Narrow and deep foundation pit supporting structure and narrow and deep foundation pit construction method

Abstract

This invention relates to a support structure and construction method for narrow, deep foundation pits, belonging to the field of foundation pit excavation and support. Its purpose is to improve the excavation and support performance of narrow, deep foundation pits and reduce safety risks. The underground diaphragm wall is formed by diaphragm segments of each stepped foundation pit. Between adjacent diaphragm segments, the root of the upper diaphragm segment interlocks with the top of the lower diaphragm segment, and is anchored by pre-stressed rebound anchors passing through the interlocking portions of the two adjacent diaphragm segments. During construction, anchor piles are driven in for pre-support, then the corresponding stepped foundation pit is excavated, and the diaphragm segment construction is completed. Finally, pre-stressed rebound anchors are driven between the diaphragm segment under construction and the adjacent upper diaphragm segment. This fundamentally realizes the principle of "supporting before excavation and strengthening support while excavating," avoiding instability caused by prolonged exposure of the exposed surface after foundation pit excavation. The pre-stressed rebound anchor increases the pre-stress value of the support wall, thereby improving the stability of the underground diaphragm wall support.

Description

Technical Field

[0001] This invention belongs to the field of foundation pit excavation and support, specifically the support structure for narrow and long deep foundation pits and the construction method for narrow and long deep foundation pits. Background Technology

[0002] Safety protection of deep foundation pits is of paramount importance in foundation pit engineering.

[0003] As the foundation of building structures, the safety and quality of deep foundation pit projects are of paramount importance. Currently, deep foundation pit construction often employs open-cut excavation with slope protection or support methods such as cast-in-place piles, sheet piles, and deep mixing piles. Long, narrow trench-shaped deep foundation pits with long longitudinal extension and small cross-sectional clearance are commonly used in urban underground utility tunnels, underground slag flushing ditches in the metallurgical industry, and underground oil and gas transportation channels. The main problems encountered are as follows:

[0004] Firstly, due to the limitations of the construction site, the deep foundation pit is close to the adjacent buildings / structures, and the excavation of the foundation pit cannot adopt the natural slope method. The excavation of the foundation pit will easily cause the original adjacent buildings to overturn, collapse or settle unevenly, which will lead to safety and quality accidents.

[0005] Secondly, the long and narrow deep foundation pit extends longitudinally, while its cross-sectional clearance is small. Large machinery lacks flexibility in excavation and transportation, and there are significant safety hazards in operating in the narrow underground space. Once the foundation pit collapses, it can easily cause casualties such as burial and suffocation.

[0006] Third, the narrow and deep foundation pit restricts the form of foundation pit support structure, and cannot realize foundation pit support methods that require the use of large construction machinery (such as jet grouting piles, steel sheet piles, deep mixing piles, etc.).

[0007] In summary, long and narrow trench-shaped deep foundation pits are not only affected by site limitations, but also pose many safety hazards. Conventional deep foundation pit support methods are difficult to meet the dual requirements of site conditions and safety protection. Summary of the Invention

[0008] The purpose of this invention is to provide a support structure and construction method for narrow and deep foundation pits, thereby improving the excavation and support performance of narrow and deep foundation pits and reducing safety risks.

[0009] The technical solution adopted in this invention is: a long and narrow deep foundation pit support structure, including underground diaphragm walls set on both sides of the deep foundation pit and extending longitudinally along the deep foundation pit; the deep foundation pit is stepped, consisting of N stepped foundation pits in the vertical direction; each stepped foundation pit has a diaphragm wall segment on its pit wall; the underground diaphragm wall is formed by the diaphragm wall segments of each stepped foundation pit, and between two adjacent diaphragm wall segments, the root of the upper diaphragm wall segment and the top of the lower diaphragm wall segment interlock, and are anchored by pre-loaded rebound anchor rods passing through the interlocking part of the two adjacent diaphragm wall segments simultaneously.

[0010] Furthermore, the preloaded rebound anchor bolt includes an anchor bolt body, with a tapered drill bit at the end of the anchor bolt body and a drill cap at the beginning of the anchor bolt body.

[0011] Furthermore, the anchor body includes a drill rod section one adjacent to the drill bit and a drill rod section two adjacent to the drill rod cap along its axial direction. The drill rod section one is provided with external threads, and the outer wall of the drill rod section two is provided with rotating blades.

