A horizontal variable stiffness foundation pit support structure and a design method thereof

Abstract

The application discloses a design method of a horizontal variable stiffness foundation pit supporting structure, which comprises the following steps: three-dimensional modeling of the foundation pit supporting structure; analysis of lateral displacement of the supporting structure and calculation of a plane strain ratio; for a region with a plane strain ratio less than 1, the structure stiffness is redundant, the corner effect of the foundation pit is considered, the row piles are connected with the concrete support, and stiffness adjustment is performed according to an optimized expression of the plane strain ratio; for a region with a plane strain ratio equal to 1, two groups of row piles are arranged and connected with the concrete support through an active stiffness adjusting device. The application further discloses the horizontal variable stiffness foundation pit supporting structure obtained by the design method. The corner effect of the foundation pit (restraint of the foundation pit deformation by a reentrant corner) is utilized, the stiffness of the supporting structure is divided into regions by introducing the plane strain ratio, and variable stiffness design is performed, so that the cost can be effectively reduced, large deformation of the middle section of the supporting structure can be reduced, and the influence of the foundation pit construction on the surrounding environment can be reduced.

Description

A horizontal variable stiffness foundation pit support structure and its design method Technical Field

[0001] This invention pertains to foundation pit support structures and their design methods, specifically a horizontal variable stiffness foundation pit support structure and its design method. Background Technology

[0002] With the deepening development and utilization of urban underground space, the number of "deep, large, compact, and near" foundation pit projects is increasing daily. The design of foundation pit projects is gradually shifting from strength control to deformation control. Currently, the design of foundation pit support structures is mainly based on two-dimensional plane strain. However, deep foundation pit projects have significant spatial effects; the corners of the pit can effectively suppress the deformation of the support structure, a point largely ignored in existing designs. Therefore, horizontal variable stiffness design for foundation pit projects is a more economical and effective technical approach. Specifically, the support stiffness near corners should be appropriately reduced, while different structural forms should be used to enhance stiffness in the mid-span area. Furthermore, when the deformation value is too large, axial force should be applied promptly to suppress further deformation, achieving real-time monitoring and servo control.

[0003] Chinese patent application number CN201811440978.4 discloses a method for foundation pit support with vertical variable stiffness support. By setting multiple vertical bodies with different heights and widths, the stiffness of the foundation pit support structure varies in the vertical direction. However, the problem is that this type of support structure requires a large amount of space outside the pit and requires the construction of multiple vertical bodies with different strengths. The process is complicated, and the contact interfaces between vertical bodies with different strengths cannot coordinate deformation. This will result in the final effect of the entire support structure not achieving the expected results.

[0004] Chinese patent application number CN 202010170614.X discloses a variable stiffness composite precast double-row pile support structure and its construction method, including a front row of piles, a rear row of piles, a pile top beam platform, a retaining wall, and buttress ribs. The front row of piles consists of a row of densely arranged precast prestressed concrete hollow square piles, and the rear row of piles consists of a row of spaced-apart precast prestressed concrete hollow piles. This structural form achieves the variation of the lateral stiffness of the support structure along the depth. However, the problem is that this method ignores the corner effect of the foundation pit. The inside corner of the foundation pit can suppress deformation, but the above invention only adjusts the vertical stiffness, and will further increase the stiffness of the support structure near the corner, which will further cause design waste.

[0005] In summary, existing variable stiffness foundation pit support structures mainly focus on vertical variable stiffness, but they all ignore the corner effect of the foundation pit and cannot be monitored in real time. Furthermore, while existing variable stiffness support structures can control the deformation of the foundation pit structure to a certain extent, they also increase the cost of the support structure near the corner of the foundation pit, resulting in waste. Summary of the Invention

[0006] Purpose of the invention: In order to overcome the shortcomings of the existing technology, the purpose of this invention is to provide a design method for a material-saving horizontal variable stiffness foundation pit support structure. Another purpose of this invention is to provide a horizontal variable stiffness foundation pit support structure that can be monitored and controlled in real time and is easy to construct.

