the spraying construction, a slurry flow meter is used to control the slurry delivery speed, and the outlet pressure of the grouting pump is maintained at 0.40MPa-0.60MPa, and the slurry delivery speed must be kept constant. During shotcrete spraying, an automatic shotcrete recorder must be used to record: shotcrete volume, drill lifting speed and bored pile length indicators.

[0212] 4) Spraying and lifting: When the cement mixing pile machine sinks to the designed reinforcement depth, after mixing and spraying at the end of the cement mixing pile for 30 seconds, it is stirred and lifted at a uniform speed of 0.3-0.5m / min, and the cement slurry is sprayed at a grouting pressure of 0.40MPa-0.60MPa. The cement slurry is continuously pressed into the silt soft soil layer 1 from the central tube of the cement mixing pile machine. The cement slurry and the silt soft soil layer 1 are mixed by the mixing blades of the mixer. The slurry is sprayed while stirring until it is 0.5m away from the top surface of the silt soft soil layer 1. The spraying is stopped, and the mixing process is completed.

[0213] 5) Re-mixing: During implementation, each cement mixing pile must repeat the previous operation of four mixing and four spraying. The speed of the first sinking mixing and lifting mixing is 0.3 or 0.4m / min; during re-mixing, the sinking mixing speed is 0.4 or 0.5m / min, and the lifting mixing speed is 0.3 or 0.4m / min, and there is a short pause between the two mixing. The normal molding time of each cement mixing pile is not less than 40min, and the spraying pressure is not less than 0.4MPa. The re-spraying and re-mixing method is conducive to making up for the insufficient amount of spraying in one time and accelerating the slurry reaction process.

[0214] 6) Shifting: After one cement mixing pile is completed, the next cement mixing pile shall be constructed in the same way. Repeat the above steps. After the reinforcement is completed every day, use clean water to clean the storage tank, mortar pump, cement mixing pile machine and corresponding pipelines for reuse.

[0215] 2. Construction of cement mixing pile retaining wall 301 outside the foundation pit; specifically, Figure 7As shown, on the top surface of the silt soft soil layer 1 outside the steel sheet pile retaining wall 4 on the four sides of the foundation pit, facing the outside of the foundation pit, a row of rectangular cement mixing pile retaining walls 301 (width 0.60m) and pile length 8.0m are constructed to abut the outside of the steel sheet pile retaining wall 4; after construction, the cement mixing pile retaining wall 301 outside the pit is firmly bonded to the steel sheet pile retaining wall 4; its construction quality requirements are: the wall top elevation of the cement mixing pile retaining wall 301 outside the pit is 4.2m, the wall bottom elevation is -3.8m, the 28d unconfined compressive strength is ≥1.0MPa, the vertical deviation is ≤+1%, and the plane position deviation is ≤50mm.

[0216] 3. Construction of the quasi-rectangular cement mixing pile retaining wall 3021 inside the foundation pit; Figure 7-9 As shown, the construction of the quasi-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 a special-shaped cement mixing pile retaining wall 30213; the special-shaped cement mixing pile retaining wall 30213 inside the foundation pit includes: an H-shaped cement mixing pile retaining wall 302131, a quasi-full-area cement mixing pile retaining wall 302132 and a straight-line cement mixing pile retaining wall 302133; its construction quality requirements are: the top elevation of the quasi-rectangular cement mixing pile retaining wall 3021 is -3.3m, the bottom elevation is -9.3m, the 28d unconfined compressive strength is ≥1.0MPa, the vertical deviation is ≤+1%, and the plane position deviation is ≤50mm.

[0217] (1) Construction of the outer rectangular cement mixing pile retaining wall 30211 inside the foundation pit: Figure 7-9 As shown, on the top surface of the silt soft soil layer 1 inside the steel sheet pile retaining wall 4 on the four sides of the foundation pit, three rows (width 1.5m) of cement mixing piles with a length of 6.0m are constructed toward the center of the foundation pit. Many cement mixing piles are interconnected longitudinally and transversely to form an outer rectangular cement mixing pile retaining wall 30211 on the inner side of 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 on the inner side of the foundation pit is firmly bonded to the steel sheet pile retaining wall 4.

