Construction method of retaining structure of small bearing platform foundation pit under water-rich soft soil geological condition

Abstract

The application discloses a kind of retaining structure construction method of small bearing platform foundation pit under water-rich soft soil geological condition, belong to foundation pit retaining technical field.The retaining structure includes the continuous lap joint of SMW method pile along the circumference of foundation pit, inserts H-shaped steel, reinforced concrete crown beam and support system.During construction, drill-in construction triaxial mixing pile and insert H-shaped steel with pile spacing;At the top of pile, crown beam is constructed, H-shaped steel is tied on both sides of the steel with main reinforcement and stirrup and double support hoop is used, main reinforcement is tied on both sides of the pile without H-shaped steel and three support hoops are used;Support system is constructed according to the size of bearing platform: when bearing platform size is less than or equal to 5m, concrete support plate reinforcement and crown beam reinforcement are tied at the same time and poured integrally;When bearing platform size is greater than 5m, crown beam embedded support embedded part is welded with main reinforcement, and H-shaped steel support is welded with embedded part before excavation.The application solves the technical problems of difficult support and easy collapse of small bearing platform foundation pit under water-rich soft soil condition, and realizes the comprehensive effect of water stop, support, rapid construction, turnover and low cost.

Description

Technical Field

[0001] This invention relates to the field of foundation pit retaining structure construction technology, and in particular to a construction method for retaining structure of small foundation pit under water-rich soft soil geological conditions. Background Technology

[0002] For large-area deep foundation pits (typically with an excavation depth ≥10m and a planar dimension of tens of meters) in soft soil and water-rich areas, a support system of diaphragm walls + triaxial mixing piles + capping beams + multiple internal bracing or anchor cables is usually adopted. This solution is technically mature, has high rigidity, and good water-stopping effect, but the components are complex, the cost is high, the construction period is long, and a large construction site and working area are required.

[0003] For foundation pits commonly found in bridges and building foundations with relatively small abutment dimensions (typically ≤5m in both length and width) and limited excavation depth (above 3m but below 7m), the aforementioned large-area solutions are clearly uneconomical. Existing technologies typically employ methods such as brick formwork construction, sloping excavation, and backfilling with stabilized soil. However, in water-rich soft soil conditions, brick formwork lacks water-stopping capabilities and has weak resistance to lateral earth pressure. Sloping excavation is limited by site constraints, and soft soil is prone to collapse, resulting in high construction safety risks. Furthermore, brick formwork cannot be reused, and the backfilling period is uncontrollable.

[0004] While the SMW (triaxial mixing pile) method (with H-beams inserted into the pile) has proven successful in large-area foundation pit cutoff wall applications, existing technologies lack a standardized, integrated support system suitable for small foundation pits with cap sizes ≤5m and excavation depths of 3-7m. Specifically, existing technologies fail to: classify and standardize support methods according to cap size; integrate the cap beam and support system during construction; and include reinforcement avoidance design for cap beams crossing H-beams. This results in a lack of a comprehensive solution for small foundation pits with cap sizes ≤5m and excavation depths of 3-7m in soft, water-rich soil conditions, which simultaneously addresses cutoff, support, rapid construction, reusability, and low cost. Summary of the Invention

[0005] The purpose of this invention is to provide a construction method for the retaining structure of a small foundation pit under water-rich soft soil geological conditions, so as to solve the technical problems of difficulty in supporting small foundation pits under water-rich soft soil geological conditions, easy collapse, high construction cost and uncontrollable construction period in the prior art.

[0006] To achieve the above objectives, this invention provides a construction method for the retaining structure of a small pile cap foundation pit under water-rich soft soil geological conditions. The retaining structure includes continuously overlapping SMW (Self-Drilling Welding) piles along the circumference of the foundation pit, H-beams inserted into the SMW piles, a reinforced concrete capping beam cast on top of the SMW piles, and a support system. The construction method includes the following steps: drilling and constructing triaxial mixing piles along the circumference of the foundation pit, and inserting H-beams at intervals within the triaxial mixing piles, with the H-beams extending 500mm above the subsequently constructed capping beam; constructing a reinforced concrete capping beam on top of the triaxial mixing piles, connecting the piles into a whole; wherein, at the location where the capping beam crosses the H-beams... The main reinforcement and stirrups of the capping beam are tied to both sides of the H-beam, with double stirrups used. Where the capping beam does not cross the H-beam, the main reinforcement of the capping beam is tied to both sides of the pile, with triple stirrups used. The support system is constructed in stages according to the pile cap size: when the pile cap size is ≤5m, the concrete support slab reinforcement is tied to the capping beam reinforcement and cast integrally, forming a rigid whole between the concrete support slab and the capping beam; when the pile cap size is >5m, embedded support components are pre-embedded in the capping beam stage, with the embedded reinforcement of the support components welded to the main reinforcement of the capping beam. Before excavation, the H-beam support is welded and fixed to the embedded support components; excavation is carried out after acceptance.

