Zero-distance anti-displacement steel plate supporting method for existing double-step bearing platform of deep foundation pit

Abstract

The invention relates to the technical field of civil construction, in particular to a zero-distance anti-displacement steel plate supporting method for an existing double-step bearing platform of a deep foundation pit. According to the technical scheme, the technical problem that under the extreme working conditions that a foundation pit with the depth of-8 m is zero in distance from the bearing platform and piling and bolt planting cannot be conducted, accurate displacement prevention protection is conducted on an existing double-step bearing platform is solved. In the construction process, firstly, layered excavation is conducted, and a supporting benchmark is arranged; hoisting an anti-displacement steel plate wall with preset grouting holes, and reinforcing a soil body at the bottom of the anti-displacement steel plate wall by injecting cement and the like; assembling a first-order reinforced occlusion hoop on the lower-order bearing platform; a three-stage multi-dimensional adjustable inclined support is installed, and graded pre-tightening force is applied; deep excavation is continued under dynamic monitoring, and the support is adjusted in time; and finally, orderly backfilling and dismantling are conducted. Through the combination of grouting reinforcement, hoop occlusion limiting and graded adjustable inclined struts, low-disturbance and high-precision displacement control over the existing bearing platform is achieved, the lower prefabricated pile is effectively protected, and construction safety and efficiency are remarkably improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of civil engineering construction technology, in particular to a deep foundation existing double-step pile cap zero-distance displacement prevention steel plate support method. BACKGROUND

[0002] In the upgrading and reconstruction process of continuous casting machines in steel enterprises, deep foundation excavation is often needed in the area close to the existing equipment foundation. Such foundation is often deep, and the distance between it and the existing double-step concrete pile cap is extremely small, even zero distance. At this time, the side of the pile cap is completely exposed, and its stability is seriously threatened by excavation unloading. How to safely and accurately complete the foundation support under the premise of not affecting the normal operation of the upper structure and not damaging the original foundation is the core technical challenge faced by such reconstruction projects.

[0003] At present, the common technical route for deep foundation support adjacent to existing foundation often relies on the combination of active or passive soil reinforcement and enclosure structure. For example, micro piles, mixing piles and other vertical reinforcement bodies are combined with soil nails or anchor rods to form a composite support system; or steel sheet piles, bored piles and other enclosure piles are used, and concrete support beams or steel supports are set at the top of the piles to form an internal support system. Attempts are made to strengthen the strength of the soil body or set a retaining structure to maintain the stability of the foundation. The mechanical principle mainly relies on the embedded action of the pile body, the uplift resistance of the anchor rod or the axial pressure of the internal support to balance the lateral soil pressure of the foundation.

[0004] However, in the extreme limited working condition of traditional zero-distance deep foundation with dense pile cap reinforcement, the existing technology exposes its inherent defects. Whether it is to drill micro piles or to set anchor rods, a certain construction operation space is needed, and the drilling and setting operation will cause damage to the pile cap structure itself and may disturb the important precast pile foundation at the bottom of the pile cap. In addition, the traditional support structure often does not closely connect with the foundation excavation process, the support force is fixed and cannot be adjusted, and it is difficult to dynamically adapt to the complex changes of soil pressure in the process of deep foundation excavation in steps, resulting in insufficient control accuracy of horizontal and settlement displacement of the existing pile cap, which cannot meet the requirements of high-precision engineering protection. SUMMARY

[0005] The purpose of the present application is to solve the problems in the background art, and to provide a deep foundation existing double-step pile cap zero-distance displacement prevention steel plate support method.

