Coastal beach substation double-layer composite foundation construction settlement control method

Abstract

The application discloses a kind of coastal beach substation double-layer composite foundation construction settlement control methods, it is related to substation foundation engineering field.The existing soft soil ground treatment method has the deficiency in controlling long-term creep, uneven settlement under tidal disturbance and economy aspect.The present application first establishes viscoelastic plasticity settlement calculation model based on soft soil structural damage factor and load partition;Then, different load area is different in sinking anticorrosion PHC pipe pile and setting different pile spacing;Then, pile cap with limiting device is poured on the top of pile, and after backfilling graded gravel, multi-layer reinforced body is laid to form shallow stress structure;At the same time, pile-raft retaining wall with the same depth as pipe pile is set around, and the reinforced body is anchored in the retaining wall.The present application optimizes the amount of pile foundation by partition design, effectively controls settlement by the cooperative work of deep pile foundation and shallow reinforced cushion, and ensures the long-term stability and safety of foundation in harsh environment through the whole process of corrosion prevention and automatic monitoring.

Description

Technical Field

[0001] This invention relates to the field of substation foundation engineering, and in particular to a method for controlling settlement during construction of a double-layer composite foundation for a coastal tidal flat substation. Background Technology

[0002] The tidal flat foundation is a deep, soft soil formed by alternating marine and terrestrial sedimentation, characterized by a thick soft layer (10–30 m), high natural water content (47.7%–60%), large porosity (1.3–1.5), and high compressibility (0.89–1.2 MPa). - ¹) The significant creep characteristics, combined with the extreme environment of high salinity and humidity, typhoons and tides, can easily lead to excessive foundation settlement, excessive differential settlement and excessive horizontal displacement, which directly threatens the safe and stable operation of the power grid. In severe cases, it can cause damage to the connection parts of electrical equipment and prevent the substation from being put into normal operation.

[0003] Existing tidal flat foundation treatment technologies have significant shortcomings: the vacuum preloading method has a construction period of 6 to 12 months, which cannot meet the needs of rapid commissioning of substations; the ordinary pipe pile sparse pile method ignores the coordination of shallow deformation, which can easily lead to equipment foundation diseases; the conventional pile-net composite foundation does not take into account the structural damage mechanism of soft soil and the heavy load characteristics of substations, resulting in insufficient settlement control accuracy and poor durability, making it difficult to meet the long-term stable operation requirements of ultra-high voltage substations. Summary of the Invention

[0004] The technical problem to be solved and the technical task proposed by this invention is to improve and refine existing technical solutions, and to provide a method for controlling settlement during the construction of a double-layer composite foundation for a coastal tidal flat substation, in order to enhance the stability of the foundation of the soft soil substation and ensure the long-term stable operation of the substation. To this end, this invention adopts the following technical solution.

[0005] A method for controlling settlement during construction of a double-layer composite foundation for a coastal tidal flat substation includes the following steps: 1) Obtain the physical and mechanical parameters of the soft soil in the tidal flats, define the structural damage factor to characterize the structural damage evolution of the soft soil, and divide the foundation into different load zones according to the load distribution characteristics of electrical equipment. Set different additional stress control requirements for different load zones. Based on the structural damage factor, the additional stress control requirements of each load zone, and the stress change of the soft soil caused by tidal periodic disturbance, establish a viscoelastic-plastic constitutive model for settlement calculation. 2) In each load zone, multiple anti-corrosion PHC pipe piles are driven using the static pressure pile driving method. The pipe piles are embedded in the bearing layer, and the pile spacing in the core load zone is smaller than the pile spacing in the secondary load zone, and the pile spacing in the secondary load zone is smaller than the pile spacing in the edge load zone. 3) Cast reinforced concrete pile caps on the top of the pipe piles. The pile caps are equipped with anti-lateral displacement limiting devices. After the pile caps pass the concealed acceptance, backfill the foundation pit between the pile caps with graded crushed stone until it is level with the top surface of the pile caps. 4) On the backfilled and leveled top surface of the foundation pit, the reinforced body is laid as a whole. Finally, graded pond slag is backfilled in layers on the reinforced body and compacted to form a shallow load-bearing structure. 5) After the composite foundation is formed, the integrity of the pile body, the bearing capacity of the composite foundation, the pull-out force of the pile body and the horizontal bearing capacity are tested, and automated settlement monitoring points are set up to automatically monitor the foundation settlement and horizontal displacement.