[0007] Technical solution: The design method of a horizontal variable stiffness foundation pit support structure according to the present invention includes the following steps:

[0008] Step 1: Use 3D finite element analysis software to create a 3D model of the foundation pit support structure;

[0009] Step 2: Analyze the lateral displacement of the support structure and calculate the ratio of the deformation value of each section to the deformation value under plane strain, thereby obtaining the plane strain ratio of the entire support section.

[0010] Step 3: For areas where the plane strain ratio is less than 1, the structural stiffness is redundant. Considering the corner effect of the foundation pit, the piles are connected to the concrete supports, and the stiffness is adjusted according to the optimized expression of the plane strain ratio. For areas where the plane strain ratio is equal to 1, the structural stiffness is insufficient. Two sets of piles are set up and connected to the concrete supports through an active stiffness adjustment device.

[0011] Furthermore, in step two, the plane strain ratio is the ratio of the maximum deformation of a cross section under three-dimensional conditions to the maximum deformation of that cross section under plane strain conditions.

[0012] Excavation of foundation pits exhibits significant spatial effects. The corners of the pit effectively suppress deformation of the support structure, a phenomenon known as the corner effect. Maximum deformation typically occurs in the mid-span of the support structure. Current design theories for foundation pit support structures are usually based on the two-dimensional plane strain assumption, which does not reflect actual foundation pit conditions. This leads to redundant stiffness in the support structure near the corners, resulting in deformation that is larger in the middle and smaller at both ends—a deformation inconsistency. Therefore, a dimensionless quantity, the plane strain ratio (PSR), is introduced. By using PSR to strategically partition the foundation pit support structure and employ different support methods, the horizontal stiffness of the foundation pit support structure can be adjusted.

[0013] Furthermore, in step three, the optimized expression for the plane strain ratio is:

[0014] D d =D d0 +PSR (b-a) / 2 (D0-D d0 )

[0015] Among them, D d The optimized support pile diameter is D0, where D is the pile diameter designed based on a two-dimensional plane. d0 The critical pile diameter is specifically the recommended minimum value given in the "Technical Specification for Foundation Pit Support" (JGJ 120-2012), PSR. (b-a) / 2 The average PSR value is the sum of the values ​​of two adjacent calculation points.

[0016] The present invention discloses a horizontal variable stiffness foundation pit support structure, comprising a stiffness redundancy zone and a stiffness deficiency zone; the stiffness redundancy zone comprises interconnected piles and concrete supports; the stiffness deficiency zone comprises piles, an active stiffness adjustment device and concrete supports, wherein two sets of piles are arranged parallel to each other, and the piles are connected to the concrete supports through the active stiffness adjustment device.

[0017] Furthermore, in the stiffness redundancy zone, each pair of piles is connected by a capping beam and reinforced soil between the piles. The height of the reinforced soil between the piles is greater than or equal to two-thirds of the excavation depth of the foundation pit, in order to improve the horizontal bearing capacity of the piles. The capping beam acts as a capping beam. Connecting protrusions are provided on the capping beams, and steel hooks are provided on the connecting protrusions for connecting with the steel reinforcement cage of the piles, making the structure more stable and the connection more secure.

[0018] Furthermore, the pile-reinforcing soil is prepared by mixing 50-60 parts of sulfoaluminate cement clinker, 10-15 parts of gypsum, 0.1-0.5 parts of hydroxypropyl methylcellulose, 0.1-0.2 parts of polycarboxylate superplasticizer, and 1-2 parts of aluminum sulfate, and is a light yellow powder.

[0019] Furthermore, the active stiffness adjustment device is connected to the concrete support through the lower steel plate and to the variable cross-section force transmission member through the upper steel plate; the variable cross-section force transmission member is connected to the capping beam; the cross-section of the variable cross-section force transmission member on the side closer to the active stiffness adjustment device is smaller than the cross-section on the side closer to the capping beam. For the additional force applied to the variable cross-section force transmission member by the active stiffness adjustment device, the variable cross-section member can achieve stress diffusion and transfer it to the capping beam, avoiding stress concentration that could lead to structural failure, thus playing a role in stress diffusion and achieving a better effect in adjusting the support stiffness.

[0020] Furthermore, the active stiffness adjustment device includes hydraulic jacks, hydraulic pipes, prestressing tendons, irregularly shaped supports, and an outer wall. The outer wall is evenly divided by the prestressing tendons, with hydraulic jacks and irregularly shaped supports spaced apart. The hydraulic pipes are connected to the hydraulic jacks, and the prestressing tendons are pre-tensioned. The cross-section of the outer wall is preferably regular hexagonal. The steel outer wall is then tightened.