[0006] The present application provides a deep foundation existing double-step pile cap zero-distance displacement prevention steel plate support method, comprising the following steps: S1, survey the range of 8m deep foundation pit and the parameters of existing double-step pile cap, and make the lower-step pile cap as the core protection object; precast displacement-resistant steel plate wall, multi-dimensional hierarchical adjustable diagonal support and first-step reinforced interlocking hoop; set up multi-dimensional monitoring points and set early warning threshold; use precipitation method to reduce underground water level to below-8.5m; S2, start layered excavation of surface soil from-4.5m elevation to-5.5m elevation; preset steel plate wall installation reference surface on the side wall of foundation pit and calibrate perpendicularity; set up temporary protection frame outside existing lower-step pile cap; S3, hoist displacement-resistant steel plate wall close to the side wall of foundation pit and existing lower-step pile cap; inject cement-silicate double liquid slurry through preset grouting hole at the bottom of steel plate wall to form reinforced layer; use temporary support to fix steel plate wall during grouting; S4, set first-step reinforced interlocking hoop on existing lower-step pile cap and splice and fix through high-strength tension bolt; S5, set up multi-dimensional adjustable diagonal support according to upper, middle and lower three levels; apply hierarchical pre-tightening force through hydraulic adjusting unit and dynamically calibrate support angle; S6, continue layered excavation to-8m design elevation; during excavation process, real-time monitor and dynamically adjust pre-tightening force of corresponding level diagonal support; S7, real-time monitor during whole construction process; when monitoring value approaches early warning threshold, suspend excavation and adjust support parameters; S8, after completion of continuous casting machine foundation construction, backfill foundation pit soil in layers; during backfill process, sequentially remove multi-dimensional hierarchical diagonal support, first-step reinforced interlocking hoop and displacement-resistant steel plate wall.

[0007] Optionally, the support system composed of precast displacement-resistant steel plate wall, multi-dimensional hierarchical adjustable diagonal support and first-step reinforced interlocking hoop and its installation and construction process do not need to pile in foundation pit and do not need to implant anchor bolt on existing double-step pile cap.

[0008] Optionally, in the steps of hoisting steel plate wall and grouting reinforcement at the bottom, the water-cement ratio of injected cement-silicate double liquid slurry is 0.5-0.6, the volume ratio of cement to silicate is 1:0.3-0.5, and the grouting pressure is 0.5-0.8MPa; the thickness of formed reinforced layer is 200-300mm, and the reinforced range extends 500-600mm along the soil on both sides of steel plate wall bottom.

[0009] Optionally, in the steps of multi-dimensional hierarchical diagonal support installation and pre-tightening, the hierarchical pre-tightening force applied through hydraulic adjusting unit is: upper level support 120-180kN, middle level support 180-240kN, and lower level support 240-300kN; the dynamic calibration angle of multi-dimensional hierarchical adjustable diagonal support with horizontal plane is 30-60°.

[0010] Optionally, during deep excavation and dynamic monitoring, the dynamic adjustment of the support preload is specifically as follows: after each layer of soil is excavated and when the monitoring value approaches the warning threshold, the preload of the corresponding layer of diagonal bracing is adjusted up or down through the hydraulic adjustment unit according to the monitoring data.

[0011] Optionally, in the first-stage reinforced interlocking clamp assembly step, the first-stage reinforced interlocking clamp is fitted onto the existing lower stage bearing platform, so that the top limiting plate of the clamp fits against the upper and lower stage connection surface of the bearing platform and the bottom support plate fits against the bottom surface of the lower stage bearing platform; the clamp body is spliced ​​and fixed by 10.9 grade high-strength tension bolts, so that the densified interlocking tooth plate on the inner side of the clamp is embedded 5-8mm into the concrete surface of the bearing platform.

[0012] Optionally, the anti-displacement steel plate wall is made of Q355B steel, with a plate thickness of 20-30mm and a plate height of 7.5-8.5m. Its bottom has pre-set grouting holes with a diameter of 30-40mm and a hole spacing of 300-500mm. The clamp body of the first-stage reinforced interlocking clamp is made of steel plate with a thickness of 18-22mm by welding. The support rod of the multi-dimensional graded adjustable oblique support is made of Q460C steel.