[0006] By introducing structural damage factors and tidal disturbance stress, a constitutive model that better reflects the actual stress and deformation characteristics of soft soil in tidal flats was established, making settlement prediction and foundation design more scientific and accurate. The foundation was divided into load zones according to the importance of the equipment, and pile spacing and additional stress control standards were set differently. Strengthening treatment in the core area ensured safety, while moderate relaxation was implemented in secondary and peripheral areas, achieving an optimal balance between safety and economy. A double-layer composite foundation was constructed using deep PHC pipe pile groups and a shallow reinforced cushion layer. The pipe piles transferred the load to the deep, good soil layer, while the shallow reinforcement uniformly diffused stress and coordinated pile-soil deformation. The two worked together to effectively control total and uneven settlement. After construction, multiple tests verified the foundation performance, and an automated monitoring system was deployed for long-term monitoring, achieving full-process safety management from construction completion to full life-cycle operation. This effectively improved the stability of the foundation of the soft soil substation in tidal flats, ensuring the long-term stable operation of the substation.

[0007] As a preferred technical means: In step 1), the physical and mechanical parameters of the soft soil in the tidal flats are obtained through field drilling and indoor triaxial creep tests. The physical and mechanical parameters of the soft soil include: a thickness of 10–30 m, a natural water content of 47.7%–60%, a void ratio of 1.3–1.5, and a compressibility coefficient of 0.89–1.2 MPa. - ¹, the structural yield stress q / p0 is 0.12–0.16, and the structural damage factor is 0.03–0.08. This method realizes the acquisition of physical and mechanical parameters of tidal flat soft soil, clarifies the structural damage factor and its value range, and incorporates the important but difficult-to-quantify structural characteristic of soft soil into the calculation model, significantly improving the reliability of settlement prediction and analysis.

[0008] As a preferred technical approach: In step 1), the indoor triaxial creep test adopts a graded loading method, with each loading stage having a stabilization time of no less than 24 hours. The creep coefficient of soft soil is determined by fitting the test data, with a value ranging from 0.85 to 0.92. Graded loading and sufficient stabilization time ensure the accuracy and reliability of the creep coefficient used in the viscoelastic-plastic constitutive model, thereby improving the accuracy of long-term settlement prediction.

[0009] As a preferred technical approach: In step 1), the foundation is divided into different load zones, including a core load zone, a secondary load zone, and a peripheral load zone. The core load zone is the main transformer and GIS area, the secondary load zone is the power distribution equipment area, and the peripheral load zone is the road and vacant area. The specific requirements for the additional stress classification control are: additional stress ≤ 0.10p0 in the core load zone, ≤ 0.12p0 in the secondary load zone, and ≤ 0.14p0 in the peripheral load zone, where p0 is the self-weight stress of the soft soil. Dividing the substation foundation into three load zones—core, secondary, and peripheral—corresponds to specific functional areas, making the zoning principle clear and easy to operate. Through a quantified additional stress classification control standard based on the self-weight stress p0 of the soft soil, a higher safety reserve is provided for the core zone, and the design parameters of each zone are optimized and refined while meeting the requirements.

[0010] As a preferred technical means: In step 2), the surface of the PHC pipe pile undergoes salt-resistant anti-corrosion treatment. This treatment uses an epoxy coal tar coating with a thickness ≥3mm and an adhesion strength ≥3.5MPa. After coating application, an electrical spark test is used to check the integrity of the coating. By applying salt-resistant anti-corrosion treatment to the PHC pipe pile and specifying the coating thickness, adhesion strength, and testing methods, corrosion from the high-salt, humid environment of coastal mudflats can be effectively resisted, greatly extending the service life of the pile foundation and the entire foundation structure.

[0011] As a preferred technical approach: In step 2), the differentiated pile spacing is specifically set as follows: the pile spacing in the core load zone is 2.0–2.4 m, the pile spacing in the secondary load zone is 2.4–3.0 m, and the pile spacing in the edge load zone is 3.0–4.0 m. The pile spacing is the smallest in the core zone to form a high-density pile group to provide maximum support, and the spacing is the largest in the edge zone. This hierarchical quantification ensures high bearing capacity in the core equipment area while significantly saving the total amount and cost of pile foundation engineering.

