A layered and segmented composite prefabricated foundation pit retaining structure and its construction method

Abstract

This invention specifically discloses a layered and segmented composite prefabricated foundation pit retaining structure and construction method. The overall technical solution follows the design concept of "dual retaining of deep mixing piles and cement-soil retaining walls, layered and segmented assembly of truss rib composite plates, anchor bolt groups traversing the deep mixing piles and anchoring into the cement-soil retaining wall, anchor bolt holes also serving as high-pressure grouting holes, anchor bolt sleeves guiding the anchor bolts into the soil and assisting in the positioning of the truss rib composite plates, and high-pressure grouting with high-strength grouting material, thus forming a pile-wall-slab-anchor bolt composite foundation pit retaining structure". This constructs a technical system in which the cement-soil retaining wall stabilizes the outer soil layer of the foundation pit, the deep mixing piles support the load, the truss rib composite plates are grouted and formed for retaining, and the anchor bolts are installed to strengthen the overall synergistic bearing capacity, stably adapting to the support needs of complex geological environments and deep foundation pits.

Description

Technical Field

[0001] This invention relates to the field of foundation pit support technology, specifically to a layered and segmented composite prefabricated foundation pit support structure and its construction method. Background Technology

[0002] In the field of foundation pit engineering, deep mixing piles have long been the core technical means for foundation pit retaining construction due to their outstanding advantages such as good economy, fast construction speed, no vibration and no soil displacement, and are widely used in various foundation projects.

[0003] However, with the continuous advancement of urban construction, foundation pit engineering is gradually facing complex working conditions such as deep excavation, complex geological conditions (such as sand, silt, and muddy soil) and proximity to sensitive buildings (structures). The limitations of the traditional deep mixing pile retaining system are becoming increasingly prominent, making it difficult to meet the actual engineering needs.

[0004] Single deep mixing pile structures have low strength and stiffness, and weak resistance to lateral displacement. Under the lateral earth and water pressure of deep foundation pits, they are prone to deformation, cracking, and even instability. At the same time, due to factors such as fluctuations in geological conditions and the precision of construction process control, the cement-soil mixing effect of deep mixing piles has large dispersion and poor quality uniformity, which can easily form weak points in local areas, significantly reducing the overall reliability of the retaining structure.

[0005] In terms of water-stopping performance, the water-stopping effect of traditional deep mixing piles depends on the quality of pile body overlap and lacks a systematic sealed water-stopping design. In water-rich strata, there is a high risk of water leakage. Moreover, the soil retaining and water-stopping functions are not coordinated enough, making it difficult to meet both needs.

[0006] In addition, in traditional construction methods, the processes of anchor installation, component hoisting, and earthwork excavation are loosely connected and lack efficient collaborative design. The accuracy of anchor insertion angle control is low, the hoisting and positioning of retaining components is difficult, and the construction process lacks a scientific layered and segmented cyclical mechanism, resulting in excessively long exposure time of the foundation pit. This not only reduces construction efficiency but also significantly increases the risk of foundation pit instability and cannot meet the construction requirements under complex geological conditions. Summary of the Invention

[0007] The purpose of this invention is to provide a layered and segmented composite prefabricated foundation pit retaining structure and construction method to solve the problems raised in the background art.

[0008] To achieve the above objectives, the present invention provides the following technical solution: a layered and segmented composite prefabricated foundation pit retaining structure, wherein the foundation pit is surrounded by layers and segments, with cement-soil retaining walls, deep mixing piles, and truss rib composite slabs arranged sequentially from the outside to the inside; anchor holes are pre-drilled in the truss rib composite slabs, and anchor sleeves are inserted into the anchor holes, which are driven into the deep mixing piles and cement-soil retaining walls, and anchors are installed inside the anchor sleeves, which are driven into the soil layer outside the foundation pit and fixed to the surface of the truss rib composite slabs by anchors; high-strength grouting material is placed in the cavity between the truss rib composite slabs and the deep mixing piles, and the high-strength grouting material is injected into the retaining structure through the anchor holes in a layered and segmented high-pressure grouting method.

[0009] Preferably, the truss reinforcement of the truss rib composite slab is designed using triangular steel bars and steel ribs, wherein the diameter of the triangular steel bars is not less than 6mm, the cross-sectional size of the steel ribs is not less than 30mm×10mm, and the length is the length of the truss rib composite slab plus 300mm at each end. The upper and lower truss rib composite slabs of the foundation pit retaining structure are assembled in sections and layers, and the truss reinforcement at corresponding positions is designed to be staggered by 80mm.

[0010] Preferably, the truss rib composite slab has multiple pre-reserved anchor holes inside. The distance between the upper and lower rows of anchor holes on each truss rib composite slab and its edge is not less than 250mm, and the spacing between adjacent anchor holes does not exceed 450mm. The diameter of each anchor hole is 80mm, and the outer diameter of the anchor sleeve is 5mm to 8mm smaller than the diameter of the anchor hole. After passing through the anchor hole, the anchor sleeve passes through the deep mixing pile and cement-soil retaining wall according to the anchor insertion angle. The cutting length of the anchor sleeve is not less than 500mm.

