Construction method for sectional composite reinforcement of end pile at orthogonal gap of pile wall

Abstract

This invention discloses a construction method for segmented composite reinforcement of end piles at orthogonal gaps between piles and walls. This method addresses the special condition where, due to noise reduction requirements, cast-in-place piles are replaced with steel pipe piles, resulting in a narrow gap of only 1.8m between the end piles and the existing building's basement exterior wall. It is implemented after the steel pipe pile row is constructed but before the new foundation pit is excavated. A side-bracing construction trench is excavated within the narrow gap, and steel pipe reinforcement piles of the same type as the end piles are driven in segments. The bottom is grouted and anchored to form a bottom anchor body. Two limiting tie rods rigidly connect the middle section to the end pile, and the top is grouted to form a top anchor body. Layered backfilling and compaction of soil provide lateral restraint, forming an independent lateral support system. This invention solves the technical problems of insufficient stiffness of steel pipe piles, lack of passive earth pressure, inability to construct conventional long piles, and the need to avoid damage to existing building exterior walls. It has the advantages of strong construction feasibility, avoidance of damage to existing buildings, multiple water-stopping guarantees, and low noise throughout the process.

Description

Technical Field

[0001] This invention belongs to the field of foundation pit engineering and existing building protection technology, and in particular, it is a construction method for segmented composite reinforcement of end piles at the orthogonal gap between piles and walls. Background Technology

[0002] In foundation pit engineering adjacent to existing buildings, cast-in-place piles are a commonly used support structure. The construction process involves drilling, lowering a reinforcing cage, and pouring concrete. Cast-in-place piles have a relatively large diameter, typically 500mm-1000mm, and high bending stiffness, enabling them to independently withstand most earth pressure loads based on their own structural strength, with low dependence on passive earth pressure from the soil behind the pile. During the concrete pouring process, the cast-in-place pile forms a close contact with the soil outside the basement wall of the existing building. After the concrete hardens, friction and interlocking on the contact surface can transfer some horizontal forces, further enhancing overall stability. Therefore, in conventional foundation pit engineering, no special connection or reinforcement measures are usually required between the cast-in-place piles and the existing basement wall.

[0003] However, the drilling of cast-in-place piles relies on impact drills and rotary drills, resulting in significant construction noise. When the foundation pit site is near noise-sensitive areas such as hospitals, schools, and residential areas, existing projects often replace some cast-in-place piles with steel pipe piles to meet environmental protection and noise reduction requirements. Steel pipe piles utilize static pressure or micro-vibration pile driving technology, and the construction noise can be controlled below 65dB. This is particularly beneficial when the exterior walls of existing building basements are oriented north-south, and the steel pipe pile rows are arranged east-west along the edge of the new foundation pit, with their outlines orthogonal in plan. When the pile type is changed at the corners, the following new technical problems arise: 1. The diameter of the steel pipe piles is much smaller than that of the cast-in-place piles. After the pile type is changed, when designing the support according to the original earth pressure, the end support piles need to be arranged closer to the existing basement exterior wall. At the same time, because their diameter is much smaller than that of the cast-in-place piles, the bending stiffness of the pile body is significantly reduced. In particular, the end steel pipe piles cannot independently bear the entire load and need to rely on the passive earth pressure of the soil behind the piles to provide lateral resistance. At this time, the end steel pipe piles are located at the end of the pile row, orthogonal to the existing basement exterior wall, forming a special working condition with a narrow gap of only 1.8m. The original soil layer in this area has a limited volume and poor mechanical properties, which cannot provide sufficient passive earth pressure. Coupled with the insufficient stiffness of the steel pipe piles themselves, it is easy to cause instability of the foundation pit sidewall. At the same time, the seepage prevention capacity of the original soil layer in this area does not meet the design requirements of the foundation pit cutoff wall.

[0004] 2. The new foundation pit is located south of the pile row and east of the basement exterior wall of the existing building. After the foundation pit is excavated and unloaded, the end steel pipe piles tend to shift towards the foundation pit side. Insufficient lateral restraint causes excessive deformation at the end of the pile row. This deformation will be transmitted to the entire row of support piles through the capping beam, affecting the overall stability of the support system.

[0005] Third, the net distance between the end steel pipe pile and the strip foundation of the existing building's basement exterior wall is less than 1m. If the process is adopted to directly drive the whole long pile, the pile body needs to pass through the narrow space on the side of the foundation during construction. Even if the verticality deviation of the pile driving is controlled within 0.5%, the deviation can reach 45mm at a depth of 9m. The pile body is easy to touch the strip foundation, causing foundation damage or pile body tilting, which poses an extremely high construction risk.

[0006] Fourth, the existing basement exterior wall structure is too weak. If the end steel pipe piles are rigidly connected to the existing basement exterior wall, the tie load will act directly on the wall, destroying the original stress system of the building and failing to effectively protect the existing building.

[0007] Fifth, existing reinforcement technologies are mostly designed for the condition where the support piles are parallel to the old exterior wall. Horizontal grouting can be used to extend to the foundation of the old building to complete the reinforcement of the old building, but it cannot provide a reference for the end reinforcement of the pile wall orthogonal in this condition. Summary of the Invention

[0008] The purpose of this invention is to provide a construction method for segmented composite reinforcement of end piles at the orthogonal gap between piles and walls. This method addresses the technical problems of insufficient bending stiffness of the pile body and the inability of the original soil layer within the narrow orthogonal gap between the end steel pipe piles and the existing basement exterior wall to meet passive earth pressure and seepage prevention requirements after replacing cast-in-place piles with steel pipe piles for noise reduction. It also solves the problems of insufficient lateral restraint of the end steel pipe piles after excavation and unloading, excessive deformation at the pile ends affecting the overall integrity of the entire pile support. Furthermore, it addresses the issues of not being able to directly construct conventional long piles within the narrow space outside the strip foundation and avoiding damage to the rigid connection of the existing basement exterior wall.