[0012] As a preferred technical means: In step 4), the reinforced body includes a graded crushed stone cushion layer, a medium-coarse sand layer, and a geogrid laid from bottom to top. Geotextile is laid on top of the geogrid, followed by graded pond slag. The longitudinal and transverse tensile strength of the geogrid is ≥80kN / m, and the overlap width is ≥300mm. The number of geogrid layers in the core load area is greater than the number of geogrid layers in the secondary load area. The multi-layer composite cushion layer composed of graded crushed stone, medium-coarse sand, high-strength geogrid, and geotextile has good drainage, stress diffusion, and reinforcement functions. The strength and overlap width of the geogrid ensure the integrity of the reinforced layer. Laying more geogrid layers in the core area further enhances the stress diffusion and resistance to uneven deformation in this area.

[0013] As a preferred technical approach: In step 5), the composite foundation bearing capacity testing employs a static load test, with no fewer than three test points evenly distributed across the core load area and secondary load area. The test loading level is 1.1 to 1.3 times the design bearing capacity, and the settlement stability standard is a settlement of ≤0.1mm per hour, lasting for more than 2 hours, ensuring the composite foundation bearing capacity is ≥180kPa. Simultaneously, the pile pull-out force and horizontal bearing capacity are tested, with the pull-out force ≥1.2 times the design value and the horizontal bearing capacity ≥1.1 times the design value. This clarifies the testing methods, quantities, locations, loading standards, and qualification indicators for the composite foundation bearing capacity, pile pull-out force, and horizontal bearing capacity, providing a quantifiable and operable strict basis for project acceptance and ensuring that the reliability of the constructed foundation fully meets the design requirements.

[0014] As a preferred technical means: In step 5), the automated settlement monitoring point adopts a fiber optic grating monitoring device with a monitoring accuracy ≤0.01mm; the monitoring frequency in the core load area is once every 1.5-2.5 hours, and in other areas once every 4-8 hours, with a settlement rate warning threshold range of 0.2-0.4mm / d and a horizontal displacement warning threshold range of 0.4-0.6mm / d. By employing high-precision fiber optic grating automated monitoring and setting differentiated monitoring frequencies for areas of different importance, real-time, accurate, and remote monitoring of foundation settlement and horizontal displacement is achieved. By setting settlement rate and horizontal displacement warning thresholds, automatic warnings are issued once the monitoring data exceeds the thresholds, facilitating timely detection of potential risks and implementation of countermeasures by the operating unit, greatly improving the safety assurance level during the substation's operation.

[0015] As a preferred technical approach, this also includes the construction of retaining walls around the substation, with pile-raft foundations set at the bottom of the retaining walls. The pile length of the pile-raft foundation is consistent with the pile length of the PHC pipe piles. The edge of the geogrid extends to the pile-raft foundation and is fixed with reverse-wrapped steel bars. The reverse-wrapped steel bars are embedded into the raft slab to a depth ≥500mm and a spacing ≤500mm. Maintaining the pile length of the retaining wall pile-raft foundation consistent with the PHC pipe piles of the main project helps control differential settlement and avoids cracking between the retaining wall and the main construction area due to uneven settlement. By reverse-wrapping and anchoring the geogrid into the retaining wall raft slab, the shallow reinforced cushion layer, deep pile foundation, and surrounding retaining wall are tightly connected into a whole, significantly enhancing the lateral restraint and overall stability of the composite foundation, preventing slippage or damage of the shallow structure at the edges, and ensuring the overall working performance of the two-layer composite system.

[0016] Beneficial effects: By establishing a viscoelastic-plastic constitutive model that considers structural damage to the soil and tidal disturbances, a scientific basis for settlement prediction is provided; the foundation is divided into core, secondary, and peripheral load zones according to the importance of the equipment, and pile spacing and additional stress control standards are set differently, which optimizes the design while ensuring high safety in the core equipment zone and achieves a balance between safety and economy; the "deep anti-corrosion PHC pipe pile group and shallow reinforced cushion layer form a double-layer composite foundation with synergistic force transmission, effectively transfer loads and coordinate deformation, and jointly control total settlement and uneven settlement; the implementation of strict anti-corrosion treatment and automated high-precision monitoring, combined with the overall anchoring of the retaining wall foundation, significantly improves the durability and overall stability of the foundation of the tidal flat soft soil substation, ensuring the long-term stable operation of the substation and the safety guarantee capability of operation and maintenance throughout its entire life cycle. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of the process of this invention.