[0218] (2) Construction of the inner rectangular cement mixing pile retaining wall 30212 inside the foundation pit: Figure 7-9 As shown, on the top surface of the silt soft soil layer 1 at a position 3.75m away from the inner edge of the inner outer rectangular cement mixing pile retaining wall 30211 on the inner side of the foundation pit, double rows (width 1.05m) of cement mixing piles with a length of 6.0m are constructed toward the center of the foundation pit, and many cement mixing pile retaining walls are connected to each other longitudinally and transversely to form the inner inner rectangular cement mixing pile retaining wall 30212 on the inner side of the foundation pit.

[0219] (3) Construction of special-shaped cement mixing pile retaining wall 30213 inside the foundation pit: On the top surface of the silt soft soil layer 1 within the range between the outer rectangular cement mixing pile retaining wall 30211 inside the foundation pit and the inner rectangular cement mixing pile retaining wall 30212 inside the foundation pit, H-shaped cement mixing pile retaining wall 302131, quasi-full-area cement mixing pile retaining wall 302132 and I-shaped cement mixing pile retaining wall 302133 are constructed to form the special-shaped cement mixing pile retaining wall 30213 inside the foundation pit. The details are as follows:

[0220] 1) Construction of H-shaped cement mixing pile retaining wall 302131: Figure 7-9 As shown, on the top surface of the silt soft soil layer 1 between the adjacent cast-in-place piles 2 on the three sides of the left half of the foundation pit along the bridge direction, double rows (width 1.05m) and 6.0m long "H"-shaped cement mixing piles are respectively constructed, and many cement mixing piles are connected to each other in the longitudinal and transverse directions to form an H-shaped cement mixing pile enclosure wall 302131, the two ends of which are respectively abutted against the outer rectangular cement mixing pile enclosure wall 30211 and the inner rectangular cement mixing pile enclosure wall 30212 on the inner side of the foundation pit, and the middle part thereof is respectively abutted against the adjacent cast-in-place piles 2 On the top surface of the silt soft soil layer 1 between the adjacent bored piles 2 on the three sides of the right half of the foundation pit along the bridge direction, double rows (width 1.05m) and 6.0m long "H"-shaped cement mixing piles are constructed, and many cement mixing piles are connected to each other longitudinally and transversely to form an H-shaped cement mixing pile retaining wall 302131, the two ends of which are respectively abutted 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, and parts thereof are respectively abutted against the adjacent bored piles 2.

[0221] 2) Construction of full-area cement mixing pile retaining wall 302132: Figure 7-8 As shown, on the top surface of the silt soft soil layer 1 at the upper left corner and the lower left corner of the inner outer rectangular cement mixing pile retaining wall 30211 of the foundation pit in the left half of the foundation pit along the bridge direction, a plurality of cement mixing piles with a pile length of 6.0m are respectively constructed, and many cement mixing piles are connected to each other in the longitudinal and transverse directions to form a quasi-full-area cement mixing pile retaining wall 302132, which respectively abuts against the inner outer rectangular cement mixing pile retaining wall 30211 of the foundation pit and the inner inner rectangular cement mixing pile retaining wall 30211 of the foundation pit. 2. On the top surface of the silt soft soil layer 1 at the upper right corner and lower right corner of the inner outer rectangular cement mixing pile retaining wall 30211 of the foundation pit on the right half of the foundation pit along the bridge direction, a plurality of cement mixing piles with a pile length of 6.0 m are respectively constructed, and many cement mixing piles are interconnected longitudinally and transversely to form a quasi-full-area cement mixing pile retaining wall 302132, which respectively abuts the inner outer rectangular cement mixing pile retaining wall 30211 and the inner inner rectangular cement mixing pile retaining wall 30212 of the foundation pit.

[0222] 3) Construction of the 302133 straight cement mixing pile retaining wall: Figure 7-8 As shown, on the top surface of the silt soft soil layer 1 in the middle of the outer rectangular cement mixing pile retaining wall 30211 and the inner rectangular cement mixing pile retaining wall 30212 on the two length sides of the foundation pit, double rows (width 1.05m) of "I"-shaped cement mixing piles with a pile length of 6.0m are respectively constructed, and many cement mixing piles are connected to each other longitudinally and transversely to form an I-shaped cement mixing pile retaining wall 302133, which respectively abuts against two opposite cast-in-place piles 2 in the middle of the foundation pit.