[0011] Preferably, the width of the cement-soil retaining wall is not less than 2000mm, and the cement-soil retaining wall is constructed using the SMW method and a diagonal anchor bolt scheme at the top of the retaining wall.

[0012] This invention also discloses a construction method for a layered and segmented composite prefabricated foundation pit retaining structure, comprising the following steps: Step S1: Measurement and layout, and site preparation; Step S2: Construction of cement-soil retaining wall; Step S3: Construction of deep mixing piles; Step S4: Excavation of the first layer of earthwork, positioning and installation of anchor bolt sleeves; Step S5: Hoisting and temporary fixing of the first-floor truss rib composite slab; Step S6: High-pressure grouting and anchor bolt installation; Step S7: Cycle construction to the bottom of the foundation pit, monitoring and information-based construction.

[0013] This invention also discloses a construction method for a layered and segmented composite prefabricated foundation pit retaining structure, including the following specific steps: Step S1: Measurement and layout and site preparation: Using equipment such as total stations and GPS, the perimeter of the foundation pit, axis control piles, and elevation benchmarks were accurately measured and marked. The specific locations of the cement-soil retaining wall, deep mixing piles, and truss rib composite slabs were marked according to the design drawings. Then, the site was leveled, hardened, and a drainage system was set up to create conditions for the entry and operation of large machinery. Process acceptance and recording were carried out. Step S2: Construction of cement-soil retaining wall: The cement-soil retaining wall is constructed first as an external water-stopping and preliminary retaining structure. A dual-axis deep mixing pile machine is used for construction according to the designed wall width and depth, with a wall width of not less than 2000mm and a depth penetrating into the relatively impermeable layer by not less than 1.5m. The cement admixture ratio is determined based on the geological survey report and tests. On-site construction adopts the "four spraying and four mixing" process to ensure the uniformity and continuity of the cement-soil retaining wall construction. During construction, the verticality of the pile driver, the lifting and sinking speed, and the grout flow rate are strictly controlled, and no less than 3 sets of test blocks are left to test the wall strength. The retaining wall construction is carried out continuously to reduce cold joints. If necessary, the joints are treated with splicing or reinforced pile technology. After the cement-soil retaining wall is completed, the top surface of the soil layer within the cement-soil retaining wall area is immediately hardened with fine stone concrete to prepare the working surface for subsequent deep mixing pile construction. Process acceptance and recording are carried out. Step S3: Construction of deep mixing piles: Inside the cement-soil retaining wall, deep mixing piles are constructed at designed intervals. As the main vertical load-bearing components, the pile diameter, length, and cement content are determined through calculation based on the pit depth and soil parameters. The deep mixing piles employ a "four-spray, four-stir" technique: First, sinking and stirring to the design elevation (no or minimal grouting during this process; primarily to break up and loosen the soil); second, lifting and grouting; third, sinking and grouting again to the design elevation (continuous grouting during this process); fourth, lifting and stirring to the borehole opening (usually no grouting during this process; primarily for secondary stirring to make the cement-soil more uniform), ensuring the grout nozzle remains at the pile bottom for at least 15 seconds to guarantee pile end quality. Accurate positioning is crucial during construction to ensure the piles are vertical, providing a precise positional relationship for subsequent anchor bolt penetration and truss plate installation. Step S4: First-layer excavation, anchor bolt sleeve positioning and installation: After the cement-soil retaining wall and deep mixing piles reach a certain strength, the first layer of earthwork excavation is carried out. This strength is usually 70% of the design strength. The excavation depth of each layer is determined according to the installation height of the truss rib composite plate and the anchor bolt installation elevation, generally controlled between 2.5 and 3.5 meters. Excavation should follow the principles of "layering, segmentation, symmetry, and balance," and over-excavation is strictly prohibited. The excavation surface should be flat, and the slope toe should be cleaned in a timely manner to provide a clean and safe working surface for subsequent processes. On the first excavation surface, according to the design position of the pre-reserved anchor bolt holes on the truss rib composite plate, the deep mixing piles and cement-soil retaining wall... The corresponding positions are accurately laid out; using an anchor drilling rig, holes are drilled to the predetermined depth at the designed angle, which is usually 10°~30°. The predetermined depth is to penetrate the cement-soil retaining wall and enter the external stable soil layer by a certain length; after drilling, the prefabricated anchor sleeves are immediately inserted; the sleeves are made of high-strength steel pipes with an outer diameter slightly smaller than the anchor hole and a length of not less than 500mm; during insertion, it is necessary to ensure that the sleeves are in close contact with the hole wall, the angle is accurate, and the height of the exposed ends is consistent, so as to facilitate the subsequent hoisting and positioning of the truss rib composite plate; after the anchor sleeves are installed, the ports can be temporarily sealed to prevent foreign objects from blocking them; Step S5: Hoisting and temporary fixing of the first-floor truss rib composite slab: After the prefabricated truss rib composite slabs from the prefabricated PC factory arrive at the site, they are first inspected to check whether their dimensions, reserved hole positions, and truss reinforcement specifications meet the design requirements. A truck crane or tower crane is then used for hoisting. Before hoisting, lubricant is applied to the exposed parts of the pre-installed anchor bolt sleeves or guide sleeves are fitted. During hoisting, the anchor bolt holes on the truss rib composite slab are aligned with the corresponding pre-installed anchor bolt sleeves, and the sleeves are slowly inserted, allowing them to pass smoothly through the corresponding anchor bolt holes. This fully utilizes the auxiliary hoisting function of the anchor bolt sleeves, namely, using the exposed parts of the anchor bolt sleeves and the self-weight of the truss rib composite slabs, supplemented by simple diagonal braces or adjustment devices, to achieve the truss... Initial positioning and temporary stabilization of the ribbed composite slab; after positioning, check the verticality, flatness and joint condition of the truss ribbed composite slab with adjacent slabs, and make timely fine adjustments. The truss reinforcement of the upper and lower slabs should be staggered by 80mm as designed to ensure reliable lap joints; the width of all joints between adjacent truss ribbed composite slabs in layers and sections is 8mm~10mm. To avoid grout leakage during subsequent grouting, a T-shaped steel cut plate is installed in the joint. The cross-sectional dimensions of the T-shaped steel cut plate are not less than 100mm×300mm×6mm. The web of the T-shaped steel cut plate is inserted into the slab joint and cuts into the interior of the enclosure structure. Its flange plate seals the joint between adjacent truss ribbed composite slabs. Step S6: High-pressure grouting and anchor bolt installation: Grouting with high-strength grout is a crucial step in forming the composite retaining structure. The grouting process uses high-strength, non-shrink grout and is carried out using a high-pressure grouting pump. First, the grouting pipe is inserted near the bottom of the hole through the anchor sleeve, ensuring a tight connection. Then, the high-pressure grouting pump is started, injecting the grout into the cavity enclosed by the truss rib composite slab, deep mixing piles, and cement-soil retaining wall. The grouting pressure is controlled according to the geological conditions, with an initial pressure of approximately 0.5~1.0 MPa and a final pressure of approximately 1.5~2.0 MPa. The grouting volume is determined by the amount of grout overflowing from the pre-set anchor sleeve or pre-set vent hole, ensuring the cavity is properly sealed. The filling is compacted; during the grouting process or before the grout initially sets, the prefabricated anchor rods are passed through the casing and pushed to the design depth using a drilling rig or jacking equipment. The anchoring section at the end of the anchor rod is located in the stable soil layer within the cement-soil retaining wall; secondary grouting and anchor sealing measures are adopted. After the anchor rods are installed, secondary pressure grouting is performed through the grouting pipe reserved in the anchor rod body to fill any possible gaps and enhance the anchoring force; after the grout reaches a certain strength, the anchor is installed on the surface of the truss rib composite plate, the design pre-tightening force is applied to the anchor rod and it is locked, and finally the anchor head and plate surface are treated with anti-corrosion and sealing. Step S7: Cycle construction to the bottom of the foundation pit, monitoring and information-based construction: Once the first-floor retaining structure reaches its designed strength, the next floor of earthwork excavation can begin. The technical process of steps S4 to S6 is repeated in this cycle until the designed bottom elevation of the foundation pit is reached. Throughout the construction process, a BeiDou / GNSS real-time dynamic monitoring system is used for full-process monitoring and early warning. Monitoring includes: surface settlement and horizontal displacement around the foundation pit, horizontal displacement of deep soil, internal forces of the support structure, anchor bolt tension, groundwater level, and settlement and tilt of adjacent buildings. Real-time feedback of monitoring data guides the construction schedule, adjusts process parameters, and enables information-based dynamic design and construction, ensuring construction safety. Finally, project acceptance and data archiving are completed.