[0007] Preferably, the diameter of the triaxial mixing pile is 650mm, the pile spacing is 450mm, and the construction method is die drilling; the water-cement ratio of the grout is 1.2-1.5, and the cement content of a single pile is ≥25%; the sinking speed of the triaxial mixing pile is ≤0.6m / min, and the lifting speed is ≤1.2m / min; wherein, when the triaxial mixing pile is used as a water-stop curtain, the sinking speed is ≤0.3m / min, and the lifting speed is ≤0.5m / min; the amount of grout sprayed during the sinking process accounts for 70%-80% of the entire pile, and the amount of grout sprayed during the lifting process accounts for 20%-30% of the entire pile, thereby ensuring uniform mixing of cement and soil and continuous sealing of the water-stop curtain through staged grout spraying and speed control, thereby improving the overall seepage prevention performance of the retaining structure.

[0008] Preferably, the grout mixing time is not less than 3 minutes, the grout density is detected by a grout hydrometer, and the grout delivery is continuous; the triaxial mixing pile adopts a two-stage grouting process, namely sinking grouting and lifting grouting, and after sinking to the bottom of the pile, it is stirred and lifted again to eliminate air bubbles, thereby eliminating pile defects through grout quality control and repeated stirring, ensuring the pile formation quality and water-stopping reliability of the triaxial mixing pile.

[0009] Preferably, the H-beams are inserted within 30 minutes after the completion of the triaxial mixing pile construction, with each H-beam inserted one at a time. The H-beams are allowed to sink slowly under their own weight. If mechanical assistance is used, the vibration frequency is 30-50Hz, and the H-beams sink at a uniform speed. Free fall and impact are strictly prohibited. This allows the steel to be inserted within the time window before the cement and soil set, ensuring that the H-beams and the cement and soil work together to bear the load, and creating conditions for the later removal and reuse of the steel.

[0010] Preferably, the dimensions of the capping beam are 1000mm × 600mm; where there is an H-beam, the main reinforcement and stirrups of the capping beam are tied to both sides of the H-beam, and the stirrups are double-stranded; where there is no H-beam, the main reinforcement of the capping beam is tied to both sides of the pile, and the stirrups are triple-stranded. This design avoids the conflict of reinforcements when the capping beam crosses the H-beam, ensuring the compactness of the capping beam concrete and its cooperative performance with the H-beam.

[0011] Preferably, when the foundation size is ≤5m, the thickness of the concrete support plate is 300mm and the plane size is 1.5m×1.5m. The reinforcing bars of the concrete support plate and the reinforcing bars of the cap beam are tied and poured together to form an integral whole. In the case of small foundations, the concrete support plate replaces the traditional steel support, realizes the integrated construction of the cap beam and the support, reduces the number of components, shortens the construction period and reduces the cost.

[0012] Preferably, when the foundation size is >5m, the H-beam support is made of H400×400×13×21 steel and is arranged at the four corners of the foundation pit with a support spacing of ≤2000mm. The H-beam support and the embedded support are connected by welding, thereby forming a stable rigid support system through the pre-embedded-welded integrated structure in the case of a large foundation, effectively constraining the lateral deformation of the retaining structure and controlling the displacement of the foundation pit.

[0013] Preferably, the dimensions of the embedded support are 700mm × 500mm, comprising a 15mm thick Q235B steel plate and nine 25mm diameter HRB400 grade steel bars. The steel bars are 400mm long and spaced 150mm apart. The steel plate and steel bars are connected by through-hole plug welding, and the embedded steel bars are welded to the main reinforcement of the capping beam. This dual anchoring mechanism of through-hole plug welding and main reinforcement welding ensures a reliable integral connection between the embedded support and the capping beam, providing a stable support for the H-beam support.