[0223] 4. Construction of the straight support cement mixing pile retaining wall 3022 inside the foundation pit; Figure 7-9 As shown, on the top surface of the silt soft soil layer 1 inside the steel sheet pile retaining wall 4 at the middle position on the length side of the foundation pit, facing the center side of the foundation pit, 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 are constructed. Many cement mixing piles are interconnected longitudinally and transversely to form a straight-supported cement mixing pile retaining wall 3022, and its two ends respectively abut the outer rectangular cement mixing pile retaining wall 30211, the I-shaped cement mixing pile retaining wall 302133 and the inner rectangular cement mixing pile retaining wall 30212 on the inner side of the foundation pit.

[0224] Preferably, after completing the step of "silt soft soil support reinforcement design and construction" and before carrying out the step of "earth excavation in the foundation pit and internal support construction", the following steps are also included:

[0225] Optionally, step "S5: monitoring and measuring" specifically includes: 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; specifically including:

[0226] 1. Monitoring content: After the silt soft soil reinforcement construction is completed and before the foundation pit excavation, monitoring points are arranged on the top of the cement mixing pile retaining wall 301 outside the pit and on the ground outside the steel sheet pile retaining wall 4 on the four sides of the foundation pit; according to the characteristics of this project, the monitoring contents include: horizontal displacement of the top of the cement mixing pile retaining wall (referred to as: retaining wall) outside the pit, vertical displacement of the top of the retaining wall, supporting axial force, water level inside and outside the retaining wall, groundwater level monitoring, horizontal displacement of deep soil, and daily inspections.

[0227] 2. Layout of monitoring points: (1) Horizontal displacement monitoring points on the top of the retaining wall are arranged along the perimeter of the foundation pit. According to Article 5.2.1 of the Technical Standard for Monitoring of Construction Pit Engineering (GB50497), the horizontal spacing of the monitoring points shall not exceed 20m. The monitoring points shall be arranged at least 3 on the length side, at least 1 on the width side, and 1 at each of the four corners of the foundation pit; (2) Vertical displacement monitoring points on the top of the retaining wall share the same position as the horizontal displacement monitoring points on the top of the retaining wall; (3) Support axial force monitoring points: 5 axial force monitoring points of the first layer of steel pipe internal support are arranged, at the middle of the straight steel pipe internal support 501 on the length side of the foundation pit and at the first inclined steel pipe internal support at the four corners of the foundation pit. A support axial force gauge is installed on each support 502, and a support axial force gauge is installed at the position of the axial force monitoring point of the second layer of steel pipe support 501 in the vertical direction corresponding to the axial force monitoring point of the first layer of steel pipe support 501; (4) Water level monitoring points inside and outside the retaining wall: a water level monitoring point on the outside of the retaining wall is arranged on the outer wall of the cement mixing pile retaining wall 301 outside the pit, and the reading can be directly read; a scale is set on the inside of the steel sheet pile retaining wall 4 to arrange the water level monitoring point on the inside of the retaining wall; (5) Groundwater level monitoring point: a water level measuring hole is arranged on the center line of each length side of the foundation pit; (6) Deep soil horizontal displacement monitoring point: a soil displacement monitoring point is arranged on the center line of each length side and width side of the foundation pit.

[0228] 3. Test methods: (1) The polar coordinate method is used to monitor the horizontal displacement of the top of the retaining wall; (2) The geometric leveling method is used to monitor the vertical displacement of the top of the retaining wall, using a digital level in combination with an Indium Wafer barcode ruler for observation; (3) The support axial force is monitored using a comprehensive tester; (4) The water level inside and outside the retaining wall is monitored by direct observation; (5) The groundwater level is monitored by direct observation; (6) The horizontal displacement of the deep soil is monitored by an inclinometer.

[0229] 4. Monitoring cycle, frequency and warning value:

[0230] (1) Monitoring period: The specific monitoring period is based on the on-site construction time, from the start of foundation pit excavation to the completion of foundation pit backfilling.

[0231] (2) Monitoring frequency and warning value, see Table 6:

[0232] Table 6 Monitoring frequency and warning value table

[0233]

[0234] The monitoring frequency should be increased when any of the following situations occurs:

[0235] ① The monitoring data changes significantly or at a faster rate;

[0236] ② The surrounding ground suddenly settles or cracks severely;

[0237] ③ The adjacent buildings suddenly experience large settlement, uneven settlement or severe cracking;

[0238] ④ Reorganize construction after an accident occurs in the project;

[0239] ⑤Other abnormal situations that affect construction safety occur.