[0012] Furthermore, a support plate is fitted onto the anchor bolt body. Along the axial direction of the anchor bolt body, the support plate is movably fitted with the anchor bolt body. Several telescopic springs are installed between the support plate and the drill pipe cap, and the several telescopic springs are evenly distributed around the anchor bolt body.

[0013] Furthermore, each wall segment is cast into a single unit with the anchor piles vertically anchored in the foundation soil of the corresponding wall segment.

[0014] Furthermore, the anchor pile includes a guide pipe inserted into the foundation soil, and the guide pipe wall is provided with grout seepage holes; the reinforcing cage is inserted into the guide pipe, and the reinforcing cage and the guide pipe are solidified into one piece by the grout injected into the guide pipe.

[0015] Furthermore, prestressing struts are installed between the underground diaphragm walls on both sides of the deep foundation pit; the prestressing struts are installed at the top of the underground diaphragm walls.

[0016] Furthermore, the preload support rod includes a horizontal support rod, with a preload spring and a preload end respectively fitted at both ends of the horizontal support rod, and a jack is provided to apply pressure to the preload end along the axial direction of the horizontal support rod towards the side where the preload spring is located; the preload end is movably connected to the horizontal support rod along the axial direction of the horizontal support rod, one end of the preload spring abuts against the preload end, and the other end abuts against the pressure-bearing anchor plate of the underground wall.

[0017] The construction method for long and narrow deep foundation pits involves constructing the deep foundation pit in N stepped stages along the vertical direction. The N stepped foundation pits are excavated sequentially from top to bottom. Before excavating each stepped foundation pit, anchor piles corresponding to the stepped foundation pits are driven into two rows along the longitudinal direction of the deep foundation pit for pre-support. Next, the corresponding stepped foundation pits are excavated, and the wall connection segments of the stepped foundation pits are constructed. Finally, pre-stressed rebound anchor rods are driven between the wall connection segments under construction and the adjacent upper wall connection segments.

[0018] Furthermore, after the construction of the wall ties in the first-stage foundation pit is completed, prestressing struts are installed.

[0019] The beneficial effects of this invention are as follows: The narrow, deep foundation pit support structure disclosed in this invention improves the stability of the foundation pit to a certain extent by creating a stepped structure. Each stepped foundation pit is protected by its own connecting wall segments, forming a stepped underground connecting wall. During construction, the corresponding connecting wall segments can be constructed immediately after the excavation of one stepped foundation pit, allowing the connecting wall segments to participate in the support function as early as possible and avoiding instability caused by prolonged exposure of the exposed surface after foundation pit excavation. Through the connection of pre-loaded rebound anchor rods, all connecting wall segments form an integral stepped underground connecting wall. Moreover, the underground connecting wall is anchored in the surrounding rock and soil behind the foundation pit by the pre-loaded rebound anchor rods, applying pre-pressure to the underground connecting wall to resist the horizontal thrust generated after the stress of the soil outside the foundation pit wall is released, thus improving the stability of the underground connecting wall support.

[0020] The method for supporting narrow, deep foundation pits disclosed in this invention fundamentally realizes the principle of "supporting before excavation and reinforcing support while excavating." It employs a stepped underground diaphragm wall support structure, achieving simultaneous excavation and support according to the number of steps during slope excavation. This avoids instability caused by prolonged exposure of the exposed surface after foundation pit excavation, preventing collapses or landslides. The method utilizes inclined pre-stressed rebound anchors to increase the pre-stress value of the support wall, counteracting the horizontal thrust generated after stress release from the soil outside the foundation pit wall. This improves the stability of the underground diaphragm wall support, enhancing the efficiency of deep foundation pit excavation and support construction while reducing safety and quality risks. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the end face of the narrow and deep foundation pit support structure disclosed in this invention;

[0022] Figure 2 This is a schematic diagram of an anchor pile structure;

[0023] Figure 3 This is a schematic diagram of a preloaded strut structure;

[0024] Figure 4 for Figure 3 Enlarged view of a portion at point A;

[0025] Figure 5 This is a schematic diagram of a preloaded rebound anchor bolt structure.

[0026] Figure 6 for Figure 5 AA section view;

[0027] Figure 7 for Figure 5 BB cross-sectional view.