[0014] Compared with the prior art, the beneficial effects of this invention are as follows: 1. This invention adopts a triple retaining system of "cement-soil retaining wall + deep mixing pile + truss rib composite plate", combined with anchor bolt anchoring and high-strength grouting material filling, which greatly improves the strength, rigidity and lateral displacement resistance of the retaining structure, and is suitable for complex geology and deep foundation pits. 2. The cement-soil retaining wall and deep mixing pile of the present invention both adopt the "four spraying and four mixing" process, and the truss rib composite plate is prefabricated in the factory to ensure the uniformity of construction quality and the precision of components, and reduce on-site quality fluctuations; 3. The cement-soil retaining wall of the present invention is embedded in a relatively impermeable layer of ≥1.5m, and with the joint sealing and high-pressure grouting, a closed water-stopping system is formed to eliminate the hidden danger of water leakage in water-rich strata. 4. The anchor sleeve of the present invention has the functions of guiding the anchor into the soil and assisting in the hoisting and positioning of the truss plate. The layered and segmented cyclic construction mode is highly efficient, shortens the exposure time of the foundation pit, and improves construction efficiency. In addition, the present invention adopts a secondary grouting and anchor locking process to enhance the anchoring force of the anchor, and with anti-corrosion sealing treatment, it extends the durability of the retaining structure. Attached Figure Description

