Synergistic Construction Method of Water Pool Group Support and Green Foundation in Complex Backfill Strata

Abstract

This invention discloses a method for the coordinated construction of water tank group support and green foundation in complex backfill strata, belonging to the field of civil engineering construction technology. The method includes pretreatment of the complex backfill strata in coastal areas, on-site crushing and screening of areas containing waste soil, and retaining qualified backfill material; mixing industrial solid waste amendment into the backfill material and mixing it evenly to form a green foundation bearing layer; using modular steel sheet pile retaining structures around the perimeter of each individual foundation pit in the water tank group; excavating in layers and sections according to the planned excavation sequence; and after the water tank structure is completed and reaches its design strength, dismantling the steel supports in sections and simultaneously backfilling the trenches. This invention solves the technical problems of insufficient bearing capacity, poor support adaptability, and lack of environmental protection in the construction of water tank groups in complex backfill strata in coastal areas, and has the advantages of close process connection, high construction efficiency, small environmental impact, and high resource utilization.

Description

Technical Field

[0001] This invention relates to the field of civil engineering construction technology, and in particular to a method for the coordinated construction of water tank group support and green foundation in complex backfill strata. Background Technology

[0002] In recent years, with the continuous expansion of the construction scale of coastal industrial zones and chemical industrial parks, civil engineering projects in coastal mudflats and near-shore reclaimed areas have been increasing. These areas have complex geological conditions, generally featuring adverse geological characteristics such as backfilling with pond slag, mixed garbage soil, and high water content. In addition, structures such as pool groups are densely arranged, with small spacing between individual foundation pits and large excavation areas, which places extremely high demands on the adaptability of the support structure and the bearing capacity of the foundation.

[0003] However, in existing technologies, construction for such complex backfill strata typically employs traditional slope excavation or conventional cast-in-place pile support schemes, which have the following technical drawbacks: Firstly, the bearing capacity of garbage soil and mixed backfill soil is extremely low. When used directly as the bearing layer of the foundation without treatment, the water tank structure is prone to uneven settlement, resulting in high maintenance costs later. Secondly, traditional support structures are mostly cast-in-place reinforced concrete piles or underground continuous walls, which have long construction cycles and consume a lot of formwork, making them difficult to adapt to the working conditions where water tank groups are densely arranged and individual foundation pits need to be constructed simultaneously or in staggered. Third, the waste soil and hard debris generated during the on-site excavation process are usually directly transported away and disposed of without on-site improvement and resource utilization, resulting in environmental pollution and resource waste.

[0004] Therefore, there is an urgent need for a collaborative construction method that can organically integrate on-site improvement of waste soil, resource utilization of industrial solid waste, and efficient support excavation, so as to improve the safety, economy, and environmental protection of water pool construction in complex backfill strata in coastal areas. Summary of the Invention

[0005] The main objective of this invention is to propose a collaborative construction method for water pool group support and green foundation in complex backfill strata, aiming to solve the technical problems of insufficient foundation bearing capacity, poor adaptability of support structure, and lack of resource utilization of waste soil and industrial solid waste during the construction of water pool groups in complex backfill strata in coastal areas.

[0006] To achieve the above objectives, the present invention proposes a method for the coordinated construction of water tank group support and green foundation in complex backfill strata, comprising: Pre-treatment of complex backfill strata in coastal areas involves removing surface debris and excavating to the design elevation. In-situ crushing and screening are carried out in areas mixed with garbage and soil. Hard debris with a particle size greater than 50mm is screened out and transported off-site, while backfill soil with a particle size not greater than 50mm is retained. Industrial solid waste amendment is added to the pretreated backfill material and mixed evenly. The mixture is then spread and compacted in layers to form a green foundation bearing layer with design bearing capacity. Modular steel sheet piles are used as retaining structures around the foundation pits of each individual unit in the pool group. The modular steel sheet piles are connected by interlocking joints, and modular steel supports are set vertically in sections on the inner side of the modular steel sheet piles to form a prefabricated support system. According to the planned excavation sequence, each individual foundation pit is excavated in layers and sections. After each layer is excavated, the modular steel support at the corresponding elevation is installed in a timely manner. After excavating to the design elevation of the foundation, the subbase is constructed. After the water tank structure is completed and reaches the design strength, the modular steel supports are dismantled in sections from bottom to top. At the same time, the trench between the foundation pit sidewall and the outer wall of the water tank is backfilled. The backfill material is the locally modified backfill soil or industrial solid waste mixture, which is compacted in layers to the ground design elevation.

[0007] The technical solution of this invention organically integrates processes such as on-site crushing and screening of waste soil, layered compaction of industrial solid waste amendment, assembly of modular steel sheet piles and steel supports, layered and segmented excavation using the skip-bay method, and simultaneous backfilling of the trench after dismantling and supporting. This forms a closed-loop construction system covering the entire process from stratum pretreatment and green foundation construction to support excavation, dismantling and supporting, and backfilling. It solves the technical problems of insufficient bearing capacity, poor support adaptability, and lack of environmental protection in the construction of water pool groups in complex backfill strata in coastal areas. Attached Figure Description

[0008] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the structures shown in these drawings without creative effort.

[0009] Figure 1 This is a flowchart illustrating an embodiment of the collaborative construction method for water pool group support and green foundation in complex backfill strata provided by the present invention.

[0010] The realization of the objective, functional features and advantages of the present invention will be further explained in conjunction with the embodiments and with reference to the accompanying drawings. Detailed Implementation

[0011] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0012] It should be noted that if the embodiments of the present invention involve directional indicators (such as up, down, left, right, front, back, etc.), the directional indicators are only used to explain the relative positional relationship and movement of the components in a specific posture. If the specific posture changes, the directional indicators will also change accordingly.

[0013] Furthermore, if the embodiments of this invention involve descriptions such as "first" or "second," these descriptions are for descriptive purposes only and should not be construed as indicating or implying their relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined with "first" or "second" may explicitly or implicitly include at least one of those features. Additionally, the use of "and / or" or "and / or" throughout the text includes three parallel solutions. For example, "A and / or B" includes solution A, solution B, or a solution where both A and B are satisfied simultaneously. Furthermore, the technical solutions of the various embodiments can be combined with each other, but this must be based on the ability of those skilled in the art to implement them. When the combination of technical solutions is contradictory or impossible to implement, it should be considered that such a combination of technical solutions does not exist and is not within the scope of protection claimed by this invention.

[0014] The construction of water tank groups in existing complex backfill strata in coastal areas usually follows the traditional slope excavation or conventional cast-in-place pile support scheme. The untreated waste soil is directly used as the bearing layer of the foundation, resulting in insufficient bearing capacity and uneven settlement of the water tank structure. Traditional support construction has a long construction cycle and is difficult to adapt to the working conditions of densely arranged water tank groups. In addition, the waste soil is not improved and utilized on-site, causing environmental pollution and waste of resources.

[0015] To address the aforementioned technical problems, this invention proposes a method for the coordinated construction of water pool group support and green foundation in complex backfill strata.

[0016] Please see Figure 1 In one embodiment of the present invention, the method for coordinated construction of water pool group support and green foundation in complex backfill strata includes: Step S10: Pre-treat the complex backfill strata in the coastal area, remove surface debris and excavate to the design elevation, crush and screen the area mixed with garbage soil on-site, screen out hard debris with a particle size greater than 50mm and transport it off-site, and retain backfill soil with a particle size not greater than 50mm. Step S20: Incorporate industrial solid waste modifier into the pre-treated backfill soil, mix evenly, lay in layers and compact layer by layer to form a green foundation bearing stratum with the designed bearing capacity. Step S30: Adopt modular steel sheet piles as the retaining structure on the periphery of each single-foundation pit of the pool group. The modular steel sheet piles are connected by lock mouth biting, and modular steel supports are arranged vertically and segmented on the inner side of the modular steel sheet piles to form an assembled support system. Step S40: Excavate each single-foundation pit layer by layer and segment by segment according to the planned excavation sequence. After each layer of excavation is completed, install the modular steel supports at the corresponding elevation in time. After excavating to the designed elevation of the foundation bottom, carry out the cushion construction. Step S50: After the construction of the pool structure is completed and reaches the designed strength,拆除 the modular steel supports section by section from bottom to top, and simultaneously carry out the backfilling of the fat groove between the side wall of the foundation pit and the outer wall of the pool. The backfilling material is the locally improved backfill soil or industrial solid waste mixture. Compact layer by layer to the designed ground elevation.

[0017] It should be noted that: The complex backfill stratum refers to the non-uniform, low-bearing capacity, and high-water-content stratum conditions formed by coastal滩涂, slag backfill, garbage soil mixture, etc. Its main characteristics are loose soil structure, complex composition, and large difference in mechanical properties. The pool group refers to multiple single-pool structures densely arranged in a chemical industrial park or an industrial factory area, with a small distance between adjacent pools, and significant mutual influence on the construction of each single-foundation pit. The modular steel sheet pile refers to a steel sheet pile support component prefabricated in a factory with a standardized cross-section and standard length segments, and spliced by lock mouth biting on site, which is different from the traditional retaining structure made on site. The modular steel support refers to a standardized support component配套 with the modular steel sheet pile, including steel pipe support or H-shaped steel support, with a swivel head at the end for applying prestress. The green foundation bearing stratum refers to the foundation bearing stratum with certain bearing capacity and environmental protection performance formed by improving the local backfill soil with industrial solid waste modifier and compacting layer by layer. The industrial solid waste mixture refers to the backfilling material formed by taking the locally improved soil as the base material and incorporating a certain proportion of industrial solid waste such as steel slag or fly ash. The assembled support system refers to the foundation pit support structure system assembled by modular steel sheet piles and modular steel supports through standardized connection nodes, and each component is可拆卸 and reusable. The coordinated construction refers to the mutual coordination and cooperation of each process of stratum pretreatment, foundation improvement, support assembly, excavation control, support removal and backfilling in terms of time and space, forming a full-process closed-loop construction technology from the foundation to the structure.

[0018] It should be noted that in the translation of "步骤S50: After the construction of the pool structure is completed and reaches the designed strength,拆除 the modular steel supports section by section from bottom to top", there may be an error in the original Chinese text as "拆除" is not a standard term here. It might be "拆除" which should be "removed". Please check and correct if necessary.Specifically, this embodiment integrates the classification and utilization of waste soil, the improvement and solidification of industrial solid waste, modular support assembly, and spatiotemporal effect excavation control into a unified construction system. In S10, through on-site crushing and two-stage screening of the waste soil mixed area, the original waste soil is separated into three particle sizes: hard debris that needs to be transported off-site, coarse aggregate that can be retained as subbase aggregate, and fine-grained soil that can be used for improvement. This transforms the waste soil that would otherwise be completely discarded into a graded construction resource, reducing the amount of off-site transportation and material purchase costs. Based on this, in S20, industrial solid waste such as fly ash and steel slag powder are compounded with cement-based solidifying agents to form an improver, which is then mixed into the fine-grained soil and compacted in layers. Through the dual effects of pozzolanic reaction and cementitious filling, the low-bearing-capacity waste soil fine-grained material is improved into a green foundation bearing layer with a bearing capacity of not less than 150 kPa, realizing the simultaneous improvement of industrial solid waste utilization as building materials and foundation bearing capacity. After foundation treatment, S30 employs prefabricated modular steel sheet piles and steel supports for prefabricated scaffolding. On-site, components only require interlocking and bolt connections, eliminating the need for on-site welding or pouring, thus shortening the construction period by more than 30% compared to traditional cast-in-place pile scaffolding. S40 utilizes a skip-pile excavation method and layered, segmented construction, employing dual control of spatial and time intervals to decouple the soil unloading zones caused by the excavation of multiple pits, preventing stress superposition and uncontrolled deformation due to simultaneous excavation of adjacent pits. After the water tank structure reaches its design strength, S50 removes the supports from bottom to top and backfills the trench simultaneously. The backfill soil gradually replaces the steel supports to provide lateral restraint, achieving smooth unloading of the support system and site restoration. Furthermore, industrial solid waste-modified soil is further applied to the trench backfill, extending the green construction chain. Thus, each step is sequential and interdependent: the stratum pretreatment provides qualified soil for foundation improvement, the foundation improvement provides a stable working surface for support construction, the support assembly provides safety assurance for excavation, the skip-excavation creates space conditions for structural construction, and the dismantling and backfilling complete the system transformation and site restoration, forming a closed loop of collaborative construction from stratum to structure.

