Construction method of staggered bite type double pile-wall-anchor rod foundation pit enclosure structure

Abstract

This invention discloses a construction method for an interlocking double-pile-wall-anchor foundation pit retaining structure. The structure includes deep mixing piles, H-shaped steel piles, a composite slab with truss ribs, and a cement-soil retaining wall. H-shaped steel piles are driven across adjacent deep mixing piles to form an interlocking pile body. The cement-soil retaining wall is located on the outside of the interlocking pile body, with the soil layer outside the foundation pit between them. Anchors are driven through the web of the H-shaped steel piles and into the soil layer outside the foundation pit and the cement-soil retaining wall. The composite slab with truss ribs is fixed to the deep mixing piles with high-strength bolts, and the enclosing cavity is filled with high-strength grout. The overall construction is carried out in layers and sections in a cyclical manner, including cement-soil retaining wall construction, deep mixing pile construction, H-beam steel pile driving and first-layer earthwork excavation, anchor bolt drilling and installation, high-strength bolt rebar installation and composite slab hoisting, high-strength grouting material tremie pipe construction and lower-layer earthwork excavation. It has the advantages of excellent water-stopping effect, strong synergistic bearing capacity, high assembly efficiency and good overall stability, and is suitable for various foundation pit retaining projects.

Description

Technical Field

[0001] This invention relates to the field of foundation pit support technology, specifically a construction method for an interlocking double-pile-wall-anchor foundation pit support structure. Background Technology

[0002] Currently, the prefabricated construction of foundation pit retaining structures mostly adopts the traditional combination of pile-beam-wall-anchor bolts, which has many technical defects in engineering practice: a single pile structure cannot form a continuous and reliable water-stop curtain, and the foundation pit is prone to leakage and water inrush risk; the connection nodes between piles and beams, and between beams and precast walls are structurally weak, with insufficient node strength and poor overall integrity, resulting in serious water leakage problems at the joints; multiple anchor bolts are prone to failure throughout their life cycle, the collective bearing capacity of the anchor group is greatly reduced, and the overall stress imbalance of the retaining structure is caused; the hoisting and positioning accuracy of each precast component is low and the placement is difficult, making it difficult to guarantee construction efficiency and quality.

[0003] The aforementioned problems seriously affect the safety, durability, and ease of construction of the foundation pit retaining structure, and restrict the promotion and application of prefabricated foundation pit retaining technology. Therefore, it is urgent to develop a new type of foundation pit retaining structure and construction method that is reliable in water stopping, stable in nodes, strong in synergistic load bearing, and efficient in assembly. Summary of the Invention

[0004] The purpose of this invention is to provide a construction method for an interlocking double-pile-wall-anchor foundation pit retaining structure to solve the problems raised in the background art.

[0005] To achieve the above objectives, the present invention provides the following technical solution: a construction method for an interlocking double-pile-wall-anchor foundation pit retaining structure, wherein the foundation pit retaining structure includes deep mixing piles, H-shaped steel piles, truss-ribbed composite slabs, and a cement-soil retaining wall. The deep mixing piles are located around the perimeter of the foundation pit, and the H-shaped steel piles are driven across adjacent deep mixing piles to form an interlocking pile body. The cement-soil retaining wall is located outside the interlocking pile body, with the outer soil layer of the foundation pit between them. Anchor rods are installed on the web of the H-shaped steel piles, and the anchor rods are driven into the outer soil layer of the foundation pit and the interior of the cement-soil retaining wall. High-strength bolts are provided on the pile body of the deep mixing piles facing the inner side of the foundation pit. The truss-ribbed composite slabs are fixed to the deep mixing piles in layers and sections by the high-strength bolts. High-strength grouting material is provided in the cavity formed by the deep mixing piles, the H-shaped steel piles, and the truss-ribbed composite slabs. The foundation pit retaining structure is constructed using the following method, with the overall steps as follows: Step S1: Measure and set out the location, then construct the cement-soil retaining wall; Step S2: Construction of deep mixing piles; Step S3: H-beam steel piles are driven across the piles, and the first layer of earthwork is excavated; Step S4: Drilling and installing anchor bolts on the web of the H-beam steel pile; Step S5: Install high-strength bolts for rebar installation, and hoist the composite slab with truss ribs; Step S6: High-strength grouting material guide pipe construction, lower layer soil excavation.