[0015] Figure 1 This is a schematic diagram of the foundation pit retaining structure of the present invention; Figure 2 This is a schematic diagram of the truss rib composite plate of the present invention; Figure 3 This is a process flow diagram of the construction method of the present invention. Detailed Implementation

[0016] This invention discloses a layered and segmented composite prefabricated foundation pit retaining structure, mainly comprising a truss rib composite slab 3, deep mixing piles 2, cement-soil retaining wall 1, anchor sleeves 4, anchors 5, and anchorages 6. For details, please refer to the attached document. Figure 1 : The foundation pit is surrounded by a layered and segmented structure consisting of truss-rib composite slabs 3, deep mixing piles 2, and cement-soil retaining walls 1. The deep mixing piles 2 are located inside the cement-soil retaining walls 1, and the truss-rib composite slabs 3 are located inside the deep mixing piles 2. That is, from the inside of the foundation pit to the outside, the structure consists of truss-rib composite slabs 3, deep mixing piles 2, and cement-soil retaining walls 1. Anchor bolt holes 5 are pre-drilled in the truss-rib composite slabs 3. Anchor bolt sleeves 4 pass through these holes and are driven into the deep mixing piles 2 and the cement-soil retaining walls 1. Anchor bolts 5 pass through the anchor bolt sleeves 4 and are driven into the soil outside the foundation pit, and are fixed to the surface of the truss-rib composite slabs 3 by anchors 6. High-strength grouting material is placed in the cavity between the truss-rib composite slabs 3 and the deep mixing piles 2. The high-strength grouting material is injected in layers and segments through the anchor bolt holes using a high-pressure grouting method to complete the formation of the retaining structure.

[0017] The overall technical solution of this invention follows the design concept of "double enclosure of deep mixing piles 2 and cement-soil retaining wall 1, layered and segmented assembly of truss rib composite plate 3, anchor bolts 5 group transversely through deep mixing piles 2 and anchored into the interior of cement-soil retaining wall 1, anchor bolt 5 holes also serving as high-pressure grouting holes, anchor bolt sleeves 4 guiding the anchor bolts 5 into the soil and assisting the truss rib composite plate 3 in positioning, high-strength grouting material high-pressure grouting, thereby forming a composite foundation pit enclosure structure of pile-wall-slab-anchor bolt 5".

[0018] See Figure 2The truss reinforcement of the truss rib composite plate 3 adopts triangular steel bars 31 and steel ribs 32 as truss reinforcement design. The diameter of the triangular steel bars is not less than 6mm, and the cross-sectional size of the steel ribs 32 is not less than 30mm×10mm. The length is the length of the truss rib composite plate 3 plus 300mm at each end. The upper and lower truss rib composite plates 3 of the foundation pit retaining structure are assembled in sections and layers. The truss reinforcement at the corresponding positions is staggered by 80mm to ensure that the upper and lower adjacent truss rib composite plates 3 can form a sufficient lap length through the staggered connection of the steel ribs.

[0019] During the prefabrication process in the PC factory, the truss rib composite slab 3 has multiple anchor bolt holes 5 pre-drilled inside. The distance between the upper and lower rows of anchor bolt holes 5 on each truss rib composite slab 3 and its edge is not less than 250mm, and the spacing between adjacent anchor bolt holes 5 does not exceed 450mm. In order to better guide the angle of the anchor bolts 5 into the soil, assist in the temporary fixing when assembling each truss rib composite slab 3, and meet the requirements of high-pressure grouting technology, the diameter of each anchor bolt hole 5 is 80mm. The outer diameter of the anchor bolt sleeve 4 is 5mm~8mm smaller than the diameter of the anchor bolt hole 5. After passing through the anchor bolt hole 5, the anchor bolt sleeve 4 passes through the deep mixing pile 2 and the cement-soil retaining wall 1 according to the angle of the anchor bolt 5 into the soil. The cutting length of the anchor bolt sleeve 4 is not less than 500mm. At the same time, when the truss rib composite slab 3 is hoisted into place, the anchor bolt sleeve 4 already in the soil can be used as a temporary support design for the truss rib composite slab 3, and also serves as a grouting hole design during the construction of high-strength grouting material.

[0020] The width of the cement-soil retaining wall 1 should not be less than 2000mm. In complex working conditions such as loose soil, long-term vibration or large vibration, and sandy soil, silty soil, and muddy soil with high water content, the cement-soil retaining wall 1 can adopt the SMW construction method + the technical solution of diagonal anchor rods 5 at the top of the retaining wall to better control the deformation of the top of the foundation pit.

[0021] For the construction method of the above-mentioned layered and segmented composite prefabricated foundation pit retaining structure, please refer to [reference needed]. Figure 3 The construction process is as follows: Step S1: Measurement and layout, and site preparation; Step S2: Construction of cement-soil retaining wall; Step S3: Construction of deep mixing piles; Step S4: Excavation of the first layer of earthwork, positioning and installation of anchor bolt sleeves; Step S5: Hoisting and temporary fixing of the first-floor truss rib composite slab; Step S6: High-pressure grouting and anchor bolt installation; Step S7: Cycle construction to the bottom of the foundation pit, monitoring and information-based construction.