[0018] Figure 2 This is a schematic diagram of the cross-sectional structure of the double-layer composite foundation of the present invention.

[0019] Figure 3 This is a schematic diagram of the reverse-wrapping structure of the geogrid in this invention.

[0020] Figure 4 This is a schematic diagram of the retaining wall structure of the present invention.

[0021] In the diagram: 1. PHC pipe pile; 2. Bearing layer; 3. Pile cap; 4. Graded crushed stone; 5. Medium-coarse sand layer; 6. Geogrid; 7. Geotextile; 8. Concrete cushion layer; 9. Graded pond slag; 10. Retaining wall; 11. Pile raft foundation; 601. Inverted wrapping; 1101. Inverted reinforcing steel; 1102. Raft slab. Detailed Implementation

[0022] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings.

[0023] like Figure 1 , Figure 2 As shown, a method for controlling settlement during construction of a double-layer composite foundation for a coastal tidal flat substation is illustrated below, using a 220kV substation project planned and constructed in a coastal tidal flat area as an example. The method includes the following steps: S1: Geological exploration and computational model establishment First, the physical and mechanical parameters of the soft soil in the tidal flats were obtained. Field drilling and sampling were conducted at the site, followed by systematic indoor geotechnical tests. The investigation revealed that the soft soil layer in this area was approximately 18m thick, with a natural water content of 52%, a void ratio of 1.41, and a compressibility coefficient of 1.05MPa. -¹ Through indoor triaxial creep tests, using a graded loading method, the structural yield stress q / p0 of the soft soil was determined to be 0.14, the structural damage factor was fitted to be 0.05, and the creep coefficient was 0.88. In this embodiment, the loading levels of the graded loading method were 0.2p0, 0.4p0, 0.6p0, 0.8p0, 1.0p0, and 1.2p0, respectively, and the stabilization time of each load level was not less than 24 hours. The graded loading and sufficient stabilization time ensured the accuracy and reliability of the creep coefficient used in the viscoelastic-plastic constitutive model.

[0024] Based on the geological survey results and the load magnitude and distribution characteristics of electrical equipment such as the main transformer, GIS switchgear, and power distribution equipment, the substation foundation is divided into three load zones: the main transformer foundation and GIS equipment area, which bear the largest load, are designated as the core load zone; the power distribution equipment area is designated as the secondary load zone; and the roads, cable trenches, and vacant areas within the substation are designated as the peripheral load zone. Different additional stress control standards are set for different load zones: additional stress ≤ 0.10p0 in the core load zone, ≤ 0.12p0 in the secondary load zone, and ≤ 0.14p0 in the peripheral load zone, where p0 is the self-weight stress of the soft soil.

[0025] Based on the above soft soil parameters, structural damage factors, zoning control standards, and considering the reciprocating shear stress disturbance of the foundation soil caused by the periodic changes in tidal water level, a viscoelastic-plastic constitutive model for settlement calculation and analysis is established to provide a theoretical basis for subsequent design.

[0026] S2: Deep PHC pipe pile construction According to the design, prestressed high-strength concrete PHC pipe piles of type PHC-550-AB-110 with a concrete strength ≥ C80 were selected. All pipe pile surfaces were treated with epoxy coal tar pitch for corrosion protection, with a coating thickness of not less than 3mm and a coating adhesion ≥ 3.5MPa. The integrity of the coating was verified using an electric spark test after construction.

[0027] Before construction, graded crushed stone 4 was used to backfill and form a construction access road. Roadbed plates were laid to prevent transport vehicles from sinking and to minimize disturbance to the deep soil.

[0028] PHC pipe piles 1 were driven into the foundation using the static pressure pile driving method. During construction, the pile driving rate was controlled at 1-2 m / min, avoiding operation during high tide periods when the groundwater level is ≤1.0 m deep. The verticality deviation of the pile body was ≤0.5%, and the pile penetration was controlled at 20-40 mm / 10 blows. The verticality and penetration of the pile body were controlled in real time to avoid disturbing the soft soil structure. For pile splicing, the MG500 pipe all-position automatic welding system was used for circumferential welding to ensure welding quality and construction efficiency, adapting to the complex construction needs of the tidal flat reclamation area.