[0028] In the diagram, the components are: underground diaphragm wall 1, diaphragm segment 11, anchor pile 2, guide pipe 21, rebar cage 22, grouting hole 23, preload strut 3, horizontal strut 31, preload spring 32, preload flat steel 33, preload end 34, jack 35, connecting lug 36, embedded bolt 37, preload rebound anchor 4, anchor body 41, drill rod section 1 41A, drill rod section 2 41B, rotary blade 42, drill bit 43, telescopic spring 44, drill rod cap 45, and support plate 46. Detailed Implementation

[0029] The present invention will be further described below with reference to the accompanying drawings and embodiments:

[0030] In this invention, the terms "longitudinal," "lateral," "vertical," "upper," "lower," "rear," "horizontal," "top," and "bottom," etc., indicate the orientation or positional relationship based on the appendix. Figure 1 The orientations or positional relationships shown are for the purpose of describing the present invention only, and are not intended to indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as limiting the present invention.

[0031] Narrow and deep foundation pit support structures, such as Figure 1 As shown, it includes underground diaphragm walls 1 set on both sides of the deep foundation pit and extending longitudinally along the deep foundation pit; the deep foundation pit is stepped and is composed of N stepped foundation pits in the vertical direction; each stepped foundation pit has a diaphragm wall segment 11 on its pit wall; the underground diaphragm wall 1 is composed of the diaphragm wall segments 11 of each stepped foundation pit, and between two adjacent diaphragm wall segments 11, the root of the upper diaphragm wall segment 11 and the top of the lower diaphragm wall segment 11 are interlocked, and anchored by pre-loaded rebound anchor rods 4 passing through the interlocking part of the two adjacent diaphragm wall segments 11.

[0032] The long and narrow deep foundation pit support structure disclosed in this invention uses underground diaphragm walls 1 for support. The deep foundation pit is stepped, allowing for stepped excavation and avoiding the drawbacks of traditional large-scale sloping excavation. The deep foundation pit is stepped, consisting of N steps vertically, where N is a positive integer greater than or equal to 2. This stepped design improves the stability of the foundation pit to some extent compared to traditional large-scale sloping excavation. Each step of the foundation pit is protected by its respective diaphragm wall segment 11, forming an underground diaphragm wall 1 that is also stepped. During construction, the corresponding diaphragm wall segment 11 can be constructed immediately after the excavation of one step of the foundation pit, allowing the diaphragm wall segment 11 to participate in the support function as early as possible. The root of the upper wall tie segment 11 is engaged with the top of the lower wall tie segment 11. This means that the root of the upper wall tie segment 11 is located behind the top of the lower wall tie segment 11, and the root of the upper wall tie segment 11 is tightly attached to the top of the lower wall tie segment 11. They are connected by a pre-loaded rebound anchor rod 4, so that all the wall tie segments 11 form an integral stepped underground wall tie 1. Moreover, the underground wall tie 1 is anchored in the surrounding rock and soil of the foundation pit behind the underground wall tie 1 by the pre-loaded rebound anchor rod 4, which applies pre-pressure to the underground wall tie 1 to resist the horizontal thrust generated after the stress of the soil outside the foundation pit wall is released, thereby improving the stability of the underground wall tie 1 support.

[0033] To increase drilling force and enable the preloaded rebound anchor 4 to be quickly driven into the surrounding rock and soil of the foundation pit, preferably, such as Figure 5 , Figure 6 and Figure 7 As shown, the preloaded rebound anchor bolt 4 includes an anchor bolt body 41, with a tapered drill bit 43 at the end of the anchor bolt body 41 and a drill cap 45 at the beginning of the anchor bolt body 41.

[0034] To further enhance drilling power, preferably, the anchor body 41 includes a drill rod section 41A adjacent to the drill bit and a drill rod section 41B adjacent to the drill rod cap 45 along its axial direction. The drill rod section 41A is provided with external threads. The external threads allow it to be quickly driven into the surrounding rock and soil of the foundation pit by a rotary impact drill. The outer wall of the drill rod section 41B is provided with rotating blades 42, which are welded to the drill rod section 41B. During the drilling of the pre-loaded rebound anchor 4, the rotating blades 42 can quickly cut the soil, playing a role in cleaning the hole; during the anchoring stage after grouting, the anchor body 41 can be firmly bonded to the surrounding rock of the foundation pit through pressure grouting, which can improve the interlocking anchoring force of the anchor body 41.