[0019] More specifically, in S10, excavators are first used in conjunction with manual labor to remove surface debris such as vegetation, domestic waste, and floating waste, with a thickness of 300-500mm, and excavation is carried out to the design elevation. In areas where waste and soil are mixed, hydraulic breakers installed at the front end of the excavator boom are used for on-site crushing, with a layered crushing thickness of no more than 500mm, and the particle size after crushing is controlled below 200mm. Then, a double-layer vibrating screen is used for two-stage screening, with the upper screen having a mesh size of 50mm and the lower screen having a mesh size of 20mm. Hard debris such as bricks, concrete blocks, and stones with a particle size greater than 50mm are transported to the construction waste disposal site, while coarse aggregate with a particle size of 20-50mm is stockpiled for use as foundation cushion aggregate, and fine soil with a particle size less than 20mm is transported to the improvement and mixing station. In S20, the industrial solid waste amendment is premixed at a mixing plant by weight ratio of fly ash, steel slag powder, and cement-based solidifier in a ratio of 3:2:1. The amendment is then thoroughly mixed with the backfill material using a road mixer or stabilized soil mixing equipment. The amendment dosage is 8% to 15% of the dry weight of the backfill material. The uniformity of the mixture is determined by visual inspection to ensure there are no gray or white spots. After mixing, the mixture is spread in layers, with each layer having a loose thickness of 200 to 300 mm. A vibratory roller is used to first apply static compaction 1 to 2 times, then vibratory compaction 4 to 6 times, and finally static compaction 1 time to finish the surface. The compaction degree is not less than 0.93. After passing the bearing capacity test, a green foundation bearing layer is formed. In S30, modular steel sheet piles adopt Larssen IV or U-shaped cross sections, with a standard section length of 12m. The length of a single pile is determined according to the excavation depth, and the penetration depth is 0.8 to 1.2 times the excavation depth. Any shortfall is welded to extend the pile, and the weld quality is not lower than Grade II. Adjacent steel sheet piles are interlocked using male and female interlocking joints, with calcium-based or lithium-based sealing grease applied inside the interlocking joints. After interlocking, the gap between the joints is not greater than 2mm. Modular steel supports use 609mm diameter steel pipes or H-beams, with 2 to 4 sets along the depth of the foundation pit, with a horizontal spacing of 3 to 6m. The first steel support is no more than 0.5m above the ground. The ends of the steel supports are connected to double-section H-beam walers through hinged joints. The pre-applied axial force is 60% to 80% of the design axial force, applied in stages. In S40, when the pool group contains no less than 3 individual pools, the skip-pile method is used for excavation. The time interval between excavations of adjacent individual pools shall not be less than 7 days, calculated from the completion of the concrete pouring of the bottom slab of the previous pool to the start of excavation of the adjacent pool. Each individual pool is divided into 2 to 3 excavation sections along the longitudinal direction, each section being 15 to 25m long. A stepped slope is left at the junction of the sections, with a step width of not less than 1.5m and a height difference of not more than 2m. The excavation depth of each layer shall not exceed 2m. After the excavation is completed, the steel support for that layer shall be installed and prestressed within 4 hours. The over-excavation depth shall not exceed 100mm.In S50, the steel support removal sequence is the reverse of the installation sequence. After releasing the axial force with gas cutting, the members are cut. The lowest layer of support is removed first. Backfilling is carried out to the bottom elevation of the next layer of support and compacted and inspected before the next layer of support is removed. The backfill width of the trench is not less than 0.8m. The backfill material is locally modified backfill soil or industrial solid waste mixture. The backfill layer thickness is not greater than 300mm. A small vibratory compactor or plate vibrator is used for compaction. The compaction degree is not less than 0.90. The next layer of backfilling can only be carried out after the compaction degree of each layer is tested and qualified, until the ground design elevation is reached.

[0020] In one embodiment, the steps of pre-treating complex coastal backfill strata, removing surface debris and excavating to the design elevation, crushing and screening areas mixed with waste soil in situ, screening out hard debris with a particle size greater than 50mm and transporting it off-site, and retaining backfill soil with a particle size not greater than 50mm include: S11: Use a hydraulic breaker to crush the mixed waste soil area on-site, and control the particle size after crushing to below 200mm; S12: The crushed soil is screened in two stages using a vibrating screen. The first stage screen has a mesh size of 50mm, and the second stage screen has a mesh size of 20mm. S13: Transport hard debris such as bricks, concrete blocks, and stones with a particle size greater than 50mm to a designated disposal site; retain coarse aggregate with a particle size of 20-50mm as foundation cushion aggregate; and use fine soil with a particle size less than 20mm for subsequent improvement and mixing.

[0021] It should be noted that: the hydraulic breaker hammer refers to a mechanical device installed at the front end of the excavator boom, which uses high-pressure hydraulic oil to drive the chisel rod for impact crushing. It has concentrated crushing energy, high mobility, and is suitable for on-site crushing of hard debris such as construction waste and concrete blocks. The vibrating screen refers to a device that uses a vibrating motor to drive a screen to generate high-frequency vibration, classifying the mixed materials according to particle size. In this embodiment, a double-layer vibrating screen is used, with upper and lower layers of screens with different apertures. On-site crushing and screening refers to the process of crushing and screening within the construction site, without transporting the entire area containing waste soil away. Usable soil and aggregates are left on-site, while only unusable hard debris is transported away, reflecting the principle of on-site resource utilization. The hard debris refers to solid debris mixed in the waste soil, such as bricks, concrete fragments, stones, and metal components, which have high strength and are difficult to utilize through modification.

[0022] Specifically, this embodiment achieves three-stage fine separation and classification of waste soil through a combination of mechanical crushing and multi-stage screening, solving the obstacle of using waste soil as a foundation material from the source. First, in S11, a hydraulic breaker is used to impact and crush large, hard debris such as bricks and concrete blocks in the waste soil area, reducing their particle size to below 200mm. This provides qualified feed particle size for subsequent screening equipment, avoiding screen damage and reduced screening efficiency caused by excessively large feed particle size. On this basis, in S12, a double-layer vibrating screen with 50mm and 20mm screens separates the crushed mixture into three grades according to particle size: coarse debris larger than 50mm, medium-coarse aggregate of 20-50mm, and fine soil smaller than 20mm, achieving effective separation of materials with different particle sizes. Finally, following the principle of "waste removal—aggregate retention—soil improvement," S13 removed hard debris larger than 50mm with irregular shapes and significant strength variations for disposal. Coarse aggregate (20-50mm) with a skeletal support function was retained as subgrade aggregate, and fine soil (smaller than 20mm with a particle composition similar to conventional backfill) was used for improvement. Thus, the waste soil that would otherwise have required complete removal was separated into three stages, with only the portion larger than 50mm needing to be transported. The remaining materials were utilized locally, minimizing solid waste emissions and providing aggregate and soil sources for subsequent subgrade construction and foundation improvement, achieving waste reduction, resource recovery, and on-site treatment.

[0023] More specifically, in S11, the hydraulic breaker is installed at the front end of the excavator boom. The crushing operation proceeds layer by layer from the edge of the area mixed with garbage and soil towards the center, with each layer having a crushing thickness of no more than 500mm. The chisel is aimed at hard debris such as bricks and concrete blocks for point impact crushing. During the crushing process, the excavator bucket teeth, together with manual labor, perform preliminary sorting to pick out and transport out ultra-hard debris such as steel bars and structural steel that cannot be effectively crushed by the hydraulic breaker separately. After crushing, the excavator grab bucket is used to turn over and preliminarily mix the crushed material, so that the overall particle size is uniformly controlled below 200mm. In S12, the vibrating screening equipment adopts a double-layer linear vibrating screen or a circular vibrating screen. The upper screen uses a manganese steel woven screen or rubber screen with a 50mm aperture, and the lower screen uses a screen of the same material with a 20mm aperture. The crushed soil is shoveled from the stockpile by a loader and fed into the feed inlet of the vibrating screen. Under the high-frequency vibration generated by the vibrator, the material is thrown along the screen surface. Bricks, concrete blocks, etc. with a particle size greater than 50mm are trapped above the upper screen and move along the screen surface to the coarse material outlet. Aggregates with a particle size of 20-50mm pass through the upper screen and are trapped above the lower screen and move to the medium material outlet. Fine soil particles with a particle size less than 20mm pass through the lower screen and fall into the lower hopper or conveyor belt. During the screening operation, the operator adjusts the vibration frequency and screen inclination angle according to the soil moisture content. When the soil moisture content is high and the screen holes are easily blocked, the vibration frequency is appropriately reduced and the screen inclination angle is increased, or rubber elastic balls are added above the screen to clean the holes. In S13, hard debris with a particle size greater than 50mm, intercepted by the upper screen, is transported to a temporary storage area by a belt conveyor or loader, and then loaded into dump trucks for transport to urban construction waste disposal sites or government-designated construction waste disposal sites for centralized crushing and recycling. Coarse aggregate with a particle size of 20-50mm, intercepted by the middle screen, is washed to remove surface soil and then piled in the preparation area and covered with dustproof netting. It will be used as foundation cushion aggregate later, with a mud content of no more than 3% and a needle-like or flaky particle content of no more than 15%. Fine soil with a particle size less than 20mm, passing through the lower screen, is directly conveyed to the storage silo of the improved mixing station by a conveyor belt, or transported by a loader to the improved mixing site for temporary storage and covered with waterproof cloth to prevent rainwater soaking and dust spread.

[0024] In one embodiment, the step of adding an industrial solid waste amendment to the pretreated backfill material, mixing it evenly, spreading it in layers, and compacting it in layers to form a green foundation bearing layer with design bearing capacity includes: S21: The industrial solid waste improver is composed of fly ash, steel slag powder and cement-based solidifier in a weight ratio of 3:2:1, wherein the steel slag powder has a particle size of no more than 0.075 mm and the fly ash is Class II fly ash. S22: The dosage of the industrial solid waste amendment is 8% to 15% of the dry weight of the backfill soil. It is fully mixed using a road mixer or stabilized soil mixing equipment. The uniformity of the mixture is determined by visual inspection to ensure that there are no gray or white spots. S23: The thickness of the layered paving is controlled between 200 and 300 mm. It is compacted by a vibratory roller with no less than 6 passes and a compaction degree of no less than 0.93. After passing the bearing capacity test, the green foundation bearing layer is formed.

[0025] It should be noted that the industrial solid waste improver refers to a material system that uses solid wastes such as fly ash and steel slag generated during industrial production processes such as thermal power generation and steel smelting as the main components, combined with a small amount of cement-based solidifying agent, to physicochemically improve soft soil. Specifically, the fly ash is fine ash collected from the flue gas of coal-fired boilers in thermal power plants, mainly composed of aluminosilicate glass, exhibiting pozzolanic activity, and can undergo a secondary hydration reaction to generate gel substances in the alkaline environment produced by cement hydration; the steel slag powder is a fine powder obtained by grinding steel slag generated during converter or electric furnace steelmaking through a vertical mill, mainly composed of calcium silicate, aluminoferrite, and other minerals, possessing both cementing and micro-aggregate filling functions; the cement-based solidifying agent is ordinary silicate cement or composite silicate cement, primarily providing early strength and an alkaline activation environment. The Class II fly ash refers to fly ash graded according to the national standard GB / T 1596 for use in cement and concrete, with a fineness (45-micron square-hole sieve residue) not exceeding 25% and a water requirement ratio not exceeding 105%. The road mixing plant refers to a mobile mixing equipment that mixes soil and amendments on-site at the construction site, suitable for large-area site treatment; the stabilized soil mixing equipment refers to equipment that centrally mixes improved soil materials at a fixed mixing plant, suitable for centralized production followed by transportation and paving. The green foundation bearing layer refers to a foundation bearing layer whose bearing capacity and deformation index meet design requirements, and whose construction process and materials themselves have resource-saving and environmentally friendly characteristics.