[0013] Optionally, 2-3 displacement monitoring points are set on each side of the existing lower step foundation, one displacement monitoring point is set on the anti-displacement steel plate wall at every 1.5-2.5m interval, and one earth pressure monitoring point is set on the soil of the foundation pit side wall at every 3m interval; the early warning threshold is set to 80% of the design limit value of each monitoring item; the monitoring frequency throughout the construction process is not less than once every 30 minutes.

[0014] Optionally, during the layered backfilling process, the thickness of each backfill layer shall be ≤300mm and the compaction degree shall be ≥95%. During the backfilling process, the multi-dimensional graded diagonal bracing shall be removed in the order of bottom to top, and finally the first-stage reinforced interlocking clamp and steel plate wall shall be removed.

[0015] Optionally, after removing the support structure, the scratches on the surface of the existing foundation caused by construction can be repaired with high-strength polymer mortar, and the exposed surface can be coated with epoxy resin anti-corrosion paint.

[0016] In summary, this application includes at least one of the following beneficial technical effects: This invention, through a combination of pre-protection and targeted support, along with bottom grouting for low-disturbance reinforcement, can effectively control the horizontal displacement of the foundation cap to within ≤1.5mm and the settlement to within ≤2.5mm, minimizing disturbance to the lower precast piles and ensuring the safety of the original structural system.

[0017] Furthermore, by combining innovative pileless and anchorless design with layered collaborative construction technology, and replacing traditional toothed plates with grouting reinforcement layers, the construction challenges under multiple constraints such as -8m deep foundation pits, zero-distance working surfaces, prohibition of bolting, and the need to protect the lower precast piles have been solved, thus broadening the application boundaries of deep foundation pit support technology for existing structures.

[0018] Finally, a three-level multi-dimensional adjustable diagonal bracing system was adopted in conjunction with dynamic monitoring throughout the entire process, which enabled real-time visibility and adjustment of the support force and allowed it to actively adapt to high lateral pressure conditions. At the same time, the processes were closely linked, with support installation and excavation occurring simultaneously, and the grouting material set quickly and hardened rapidly, resulting in a significant reduction in the overall construction period compared to traditional methods, and achieving both excellent safety and economy. Attached Figure Description

[0019] Figure 1 A flowchart of the present invention is provided for a method for zero-distance anti-displacement steel plate support for existing double-stage foundation caps in deep foundation pits. Detailed Implementation

[0020] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0021] like Figure 1 As shown, this invention proposes a method for zero-distance anti-displacement steel plate support for existing double-stage foundation caps in deep foundation pits, comprising a support system consisting of prefabricated anti-displacement steel plate walls, multi-dimensional graded adjustable diagonal supports, and first-stage reinforced interlocking clamps, as well as its installation and construction process. As one implementation method, the support system consisting of prefabricated anti-displacement steel plate walls, multi-dimensional graded adjustable diagonal supports, and first-stage reinforced interlocking clamps requires no piling within the foundation pit, nor does it require the insertion of anchor bolts into the existing double-stage foundation caps. The support system is described in detail below: In this embodiment, a steel plate wall is hoisted and tightly attached to the sidewall of the foundation pit to directly bear the active earth pressure. A first-stage reinforced interlocking clamp tightly wraps around and interlocks with the existing lower-stage foundation, transforming it into a stable load-bearing whole and internal support base. Multi-dimensional, graded, adjustable diagonal supports effectively transfer the earth pressure borne by the steel plate wall to the clamps and the foundation body. This achieves zero-distance physical contact with the existing foundation and avoids any damage to the foundation's concrete structure from drilling and rebar installation, thus perfectly adapting to extreme working conditions where space is extremely limited and the integrity of the original foundation cannot be compromised. The stability of the entire system does not depend on the embedment of the piles or the pull-out resistance of the anchors, but rather on the statically determinate or statically indeterminate support structure formed between the aforementioned components.