[0014] Preferably, the construction method comprises the following steps: base surface cleaning → positioning and layout → construction of three-axis mixing piles → insertion of H-beams → excavation of cap beams → construction of cap beams and support system → acceptance → excavation of foundation pits. This standardized construction sequence ensures close connection between each process, shortening the formation period of the retaining structure while guaranteeing construction quality.

[0015] This invention also provides a small-scale foundation pit retaining structure, comprising: SMW method piles continuously overlapping along the circumference of the foundation pit; H-beams inserted into the SMW method piles; a reinforced concrete capping beam cast on the top of the SMW method piles, wherein, at the location where the capping beam crosses the H-beam, the main reinforcement and stirrups of the capping beam are tied to both sides of the H-beam, and the stirrups adopt double-strand stirrups; at the location where the capping beam does not cross the H-beam, the main reinforcement of the capping beam is tied to both sides of the pile, and the stirrups adopt triple-strand stirrups; the H-beam extends 500mm above the capping beam; and a support system, comprising: when the foundation size is ≤5m, a concrete support slab tied simultaneously with the capping beam reinforcement and integrally cast, the concrete support slab forming a rigid whole with the capping beam; or when the foundation size is >5m, a support embedded part pre-embedded in the capping beam, and an H-beam support welded and fixed to the support embedded part.

[0016] Compared with traditional brick formwork construction and slope excavation with solidified soil backfilling, this invention replaces the traditional separate setup of brick formwork and independent water-stop curtain with a composite structure of continuous overlapping triaxial mixing piles and inserted H-beams. This transforms the retaining structure from a loose combination of multiple components into an integrated pile-steel composite water-stop support structure. The self-supporting retaining structure using SMW method piles eliminates the reliance on slope space for slope excavation, enabling vertical excavation even under site constraints. Simultaneous binding and integral casting of the concrete support slab reinforcement and the capping beam reinforcement transforms the capping beam and support from a traditional post-installed connection to a cast-in-place rigid whole, eliminating the uncertainty of post-installation nodes. The separable inserted structure of H-beams and cement-soil allows for the non-destructive removal of the steel after the retaining structure's service life, transforming the support system from a one-time consumable to a partially reusable system. Therefore, in the construction of small foundation pits under water-rich soft soil geological conditions, this invention significantly improves the overall rigidity, water-stopping reliability, and construction safety of the retaining structure through structural integration, functional integration, and coordinated construction sequence. At the same time, it shortens the formation cycle of the retaining structure, reduces construction costs, and minimizes site occupation. Attached Figure Description

[0017] Figure 1 This is a plan view of the foundation enclosure structure when the foundation size is ≤5m.

[0018] Figure 2 yes Figure 1 Section 1-1 of the enclosure structure is shown.

[0019] Figure 3 This is a plan view of the foundation enclosure structure when the foundation size is >5m.

[0020] Figure 4 This is a schematic diagram of the reinforcement structure of the cap beam of the present invention.

[0021] Figure 5 yes Figure 3 Section 2-2 of the enclosure structure is shown.

[0022] Figure 6 This is a schematic diagram of the structure of the embedded support component of the present invention. Detailed Implementation

[0023] To further illustrate the technical means and effects of the present invention in achieving its intended purpose, the following detailed description of the specific implementation methods, structures, features, and effects of the present invention, in conjunction with the accompanying drawings and preferred embodiments, is provided below.

[0024] like Figures 1 to 6 As shown, this invention provides a construction method for the retaining structure of a small foundation pit under water-rich soft soil geological conditions. The retaining structure includes three-axis mixing piles 1 continuously overlapping along the circumference of the foundation pit, H-beams 2 inserted into the three-axis mixing piles 1, a capping beam 3 cast on the top of the three-axis mixing piles 1, and a support system.