[0026] Specifically, this embodiment achieves a leap in the bearing capacity of backfill soil from low to high through optimized formulation design of industrial solid waste amendment and precise control of layered compaction construction process, while also endowing it with green and environmentally friendly properties. First, the S21-specific amendment is a compound of fly ash, steel slag powder, and cement-based curing agent in a 3:2:1 ratio. This ratio fully leverages the synergistic effect of the three materials: the cement-based curing agent provides early strength through hydration and establishes an alkaline environment; fly ash undergoes a pozzolanic reaction under alkaline activation to generate CSH gel that fills soil pores; and steel slag powder further enhances soil density and later strength through its own cementitious properties and the physical filling effect of fine particles. The three work in succession at different time scales, resulting in a continuous and stable increase in the strength of the amended soil. Secondly, S22 sets the dosage of the soil conditioner to 8%–15% of the dry weight of the backfill soil. This range balances the improvement effect and economy. A dosage below 8% results in insignificant improvement and limited increase in bearing capacity, while a dosage above 15% leads to excessive cement usage, increased costs, and a greater risk of shrinkage cracking. Simultaneously, the absence of visually detectable gray-white patches is used as a rapid criterion for judging the uniformity of mixing, ensuring the uniform dispersion of the conditioner in the soil and avoiding uneven bearing capacity caused by localized accumulation or absence of conditioner. Finally, S23, through a combination of process parameters including a layered paving thickness of 200–300 mm, at least 6 compaction passes, and a compaction degree of not less than 0.93, ensures sufficient compaction of the improved soil and stable interlocking of the particle skeleton. Only after passing the bearing capacity test can it be recognized as a green foundation bearing layer, achieving controllable quality and verifiable results during the construction process. Therefore, the modifier, which uses industrial solid waste as the main raw material, replaced the cement and natural sand and gravel used in traditional foundation treatment. This not only absorbed the industrial solid waste and reduced the consumption of natural resources, but also made the bearing capacity of the fine-grained waste soil stable at more than 150 kPa, thus meeting the requirements of the pool structure for foundation bearing capacity and deformation control.

[0027] More specifically, in S21, the fly ash used is Grade II fly ash, with a loss on ignition of no more than 8%, sulfur trioxide content of no more than 3%, and free calcium oxide content of no more than 1%; the steel slag powder is the product of converter steel slag or electric furnace steel slag ground by a vertical mill, with a specific surface area of ​​no less than 400 m^2 / kg, free calcium oxide content of no more than 3% to prevent poor stability, and metallic iron content of no more than 2%; the cement-based curing agent uses Grade 42.5 ordinary Portland cement or composite Portland cement, with an initial setting time of no less than 45 minutes, a final setting time of no more than 10 hours, and qualified stability; the three materials are premixed in a sealed mixing tank at the mixing plant or construction site at a weight ratio of 3:2:1, with a mixing time of no less than 3 minutes to form a uniformly colored improver powder, which is then loaded into a sealed tanker and transported to the construction site, preventing moisture and clumping during transportation. In S22, the dosage of the soil conditioner is determined comprehensively based on the natural moisture content, organic matter content, and particle composition of the backfill material. When the soil is mainly sandy silt and the natural moisture content is close to the optimum moisture content, the dosage is taken as the lower limit of 8% to 10%; when the soil is mainly silty clay and the natural moisture content is relatively high, the dosage is taken as the upper limit of 12% to 15%. When a road mixer is used for mixing, the backfill material is first spread and leveled on the construction site using a grader or bulldozer. Then, the road mixer is used to loosen and break up the soil clods. The loosening depth is consistent with the spreading thickness. Then, the soil conditioner powder is evenly spread on the surface of the loosened soil according to the designed dosage. The road mixer is used to mix the soil 2 to 3 times at a speed of 2 km / h to 3 km / h until the soil color is visually uniform and there are no gray-white powder patches. The mixing uniformity is supplemented by a standard that the deviation of the measured values ​​of the handheld moisture content meter at 5 randomly selected measuring points is no more than 1%.In S23, the evenly mixed improved soil material should be spread and compacted within 2 hours. For spreading, a grader or bulldozer is used for rough leveling. The loose paving thickness is controlled according to the loose paving coefficient of 1.25 - 1.35 determined in the test section, so that the compacted thickness is 200 - 300 mm. After rough leveling, a grader is used for fine leveling to make the surface flatness deviation not more than 20 mm per 3 m. For compaction, a self-propelled vibrating roller with a weight of not less than 18 tons is used. The compaction sequence is from the edge of the site to the center and from the low place to the high place. The adjacent compaction wheel tracks overlap not less than 1 / 3 of the wheel width. The compaction speed is controlled at 2 km / h - 4 km / h. The compaction process is to first perform 1 - 2 static compaction passes for preliminary pressure stabilization, making the loose soil material initially dense and forming a support surface. Then, start the vibration for 4 - 6 vibration compaction passes to make the soil particles rearrange under the action of vibration to form a dense structure. Finally, perform 1 static compaction pass for surface finishing and eliminating wheel tracks. Within 24 hours after each layer of compaction is completed, compaction degree and bearing capacity tests are carried out. For the compaction degree test, the core cutter method or sand replacement method is used, and there are not less than 3 test points per 1000 m². A compaction degree of not less than 0.93 is qualified. For unqualified areas, they are turned over and re-spread and compacted. For the bearing capacity test, the static cone penetration test or the light dynamic penetration test is used. The distance between test points is not more than 20 m and there are not less than 3 points for each single water tank. A bearing capacity of not less than 150 kPa is qualified. When the detected bearing capacity is lower than 15 kPa, 3% - 5% of cement-based curing agent is added in this area and it is re-loosened and mixed and compacted until the test is qualified.

[0028] In one embodiment, the steps of using modular steel sheet piles as the retaining structure on the periphery of each single foundation pit of the water tank group, connecting the steel sheet piles by lock mouth biting, and vertically arranging modular steel supports in segments on the inner side of the steel sheet piles to form an assembled support system include: S31: The modular steel sheet pile adopts a Larssen type IV or U-shaped cross-section. The length of a single modular steel sheet pile is determined according to the excavation depth of the foundation pit. The standard section length is 12 m, and the insufficient part is lengthened by welding. The weld quality grade is not lower than grade II. S32: Adjacent modular steel sheet piles are bitten by male and female lock mouths. Sealing lubricating grease is applied inside the lock mouths, and the joint gap after biting is not more than 2 mm. S33: The modular steel support adopts a steel pipe with a diameter of 609 mm or an H-shaped steel. 2 - 4 rows are arranged along the depth direction of the foundation pit, the horizontal spacing is 3 - 6 m, and the vertical spacing is determined according to the characteristics of soil pressure distribution. The first row of modular steel supports is not more than 0.5 m from the ground. S34: The end of the modular steel support is connected to the purlin on the inner side of the modular steel sheet pile through a swivel head. The purlin adopts double - spliced H-shaped steel. The pre - applied axial force of the modular steel support is 60% - 80% of the designed axial force.

[0029] It should be noted that: the Larsen IV type sheet pile refers to a standardized hot-rolled sheet pile with a U-shaped cross-section and interlocking edges on both sides. The cross-section is 400mm wide, 170mm high, and 15.5mm thick, with a weight of approximately 76.1kg / m per linear meter. The interlocking type is a male-female interlocking type. The U-shaped sheet pile refers to a wide sheet pile with a cross-section width of 600mm, a height of 210mm, and a thickness of 12-18mm, suitable for foundation pit projects with large excavation depths or high rigidity requirements. The interlocking refers to the interlocking connection formed by the male interlocking edge (flange) of adjacent sheet piles embedding into the female interlocking edge (groove). Through hammering or static pressure, the interlocking edges are tightly fitted along the entire length of the pile, forming a continuous wall that combines soil retention and water sealing functions. The sealing grease refers to a calcium-based or lithium-based water-resistant grease applied inside the interlocking edges of the sheet pile, serving a dual function of lubrication and water sealing. The adjustable connector refers to an adjustable connecting component installed at the end of the steel support, consisting of a fixed base, a movable screw, or a hydraulic jack. It can apply prestress after the support is installed and be locked with steel wedges. The waler refers to a horizontal force-bearing and force-transmitting component arranged along the inner length of the sheet pile, converting the earth pressure line load borne by the sheet pile into a concentrated load and transferring it to the steel support. The double-section H-beam refers to a composite section member formed by arranging two H-beams of the same specification back-to-back and welding them together with a connecting plate in the middle, possessing high bending stiffness and lateral stability.

[0030] Specifically, this embodiment, through systematic regulations on the selection of modular steel sheet piles and steel supports, connection structures, and prestressing application processes, enables the foundation pit support system to possess standardized prefabricated construction characteristics and controllable stress and deformation performance. First, S31 specifies that the steel sheet piles adopt standard sections of Larssen IV or U-shape, with a standard section length uniformly set at 12m. This achieves mass production of components in the factory and direct on-site application, eliminating the on-site processing workload caused by inconsistent cross-sectional dimensions in traditional support systems. Extensions of insufficient length are achieved through welding, requiring weld quality no lower than Grade II, ensuring that the overall pile strength after extension is consistent with the parent material. Second, S32, through the dual requirements of applying sealing grease to the interlocking joints and ensuring an interlocking gap no greater than 2mm, enables the steel sheet pile continuous wall to possess reliable water-stopping function while retaining soil, preventing water accumulation inside the pit and soil erosion outside the pit due to seepage in coastal high groundwater level strata. Secondly, S33 determines the number and spacing of steel supports based on the depth of the foundation pit and the distribution characteristics of soil pressure. The first support is no more than 0.5m above the ground, ensuring effective support is formed immediately after the surface soil is excavated and unloaded, minimizing the cantilever stress section length and pile top displacement of the sheet piles. Finally, S34, through the cooperation of the flexible joint and double H-beam walers, ensures that the pre-applied axial force of the steel supports is uniformly and reliably transferred to the sheet piles. With a pre-applied level of 60%–80% of the design axial force, it immediately provides reverse support force to the sheet piles after support installation, actively compensating for the stress release caused by soil excavation and unloading, thus transforming the deformation of the support system from passive resistance to active control. Therefore, the modular prefabricated support system transforms a large amount of on-site wet work and welding work in traditional support into factory prefabrication and on-site assembly, significantly improving construction speed and component quality. Simultaneously, the active control of prestress keeps the foundation pit deformation within a small range, providing a reliable guarantee for the safe construction of dense water tank groups.