[0022] likeFigure 1 As shown, this support construction method also includes steel plate wall hoisting and bottom grouting reinforcement steps. In one implementation method, during the steel plate wall hoisting and bottom grouting reinforcement steps, the injected cement-water glass dual-liquid grout has a water-cement ratio of 0.5-0.6, a cement to water glass volume ratio of 1:0.3-0.5, and a grouting pressure of 0.5-0.8 MPa. The resulting reinforcement layer is 200-300 mm thick, and the reinforcement range extends 500-600 mm along both sides of the bottom of the steel plate wall. The steel plate wall hoisting and bottom grouting reinforcement steps are described in detail below: In this embodiment, firstly, the two-component grout has controllable rapid setting characteristics (initial setting time ≤ 4 hours), enabling it to quickly solidify in the soil at the base of the steel plate wall. Secondly, by controlling the water-cement ratio (0.5-0.6), the cement to water glass volume ratio (1:0.3-0.5), and the grouting pressure (0.5-0.8MPa), the grout can effectively penetrate and compact the soil around the base of the steel plate wall (within a range of 500-600mm on both sides) under pressure, displacing pore water and air. Finally, after solidification, a dense, 200-300mm thick plate-like reinforcement layer is formed. The working principle of this reinforcement layer is equivalent to providing an extended foundation and anti-slip key for the steel plate wall. It greatly increases the vertical bearing capacity and horizontal anti-slip resistance at the base of the steel plate wall, diffusing the concentrated stress originally acting on the base of the steel plate wall to a wider range of soil, thereby effectively suppressing the settlement and skirting displacement of the steel plate wall, which is a key link in ensuring the stability of the bottom of the support system.

[0023] like Figure 1 As shown, this support construction method also includes a multi-dimensional graded diagonal brace installation and pre-tightening step. In one implementation, the graded pre-tightening force applied by the hydraulic adjustment unit in the multi-dimensional graded diagonal brace installation and pre-tightening step is: 120-180kN for the upper support, 180-240kN for the middle support, and 240-300kN for the lower support. The dynamic calibration of the angle between the multi-dimensional graded adjustable diagonal brace and the horizontal plane is 30-60°. The multi-dimensional graded diagonal brace installation and pre-tightening step is explained in detail below: In this embodiment, a hydraulic adjustment unit applies graded pre-tightening forces to each level of the support (upper level 120-180kN, middle level 180-240kN, lower level 240-300kN), enabling the active application of a support force opposite to the expected earth pressure to the support system before or at the initial stage of excavation. This can pre-counteract some soil deformation, significantly reducing the actual displacement of the retaining structure during the excavation process. The dynamic calibration of the support's angle with the horizontal plane to 30-60° optimizes the stress state of the support members, placing them in a more reasonable state of predominant axial compression, and facilitating the decomposition and transmission of horizontal earth pressure to the clamp-and-pillar system serving as the fulcrum. The graded setting allows for differentiated and refined balance control of earth pressure at different depths and development stages.

[0024] like Figure 1 As shown, this support construction method also includes a deep excavation and dynamic monitoring process. As one implementation method, during the deep excavation and dynamic monitoring process, the dynamic adjustment of the support preload is specifically as follows: after each layer of soil is excavated and when the monitoring value approaches the warning threshold, the preload of the corresponding level of diagonal bracing is adjusted upwards or downwards via a hydraulic adjustment unit based on the monitoring data. The deep excavation and dynamic monitoring process is explained in detail below: In this embodiment, after each layer of soil is excavated (≤1.5m), the soil stress state changes. Data such as support axial force and pile cap displacement are collected in real time through a monitoring system. This data is compared and analyzed with early warning thresholds. If the data change rate is abnormal or close to the threshold, the preload of the corresponding layer's diagonal bracing is adjusted upwards or downwards via a hydraulic adjustment unit. This makes the support system an adaptive system. It can respond to the redistribution of soil pressure caused by excavation, dynamically adjusting the internal force system to maintain a dynamic balance between the support structure and soil pressure. This keeps the displacement of the pile cap and retaining structure within millimeter range, achieving controllability and safety during construction.