[0009] To achieve the above objectives, the present invention adopts the following technical solution: A construction method for segmented composite reinforcement of end piles at the orthogonal gap between piles and walls, characterized in that: Step 1, Construction condition collection and preliminary design of steel pipe piles: The outline of the steel pipe piles for the new foundation pit support and the old exterior wall of the existing building basement are orthogonal in the plane. The steel pipe piles are set along the left and right direction, and the old exterior wall is set along the front and back direction. The front side of the steel pipe piles is the new foundation pit to be excavated. A narrow gap is formed in the overlapping area of ​​the end piles of the steel pipe piles and the old exterior wall in terms of height. The individual piles of the steel pipe pile row are connected by three or more waist beams. The top surface of the first waist beam from top to bottom is lower than the top surface of the strip foundation at the bottom of the old outer wall. The anchoring in the soil between the individual piles is an anchor cable driven into the soil from the waist beam to the rear side. Step 2, Pre-design of reinforcement piles: Based on the existing building's basement old exterior wall, strip foundation, steel pipe pile row and narrow gap design dimensions and elevation, design the reinforcement pile dimensions, bottom pile depth, number of segments and corresponding limit tie rod dimensions and installation position; Step 3, Pre-design of backfill anchoring: Design the bottom grouting method and grouting range of the reinforced pile, design the thickness of the bottom concrete cover layer, design the top grouting method and grouting range of the reinforced pile, and design the backfilling method of the backfill soil; Step 4: The construction timing for the segmented composite reinforcement of end piles is after the steel pipe pile row construction is completed and before the new foundation pit is excavated. The steel pipe pile row is constructed to the design depth on the right side of the old exterior wall of the existing building basement using the hydraulic static pressure process. Then, the pile cap beam is constructed on the top of the steel pipe pile row. Step 5: Excavate the original soil layer in the narrow gap between the end pile and the old outer wall. Excavation should proceed from the ground downwards, forming a side-bracing construction trench. Temporary supports should be erected in layers between the trench walls as the excavation progresses. These temporary supports will be removed layer by layer as the backfill soil is added upwards. Excavate downwards to the top surface of the strip foundation. The excavation length in the front and back directions is greater than the maximum width of the narrow gap, covering the end piles and the operating space on both sides. Excavate along the right side of the old exterior wall and retain a layer of soil protection layer without exposing the wall surface. Excavate along the left side of the end piles and expose the left side surface. The exposed area does not exceed the diameter of the end piles. Step 6: Construct the first pile segment of the reinforcement pile in the side support construction trench. The pile is driven into the bottom soil of the trench between the strip foundation and the end pile, along the extension line of the steel pipe pile row in the left and right direction. The pile bottom is driven into the underlying stable bearing layer and the pile bottom elevation is lower than the bottom surface of the strip foundation. Then, the bottom grouting pipe is buried in the bottom soil of the trench. Step 7: Pour a concrete cover layer inside the side support construction trench and above the bottom soil of the trench; Step 8: After the concrete cover layer has initially set, grouting is carried out in the side support construction trench through the bottom grouting pipe to form a bottom anchor body from the right side surface of the strip foundation. The grouting depth of the bottom anchor body enters the bearing layer and is not less than twice the height of the strip foundation. The bottom anchor body wraps the bottom of the first pile segment of the reinforcement pile. Step 9: Erect a temporary construction platform on the concrete cover layer, then extend the second pile segment on the top of the first pile segment, and set the first limiting tie rod between the end pile and the second pile segment for rigid connection. Step 10: Remove the temporary construction platform, then backfill the first section of soil in layers in the side support construction trench and compact it, backfilling to a level exceeding the top elevation of the first limit tie rod; Step 11: Erect a temporary construction platform on the first section of soil, then extend the third pile segment on the top of the second pile segment, and install a second limiting tie rod between the end pile and the third pile segment for rigid connection. The installation of the reinforced pile body is now complete. Step 12: Remove the temporary construction platform, then backfill the second section of soil in layers in the side support construction trench and compact it, backfill to the top elevation of the third pile section, and bury the top grouting pipe in the second section of soil. Step thirteen: Inject grout into the pile body of the segmented reinforced pile; Step fourteen: Continue backfilling the third section of soil in layers until it is flush with the top surface of the pile cap beam; Step 15: Grouting is performed from the top surface of the third section of soil downwards through the top grouting pipe in the side bracing construction trench to form a top anchor body. The grouting depth of the top anchor body is greater than the height of the pile cap beam, and it wraps around the top of the reinforced pile. Step 16: Excavate the new foundation pit until it is below the design position of the first wainscoting. Then construct the first wainscoting and drive the first anchor cable into the soil on the rear side of the pile between the piles on the first wainscoting. Step 17: Continue excavating the new foundation pit downwards until it reaches below the design position of the second wainscoting. Then, construct the second wainscoting and drive the second anchor cable into the soil behind the piles on the second wainscoting. Repeat this step until all wainscoting and anchor cables are constructed.

[0010] In step sixteen, the first anchor cable is not installed on the side of the end pile adjacent to the old outer wall, or the horizontal net distance between the anchoring section of the first anchor cable and the reinforced pile is not less than 1m.

[0011] The reinforcing piles are steel pipe piles of the same type as the end piles. The distance between the two piles is equal to the distance between the individual piles of the steel pipe pile row. The minimum distance between the reinforcing piles and the strip foundation is not less than 200mm. The verticality deviation of the first pile segment 71 is not greater than 0.3%, and the horizontal positioning deviation is not greater than 15mm. The grouting material is the same as that of the steel pipe pile row.

[0012] The limiting tie rod is a double-channel steel tie rod of the same model as the waist beam. The two ends of the limiting tie rod are equipped with end plates and are fixedly connected to the connecting supports at the corresponding positions of the reinforcing pile and the end pile by high-strength bolts. After assembly, an initial preload of 20%-30% of the design tension is applied.

[0013] The bottom anchor body is grouted in layers using a sleeve valve pipe. The grouting material is P·O42.5 cement grout, and the grouting pressure is 0.3MPa-0.5MPa.

[0014] The top anchor body is grouted with a cement-water glass double-liquid grout at a pressure of 0.2MPa-0.3MPa.

[0015] The thickness of the soil protective layer is 50mm-100mm, and the concrete cover layer is plain concrete of C25 or higher, with a thickness of 100mm-300mm.

[0016] In step three, the backfill soil is judged based on the seepage prevention performance of the original soil layer within the narrow gap. The backfill soil is either original soil or replacement backfill, and is backfilled and compacted in layers. The thickness of each layer is 100mm-300mm, and the compaction coefficient is not less than 0.94. When the seepage prevention performance of the original soil layer is not up to standard, the backfill soil is replaced with cement-stabilized soil, and the cement content is 5%-8%.

[0017] The first, second, and third pile segments are connected by butt welding or by end flanges and high-strength bolts.

[0018] A horizontal displacement monitoring point is set at the top of the end pile, a settlement monitoring point is set at the top of the strip foundation, a settlement monitoring point is set at the top of the reinforcement pile, and a strain monitoring point is set on the first and second limit tie rods. During the construction of the side support trench, the monitoring is carried out once every 4 hours. During the excavation of the new foundation pit, the monitoring is carried out once every 2 hours. After the foundation slab of the new building is poured, the monitoring is carried out once a day.