[0006] This invention also provides a construction method for an interlocking double-pile-wall-anchor foundation pit retaining structure, the specific steps of which are as follows: Step S1: Measurement and layout to determine location, construction of cement-soil retaining wall: The cement-soil retaining wall is located on one side of the soil layer outside the foundation pit. It performs local solidification treatment on the soil layer around the foundation pit. The cement-soil retaining wall is closed with the foundation pit as the center. During the on-site construction, the total station is first used to accurately measure and mark the planar position relationship of the deep mixing piles, H-shaped steel piles and cement-soil retaining wall. The width of the cement-soil retaining wall is not less than 3.0m and the depth is not less than 1.2 times the excavation depth of the foundation pit. The cement content of the cement-soil retaining wall shall not be less than 12%, the water-cement ratio shall be 0.45 and 0.5% early strength agent shall be added, and the on-site shotcrete lifting or sinking speed shall not exceed 0.4m / min; the location of the cement-soil retaining wall shall take into account the length of the anchor bolts and the requirements for the length of the anchor bolts into the cement-soil retaining wall from the perspective of stress, and the length of the anchor bolts into the cement-soil retaining wall shall not be less than 600mm. Step S2: Construction of deep mixing piles: The deep mixing piles were constructed symmetrically from both sides towards the middle. After the cement-soil retaining wall was completed, the deep mixing piles were constructed one by one by machinery according to the measured plane position relationship between the deep mixing piles and H-shaped steel piles. The on-site deep mixing pile construction adopts low-frequency eddy pulse grouting technology. The eddy pulse grouting system includes a variable frequency pulse pump, an accumulator, and a high-pressure pipeline, which can generate square wave or sawtooth wave pressure pulses. That is, a pulse grouting pump is used to intermittently spray grout at a frequency of 0.5~2Hz and a peak pressure 1.2~1.5 times that of the conventional method. The generated pressure wave drives the grout to form a local "eddy" in the soil, promotes the diffusion of grout into micro-cracks, inhibits the grout from floating along the pile wall, and achieves uniform strength in the upper, middle and lower layers. Step S3: H-beam steel pile driving and first-layer excavation: After the deep mixing piles are constructed one by one, H-shaped steel piles are driven between adjacent deep mixing piles after the initial setting and before the final setting of the deep mixing piles. The flange plates of the H-shaped steel piles are embedded into the interior of the deep mixing piles, thus forming a cross-interlocking arrangement of deep mixing piles and H-shaped steel piles. After the H-shaped steel piles are constructed and the deep mixing piles are completely solidified, the first layer of earthwork excavation can be carried out. Then, the soil between the piles and the reserved holes on the web of the H-shaped steel piles are cleaned, and the process acceptance work is carried out. Step S4: Drilling and installing anchor bolts on the web of the H-beam pile: The web of the H-beam steel pile has pre-reserved holes distributed at equal intervals. The positions of the pre-reserved holes correspond one-to-one with the designed anchor positions. The spacing between adjacent pre-reserved holes does not exceed 0.8m, and the diameter of the pre-reserved holes is 5mm~8mm larger than the designed anchor diameter. After the first layer of earthwork is excavated and the soil between the piles and the pre-reserved hole positions are cleared, the installation of multiple rows of anchors can be carried out. The anchor installation adopts a self-advancing hollow drilling and grouting integrated rod, driven by a special drilling rig. The front end of the anchor body passes through the hole in the web of the H-beam steel and directly reaches the critical surface of the soil layer. The drilling rig drives the anchor body to rotate and drill. The self-advancing anchor automatically opens a channel in the soil layer with the front drill bit and advances forward synchronously until the designed anchoring depth is reached. The anchor body adopts a hollow design, which serves as both a drill rod, a grouting pipe, and a reinforcing bar. After drilling to the designed anchoring depth, the tail end of the anchor rod is connected to the grouting pump via a quick grouting connector to start the grouting operation. The grout is ejected from the outlet at the front drill bit through the internal channel of the hollow rod body, filling from the bottom of the hole upwards—that is, the grout gradually fills from the deepest point upwards, pushing out residual soil and gravel from the hole by the grout pressure, ensuring that the grout is full and dense. The grouting pressure is controlled between 0.5 and 2.0 MPa according to the soil conditions. Under pressure, the grout penetrates into the surrounding soil gaps, reinforcing the soil around the hole wall and forming a full-length anchor body integrating the "rod body-grout-soil layer," providing continuous anchoring force. A grout stop plug is installed at the hole opening to ensure that the grout does not leak during grouting. After grouting is completed, once the grout has cured to the designed strength, the pad and nut are installed, prestress is applied according to the design requirements, and the grout is locked. Step S5: Install high-strength bolts for rebar installation, and hoist the composite slab with truss ribs: The truss-ribbed composite plate is installed and fixed to the deep mixing pile by high-strength bolts, and several reserved connection holes are provided at the contact point between the truss-ribbed composite plate and the deep mixing pile. After the anchor bolts on the web of the H-beam steel piles are drilled and installed, the excess soil in each deep mixing pile body is cleaned a second time. First, drilling equipment is used to drill holes according to the reserved connection holes at the contact point between the composite plate with truss ribs and the deep mixing pile. The drilling depth is not less than 160mm. Then, "sulfur mortar" is used to fill the holes drilled on the deep mixing piles. High-strength bolts are inserted into the pile body holes one by one to reduce vibration and strengthen maintenance. After the high-strength bolts are installed one by one, the truss rib composite plate is slowly lowered to the deep mixing pile body location using a tower crane. The reserved connection holes on the truss rib composite plate are aligned with the high-strength bolts in modules, so that all the high-strength bolts are inserted into the reserved connection holes on the truss rib composite plate. Then, additional steel pads and nuts are installed to fix it. At the horizontal or vertical joints between adjacent truss ribbed composite slabs, each truss ribbed composite slab should be filled with a water-swellable polymer sealing rubber strip during hoisting, and process acceptance should be carried out. Step S6: High-strength grouting material guide pipe method construction, lower layer soil excavation: Within each grouting section, grouting holes are provided on the truss-ribbed composite plate at the bottom of each section, and overflow holes are provided on the truss-ribbed composite plate at the top of each section. The grouting holes and overflow holes are diagonally related within each section. After the truss-ribbed composite slabs are hoisted, high-pressure grouting is carried out in the internal cavities of the truss-ribbed composite slabs in each section. High-strength grout is injected into the cavities using a guide pipe method on site. After the high-strength grout is injected into the guide pipe method, the excavation of the lower layer of earthwork can be carried out. Steps S4 to S6 above are repeated until the construction of the entire retaining structure is completed. After all the earthwork inside the foundation pit is excavated and the retaining structure is completed, the concrete cushion layer at the bottom of the foundation pit is poured immediately, thus completing all the work on the overall foundation pit retaining structure.

[0007] Preferably, the specific construction process of deep mixing piles in step S2 includes seven steps: The first step is to position the pile driver and perform intelligent vertical adjustment, moving the pile driver to the pile location; The second step is to activate the automatic vertical adjustment system to ensure that the verticality deviation of the guide frame is ≤0.5%. The third step is to pre-stir and sink the mixture. With all the radial telescopic stirring teeth retracted, start the stirring head and let it sink while rotating at a speed of ≤0.6m / min. The fourth step is to monitor the torque in real time and automatically reduce the sinking speed and slightly increase the rotation speed when encountering hard soil layers to prevent the drill from getting stuck. Step 5: Pulse grouting + radial telescopic mixing and lifting. After sinking to the designed depth, the eddy current pulse grouting pump is turned on, with a grouting pressure of 0.6~1.0MPa and a frequency of 1Hz. At the same time, all radial telescopic mixing teeth are extended to start lifting. During the lifting process, the telescopic teeth continuously extend and retract at a frequency of 0.5~1Hz to shear the soil. The lifting speed is controlled at 0.4~0.6m / min to match the pulse frequency, ensuring that the soil and grout at each depth of the pile section are mixed and sheared 15~20 times. Step 6: Re-mix. After raising the pile to 0.5m above the designed elevation of the pile top, stop grouting, retract the telescopic teeth, and sink and mix again. If the sensor shows that the strength of a certain section of the pile is uneven, low-frequency pulse grouting can be activated during re-mixing to supplement 10-20%. Step 7: Cleaning and relocation, lifting the drill out of the ground, flushing the pipeline and mixing head with high-pressure water, and moving it to the next pile location.