[0025] like Figure 1 As shown, this support construction method also includes a first-stage reinforced interlocking clamp assembly step. In one implementation, the first-stage reinforced interlocking clamp is fitted onto the existing lower-level foundation, with the top limiting plate of the clamp fitting against the connecting surface between the upper and lower levels of the foundation, and the bottom support plate fitting against the bottom surface of the lower-level foundation. The clamp body is then fixed using 10.9 grade high-strength tension bolts, so that the densely interlocking teeth on the inner side of the clamp are embedded 5-8mm into the concrete surface of the foundation. The assembly steps of the first-stage reinforced interlocking clamp are described in detail below: In this embodiment, a top limiting plate is tightly fitted to the connecting step surfaces of the upper and lower steps of the foundation to resist possible upward lifting or horizontal rotation deformation of the foundation. A bottom support plate fits against the bottom surface of the lower step of the foundation, providing vertical support and preventing minor settlement of the foundation caused by the unloading of the foundation soil due to excavation. The inner, densely packed interlocking teeth are pressed 5-8mm into the concrete surface of the foundation under a torque of 800-1000 N·m applied by 10.9 grade high-strength tension bolts, generating strong static friction and mechanical interlocking force, tightly binding the clamp to the foundation into a single unit. The clamp body then serves as a unified support for all diagonal bracing members, uniformly and reliably transmitting the supporting force to the entire foundation cross-section.

[0026] like Figure 1As shown, this support construction method also includes an anti-displacement steel plate wall. In one implementation method, the anti-displacement steel plate wall uses Q355B steel, with a plate thickness of 20-30mm and a plate height of 7.5-8.5m. Its bottom has pre-set grouting holes with a diameter of 30-40mm and a spacing of 300-500mm. The clamp body of the first-stage reinforced interlocking clamp is made of steel plates with a thickness of 18-22mm welded together. The support rod of the multi-dimensional graded adjustable oblique support uses Q460C steel. The anti-displacement steel plate wall is described in detail below: In this embodiment, the anti-displacement steel plate wall uses Q355B steel with a plate thickness of 20-30mm, ensuring sufficient bending strength and stiffness as the main retaining component to resist the enormous lateral earth pressure generated by the -8m deep foundation pit. Pre-set grouting holes with a diameter of 30-40mm and a spacing of 300-500mm at the bottom ensure the formation of a grouting reinforcement layer. The first-stage reinforced interlocking clamp is made of welded steel plates with a thickness of 18-22mm, ensuring its strength and integrity as a force transmission node. The support rods of the multi-dimensional graded diagonal bracing use higher-strength Q460C steel, ensuring no instability or yielding occurs when subjected to preloads of up to 240-300kN and possible additional loads. The combination of materials and structural parameters provides a fundamental material basis for the safe operation of the entire support system under extreme working conditions.

[0027] like Figure 1 As shown, this support construction method also includes displacement monitoring points. As one implementation method, 2-3 displacement monitoring points are set up on each side of the existing lower step foundation, one displacement monitoring point is set up every 1.5-2.5m on the anti-displacement steel plate wall, and one earth pressure monitoring point is set up every 3m in the soil of the foundation pit sidewall. The early warning threshold is set to 80% of the design limit value of each monitoring item. The monitoring frequency throughout the construction process is no less than once every 30 minutes. The displacement monitoring points are explained in detail below: In this embodiment, 2-3 displacement monitoring points are set up on each side of the existing lower-level foundation to directly monitor the displacement state of the protected object, which is the final indicator for evaluating the support effect. A displacement monitoring point is set up at every 1.5-2.5m interval on the anti-displacement steel plate wall to plot the overall deformation curve of the retaining structure and determine whether its stress is uniform. A soil lateral pressure monitoring point is set up at every 3m interval in the soil of the foundation pit sidewall to sense the actual magnitude and distribution of soil pressure and verify design assumptions. Setting the early warning threshold to 80% of the design limit value is based on the principle of safety redundancy, reserving time and space for intervention and adjustment. The monitoring frequency throughout the construction process is no less than once every 30 minutes, ensuring the continuity of data collection and enabling timely capture of any subtle changes caused by construction activities, providing real-time basis for decision-making.