[0021] Furthermore, the hydraulic oil pipe includes an oil pipe valve, a flexible oil pipe interface, and an oil pipe plug. The flexible oil pipe interface is equipped with an oil pipe valve and an oil pipe plug, which can achieve complete sealing of the oil circuit.

[0022] Furthermore, the irregularly shaped support structure includes a support rod, an axial force monitoring rod, and a resistance strain gauge. The support rod and the axial force monitoring rod form a symmetrical structure with the same centroid. The axial force monitoring rod is mounted on the support rod, and the resistance strain gauge is mounted on the axial force monitoring rod. The resistance strain gauge is connected to a strain acquisition instrument via wires to achieve real-time monitoring of deformation. The center position of the axial force monitoring rod is the same as that of the support rod, and both are made of materials with high compressive stiffness. Both bear external loads, and their length is half the height of the support rod. Both can withstand vertical forces.

[0023] Working principle: When the monitored deformation value reaches the preset alarm value, the hydraulic oil pump is turned on, and the hydraulic oil is pumped into the hydraulic jack through the hydraulic oil pipe. The hydraulic jack applies a horizontal thrust to the support piles outward of the foundation pit. After stabilization, the flow of hydraulic oil in the hydraulic oil pipe is blocked by the oil pipe valve, flexible oil pipe interface and oil pipe plug, locking the hydraulic jack. In this way, servo deformation control of the foundation pit support structure can be realized.

[0024] A resistance strain gauge is attached to the displacement monitoring rod, and a strain acquisition device is connected to its outer end for real-time monitoring. Simultaneously, the axial force of the rod can be calculated based on the bending stiffness EA of the axial force monitoring rod. When the axial force or deformation reaches the alarm value, hydraulic oil is pumped into the hydraulic jack via a hydraulic pump to further increase the supporting axial force, achieving servo control of the support system. After this operation is completed, the hydraulic jack can be locked by sealing the hydraulic oil in the pipeline using a pipe valve, flexible pipe interface, and pipe plug.

[0025] Beneficial effects: Compared with the prior art, the present invention has the following significant features:

[0026] 1. No longer confined to the traditional design theory of foundation pit design (assuming a two-dimensional plane), but considering the spatial effect of the foundation pit, utilizing the corner effect of the foundation pit (the inner corner suppresses the deformation of the foundation pit), a dimensionless quantity (plane strain ratio, PSR) is introduced to divide the stiffness of the support structure into zones for variable stiffness design. For areas with excessive stiffness, the optimization expression based on PSR can effectively reduce construction costs while ensuring that the deformation remains almost constant. For areas with insufficient stiffness, by increasing the stiffness, the large deformation of the support structure in the middle span is reduced, and the impact of foundation pit construction on the surrounding environment is reduced, effectively realizing the horizontal variable stiffness design of the foundation pit support structure.

[0027] 2. For areas with insufficient stiffness, stiffness is enhanced through two effective means. First, the soil between the double-row piles is reinforced. The reinforcement depth is determined by the deformation characteristics of the foundation pit retaining structure and should not be less than 2 / 3 of the excavation depth. This method can effectively improve the horizontal bearing capacity of the support piles. Second, a prefabricated active stiffness adjustment device is added to the support structure. This device can monitor structural deformation in real time and adjust stiffness as needed, apply axial force, improve the stiffness of the support system, and achieve the purpose of servo control.

[0028] 3. The active stiffness adjustment device can adjust the support stiffness in real time as needed, overcoming the drawback of passive devices that can only be manufactured and installed according to the preset bearing capacity and deformation limits.

[0029] 4. The hydraulic jacks and irregular support bodies in the stiffness adjustment device are arranged at intervals. Each hydraulic jack corresponds to a hydraulic oil pump. When adjusting the support stiffness, eccentric adjustment can be achieved. At the same time, the oil pipe valve, oil pipe plug and flexible oil pipe interface on the hydraulic oil pipe can seal the hydraulic oil in the hydraulic oil pipe in emergency situation or when the stiffness requirement is met, so as to lock the displacement of the jack. Attached Figure Description

[0030] Figure 1 is a flowchart of the design process of this invention.