[0031] More specifically, in S31, the cross-sectional form of the sheet piles is selected based on the geological survey report and the excavation depth. For excavations with an excavation depth not exceeding 10m, the Larsen IV type is preferred; for excavations with an excavation depth of 10-15m, the U-type is selected. The total length of a single sheet pile is determined by "excavation depth + embedment depth," and the embedment depth is verified using the circular arc sliding method or the elastic support method, generally being 0.8-1.2 times the excavation depth, with the upper limit taken for soft soil strata. Sheet piles are supplied from the factory in 12m standard sections. When the required pile length exceeds 12m, on-site installation is used... Two or more standard sections are welded together, with the joint located at a point where the bending moment of the pile is relatively small, i.e., at a distance of not less than 1 / 3 of the pile length from the top or bottom of the pile. The number of joints on the same cross section shall not exceed 50%. Welding shall be carried out using CO2 gas shielded welding or submerged arc welding. The welding wire shall be of a type that matches the strength of the base material. The weld type shall be butt V-groove welding. The weld shall be preheated to 100-150 degrees Celsius before welding. After welding, ultrasonic testing shall be performed. The weld quality grade shall not be lower than the Class II standard specified in GB50205. The proportion of welds tested in the same batch shall not be less than 20%. In S32, steel sheet pile construction uses a crawler-mounted pile driver equipped with a hydraulic vibratory hammer or a hydraulic static pressure pile driver for pile driving. Before driving the pile, a sealing grease of 1-2 mm thickness is evenly applied to the interlock of each pile using a brush. During pile driving, the male interlock is aligned with the female interlock that has already been driven into the pile. The verticality deviation is controlled to be no more than 0.5% by the pile driver guide frame. The pile is first slowly pressed in for 1-2 m to ensure that the interlock is properly engaged, and then the pile is continuously driven to the design elevation. During the pile driving process, the condition of the interlock joint is monitored at any time. After engagement, the joint gap is checked with a feeler gauge. A gap of no more than 2 mm at any position is considered qualified. For areas with gaps exceeding the standard, a water-swellable sealing rubber strip is inserted into the inside of the interlock or post-pile grouting is performed for sealing. In S33, modular steel supports are selected based on stress calculation results. When the support span is no more than 6m and the axial force design value is no more than 2000kN, spiral welded pipes with a diameter of 609mm and a wall thickness of 16mm are used; when the support span is 6-8m and the axial force design value is no more than 3000kN, hot-rolled H400×400×13×21 steel is used. The number of steel supports is determined according to the excavation depth of the foundation pit. Two supports are set for excavation depths no more than 6m, three supports for 6-10m, and four supports for depths greater than 10m. The vertical position of each support is determined by the support design drawings based on the earth pressure distribution and the construction requirements of the main structure. The horizontal spacing is adjusted within the range of 3-6m according to the foundation pit plane dimensions and the support bearing capacity. When the plane dimensions exceed the applicable span of the support, intermediate steel columns are set to reduce the calculated span of the support. The distance between the top surface of the first steel support and the top surface of the steel sheet pile or the ground is no more than 0.5m to constrain the pile top displacement in a timely manner.In S34, the waler consists of two H-beams of the same specification arranged back-to-back, with a web spacing of 200-300mm. Along the longitudinal direction, every 1-1.5m, the flanges of the two steel sections are welded together using gusset plates with a thickness of not less than 12mm. The gusset plates and steel flanges are fully welded using fillet welds. The waler is supported on the inside of the sheet piles by steel brackets or welded angle steel supports. The gap between the waler and the sheet piles is filled tightly with steel wedges or fine aggregate concrete to ensure reliable force transmission. A hinge is installed at each end of the steel support. The movable end of the hinge is prestressed using a hydraulic jack. The prestressed axial force is controlled at 60%-80% of the design axial force. A graded loading method is used, with 20% of the design axial force applied at each stage. After each stage of loading, the displacement and axial force changes are observed for 5 minutes. If no abnormalities are found, the next stage is applied. Once the target value is reached, steel wedges are driven in to lock the waler and spot-welded to prevent loosening.

[0032] In one embodiment, the steps of excavating each individual foundation pit in layers and sections according to the planned excavation sequence, installing steel supports at the corresponding elevation after each layer is excavated, and constructing the foundation layer after excavating to the designed elevation of the base include: S41: When the pool group includes no less than 3 individual pools, the skip-cell method shall be used for excavation. The time interval between excavations of adjacent individual pools shall not be less than 7 days. The time interval between excavations refers to the time from the completion of the concrete pouring of the bottom slab of the previous individual pool to the start of excavation of the adjacent individual pool. S42: Each of the aforementioned individual foundation pits is divided into 2 to 3 excavation sections along the longitudinal direction, each section being 15 to 25m long. A stepped slope is left at the junction of the excavation sections, with a step width of not less than 1.5m and a height difference of not more than 2m. S43: The excavation depth of each layer shall not exceed 2m. After the excavation is completed, the modular steel support of the layer shall be installed and prestressed within 4 hours. The over-excavation depth shall not exceed 100mm.

[0033] It should be noted that the skip-excavation method refers to a construction method in which, in the construction of a group of multiple individual foundation pits, excavation is not carried out continuously in an adjacent order, but rather in batches according to a pre-planned skip-excavation sequence, selecting individual foundation pits at intervals for excavation and structural construction. The adjacent batch of foundation pits is excavated only after the bottom slab concrete of the first batch of foundation pits has been poured and reached a certain strength. The excavation time interval refers to the number of days elapsed from the date the bottom slab concrete of the first foundation pit in two adjacent individual foundation pits is poured to the date the earthwork excavation of the subsequent foundation pit begins. This interval is used to ensure that the bottom slab concrete of the first foundation pit hardens and forms sufficient lateral stiffness. The stepped slope refers to the practice of retaining a section of soil at the boundary between the current excavation section and the next section to be excavated during segmented excavation, without excavating to the design elevation, forming a stepped temporary slope. The top surface of the step allows passage for construction personnel and equipment, while also providing lateral restraint for the next section of excavation. The over-excavation depth refers to the difference between the actual excavation surface and the designed excavation elevation. Excessive over-excavation will cause the stress state of the support structure to deviate from the design assumptions and increase the amount of cushion material used.

[0034] Specifically, this embodiment constructs a spatiotemporal effect collaborative control system for multi-pit construction by using spatial intervals for skip-excavation, longitudinal constraints for segmented excavation, and time control for time-limited support. This minimizes the cumulative soil unloading and support deformation caused by simultaneous excavation of multiple pits. First, S41 stipulates that when a pool group contains no fewer than three individual pools, skip-excavation must be used, and the time interval between excavations of adjacent pits must be no less than seven days, starting from the completion of the concrete pouring of the bottom slab of the first pit. This ensures, from a temporal perspective, that the first pit has sufficient structural rigidity before the excavation of adjacent pits, effectively resisting the lateral earth pressure transmission caused by the unloading of adjacent pits during excavation, and avoiding the superposition of stress and uncontrolled deformation of the support structure caused by simultaneous excavation of adjacent pits. Secondly, S42 divides each individual foundation pit longitudinally into 2-3 excavation sections, limiting the section length to 15-25m. Stepped slopes are left at the section boundaries, thus spatially controlling the volume and longitudinal range of soil unloaded in one go. The stepped soil provides lateral constraints for subsequent excavation sections and also provides a working platform for construction equipment, reducing the longitudinal exposure length of the foundation pit and the accumulation of deformation. Finally, S43 limits the excavation depth of each layer to within 2m and completes the installation of steel supports and prestressing within 4 hours of excavation completion. This ensures, in terms of construction rhythm, that the support structure can quickly enter a stressed state after each layer of soil is unloaded, minimizing the unsupported exposure time. Over-excavation depth not exceeding 100mm ensures the consistency between the actual stress state of the support structure and the design conditions. The synergistic effect of the three elements decouples the soil displacement field in both time and space during the construction of the multi-pit pits. The support system always operates within the designed stress state, and the deformation of the foundation pits is significantly reduced compared with conventional continuous excavation methods, ensuring the structural safety and stability of the surrounding environment during the construction of the dense pool group.

[0035] More specifically, in S41, the specific implementation steps of the skip-cell excavation method are as follows: First, based on the overall layout plan of the pool group and the planar position relationship of each individual pool, all individual pools are divided into the first and second batches according to the principle of "excavating one every three" or "excavating one every two". When the pool group is arranged in rows and columns, the selection is generally based on the row and column intervals. When the pool group is arranged in a ring or irregular pattern, the individual pool with the largest spacing is selected as the first batch. The foundation pit of the first batch of individual pools is excavated in layers and sections according to the provisions of S42 and S43, and the bottom slab concrete is poured. The excavation of the second batch of adjacent single foundation pits can only begin after the concrete strength reaches more than 70% of the design strength standard value through testing of test blocks cured under the same conditions. The time interval between excavations of adjacent single foundation pits shall be calculated from the date of completion of the previous batch of foundation slab concrete pouring, and the actual interval shall be confirmed by the construction log and shall not be less than 7 days. In the implementation of the skip-pour method, if there is a close-range working condition where the distance between adjacent single foundation pits is less than twice the excavation depth, the skip-pour interval of this section shall be appropriately extended to no less than 10 days or the adjacent foundation pit shall be excavated after the sidewall of the previous foundation pit has been backfilled. In S42, the longitudinal segmentation of each individual foundation pit is determined based on the pit's planar length. If the length is no more than 30m, it is divided into 2 segments; if it is 30-50m, it is divided into 3 segments. The segmentation location is preferentially chosen at the design post-cast strip or construction joint of the main structure's base slab, ensuring the segment joints coincide with structural joints to minimize the impact on the overall structure. The stepped slope at the junction of segment excavations is maintained as follows: After the first excavation segment reaches the current layer's design elevation, a step with a width of no less than 1.5m is retained at the longitudinal end. The top elevation of the step is the current excavation layer's bottom elevation, and the step's elevation difference does not exceed 2m. The step slope is determined based on the soil's self-stabilizing capacity, generally 1:0.5 to 1:1. The step slope and top surface are temporarily protected using wire mesh spraying or waterproof fabric covering to prevent rainwater erosion and cracking / collapse. The subsequent excavation segment continues excavation from the top of the step, and the step soil is removed along with the excavated soil during the construction of the subsequent excavation segment. In S43, each layer of excavation is carried out sequentially by excavators from top to bottom, from the center to the edge, or from one end to the other. During the excavation process, measuring instruments are used to control the excavation elevation. When the elevation is 200-300mm below the designed bottom elevation of the layer, manual labor combined with small machinery is used to clear the bottom to prevent over-excavation. Immediately after the excavation of each layer is completed, the following work must be completed within 4 hours: hoisting the corresponding steel support components to the installation position, aligning and connecting the flexible joints at the ends of the steel supports with the walers, and installing hydraulic jacks and loading them according to the graded loading procedure. Apply prestress of 60% to 80% of the design axial force, drive in steel wedges to lock the movable head, and fix it with spot welding; if the support installation cannot be completed within 4 hours due to special reasons, emergency measures such as temporary support or soil backfilling should be taken to limit the displacement of the steel sheet piles, and the cause and treatment measures should be recorded; the over-excavation depth should be measured with a steel ruler or level, and it is considered qualified if the over-excavation is not greater than 100mm at any measuring point. If the over-excavation exceeds the allowable value, backfill with concrete or graded sand and gravel of the same grade as the cushion layer to the design elevation and compact it. Loose soil must not be used for backfilling.

[0036] In one embodiment, the layered paving thickness is controlled at 200-300 mm, and a vibratory roller is used for compaction, with no less than 6 passes and a compaction degree of no less than 0.93. The steps for forming a green foundation bearing layer after passing the bearing capacity test include: S61: The bearing capacity of the foundation after each layer of compaction is tested by static cone penetration test or light dynamic cone penetration test. The spacing between test points is no more than 20m, and there are no less than 3 test points for each individual water tank. S62: When the tested bearing capacity is lower than 150 kPa, add 3% to 5% of the cement-based curing agent to the area and remix and compact until the bearing capacity meets the design requirements; S63: After each layer is compacted, the compaction degree is tested by the ring cutter method or the sand cone method. The compaction degree is not less than 0.93 to be qualified. If the unqualified area is turned over and re-laid and compacted until the compaction degree is qualified.

[0037] It should be noted that: the static cone penetration test refers to an in-situ testing method that uses a static cone penetrometer to press a conical probe into the soil at a constant rate, and measures the changes in the cone tip resistance and sidewall friction with depth using sensors to evaluate the soil strength and stratification characteristics. The light dynamic cone penetration test refers to an in-situ testing method that uses a 10kg drop hammer to fall freely from a height of 500mm, driving the conical probe into the soil, and recording the number of blows N10 for every 100mm of penetration, thereby calculating the soil bearing capacity. The ring cutter method refers to a laboratory testing method that uses a standard ring cutter of known volume and mass to sample from the compacted layer, weighs the soil sample inside the ring cutter, determines the moisture content, calculates the wet density and dry density of the soil sample, and then obtains the degree of compaction. The sand cone method refers to a field testing method that excavates a regular test pit on the surface of the compacted layer, pours standard sand into the test pit, and measures the mass and volume of the poured sand to calculate the volume of the test pit and the density and degree of compaction of the soil sample excavated from the pit. The cement-based curing agent admixture refers to a targeted reinforcement process in which a certain proportion of cement-based curing agent is added to local areas where the bearing capacity does not meet the requirements, and then the mixture is mixed and compacted a second time.