[0022] The specific plan for the above construction method is as follows: Step S1: Measurement and layout and site preparation: According to the technical plan, total stations, GPS and other equipment were used to accurately measure and mark the perimeter of the foundation pit, axis control piles and elevation benchmarks. According to the design drawings, the specific locations of the cement-soil retaining wall, deep mixing piles and truss rib composite slabs were marked. Then, the site was leveled, hardened and a drainage system was set up to create conditions for the entry and operation of large machinery. Process acceptance and recording were carried out.

[0023] Step S2: Construction of cement-soil retaining wall: According to the technical plan, the cement-soil retaining wall is constructed first as the outer waterproofing and preliminary retaining structure. A twin-axis deep mixing pile machine is used, and construction is carried out according to the designed wall width (not less than 2000mm) and depth (entering the relatively impermeable layer by not less than 1.5m). The cement admixture ratio is determined based on the geological survey report and tests. On-site construction employs a "four-spray, four-mix" process to ensure the uniformity and continuity of the cement-soil retaining wall construction. During construction, the verticality of the pile driver, the lifting and sinking speed, and the grout flow rate are strictly controlled, and no less than three sets of test blocks are retained to test the wall strength. The retaining wall construction should be continuous to reduce cold joints; if necessary, splicing or reinforced pile techniques are used to treat the joints. After the cement-soil retaining wall construction is completed, the top surface of the soil layer within the cement-soil retaining wall area is immediately hardened with fine aggregate concrete to prepare the working surface for subsequent deep mixing pile construction. Process acceptance and recording are carried out.

[0024] Step S3: Construction of deep mixing piles: According to the technical plan, deep mixing piles are constructed on the inner side of the cement-soil retaining wall at designed intervals. As the main vertical load-bearing components, the pile diameter, pile length, and cement content are determined through calculation based on the pit depth and soil parameters. The deep mixing piles employ a "four-spray, four-mix" technology: First, during the initial sinking and mixing (pre-mixing), the piles are drilled to the design elevation. No grouting or only a small amount of grout is sprayed during this process; the main purpose is to break up and loosen the soil. Second, during the initial lifting and grouting mixing, grouting continues during the lifting process. Third, during the second sinking and grouting mixing, the piles are lowered again to the design elevation, with continuous grouting during this process. Finally, during the second lifting and mixing (re-mixing), the piles are lifted to the borehole opening. Grouting is usually not sprayed during this process; the main purpose is secondary mixing to make the cement-soil more uniform. It is ensured that the grout nozzle remains at the bottom of the pile for at least 15 seconds to guarantee the quality of the pile tip. Accurate positioning is crucial during construction to ensure the piles are vertical, providing accurate positional relationships for subsequent anchor bolt sleeve penetration and truss plate installation.

[0025] Step S4: First-layer excavation, anchor bolt sleeve positioning and installation: According to the technical plan, after the cement-soil retaining wall and deep mixing piles reach a certain strength (usually 70% of the design strength), the first layer of earthwork excavation will be carried out. The excavation depth of each layer will be determined based on the installation height of the truss rib composite slab and the anchor bolt installation elevation, generally controlled between 2.5 and 3.5 meters. Excavation must adhere to the principles of "layering, segmentation, symmetry, and balance," and over-excavation is strictly prohibited. The excavation surface should be flat, and the slope toe should be cleaned promptly to provide a clean and safe working surface for subsequent processes. On the first excavation surface, based on the design positions of the pre-reserved anchor bolt holes on the truss rib composite slab, precise layout will be carried out at the corresponding positions of the deep mixing piles and cement-soil retaining wall. An anchor bolt drilling rig will be used to drill holes to the predetermined depth (penetrating the cement-soil retaining wall to a certain length into the external stable soil layer) at the design angle (usually 10° to 30°). After drilling, prefabricated anchor bolt sleeves will be immediately inserted. The casing is made of high-strength steel pipe, with an outer diameter slightly smaller than the anchor bolt hole (e.g., 75mm for an 80mm hole), and a length of not less than 500mm. During installation, ensure the casing fits tightly against the hole wall, the angle is accurate, and the exposed ends are at a consistent height to facilitate the subsequent hoisting and positioning of the truss rib composite plate. After the anchor bolt casing is installed, the port can be temporarily sealed to prevent blockage by foreign objects.