[0031] Figure 2 is a schematic diagram of the structure of the present invention.

[0032] Figure 3 is a cross-sectional view of the pile 1 of the present invention.

[0033] Figure 4 is a structural schematic diagram of the pressure cap beam 11 of the present invention.

[0034] Figure 5 is a schematic diagram of the connection between the concrete support 7 and the capping beam 11 of the present invention.

[0035] Figure 6 is a schematic diagram of the active stiffness adjustment device 4 of the present invention.

[0036] Figure 7 is a schematic diagram of the irregular support 44 of the present invention. Detailed Implementation

[0037] As shown in Figure 1, the design method for a horizontal variable stiffness foundation pit support structure includes the following steps:

[0038] ① The foundation pit was modeled in three dimensions using finite element analysis software based on the preliminary design results.

[0039] ②Analyze the lateral displacement of the support structure and calculate the ratio of the deformation value of each section to the deformation value under plane strain, thereby obtaining the plane strain ratio of the entire support section.

[0040] ③ For areas where PSR < 1, the structural stiffness is considered redundant, referred to as the stiffness excess zone. The corner effect of the foundation pit should be considered, and the conventional support structure of single-row pile 1 + concrete support 7 should still be used, but stiffness adjustment is required according to the proposed design optimization expression. For areas where PSR = 1, the stiffness is considered insufficient, referred to as the stiffness deficiency zone. Therefore, to achieve coordinated deformation of the support structure, minimize the maximum deformation value, and reduce the impact on the surrounding environment, a suitable support structure of double-row pile 1 + active stiffness adjustment device 4 + concrete support 7 is adopted to enhance the support stiffness in the mid-span area and achieve variable stiffness design of the support structure in the horizontal direction. After optimization and adjustment, the support structure at the corner of the foundation pit can save materials while ensuring no significant increase in lateral deformation, while the deformation of the middle support structure can be effectively reduced, achieving horizontal variable stiffness leveling of the support structure.

[0041] Optimized expression based on PSR:

[0042] D d =D d0 +PSR (b-a) / 2 (D0-D d0 )

[0043] Among them, D d The optimized support pile diameter is D0, where D is the pile diameter designed based on a two-dimensional plane. d0 The critical pile diameter is specifically the recommended minimum value given in the "Technical Specification for Foundation Pit Support" (JGJ 120-2012), PSR. (b-a) / 2 The average PSR value is the sum of the values ​​of two adjacent calculation points.

[0044] As shown in Figures 2-4, the horizontal variable stiffness foundation pit support system includes pile rows 1, variable cross-section force transmission components 3, active stiffness adjustment devices 4, and concrete supports 7. In the stiffness redundancy zone 8 where PSR < 1, pile rows 1 adopt a single-row pile structure. In the stiffness deficiency zone 9 where PSR = 1, a double-row pile structure is adopted, and the two parallel rows of piles 1 should be used for soil reinforcement 2 from a depth not less than 2 / 3 times the excavation depth below the ground surface. The foundation pit has a corner effect, which was not considered in the existing design, which was based on a two-dimensional plane strain state, resulting in structural stiffness redundancy at the corners. The solidified soil material is prepared by mixing 50-60 parts of sulfoaluminate cement clinker, 10-15 parts of gypsum, 0.1-0.5 parts of hydroxypropyl methylcellulose, 0.1-0.2 parts of polycarboxylate superplasticizer, and 1-2 parts of aluminum sulfate, and is a pale yellow powder. The top of the pile 1 is provided with a post-cast groove 12, and the capping beam 11 is provided with a connecting protrusion 111 and a steel bar hook 112 that match the post-cast groove 12. The steel bar hook 112 can be connected to the steel bar skeleton inside the pile 1, making the structure more stable and the pile 1 can provide greater horizontal bearing capacity.