[0008] Preferably, in step S6, the high-pressure grouting operation of the high-strength grouting material is carried out on site using the guide pipe method. Specifically, the grouting holes are first plugged with hollow rubber plugs that are 2mm to 3mm smaller than the diameter of the grouting holes on the truss-ribbed composite plate. The diameter of the hole in the middle of the hollow rubber plug is also 1mm to 2mm smaller than the diameter of the grouting guide pipe. Then, the grouting guide pipe is inserted into the hole in the middle of the hollow rubber plug and extends into the cavity between the truss-ribbed composite plate and the soil layer outside the foundation pit. The high-pressure grouting machine is connected to the grouting guide pipe to carry out the construction of high-pressure grouting material. After the grout overflows from the grouting hole at the top of the truss-ribbed composite plate, the overflow hole is plugged with a sealing rubber plug. The high-pressure grouting is maintained and the pressure is stabilized for 2 minutes. Finally, the guide pipe is pulled out and immediately sealed with a sealing rubber plug that is 2mm smaller than the diameter of the hole in the middle of the hollow rubber plug for enhanced curing.

[0009] Compared with the prior art, the beneficial effects of this invention are as follows: 1. This invention uses deep mixing piles and H-shaped steel piles interlocked to form a closed and continuous retaining and water-stopping curtain. Compared with the traditional single row of piles, the water-stopping effect is more prominent, and the composite pile body has a stronger ability to resist the soil pressure on the pile side. 2. The anchor rod of the present invention is directly anchored into the interior of the outer cement-soil retaining wall, effectively connecting the interlocking piles, cement-soil retaining wall and soil layer outside the foundation pit into a whole, strengthening the synergistic force bearing of each component, and avoiding the problem of reduced working capacity of the group anchor caused by the failure of a single anchor rod; 3. The present invention sets water-swellable sealing rubber strips at the joints of composite slabs with truss ribs, and uses a conduit method to inject high-strength grout into the cavity under high pressure, which effectively prevents joint leakage and improves the overall density and stability of the enclosure structure. Attached Figure Description

[0010] Figure 1 This is a schematic diagram of the foundation pit retaining structure of the present invention; Figure 2 This is a flowchart of the construction method of the present invention. Detailed Implementation

[0011] This invention discloses a construction method for an interlocking double-pile-wall-anchor foundation pit retaining structure. The retaining structure consists of a cement-soil retaining wall, deep mixing piles 1, H-shaped steel piles 2, anchors 3, high-strength bolts 4, a composite plate with truss ribs 5, and high-strength grouting material. The overall technical solution is based on the following: "The cement-soil retaining wall stabilizes the surrounding environment; the deep mixing piles and H-shaped steel piles are interlocked and interlocked to retain soil and stop water; multiple rows of anchors are drilled and anchored inside the cement-soil retaining wall outside the foundation pit to improve the pull-out resistance of the anchors; a high-strength bolt drilling method is used for rebar installation; the composite plate with truss ribs is linked to each deep mixing pile and assisted by wet grouting; and high-strength grouting material is injected in layers and sections to form a new technology system for an interlocking double-pile-wall-anchor foundation pit retaining structure."

[0012] See Figure 1 The specific structure of the foundation pit retaining structure is as follows: multiple deep mixing piles 1 are provided around the foundation pit, and H-shaped steel piles 2 are driven between two adjacent deep mixing piles 1. That is, the two flange plates of the H-shaped steel piles 2 are respectively embedded in the deep mixing piles 1 on both sides, forming an interlocking pile body. In this invention, the "interlocking pile body" refers to a composite pile body formed by the interlocking of deep mixing piles and H-shaped steel piles driven between two adjacent deep mixing piles, which is different from the traditional "interlocking pile" (arc interlocking between adjacent cast-in-place piles).

[0013] A cement-soil retaining wall is installed on the outer side of the staggered pile body. The soil layer outside the foundation pit is between the staggered pile body and the cement-soil retaining wall. Depending on the soil conditions, the safe distance between the deep mixing pile and the cement-soil retaining wall is generally controlled between 2000mm and 5000mm to avoid mutual interference during construction. The web of the H-shaped steel pile 2 has reserved holes. The anchor rod 3 passes through the reserved holes on the web of the H-shaped steel pile 2 and is driven into the soil layer outside the foundation pit and the interior of the cement-soil retaining wall. High-strength bolts 4 are installed on the pile body of the deep mixing pile 1 facing the inside of the foundation pit. The composite plate 5 with truss ribs is fixed to the deep mixing pile 1 in layers and sections by the high-strength bolts 4. High-strength grouting material is installed in the cavity formed by the deep mixing pile 1, the H-shaped steel pile 2, and the composite plate 5 with truss ribs.

[0014] The aforementioned foundation pit retaining structure is achieved through the following construction method, please refer to [reference needed]. Figure 2 The overall steps are as follows: Step S1: Measure and set out the location, then construct the cement-soil retaining wall; Step S2: Construction of deep mixing piles; Step S3: H-beam steel piles are driven across the piles, and the first layer of earthwork is excavated; Step S4: Drilling and installing anchor bolts on the web of the H-beam steel pile; Step S5: Install high-strength bolts for rebar installation, and hoist the composite slab with truss ribs; Step S6: High-strength grouting material guide pipe construction, lower layer soil excavation.