[0038] Specifically, in this embodiment, a dual-index parallel detection system for bearing capacity and compaction degree is established, and reinforcement treatment plans are given respectively for unqualified situations, so that the construction quality of the green foundation bearing stratum extends from "process control" to the closed-loop management of "result verification" and "defect repair". First, in S61, in-situ testing methods such as static cone penetration test or light dynamic cone penetration test are used to measure the bearing capacity of the foundation after each layer of compaction. Compared with indoor geotechnical tests, in-situ testing can reflect the true bearing performance of the soil under the actual stress state. The spacing between detection points is not greater than 20 m and there are no less than 3 detection points for each single water tank, ensuring the representativeness and coverage density of the detection data in the plane space. Secondly, in S62, for local weak areas with a bearing capacity lower than 150 kPa, reinforcement measures of adding 3% - 5% cement-based curing agent and remixing and compacting are specified. This additional admixture amount is a secondary supplement based on the original modifier, aiming to rapidly improve the strength of the weak area by increasing the dosage of cementitious materials, and the additional admixture range is for local non-compliant areas rather than large-area rework, taking into account both the reinforcement effect and construction economy. Finally, in S63, compaction degree detection is used as a parallel mandatory inspection index for each layer of compaction, and the compaction degree data is obtained by the core cutter method or sand replacement method. The compaction degree not less than 0.93 is qualified, and the unqualified area is completely turned over and re-spread and compacted, avoiding the hidden danger of "solid on the surface and loose inside" caused by only surface repair. The dual indexes of bearing capacity and compaction degree evaluate the foundation quality from two dimensions of strength and density respectively, and they complement and verify each other. Qualified compaction degree is the basis of bearing capacity, and qualified bearing capacity is the goal of compaction degree, forming a complete quality assurance system to ensure that the water tank structure does not undergo excessive settlement and uneven deformation under long-term service loads.

[0039] More specifically, in S61, bearing capacity testing is conducted 24 hours after each layer is compacted and allowed to stand for a period of time to allow pore water pressure in the soil to dissipate fully and cementitious materials to undergo initial hydration. Testing points are arranged using a grid method or a staggered pattern, with a spacing of no more than 20m between adjacent points. Testing points are appropriately densified in areas with significant stress changes, such as the edges and corners of the foundation pit. Each individual water tank has at least three testing points. When using static cone penetration testing, the probe is continuously driven in at a constant rate of 20mm / s, with data acquisition intervals no greater than 10 seconds. 0mm, the cone tip resistance qc and the bearing capacity characteristic value fak are converted according to the regional empirical formula fak equals qc divided by Nk, and the value of Nk is taken as 8~15 according to the soil properties; when using light dynamic penetration test, the hammer falls freely and the hammering energy is kept consistent. Record the number of hammer blows N10 for every 100mm penetration. The bearing capacity characteristic value is initially estimated by fak equals 8 multiplied by N10 minus 20 (kPa); if abnormal resistance such as rocks or hard layers is encountered during the test and causes a sudden change in data, it should be noted in the record and a new test point should be added nearby. In S62, when the bearing capacity test value at a certain test point is lower than 150 kPa, the area extending outwards from that test point by at least one test point spacing is designated as the substandard area. First, an excavator or road mixer is used to loosen all the compacted improved soil material in this area, with the loosening depth consistent with the layer thickness. After loosening, the soil clods are broken up to restore the soil to a loose state. Then, the amount of cement-based hardener added is calculated based on 3%–5% of the dry weight of the soil material in the substandard area. When the bearing capacity test value is within the specified range... When the pressure is between 120 and 150 kPa, take 3%; when the pressure is between 100 and 120 kPa, take 5%; when the pressure is below 100 kPa, expand the area of ​​non-compliance and re-evaluate the improvement plan. Spread the cement-based hardener evenly on the surface of the loosened soil, mix it with a road mixer at least twice to ensure that the admixture is fully mixed with the soil. After mixing, re-compact the soil according to the paving thickness and compaction process specified in S23. After compaction, conduct the bearing capacity test again until the test value reaches above 150 kPa.In S63, compaction degree testing and bearing capacity testing are conducted simultaneously, with no fewer than 3 ring cutter or sand cone testing points per 1000 m² for each layer. For ring cutter sampling, the top 10-20 mm of loose soil is removed first, the ring cutter is vertically pressed into the compacted layer, and after removal, excess soil at both ends is trimmed and weighed. For sand cone testing, a circular test pit with a diameter of approximately 200 mm and a depth consistent with the thickness of the compacted layer is dug on the surface of the compacted layer using a chisel and small shovel. All soil excavated from the test pit is collected and weighed, and standard sand is poured into the test pit at a constant drop height to measure the pit volume. Compaction degree is calculated based on the dry density and the maximum dry density determined by the indoor compaction test. The maximum dry density test is conducted according to the geotechnical... The test method shall be conducted according to standard procedures, using heavy compaction or vibratory compaction. A compaction degree of not less than 0.93 is considered acceptable. For areas with a compaction degree lower than 0.93, an excavator shall be used to turn over the entire layer of improved soil in the unacceptable area, turning it over to the bottom surface of the layer. The turned-over soil shall be broken up and remixed evenly using a road mixer. If necessary, water shall be added or an amendment shall be added to adjust the moisture content and the amount of amendment. Then, the soil shall be laid and compacted again according to the paving thickness and compaction process specified in S23. After compaction, the compaction degree shall be tested again until it is acceptable. If the same area is over-excavated and re-paved more than twice and still fails to meet the requirements, the cause shall be analyzed and the amendment ratio or paving thickness shall be adjusted.

[0040] In one embodiment, the dosage of the amendment is 8% to 15% of the dry weight of the backfill soil, and the mixture is thoroughly mixed using a road mixer or stabilized soil mixing equipment. The step of ensuring the uniformity of the mixture is visually free of gray-white patches includes: S71: Measure the natural moisture content of the backfill material before mixing. If the moisture content is higher than the optimum moisture content +2%, turn it over and dry it to reduce the moisture content. If the moisture content is lower than the optimum moisture content -2%, add water to moisten it. S72: In rainy season construction conditions, the improved soil material that has been laid should be dried until the moisture content is no more than +2% of the optimum moisture content before it can be compacted. S73: The improved soil material shall be laid and compacted within 2 hours after mixing. If the time limit is exceeded, the improved soil material shall be loosened and mixed again before construction.

[0041] It should be noted that: the optimal moisture content refers to the moisture content at which the improved soil material reaches its maximum dry density under specified compaction work, determined by indoor heavy compaction tests or vibratory compaction tests, and is a core parameter for controlling the compaction quality of improved soil material. The natural moisture content refers to the moisture content of the backfill soil material under natural conditions, determined by the drying method or alcohol combustion method. The sun-drying refers to construction measures that utilize solar radiation and natural wind to reduce the moisture content of the soil material, mechanically spreading soil with excessive moisture content into a thin layer and periodically turning it over to accelerate moisture evaporation. The watering and wetting refers to construction measures that use water trucks or spray equipment to evenly replenish moisture to soil with low moisture content and allow it to infiltrate. The rainy season construction environment refers to construction operations carried out during the rainy season or in weather conditions where rainfall is forecast. In such cases, the improved soil material that has been spread but not compacted is at risk of being soaked by rainwater, leading to excessive moisture content. The term "overtime soil material" refers to improved soil material that has not been laid and compacted within 2 hours of mixing. At this time, the cement-based curing agent has undergone a preliminary hydration reaction, reducing the plasticity of the soil material and making compaction difficult.

[0042] Specifically, this embodiment uses moisture content and post-mixing aging as two core control lines, running through the entire process of improved soil from pre-mixing treatment to compaction, ensuring that the improved soil is always in a compactable state and eliminating compaction quality defects caused by moisture deviation and initial cement setting. First, S71 measures and pre-regulates the natural moisture content of the backfill soil before mixing, using the optimal moisture content as the center and ±2% as the allowable deviation range. If the moisture content is too high, it is reduced by sun-drying; if it is too low, it is supplemented by watering, so that the soil has a near-optimal moisture content before mixing, laying the foundation for uniform dispersion of the improver and subsequent compaction. Second, S72 specifies the moisture content standard and treatment method for resuming work after rain in the case of improved soil that has been laid during rainy season construction. It requires that the soil soaked by rainwater be dried until the moisture content is no more than the optimal moisture content +2% before continuing to compact, preventing the "springy soil" phenomenon and insufficient compaction caused by excessive moisture content. Finally, S73 strictly limits the working time window from the completion of mixing to the completion of paving and compaction to 2 hours. Improved soil materials exceeding this time limit must be loosened and mixed again before construction, as the cement-based curing agent has already begun to set and the plasticity of the soil material has significantly decreased, to ensure compaction effect and strength uniformity. The three control lines correspond to three key nodes: before mixing, before compaction, and after compaction. From the two dimensions of moisture control and time management, a preventative and process-interrupting mechanism for the construction quality of improved soil materials is constructed, reliably ensuring the compaction degree and uniformity of the green foundation bearing layer and avoiding subsequent foundation settlement and uneven deformation caused by compaction defects.

[0043] More specifically, in S71, the natural moisture content of the backfill material is determined by drying or alcohol combustion before mixing. Sampling points are arranged at least one per 500 m², and the average moisture content of each point is taken as the representative value of the natural moisture content of that batch of soil. The optimum moisture content is determined by indoor heavy compaction tests, conducted according to geotechnical testing methods, using a standard compaction work of 5 layers and 56 blows per layer. When the natural moisture content is higher than the optimum moisture content by 2%, a grader or bulldozer is used to spread the backfill material into a thin layer of 200–300 mm thickness on the construction site, and it is then turned over and dried using sunlight and natural wind every 1–2 days. Turn the soil over every 2 hours with a plow or road mixer to accelerate moisture evaporation. During the turning and drying process, continuously monitor the moisture content changes with a rapid moisture content meter until the moisture content drops to within the range of optimum moisture content +2% before proceeding to the mixing process. When the natural moisture content is lower than the optimum moisture content -2%, use a water truck with spray nozzles to evenly spray water on the soil. The amount of water sprayed is calculated and determined based on the moisture content difference, soil dry density, and spreading area. After spraying water, turn the soil over once with a road mixer or plow to make the moisture initially uniform. Then let it stand for 30 to 60 minutes to allow the moisture to fully penetrate and diffuse. Measure the moisture content again to confirm that it has reached or exceeded the range of optimum moisture content -2% before proceeding to the mixing process. In S72, during the rainy season, weather forecasts should be checked before construction to avoid spreading the improved soil material before forecasted rainfall. If rain occurs during construction, spreading and compaction should be stopped immediately. The spread but uncompacted improved soil material should be completely covered with waterproof cloth or plastic film, and temporary drainage ditches should be dug around the site to drain accumulated water. After the rain stops, the covering should be removed to check the moisture content of the improved soil material. If the moisture content is higher than the optimum moisture content + 2%, the drying method described in S71 should be used. Compaction can only be resumed after the moisture content is no higher than the optimum moisture content + 2%. If continuous rainy weather makes drying difficult, a temporary rain shelter should be erected above the improved soil material, or water-absorbing materials such as quicklime powder should be added to accelerate the reduction of moisture content. The amount of quicklime added should be controlled at 2% to 5% of the dry soil weight, depending on the degree of moisture content exceeding the standard. After adding quicklime, the mixture should be mixed evenly with a road mixer and left to stand for 24 hours. The moisture content should be retested after it is qualified before compaction.In S73, the time when the mixing equipment completes the last mixing and leaves the work surface is taken as the starting point for the timing of the completion of mixing. The construction management personnel should record this time in the construction log. Within 2 hours after the mixing is completed, all of the following operations should be completed: spreading, rough leveling and fine leveling, and all passes of the rolling process (static compaction, vibratory compaction and finishing). If the rolling cannot be completed within 2 hours due to uncontrollable reasons such as construction machinery failure or sudden weather changes, the batch of improved soil that has exceeded the time limit should be treated as follows: Use a road mixer or rotary tiller to loosen the spread or partially rolled improved soil to the full depth, with the loosening depth consistent with the spreading thickness. After loosening, break up the lumps formed by cement hydration to restore the soil to a loose state. Immediately after loosening, take a sample to determine the moisture content and visually check the uniformity of the improver. If the moisture content is too low, add water appropriately. If the improver is unevenly distributed, mix 1-2 more times. After the treatment, the improved soil should be spread and rolled again according to the spreading thickness and rolling process specified in S23. Rolling must be completed within 2 hours of the restart time. If the same area is subjected to repeated treatments due to exceeding the time limit more than twice, the batch of improved soil should be discarded and re-prepared for construction.