[0026] Step S5: Hoisting and temporary fixing of the first-floor truss rib composite slab: According to the technical plan, after the prefabricated truss-rib composite slabs from the prefabricated PC factory arrive at the site, they will first undergo acceptance testing to check whether their dimensions, reserved hole positions, and truss reinforcement specifications meet the design requirements. A truck crane or tower crane will be used for hoisting. Before hoisting, lubricant will be applied to the exposed parts of the pre-installed anchor bolt sleeves or guide sleeves will be fitted. During hoisting, the anchor bolt holes on the truss-rib composite slab will be aligned with the corresponding pre-installed anchor bolt sleeves, and the sleeves will be slowly inserted, allowing them to pass smoothly through the corresponding anchor bolt holes. The auxiliary hoisting function of the anchor bolt sleeves will be fully utilized; that is, the exposed parts of the anchor bolt sleeves and the self-weight of the truss-rib composite slab, supplemented by simple diagonal braces or adjustment devices, will achieve the initial positioning and temporary stability of the truss-rib composite slab. After positioning, the verticality, flatness, and joint condition with adjacent slabs of the truss-rib composite slab will be checked, and minor adjustments will be made as needed. The truss reinforcement of the upper and lower slabs must be staggered by 80mm as designed to ensure reliable lap joints. The width of all joints between adjacent truss rib composite slabs in the layered and segmented sections is 8mm~10mm. To avoid grout leakage during subsequent grouting, the technology adopts T-shaped steel cut plates inside the joints. The cross-sectional dimensions of the T-shaped steel cut plates are not less than 100mm×300mm×6mm. The web of the T-shaped steel cut plate is inserted into the plate joint and cuts into the interior of the enclosure structure, and its flange plates seal the joints between adjacent truss rib composite slabs.

[0027] Step S6: High-pressure grouting and anchor bolt installation: According to the technical plan, high-strength grouting is a key step in forming the composite retaining structure. The grouting process uses high-strength, non-shrink grout, delivered via a high-pressure grouting pump. First, the grouting pipe is inserted through the anchor sleeve to near the bottom of the hole, ensuring a tight connection. The high-pressure grouting pump is then started, injecting the grout into the cavity enclosed by the truss rib composite slab, deep mixing piles, and cement-soil retaining wall. The grouting pressure is controlled according to the geological conditions, with an initial pressure of approximately 0.5~1.0 MPa and a final pressure of approximately 1.5~2.0 MPa. The grouting volume is determined by the overflow of grout from the pre-set anchor sleeve or pre-set vent hole, ensuring a dense filling of the cavity. During grouting or before the grout initially sets, prefabricated anchor rods (usually steel strands or precision-rolled threaded steel bars) are passed through the sleeve and pushed to the designed depth using a drilling rig or jacking equipment. The anchoring section at the end of the anchor rod is located in the stable soil layer within the cement-soil retaining wall. A secondary grouting and anchor sealing method is adopted. After the anchor bolts are installed, secondary pressure grouting is performed through the grouting pipes reserved in the anchor bolt body to fill any possible gaps and enhance the anchoring force. After the grout reaches a certain strength, anchorages (such as anchor plates and nuts) are installed on the surface of the truss rib composite plate, the designed preload is applied to the anchor bolts and locked, and finally the anchor head and plate surface are treated with anti-corrosion and sealing treatment.

[0028] Step S7: Cycle construction to the bottom of the foundation pit, monitoring and information-based construction: According to the technical plan, once the first-layer retaining structure (slab-grout-anchor) reaches its design strength, the next layer of earthwork excavation can begin. Steps S4 to S6 are repeated in this cycle until the designed bottom elevation of the foundation pit is reached. Throughout the construction process, a BeiDou / GNSS real-time dynamic monitoring system is used for full-process monitoring and early warning. Monitoring includes: surface settlement and horizontal displacement around the foundation pit, horizontal displacement of deep soil (inclinometer), internal forces of the support structure, anchor bolt tension, groundwater level, and settlement and tilt of adjacent buildings. Monitoring data is fed back in real time to guide the construction schedule, adjust process parameters, achieve information-based dynamic design and construction, ensure construction safety, and finally, ensure project acceptance and document archiving.

[0029] In summary, this invention constructs a technical system that stabilizes the outer soil layer of the foundation pit with cement-soil retaining walls, supports the load with deep mixing piles, forms grouted truss rib composite slab enclosures, and reinforces the overall synergistic load-bearing capacity with anchor bolt installation. This system is stable and adaptable to the support needs of complex geological environments and deep foundation pits. The standardized process effectively ensures the uniformity of construction quality and the precision of components, reducing quality fluctuations during on-site construction.

[0030] 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 layered and segmented composite prefabricated foundation pit retaining structure, characterized in that: The foundation pit is surrounded by layers and sections, with cement-soil retaining walls, deep mixing piles and truss rib composite slabs arranged sequentially from the outside to the inside. The width of the cement-soil retaining walls is not less than 2000mm. As the main soil retaining and water-stopping structure, the cement-soil retaining walls and deep mixing piles form a double retaining structure. Anchor bolt holes are pre-drilled in the truss rib composite slab, and anchor bolt sleeves are inserted into the anchor bolt holes. The anchor bolt sleeves are driven into the deep mixing piles and cement-soil retaining walls. Anchor bolts are installed inside the anchor bolt sleeves and are driven into the soil layer outside the foundation pit and fixed to the surface of the truss rib composite slab by anchors. High-strength grouting material is installed in the cavity between the truss rib composite slab and the deep mixing piles. The high-strength grouting material is injected into the retaining structure in layers and sections through the anchor bolt holes using a high-pressure grouting method.