[0240] (3) During the construction process, the state of the foundation pit should be observed in real time. If there is any abnormal noise or sudden increase in deformation, the construction should be stopped immediately and personnel should be evacuated. The construction can be continued only after the cause is identified and effective measures are taken.

[0241] 5. Early warning value and control value: The early warning of the engineering risk safety status during the construction process is divided into three categories: monitoring early warning, patrol early warning and comprehensive early warning, as follows:

[0242] (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 from small to large in terms of severity: yellow monitoring and early warning, orange monitoring and early warning, and red monitoring and early warning.

[0243] 1) Yellow monitoring warning: When both the "dual control" indicators (cumulative change and rate) reach between 70% and 85% of the monitoring control value (including 85% of the monitoring control value), or when one of the dual control indicators reaches between 85% of the monitoring control value and the control value (excluding the monitoring control value), and the other indicator does not reach 85% of the monitoring control value;

[0244] 2) Orange monitoring and early warning: When both "dual control" indicators reach 85% to 100% of the monitoring control value (including the monitoring control value), or when one of the dual control indicators reaches or exceeds the monitoring control value;

[0245] 3) Red monitoring warning: When both “dual control” indicators reach or exceed the monitoring control values, and the measured rate of change increases sharply.

[0246] (2) Patrol warning: A warning issued when safety hazards or unsafe conditions are discovered during the construction process. It is divided into three levels from small to large in terms of severity: yellow patrol warning, orange patrol warning, and red patrol warning.

[0247] (3) Comprehensive warning: During the construction process, the risk of unsafe conditions in the project is determined in a timely and comprehensive manner based on the on-site monitoring and inspection information, and through on-site verification, comprehensive analysis, consultations with all parties involved in the construction, and expert demonstration. The comprehensive warning is divided into four levels from small to large in severity: blue comprehensive warning, yellow comprehensive warning, orange comprehensive warning, and red comprehensive warning.

[0248] Optionally, step “S6: excavation of earthwork in foundation pit and construction of internal support” specifically includes:

[0249] S601: Excavation of earthwork in foundation pit; specifically including:

[0250] 1. Lay out water collection wells: On the foundation pit plane, according to the excavation sequence and excavation divisions, set up interconnected drainage ditches around the foundation pit, and leave several water collection wells in the parts with large seepage in each area. Place submersible pumps in the water collection wells to drain water, so as to achieve the purpose of drainage and precipitation; the water collection well is set on the inner side of the steel sheet pile retaining wall 4, with a size of 800×800×400mm.

[0251] 2. Excavation of earthwork in foundation pit: Figure 3 As shown, the excavation of the earthwork in the foundation pit can only be carried out after the conditions for excavation of the earthwork in the foundation pit have been accepted; the earthwork in the foundation pit is excavated in three steps from top to bottom. During the excavation of the earthwork in the foundation pit, a water collection well must be set up in time for drainage to ensure a smooth excavation process in the foundation pit. The specific construction steps are as follows:

[0252] Step 1: When the first layer of earthwork is excavated to 1.0m below the center line of the first layer of steel pipe support 5, cut the steel casing and break the over-cast pile foundation pile head in time, clean the soil on the steel sheet pile retaining wall 4 and the bored pile 2, and install the first layer of steel corbels 5011, steel purlins 5012 and steel pipe support rods 5013.

[0253] Step 2: When the second layer of earthwork is excavated to 1.0m below the center line of the second layer of steel pipe support 5, cut the steel casing and break the over-cast pile foundation pile head in time, clean the soil on the steel sheet pile retaining wall 4 and the bored pile 2, and install the second layer of steel corbels 5011, steel purlins 5012 and steel pipe support rods 5013.

[0254] Step 3: When the third layer of earthwork is excavated to the design elevation of -3.3m at the bottom of the foundation pit, cut the steel casing in time and break the over-cast pile foundation head to the design elevation of -2.3m at the top of the bored pile 2, and clean the soil on the steel sheet pile retaining wall 4 and the bored pile 2.

[0255] 3. Principles of earth excavation: The earth excavation in the foundation pit shall strictly follow the "time and space effect" theory and be carried out according to the principles of "overall segmentation, longitudinal segmentation, vertical stratification, synchronous retreat of excavators, support as excavation progresses, and time-limited balance".