[0044] In one embodiment, the step of using modular steel sheet piles as retaining structures around the perimeter of each individual foundation pit in the pool group, with interlocking connections between the steel sheet piles, and vertically segmented modular steel supports installed inside the steel sheet piles to form a prefabricated support system includes: S81: When the distance between adjacent individual water tanks is less than twice the excavation depth of the foundation pit, the common support area between the two individual foundation pits shall adopt the reinforcement measures of double rows of modular steel sheet piles or deepening the penetration depth of the modular steel sheet piles. S82: The spacing between the rows of the modular steel sheet piles is 1.5 to 2.5 m, and tie rods or steel tie rods are installed between the rows. The horizontal spacing of the tie rods is 2 to 3 m. S83: The first modular steel support of the reinforced section adopts a bracing form to connect two adjacent foundation pits, forming an overall load-bearing frame.

[0045] It should be noted that: the shared support zone refers to the area where the unloading areas of the excavated soil from two adjacent individual foundation pits overlap due to insufficient spacing. The support structure in this area must simultaneously resist the soil pressure from both foundation pits. The double-row modular sheet piles refer to a support form where one row of modular sheet piles is installed on each side of the shared support zone, and the two rows of piles are connected by connecting members to form a frame-like spatial load-bearing structure. The tie rods are tension members connecting the double rows of sheet piles, typically made of finely rolled threaded steel bars or steel strands, with both ends anchored to the walers of the two rows of sheet piles by anchorages, coordinating the deformation of the piles on both sides and ensuring they share the load. The steel tie rods are rigid tie rod members made of round steel or shaped steel, connecting the double rows of sheet piles, with both ends connected to the walers of the two rows of sheet piles by welding or bolts. The aforementioned bracing refers to a steel support that starts from the waler of one side of the foundation pit, runs through the shared support section, and connects to the waler of the other side of the foundation pit, forming a continuous horizontal force transmission member that balances and cancels out the soil pressure on both sides of the foundation pit.

[0046] Specifically, this embodiment addresses the situation where the close proximity of adjacent foundation pits in a pool group leads to superimposed stress on the support structure, and proposes a graded reinforcement scheme for the shared support zone. First, S81 establishes the initiation criterion for reinforcement measures: the distance between adjacent individual pools is less than twice the excavation depth of the foundation pit. In this case, the active earth pressure zone and passive resistance zone caused by the excavation of the foundation pits on both sides intersect, and a single row of sheet piles cannot simultaneously resist the unloading effect of the soil on both sides, necessitating reinforcement measures. Optional solutions include double-row sheet piles or increasing the depth of the piles. The former increases the overall stiffness by increasing the number of rows and the connections between rows, while the latter increases the resistance to overturning by increasing the length of the resistance arm in the passive zone in front of the piles. Secondly, S82 specifies key parameters for the double-row scheme: a row spacing of 1.5–2.5m ensures an effective framework for load-bearing space between the double rows of piles. Too small a spacing results in the double rows of piles being approximately subjected to single-row loads, with insignificant reinforcement; too large a spacing leads to excessively long connecting members and reduced stiffness. A horizontal spacing of 2–3m for the tie rods or steel tie rods ensures effective force transmission density and uniformity between rows, coordinating the displacement of the two rows of piles. Finally, S83 adopts a through-bracing method for the first steel support in the reinforced section, connecting two adjacent foundation pits. This through-bracing converts the earth pressure on both sides into axial force and self-balances within the support rod, avoiding the eccentric load effect caused by the walers and sheet piles in the shared section bearing unilateral earth pressure. This creates a unified load-bearing framework on both sides, significantly improving overall stiffness. Therefore, by combining double-row piles with tie rods or by increasing the depth of the piles and adding through bracing, the two independent foundation pits that originally interfered with each other can be transformed into a unified support system that works together to resist force. This effectively suppresses horizontal displacement and surface settlement in the shared section and ensures the safety of construction of the close-knit group of pits.

[0047] More specifically, in S81, when the distance between adjacent individual water tanks is less than twice the excavation depth of the foundation pit, a special design calculation is first performed on the shared support area. Using the elastic support method or finite element numerical analysis method, an overall model including the two adjacent foundation pits and the intermediate soil is established to calculate the internal forces and deformations of the shared section support structure under the most unfavorable excavation conditions. When the calculated deformation exceeds the allowable value, reinforcement measures are taken. When adopting the double-row modular steel sheet pile scheme, the piles closer to the first excavated foundation pit are the front row piles, and the piles closer to the second excavated foundation pit are the rear row piles. The top elevations of the two rows of piles are consistent, and the penetration depth is the same, or the rear row piles are appropriately deepened according to the stress conditions. When adopting the scheme of increasing the penetration depth, the penetration depth of the modular steel sheet piles in the shared section is increased to 1.5 to 2.0 times the conventional penetration depth. The overturning and sliding safety factors are improved by deepening the passive earth pressure zone in front of the piles. In S82, the spacing between double-row modular steel sheet piles is controlled at 1.5 to 2.5 m. The smaller value is used when the width of the shared section (distance between adjacent pools) is small, and the larger value is used when the width is large. The tie rods between rows are made of finely rolled threaded steel bars with a diameter of 25 to 32 mm and a strength grade of not less than PSB785. The ends of the tie rods are anchored to the walers on the inner side of the two rows of piles through anchor plates and nuts. The prestress applied to the tie rods is 50% to 70% of the design tension, and is applied using a torque wrench or a through-hole jack. If steel tie rods are used, round steel or steel sections with a diameter of not less than 30 mm are selected. The ends of the tie rods are welded to the walers, and the length and height of the weld are calculated according to the equal strength connection. The tie rods or steel tie rods are set vertically according to the position of each waler, with a horizontal spacing of 2 to 3 m, and are evenly distributed within the length of the shared section. In S83, the first steel support of the reinforced section adopts the form of a bracing system. The bracing system uses steel pipes or H-beams of the same specification as the individual foundation pit steel support or one size larger. The length is equal to the width of the shared section plus the distance from the outer edge of the walers on both sides to the end of the steel support. The two ends of the bracing system are connected to the walers of the foundation pit on both sides through flexible joints. The pre-stressed axial force is the same as that of the individual foundation pit steel support, and is applied in stages from 60% to 80% of the design axial force. After the prestress is applied to the bracing system, the soil pressure on both sides is transmitted to the bracing system through the walers, and an axial pressure self-balance is formed within the bracing system. The walers on both sides and the sheet piles no longer bear unidirectional eccentric loads. When the length of the shared section is greater than 15m, a steel pipe column or lattice column is set in the middle of the bracing system to reduce the calculated length of the bracing system. The lower end of the column is supported on a concrete pier or steel pad on the bottom of the foundation pit.

[0048] In one embodiment, after the step of using modular steel sheet piles as retaining structures around the perimeter of each individual foundation pit in the pool group, with the steel sheet piles connected by interlocking joints, and modular steel supports installed vertically in segments inside the steel sheet piles to form a prefabricated support system, the method further includes: S91: Deformation observation points are set at the top of the modular steel sheet pile, the end of the modular steel support, and the ground around the foundation pit. The distance between the observation points is 15-25m. Displacement and settlement are observed using a level and a theodolite. The observation frequency is once a day during the excavation period and once every two days during the dismantling and backfilling period. S92: When the horizontal displacement of the top of the modular steel sheet pile exceeds 0.3% of the excavation depth of the foundation pit or the daily change rate exceeds 2mm / day, the excavation of that section of the foundation pit shall be stopped immediately, and temporary steel supports or counter-pressure earth platforms shall be added at the corresponding positions. Construction can only continue after the deformation has stabilized. S93: Add diagonal steel supports or corner reinforcements at the corners and stress concentration points of the foundation pit, wherein the angle between the diagonal steel supports and the horizontal modular steel supports is 45°~60°.

[0049] It should be noted that: the deformation observation points refer to fixed markers set in the support structure and surrounding environment for measuring displacement and settlement, usually constructed by pre-embedding steel bars, attaching reflectors, or setting measuring prisms. The level refers to an optical or electronic measuring instrument used to measure elevation differences, obtaining settlement data by observing changes in elevation between each measuring point and a benchmark. The theodolite refers to an optical or electronic measuring instrument used to measure horizontal and vertical angles, obtaining horizontal displacement data of the measuring points through intersection measurements or polar coordinate methods. The daily rate of change refers to the rate of change obtained by dividing the displacement change between two adjacent observations by the time interval, used to determine whether the deformation is in an accelerated development stage. The temporary steel support refers to additional steel support components added in an emergency when deformation exceeds limits, usually using steel pipes or H-beams of the same specifications as the permanent support, installed at or near the point of maximum deformation. The counterweight earth platform refers to temporary soil piled up at the point of significant deformation inside the pit, using the reverse pressure generated by the soil's own weight to limit the continued displacement of the support structure into the pit. The inclined steel support refers to a steel support arranged along the bisector of the corner of the foundation pit, with one end connected to the waler at the corner and the other end supported by embedded parts on the opposite waler or the bottom plate of the foundation pit. The corner reinforcement brace refers to a support component specifically used to strengthen the rigidity of the corner of the foundation pit, which can be a steel truss or a cast-in-place reinforced concrete support.

[0050] Specifically, this embodiment establishes an information-based monitoring and dynamic control mechanism to feed back deformation data during construction to excavation decisions, forming a closed-loop control of "monitoring—early warning—handling—retesting," ensuring the stress safety of the prefabricated support system throughout the entire construction cycle. First, S91 sets up an observation network covering key parts of the support structure and the surrounding ground surface. Observation points are arranged along the top of the sheet piles, the ends of the steel supports, and the ground around the foundation pit, with a spacing of 15–25 m, ensuring complete capture of the overall deformation pattern of the foundation pit. The observation frequency is differentiated according to the risk level of different construction stages: once a day during excavation to capture rapid changes, and once every two days during the dismantling and backfilling period to adapt to a slower pace of change. Secondly, S92 clearly defines dual early warning thresholds: a cumulative displacement threshold (0.3% of excavation depth) to control the ultimate bearing capacity of the support structure, and a daily rate of change threshold (2 mm / day) to capture the accelerating trend of deformation. Triggering either threshold initiates an emergency response procedure, including stopping excavation, adding temporary supports, or constructing a counterweight platform. Work can only resume after the deformation rate drops to a safe level and stabilizes, thus proactively intervening before the support structure approaches its ultimate limit. Finally, S93 addresses structural reinforcement at pit corners and stress concentration areas by installing diagonal steel supports along the angle bisector at corners, forming an angle of 45°~60° with the horizontal supports. This transforms the out-of-plane force at the corner into axial force from the diagonal supports, improving the stress state and deformation constraints at the corners. These three elements work together to form a complete safety assurance chain from real-time sensing to rapid response to structural reinforcement, controlling the maximum horizontal displacement of the pit within 0.3% of the excavation depth. Compared to construction methods without a monitoring and control system, this significantly improves deformation control accuracy and emergency response speed.

