Soft soil non-equal-height foundation pit structure based on fluid-structure interaction and construction method

Abstract

The invention relates to the field of constructional engineering, and discloses a soft soil non-equal-height foundation pit structure based on fluid-structure interaction and a construction method.The soft soil non-equal-height foundation pit structure comprises a foundation pit, a supporting module, a reinforcing module, a drainage module and a monitoring point, and the foundation pit is divided into a low elevation side and a high elevation side; the supporting module comprises supporting piles and an inner supporting system, and the supporting piles are arranged along the periphery of a foundation pit and used for bearing lateral pressure of a soil body. The inner supporting system comprises an angle brace and a cross-shaped pair support, the angle brace is arranged at the vertex angle of the foundation pit, and the cross-shaped pair support is arranged at the center of the foundation pit. According to the method, the novel excavation construction method is adopted in the soft soil non-equal-height foundation pit, the optimal scheme is selected through fluid-solid coupling model analysis, soil pressure and deformation displacement borne by foundation pit supporting piles and supports are reduced as much as possible in the excavation process, and rapid, effective and low-cost foundation pit supporting construction is achieved; the complex situation in the soft soil foundation excavation and supporting process is avoided, and the serious construction problem caused by untimely emergency treatment is avoided.

Description

Technical Field

[0001] The present invention relates to the technical field of construction engineering, and particularly relates to a soft soil non-equal-height foundation pit structure and a construction method based on fluid-solid coupling. Background Technique

[0002] In recent years, with the development of urban construction, a large number of high-rise and super high-rise buildings have emerged in urban construction, and there are more and more deep foundation pit projects. The support of deep foundation pits is difficult and the safety risks are high. Especially for deep foundation pit operations in soft soil areas, construction deformation and risk control are particularly crucial. For deep foundation pits with complex surrounding environments in the city, the support form of retaining piles + internal supports has also become one of the main support forms for deep foundation pits.

[0003] Complex surrounding environments often cause differences in the earth pressure and deformation borne by the support structure, resulting in safety problems in the foundation pit. When the foundation pit has a large height difference and the internal support form is adopted, there is an imbalance in the earth pressure behind the retaining piles on both sides of the support. Under the soft soil foundation pit, the non-equal height difference of the foundation pit has a great impact on the stability of the foundation pit. Summary of the Invention

[0004] In order to make up for the above deficiencies, the present invention provides a soft soil non-equal-height foundation pit structure and a construction method based on fluid-solid coupling. In view of the special soil characteristics in soft soil areas, the optimal excavation construction method is analyzed by combining the fluid-solid coupling model, reducing the horizontal force of the upper section of the retaining pile towards the pit, and reducing the horizontal displacement of the foundation pit and the settlement of the ground surface outside the pit.

[0005] In order to achieve the above object, the present invention adopts the following technical scheme: A soft soil non-equal-height foundation pit structure based on fluid-solid coupling, including a foundation pit, a support module, a reinforcement module, a drainage module and monitoring points. The foundation pit is divided into a low elevation side and a high elevation side; the support module includes retaining piles and an internal support system. The retaining piles are arranged along the periphery of the foundation pit to bear the lateral earth pressure; the internal support system includes corner braces and cross braces. The corner braces are arranged at the top corners of the foundation pit, and the cross braces are arranged at the center of the foundation pit;

[0006] The reinforcement module includes reinforcement in the passive area inside the pit and reinforcement in the active area behind the piles. The reinforcement area of the reinforcement in the passive area inside the pit is the bottom of the inner periphery of the foundation pit, and the reinforcement area of the reinforcement in the active area behind the piles is the active area soil in the middle of the low elevation side and the high elevation side;

[0007] The drainage module includes a cut-off curtain and a drainage channel. The drainage channel is arranged on the low elevation side for surface drainage, and the cut-off curtain is arranged along the periphery of the foundation pit to block the seepage of groundwater;

[0008] The monitoring points include deep displacement monitoring points, axial force horizontal displacement monitoring points and axial force measuring points to realize the comprehensive monitoring of the deformation and internal force of the foundation pit support module.

[0009] Preferably, it further includes a fluid-structure interaction model which establishes a coupling model through COMSOL to analyze the interaction between the seepage field and the stress field, and control the foundation pit deformation and the influence of groundwater.