[0256] 4. Overall excavation method for foundation pit earthwork: Use excavators to excavate the first layer of earthwork inside the foundation pit, and use long-arm excavators to excavate the second and third layers of earthwork outside the foundation pit. In order to ensure the safety of the machinery during the construction process, the earthwork excavation is carried out in a layered step method, with a step width of ≥4m. Loading is prohibited within 2m outside the foundation pit.

[0257] S602: Support construction in foundation pit; Figure 2 , Figure 3As shown, the support construction content in the foundation pit includes: installation of steel corbels 5011, installation of steel purlins 5012, installation of steel pipe support rods 5013, and construction of bottom sealing concrete 6, as follows:

[0258] 1. Construction process, including:

[0259] (1) Construction sequence: measurement and layout → installation of steel corbel 5011 → installation of steel purlin 5012 → installation of steel pipe support rod 5013 → construction of bottom concrete 6.

[0260] (2) Installation of steel corbel 5011: Steel corbel 5011 is welded manually.

[0261] (3) Installation of steel purlin 5012: It is installed manually with the help of crawler crane. The sections of steel purlin 5012 are connected by butt welding, and steel plates are used for reinforcement welding at the welds. The four gaps between steel purlin 5012 and the steel sheet pile retaining wall are supported horizontally by welding I-beams.

[0262] (4) Installation of steel pipe support rod 5013: Several sections of steel pipe are spliced ​​together on the ground to form a segment of steel pipe main rod, and then spliced ​​with a steel pipe main rod with a flexible end and a steel pipe main rod with a fixed end to form a whole steel pipe support rod 5013, which is hoisted into place as a whole by manual labor and crawler crane.

[0263] (5) Construction of bottom seal concrete 6: Pour bottom seal concrete 6 with a thickness of 0.8m and strength grade C30 on the bottom surface of the foundation pit.

[0264] (6) Dismantling sequence: dismantling of steel pipe support rod 5013 → dismantling of steel purlin 5012 → dismantling of steel corbel 5011.

[0265] 2. Material and processing of steel corbels, steel purlins and steel pipes, including:

[0266] In this optional solution, the steel corbel 5011, steel purlin 5012, and steel pipe support rod 5013 are all made of purchased semi-finished products and assembled and welded on site. The steel corbel 5011 is made of I-beam of Ⅰ25a, 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 is composed of two parts: a support rod system and ancillary components, in which: the support rod system includes: a steel pipe main rod, a flexible end, and a fixed end; the ancillary components include: flanges, high-strength bolts, and steel wedges, in which: the steel pipe main rod is assembled and connected by multiple sections of steel pipes, and the multiple sections of steel pipes are connected by flanges and high-strength bolts; the first layer of steel purlin 5012 is made of I-beam of Q235 and 3Ⅰ56a, the steel pipe used for the straight steel pipe inner support 501 is φ630*10mm steel pipe, and the inclined steel pipe inner support 5 The steel pipe used in 02 is φ480*8mm steel pipe; the second-layer steel purlin 5012 uses Q235, 3HN700*300 H-shaped steel, the steel pipe used in the straight steel pipe inner support 501 is φ1000*12mm steel pipe, and the steel pipe used in the inclined steel pipe inner support 502 is φ480*8mm steel pipe; before processing, a cutting list is made according to the use location of the steel purlin 5012, and the materials are cut according to the cutting list and stacked neatly in categories; all the directly load-bearing steel plate end faces of the steel purlin 5012 are milled flat in advance; the connection between the sections of the steel purlin 5012 must be butt-welded and extended using the equal strength connection method when the steel purlin 5012 is assembled.

[0267] 3. Installation of steel corbels, steel purlins, and steel pipe support rods, including:

[0268] (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 inner support 5, the engineering and technical personnel will measure and set out to determine the position of the first layer of steel pipe inner support 5, promptly clean the soil on the steel sheet pile retaining wall 4 and 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 inner support 5, the engineering and technical personnel will measure and set out to determine the position of the second layer of steel pipe inner support 5, promptly clean the soil on the steel sheet pile retaining wall 4 and cast-in-place pile 2, and weld the second layer of steel bracket 5011. The steel bracket 5011 is triangular in shape.