[0028] like Figure 1As shown, this support construction method also includes layered backfilling. As one implementation method, during layered backfilling, each layer has a thickness ≤300mm and a compaction degree ≥95%. During backfilling, the multi-dimensional graded diagonal bracing is removed in a bottom-to-top sequence, and finally, the first-stage reinforced interlocking clamps and steel plate wall are removed. The following is a detailed explanation of the layered backfilling: In this embodiment, the passive earth pressure gradually generated by the layered backfill soil is used to replace and unload the active support force provided by the diagonal bracing system. Each backfill layer is ≤300mm thick and compacted to ≥95%, ensuring that the backfill soil itself has sufficient strength and stability. The multi-dimensional tiered diagonal bracing is removed in a bottom-up order because as the backfill surface rises, the supporting effect of the lower soil on the steel plate wall becomes stronger, making it safe to remove the lowest layer of support first. Once the backfill reaches a certain height and the constraint provided by the soil is sufficient to maintain stability, the clamps and steel plate walls are then removed sequentially. This strictly follows the principle of gradual transfer of mechanical equilibrium, effectively avoiding the risk of secondary displacement of the foundation caused by sudden stress release due to improper removal.

[0029] like Figure 1 As shown, this support construction method also includes the process of dismantling the support structure. In one implementation method, after dismantling the support structure, scratches caused by construction on the surface of the existing foundation are repaired with high-strength polymer mortar, and epoxy resin anti-corrosion coating is applied to the exposed surfaces. The dismantling process of the support structure is described in detail below: In this embodiment, during construction, interlocking of clamps or collisions between components may leave minor scratches on the surface of the foundation. Repairing these scratches with high-strength polymer mortar is effective because its high bonding strength with the concrete substrate and low shrinkage can effectively seal surface defects and prevent moisture and corrosive media from penetrating the reinforcing steel. Applying epoxy resin anti-corrosion coating to the exposed surface forms a continuous, dense, and highly adhesive protective film, isolating the concrete from the external corrosive environment, thereby extending the service life of the existing foundation and ensuring the long-term benefits of the entire renovation project.

[0030] like Figure 1 As shown, the existing double-stage foundation cap zero-distance anti-displacement steel plate support method for this deep foundation pit also includes the following steps: S1. Investigate the scope of the -8m deep foundation pit and the parameters of the existing double-stage foundation, and identify the lower stage foundation as the core protection object; prefabricate anti-displacement steel plate walls, multi-dimensional graded adjustable diagonal supports and first-stage reinforced interlocking clamps; set up multi-dimensional monitoring points and set early warning thresholds; use dewatering to lower the groundwater level to below -8.5m; S2. Excavate the surface soil in layers from the -4.5m elevation to the -5.5m elevation; install a reference surface on the pre-installed steel plate wall on the side wall of the foundation pit and calibrate its verticality; erect a temporary protective frame on the outside of the existing lower step foundation. S3. Hoist the anti-displacement steel plate wall close to the side wall of the foundation pit and the existing lower step foundation; inject cement-water glass double liquid grout through the pre-set grouting holes at the bottom of the steel plate wall to form a reinforcement layer; use temporary supports to fix the steel plate wall during grouting. S4. The first-stage reinforced interlocking clamp is fitted onto the existing lower stage bearing platform and fixed by splicing with high-strength tension bolts. S5. Multi-dimensional adjustable diagonal supports are arranged in three levels: upper, middle, and lower. Graded preload is applied through a hydraulic adjustment unit, and the support angle is dynamically calibrated. S6. Continue excavating in layers to the -8m design elevation; during the excavation process, monitor and dynamically adjust the preload of the diagonal bracing at the corresponding layer in real time; S7. Real-time monitoring will be conducted throughout the entire construction process. When the monitoring value approaches the warning threshold, excavation will be suspended and the support parameters will be adjusted. S8. After completing the foundation construction of the continuous casting machine, backfill the foundation pit in layers; during the backfilling process, remove the multi-dimensional graded diagonal bracing, the first-stage reinforced interlocking clamp and the anti-displacement steel plate wall in sequence.