[0035] To generate preload on the diaphragm wall 1 and further improve its stability, preferably, a support plate 46 is fitted onto the anchor bolt body 41. Along the axial direction of the anchor bolt body 41, the support plate 46 is movably fitted with the anchor bolt body 41. Several telescopic springs 44 are evenly distributed around the anchor bolt body 41 between the support plate 46 and the drill pipe cap 45. During construction, after the anchor bolt grouting is completed, the support plate 46 is moved along the anchor bolt body 41 closer to the drill pipe cap 45, compressing the telescopic springs 44. After the grout has fully solidified and achieved sufficient anchoring, the force of moving the support plate 46 is released, and the telescopic springs 44 return to their original deformation, generating preload on the diaphragm wall 1. This preload balances the lateral pressure of the backfill soil on the back side of the foundation pit. Of course, during construction, multiple smaller springs can be distributed circumferentially around the anchor body 41, or a relatively larger single spring can be directly fitted onto the anchor body 41. The main consideration is to match the depth of the foundation pit and the required rebound preload.

[0036] To improve the stability of the connecting wall segment 11, preferably, each connecting wall segment 11 is cast integrally with the anchor piles 2 vertically anchored in the foundation soil of the corresponding connecting wall segment 11. The anchor piles 2 serve as pre-support for the deep foundation pit. Before the excavation of each connecting wall segment 11, two rows of anchor piles 2 are driven longitudinally into the deep foundation pit for pre-support. Then, construction machinery is used to excavate the stepped foundation pit. After excavation, when casting the connecting wall segment 11, the exposed section of the anchor pile 2 is cast together with the connecting wall segment 11. The anchor piles 2 first serve as pre-support, reducing the risk of collapse caused by excavation; then, they are cast integrally with the connecting wall segment 11, strengthening the connecting wall segment 11 and anchoring the connecting wall segment 11 to the foundation soil, increasing the stability of the connecting wall segment 11.

[0037] Anchor pile 2 can adopt a conventional structure, which involves inserting a reinforcing cage into the pile hole and injecting grout into the pile hole to solidify. In this invention, to prevent hole collapse after formation, preferably, as follows: Figure 2 As shown, the anchor pile 2 includes a guide pipe 21 inserted into the foundation soil, and grouting holes 23 are distributed on the pipe wall of the guide pipe 21. A rotary impact drilling rig is used for drilling the anchor pile. To prevent hole collapse after drilling, the guide pipe 21 is used for protection. To achieve the required anchoring depth, a steel cage 22 welded from steel bars is inserted into the guide pipe 21. A grouting pipe is inserted into the center of the guide pipe 21, and grouting is performed using a pressure grouting machine. The grout seeps into the foundation soil through the grouting holes 23 on the pipe wall of the guide pipe 21, thereby increasing its anchoring strength and enhancing the stability of the foundation soil. The grout filling the guide pipe 21 solidifies, binding the steel cage 22 and the guide pipe 21 together to form the anchor pile 2.

[0038] Considering the significant lateral pressure of the foundation soil on both sides of a long and narrow deep foundation pit, which could easily cause the foundation pit support structure to slide or overturn from both sides towards the middle, resulting in a collapse and burial accident, this invention includes prestressing struts 3 installed between the underground diaphragm walls 1 on both sides of the deep foundation pit. The prestressing struts 3 are located at the top of the underground diaphragm walls 1. The prestressing struts 3 provide prestress to balance the lateral pressure of the foundation soil on both sides. Since the prestressing struts 3 are located at the top of the underground diaphragm walls 1, the construction of the prestressing struts 3 can begin after the construction of the topmost diaphragm wall segment 11, allowing the prestressing struts 3 to provide support as early as possible and creating favorable conditions for subsequent excavation and construction.

[0039] The preload strut 3 can be any member with elastic deformation properties, as long as it can increase the preload. In this embodiment, for example... Figure 3 and Figure 4 As shown, the preload support rod 3 includes a horizontal support rod 31, with a preload spring 32 and a preload end 34 respectively fitted at both ends of the horizontal support rod 31, and a jack 35 is provided to apply pressure to the preload end 34 along the axial direction of the horizontal support rod 31 towards the side where the preload spring 32 is located; the preload end 34 is movably connected to the horizontal support rod 31 along the axial direction of the horizontal support rod 31, one end of the preload spring 32 abuts against the preload end 34, and the other end abuts against the pressure-bearing anchor plate of the underground diaphragm wall 1. The pressure-bearing anchor plate and the pre-embedded bolts 37 are pre-embedded during the pouring of the underground diaphragm wall 1.