[0269] (2) Installation of steel purlin 5012: After the first layer of steel corbel 5011 is installed, the first layer of steel purlin 5012 is hoisted and installed; after the second layer of steel corbel 5011 is installed, the second layer of steel purlin 5012 is hoisted and installed. The processed steel purlin 5012 is installed on the steel corbel 5011 bracket in sections by manual operation and a 50t crawler crane. It is leveled in place and small steel sheets are used to support the uneven parts on the steel corbel 5011 bracket. The specific installation is as follows:

[0270] The connection between the segments of the steel purlin 5012 is made of equal strength and butt welding to form a continuous steel beam, ensuring that there is no cantilever form and ensuring the overall stability of the steel purlin 5012. Steel plates are used to reinforce the welding connection at the weld. The location of the welded joint of the steel purlin 5012 avoids the center of the steel tube inner support 5 and must be within the range of 1 / 6 of the steel tube inner support 5 spacing on the left and right of the center line of the steel tube inner support 5. The gap between the steel purlin 5012 and the steel sheet pile retaining wall 4 is supported horizontally by welding I-beams to make the steel purlin 5012 evenly stressed. Six and three steel purlin 5012 anti-fall protection devices are arranged on the length side and width side of the foundation pit respectively. A 16 mm thick perforated steel plate is welded to the steel purlin 5012 and the steel sheet pile enclosure wall 4 respectively, and a 16 mm diameter steel wire rope and a snap lock are used to connect the steel purlin 5012 and the steel sheet pile enclosure wall 4 to prevent the steel purlin 5012 from falling.

[0271] (3) Installation of steel pipe support rods 5013. Before installing the steel pipe support rods 5013, calculate or measure the actual length of the steel pipe support rods 5013 at each installation location. The total length of the spliced ​​steel pipe support rods 5013 is slightly less than the actual length. After the supporting axial force is applied, the gap between the steel pipe support rods 5013 and the steel purlin 5012 is wedged and locked with a specially shaped steel wedge.

[0272] 1) After the first layer of steel purlin 5012 is installed, the center position of the straight steel pipe inner support 501 is determined by marking out on the steel purlin 5012 on both length sides of the foundation pit, and the cross-line method is used for accurate positioning. On the side of the steel purlin 5012 facing the center of the foundation pit, a triangular steel plate support with a welded end face perpendicular to the axis of the steel pipe support rod 5013 is installed, and the steel pipe support rod 5013 is installed. During installation, a crawler crane is used for overall lifting, and both ends of the steel pipe support rod 5013 are placed on the triangular steel plate supports on the steel purlin 5012 on both length sides of the foundation pit. The center position of the inclined steel pipe inner support 502 is determined by setting out on the steel purlins 5012 on the adjacent length and width sides of the four corners of the foundation pit, and the cross-line method is used for accurate positioning. On the side of the steel purlins 5012 facing the center of the foundation pit, a wedge-shaped triangular steel plate support with an end face perpendicular to the axis of the steel pipe support rod 5013 is welded, and the steel pipe support rod 5013 is installed. During installation, the crawler crane is used for overall lifting, and the two ends of the steel pipe support rod 5013 are placed on the wedge-shaped triangular steel plate supports on the steel purlins 5012 on the adjacent length and width sides of the four corners of the foundation pit. When the second layer of steel purlins 5012 is installed, the second layer of steel pipe support rods 5013 are installed in the same way.

[0273] 2) Weld a 16 mm thick perforated steel plate on the steel pipe support rod 5013 and the steel sheet pile retaining wall 4 respectively, and use a 16 mm diameter steel wire rope and a snap 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.

[0274] 3) When hoisting the steel pipe support rod 5013, two-point hoisting is adopted, and the hoisting point is arranged at a distance of 0.2 times the total length of the steel pipe support rod 5013 from the end of the steel pipe support rod 5013. During the process of applying the supporting axial force of the steel pipe support rod 5013, the crawler crane cannot lower the steel pipe support rod 5013, and the crawler crane can be removed only after the steel pipe support rod 5013 is locked. The maximum weight of the first layer of steel pipe support rod 5013 is about 3.4t, and the hoisting distance is 16m. A 50t crawler crane is used for hoisting; the maximum weight of the second layer of steel pipe support rod 5013 is about 6.4t, and the hoisting distance is 16m. A 75t crawler crane is used for hoisting.