[0019] Compared with the prior art, the present invention has the following features and beneficial effects: This invention addresses the unique challenge of a narrow gap of only 1.8m between the end steel pipe pile and the existing basement exterior wall, where they are orthogonal. It constructs an independent lateral composite reinforcement system for the end pile, employing segmented reinforcement piles, double-layer limiting tie rods, bidirectional grouting anchor bodies, and backfill soil to collaboratively bear the load. The reinforcement pile is rigidly connected to the end steel pipe pile via the double-layer limiting tie rods, serving as the lateral support point for the end pile. The tie rods transfer the earth pressure borne by the end pile to the reinforcement pile, which then distributes it to the bottom anchor body and surrounding backfill soil, ultimately reaching deeper soil layers. This enhances the overall lateral stiffness of the end pile, reduces the maximum bending moment and pile top displacement, and minimizes the lateral pressure transferred through the backfill soil. The reinforcement system generates minimal lateral pressure, without imposing additional loads on the existing building walls. This solution addresses the issue of insufficient passive earth pressure on the end steel pipe pile in limited soil conditions, controlling its lateral displacement and providing a solution for similar L-shaped corner construction scenarios. Details are as follows: This invention involves excavating and installing reinforcing piles within narrow gaps. The piles are three-section, modular structures. The first pile section is embedded in the soil below the bottom of the strip foundation, forming a bottom anchor body through bottom grouting. This anchor body secures the bottom of the pile to the bearing layer. A concrete capping layer prevents grout leakage, seals gaps at the bottom of the trench, and ensures the quality of the bottom anchor body. At the pile top, top grouting forms a top anchor body that wraps around the pile top. The grouting depth is greater than the height of the capping beam. The top and bottom anchor sections form a force couple, jointly resisting the horizontal force transmitted from the end pile. The top anchor body reinforces the soil around the capping beam, enhancing the overall stability of the capping beam end and preventing the pile top from being exposed or pulled up after backfill settlement, thus resisting slippage and overturning. Layered compaction of the backfill provides lateral support for the reinforcing pile, preventing out-of-plane instability, and also shares some of the soil pressure. Double-layered tie rods rigidly connect the end steel pipe pile to the reinforcing pile. Lateral constraint points are formed on the pile body of the end steel pipe pile to enhance the overall rigidity. After the limit tie rod is assembled, an initial preload of 20%-30% of the design tension is applied to eliminate assembly gaps and ensure that the tie rod immediately enters the working state after the new foundation pit is excavated. The displacement of the end pile is converted into the load of the reinforced pile through the tie rod, and then transmitted to the deep stable soil through the bottom anchor section of the reinforced pile and the frictional resistance of the backfill soil on the pile side. This limits the horizontal displacement of the end steel pipe pile after the foundation pit is unloaded, avoids excessive deformation of the pile end, and thus affects the deformation control and overall support integrity of the entire pile row through the cap beam.

[0020] This invention provides multiple layers of water-stopping protection. The bottom anchor body forms a bottom seepage-proof curtain, cutting off the channel for groundwater to seep around from the bottom. The middle layer of compacted backfill soil can also be replaced with cement-stabilized soil to reduce the soil permeability coefficient, block lateral seepage, and extend the seepage path. The top anchor body seals the surface leakage, preventing surface water or shallow groundwater from seeping in and ensuring the integrity of the foundation pit water-stopping curtain.

[0021] The invention employs a first pile segment driven into the ground and anchored by bottom grouting, which reduces the risk of long piles being driven off-center and hitting the foundation. Subsequent pile segments are spliced ​​in the backfilled stable soil layer, significantly reducing construction risks.

[0022] The present invention preserves a soil protective layer between existing exterior walls, and the reinforcing piles and limiting tie rods are not connected to the old exterior walls. The soil pressure of the end steel pipe piles is transmitted through the path of steel pipe pile → tie rod → reinforcing pile. The old wall only bears the normal lateral pressure of the backfill soil and does not bear the tie load, thus achieving the protection of the existing building.

[0023] This invention features highly versatile components. The reinforcing piles and end piles use steel pipes of the same specifications, and the limiting tie rods are consistent with the profiles of the foundation pit wainscoting. On-site materials are universal and processing is convenient. The reinforcement work is arranged to be completed before the foundation pit excavation, seamlessly connecting with the construction sequence of the original wainscoting and anchor cables, without disrupting the overall construction plan, and the construction period is controllable. During construction, the reinforcing piles are driven by static pressure, the limiting tie rods are connected by bolts, the backfill soil is compacted by a small tamper, and the grouting is done with a low-noise grouting pump. The construction noise is controlled below the requirements, meeting the requirements of sensitive areas such as hospitals and schools. The construction sequence of segmented pile splicing, layered installation of tie rods, and layered backfilling is adopted, and temporary supports are removed layer by layer to achieve a smooth load transfer during the construction stage, avoid instantaneous load impact, reduce structural risks during construction, and ensure reasonable process design and controllable construction safety. Attached Figure Description

[0024] The present invention will now be described in further detail with reference to the accompanying drawings.

[0025] Figure 1 This is a plan view of a certain project according to an embodiment of the present invention.

[0026] Figure 2 This is a schematic diagram of the structure after the completion of the steel pipe pile row construction in step four of this invention.

[0027] Figure 3 This is a schematic diagram of the structure after step five of the present invention, in which the narrow-gap original soil layer is excavated to form a side-support construction trench.

[0028] Figure 4 This is a schematic diagram of the structure after the construction of the first section of the reinforcement pile and the installation of the bottom grouting pipe in step six of this invention.

[0029] Figure 5 This is a schematic diagram of the structure after step seven of the present invention, which involves pouring the concrete cover layer.

[0030] Figure 6 This is a schematic diagram of the structure after the bottom grouting and anchoring is completed in step eight of the present invention.

[0031] Figure 7 This is a schematic diagram of the structure after step nine of the present invention, which involves extending the second pile segment and installing the first limiting tie rod.

[0032] Figure 8 This is a schematic diagram of the structure after the first backfilling is completed in step ten of this invention.

[0033] Figure 9 This is a schematic diagram of step eleven of the present invention, which involves extending the third pile segment and installing the second limiting tie rod.

[0034] Figure 10 This is a schematic diagram of the structure after the completion of step twelfth of the present invention, namely the second backfilling and the installation of the top grouting pipe.