[0031] Specifically, in the S1 construction preparation phase, precise surveying and dewatering create a stable foundation for subsequent operations. S2 surface excavation and S3 steel plate wall installation and grouting form a preliminary coordinated approach of support before excavation and simultaneous support and excavation, quickly establishing the first line of defense. S4 clamp assembly and S5 diagonal brace installation and pre-tightening form the core load-bearing framework integrating the retaining wall and the foundation, transforming the support from passive bearing to active control. S6 deep excavation and S7 dynamic monitoring and control constitute the core execution and feedback loop, ensuring precise management of force and deformation during excavation through information technology. S8 backfilling and removal safely and orderly dismantle the temporary support system, completing a smooth transition of the load-bearing path. Each stage creates conditions for the next and is adjusted by its feedback, collectively solving comprehensive technical challenges under stringent constraints such as zero-distance, pileless and boltless foundation pits, protection of existing double-stage foundations and lower precast piles, achieving the comprehensive goals of safety, precision, and efficiency.

[0032] The above specific embodiments are merely several optional embodiments of the present invention. Based on the technical solutions of the present invention and the relevant teachings of the above embodiments, those skilled in the art can make various alternative improvements and combinations to the above specific embodiments.

Claims

1. A method for zero-distance anti-displacement steel plate support for existing double-stage foundation caps in deep foundation pits, characterized in that, The method includes: S1. Investigate the scope of the -8m deep foundation pit and the parameters of the existing double-stage foundation, and identify the lower stage foundation as the core protection object; prefabricate anti-displacement steel plate walls, multi-dimensional graded adjustable diagonal supports and first-stage reinforced interlocking clamps; set up multi-dimensional monitoring points and set early warning thresholds; use dewatering to lower the groundwater level to below -8.5m; S2. Excavate the surface soil in layers from the -4.5m elevation to the -5.5m elevation; install a reference surface on the pre-installed steel plate wall on the side wall of the foundation pit and calibrate its verticality; erect a temporary protective frame on the outside of the existing lower step foundation. S3. Hoist the anti-displacement steel plate wall close to the side wall of the foundation pit and the existing lower step foundation; inject cement-water glass double liquid grout through the pre-set grouting holes at the bottom of the steel plate wall to form a reinforcement layer; use temporary supports to fix the steel plate wall during grouting. S4. The first-stage reinforced interlocking clamp is fitted onto the existing lower stage bearing platform and fixed by splicing with high-strength tension bolts. S5. Multi-dimensional adjustable diagonal supports are arranged in three levels: upper, middle, and lower. Graded preload is applied through a hydraulic adjustment unit, and the support angle is dynamically calibrated. S6. Continue excavating in layers to the -8m design elevation; during the excavation process, monitor and dynamically adjust the preload of the diagonal bracing at the corresponding layer in real time; S7. Real-time monitoring will be conducted throughout the entire construction process. When the monitoring value approaches the warning threshold, excavation will be suspended and the support parameters will be adjusted. S8. After completing the foundation construction of the continuous casting machine, backfill the foundation pit in layers; during the backfilling process, remove the multi-dimensional graded diagonal bracing, the first-stage reinforced interlocking clamp and the anti-displacement steel plate wall in sequence.

2. The method for zero-distance anti-displacement steel plate support of existing double-stage foundation caps in deep foundation pits according to claim 1, characterized in that, The support system consisting of the prefabricated anti-displacement steel plate wall, multi-dimensional graded adjustable diagonal support, and first-stage reinforced interlocking clamp, as well as its installation and construction process, does not require pile driving in the foundation pit, nor does it require the implantation of anchor bolts on the existing double-stage bearing platform.