[0275] 4) On the flexible end face of the steel pipe support rod 5013 where the support axial force monitoring point needs to be arranged, the support axial force meter is welded and installed. At the flexible end of the steel pipe support rod 5013, two 500t hydraulic jacks are used to apply the support axial force prestress to the steel pipe support rod 5013. After the axial force prestress of the steel pipe support rod 5013 is applied to the design value and stabilized, a steel wedge limiter is driven into the flexible end to complete the application of the axial force prestress of the steel pipe support rod 5013. The axial force of the first layer of steel pipe support rod 5013 is 1116kN, and the axial force of the second layer of steel pipe support rod 5013 is 5010kN.

[0276] 4. Installation quality requirements

[0277] The installation deviation of the steel tube support rod 5013 must be within the allowable range of the specification: the difference between the center elevation of the steel tube support rod 5013 and the top surface elevation of the steel tube support rod 5013 on the same floor is ±30mm; the difference between the elevations at both ends of the steel tube support rod 5013 is ≤20mm and 1 / 600 of the length of the steel tube support rod 5013, whichever is smaller; the deflection of the steel tube support rod 5013 is ≤1 / 1000 of the length of the steel tube support rod 5013; the horizontal axis deviation of the steel tube support rod 5013 is ≤30mm.

[0278] Optionally, step “S7: pit construction and support system conversion” specifically includes:

[0279] S701: Construction of bottom concrete 6, including:

[0280] The bottom sealing concrete 6 in the foundation pit and the rectangular cement mixing pile retaining wall 3021 and the straight support cement mixing pile retaining wall 3022 on the inner side of the foundation pit form a rectangular "frame" type rigid anti-floating structure, which effectively increases the weight of the bottom sealing components of the foundation pit, prevents the sludge soft soil at the bottom of the foundation pit from uplifting and water gushing accidents, thereby improving the anti-uplift safety factor of the sludge soft soil at the bottom of the foundation pit; and then 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 on the inner side of the foundation pit, the straight support cement mixing pile retaining wall 3022, the first The first layer of steel tube inner support 5 and the second layer of steel tube inner support 5 constitute a steel tube concrete support system, which effectively enhances the rigidity of the deep foundation pit support system and resists the sliding and collapse of the soil on the side walls around the foundation pit; it also forms a rectangular "water cup" type waterproof structure with the quasi-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, preventing groundwater from seeping into the foundation pit or construction sewage in the foundation pit from seeping into the nearby river outside the foundation pit and polluting the river water quality, thereby improving the water-stopping performance of the silt soft soil layer 1.

[0281] 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 is cut in time and the over-cast pile foundation head is broken to the design elevation of -2.3m at the top of the bored pile 2, and the soil on the steel sheet pile retaining wall 4 and the bored pile 2 is cleaned in time.

[0282] 2. Level the bottom surface of the foundation pit, and inspect and accept the plane position, geometric dimensions, and bottom elevation of the foundation pit. After acceptance, pour 0.8m thick C30 bottom seal concrete 6 on the bottom surface of the foundation pit. The pouring order of the bottom seal concrete 6 is: pour from both ends of the width side of the foundation pit to the middle of the foundation pit.

[0283] 3. After the strength of the bottom concrete 6 reaches the design strength, install the base reinforcement.

[0284] S702: Capping construction and support system conversion, including:

[0285] 1. Removal of the second layer of steel pipe support rods 5013: After the strength of the bottom concrete 6 reaches the design requirements, the second layer of steel pipe support rods 5013, steel purlins 5012, and steel corbels 5011 can be removed to gradually complete the first conversion of the deep foundation pit support system.

[0286] 2. First foundation construction: After the second layer of steel pipe support rods 5013, steel purlins 5012 and steel corbels 5011 are removed, the first 3m high steel bars, embedded parts, formwork installation and foundation C40 high performance mass concrete are completed to a position 2.5m away from the bottom surface of the first layer of steel pipe support rods 5013.

[0287] 3. First backfill of the cavity: After the strength of the first cap concrete reaches 75% of the design strength, use clay soil to backfill the cavity between the cap concrete and the steel sheet pile retaining wall 4 in layers. The thickness of each layer of backfill soil is 300mm and the density is ≥90%.