[0051] More specifically, in S91, deformation observation points are set up and observed as follows: A displacement observation point is set up every 15-25m at the top of the modular steel sheet piles. The observation points are L-shaped steel bar heads welded to the inner side of the pile top or attached reflective sheets. The top surface of the observation point is 10-20mm above the pile top and marked with red paint. An observation point is set up at the end of each modular steel support's hinge to directly observe the displacement of the hinge locking wedge or the compression of the support member. Within a range of at least one excavation depth from the edge of the pit, a surface settlement observation point is set up every 15-25m around the perimeter of the pit. The observation points are made of stainless steel leveling stones with protective covers buried in the undisturbed soil layer, with a burial depth of at least 500mm. Outside the influence zone of the pit, [further details are needed]. Establish no fewer than 3 fixed leveling benchmarks and 2 horizontal control points. The benchmarks should be located at least 3 times the excavation depth from the edge of the pit. Leveling should be performed using a DSZ2 or higher precision level instrument with an invar steel tape, following the second-order leveling specifications. The closure error should not exceed 0.3 mm multiplied by the square root of the number of stations. Horizontal displacement should be observed using a DJ2 or higher precision theodolite or total station, following the polar coordinate method or intersection method. The measurement error should not exceed 2 mm. The observation frequency should strictly adhere to once a day during excavation and once every two days during the dismantling and backfilling period. In case of increased deformation rate or abnormal conditions such as heavy rain, the frequency should be increased to twice a day or continuous observation should be performed as needed. After each observation, the data should be promptly processed, and cumulative displacement-time curves and deformation rate-time curves should be plotted to determine the deformation trend. In S92, the early warning and response procedures are as follows: After daily observation data is calculated, it is first determined whether the cumulative horizontal displacement has reached or exceeded 0.3% of the excavation depth H of the foundation pit. For example, when H is 8m, the early warning value is 24mm. At the same time, the daily change rate of the most recent 24 hours is calculated to determine whether it has reached or exceeded 2mm / day. If either of the above two conditions is met, the construction supervisor immediately issues an order to stop the excavation of that section of the foundation pit, all excavation machinery and personnel are evacuated from that section of the foundation pit, and the monitoring personnel are notified to increase the frequency of observations. The emergency response team arrives at the scene within 2 hours and formulates a response plan based on the location and degree of deformation exceeding the limit: if the deformation is concentrated in a certain missing or not installed support in time... In the affected area, immediately hoist emergency steel supports to the location, apply 50%–70% of the design axial force prestress at both ends using jacks, and then lock them in place. If the deformation range is large and it is difficult to add additional supports, build a counterweight earth platform inside the pit. The height of the platform should not be less than 1 / 3 of the pit depth of the deformed section, the top width should not be less than 2m, and the slope should be 1:1 to 1:1.5. Use an excavator to take soil from nearby areas, pile it up, and lightly compact it. The soil of the counterweight earth platform will be removed later after the formal supports are installed. After the treatment measures are implemented, continuously monitor the deformation changes. Once the daily change rate drops below 0.5mm / day and remains stable at this level for 3 consecutive days, the technical supervisor must confirm that the deformation has converged before excavation of that section of the pit can resume.In S93, the implementation plan for corner reinforcement of the foundation pit is as follows: At the four corners and plane turning points of the foundation pit, diagonal steel supports are installed along the angle bisector. One end of the diagonal support is connected to the waler at the corner via a specially welded diagonal support bracket. The bracket is made of steel plate with a thickness of not less than 20mm and is fully welded to the waler, with a weld quality of not less than Grade II. The other end of the diagonal support is supported on a steel plate embedded in the opposite waler or the foundation pit bottom slab concrete. The embedded steel plate has a specification of not less than 300mm × 300mm × 16mm and is embedded and anchored according to the design position before the foundation layer or bottom slab is poured. The angle between the diagonal support and the horizontal direction is 45°~60°, with 45° used when the side lengths on both sides of the corner are similar. When the difference in side lengths is large, take the smaller included angle closer to the longer side; the diagonal bracing uses steel pipes of the same specifications as the formal steel bracing or H-beams one grade larger, and the pre-applied axial force is 50% to 60% of the design axial force, lower than the pre-applied axial force of the formal horizontal bracing, to avoid generating excessive thrust on the main structure; for the corner of the foundation pit with small planar dimensions (side length less than 10m), corner reinforcement bracing can be used instead. The corner reinforcement bracing uses two steel sections welded at the corner to form a triangular planar truss, and the two legs are welded to the walers on both sides of the corner to form a rigid corner node; the diagonal bracing and corner reinforcement bracing are installed layer by layer as the foundation pit is excavated, and the installation time is synchronized with the horizontal steel bracing of that layer.

[0052] In one embodiment, after the construction of the water tank structure is completed and reaches the design strength, the steel supports are dismantled in sections from bottom to top, and the trench between the foundation pit sidewall and the outer wall of the water tank is backfilled simultaneously. The backfill material is locally modified backfill soil or industrial solid waste mixture, and the steps of compacting it in layers to the design ground elevation include: S101: The dismantling sequence of the modular steel support is the reverse of the installation sequence. First, dismantle the lowest layer of the modular steel support, backfill to the bottom elevation of the upper layer of the modular steel support, and then dismantle the upper layer of the modular steel support. S102: The backfill width of the fertilizer trench shall not be less than 0.8m. When the industrial solid waste mixture is used, the industrial solid waste mixture shall be composed of 20% to 30% steel slag or fly ash mixed with locally improved soil. The locally improved soil refers to the backfill soil treated with the industrial solid waste improver. S103: The thickness of each backfill layer shall not exceed 300mm. A small vibratory compactor or plate vibrator shall be used for compaction, with a compaction degree of not less than 0.90. After each layer of backfill is completed, a density test shall be conducted, and the next layer of backfill can only be carried out after the test is qualified.

[0053] It should be noted that: the "fertilizer trench" refers to the narrow space reserved between the side wall of the foundation pit and the outer wall of the water tank structure during the excavation of the foundation pit. Its width must meet the space requirements for construction operations and the construction of the waterproof layer on the outer wall of the water tank. The simultaneous removal of supports and backfilling means that after each support is removed, the area below that support is immediately backfilled, using the lateral constraint of the backfill soil to replace the removed steel supports, achieving a gradual transfer of stress on the support structure. The industrial solid waste mixture, as an option for backfill material, is made by mixing 20%–30% steel slag or fly ash with on-site improved soil treated with industrial solid waste modifiers, thus extending the green foundation construction chain. The small vibratory compactor refers to compaction equipment suitable for operation in narrow spaces, such as a walk-behind vibratory compactor or a frog-type rammer. Its base plate width is usually no more than 600mm, allowing it to move and operate within the fertilizer trench. The plate vibrator refers to compaction equipment that uses an electric motor to drive an eccentric block to generate vibration force, which is transmitted to the soil through a plate. It is suitable for compacting granular soil materials. The compaction test refers to the test to inspect the compaction quality of the backfill layer, which can be carried out by the ring cutter method, sand cone method or nuclear density meter method.

[0054] Specifically, this embodiment achieves a smooth transition of stress transformation in the support system through a simultaneous operation of "removing one layer of support and filling one layer of soil" and the green utilization of backfill materials. Simultaneously, industrial solid waste-modified soil is further applied to the backfilling of the fertilizer trench. First, S101 specifies a strict sequence and correspondence between support removal and backfilling. The steel support removal sequence must be the complete reverse of the installation sequence, proceeding layer by layer from the bottom. After each layer of support is removed, the corresponding fertilizer trench area must be backfilled to the bottom elevation of the previous layer's support, ensuring the backfill soil provides lateral support to the steel sheet piles before the next layer of support can be removed. This procedure avoids the risk of the steel sheet piles being in a cantilevered state due to lack of lateral support after support removal. The backfill soil gradually replaces the steel support, providing lateral constraint and achieving a smooth transition of the support system from temporary support to permanent backfilling. Second, S102 clarifies the minimum working width for fertilizer trench backfilling and the composition of the industrial solid waste mixture. A backfill width of no less than 0.8m ensures that small compaction machinery can enter the trench to operate, avoiding the inefficiency and unstable quality problems caused by manual compaction due to insufficient width. The industrial solid waste mixture consists of locally improved soil mixed with 20%–30% steel slag or fly ash. The secondary addition of steel slag or fly ash further improves the strength and stability of the backfill material, while extending industrial solid waste improvement technology from the foundation to the backfilling stage, realizing green material utilization throughout the entire process. Finally, based on the narrow space of the trench, S103 specifies layer thickness and compaction standards that match the construction conditions and equipment capabilities. The layer thickness is no more than 300mm, which is compatible with the effective compaction depth of small compaction equipment, and the compaction degree is no less than 0.90. While taking into account the backfilling period and cost, the density and stability of the backfill body are guaranteed. Each layer of backfilling can only be carried out after the density test of each layer is qualified, and process inspection ensures the final backfill quality, avoiding excessive settlement and collapse after backfilling.

[0055] More specifically, in S101, the specific implementation process for dismantling supports and backfilling is as follows: First, confirm that the bottom slab, walls, and top slab of the water tank structure have all been poured, and that the concrete strength has reached more than 75% of the design strength standard value through testing with test blocks cured under the same conditions; then, clearly define the number, elevation, and dismantling sequence of each steel support in the construction plan, and provide technical instructions to the construction personnel; the dismantling operation starts from the lowest layer of support (e.g., the fourth layer), first use hydraulic jacks to unload the support hinge head, loosen the locking wedges, release the support axial force, and after confirming that the axial force has been completely released, use gas cutting or hydraulic shearing to cut the connection between the support rod and the hinge head, and lift the support rod out of the pit in sections; immediately after the support is dismantled, begin backfilling the trench for that layer, and clear the trench before backfilling. Remove accumulated water, silt, and debris from the tank, and protect the waterproof layer on the outer wall of the tank (e.g., by laying protective boards or geotextiles). During backfilling, use a small excavator or manual labor to evenly deliver the backfill material from the top of the trench into the trench, spread it in layers, and compact it in layers. Stop backfilling when it reaches 100-200mm below the bottom elevation of the upper support (e.g., the third layer), leaving room for the upper support to be removed. After backfilling to this elevation, compact and level the top surface of the backfill. Only after the density test is qualified can the upper steel support be removed. Proceed upwards layer by layer in this order: remove the fourth layer, backfill to the bottom elevation of the third layer, remove the third layer, backfill to the bottom elevation of the second layer, remove the second layer, backfill to the bottom elevation of the first layer, remove the first layer, and backfill to the design elevation of the ground. In S102, the backfill width of the fertilizer trench is measured from the outer edge of the waterproof layer of the outer wall of the pool to the inner side of the sheet pile, and its minimum value shall not be less than 0.8m. When the designed fertilizer trench width is less than 0.8m, the backfilling operation space shall be ensured by adjusting the excavation size of the foundation pit or the position of the outer wall of the pool, or, with the consent of the designer, self-compacting backfill material (such as fluidized solidified soil or foamed concrete) shall be used for backfilling. When industrial solid waste mixture is used as backfill material, its preparation method is as follows: the improved soil material treated with industrial solid waste improver and tested as qualified in S20 shall be used as the base material (i.e., in-situ improved soil material). Add steel slag or fly ash at a ratio of 20% to 30% of the dry weight of the base material. The steel slag should be aged steel slag with a particle size of no more than 20 mm, and its free calcium oxide content should be no more than 3% and its water immersion expansion rate should be no more than 2%. The fly ash should be Grade II fly ash, with the same technical indicators as described in S21. Mix the improved soil material with the steel slag or fly ash thoroughly 2 to 3 times at the mixing site using a road mixer or stabilized soil mixing equipment. During the mixing process, add water as needed according to the moisture content of the mixture, and control the moisture content after mixing to be within the range of the optimum moisture content ±3%. After mixing evenly, transport it to the fertilizer backfilling point for use.In S103, during backfilling, the loose paving thickness for layered paving is controlled at 350 - 400 mm, so that the thickness after ramming is not greater than 300 mm; the ramming equipment uses a walking type vibrating rammer (self-weight not less than 80 kg, exciting force not less than 15 kN) or a plate vibrator (power not less than 2.2 kW, bottom plate size not less than 500 mm × 400 mm). When the width of the fat pocket is relatively large (not less than 1.5 m), a small walking type vibrating roller can also be used; the number of ramming passes is not less than 4 times. Taking the surface without obvious subsidence and wheel tracks as the visual standard, the ramming is carried out reciprocally along the longitudinal direction of the fat pocket, and the adjacent ramming tracks overlap not less than 1 / 3 of the bottom plate width; after each layer of backfilling and ramming is completed, a density test is carried out. The test method uses the core cutting method or the sand replacement method. There is not less than 1 test point per 50 running meters for each layer. The compaction degree not less than 0.90 is qualified; after the density test is qualified, it is reported to the supervision engineer for acceptance and signature before the next layer of backfilling can be carried out; after backfilling to the designed ground elevation, the top surface of the backfilled body is finally leveled and compacted. After the compaction degree and elevation are detected to meet the requirements of the design and specifications, the fat pocket backfilling process is completed, and the system conversion between support and backfilling is all completed.