2. The foundation pit retaining structure according to claim 1, characterized in that: The truss reinforcement of the truss rib composite slab is designed with triangular steel bars and steel ribs. The diameter of the triangular steel bars is not less than 6mm, the cross-sectional size of the steel ribs is not less than 30mm×10mm, and the length is the length of the truss rib composite slab plus 300mm at each end. The upper and lower truss rib composite slabs of the foundation pit retaining structure are assembled in sections and layers, and the truss reinforcement at corresponding positions is designed to be staggered by 80mm.

3. The foundation pit retaining structure according to claim 1, characterized in that: The truss rib composite slab has multiple pre-drilled anchor holes inside. The distance between the upper and lower rows of anchor holes on each truss rib composite slab and its edge is not less than 250mm, and the spacing between adjacent anchor holes does not exceed 450mm. The diameter of each anchor hole is 80mm, and the outer diameter of the anchor sleeve is 5mm to 8mm smaller than the diameter of the anchor hole. After passing through the anchor hole, the anchor sleeve passes through the deep mixing pile and cement-soil retaining wall according to the anchor insertion angle. The cutting length of the anchor sleeve is not less than 500mm.

4. The foundation pit retaining structure according to claim 1, characterized in that: The cement-soil retaining wall is constructed using the SMW method and a diagonal anchor bolt installation scheme at the top of the retaining wall.

5. A construction method for a layered and segmented composite prefabricated foundation pit retaining structure, characterized in that, Includes the following steps: Step S1: Measurement and layout, and site preparation; Step S2: Construction of cement-soil retaining wall; Step S3: Construction of deep mixing piles; Step S4: Excavation of the first layer of earthwork, positioning and installation of anchor bolt sleeves; Step S5: Hoisting and temporary fixing of the first-floor truss rib composite slab; Step S6: High-pressure grouting and anchor bolt installation; Step S7: Cycle construction to the bottom of the foundation pit, monitoring and information-based construction.