[0040] To reduce preload loss and prevent injury from excessive rebound due to excessive preload when the jack 35 is unloading, an arched preload flat steel 33 is installed between the preload ends 34 at both ends. The preload flat steel 33 can be bolted to the preload ends 34. The horizontal support rod 31 is securely bolted to both ends. Preload is applied to both ends using the jack 35. After adjusting the compression of the preload spring 32, the arched preload flat steel 33 is then fitted on to maintain the necessary preload on the walls on both sides and resist the lateral pressure of the pit walls. Of course, a plastic flexible hose is fitted over the preload spring 32 for protection to prevent it from detaching and causing injury or damage.

[0041] Vertically, the prestressed flat steel 33 is located above the horizontal strut 31. In actual construction, a rain shelter can also be erected on the top of the arched prestressed flat steel 33 to achieve a good rain protection effect, reduce water accumulation in the deep foundation pit caused by heavy rain, and provide good construction conditions for rainy day operations.

[0042] The present invention discloses a method for constructing a long and narrow deep foundation pit, which involves constructing the deep foundation pit in N vertical steps.

[0043] If the N-step foundation pit is excavated sequentially from top to bottom, that is, if the N-step foundation pit is divided into the first step foundation pit, the second step foundation pit, the third step foundation pit, and so on from top to bottom, then the excavation shall be carried out in the order of the first step foundation pit, the second step foundation pit, the third step foundation pit, and so on.

[0044] Before excavating the foundation pit of each step, firstly, two rows of anchor piles 2 corresponding to the step foundation pit are driven in the longitudinal direction of the deep foundation pit for pre-support; then, the corresponding step foundation pit is excavated and the construction of the wall connection segment 11 of the step foundation pit is completed; finally, pre-stressed rebound anchor rods 4 are driven between the wall connection segment 11 under construction and the adjacent upper wall connection segment 11. Before excavating the first-stage foundation pit, anchor piles 2 are driven into both sides of the first-stage foundation pit. Then, the first-stage foundation pit is excavated, and the wall-connecting segment 11 is poured. After the wall-connecting segment 11 of the first-stage foundation pit is poured, anchor piles 2 are driven into both sides of the second-stage foundation pit. Then, the second-stage foundation pit is excavated, and the wall-connecting segment 11 of the second-stage foundation pit is poured. After the wall-connecting segment 11 of the second-stage foundation pit is poured, pre-stressed rebound anchor rods 4 are driven into the interlocking points of the wall-connecting segments 11 of the first and second-stage foundation pits. Next, following the construction method of the first-stage foundation pit, the wall-connecting segment 11 of the third-stage foundation pit is poured, and pre-stressed rebound anchor rods 4 are driven into the interlocking points of the wall-connecting segments 11 of the third and second-stage foundation pits. This process continues until the final wall-connecting segment 11 is completed.

[0045] In this invention, the conical drill bit 43 and the threaded drill rod section 41A at the end of the pre-stressed rebound anchor rod 4 have strong drilling power, allowing it to be quickly driven into the surrounding rock and soil of the foundation pit using a rotary impact drill. The drill rod section 41B has several spiral-shaped rotary blades 42 welded on, which can quickly cut the soil during drilling of the pre-stressed rebound anchor rod 4, thus cleaning the hole. During the post-grouting anchoring stage, pressure injection will firmly bond the anchor rod to the surrounding rock of the foundation pit, improving the interlocking anchoring force of the pre-stressed rebound anchor rod 4. The telescopic spring 44 at the end, also called a rebound spring, is compressed after the anchor rod is injected with grout. After the grout has fully solidified and achieved sufficient anchoring effect, the telescopic spring 44 is released. When the telescopic spring 44 returns to its original deformation, it generates pre-pressure on the underground diaphragm wall, thereby balancing the lateral pressure of the backfill soil in the foundation pit.