[0035] Figure 11 This is a schematic diagram of the structure after step thirteen of the present invention, which involves injecting grout into the reinforced pile.

[0036] Figure 12 This is a schematic diagram of the structure after the third backfilling is completed in step fourteen of the present invention.

[0037] Figure 13 This is a schematic diagram of the structure after the top grouting and wrapping are completed in step fifteen of the present invention.

[0038] Figure 14 This is a schematic diagram of the structure after the first waist beam and anchor cable construction is completed in step sixteen of this invention.

[0039] Figure 15 This is a schematic diagram of the structure after the construction of the second waist beam and anchor cable is completed in step seventeen of this invention.

[0040] Figure 16 This is a schematic diagram of the structure after the completion of the construction of the third waist beam and anchor cable in step seventeen of this invention.

[0041] Figure 17 yes Figure 2 A schematic diagram of the planar structure layout.

[0042] Figure 18 yes Figure 4 A schematic diagram of the planar structure layout.

[0043] Figure 19 yes Figure 6 A schematic diagram of the planar structure layout.

[0044] Figure 20 yes Figure 12 A schematic diagram of the planar structure layout.

[0045] Figure 21 yes Figure 16 A schematic diagram of the planar structure layout.

[0046] Attached reference numerals: 1 - Steel pipe pile bank, 2 - Existing building exterior wall, 3 - New foundation pit, 4 - Waist beam, 41 - First waist beam, 42 - Second waist beam, 43 - Third waist beam, 5 - Strip foundation, 6 - Anchor cable, 61 - First anchor cable, 62 - Second anchor cable, 63 - Third anchor cable, 7 - Reinforcement pile, 71 - First pile segment, 72 - Second pile segment, 73 - Third pile segment, 8 - Limiting tie rod, 81 - First limiting tie rod, 8 2 - Second limiting tie rod, 9 - Pile capping beam, 10 - Ground, 11 - End pile, 20 - Side bracing construction trench, 21 - Soil protective layer, 22 - Soil at the bottom of the trench, 23 - Concrete cover layer, 24 - Bottom grouting pipe, 25 - Top grouting pipe, 26 - Bottom anchor body, 27 - Top anchor body, 28 - Narrow gap, 29 - Original soil layer, 30 - Backfill soil, 31 - First section of soil, 32 - Second section of soil, 33 - Third section of soil. Detailed Implementation

[0047] See the examples. Figure 1 As shown, a hospital expansion project involves a new two-story underground multi-purpose building on the east and right sides. Adjacent to the west side of the foundation pit is an old hospital ward building, which has six floors above ground and one below. The basement walls are 400mm thick C30 reinforced concrete walls with concrete strip foundations at the bottom. The top and bottom elevations of these strip foundations are -6m and -7m respectively, with a width of 2.2m. The multi-purpose building is designed with two underground floors. The original foundation pit support design used cast-in-place piles with a diameter of 500mm, a length of 14.5m, and a spacing of 750mm. However, due to measured noise levels of 92dB during pile drilling, far exceeding the daytime noise limits for the hospital area, the cast-in-place pile design was changed to steel pipe piles to meet environmental noise reduction requirements. The revised design uses seamless steel pipe Φ273mm×14mm steel pipe piles, with a pile length of 14.5m, driven into moderately weathered dolomite layer, with a pile spacing of 750mm and a hole diameter of 330mm. M30 cement grout is injected around the steel pipe to form a composite pile body.

[0048] Step 1: Collection of construction conditions and preliminary design of steel pipe pile banks: Before construction, a total station was used to measure the plan position and elevation of the existing building's basement exterior walls and strip foundation 5 to confirm the orientation of the basement exterior walls and the steel pipe pile arrangement. (See attached diagram.) Figure 1As shown, taking point A as an example, the basement exterior wall is oriented north-south (front-to-back), while the steel pipe pile row 1 is oriented east-west (left-to-right), and the two are orthogonal in plan. The net distance between the end pile 11 of the steel pipe pile row 1 and the old exterior wall 2 is measured to be 1.8m, and the net distance between the outer edge of the strip foundation 5 and the end pile 11 is 0.9m. A narrow gap 28 is formed in the overlapping area of ​​the end pile 11 and the old exterior wall 2 in terms of height. That is, the working range of this construction method is from the top surface of the strip foundation 5 to the top surface of the pile cap beam 9. The front side of the steel pipe pile row 1 is the new foundation pit 3 to be excavated. The excavation depth of the new foundation pit 3, the design elevation of the waist beam 4, and the design position of the anchor cable 6 are verified. When the new foundation pit 3 is excavated, the construction of the waist beam 4 and the anchor cable 6 will not conflict with the setting of the reinforcement piles 7.

[0049] A steel mesh with 6mm thick reinforcing bars and a mesh size of 200mm x 200mm is hung near the foundation pit between the steel pipe piles, and C20 concrete is sprayed over it. (See also...) Figure 16 and Figure 21 As shown, the capping beam of the steel pipe pile is 800mm×600mm. The steel pipe pile adopts the static pressure pile driving process. Three double-channel steel waist beams 4 are set on the steel pipe pile. From top to bottom, three anchor cables are driven in at -5m, -7m and -9m respectively. The first anchor cable 61 is driven at an angle of 30 degrees, each with two bundles of steel strands with a diameter of 17.8mm, a total length of 15m, an anchorage length of 10m, and a locking load of 150kN. The second anchor cable 62 is driven at an angle of 25 degrees, each with two bundles of steel strands with a diameter of 17.8mm, a total length of 12m, an anchorage length of 7m, and a locking load of 120kN. The third anchor cable 63 is driven at an angle of 25 degrees, each with two bundles of steel strands with a diameter of 17.8mm, a total length of 10m, an anchorage length of 7m, and a locking load of 100kN. The top surface of the first waist beam 41 is lower than the top surface of the strip foundation 5 at the bottom of the old outer wall 2. The anchoring in the soil between the piles is an anchor cable 6 driven into the soil from the waist beam 4 to the rear.