[0288] 4. Removal of the first layer of steel pipe support rods 5013: When the backfill soil in the cavity reaches the top surface of the first cap, the first layer of steel pipe support rods 5013, steel purlins 5012, and steel corbels 5011 can be removed. The first cap structure that has been constructed and the backfill soil in the cavity are used as the support in the foundation pit to complete the second conversion of the deep foundation pit support system.

[0289] 5. Second pedestal construction: After the first layer of steel pipe support rods 5013, steel purlins 5012, and steel corbels 5011 are removed, continue to complete the second 2m height of the pedestal, install the steel bars, embedded parts, formwork, and C40 high-performance large-volume concrete of the pedestal to the top surface of the pedestal.

[0290] 6. Second backfill of the cavity: After the strength of the second cap concrete reaches 75% of the design strength, use clay soil to backfill the cavity between the cap concrete and the steel sheet pile retaining wall 4 in layers. The thickness of each layer of backfill soil is 300mm and the density is ≥90%. The constructed second cap structure and the backfill soil in the cavity are used as the support in the foundation pit to complete the third conversion of the deep foundation pit support system.

[0291] 7. Stop the foundation pit monitoring and measurement, and pull out the steel sheet pile retaining wall 4: After the first, second and third conversions of the deep foundation pit support system mentioned above, the foundation pit monitoring and measurement can be stopped, the steel sheet pile retaining wall 4 can be pulled out, and the construction of the 7# main pier cap of this bridge can be successfully completed.

[0292] Optionally, the construction method of the present invention can be applied to the foundation pit construction of underground structure projects such as urban rail transit underground stations, underground pipe corridors, underground passages, underground culverts, industrial and civil building basements, in addition to the bridge pedestal foundation pit construction field of the above-mentioned embodiment.

[0293] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A deep foundation pit support system and construction method in a silt soft soil environment, characterized in that: The following steps are involved: Construction preparation: Obtain the distribution of the strata at the foundation to be constructed and construct the cast-in-place piles below the foundation (2); Construction scheme design and feasibility assessment: prepare construction schemes and use soil mechanics formulas and computer software to model, analyze and verify to assess whether the stability of the deep foundation pit support system meets the regulatory requirements; Design and construction of silt soft soil support and reinforcement: designing a structure for supporting and reinforcing silt soft soil in a deep foundation pit support system, constructing a steel sheet pile retaining wall (4) extending along the perimeter of the deep foundation pit to support the silt soft soil, and constructing an outer pit cement mixing pile retaining wall (301) arranged close to the outer perimeter of the steel sheet pile retaining wall (4) and an inner pit cement mixing pile retaining wall (302) located below the deep foundation pit to cooperate to reinforce the silt soft soil; Excavation of earthwork and construction of internal supports in the foundation pit: Excavation of earthwork is carried out in the steel sheet pile retaining wall (4), and the excavated foundation pit is supported by steel pipe internal supports (5); Construction in the pit and conversion of the support system: pouring the bottom concrete (6) at the bottom of the excavated deep foundation pit, and pouring the pedestal on the bottom concrete (6) and removing the steel pipe inner support (5) during the pouring process.

2. The deep foundation pit support system and construction method in a silt soft soil environment according to claim 1 is characterized in that: The step "Construction scheme design and feasibility assessment" specifically includes: Preparation of construction plans; Verification of construction plan: Use soil mechanics formulas and MIDAS / CIVIL finite element analysis software modeling to calculate or verify the load, stability and strength of the deep foundation pit support system in the construction plan; Expert review and approval of construction plans.

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

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

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

8. The deep foundation pit support system and construction method in a silt soft soil environment according to claim 1, characterized in that: After completing the step "Silt soft soil support reinforcement design and construction", and before carrying out the step "Excavation of earthwork in the foundation pit and internal support construction", the following steps are also included: Monitoring and measurement: monitoring points and early warning prompts are arranged 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 silt soft soil environment according to claim 1, characterized in that: The step "excavation of earthwork in the foundation pit and internal support construction" specifically includes: Excavation of earthwork in foundation pit; Construction of steel pipe internal support (5) in foundation pit.

10. The deep foundation pit support system and construction method in a silt soft soil environment according to claim 1, characterized in that: The step "construction in the pit and conversion of the support system" specifically includes: Construction of bottom seal concrete (6); Cap construction and support system conversion.

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