[0015] The specific steps of this construction method are as follows: Step S1: Measurement and layout to determine location, construction of cement-soil retaining wall: The cement-soil retaining wall is located on one side of the soil layer outside the foundation pit. Its main function is to locally solidify the soil layer surrounding the foundation pit. The cement-soil retaining wall forms a closed loop with the foundation pit as its center. During on-site construction, a total station is first used to accurately measure and mark the planar positional relationship between the deep mixing piles, H-beam steel piles, and the cement-soil retaining wall. The width of the cement-soil retaining wall is no less than 3.0m, and its depth is no less than 1.2 times the excavation depth of the foundation pit.

[0016] The cement content of the cement-soil retaining wall shall not be less than 12%, the water-cement ratio shall be 0.45, and 0.5% early-strength agent shall be added. The on-site shotcrete lifting or sinking speed shall not exceed 0.4 m / min. The location of the cement-soil retaining wall shall take into account the design length of the anchor bolts and the requirements for the anchor bolts' embedment length into the cement-soil retaining wall from the perspective of stress, and the anchor bolt embedment length into the cement-soil retaining wall shall not be less than 600 mm.

[0017] Step S2: Construction of deep mixing piles: The on-site construction of deep mixing piles adopts a "symmetrical construction from both sides to the center" technical scheme. That is, for strip-shaped foundation pits, construction is carried out symmetrically from both sides of the long side of the foundation pit towards the geometric center; for circular or irregular-shaped foundation pits, construction is carried out symmetrically from the opposite ends or two points towards the central area. After the cement-soil retaining wall is completed, the deep mixing piles are constructed one by one using machinery according to the measured plane position relationship between the deep mixing piles and H-beam steel piles.

[0018] The deep mixing pile construction on site adopts a low-frequency eddy pulse grouting process. The eddy pulse grouting system includes a variable frequency pulse pump, an accumulator, and high-pressure pipelines, which can generate square wave or sawtooth wave pressure pulses. That is, a pulse grouting pump is used to intermittently spray grout at a frequency of 0.5~2Hz and a peak pressure 1.2~1.5 times that of the conventional method. The generated pressure waves drive the grout to form a local "eddy" in the soil, promoting the diffusion of the grout into micro-cracks and inhibiting the grout from floating along the pile wall. This achieves uniform strength in the upper, middle, and lower sections. This strength description is in contrast to the "stronger at the top and weaker at the bottom" phenomenon caused by the grout flowing along the pile wall in traditional continuous grouting processes. The pulse grouting process of this invention significantly improves the strength uniformity of the pile body along the depth direction by driving the grout to diffuse laterally and inhibiting floating through pressure waves.

[0019] The following preparations are required for the construction of deep mixing piles: The radial telescopic stirring teeth consist of a toothed rod, tooth tips, and a return spring assembly made of high wear-resistant alloy steel. They are arranged in layers along the axial direction of the stirring head, with 4 to 6 telescopic teeth symmetrically arranged in each layer along the circumference. Each telescopic tooth is independently installed in a radial guide hole on the stirring head body. The inner wall of the guide hole is equipped with a sealing bushing and a dustproof ring to prevent slurry intrusion.

[0020] Drive method: Hydraulic drive is adopted. The rear end of each telescopic tooth is directly connected to a miniature hydraulic cylinder (25mm diameter, 55mm stroke). Hydraulic oil is introduced through a high-pressure rotary joint on the top of the mixing head, and the telescopic movement of the cylinder is controlled by an electronically controlled proportional valve. The hydraulic system operates at a pressure of 8~12MPa, achieving a continuous telescopic frequency of 0.5~1Hz, with a fixed extension of 50mm. The control system can automatically adjust the telescopic frequency based on the lifting speed of the mixing head and feedback from soil resistance, ensuring that each depth section is disturbed 15~20 times.

[0021] Tooth profile and dimensions: Tooth bar diameter: 30mm, length: 120mm; Tooth tip is truncated cone shape, front diameter 8mm, root diameter 20mm, tooth tip length: 25mm; Tooth tip surface is overlaid with tungsten carbide wear-resistant layer (thickness 2mm), Rockwell hardness ≥60HRC; Return spring is a rectangular section helical spring, installed in the countersunk hole at the rear end of the tooth bar, used to automatically retract the telescopic tooth to the initial position (fully retracted state) after hydraulic oil pressure is released. Sealing and anti-jamming design: Two V-shaped combined sealing rings and one mud scraper ring are provided between the tooth bar and the guide hole, which can effectively prevent cement slurry from entering the hydraulic cavity; at the same time, a limiting shoulder is set at the bottom of the guide hole to ensure that the maximum extension of the telescopic tooth is precisely controlled at 50mm to prevent over-travel damage.

[0022] Control Logic: During the mixing and lifting process, the hydraulic system periodically drives the telescopic teeth to extend outward and immediately retract at a preset frequency (e.g., 1Hz), creating a pulsed shearing and squeezing effect on the soil. When the sensor detects an abnormal increase in torque at a certain cross-section, the control system can temporarily increase the telescopic frequency to 2Hz to enhance the disturbance effect and prevent the drill from getting stuck.

[0023] Intelligent vertical adjustment system: Dual-axis tilt sensors are installed at the top, middle, and bottom of the pile driver guide frame, and laser displacement sensors are installed at the four corners of the pile driver chassis. A prism target is installed at the top of the guide frame to cooperate with a ground total station for absolute verticality verification. The controller is a programmable logic controller that collects tilt data in real time and calculates the overall tilt of the guide frame. When the tilt exceeds the set value, the automatic vertical adjustment program is activated: the controller controls the extension and retraction of the chassis hydraulic outriggers according to the tilt direction, and at the same time drives the electric screw fine adjustment mechanism in the middle of the guide frame to perform reverse correction until the verticality deviation is controlled within 0.5%.