[0029] The pile tip must be embedded in the underlying silty clay bearing layer to a depth of not less than 1m. The pile spacing is differentiated according to load zones: in the core load zone, the pile spacing is designed to be 2.2m; in the secondary load zone, the pile spacing is designed to be 2.8m; and in the edge load zone, the pile spacing is designed to be 3.5m. This arrangement ensures a higher pile support density in areas of concentrated load.

[0030] S3: Construction of Pile Cap 3 and Backfilling of Foundation Pit After the pipe piles are driven, a reinforced concrete pile cap 3 is poured on top of the pile. A lateral displacement limiting device is pre-installed inside the pile cap 3, with a horizontal bearing capacity ≥200kN / m. In this embodiment, the lateral displacement limiting device uses a corrosion-resistant steel bracket. The concrete for the pile cap 3 is C30, but the grade can be increased according to design requirements. The pile cap 3 cushion layer is a C15 / C20 concrete cushion layer 8. After the concrete strength of the pile cap 3 reaches the design requirements and passes acceptance, a 100mm thick layer of stone powder is first laid in the foundation pit between the pile caps 3 to enhance soil density, adjust moisture content, and improve stability. Then, graded crushed stone 4 is backfilled. Backfilling requires layered compaction, with a layer thickness ≤300mm and a compaction coefficient ≥0.96, until the elevation is flush with the top surface of the pile cap 3, forming a flat working surface.

[0031] S4: Laying reinforced structures to form shallow load-bearing structures After backfilling and leveling the top surface of the foundation pit, a reinforced structure is laid as a whole to form a shallow load-bearing structure. This structure, from bottom to top, includes: a first layer of 80mm thick graded crushed stone 4; a second layer of 20mm thick medium-coarse sand 5, with compaction sampling and testing showing a compaction coefficient of 0.97; and a third layer of bidirectional salt-resistant high-strength geogrid 6, with a tensile strength of not less than 80kN / m and an overlap width ≥300mm. Except for the cable layer, the entire substation is fully covered with geogrid 6. The geogrid 6 is reinforced with backing 601 at the fire water tank, emergency oil tank, and rainwater pump well, with an edge backing 601 length of 1.5m. Figure 3 As shown. To improve overall integrity, two layers of geogrid 6 are laid in the core load area, more than in the secondary load area. Geotextile 7 is then laid on top of the geogrid 6 for isolation and protection. Finally, graded pond slag 9 is backfilled and compacted in layers on top of the geotextile 7, forming a flexible, high-strength shallow load-bearing layer to diffuse the upper load and coordinate pile-soil deformation.

[0032] S5: Deployment of Quality Inspection and Automated Monitoring System After the composite foundation was formed, tests were conducted on pile integrity, composite foundation bearing capacity, pile pull-out force, and horizontal bearing capacity. During the tests, static load tests were performed on the composite foundation at four points in the core load area and secondary load area, with the load increased to 1.2 times the design bearing capacity. The settlement stability standard was ≤0.1mm settlement per hour. After settlement stabilization, the characteristic values ​​of the bearing capacity were all greater than 180kPa, meeting the requirements. Simultaneously, tests were conducted on pile pull-out force and horizontal bearing capacity. The pull-out force was ≥1.2 times the design value, and the horizontal bearing capacity was ≥1.1 times the design value.

[0033] Automated settlement monitoring points using fiber optic grating technology are deployed at key locations within the station, such as the main transformer foundation, GIS equipment area, and the 10-degree corner of the retaining wall, to detect settlement and horizontal displacement. The monitoring system has an accuracy of 0.01 mm. The monitoring frequency is set to once every 2 hours in the core load area and once every 6 hours in other areas. The early warning thresholds for settlement rate and horizontal displacement are set at 0.3 mm / d and 0.5 mm / d, respectively. The monitoring cycle is no less than 5 years after construction, achieving automated real-time monitoring of foundation deformation and ensuring timely detection of potential safety hazards.