[0056] The above description is only an exemplary embodiment of the present invention, and does not limit the protection scope of the present invention accordingly. Any equivalent structural transformation made under the technical concept of the present invention by using the content of the specification and drawings of the present invention, or any direct / indirect application in other related technical fields is included in the protection scope of the present invention.

Claims

1. A method for the coordinated construction of water tank group support and green foundation in complex backfill strata, characterized in that, The method for coordinating the construction of water tank group support and green foundation in complex backfill strata includes: Pre-treatment of complex backfill strata in coastal areas involves removing surface debris and excavating to the design elevation. In-situ crushing and screening are carried out in areas mixed with garbage and soil. Hard debris with a particle size greater than 50mm is screened out and transported off-site, while backfill soil with a particle size not greater than 50mm is retained. Industrial solid waste amendment is added to the pretreated backfill material and mixed evenly. The mixture is then spread and compacted in layers to form a green foundation bearing layer with design bearing capacity. Modular steel sheet piles are used as retaining structures around the foundation pits of each individual unit in the pool group. The modular steel sheet piles are connected by interlocking joints, and modular steel supports are set vertically in sections on the inner side of the modular steel sheet piles to form a prefabricated support system. According to the planned excavation sequence, each individual foundation pit is excavated in layers and sections. After each layer is excavated, the modular steel support at the corresponding elevation is installed in a timely manner. After excavating to the design elevation of the foundation, the subbase is constructed. After the water tank structure is completed and reaches the design strength, the modular steel supports are dismantled in sections from bottom to top. At the same time, the trench between the foundation pit sidewall and the outer wall of the water tank is backfilled. The backfill material is the locally modified backfill soil or industrial solid waste mixture, which is compacted in layers to the ground design elevation.

2. The method for coordinated construction of water tank group support and green foundation in complex backfill strata as described in claim 1, characterized in that, The steps of pre-treating complex backfill strata in coastal areas, removing surface debris and excavating to the design elevation, crushing and screening areas mixed with waste soil in situ, separating hard debris with a particle size greater than 50mm and transporting it off-site, and retaining backfill soil with a particle size no greater than 50mm include: The area containing the mixed waste soil was crushed in situ using a hydraulic breaker, and the particle size after crushing was controlled to be below 200mm. The crushed soil was screened in two stages using a vibrating screen. The first stage screen had a mesh size of 50 mm, and the second stage screen had a mesh size of 20 mm. Hard debris such as bricks, concrete blocks, and stones with a particle size greater than 50mm will be transported to a designated disposal site. Coarse aggregate with a particle size of 20-50mm will be retained as foundation cushion aggregate, and fine soil with a particle size less than 20mm will be used for subsequent improvement and mixing.

3. The method for coordinated construction of water tank group support and green foundation in complex backfill strata as described in claim 1, characterized in that, The steps of adding industrial solid waste amendment to the pretreated backfill material, mixing it evenly, spreading it in layers, and compacting it in layers to form a green foundation bearing layer with design bearing capacity include: The industrial solid waste improver is composed of fly ash, steel slag powder and cement-based solidifier in a weight ratio of 3:2:1, wherein the steel slag powder has a particle size of no more than 0.075 mm and the fly ash is Class II fly ash. The dosage of the industrial solid waste amendment is 8% to 15% of the dry weight of the backfill soil. It is thoroughly mixed using a road mixer or stabilized soil mixing equipment. The uniformity of the mixture is determined by visual inspection to ensure that there are no gray or white spots. The thickness of the layered paving is controlled between 200 and 300 mm. It is compacted by a vibratory roller with no less than 6 passes and a compaction degree of no less than 0.

93. After passing the bearing capacity test, the green foundation bearing layer is formed.

4. The method for coordinated construction of water tank group support and green foundation in complex backfill strata as described in claim 1, characterized in that, The steps of using modular steel sheet piles as retaining structures around the perimeter of each individual foundation pit in the water tank group, with interlocking connections between the sheet piles, and vertically segmented modular steel supports installed inside the sheet piles to form a prefabricated support system include: The modular steel sheet piles adopt Larssen IV or U-shaped cross sections. The length of a single pile is determined according to the excavation depth of the foundation pit. The standard section length is 12m. Any short section is extended by welding. The weld quality grade is not lower than Grade II. Adjacent modular steel sheet piles are engaged by interlocking male and female locking jaws, with sealing grease applied inside the locking jaws, and the gap between the joints after engagement is no more than 2mm. The modular steel supports are made of φ609mm steel pipes or H-beams, and 2 to 4 rows are set along the depth of the foundation pit, with a horizontal spacing of 3 to 6m and a vertical spacing determined according to the soil pressure distribution characteristics. The first row of modular steel supports is no more than 0.5m above the ground. The end of the modular steel support is connected to the waler on the inner side of the modular steel sheet pile via a hinged joint. The waler is made of double H-beams, and the pre-applied axial force of the modular steel support is 60% to 80% of the design axial force.

5. The method for coordinated construction of water tank group support and green foundation in complex backfill strata as described in claim 1, characterized in that, The steps of excavating each individual foundation pit in layers and sections according to the planned excavation sequence, installing steel supports at the corresponding elevation after each layer is excavated, and constructing the foundation layer after excavating to the designed elevation of the base include: When the pool group includes no less than 3 individual pools, the skip-pool method shall be used for excavation, and the time interval between excavations of adjacent individual pools shall not be less than 7 days. Each of the aforementioned individual foundation pits is divided into 2 to 3 excavation sections along the longitudinal direction, each section being 15 to 25m long. A stepped slope is left at the junction of the excavation sections, with a step width of not less than 1.5m and a height difference of not more than 2m. The excavation depth of each layer shall not exceed 2m. After the excavation is completed, the modular steel support of the layer shall be installed and prestressed within 4 hours. The over-excavation depth shall not exceed 100mm.

6. The method for coordinated construction of water tank group support and green foundation in complex backfill strata as described in claim 3, characterized in that, The steps for forming a green foundation bearing layer, where the layer thickness is controlled between 200 and 300 mm, and compacted using a vibratory roller with at least 6 passes and a compaction degree of not less than 0.93, and after passing the bearing capacity test, include: The bearing capacity of the foundation after each layer of compaction is tested by static cone penetration test or light dynamic cone penetration test. The spacing between test points is no more than 20m, and there are no less than 3 test points for each individual water tank. When the bearing capacity is found to be lower than 150 kPa, 3% to 5% of the cement-based curing agent is added to the area and the mixture is remixed and compacted until the bearing capacity meets the design requirements. After each layer is compacted, the compaction degree is tested using the ring cutter method or the sand filling method. The compaction degree is considered qualified if it is not less than 0.

93. If the unqualified area is not qualified, it should be turned over and re-laid and compacted.

7. The method for coordinated construction of water tank group support and green foundation in complex backfill strata as described in claim 3, characterized in that, The amendment dosage is 8% to 15% of the dry weight of the backfill soil. It is thoroughly mixed using a road mixing machine or stabilized soil mixing equipment. The mixing uniformity is determined by visually eliminating any gray or white patches. The steps include: Before mixing, the natural moisture content of the backfill material is measured. If the moisture content is higher than the optimum moisture content +2%, it is first turned over and dried to reduce the moisture content. If the moisture content is lower than the optimum moisture content -2%, water is sprinkled to moisten it. In rainy season construction conditions, the improved soil material that has been laid should be dried until the moisture content is no more than +2% of the optimum moisture content before it can be compacted. The improved soil should be spread and compacted within 2 hours after mixing. If the time limit is exceeded, the improved soil should be loosened and mixed again before construction.

8. The method for coordinated construction of water tank group support and green foundation in complex backfill strata as described in claim 1, characterized in that, The steps of using modular steel sheet piles as retaining structures around the perimeter of each individual foundation pit in the water tank group, with interlocking connections between the sheet piles, and vertically segmented modular steel supports installed inside the sheet piles to form a prefabricated support system include: When the distance between adjacent individual water tanks is less than twice the excavation depth of the foundation pit, the common support area between the two individual foundation pits shall be reinforced by double rows of modular steel sheet piles or by increasing the depth of the modular steel sheet piles in the soil. The modular steel sheet piles in the double rows are spaced 1.5 to 2.5 m apart, and are connected by tie rods or steel tie rods between the rows. The horizontal spacing of the tie rods is 2 to 3 m. The first modular steel support of the reinforced section adopts a bracing form to connect two adjacent foundation pits, forming an overall load-bearing frame.

9. The method for coordinated construction of water tank group support and green foundation in complex backfill strata as described in claim 1, characterized in that, The step of using modular steel sheet piles as retaining structures around the perimeter of each individual foundation pit in the pool group, with interlocking connections between the sheet piles, and modular steel supports installed vertically in segments inside the sheet piles to form a prefabricated support system, further includes: Deformation observation points are set at the top of the modular steel sheet pile, the end of the modular steel support, and the ground around the foundation pit. The observation points are spaced 15-25m apart. Displacement and settlement are observed using a level and a theodolite. The observation frequency is once a day during excavation and once every two days during the dismantling and backfilling of the support. When the horizontal displacement of the top of the modular steel sheet pile exceeds 0.3% of the excavation depth of the foundation pit or the daily change rate exceeds 2 mm / day, the excavation of that section of the foundation pit shall be stopped immediately, and temporary steel supports or counterweight platforms shall be added at the corresponding positions. Construction can only continue after the deformation has stabilized. Diagonal steel supports or corner reinforcements are added at the corners and stress concentration points of the foundation pit, with the angle between the diagonal steel supports and the horizontal modular steel supports being 45°~60°.

10. The method for coordinated construction of water tank group support and green foundation in complex backfill strata as described in claim 1, characterized in that, After the water tank structure is completed and reaches its design strength, the steel supports are dismantled in sections from bottom to top. Simultaneously, the trench between the foundation pit sidewall and the outer wall of the water tank is backfilled. The backfill material is locally modified backfill soil or industrial solid waste mixture. The steps of compacting the backfill to the ground design elevation in layers include: The dismantling sequence of the modular steel supports is the reverse of the installation sequence. First, the lowest layer of modular steel supports is dismantled, and the backfill is moved to the bottom elevation of the upper layer of modular steel supports before the upper layer of modular steel supports is dismantled. The backfill width of the fertilizer trench shall not be less than 0.8m. When the industrial solid waste mixture is used, the industrial solid waste mixture shall be composed of 20% to 30% steel slag or fly ash mixed with locally improved soil. The backfill layer thickness should not exceed 300mm. A small vibratory compactor or plate vibrator should be used for compaction, with a compaction degree of not less than 0.

90. After each layer of backfill is completed, a density test should be conducted, and the next layer of backfill can only be carried out after the test is qualified.

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