[0046] In this invention, anchor piles 2 are driven longitudinally in two rows at the front of the deep foundation pit for pre-support before excavation, thus realizing the principle of "support before excavation". During excavation, the deep foundation pit is excavated in steps, and after the excavation of one step is completed, the corresponding wall ties 11 are constructed, thus realizing the principle of "strengthening support while excavating". The bottom of the wall ties 11 of the upper step is in close contact with the top of the wall ties 11 of the lower step, and they are connected by pre-stressed rebound anchor rods 4 to form an integral stepped underground wall ties support structure, further realizing the principle of "strengthening support". Therefore, this invention fundamentally realizes the principle of "support before excavation and reinforcement of support while excavating." It adopts a stepped underground diaphragm wall support structure, which achieves simultaneous excavation and support according to the number of steps during the slope excavation process. This avoids instability caused by prolonged exposure of the exposed surface after the foundation pit is excavated, thus preventing collapse or landslide accidents. By using inclined pre-stressed rebound anchor rods 4, the pre-stress value of the support wall is increased to counteract the horizontal thrust generated after the stress of the soil on the outside of the foundation pit wall is released, thereby improving the stability of the underground diaphragm wall support. This is beneficial to improving the construction efficiency of deep foundation pit excavation and support, and reducing safety and quality risks.

[0047] Considering the significant lateral pressure of the foundation soil on both sides of the narrow, deep foundation pit, which could easily cause the pit support structure to slide or overturn from both sides towards the middle, resulting in a collapse and burial accident, prestressing struts 3 are installed after the construction of the wall ties 11 of the first-step foundation pit is completed. The prestressing struts 3 provide preload in a timely manner to balance the lateral pressure of the foundation soil on both sides.

[0048] Typically, without affecting the construction operation space, the pre-stressing strut 3 can be installed after the completion of the wall ties 11 of the first-stage foundation pit. The pre-stressing strut 3 includes a horizontal strut 31, with a pre-stressing spring 32 and a pre-stressing end 34 respectively fitted at both ends of the horizontal strut 31, and a jack 35 is provided to apply pressure to the pre-stressing end 34 along the axial direction of the horizontal strut 31 towards the side where the pre-stressing spring 32 is located. In specific construction, before pouring the first-stage foundation pit, the positions of the support surfaces on both sides are determined, and the pressure-bearing anchor plates of the pressure-bearing surfaces are embedded in advance. The end plates at both ends of the horizontal strut 31 of the pre-stressing strut 3 are fixed to the pressure-bearing anchor plates with bolts. One end of the pre-stressing spring 32 fitted on the horizontal strut 31 abuts against the pressure-bearing anchor plate, and the other end abuts against the pre-stressing end 34. The jack 35 applies pressure and thrust to the pre-stressing end 34 from the opposite side of the pre-stressing spring 32 until the compression of the pre-stressing spring 32 meets the requirements. Finally, the two ends of the arched pre-compression flat steel 33 are connected together with the connecting lugs 36 on the pre-compression ends 34 at both ends of the horizontal strut 31 by connecting bolts. The pre-compression flat steel 33 plays a certain role in limiting the position of the pre-compression ends 34 on the horizontal strut 31.

[0049] The specific construction process of the present invention is described below through an example:

[0050] First, site dewatering should be carried out before construction. In confined sites, considering the loose and unstable foundation soil before excavation, and especially in coastal areas, river valleys, and wetlands where there is a large amount of silty soil with high water content, drainage measures should be implemented first to reduce the adverse effects of groundwater on the lubrication and softening of the foundation soil. Simultaneously, considering the long construction period of narrow and elongated foundation pits, site planning and excavation schemes should be prepared before excavation, and surface water dewatering measures should be implemented to minimize the adverse effects of surface water infiltration and rainwater erosion.

[0051] Next, pre-support construction is carried out before the excavation of the foundation pit. Before excavating a long and narrow deep foundation pit, the excavation direction and cross-sectional dimensions should be marked out, taking into account the construction operation space, slope, and the space occupied by the support structure. After determining the excavation cross-section and the ground excavation boundary line, the positions of the anchor piles and the spacing between adjacent anchor piles are determined on both sides of the long and narrow deep foundation pit. The diameter, depth, and internal reinforcement of the anchor piles are determined based on the slope steps and mechanical calculations. Generally, anchor piles with a diameter of about 300-500mm are used, and the reinforcement is HRB400 or HRB500.

[0052] Next, the pit was excavated in steps and reinforced with support. After the pre-support was completed, a small excavator, matched to the available space, was used for step-slope excavation. Support was immediately installed after each step was excavated, forming a stepped protective structure with interlocking upper and lower steps. Sleeves were pre-installed at the interlocking points, and anchor bolt holes were drilled using a pneumatic impact drill at the depth and angle determined by mechanical calculations. The pre-compressed rebound anchor bolts were directly drilled into the surrounding rock behind the retaining wall. A pre-compression was applied to the rebound springs fitted at the ends, compressing them, and then secured with temporary bolts. After the pipe was filled with pressure grout and fully cured to provide the necessary anchoring force, the temporary bolts were loosened, allowing the springs to relax and rebound. To overcome this rebound force, a certain rebound pressure was generated on the retaining wall. This significantly enhanced the stability of the stepped retaining wall.