3. The method for zero-distance anti-displacement steel plate support of existing double-stage foundation caps in deep foundation pits according to claim 1, characterized in that, In the steel plate wall hoisting and bottom grouting reinforcement steps, the injected cement-water glass dual-liquid grout has a water-cement ratio of 0.5-0.6, a cement to water glass volume ratio of 1:0.3-0.5, and a grouting pressure of 0.5-0.8 MPa; the resulting reinforcement layer has a thickness of 200-300 mm, and the reinforcement range extends 500-600 mm along both sides of the bottom of the steel plate wall.

4. The method for zero-distance anti-displacement steel plate support of existing double-stage foundation caps in deep foundation pits according to claim 1, characterized in that, In the multi-dimensional graded diagonal brace installation and pre-tightening step, the graded pre-tightening force applied by the hydraulic adjustment unit is: 120-180kN for the upper support, 180-240kN for the middle support, and 240-300kN for the lower support; the dynamic calibration of the angle between the multi-dimensional graded adjustable diagonal support and the horizontal plane is 30-60°.

5. The method for zero-distance anti-displacement steel plate support of existing double-stage foundation caps in deep foundation pits according to claim 1, characterized in that, During deep excavation and dynamic monitoring, the dynamic adjustment of the support preload specifically involves adjusting the preload of the corresponding layer of diagonal bracing by means of the hydraulic adjustment unit after each layer of soil is excavated and when the monitoring value approaches the warning threshold.

6. The method for zero-distance anti-displacement steel plate support of existing double-stage foundation caps in deep foundation pits according to claim 1, characterized in that, In the assembly step of the first-stage reinforced interlocking clamp, the first-stage reinforced interlocking clamp is fitted onto the existing lower stage bearing platform, so that the top limiting plate of the clamp is in contact with the upper and lower stage connection surface of the bearing platform and the bottom support plate is in contact with the bottom surface of the lower stage bearing platform; the clamp body is spliced ​​and fixed by 10.9 grade high-strength tension bolts, so that the dense interlocking tooth plate on the inner side of the clamp is embedded 5-8mm into the concrete surface of the bearing platform.

7. A method for zero-distance anti-displacement steel plate support of existing double-stage foundation caps in deep foundation pits according to claim 1, characterized in that, The anti-displacement steel plate wall is made of Q355B steel, with a plate thickness of 20-30mm and a plate height of 7.5-8.5m. Its bottom has pre-set grouting holes with a diameter of 30-40mm and a hole spacing of 300-500mm. The clamp body of the first-stage reinforced interlocking clamp is made of steel plate with a thickness of 18-22mm by welding. The support rod of the multi-dimensional graded adjustable oblique support is made of Q460C steel.

8. A method for zero-distance anti-displacement steel plate support of existing double-stage foundation caps in deep foundation pits according to claim 1, characterized in that, Two to three displacement monitoring points are set up on each side of the existing lower step foundation, one displacement monitoring point is set up every 1.5 to 2.5m on the anti-displacement steel plate wall, and one earth pressure monitoring point is set up every 3m in the soil of the foundation pit side wall; the early warning threshold is set to 80% of the design limit value of each monitoring item; the monitoring frequency throughout the construction process is no less than once every 30 minutes.

9. A method for zero-distance anti-displacement steel plate support for existing double-stage foundation caps in deep foundation pits according to claim 1, characterized in that, During the layered backfilling process, the thickness of each backfill layer is ≤300mm and the compaction degree is ≥95%. During the backfilling process, the multi-dimensional graded diagonal bracing is removed in the order of bottom to top, and finally the first-stage reinforced interlocking clamp and steel plate wall are removed.

10. A method for zero-distance anti-displacement steel plate support for existing double-stage foundation caps in deep foundation pits according to claim 1, characterized in that, After the support structure is removed, the scratches on the surface of the existing foundation caused by construction are repaired with high-strength polymer mortar, and the exposed surface is coated with epoxy resin anti-corrosion paint.

Images