[0025] The three-axis mixing pile 1 is constructed using a casing drilling method, with a diameter of 650mm and a pile spacing of 450mm. The water-cement ratio of the grout is 1.2-1.5, and the cement content of a single pile is ≥25%. During construction, after the drilling rig is in place, the horizontal position of the chassis and the verticality of the guide frame should be checked. The deviation between the center of the drill rod and the pile position should not exceed 40mm, and the verticality deviation of the pile frame should not exceed 1 / 150. The grout mixing time should not be less than 3 minutes, and the grout density should be tested using a grout hydrometer. Grout delivery should be continuous. The three-axis mixing pile 1 adopts a two-stage grouting process: the first grouting is for sinking, with a sinking speed ≤0.6m / min and a grouting volume of approximately 70%-80% of the entire pile; the second grouting is for lifting, with a lifting speed ≤1.2m / min and a grouting volume of approximately 20%-30% of the entire pile. When the three-axis mixing pile 1 is used as a water-stop curtain, the sinking speed is ≤0.3m / min, and the lifting speed is ≤0.5m / min. After sinking to the bottom of the pile, it is stirred and lifted again to eliminate air bubbles and blind spots in the mixing process, thereby increasing the density. During construction, a designated person is responsible for recording the number of cement grout mixing tanks used, the amount of cement and admixtures, the pumping time of the grout, the speed and number of times the mixer is lifted, the amount of grout sprayed, and the mixing depth for each pile. After completing one pile, the piling machine is moved to the next pile location, and the pile foundation is drilled again with an overlap length of 250mm to ensure the continuity of the water-stop curtain and gravity retaining wall and the construction quality of the overlap.

[0026] The insertion of H-beam 2 must be completed within 30 minutes after the completion of the three-axis mixing pile 1 construction, and the maximum time shall not exceed 60 minutes. Piles cannot be inserted smoothly after the cement-soil has initially set. H-beam 2 is made of H500×300×11×18 steel and is inserted at the first pile location and subsequent drilled holes, i.e., every other pile. H-beam 2 should sink slowly under its own weight, and free fall and forceful impact are prohibited to prevent pile cracking and steel deviation. If mechanical assistance is used, the vibration frequency should be 30-50Hz, and the pile should sink at a uniform speed. High-frequency strong vibration is strictly prohibited to disturb the surrounding soil and damage the integrity of the cement-soil mixture. Vibration should be stopped immediately after sinking to the design bottom elevation, and the verticality and elevation should be re-measured. Any deviations should be promptly fine-tuned and corrected.

[0027] Excavation of the capping beam 3 can only proceed after the completion and inspection of the three-axis mixing pile 1. The capping beam 3 measures 1000mm × 600mm. During excavation, the bottom elevation of the capping beam 3 must be checked, and care must be taken to protect the H-beam 2, which extends 3500mm above the capping beam. Excavation in the vicinity should be done manually, including bottom elevation cleaning and flatness inspection. The reinforcing bars of the capping beam 3 and the supporting system should then be tied. The outer side of the main reinforcement uses six 28mm diameter HRB400 grade steel bars spaced 100mm apart, while the inner side uses four 25mm diameter HRB400 grade steel bars spaced 150mm apart. Double rows of steel bars are placed on both sides of the pile. Stirrups use 10mm diameter HRB400 grade steel bars spaced 150mm apart. For pile locations with H-beam 2, the main reinforcement and stirrups of the capping beam 3 are tied to both sides of the H-beam 2, with double-strand stirrups. For pile locations without H-beam 2, the main reinforcement is tied to both sides of the pile, with triple-strand stirrups. After the reinforcing bars are tied and inspected and approved, concrete is poured.

[0028] Construction support system is categorized according to the size of the foundation: Variation Example 1 like Figure 1 and Figure 2 As shown, when the foundation size is ≤5m, an integrated construction scheme of concrete support slab 4 and capping beam 3 is adopted. The concrete support slab 4 is 300mm thick and has a planar dimension of 1.5m × 1.5m. During the reinforcement binding stage of capping beam 3, the reinforcement of concrete support slab 4 and capping beam 3 are bound together and cast as a whole, so that concrete support slab 4 and capping beam 3 form a rigid whole. This scheme is suitable for small foundation pits with an excavation depth of more than 3m and less than 6.82m, and a foundation size of less than 5m. In this variation, by casting concrete support slab 4 and capping beam 3 as a whole, the installation process of traditional steel support is eliminated, the number of components is reduced, and the formation cycle of the retaining structure is shortened. Thus, while ensuring the lateral stability of the foundation pit, rapid construction and low-cost control of the retaining structure of small foundation pits are achieved.