Claims

1. A long and narrow deep foundation pit support structure, comprising underground diaphragm walls (1) disposed on both sides of the deep foundation pit and extending longitudinally along the deep foundation pit; characterized in that: The deep foundation pit is stepped, consisting of N steps in the vertical direction; each step of the foundation pit has a wall-connecting segment (11) on its wall; the wall-connecting segments (11) of each step of the foundation pit together form the underground wall (1), and between two adjacent wall-connecting segments (11), the root of the upper wall-connecting segment (11) and the top of the lower wall-connecting segment (11) are interlocked, and anchored by a pre-loaded rebound anchor rod (4) passing through the interlocking part of the two adjacent wall-connecting segments (11). Prestressing struts (3) are installed between the underground diaphragm walls (1) on both sides of the deep foundation pit; the prestressing struts (3) are installed at the top of the underground diaphragm walls (1); The preload support rod (3) includes a horizontal support rod (31), with a preload spring (32) and a preload end (34) respectively fitted at both ends of the horizontal support rod (31), and a jack (35) is provided to apply pressure to the preload end (34) along the axial direction of the horizontal support rod (31) towards the side where the preload spring (32) is located; an arched preload flat steel (33) is provided between the preload ends (34) at both ends; The pre-compression end (34) is movably connected to the horizontal support rod (31) along the axial direction of the horizontal support rod (31). One end of the pre-compression spring (32) abuts against the pre-compression end (34), and the other end abuts against the pressure-bearing anchor plate of the underground wall (1).

2. The narrow and deep foundation pit support structure as described in claim 1, characterized in that: The preloaded rebound anchor bolt (4) includes an anchor bolt body (41), the end of which is provided with a tapered drill bit (43), and the beginning of which is provided with a drill cap (45).

3. The narrow and deep foundation pit support structure as described in claim 2, characterized in that: The anchor body (41) includes a drill rod section one (41A) adjacent to the drill bit and a drill rod section two (41B) adjacent to the drill rod cap (45) along its axial direction. The drill rod section one (41A) is provided with external threads, and the outer wall of the drill rod section two (41B) is provided with rotating blades (42).

4. The narrow and deep foundation pit support structure as described in claim 2, characterized in that: A support plate (46) is fitted on the anchor body (41). Along the axial direction of the anchor body (41), the support plate (46) and the anchor body (41) are movably matched. Several telescopic springs (44) are set between the support plate (46) and the drill pipe cap (45). The several telescopic springs (44) are evenly distributed around the anchor body (41).

5. The narrow and deep foundation pit support structure as described in any one of claims 1-4, characterized in that: Each wall segment (11) is cast into a single unit with the anchor piles (2) vertically anchored in the foundation soil of the corresponding wall segment (11).

6. The narrow and deep foundation pit support structure as described in claim 5, characterized in that: The anchor pile (2) includes a guide pipe (21) inserted into the foundation soil, and grouting holes (23) are distributed on the pipe wall of the guide pipe (21); the reinforcing cage (22) is inserted into the guide pipe (21), and the reinforcing cage (22) and the guide pipe (21) are solidified into one by the grout injected into the guide pipe (21).

7. A construction method for a narrow, deep foundation pit based on the narrow, deep foundation pit support structure according to claim 1, characterized in that: The deep foundation pit is constructed vertically in N-step steps. The N-step foundation pit is excavated sequentially from top to bottom; Before excavating the foundation pit of each step, firstly, two rows of anchor piles (2) corresponding to the step foundation pit are driven in the longitudinal direction of the deep foundation pit for pre-support; then, the corresponding step foundation pit is excavated and the construction of the wall connection segment (11) of the step foundation pit is completed; finally, pre-stressed rebound anchor rods (4) are driven between the wall connection segment (11) under construction and the adjacent upper wall connection segment (11).

8. The construction method for a narrow, deep foundation pit as described in claim 7, characterized in that: After the construction of the wall ties (11) of the first-stage foundation pit is completed, the prestressing struts (3) are installed.

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