6. A construction method for a layered and segmented composite prefabricated foundation pit retaining structure, characterized in that, The specific steps include the following: Step S1: Measurement and layout and site preparation: Using equipment such as total stations and GPS, the perimeter of the foundation pit, axis control piles, and elevation benchmarks were accurately measured and marked. The specific locations of the cement-soil retaining wall, deep mixing piles, and truss rib composite slabs were marked according to the design drawings. Then, the site was leveled, hardened, and a drainage system was set up to create conditions for the entry and operation of large machinery. Process acceptance and recording were carried out. Step S2: Construction of cement-soil retaining wall: The cement-soil retaining wall is constructed first as an external water-stopping and preliminary retaining structure. A dual-axis deep mixing pile machine is used for construction according to the designed wall width and depth, with a wall width of not less than 2000mm and a depth penetrating into the relatively impermeable layer by not less than 1.5m. The cement admixture ratio is determined based on the geological survey report and tests. On-site construction adopts the "four spraying and four mixing" process to ensure the uniformity and continuity of the cement-soil retaining wall construction. During construction, the verticality of the pile driver, the lifting and sinking speed, and the grout flow rate are strictly controlled, and no less than 3 sets of test blocks are left to test the wall strength. The retaining wall construction is carried out continuously to reduce cold joints. If necessary, the joints are treated with splicing or reinforced pile technology. After the cement-soil retaining wall is completed, the top surface of the soil layer within the cement-soil retaining wall area is immediately hardened with fine stone concrete to prepare the working surface for subsequent deep mixing pile construction. Process acceptance and recording are carried out. Step S3: Construction of deep mixing piles: Inside the cement-soil retaining wall, deep mixing piles are constructed at the designed intervals. As the main vertical load-bearing components, the pile diameter, pile length, and cement content are determined by calculation based on the pit depth and soil parameters. The deep mixing piles adopt the "four-spraying and four-stirring" technology, namely, the first sinking and stirring, drilling to the design elevation, during which no grout is sprayed or only a small amount of grout is sprayed, the main purpose of which is to break up and loosen the soil; the first lifting and grouting stirring, with grouting continuously sprayed during the lifting process. The second sinking and grouting process involves sinking the pile again to the design elevation, with continuous grouting during the process. The second lifting and mixing process involves lifting the pile to the borehole opening. Grouting is usually not performed during this process; the main purpose is secondary mixing to make the cement and soil more uniform. It is essential to ensure that the grout nozzle remains at the bottom of the pile for at least 15 seconds to guarantee the quality of the pile end. During construction, accurate positioning is required to ensure that the pile is vertical, providing an accurate positional relationship for subsequent anchor sleeve penetration and truss plate installation. Step S4: First-layer excavation, anchor bolt sleeve positioning and installation: After the cement-soil retaining wall and deep mixing piles reach a certain strength, the first layer of earthwork excavation is carried out. This strength is usually 70% of the design strength. The excavation depth of each layer of earthwork is determined according to the setting height of the truss rib composite plate and the setting elevation of the anchor bolts, and is generally controlled between 2.5 and 3.5 m. The excavation should follow the principles of "layering, segmentation, symmetry, and balance", and over-excavation is strictly prohibited. The excavation surface should be flat, and the slope toe should be cleaned in time to provide a clean and safe working surface for subsequent processes. On the first excavation face, based on the design positions of the pre-reserved anchor bolt holes on the truss rib composite slab, precise layout is carried out at the corresponding positions of the deep mixing piles and cement-soil retaining walls. An anchor bolt drilling rig is used to drill to the predetermined depth at the designed angle, typically 10°~30°. The predetermined depth is the length of the anchor bolt that penetrates the cement-soil retaining wall into the external stable soil layer. After drilling, prefabricated anchor bolt sleeves are immediately inserted. The sleeves are made of high-strength steel pipes with an outer diameter slightly smaller than the anchor bolt hole and a length of not less than 500mm. During insertion, it is necessary to ensure that the sleeve is tightly fitted to the hole wall, the angle is accurate, and the height of the exposed end is consistent, so as to facilitate the subsequent hoisting and positioning of the truss rib composite slab. After the anchor bolt sleeves are installed, the ports can be temporarily sealed to prevent blockage by foreign objects. Step S5: Hoisting and temporary fixing of the first-floor truss rib composite slab: After the prefabricated truss rib composite slabs from the prefabricated PC factory arrive at the site, they are first inspected to check whether their dimensions, reserved hole positions, and truss reinforcement specifications meet the design requirements. A truck crane or tower crane is then used for hoisting. Before hoisting, lubricant is applied to the exposed parts of the pre-installed anchor bolt sleeves or guide sleeves are fitted. During hoisting, the anchor bolt holes on the truss rib composite slab are aligned with the corresponding pre-installed anchor bolt sleeves, and the sleeves are slowly inserted, allowing them to pass smoothly through the corresponding anchor bolt holes. This fully utilizes the auxiliary hoisting function of the anchor bolt sleeves, namely, using the exposed parts of the anchor bolt sleeves and the self-weight of the truss rib composite slabs, supplemented by simple diagonal braces or adjustment devices, to achieve the truss... Initial positioning and temporary stabilization of the ribbed composite slab; after positioning, check the verticality, flatness and joint condition of the truss ribbed composite slab with adjacent slabs, and make timely fine adjustments. The truss reinforcement of the upper and lower slabs should be staggered by 80mm as designed to ensure reliable lap joints; the width of all joints between adjacent truss ribbed composite slabs in layers and sections is 8mm~10mm. To avoid grout leakage during subsequent grouting, a T-shaped steel cut plate is installed in the joint. The cross-sectional dimensions of the T-shaped steel cut plate are not less than 100mm×300mm×6mm. The web of the T-shaped steel cut plate is inserted into the slab joint and cuts into the interior of the enclosure structure. Its flange plate seals the joint between adjacent truss ribbed composite slabs. Step S6: High-pressure grouting and anchor bolt installation: Grouting with high-strength grout is a crucial step in forming the composite retaining structure. The grouting process uses high-strength, non-shrink grout and is carried out using a high-pressure grouting pump. First, the grouting pipe is inserted near the bottom of the hole through the anchor sleeve, ensuring a tight connection. Then, the high-pressure grouting pump is started, injecting the grout into the cavity enclosed by the truss rib composite slab, deep mixing piles, and cement-soil retaining wall. The grouting pressure is controlled according to the geological conditions, with an initial pressure of approximately 0.5~1.0 MPa and a final pressure of approximately 1.5~2.0 MPa. The grouting volume is determined by the amount of grout overflowing from the pre-set anchor sleeve or pre-set vent hole, ensuring the cavity is properly sealed. The filling is compacted; during the grouting process or before the grout initially sets, the prefabricated anchor rods are passed through the casing and pushed to the design depth using a drilling rig or jacking equipment. The anchoring section at the end of the anchor rod is located in the stable soil layer within the cement-soil retaining wall; secondary grouting and anchor sealing measures are adopted. After the anchor rods are installed, secondary pressure grouting is performed through the grouting pipe reserved in the anchor rod body to fill any possible gaps and enhance the anchoring force; after the grout reaches a certain strength, the anchor is installed on the surface of the truss rib composite plate, the design pre-tightening force is applied to the anchor rod and it is locked, and finally the anchor head and plate surface are treated with anti-corrosion and sealing. Step S7: Cycle construction to the bottom of the foundation pit, monitoring and information-based construction: Once the first-floor retaining structure reaches the design strength, the next floor of earthwork excavation can begin. Repeat steps S4 to S6, and continue this cycle until the designed bottom elevation of the foundation pit is reached. Throughout the construction process, a BeiDou / GNSS real-time dynamic monitoring system was used to implement full-process monitoring and early warning operations. The monitoring content included: surface settlement and horizontal displacement around the foundation pit, horizontal displacement of deep soil, internal forces of the support structure, anchor bolt tension, groundwater level, settlement and tilt of adjacent buildings, etc. Real-time monitoring data feedback guides construction pace, adjusts process parameters, enables dynamic information-based design and construction, ensures construction safety, and ultimately facilitates project acceptance and document archiving.

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