[0050] Step 2, see Figure 16 and Figure 21 As shown, the preliminary design for reinforcing piles: Based on the design dimensions and elevation of the existing building's basement exterior wall 2, strip foundation 5, steel pipe pile row 1, and narrow gap 28, the dimensions, bottom penetration depth, number of segments, and corresponding limiting tie rod 8 of the reinforcing pile 7 are designed. In this embodiment, the reinforcing pile 7 is designed using seamless steel pipe of the same type as the end pile 11, Φ273mm×14mm, Q355B, and the grouting material is the same as that used for the steel pipe pile row 1. During subsequent construction, along the extension line of the steel pipe pile row 1, the distance between the reinforcing pile 7 and the end pile 11 is equal to the spacing between the individual piles of the steel pipe pile row 1, i.e., 750mm. The minimum spacing between the reinforcing pile 7 and the strip foundation 5 is controlled at 200mm. The entire structure is divided into three segments. The first segment 71 is 2m long with a bottom penetration depth of 1.5m. The verticality deviation of the first segment 71 is no greater than 0.3%, and the horizontal positioning deviation is no greater than 15mm. The second segment 72 and the third segment 73 are both 1.7m long, with a total length of 5.4m.

[0051] Two limiting tie rods 8 are designed, using the same double-channel steel as the wainscoting 4, with a vertical spacing of 1.7m. The construction direction of the limiting tie rods is from bottom to top; therefore, the vertical distance between the first limiting tie rod 81 and the top surface of the strip foundation 5 is 1.8m, and the vertical distance between the first limiting tie rod 81 and the second limiting tie rod 82 is 1.7m. End plates are installed at both ends of the limiting tie rods 8, and they are fixedly connected to the connecting supports at corresponding positions on the reinforcing piles 7 and end piles 11 using high-strength bolts. After assembly, an initial preload of 20%-30% of the design tension is applied. In subsequent construction, the connecting supports are pre-welded and positioned on the end piles and reinforcing piles outside the trench; only the final tightening of bolts and the application of preload are performed inside the trench.

[0052] Step 3, see Figure 16 and Figure 21 As shown, the pre-design for backfill anchoring: The design includes the bottom grouting method and grouting range of the reinforcing pile 7, the thickness of the bottom concrete cap 23, the top grouting method and grouting range of the reinforcing pile 7, and the backfilling method of the backfill soil 30. In this embodiment, the bottom grouting range is designed to be 2.0m deep, measured from the bottom surface of the concrete cap 23, twice the height of the strip foundation 5, with a lateral diffusion radius of filling the narrow gap and a front-back diffusion radius of not less than 0.5m. The top grouting range is designed to be 1.0m deep, with a lateral diffusion radius of filling the narrow gap and a front-back diffusion radius of not less than 0.5m, measured from the top surface of the backfill soil, covering the top of the reinforcing pile 7.

[0053] The bottom concrete cover layer 23 is designed to be 150mm thick, and the concrete strength grade is C30.

[0054] According to the results of the in-situ soil permeability test, the original soil layer within the narrow gap was silty clay with a permeability coefficient of approximately 1.2 × 10⁻⁻⁻⁶. 4cm / s, does not meet the requirement of ≤1.0×10⁻ 6 The design requires a water-stop curtain with a flow rate of cm / s. Therefore, the backfill soil is designed to be cement-stabilized soil with a cement content of 6%, and the backfill thickness is 100mm-300mm in layers, with a compaction coefficient ≥0.94.

[0055] The construction timing for the segmented composite reinforcement of end piles in this invention is after the steel pipe pile row construction is completed and before the excavation of the new foundation pit 3.

[0056] Step four, see Figure 2 and Figure 18 As shown, the construction of steel pipe pile groups: A hydraulic static pressure process was used to construct a row of steel pipe piles 1 to the designed depth on the right side of the old exterior wall 2 of the existing building's basement. Then, a capping beam 9 was constructed on top of the piles 1, with a borehole diameter of 330mm and a verticality deviation of no more than 0.5%. After drilling, Φ273mm×14mm Q355B steel pipes were inserted, with butt welding at the pipe joints. The weld quality was controlled according to the secondary weld standard, and 5% were randomly inspected using ultrasonic testing. After placement, M30 cement grout was injected between the steel pipes and the borehole wall at a pressure of 0.3MPa-0.5MPa until grout returned to the borehole opening. The pile top elevation was controlled to be 500mm below the bottom of the capping beam to facilitate connection with the capping beam reinforcement.

[0057] The position of end pile 11 was precisely laid out using a total station to ensure that its distance from the existing outer wall was 1.8m, with an allowable deviation of ±50mm. After all steel pipe piles were constructed, the reinforcing bars of pile cap beam 9 were tied to the top of the piles, and C35 concrete was poured for pile cap beam 9. After the concrete of pile cap beam 9 was poured, it was covered with geotextile for 7 days of moisture curing.

[0058] Step 5, see Figure 3 As shown, a side-support construction trench is formed by excavating the original soil layer with narrow gaps: After the pile cap beam 9 reaches its design strength, the original soil layer 29 in the narrow gap 28 between the end pile 11 and the old outer wall 2 is excavated. During excavation, construction starts from the ground 10 downwards, forming a side-support construction trench 20. Temporary supports are erected in layers between the trench walls while excavating. The temporary supports are removed layer by layer when the backfill soil 30 is backfilled upwards later.

[0059] The excavation depth extends downwards to the top surface of the strip foundation 5. The excavation length in the front-to-back direction is greater than the maximum width of the narrow gap 28, covering the end pile 11 and the operating space on both sides. The excavation extends along the right side of the old outer wall 2 and retains a soil protective layer 21, without exposing the wall surface. The excavation extends along the left side of the end pile 11 and exposes the left side surface, with the exposed area not exceeding the diameter of the end pile.

[0060] During the excavation, a 3m long section was excavated along the outer wall, centered on the end steel pipe pile, covering the 273mm diameter steel pipe pile and approximately 1.35m of operating space on each side. The excavation width perpendicular to the outer wall was 1.7m. The soil protective layer 21 was 100mm thick, and the excavation depth reached the top surface of the strip foundation 5. This protective layer naturally peeled off during the subsequent excavation of the new foundation pit, preventing construction machinery from directly contacting the wall. Excavation continued along the left side of the end pile 11 until the left surface of the pile was exposed, with the exposed area controlled within 1 / 3 of the pile diameter, approximately 90mm, to ensure sufficient welding space for the subsequent tie rod connection plates without compromising the pile's stability.

[0061] Each layer is excavated to a depth of 500mm. After excavation, temporary transverse supports are immediately installed between the end piles and the outer side of the protective layer. φ100×5mm steel pipes can be used, with adjustable top supports at both ends. After excavating to the top surface of the strip foundation 5, the loose soil is cleared, and the foundation is compacted using a small flat rammer. A sump is set at the bottom of the trench, and safety measures for confined space operations are implemented.