[0045] As shown in Figure 5, the support system includes a variable cross-section force transmission component 3, an active stiffness adjustment device 4, an upper steel pad 5, a lower steel pad 6, and a concrete support 7. One end of the variable cross-section force transmission component 3 is fixed to the capping beam 13, with a larger cross-section, while the other end is fixed to the upper steel pad 5, with a smaller cross-section. This special structural form enables stress diffusion. The active stiffness adjustment device 4 is fixed at both ends to the upper steel pad 5 and the lower steel pad 6, respectively. The lower steel pad 6 is then fixed to the concrete support 7, thus forming a complete servo support system capable of active control.

[0046] As shown in Figures 6 and 7, the active stiffness adjustment device 4 includes hydraulic jacks 41, hydraulic oil pipes 42, prestressing tendons 43, irregularly shaped support bodies 44, and a steel outer wall 45. The active stiffness adjustment device 4 has a regular hexahedron cross section. The hydraulic jacks 41 and irregularly shaped support bodies 44 are fixedly arranged at intervals inside the active stiffness adjustment device 4. The prestressing tendons 43 are placed horizontally in the middle of the active stiffness adjustment device 4 and are pre-stressed to give them a tendency to contract initially, which can tighten the entire steel outer wall 45, making the overall structure more stable. Each hydraulic jack 41 is connected to a hydraulic oil pipe 42, and hydraulic oil can be pumped by a hydraulic pump to make the hydraulic jack 41 start working. The irregular support 44 includes a support rod 441, an axial force monitoring rod 442, and a resistance strain gauge 443. The support rod 441 and the axial force monitoring rod 442 are made of the same material, both with high bending stiffness. The center point of the axial force monitoring rod 442 coincides with the center point of the support rod 441, and its length is 0.5 times that of the support rod 441. A resistance strain gauge 443 is attached to the axial force monitoring rod 442 and connected to a strain acquisition instrument, enabling real-time monitoring of the generated strain or axial force. When the change reaches the alarm value, the hydraulic oil pump is activated, pumping hydraulic oil to each hydraulic jack 41 according to actual needs. This generates a larger supporting axial force in the entire support structure, thereby improving the horizontal support stiffness of the support pile. Once the safety requirements are met, the hydraulic jacks 41 are locked by controlling the oil pipe valve 421, flexible oil pipe interface 422, and oil pipe plug 423 installed on the hydraulic oil pipe 42. Through this method, servo control / active control of the support system is achieved.

[0047] The construction method for the above-mentioned horizontally variable stiffness foundation pit support system specifically includes the following steps:

[0048] Step 1: Using three-dimensional finite element analysis software, model and analyze the foundation pit support structure obtained from the two-dimensional planar design, calculate the lateral deformation characteristics of the foundation pit support structure, and further obtain a dimensionless quantity: plane strain ratio (PSR).

[0049] Step 2: For the region where PSR is less than 1, it is called the stiffness redundancy zone 8. The foundation pit support structure in this region is optimized using an optimization expression based on PSR control. For the region where PSR = 1, it is called the stiffness deficiency zone 9. The foundation pit support structure in this region should be designed to enhance stiffness.

[0050] Step 3: Lay out and position the foundation pit support structure. For areas with excessive stiffness, adopt the single-row pile support structure of the original design, with conventional concrete supports 7. For areas with insufficient stiffness, adopt a combined support structure of pile system 1 + active stiffness adjustment device 4 + concrete supports 7. Reinforce the soil within a range of not less than 2 / 3 times the excavation depth between the two rows of piles 11 to improve the horizontal bearing capacity of the support piles. At the same time, construct the capping beam 11, and ensure that the steel bar hooks 112 of the capping beam 11 are engaged with the steel bar skeleton of the support piles.

[0051] Step four, construction support structure, including the variable cross-section force transmission component 3, upper steel pad 5, prefabricated active stiffness adjustment device 4, lower steel pad 6, and concrete support 7 arranged sequentially. The hydraulic jack 41 inside the active stiffness adjustment device 4 is connected to the hydraulic pump through the hydraulic oil pipe 42, and the resistance strain gauge 443 on the axial force monitoring rod 442 is connected to the strain detection device through the wire to achieve the effect of real-time monitoring of deformation.