[0010] Preferably, after the fluid-structure interaction model is constructed, the numerical simulation method is used for slope stability analysis, effectively combining the finite element theory with numerical software, and adopting the Midas geotechnical finite element analysis software.

[0011] Preferably, the deep displacement monitoring points are selected at the middle part of the retaining pile as the monitoring points, the axial force and horizontal displacement monitoring points are selected at the top of the retaining pile as the monitoring points, and the axial force measuring points are selected at the stress concentration area of the cross bracing and the shear force concentration area of the corner bracing as the monitoring points according to the layout form of the internal support system.

[0012] Preferably, the insertion depth of the water-stop curtain is determined according to the fluid-structure interaction model, and its control effect on the foundation pit deformation is verified by numerical simulation.

[0013] Preferably, the passive area inside the pit is reinforced by using a high-pressure jet grouting pile grid to improve the strength of the passive soil mass, and the active area behind the pile is reinforced by using a high-pressure jet grouting pile to reduce the stress difference on both sides.

[0014] Preferably, the internal support system balances the asymmetric loads on both sides by optimizing the support spacing and cross-sectional dimensions.

[0015] A construction method for a soft soil non-equal-height foundation pit structure based on fluid-structure interaction includes the following steps:

[0016] Step S1: Site pretreatment, slope the upper part of the foundation pit, use mechanical excavation, drive retaining piles along the perimeter of the foundation pit, and excavate a drainage channel on the low-elevation side of the foundation pit;

[0017] Step S2: Fluid-structure interaction modeling and analysis. First, collect soil samples on site and complete indoor geotechnical tests. Then, use COMSOL to establish a three-dimensional model, input parameters and simulate the excavation plan. Finally, compare the simulation results to determine the optimal plan. The geotechnical tests strictly follow the standard process to ensure the accuracy of the parameters. Multiple plan simulations are fully calculated to avoid plan mistakes caused by rough data;

[0018] Step S3: Use the numerical simulation method for slope stability analysis, effectively combine the finite element theory with numerical software, conduct a stability evaluation to obtain the main factors affecting the deformation of the non-equal-height foundation pit, and determine the foundation pit excavation and the construction method of each support;

[0019] Step S4: Set up a water-stop curtain around the foundation pit according to the optimal plan simulated by the fluid-structure interaction model;

[0020] Step S5: Asymmetric excavation and support construction,

[0021] First, excavate the foundation pit on the low elevation side and construct three supports in sequence. Then, excavate the foundation pit on the high elevation side to restrain the displacement inside the high elevation side pit and the displacement outside the low elevation side pit;

[0022] Step S6: Soil reinforcement,

[0023] Reinforcement of the passive area inside the pit: Construct by using high-pressure jet grouting piles;

[0024] Reinforcement of the active area behind the piles: Synchronously construct jet grouting piles behind the piles;

[0025] Step S7: Full-process monitoring (synchronous with construction),

[0026] Axial force and horizontal displacement: Monitor twice a day during excavation and once a day during the support curing period;

[0027] Deep displacement and axial force: Monitor once before and after each layer of excavation. When abnormal, increase the frequency to once every four hours.

[0028] Preferably: The construction in step S5 includes the following contents:

[0029] First, carry out the excavation and support construction of the foundation pit on the low elevation side,

[0030] First layer excavation: Excavate the soil on the low elevation side. Use mechanical excavation and quickly excavate to reduce the exposure time of the soil, avoiding long-term loading deformation of the soft soil;

[0031] First support construction: Tie steel bars → Install formwork → Pour C35 concrete → Cure. The concrete curing requires at least 7 days (the strength reaches more than 70% under standard curing conditions) to ensure that the support has bearing capacity;

[0032] Second layer excavation: Excavate to the bottom of the second support. Avoid excessive one-time excavation. Control the stress release amount for each layer during layered excavation and let the soil stress redistribute every three days between each layer;

[0033] Second support construction: Follow the same process as the first support. During the curing period, the installation of monitoring points can be carried out synchronously. The installation of monitoring points is parallel to the support curing. Arrange monitoring points at the top of the pile, the deep layer of the pile body, and the key parts of the support;

[0034] Third layer excavation: Excavate to the bottom of the third support. Check the displacement of the retaining piles while excavating. High-frequency monitoring ensures the safety of excavation. If any abnormality is found, the construction can be immediately suspended;

[0035] Third support construction: Follow the same process as the previous two supports. During the curing period, complete the pouring of the bottom cushion of the pit. The cushion timely seals the bottom of the pit to prevent the soft soil from heaving;

[0036] Excavate the bottom of the pit until the design elevation is reached, leaving 20 cm for manual bottom cleaning, and complete the pouring of the cushion. Manual bottom cleaning avoids mechanical disturbance of the original soil, and the rapid sealing of the cushion controls the deformation of the pit bottom.