[0062] Step Six, see Figure 4 and Figure 18 As shown, the construction of the first section of reinforcement piles and the installation of the bottom grouting pipe: The first pile segment 71 of the reinforcement pile 7 is constructed in the side support construction trench 20. The pile is driven into the bottom soil 22 between the strip foundation 5 and the end pile 11 along the extension line of the steel pipe pile row 1 in the left and right direction. The pile bottom is driven into the underlying stable bearing layer and the pile bottom elevation is lower than the bottom surface of the strip foundation 5. In this embodiment, the bottom pile depth is 1.5m. Then, the bottom grouting pipe 24 is buried in the bottom soil 22.

[0063] Specifically, a total station was used for precise layout, and a small static pressure pile driver was used to statically press the first pile segment 71 into the soil. The verticality was measured every 500mm of pressing, and any deviations were adjusted immediately. The verticality deviation was controlled to be no more than 0.3%. Pressing was stopped after reaching the design elevation, and bottom grouting pipes 24 were buried in the soil 22 at the bottom of the trench around the reinforcement pile.

[0064] Step 7, see Figure 5 As shown, pour the concrete cover layer: A concrete cover layer 23 is poured inside the side-support construction trench 20 and above the bottom soil 22.

[0065] Step 8, see Figure 6 and Figure 19 As shown, bottom grouting anchoring: After the concrete cover layer 23 has initially set, grouting is carried out in the side support construction trench 20 through the bottom grouting pipe 24 to form the bottom anchor body 26 from the right side surface of the strip foundation 5. The grouting depth of the bottom anchor body 26 enters the bearing layer and is not less than twice the height of the strip foundation. The bottom anchor body 26 wraps the bottom of the first pile segment 71 of the reinforcement pile 7. Specifically, a sleeve valve grouting system is used, with P·O42.5 cement grout, a water-cement ratio of 0.6 to 0.8, a grouting depth of 2m, a grouting pressure of 0.5MPa for the lower 1m, 0.4MPa for the middle 0.5m, and 0.3MPa for the upper 0.5m. Grouting is performed at intervals, with an interval of not less than 12 hours.

[0066] Step nine, see Figure 7 As shown, extend the second pile segment and install the first limiting tie rod: After the bottom grouting is completed and the concrete cover reaches the design strength, the pile splicing and tie rod installation are carried out. A temporary construction platform is erected on the concrete cover 23, and then the second pile segment 72 is extended on the top of the first pile segment 71. The first limiting tie rod 81 is set between the end pile 11 and the second pile segment 72 for rigid connection.

[0067] At the corresponding elevations of end pile 11 and reinforcement pile 7, steel plates and connecting supports are pre-welded, using double-sided fillet welds. An end plate is pre-welded onto the first limiting tie rod 81, with four M20 bolt holes on the end plate. After hoisting the tie rod, align the end with the bolt holes, insert four high-strength bolts, and apply an initial pre-tightening force of 25% of the design tension to ensure that the tie rod can immediately bear the force after the subsequent excavation of the foundation pit.

[0068] Step 10, see Figure 8 As shown, the first backfill: The temporary construction platform was dismantled, and then the first section of soil 31 was backfilled and compacted in layers in the side support construction trench 20, and backfilled to the top elevation of the first limit tie rod 81. In this embodiment, cement-stabilized soil is used, with a cement content of 6% by weight. Backfilling is done in layers, each layer having a loose thickness of 250mm. A small vibratory plate compactor is used for compaction, with each layer compacted four times, requiring a compaction coefficient ≥0.94. Backfilling extends 200mm above the top surface of the first limiting tie rod.

[0069] Step 11, see Figure 9 As shown, extend the third pile segment and install the second limiting tie rod: A temporary construction platform is erected on the first section of soil 31. Then, the third pile section 73 is extended on the top of the second pile section 72. A second limiting tie rod 82 is installed between the end pile 11 and the third pile section 73 for rigid connection. The pile body of the reinforcement pile 7 is installed. The specific construction method is the same as step nine. After both tie rods are installed and tightened, check that the connection nodes between the tie rods and the end piles and reinforcement piles are not loose.

[0070] Step 12, see Figure 10 As shown, the second backfilling and the installation of the top grouting pipe: Remove the temporary construction platform, then backfill the second section of soil 32 in layers in the side support construction trench 20 and compact it, backfilling to the top elevation of the third pile section 73; the specific construction method is the same as step ten, and the top grouting pipe 25 is buried around the top of the pile in the second section of soil 32 to reinforce it.

[0071] Step thirteen, see Figure 11 As shown, grout is injected into the pile body of reinforced pile 7: The same grouting material as steel pipe pile 1 was used, and the pile top was covered with wet geotextile and cured.

[0072] Step fourteen, see Figure 12 and Figure 20 As shown, the third backfill: Continue backfilling the third section of soil (33) in layers until it is flush with the top surface of the pile cap beam (9). The specific construction method is the same as in step ten.

[0073] Step 15, see Figure 13 As shown, the top is grouted and wrapped: Grouting is performed from the top surface of the third section of soil 33 downwards through the top grouting pipe 25 within the side-support construction trench 20, forming a top anchor body 27. The grouting depth of the top anchor body 27 is greater than the height of the pile cap beam 9, wrapping around the top of the reinforced pile 7. Specifically, after the third section of soil is backfilled to be level with the top of the cap beam, a grouting groove is reserved around the pile top. The grouting pipe extends from the bottom of the groove into the pile-soil gap until grout returns from the adjacent holes, ultimately forming a cap around the pile top. Cement-water glass double-liquid grout is used for grouting, with a grouting pressure of 0.2MPa-0.3MPa. Grouting is performed sequentially from the central hole to the side holes, stopping when the pressure in each hole suddenly increases or grout returns from the adjacent hole. During the grouting process, observe whether there is grout seepage at the joint between the cap beam and the outer wall. If leakage occurs, grouting is suspended and quick-drying cement is used for sealing.

[0074] Step sixteen, see Figure 14 As shown, the construction of the first joist beam and anchor cables: After the top grouting is completed and the grout reaches the design strength, the excavation of the new foundation pit 3 begins, continuing until below the design position of the first waist beam 41. Then, the first waist beam 41 is constructed, and the first anchor cable 61 is driven into the soil behind the piles on the first waist beam 41, within the inter-pile soil between the individual piles. The first anchor cable 61 is not installed on the side of the end pile 11 adjacent to the old outer wall 2, or the horizontal clearance between the anchoring section of the first anchor cable 61 and the reinforced pile 7 is not less than 1m, to avoid damaging the reinforced pile or the bottom anchor body during drilling. In this embodiment, the first anchor cable 61 is not installed on the side of the end pile 11 adjacent to the old outer wall 2.