[0024] Torque monitoring and automatic adjustment of sinking speed: A flange-type dynamic torque sensor is installed between the output shaft of the main drive motor of the mixing head and the reduction gearbox. A strain gauge torque measurement ring is also installed on the low-speed output shaft of the reduction gearbox for redundancy. The sensor transmits the torque signal to the controller in real time. The controller has normal torque, warning torque, and upper limit torque values. When the measured torque does not exceed the normal value, the original sinking speed is maintained; when the torque exceeds the normal value but does not reach the warning value, the system automatically and gradually reduces the sinking speed and slightly increases the mixing head speed; when the torque exceeds the warning value but does not reach the upper limit value, the system immediately reduces the sinking speed to a lower level and initiates a high-frequency micro-amplitude drilling action, while simultaneously increasing the grouting flow rate; when the torque exceeds the upper limit value, the system automatically stops urgently and slightly raises the mixing head, waiting for manual confirmation before continuing. All data is stored in real time to form a construction log curve.

[0025] Through the above-mentioned specific structural design, the radial telescopic mixing teeth achieve periodic radial compression and shearing of the soil around the pile during the lifting process. Combined with low-frequency eddy pulse grouting, this maximizes the uniformity of grout diffusion and the uniformity of pile strength.

[0026] Based on the above preparations, the construction process includes the following seven steps: The first step is to position the piling machine and perform intelligent vertical adjustment, moving the piling machine to the pile location.

[0027] The second step is to activate the automatic vertical adjustment system to ensure that the verticality deviation of the guide frame is ≤0.5%.

[0028] The third step is pre-stirring and sinking (contraction tooth mode). With all radial telescopic stirring teeth retracted, start the stirring head and sink it while rotating at a speed of ≤0.6m / min.

[0029] The fourth step is to monitor the torque in real time and automatically reduce the sinking speed and slightly increase the rotation speed when encountering hard soil layers to prevent the drill from getting stuck.

[0030] Step 5: Pulse grouting + radial telescopic mixing and lifting. After sinking to the designed depth, the vortex pulse grouting pump is turned on, with a grouting pressure of 0.6~1.0MPa (peak value) and a frequency of 1Hz. Simultaneously, all radial telescopic mixing teeth are extended (50mm extension) to begin lifting. During the lifting process, the radial telescopic mixing teeth continuously extend and retract at a frequency of 0.5~1Hz (i.e., once every 1~2 seconds), performing a shearing action on the soil. The lifting speed is controlled at 0.4~0.6m / min, matching the pulse frequency, to ensure that the soil and grout at each pile depth are mixed and sheared 15~20 times.

[0031] Step 6: Re-stir (select pulse grouting). After raising the pile to 0.5m above the design elevation of the pile top, stop grouting, retract the telescopic teeth, and sink and stir again. If the sensor shows that the strength of a certain section of the pile body is uneven, low-frequency pulse (0.3Hz) grouting can be turned on during re-stirring to add 10~20%.

[0032] Step 7: Cleaning and relocation, lifting the drill out of the ground, flushing the pipeline and mixing head with high-pressure water, and moving it to the next pile location.

[0033] To verify the effect of low-frequency eddy current pulse grouting combined with radial expansion and contraction mixing technology on improving the uniformity of pile strength, a comparative test was conducted on a typical silty clay site. The test was divided into two groups: the control group used conventional mixing pile technology (uniform grouting, no pulse, no expansion teeth); the experimental group used the technology of this invention (pulse frequency 1 Hz, peak pressure 1.3 times that of conventional, expansion tooth frequency 0.8 Hz). Both groups had pile diameters of 600 mm, pile lengths of 12 m, cement content of 15%, and a water-cement ratio of 0.45. Core samples were taken 28 days after pile formation, with three core samples taken from each of the three depth ranges: pile top, pile middle, and pile bottom. The average unconfined compressive strength was measured as follows: Control group: 2.1 MPa at the top of the pile, 1.8 MPa in the middle of the pile, 1.4 MPa at the bottom of the pile, range 0.7 MPa, coefficient of variation 0.18; Test group: 2.3 MPa at the top of the pile, 2.2 MPa in the middle of the pile, 2.2 MPa at the bottom of the pile, range 0.1 MPa, coefficient of variation 0.03.

[0034] The strength of the piles in the test group remained largely consistent from top to bottom, while the strength of the control group decreased significantly with depth. Furthermore, in a sandy interlayer site, the strength at the bottom of the test piles still reached over 95% of the strength at the top, and the core samples showed good integrity, with no strength stratification caused by grout rising. These data demonstrate that the process of this invention can indeed achieve maximum uniformity of strength across the entire pile structure.

[0035] Step S3: H-beam steel pile driving and first-layer excavation: After the deep mixing piles are constructed one by one, after the deep mixing piles have initially set but before they have finally set (using the "penetration resistance method," when the penetration resistance reaches 0.5MPa~3.5MPa, this period is the window period for driving H-beam steel piles), H-beam steel piles are driven between adjacent deep mixing piles, so that the flange plates of the H-beam steel piles are embedded into the interior of the deep mixing piles, thus forming a state in which the deep mixing piles and H-beam steel piles are interlocked and arranged.

[0036] The technical solution of driving H-shaped steel piles before the deep mixing piles are fully solidified effectively utilizes the incompletely solidified deep mixing piles to provide conditions for the driving of H-shaped steel piles, reduces the resistance to the driving of H-shaped steel piles into the soil, and better realizes the positioning of H-shaped steel piles. At the same time, it ensures that the flange plate and part of the web plate of the H-shaped steel pile can be reliably embedded into the interior of the deep mixing pile. After the deep mixing pile is fully solidified, a closed retaining and water-stopping curtain composed of deep mixing piles and H-shaped steel piles is formed.

[0037] After the H-beam steel piles are completed and the deep mixing piles are fully solidified, the first layer of earthwork excavation can be carried out. Then, the soil between the piles and the reserved holes on the web of the H-beam steel piles should be cleaned, and the process acceptance work should be carried out.