[0052] Step 5: When the monitored deformation value reaches the preset alarm value, turn on the hydraulic oil pump and pump the hydraulic oil into the hydraulic jack 41 through the hydraulic oil pipe 42. The hydraulic jack 41 applies a horizontal thrust to the support piles towards the outside of the foundation pit. After stabilization, the flow of hydraulic oil in the hydraulic oil pipe is blocked through the oil pipe valve 421, the flexible oil pipe interface 422 and the oil pipe plug 423, and the hydraulic jack 41 is locked. In this way, servo deformation control of the foundation pit support structure can be realized.

Claims

1. A design method for a horizontally variable stiffness foundation pit support structure, characterized in that, Includes the following steps: Step 1: Use three-dimensional finite element analysis software to perform three-dimensional modeling of the foundation pit support structure; Step 2: Analyze the lateral displacement of the support structure and calculate the ratio of the deformation value of each section to the deformation value of the plane strain state, thereby obtaining the plane strain ratio PSR of the entire support section; Step 3: For areas where the plane strain ratio is less than 1, the structural stiffness is redundant. Considering the corner effect of the foundation pit, connect the pile (1) with the concrete support (7) and adjust the stiffness according to the optimization expression of the plane strain ratio; For regions where the plane strain ratio is equal to 1, the structural stiffness is insufficient. Two sets of piles (1) are set up and connected to the concrete support (7) through an active stiffness adjustment device (4). In step two, the plane strain ratio is the ratio of the maximum deformation of a section under three-dimensional conditions to the maximum deformation of that section under plane strain conditions. In step three, the optimized expression for the plane strain ratio is: Among them, D d The optimized support pile diameter is D0, where D is the pile diameter designed based on a two-dimensional plane. d0 For critical pile diameter, PSR (b-a) / 2 The average PSR value of two adjacent calculation points; the active stiffness adjustment device (4) includes a hydraulic jack (41), a hydraulic oil pipe (42), a prestressing tendon (43), a shaped support body (44) and an outer wall (45). The outer wall (45) is evenly divided by the prestressing tendon (43) and the hydraulic jack (41) and the shaped support body (44) are arranged at intervals. The hydraulic oil pipe (42) is connected to the hydraulic jack (41). The prestressing tendon (43) is pre-stressed. The shaped support body (44) includes a support rod (441), an axial force monitoring rod (442) and a resistance strain gauge (443). The support rod (441) and the axial force monitoring rod (442) form a symmetrical structure with the same centroid. The resistance strain gauge (443) is arranged on the axial force monitoring rod (442) and is connected to the strain acquisition instrument.

2. The foundation pit support structure obtained by the design method of a horizontal variable stiffness foundation pit support structure according to claim 1, characterized in that: It includes a stiffness redundancy zone (8) and a stiffness deficiency zone (9); the stiffness redundancy zone (8) includes interconnected piles (1) and concrete supports (7); the stiffness deficiency zone (9) includes piles (1), an active stiffness adjustment device (4) and concrete supports (7), the piles (1) are arranged in two sets and are parallel to each other, and the piles (1) are connected to the concrete supports (7) through the active stiffness adjustment device (4).

3. The horizontal variable stiffness foundation pit support structure according to claim 2, characterized in that: The piles (1) in the stiffness-deficient area (9) are connected in pairs by a capping beam (11) and a pile reinforcement soil (2), the height of which is greater than or equal to two-thirds of the excavation depth of the foundation pit.

4. A horizontal variable stiffness foundation pit support structure according to claim 3, characterized in that: The top beam (11) is provided with a connecting protrusion (111), and the connecting protrusion (111) is provided with a steel hook (112) for connecting with the steel reinforcement skeleton of the pile (1).

5. A horizontal variable stiffness foundation pit support structure according to claim 3, characterized in that: The active stiffness adjustment device (4) is connected to the concrete support (7) through the lower steel pad (6) and to the variable cross-section force transmission member (3) through the upper steel pad (5); the variable cross-section force transmission member (3) is connected to the top beam (11); the cross-section of the variable cross-section force transmission member (3) on the side closer to the active stiffness adjustment device (4) is smaller than the cross-section on the side closer to the top beam (11).

6. A horizontal variable stiffness foundation pit support structure according to claim 2, characterized in that: The hydraulic oil pipe (42) includes an oil pipe valve (421), a flexible oil pipe interface (422), and an oil pipe plug (423). The flexible oil pipe interface (422) is provided with an oil pipe valve (421) and an oil pipe plug (423).

Images