[0037] Then, excavate the foundation pit on the high elevation side.

[0038] Excavate in layers, wait for two days after each layer is excavated, and continue to excavate when the monitoring data is stable.

[0039] The present invention has the following beneficial effects:

[0040] 1. In the present invention, first, a new excavation construction method is adopted in the soft soil non-equal-height foundation pit, and the optimal scheme is selected through the analysis of the fluid-solid coupling model. During the excavation process, the inward deformation of the foundation pit and the settlement of the outer surface are reduced, thereby improving the stability of the foundation pit support, realizing rapid, effective, and low-cost foundation pit support construction, and adopting the support form of passive soil reinforcement in the pit and active soil reinforcement outside the pile to reduce the stress difference on both sides of the support and restrain the displacement of the surrounding soil, solving the complex situations in the excavation and support process of soft soil foundations, and avoiding serious construction problems due to untimely emergency treatment.

[0041] 2. In the present invention, the numerical simulation method is used for slope stability analysis, the finite element theory is effectively combined with numerical software, and the main factors affecting the deformation of the non-equal-height foundation pit are obtained through stability evaluation, solving the problems of uneven settlement of the foundation pit and displacement of the foundation pit support under the unbalanced conditions of the non-equal-height difference foundation pit; by analyzing before excavation, it is determined whether the construction method meets the stability requirements, avoiding the occurrence of construction problems and saving construction costs. Description of the Drawings

[0042] Figure 1 It is a schematic diagram of the foundation pit structure of a soft soil non-equal-height foundation pit structure and construction method based on fluid-solid coupling proposed by the present invention;

[0043] Figure 2 It is a monitoring schematic diagram of a soft soil non-equal-height foundation pit structure and construction method based on fluid-solid coupling proposed by the present invention;

[0044] Figure 3 It is a flow chart of constructing a fluid-solid coupling model of a soft soil non-equal-height foundation pit structure and construction method based on fluid-solid coupling proposed by the present invention.

[0045] Legend:

[0046] 1. Support pile; 2. Internal support system; 2-1. Corner brace; 2-2. Cross bracing 3. Low elevation side; 4. High elevation side; 5. Cut-off curtain; 6. Active area reinforcement behind the pile; 7. Passive area reinforcement in the pit; 8. Drainage channel; 9. Deep displacement monitoring point; 10. Axial force horizontal displacement monitoring point; 11. Axial force measuring point. Detailed implementation manners

[0047] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0048] Example 1, referring to Figure 1 and Figure 2 , a soft soil non-uniform elevation foundation pit structure based on fluid-structure interaction, including a foundation pit, a support module, a reinforcement module, a drainage module and monitoring points. The foundation pit is divided into a low elevation side 3 and a high elevation side 4; the support module includes support piles 1 and an internal support system 2. The support piles 1 are bored cast-in-place piles arranged along the perimeter of the foundation pit, with a pile diameter of 1.0 m, a pile spacing of 1.2 m, and a pile length extending 5 m below the bottom of the pit, for bearing the lateral pressure of the soil mass; the internal support system 2 includes corner braces 2-1 and cross braces 2-2. The corner braces 2-1 are arranged at the top corners of the foundation pit, with a cross-sectional size of 1.0 m * 1.0 m and a support point spacing of 9 m, using a C35 reinforced concrete structure. The cross braces 2-2 are arranged at the center of the foundation pit, with a cross-sectional size of 1.1 m * 1.1 m and a main brace spacing of 15 m. The steel columns are composed of angle steel combined square columns (L125 * 10 angle steel welded into a 400 mm * 400 mm square column). By optimizing the support spacing and cross-sectional size, the asymmetric loads on both sides are balanced;