[0034] S6: Overall anchorage of retaining wall In addition, such as Figure 4 As shown, the project also includes the construction of a retaining wall 10 around the substation. A pile-raft foundation 11 is set at the bottom of the retaining wall 10. The pile length of the pile-raft foundation 11 is consistent with the pile length of the PHC pipe pile. The edge of the geogrid 6 extends to the pile-raft foundation 11 of the retaining wall 10 and is fixed with a reverse-wrapped steel bar 1101. The reverse-wrapped steel bar 601 of the edge of the retaining wall 10 is 1.25m long. The reverse-wrapped steel bar 1101 is embedded into the raft slab 1102 at a depth of ≥500mm and a spacing of ≤500mm to form a closed constraint system, which improves the overall anti-sliding capacity and is suitable for the tidal flat environment where typhoons and tides occur frequently. By keeping the pile length of the retaining wall 10 pile raft foundation 11 consistent with that of the main project PHC pipe piles, it is beneficial to control differential settlement and avoid cracking of the retaining wall 10 and the main building area due to uneven settlement. By wrapping the geogrid 6 around 601 and anchoring it into the raft slab 1102, the shallow reinforced cushion layer, deep pile foundation and surrounding retaining wall 10 are tightly connected into a whole, which significantly enhances the lateral restraint and overall stability of the composite foundation, prevents the shallow structure from sliding or being damaged at the edge, and ensures the overall working performance of the double-layer composite system.

[0035] In this embodiment, the bidirectional salt-resistant high-strength geogrid 6 is made of high-density polyethylene and modified with anti-ultraviolet and anti-salt corrosion agents. After being soaked in a 5% NaCl solution for 720 hours, the tensile strength retention rate is ≥90% and the elongation at break is ≤15%.

[0036] In this embodiment, the pile cap 3 is pre-embedded with a corrosion-resistant high-strength anchor bolt assembly for connection with various components and electrical equipment. The anchor bolt diameter is ≥20mm, the insertion depth into the pile cap 3 is ≥150mm, and it is fixed using a chemical anchoring process. The anchoring adhesive has a curing time of not less than 72 hours, and a pull-out test is performed after curing. The pull-out force is ≥1.2 times the design value to ensure connection reliability.

[0037] In this embodiment, the graded crushed stone 4 is a continuous gradation of 5-31.5mm with a mud content of ≤3%; the compaction construction adopts a vibratory roller for layered compaction, with each layer having a thickness of ≤100mm and a compaction pass of ≥6 passes. After compaction, the compaction degree is tested by the ring cutter method to ensure that the compaction quality meets the standards.

[0038] In this embodiment, the construction implements a "three-control, three-inspection, and one-early warning" quality control system. The three controls are controlling the pile penetration, the compaction of the subbase, and the quality of the grid overlap; the three inspections are material inspection before construction, random inspection during construction, and overall acceptance after construction; material inspection requires the issuance of a third-party testing report, and the random inspection frequency is no less than one testing point per 100m².

[0039] In this method, the specific method for obtaining the structural damage factor is as follows: First, undisturbed triaxial creep tests were conducted on undisturbed soft soil samples from the tidal flats. The tests employed a graded loading method, applying deviatoric stresses step-by-step under consolidated and drained conditions, ensuring that each load level remained stable for at least 24 hours to record the complete creep curve of the soil sample's strain over time under each constant load level. The tests yielded a series of strain-time relationship data under different stress levels.

[0040] The structural damage factor, denoted by the symbol D, is an internal state variable in soil mechanics used to quantify the degree of gradual failure of the original soil structure under load. Its simplified definition is: D = 1 - E0 / E t E0 represents the tangent modulus of the soil at the initial or low stress level when the structure is not significantly damaged, or a certain reference modulus. t This represents the instantaneous tangent modulus or secant modulus of soil after damage at a specific stress level or time. When D=0, it indicates that the structure is intact; the larger the D value (approaching 1), the more severe the structural damage.