[0075] Step 17, see Figure 15-16 , Figure 21 As shown, the downward excavation and subsequent construction of the lintel and anchor cables: Continue excavating the new foundation pit 3 downwards until it reaches below the design position of the second waist beam 42. Then construct the second waist beam 42 and drive the second anchor cable 62 into the soil between the piles on the second waist beam 42. Then construct the third waist beam 43 and drive the third anchor cable 63 into the soil between the piles on the third waist beam 43. All waist beams and anchor cables are then constructed.

[0076] The monitoring points are arranged as follows: a horizontal displacement monitoring point is set at the top of the end pile 11, a settlement monitoring point is set at the top of the strip foundation 5, a settlement monitoring point is set at the top of the reinforcing pile 7, and strain monitoring points are set on the first limiting tie rod 81 and the second limiting tie rod 82. During the construction of the side support construction trench 20, monitoring is conducted once every 4 hours. During the excavation of the new foundation pit 3, monitoring is conducted once every 2 hours. After the foundation slab of the new building is poured, monitoring is conducted once a day.

[0077] The warning values ​​are: cumulative horizontal displacement of end pile 11 ≤ 30mm or daily change ≥ 5mm; cumulative settlement of strip foundation ≤ 15mm or daily change ≥ 3mm; strain of limit tie rod exceeds 120% of the design value. When the warning values ​​are reached, excavation shall be stopped immediately, temporary diagonal bracing shall be added, supplementary grouting shall be carried out for reinforcement, and the monitoring frequency shall be increased to once every 1 hour. Construction may only resume after the design has been reviewed and confirmed to be safe.

[0078] During construction, please note the following: The thickness of the soil protective layer retained in step five should not be less than 50mm or more than 100mm. Too thin a layer will not provide adequate protection, while too thick a layer will affect the installation space for subsequent tie rods. The verticality deviation of the first pile segment must be strictly controlled within 0.3%; if it exceeds this, it should be corrected immediately or the pile should be removed and re-driven. When grouting the bottom, start with 0.2MPa-0.3MPa near the foundation edge, then gradually increase the pressure. The pressure should not exceed 0.5MPa to prevent grout from seeping into the room through the gaps at the bottom of the foundation and causing pollution. If grout leakage is found on the exterior wall or foundation, grouting should be stopped immediately and sealed with quick-drying cement. Due to the trench width of only 1.8m, large compaction equipment cannot enter; a small vibratory plate compactor must be used. The thickness of each backfill layer should be strictly controlled, and the number of compaction passes should not be less than 4. Temporary supports must be removed only after the backfill compaction meets the design requirements, and backfilling should be done up to 200mm below the support layer before removal; it is strictly forbidden to remove supports before backfilling.

[0079] In other embodiments, when the permeability coefficient of the original soil layer within the narrow gap meets the requirements as determined by field tests, the backfill soil can be the original soil layer, without the need to replace it with cement-stabilized soil, but the compaction coefficient should still be no less than 0.94.

[0080] In other embodiments, when the height of the strip foundation 5 is greater than 1.5m, the bottom grouting depth can be adjusted to 1.5 times the foundation height, but should not be less than 1.0m. In this case, the driving depth of the first pile segment 71 should also be increased accordingly to ensure that the pile bottom enters the grouting anchorage area.

[0081] In other embodiments, when on-site welding conditions are limited, pile splicing can be achieved using end flanges and high-strength bolts. The flange thickness is 20mm, and the bolts are M20 grade 8.8, with a minimum of 6 bolts. The flange connection surfaces should be coated with epoxy resin to enhance sealing.

[0082] In other embodiments, for soft soil foundations or conditions requiring higher deformation control, the initial preload of the tie rod can be increased to 30%–40% of the design tension to establish lateral restraint in advance and reduce the displacement of the end piles in the early stages of foundation pit excavation. However, the preload must not exceed 50% of the design tension to prevent the tie rod or connection node from yielding prematurely.

[0083] In other embodiments, when a more reliable connection is needed between the pile cap beam 9 and the top anchor body 27, the top grouting pipe 25 can be pre-embedded during the reinforcement binding of the pile cap beam 9, and the top grouting can be carried out after the cap beam concrete is poured and reaches the design strength. At this time, the top anchor body 27 will simultaneously wrap the top of the reinforcing pile 7 and the pile cap beam 9, forming an overall seal.