[0038] Step S4: Drilling and installing anchor bolts on the web of the H-beam pile: To facilitate the installation of multiple rows of anchor bolts, the web of the H-beam steel piles has pre-drilled holes spaced at equal intervals. The positions of these holes correspond one-to-one with the designed anchor bolt positions, with the spacing between adjacent holes not exceeding 0.8m. The diameter of the pre-drilled holes is 5mm to 8mm larger than the designed anchor bolt diameter. Installing the anchor bolts on the web of the H-beam steel piles between two adjacent deep mixing piles optimizes the load-bearing path of the combined foundation pit retaining structure. Furthermore, utilizing the web of the H-beam steel piles as support points or bearings for the anchor bolt installation improves the local anchoring compressive strength after installation.

[0039] After the first layer of earthwork is excavated and the soil between piles and the reserved hole locations are cleared, the installation of multiple rows of anchor rods can begin. The anchor rods are installed using self-propelled hollow drilling and grouting integrated rods (consisting of a hollow anchor rod body, a front-end alloy drill bit, a full-length corrugated thread, a grout stop plug, a washer plate, and a nut), driven by a specialized drilling rig. The front end of the anchor rod body passes through a hole in the web of the H-beam and directly reaches the critical surface of the soil layer. The drilling rig drives the anchor rod body to rotate and drill simultaneously, eliminating the need for pre-drilling or casing. The self-propelled anchor rod automatically creates a channel in the soil layer using the front-end drill bit and advances synchronously until the designed anchoring depth is reached. During drilling, the drilling progress directly reflects the channel condition, eliminating the problem of disconnection between drilling and rod insertion. The anchor rod body features a hollow design, serving as both a drill rod, a grouting pipe, and a reinforcing bar.

[0040] After drilling to the designed anchoring depth, the tail end of the anchor rod is connected to the grouting pump via a quick-connect grouting joint to initiate the grouting operation. Grout is ejected from the outlet at the front drill bit through the internal channels of the hollow rod, filling the hole from the bottom upwards—that is, the grout gradually fills from the deepest point upwards, pushing out residual soil and gravel from the hole by the grout pressure, ensuring a full and dense grout layer. The grouting pressure is controlled between 0.5 and 2.0 MPa depending on the soil conditions. Under pressure, the grout penetrates into the surrounding soil gaps, reinforcing the soil around the hole wall and forming a continuous anchor body consisting of the rod, grout, and soil, providing continuous anchoring force. A grout stop plug is installed at the hole opening to prevent grout leakage during grouting.

[0041] After grouting is completed, wait for the grout to cure to the design strength, install the pads and nuts, apply prestress according to the design requirements, and lock them.

[0042] Step S5: Install high-strength bolts for rebar installation, and hoist the composite slab with truss ribs: The truss-ribbed composite plate is installed and fixed to the deep mixing pile with high-strength bolts, and several reserved connection holes are provided at the contact point between the truss-ribbed composite plate and the deep mixing pile.

[0043] After the anchor bolts on the web of the H-beam steel piles are drilled and installed, the excess soil in each deep mixing pile is cleaned a second time. First, drilling equipment is used to drill holes at the contact points between the truss-ribbed composite plate and the deep mixing pile, with a drilling depth of not less than 160mm. Then, sulfur mortar is used to fill the drilled holes in the deep mixing piles, and high-strength bolts are inserted into the pile holes one by one to reduce vibration and enhance curing. In this invention, sulfur mortar mainly serves as temporary fixing and positioning; the final load is borne by the mechanical anchoring of the high-strength bolts and the subsequent high-strength grouting material.

[0044] After the high-strength bolts are installed one by one, the truss-ribbed composite plate is slowly lowered to the deep mixing pile position using a tower crane. The pre-reserved connection holes on the truss-ribbed composite plate are aligned with the high-strength bolts in modules, so that all the high-strength bolts are inserted into the pre-reserved connection holes on the truss-ribbed composite plate. Then, additional steel pads and nuts are installed to fix it.

[0045] To prevent problems during subsequent grouting operations, when hoisting each truss-ribbed composite slab, the horizontal or vertical joints between adjacent truss-ribbed composite slabs should be filled with water-swellable polymer sealing rubber strips, and process acceptance should be carried out.

[0046] Step S6: High-strength grouting material guide pipe method construction, lower layer soil excavation: To facilitate internal grouting after the truss-ribbed composite slab is hoisted, after each layer of excavation, the height range between adjacent upper and lower anchor bolts is divided into an independent grouting zone based on the vertical distribution of the deep mixing piles and H-beams. The vertical height of each grouting zone is generally 2000mm to 3000mm, and the horizontal width is defined by the center-to-center distance between two adjacent H-beams, typically 1000mm to 1500mm.

[0047] Grouting-stopping measures are implemented between each grouting section: At the boundary between adjacent sections, i.e., at the joint between the top truss-ribbed composite slab of the lower section and the bottom truss-ribbed composite slab of the upper section, water-swellable sealing strips or grouting-specific grouting-stopping rubber tapes are pre-attached to the inside of the composite slabs. Simultaneously, a layer of fast-drying epoxy resin interface agent with a thickness of not less than two millimeters is applied to the deep mixing pile body at the section boundary before grouting to form a grout-stopping barrier layer.

[0048] Within each grouting section, grouting holes are provided on the truss-ribbed composite plate at the bottom of each section, and overflow holes are provided on the truss-ribbed composite plate at the top of each section. The grouting holes and overflow holes are diagonally related within each section.

[0049] After the truss-ribbed composite slabs are hoisted, high-pressure grouting can be carried out in the internal cavities of the truss-ribbed composite slabs in each section. High-strength grouting material will be injected into the cavities using a guide pipe method on site. Specifically: First, use a hollow rubber plug with a diameter 2mm~3mm smaller than the grouting hole diameter on the truss-ribbed composite plate to plug the grouting hole. The diameter of the hole in the middle of the hollow rubber plug should also be 1mm~2mm smaller than the grouting guide pipe. Then, insert the grouting guide pipe into the hole in the middle of the hollow rubber plug and extend it into the cavity between the truss-ribbed composite plate and the soil layer outside the foundation pit. Use a high-pressure grouting machine to connect the grouting guide pipe to carry out high-pressure grouting. After the grout overflows from the grouting hole at the top of the truss-ribbed composite plate, plug the overflow hole with a sealing rubber plug and maintain high-pressure grouting continuously for 2 minutes. Finally, pull out the guide pipe and immediately seal it with a sealing rubber plug with a diameter 2mm smaller than the hole diameter in the middle of the hollow rubber plug, and strengthen the curing.