[0041] Substitute the creep curve data obtained from the test into a viscoelastic-plastic constitutive model considering damage (for example, introduce the damage factor into the Burgers model, fractional derivative model or other rheological models for fitting). The model usually includes components such as elasticity, viscosity, plasticity and damage evolution. Using numerical analysis software (such as MATLAB, Python, etc.), adjust each parameter in the model (including damage evolution parameters, creep parameters, etc.) through the least squares method or optimization algorithm, so that the "strain-time" curve calculated by the model theory is in the best fit with the measured test curve. In this process, the parameters describing the damage development law (such as the coefficients in the damage evolution equation) will be determined. Then, according to the model and the determined parameters, calculate the typical value or value range of the structural damage factor corresponding to the soil mass within the range of the structural yield stress q / p0 of 0.12 to 0.16.

[0042] In this embodiment of the method, the post-construction settlement can be ≤25 mm, the differential settlement can be ≤3.2 mm, and the settlement calculation accuracy is improved by more than 30% compared with the traditional method, fully meeting the stringent settlement requirements of electrical equipment in ultra-high voltage substations; the bearing capacity of the composite foundation is ≥180 kPa, and the uplift resistance and horizontal bearing capacity of the pile body meet the design requirements, which can effectively resist extreme loads such as typhoons and tides, ensuring the safe operation of the substation; specifically solve the problems of construction disturbance, access road paving, drainage, etc. in the tidal flat filling area, and the construction period is shortened by 35% - 40% compared with the traditional plan, adapting to the complex construction environment of the tidal flat; the key components such as pile bodies and anchor bolts adopt professional anti-salt and anti-corrosion designs, adapting to the high-salt and high-humidity tidal flat environment, and the service life of the foundation is extended to more than 30 years, greatly reducing the operation and maintenance costs; the total life cycle cost of the project is reduced by more than 25%, applicable to the coastal tidal flat soft soil geology with a natural foundation bearing capacity of 50 - 60 kPa and a soft soil thickness of 10 - 30 m; it can be extended and applied to similar soft soil infrastructure projects such as tidal flat port power facilities and offshore wind power booster stations supporting ultra-high voltage cross-sea networking, and when extended and applied, the pile body parameters, grid layer numbers and monitoring frequencies can be adjusted according to the actual load and geological conditions.

[0043] The above are specific embodiments of the present invention, which have reflected the prominent substantive features and significant progress of the present invention. According to the actual use needs, under the inspiration of the present invention, equivalent modifications can be made to its shape, structure, etc., all within the protection scope of this solution.

Claims

1. A method for controlling settlement during construction of a double-layer composite foundation for a coastal tidal flat substation, characterized in that, Includes the following steps: 1) Obtain the physical and mechanical parameters of the soft soil in the tidal flats, define the structural damage factor to characterize the structural damage evolution of the soft soil, and divide the foundation into different load zones according to the load distribution characteristics of electrical equipment. Set different additional stress control requirements for different load zones. Based on the structural damage factor, the additional stress control requirements of each load zone, and the stress change of the soft soil caused by tidal periodic disturbance, establish a viscoelastic-plastic constitutive model for settlement calculation. 2) In each load zone, multiple anti-corrosion PHC pipe piles are driven using the static pressure pile driving method. The pipe piles are embedded in the bearing layer, and the pile spacing in the core load zone is smaller than the pile spacing in the secondary load zone, and the pile spacing in the secondary load zone is smaller than the pile spacing in the edge load zone. 3) Cast reinforced concrete pile caps on the top of the pipe piles. The pile caps are equipped with anti-lateral displacement limiting devices. After the pile caps pass the concealed acceptance, backfill the foundation pit between the pile caps with graded crushed stone until it is level with the top surface of the pile caps. 4) On the backfilled and leveled top surface of the foundation pit, the reinforced body is laid as a whole. Finally, graded pond slag is backfilled in layers on the reinforced body and compacted to form a shallow load-bearing structure. 5) After the composite foundation is formed, the integrity of the pile body, the bearing capacity of the composite foundation, the pull-out force of the pile body and the horizontal bearing capacity are tested, and automated settlement monitoring points are set up to automatically monitor the foundation settlement and horizontal displacement.

2. The method for controlling settlement during construction of a double-layer composite foundation for a coastal tidal flat substation according to claim 1, characterized in that: In step 1), the physical and mechanical parameters of the tidal flat soft soil were obtained through field drilling and indoor triaxial creep tests. These parameters included: a soft soil thickness of 10–30 m, a natural water content of 47.7%–60%, a void ratio of 1.3–1.5, and a compressibility coefficient of 0.89–1.2 MPa. - ¹, the structural yield stress q / p0 is 0.12 to 0.16, and the structural damage factor is 0.03 to 0.