Claims

1. A construction method for segmented composite reinforcement of end piles at the orthogonal gap between piles and walls, characterized in that: Step 1, Construction condition collection and preliminary design of steel pipe pile row: The outline of the steel pipe pile row (1) for the new foundation pit support and the old exterior wall (2) of the existing building basement are orthogonal in the plane. The steel pipe pile row (1) is set in the left and right direction, and the old exterior wall (2) is set in the front and back direction. The front side of the steel pipe pile row (1) is the new foundation pit (3) to be excavated. A narrow gap (28) is formed in the overlapping area of ​​the end pile (11) of the steel pipe pile row (1) and the old exterior wall (2) in terms of height. The single piles of the steel pipe pile row (1) are connected by three or more waist beams (4). The top surface of the first waist beam (41) from top to bottom is lower than the top surface of the strip foundation (5) at the bottom of the old outer wall (2). The anchoring in the soil between the single piles is an anchor cable (6) driven into the soil from the waist beam (4) to the rear side. Step 2, Pre-design of reinforcement piles (7): Based on the design dimensions and elevation of the old exterior wall (2), strip foundation (5), steel pipe pile row (1) and narrow gap (28) of the existing building basement, design the dimensions of reinforcement piles (7), bottom pile depth, number of segments and the dimensions and installation position of corresponding limit tie rods (8); Step 3, Pre-design of backfill anchoring: Design the bottom grouting form and grouting range of the reinforcing pile (7), design the thickness of the bottom concrete cover layer (23), design the top grouting form and grouting range of the reinforcing pile (7), and design the backfilling form of the backfill soil (30); Step 4: The construction timing for the end pile segment composite reinforcement is after the steel pipe pile row (1) is completed and before the new foundation pit (3) is excavated; the steel pipe pile row (1) is constructed to the design depth on the right side of the old exterior wall (2) of the basement of the existing building using the hydraulic static pressure process, and then the pile cap beam (9) is constructed on the top of the steel pipe pile row (1). Step 5: Excavate the original soil layer (29) in the narrow gap (28) between the end pile (11) and the old outer wall (2). During excavation, work should proceed from the ground (10) downwards to form a side-support construction trench (20). Temporary supports should be erected in layers between the trench walls while excavating. The temporary supports should be removed layer by layer as the backfill soil (30) is backfilled upwards. The excavation depth extends down to the top surface of the strip foundation (5), and the excavation length in the front and back directions is greater than the maximum width of the narrow gap (28), covering the end pile (11) and the operating space on both sides. The excavation is carried out along the right side of the old outer wall (2) and a soil protective layer (21) is retained, without exposing the wall surface. The excavation is carried out along the left side of the end pile (11) and the left side surface is exposed, with the exposed range not exceeding the diameter of the end pile. Step 6: Construct the first pile segment (71) of the reinforcement pile (7) in the side support construction trench (20). The pile is driven into the bottom soil (22) between the strip foundation (5) and the end pile (11) along the extension line of the steel pipe pile row (1) in the left and right direction. The pile bottom is driven into the underlying stable bearing layer and the pile bottom elevation is lower than the bottom surface of the strip foundation (5). Then, the bottom grouting pipe (24) is buried in the bottom soil (22). Step 7: Pour a concrete cover layer (23) inside the side support construction trench (20) and above the bottom soil (22). Step 8: After the concrete cover layer (23) has initially set, grouting is carried out in the side support construction trench (20) through the bottom grouting pipe (24) to form a bottom anchor body (26) from the right side surface of the strip foundation (5). The grouting depth of the bottom anchor body (26) enters the bearing layer and is not less than 2 times the height of the strip foundation (5). The bottom anchor body (26) wraps the bottom of the first pile segment (71) of the reinforcement pile (7). Step 9: A temporary construction platform is erected on the concrete cover (23), and then the second pile segment (72) is extended on the top of the first pile segment (71), and a first limiting tie rod (81) is set between the end pile (11) and the second pile segment (72) for rigid connection. Step 10: Remove the temporary construction platform, then backfill the first section of soil (31) in layers in the side support construction trench (20) and compact it, backfilling to the top elevation of the first limit tie rod (81); Step 11: A temporary construction platform is erected on the first section of soil (31). Then, the third pile section (73) is extended on the top of the second pile section (72). A second limiting tie rod (82) is set between the end pile (11) and the third pile section (73) for rigid connection. The installation of the reinforced pile (7) is completed. Step 12: Remove the temporary construction platform, then backfill the second section of soil (32) in layers in the side support construction trench (20) and compact it, backfill to the top elevation of the third pile section (73), and bury the top grouting pipe (25) in the second section of soil (32). Step thirteen: Inject grout into the pile body of the segmented reinforced pile (7); Step 14: Continue backfilling the third section of soil (33) in layers until it is flush with the top surface of the pile cap beam (9); Step 15: Grouting is performed from the top surface of the third section of soil (33) downwards through the top grouting pipe (25) in the side support construction trench (20) to form a top anchor body (27). The grouting depth of the top anchor body (27) is greater than the height of the pile cap beam (9) and wraps around the top of the reinforced pile (7). Step 16: Excavate the new foundation pit (3) until it is below the design position of the first waist beam (41), then construct the first waist beam (41), and drive the first anchor cable (61) into the soil on the rear side of the pile between the single piles on the first waist beam (41). Step 17: Continue to excavate the new foundation pit (3) downwards until it is below the design position of the second waist beam (42). Then construct the second waist beam (42) and drive the second anchor cable (62) into the soil on the rear side of the pile between the single piles on the second waist beam (42). Repeat this step until all waist beams and anchor cables are constructed.

2. The construction method according to claim 1, characterized in that: In step sixteen, the first anchor cable (61) is not installed on the side of the end pile (11) adjacent to the old outer wall (2), or the horizontal net distance between the anchoring section of the first anchor cable (61) and the reinforcing pile (7) is not less than 1m.

3. The construction method according to claim 1, characterized in that: The reinforcing pile (7) is a steel pipe pile of the same type as the end pile (11). The distance between the two piles is equal to the distance between the single piles of the steel pipe pile row (1). The minimum distance between the reinforcing pile (7) and the strip foundation (5) is not less than 200mm. The verticality deviation of the first pile segment (71) is not greater than 0.3%, and the horizontal positioning deviation is not greater than 15mm. Its grouting material is the same as that of the steel pipe pile row (1).

4. The construction method according to claim 1, characterized in that: The limiting tie rod (8) is a double channel steel tie rod of the same model as the waist beam (4). The two ends of the limiting tie rod (8) are provided with end plates and are fixedly connected to the connecting supports at the corresponding positions of the reinforcing pile (7) and the end pile (11) by high-strength bolts. After assembly, an initial preload of 20%-30% of the design tension is applied.

5. The method of construction according to claim 1, wherein: The bottom anchor body (26) is grouted in layers using sleeve valve pipes. The grouting material is P·O42.5 cement grout, and the grouting pressure is 0.3MPa-0.5MPa.

6. The method of construction according to claim 1, wherein: The top anchor body (27) is grouted with cement-water glass double liquid grout at a pressure of 0.2MPa-0.3MPa.

7. The method of construction according to claim 1, wherein: The thickness of the soil protective layer (21) is 50mm-100mm, and the concrete cover layer (23) is plain concrete of C25 or above with a thickness of 100mm-300mm.

8. The method of construction according to claim 1, wherein: In step three, the backfill soil (30) is judged based on the seepage prevention performance of the original soil layer in the narrow gap (28). The backfill soil is backfilled in layers or replaced and compacted. The thickness of each layer is 100mm-300mm and the compaction coefficient is not less than 0.

94. When the seepage prevention performance of the original soil layer is not up to standard, the backfill soil (30) is replaced with cement stabilized soil and the cement content is 5%-8%.

9. The method of construction according to claim 1, wherein: The first pile segment (71), the second pile segment (72), and the third pile segment (73) are connected by butt welding or by end flanges and high-strength bolts.

10. The method of construction according to claim 1, wherein: A horizontal displacement monitoring point is set at the top of the end pile (11), a settlement monitoring point is set at the top of the strip foundation (5), a settlement monitoring point is set at the top of the reinforcement pile (7), and strain monitoring points are set on the first limiting tie rod (81) and the second limiting tie rod (82). During the construction of the side support construction trench (20), the monitoring is conducted once every 4 hours. During the excavation of the new foundation pit (3), the monitoring is conducted once every 2 hours. After the bottom slab of the new building is poured, the monitoring is conducted once a day.

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