[0029] Variation Example 2 like Figure 3 , Figure 5 and Figure 6As shown, when the foundation size is >5m, a welding scheme of H-beam support 5 and support embedded part 6 is adopted. The support embedded part 6 is installed after the reinforcement of the cap beam 3 is tied. The support embedded part 6 is 700mm×500mm in size, including a 15mm thick Q235B steel plate and 9 HRB400 grade steel bars with a diameter of 25mm, a length of 400mm, and a spacing of 150mm. The steel plate and steel bars are connected by through-hole plug welding, and the embedded steel bars are welded to the main reinforcement of the cap beam 3 to ensure the stability of the embedded part. The spacing of the four corner support embedded parts 6 is ≤2000mm. The H-beam support 5 uses H400×400×13×21 steel and is arranged at the four corners of the foundation pit. Before the foundation pit is excavated, the H-beam support 5 is welded to the support embedded part 6. The foundation pit can only be excavated after the H-beam support 5 is installed and accepted. This scheme is applicable to small foundation pits with an excavation depth of 3m or more but less than 6.82m and a foundation size of 5m or more. In this modified example, by welding the embedded support 6 to the main reinforcement of the cap beam 3 and welding the H-beam support 5 to the embedded support 6, an integrated rigid support system of embedded and welded components is formed. This effectively constrains the lateral deformation of the retaining structure and controls the displacement of the foundation pit, ensuring the excavation safety of large-size foundation pits.

[0030] Variation Example 3 Under water-rich soft soil geological conditions, the pile length of the triaxial mixing pile 1 is standardized according to the excavation depth of the foundation pit. When the excavation depth is 3m ≤ H < 4m, the depth of the triaxial mixing pile 1 is L = 9m; when the excavation depth is 4m ≤ H < 5m, the depth of the triaxial mixing pile 1 is L = 11m; when the excavation depth is 5m ≤ H < 6m, the depth of the triaxial mixing pile 1 is L = 13m; and when the excavation depth is 6m ≤ H < 7m, the depth of the triaxial mixing pile 1 is L = 16m. In this modified example, the standardized correspondence between excavation depth and pile length ensures that the penetration depth of the triaxial mixing pile 1 meets the requirements for overturning and heave resistance of the small foundation pit, thereby ensuring the overall stability and water-stopping reliability of the retaining structure under different depth conditions.

[0031] Compared with traditional brick formwork construction and slope excavation with solidified soil backfilling, the retaining structure construction method of this invention eliminates multiple processes such as brick formwork construction, plastering, and waterproofing, as well as the large-area site leveling and solidified soil backfilling required for slope excavation. It also eliminates the need for additional water-stop curtain construction, thus simplifying the multi-process, multi-material, and multi-trade cross-operation of traditional methods into integrated continuous construction of the retaining structure. Furthermore, when the foundation size is ≤5m, the integrated casting of the concrete support slab 4 and the cap beam 3 further eliminates the installation, welding, and dismantling of steel supports; the H-beam 2 can be removed and reused later, reducing the one-time steel input in traditional support systems. Therefore, in the construction of small foundation pits in water-rich soft soil conditions, this invention significantly shortens the formation cycle of the retaining structure, reduces construction costs, minimizes site occupation, and simultaneously achieves the dual functions of water-stopping and support in the construction of small foundation pits, through streamlined processes, reduced materials, and integrated structure, thereby improving construction safety and reliability.

[0032] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-disclosed technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. A construction method for the retaining structure of a small foundation pit under water-rich soft soil geological conditions, characterized in that, The retaining structure includes SMW method piles continuously overlapping along the circumference of the foundation pit, H-beams inserted into the SMW method piles, reinforced concrete cap beams cast on the top of the SMW method piles, and a support system; the construction method includes the following steps: Triaxial mixing piles are drilled around the perimeter of the foundation pit, and H-beams are inserted into the triaxial mixing piles at intervals. The H-beams are 500mm higher than the capping beam constructed later. A reinforced concrete capping beam is constructed on top of the three-axis mixing piles to connect the piles into a whole. Where the capping beam crosses the H-beam, the main reinforcement and stirrups of the capping beam are tied to both sides of the H-beam, and the stirrups are double-stranded. Where the capping beam does not cross the H-beam, the main reinforcement of the capping beam is tied to both sides of the pile, and the stirrups are triple-stranded. The support system is constructed in stages according to the dimensions of the foundation: When the foundation size is ≤5m, the concrete support plate reinforcement and the cap beam reinforcement are tied together and cast as a whole, so that the concrete support plate and the cap beam form a rigid whole. When the foundation size is >5m, a support embedded part is pre-embedded in the capping beam stage. The embedded steel bars of the support embedded part are welded to the main reinforcement of the capping beam. Before the foundation pit is excavated, the H-shaped steel support is welded and fixed to the support embedded part. After passing the acceptance inspection, the foundation pit will be excavated.