08.

3. The method for controlling settlement during construction of a double-layer composite foundation for a coastal tidal flat substation according to claim 2, characterized in that: In step 1), the indoor triaxial creep test adopts a graded loading method, with each loading stage having a stabilization time of no less than 24 hours. The creep coefficient of soft soil is determined by fitting the test data, with a value range of 0.85 to 0.

92.

4. The method for controlling settlement during construction of a double-layer composite foundation for a coastal tidal flat substation according to claim 1, characterized in that: In step 1), the foundation is divided into different load zones, including a core load zone, a secondary load zone, and an edge load zone. The core load zone is the area for main transformers and GIS, the secondary load zone is the area for power distribution equipment, and the edge load zone is the area for roads and vacant areas. The specific requirements for the graded control of additional stress are: additional stress in the core load zone ≤ 0.10p0, additional stress in the secondary load zone ≤ 0.12p0, and additional stress in the edge load zone ≤ 0.14p0, where p0 is the self-weight stress of the soft soil.

5. The method for controlling settlement during construction of a double-layer composite foundation for a coastal tidal flat substation according to claim 1, characterized in that: In step 2), the surface of the PHC pipe pile is subjected to salt-resistant anti-corrosion treatment. The salt-resistant anti-corrosion treatment adopts an epoxy coal tar coating with a coating thickness ≥3mm and a coating adhesion ≥3.5MPa. After the coating is applied, the integrity is tested by the electric spark test method.

6. The method for controlling settlement during construction of a double-layer composite foundation for a coastal tidal flat substation according to claim 1, characterized in that: In step 2), the differentiated setting of pile spacing is as follows: the pile spacing in the core load area is 2.0 to 2.4 m, the pile spacing in the secondary load area is 2.4 to 3.0 m, and the pile spacing in the edge load area is 3.0 to 4.0 m.

7. The method for controlling settlement during construction of a double-layer composite foundation for a coastal tidal flat substation according to claim 1, characterized in that: In step 4), the reinforced body includes a graded crushed stone cushion layer, a medium-coarse sand layer and a geogrid laid from bottom to top. Geotextile is laid on top of the geogrid and then graded crushed stone is laid. The longitudinal and transverse tensile strength of the geogrid is ≥80kN / m and the overlap width is ≥300mm. The number of geogrid layers in the core load area is more than the number of geogrid layers in the secondary load area.

8. The method for controlling settlement during construction of a double-layer composite foundation for a coastal tidal flat substation according to claim 1, characterized in that: In step 5), the composite foundation bearing capacity test adopts a static load test, with no less than 3 test points, evenly distributed in the core load area and secondary load area; the test loading level is 1.1 to 1.3 times the design bearing capacity, the settlement stability standard is settlement ≤0.1mm per hour, lasting for more than 2 hours, to ensure that the composite foundation bearing capacity is ≥180kPa; the pile pull-out force and horizontal bearing capacity are tested simultaneously, with the pull-out force ≥1.2 times the design value and the horizontal bearing capacity ≥1.1 times the design value.

9. The method for controlling settlement during construction of a double-layer composite foundation for a coastal tidal flat substation according to claim 1, characterized in that: In step 5), the automated settlement monitoring point adopts a fiber optic grating monitoring device with a monitoring accuracy of ≤0.01mm; the monitoring frequency in the core load area is once every 1.5-2.5h, and in other areas once every 4-8h, with a settlement rate warning threshold range of 0.2-0.4mm / d and a horizontal displacement warning threshold range of 0.4-0.6mm / d.

10. The method for controlling settlement during construction of a double-layer composite foundation for a coastal tidal flat substation according to claim 7, characterized in that: It also includes the construction of retaining walls around the substation, with a pile-raft foundation set at the bottom of the retaining wall. The pile length of the pile-raft foundation is consistent with the pile length of the PHC pipe piles. The edge of the geogrid extends to the pile-raft foundation and is fixed with reverse-wrapped steel bars. The reverse-wrapped steel bars are embedded in the raft slab to a depth of ≥500mm and a spacing of ≤500mm.

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