[0050] After the high-strength grouting material guide pipe method construction is completed, the excavation of the lower layer of soil can be carried out, and steps S4 to S6 above will be repeated until the construction of the entire retaining structure is completed. After all the soil excavation inside the foundation pit is completed and the retaining structure is completed, the concrete cushion layer at the bottom of the foundation pit will be poured immediately, thus completing all the work on the overall foundation pit retaining structure.

[0051] In summary, this invention significantly improves water-stopping and lateral pressure resistance, strengthens the overall load-bearing capacity of the group anchors, and improves component assembly efficiency through the collaborative construction of interlocking double piles, cement-soil retaining walls, anchors, and composite slabs. The structure is stable, seepage-proof, highly applicable, and easy to promote.

[0052] Finally, it should be noted that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A construction method for an interlocking double-pile-wall-anchor foundation pit retaining structure, characterized in that: The foundation pit retaining structure includes deep mixing piles, H-shaped steel piles, truss-ribbed composite slabs, and a cement-soil retaining wall. The deep mixing piles are located around the perimeter of the foundation pit, with H-shaped steel piles driven between adjacent deep mixing piles to form an interlocking pile body. The cement-soil retaining wall is located outside the interlocking pile body, with the outer soil layer of the foundation pit between them. Anchor rods are installed on the web of the H-shaped steel piles, and the anchor rods are driven into the outer soil layer of the foundation pit and the interior of the cement-soil retaining wall. High-strength bolts are installed on the pile body of the deep mixing piles facing inwards from the foundation pit. The truss-ribbed composite slabs are fixed to the deep mixing piles in layers and sections by the high-strength bolts. High-strength grouting material is installed in the cavity formed by the deep mixing piles, the H-shaped steel piles, and the truss-ribbed composite slabs. The foundation pit retaining structure is constructed using the following method, with the overall steps as follows: Step S1: Measure and set out the location, then construct the cement-soil retaining wall; Step S2: Construction of deep mixing piles; Step S3: H-beam steel piles are driven across the piles, and the first layer of earthwork is excavated; Step S4: Drilling and installing anchor bolts on the web of the H-beam steel pile; Step S5: Install high-strength bolts for rebar installation, and hoist the composite slab with truss ribs; Step S6: High-strength grouting material guide pipe construction, lower layer soil excavation.

2. A construction method for an interlocking double-pile-wall-anchor foundation pit retaining structure, characterized in that, The specific steps are as follows: Step S1: Measurement and layout to determine location, construction of cement-soil retaining wall: The cement-soil retaining wall is located on one side of the soil layer outside the foundation pit. It performs local solidification treatment on the soil layer around the foundation pit. The cement-soil retaining wall is closed with the foundation pit as the center. During the on-site construction, the total station is first used to accurately measure and mark the planar position relationship of the deep mixing piles, H-shaped steel piles and cement-soil retaining wall. The width of the cement-soil retaining wall is not less than 3.0m and the depth is not less than 1.2 times the excavation depth of the foundation pit. The cement content of the cement-soil retaining wall shall not be less than 12%, the water-cement ratio shall be 0.45 and 0.5% early strength agent shall be added, and the on-site shotcrete lifting or sinking speed shall not exceed 0.4m / min; the location of the cement-soil retaining wall shall take into account the length of the anchor bolts and the requirements for the length of the anchor bolts into the cement-soil retaining wall from the perspective of stress, and the length of the anchor bolts into the cement-soil retaining wall shall not be less than 600mm. Step S2: Construction of deep mixing piles: The deep mixing piles were constructed symmetrically from both sides towards the middle. After the cement-soil retaining wall was completed, the deep mixing piles were constructed one by one by machinery according to the measured plane position relationship between the deep mixing piles and H-shaped steel piles. The on-site deep mixing pile construction adopts low-frequency eddy pulse grouting technology. The eddy pulse grouting system includes a variable frequency pulse pump, an accumulator, and a high-pressure pipeline, which can generate square wave or sawtooth wave pressure pulses. That is, a pulse grouting pump is used to intermittently spray grout at a frequency of 0.5~2Hz and a peak pressure 1.2~1.5 times that of the conventional method. The generated pressure wave drives the grout to form a local "eddy" in the soil, promotes the diffusion of grout into micro-cracks, inhibits the grout from floating along the pile wall, and achieves uniform strength in the upper, middle and lower layers. Step S3: H-beam steel pile driving and first-layer excavation: After the deep mixing piles are constructed one by one, H-shaped steel piles are driven between adjacent deep mixing piles after the initial setting and before the final setting of the deep mixing piles. The flange plates of the H-shaped steel piles are embedded into the interior of the deep mixing piles, thus forming a cross-interlocking arrangement of deep mixing piles and H-shaped steel piles. After the H-shaped steel piles are constructed and the deep mixing piles are completely solidified, the first layer of earthwork excavation can be carried out. Then, the soil between the piles and the reserved holes on the web of the H-shaped steel piles are cleaned, and the process acceptance work is carried out. Step S4: Drilling and installing anchor bolts on the web of the H-beam pile: The web of the H-beam steel pile has pre-reserved holes distributed at equal intervals. The positions of the pre-reserved holes correspond one-to-one with the designed anchor positions. The spacing between adjacent pre-reserved holes does not exceed 0.8m, and the diameter of the pre-reserved holes is 5mm~8mm larger than the designed anchor diameter. After the first layer of earthwork is excavated and the soil between the piles and the pre-reserved hole positions are cleared, the installation of multiple rows of anchors can be carried out. The anchor installation adopts a self-advancing hollow drilling and grouting integrated rod, driven by a special drilling rig. The front end of the anchor body passes through the hole in the web of the H-beam steel and directly reaches the critical surface of the soil layer. The drilling rig drives the anchor body to rotate and drill. The self-advancing anchor automatically opens a channel in the soil layer with the front drill bit and advances forward synchronously until the designed anchoring depth is reached. The anchor body adopts a hollow design, which serves as both a drill rod, a grouting pipe, and a reinforcing bar. After drilling to the designed anchoring depth, the tail end of the anchor rod is connected to the grouting pump through a quick grouting connector to start the grouting operation. The grout is sprayed out from the grout outlet at the front drill bit through the internal channel of the hollow rod body, filling from the bottom of the hole upwards. That is, the grout gradually fills from the deepest point upwards, pushing out the residual soil and gravel in the hole by the grout pressure, ensuring that the grout is full and dense. The grouting pressure is controlled between 0.5 and 2.0 MPa according to the soil conditions. Under the action of pressure, the grout penetrates into the gaps in the surrounding soil layer, reinforcing the soil around the hole wall and forming a whole anchor body of "rod body-grout-soil layer", providing continuous anchoring force. The grout stop plug is installed at the orifice to ensure that grout does not leak during grouting; after grouting is completed, wait for the grout to cure to the design strength, install the pad and nut, apply prestress according to the design requirements and lock it; Step S5: Install high-strength bolts for rebar installation, and hoist the composite slab with truss ribs: The truss-ribbed composite plate is installed and fixed to the deep mixing pile by high-strength bolts, and several reserved connection holes are provided at the contact point between the truss-ribbed composite plate and the deep mixing pile. After the anchor bolts on the web of the H-beam steel piles are drilled and installed, the excess soil in each deep mixing pile body is cleaned a second time. First, drilling equipment is used to drill holes according to the reserved connection holes at the contact point between the composite plate with truss ribs and the deep mixing pile. The drilling depth is not less than 160mm. Then, "sulfur mortar" is used to fill the holes drilled on the deep mixing piles. High-strength bolts are inserted into the pile body holes one by one to reduce vibration and strengthen maintenance. After the high-strength bolts are installed one by one, the truss rib composite plate is slowly lowered to the deep mixing pile body location using a tower crane. The reserved connection holes on the truss rib composite plate are aligned with the high-strength bolts in modules, so that all the high-strength bolts are inserted into the reserved connection holes on the truss rib composite plate. Then, additional steel pads and nuts are installed to fix it. At the horizontal or vertical joints between adjacent truss ribbed composite slabs, each truss ribbed composite slab should be filled with a water-swellable polymer sealing rubber strip during hoisting, and process acceptance should be carried out. Step S6: High-strength grouting material guide pipe method construction, lower layer soil excavation: Within each grouting section, grouting holes are provided on the truss-ribbed composite plate at the bottom of each section, and overflow holes are provided on the truss-ribbed composite plate at the top of each section. The grouting holes and overflow holes are diagonally related within each section. After the truss-ribbed composite slabs are hoisted, high-pressure grouting is carried out in the internal cavities of the truss-ribbed composite slabs in each section. High-strength grout is injected into the cavities using a guide pipe method on site. After the high-strength grout is injected into the guide pipe method, the excavation of the lower layer of earthwork can be carried out. Steps S4 to S6 above are repeated until the construction of the entire retaining structure is completed. After all the earthwork inside the foundation pit is excavated and the retaining structure is completed, the concrete cushion layer at the bottom of the foundation pit is poured immediately, thus completing all the work on the overall foundation pit retaining structure.