2. The construction method for the retaining structure of a small foundation pit under water-rich soft soil geological conditions according to claim 1, characterized in that, The three-axis mixing pile has a diameter of 650mm and a pile spacing of 450mm, and is constructed using a casing drilling method; the water-cement ratio of the grout is 1.2-1.5, and the cement content of a single pile is ≥25%; the sinking speed of the three-axis mixing pile is ≤0.6m / min, and the lifting speed is ≤1.2m / min; wherein, when the three-axis mixing pile is used as a water-stop curtain, the sinking speed is ≤0.3m / min, and the lifting speed is ≤0.5m / min; the amount of grout sprayed during the sinking process accounts for 70%-80% of the total pile volume, and the amount of grout sprayed during the lifting process accounts for 20%-30% of the total pile volume.

3. The construction method for the retaining structure of a small foundation pit under water-rich soft soil geological conditions according to claim 1 or 2, characterized in that, The grout mixing time shall not be less than 3 minutes, and the grout density shall be detected by a grout hydrometer. The grout delivery shall be continuous. The three-axis mixing pile adopts a two-stage grouting process, namely sinking grouting and lifting grouting. After sinking to the bottom of the pile, it is stirred and lifted again to eliminate air bubbles.

4. The construction method for the retaining structure of a small foundation pit under water-rich soft soil geological conditions according to claim 1, characterized in that, The H-beams should be inserted within 30 minutes after the completion of the three-axis mixing pile construction, with one inserted every other. The H-beams should sink slowly under their own weight. If mechanical assistance is used, the vibration frequency should be 30-50Hz, and the H-beams should sink at a uniform speed. Free fall and sudden impact are strictly prohibited.

5. The construction method for the retaining structure of a small foundation pit under water-rich soft soil geological conditions according to claim 1, characterized in that, The dimensions of the capping beam are 1000mm × 600mm; where there is H-beam, the main reinforcement and stirrups of the capping beam are tied to both sides of the H-beam, and the stirrups are double-stranded; where there is no H-beam, the main reinforcement of the capping beam is tied to both sides of the pile, and the stirrups are triple-stranded.

6. The construction method for the retaining structure of a small foundation pit under water-rich soft soil geological conditions according to claim 1, characterized in that, When the foundation size is ≤5m, the thickness of the concrete support slab is 300mm, and the plane size is 1.5m×1.5m. The reinforcing bars of the concrete support slab and the reinforcing bars of the cap beam are tied and poured together to form an integral whole.

7. The construction method for the retaining structure of a small foundation pit under water-rich soft soil geological conditions according to claim 1, characterized in that, When the foundation size is greater than 5m, the H-beam support is made of H400×400×13×21 steel and is arranged at the four corners of the foundation pit with a support spacing of ≤2000mm. The H-beam support and the embedded support are connected by welding.

8. The construction method for the retaining structure of a small foundation pit under water-rich soft soil geological conditions according to claim 7, characterized in that, The dimensions of the embedded support are 700mm×500mm, including a 15mm thick Q235B steel plate and 9 HRB400 grade steel bars with a diameter of 25mm. The steel bars are 400mm long and spaced at 150mm intervals. The steel plate and steel bars are connected by through-hole plug welding, and the embedded steel bars are electrically welded to the main reinforcement of the cap beam.

9. The construction method for the retaining structure of a small foundation pit under water-rich soft soil geological conditions according to claim 1, characterized in that, The construction method consists of the following steps and sequence: base surface cleaning, positioning and layout, construction of three-axis mixing piles, insertion of H-beams, excavation of the cap beam, construction of the cap beam and support system, acceptance, and excavation of the foundation pit.

10. A small-scale foundation pit retaining structure, characterized in that, include: SMW method piles continuously overlap along the circumference of the foundation pit; H-beams inserted into the SMW method piles; A reinforced concrete capping beam is cast atop the SMW method piles. Where the capping beam crosses the H-beam, the main reinforcement and stirrups of the capping beam are tied to both sides of the H-beam, with double-strand stirrups. Where the capping beam does not cross the H-beam, the main reinforcement of the capping beam is tied to both sides of the pile, with triple-strand stirrups. The H-beam extends 500mm above the capping beam. The support system includes: When the foundation size is ≤5m, a concrete support slab is tied to and cast integrally with the capping beam reinforcement, forming a rigid whole with the capping beam; or When the foundation size is greater than 5m, the supporting embedded parts are pre-embedded in the crown beam, and the H-shaped steel supports are welded and fixed to the supporting embedded parts.

Images