3. The construction method according to claim 2, characterized in that, The specific construction process of deep mixing piles in step S2 includes seven steps: The first step is to position the pile driver and perform intelligent vertical adjustment, moving the pile driver to the pile location; The second step is to activate the automatic vertical adjustment system to ensure that the verticality deviation of the guide frame is ≤0.5%. The third step is to pre-stir and sink the mixture. With all the radial telescopic stirring teeth retracted, start the stirring head and let it sink while rotating at a speed of ≤0.6m / min. The fourth step is to monitor the torque in real time and automatically reduce the sinking speed and slightly increase the rotation speed when encountering hard soil layers to prevent the drill from getting stuck. Step 5: Pulse grouting + radial telescopic mixing and lifting. After sinking to the design depth, turn on the vortex pulse grouting pump with a grouting pressure of 0.6~1.0MPa and a frequency of 1Hz. At the same time, extend all radial telescopic mixing teeth and start lifting. During the lifting process, the telescopic teeth continuously extend and retract at a frequency of 0.5~1Hz, exerting a shearing effect on the soil. The lifting speed is controlled at 0.4~0.6m / min, matched with the pulse frequency, to ensure that the soil and grout at each depth are stirred and sheared 15~20 times. Step 6: Re-mix. After raising the pile to 0.5m above the designed elevation of the pile top, stop grouting, retract the telescopic teeth, and sink and mix again. If the sensor shows that the strength of a certain section of the pile is uneven, low-frequency pulse grouting can be activated during re-mixing to supplement 10-20%. Step 7: Cleaning and relocation, lifting the drill out of the ground, flushing the pipeline and mixing head with high-pressure water, and moving it to the next pile location.

4. The construction method according to claim 2, characterized in that, In step S6, the high-pressure grouting operation of high-strength grouting material is carried out on site using the guide pipe method. Specifically, the grouting holes are first plugged with hollow rubber plugs that are 2mm to 3mm smaller in diameter than the grouting holes on the truss-ribbed composite plate. The diameter of the hole in the middle of the hollow rubber plug is also 1mm to 2mm smaller than the grouting guide pipe. Then, the grouting guide pipe is inserted into the hole in the middle of the hollow rubber plug and extends into the cavity between the truss-ribbed composite plate and the soil layer outside the foundation pit. The high-pressure grouting machine is connected to the grouting guide pipe to carry out the high-pressure grouting. After the grout overflows from the grouting hole at the top of the truss-ribbed composite plate, the overflow hole is plugged with a sealing rubber plug. The high-pressure grouting is maintained and the pressure is stabilized for 2 minutes. Finally, the guide pipe is pulled out and immediately sealed with a sealing rubber plug that is 2mm smaller in diameter than the hole in the middle of the hollow rubber plug. Curing is then strengthened.

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