[0049] The reinforcement module includes passive zone reinforcement 7 in the pit and active zone reinforcement 8 behind the piles. The passive zone reinforcement 7 in the pit is a high-pressure jet grouting pile grid-like reinforcement at the bottom of the inner perimeter of the foundation pit, with a pile diameter of 800 mm, a pile spacing of 1.2 m, 3 rows longitudinally and 2 rows transversely, and a reinforcement depth of 3 m below the bottom of the pit to improve the strength of the passive soil mass. The active zone reinforcement 6 behind the piles is a high-pressure jet grouting pile reinforcement of the active zone soil in the middle of the low elevation side 3 and the high elevation side 4 of the foundation pit, with a pile diameter of 800 mm and a reinforcement range from the ground surface to 2 m below the bottom of the pit to reduce the stress difference between the two sides;

[0050] The drainage module includes a water stop curtain 5 and a drainage channel 8. The drainage channel 8 is arranged on the low elevation side 3 of the foundation pit, with a width of 2 m and a depth of 1.5 m, lined with C20 concrete for surface drainage; the water stop curtain 5 is arranged around the perimeter of the foundation pit to block the seepage of groundwater;

[0051] The monitoring points include the deep displacement monitoring point 9, the axial force and horizontal displacement monitoring point 10, and the axial force measuring point 11. The deep displacement monitoring point 9 is selected at the middle part of the retaining pile 1. If the deep displacement curve shows a sudden change in the displacement at the middle part of the pile (near the excavation surface of the foundation pit), it indicates insufficient reinforcement of the passive zone soil. If the displacement of the pile body is significant, it reflects insufficient penetration depth of the retaining pile (1), and can intuitively reflect the reinforcement effect. The axial force and horizontal displacement monitoring point 10 is selected at the top of the retaining pile 1. The displacement at the pile top is a key signal in the early stage of foundation pit instability. Real-time monitoring can immediately suspend construction when the displacement exceeds the warning value (such as 30 mm) to avoid the expansion of the situation. According to the layout form of the internal support system 2, the axial force measuring point 11 is selected at the mid-span (stress concentration area) of the cross-shaped bracing 2-2 and the shear force concentration area of the corner bracing 2-1 node. The internal support system 2 is the "lifeline" of the foundation pit support. Axial force monitoring can provide real-time feedback on whether the support is overloaded (warning when exceeding 80% of the design value). By comparing the support axial force data on the high elevation side 4 and the low elevation side 3, the construction rhythm of the support can be dynamically adjusted to achieve comprehensive monitoring of the deformation and internal force of the support module.

[0052] Example 2, referring to Figure 3 , a three-dimensional fluid-solid coupling model was established by COMSOL to analyze the interaction between the seepage field and the stress field. The model construction process includes: geological exploration and parameter determination → COMSOL model geometric modeling → seepage field and stress field setting → mesh generation and boundary conditions → multi-condition simulation calculation → judging whether the result converges? If the result converges, output displacement / seepage / stress data and compare and verify with the measured data; if the result does not converge, adjust the parameters and recalculate. The model parameters include: soil moisture content 35%, compression modulus 2.5 MPa, permeability coefficient 5*10 -7 m / s; elastic modulus of the retaining pile 28 GPa, Poisson's ratio 0.2; elastic modulus of the internal support concrete 30 GPa. Three excavation schemes (simultaneous excavation, the low elevation side 3 excavates 3 m first, the low elevation side 3 excavates 5 m first) were simulated, and the optimal scheme was obtained as the low elevation side 3 excavates 3 m first; the insertion depth of the water-stop curtain 5 was determined according to the fluid-solid coupling model, and its control effect on the foundation pit deformation was verified by numerical simulation. The simulation results are as follows:

[0053] Simulation 1: When the insertion depth is 3 m, the pile top displacement of the high elevation side 4 is 58 mm, and the settlement outside the pit is 40 mm;

[0054] Simulation 2: When the insertion depth is 5 m, the pile top displacement of the high elevation side 4 is 45 mm, and the settlement outside the pit is 32 mm;

[0055] Simulation 3: When the insertion depth is 7 m, the pile top displacement of the high elevation side 4 is 42 mm, and the settlement outside the pit is 30 mm;

[0056] Conclusion: By comparing the simulation results and combining them with the actual construction costs, it can be concluded that the balance effect of simulation 2 is good and meets the actual requirements; the water-proof effect of simulation 1 is insufficient and the seepage effect is significant; the displacement of simulation 3 is reduced but the cost increases by 20%, which is low in cost performance;

[0057] After the fluid-solid coupling model was constructed, the numerical simulation method was used to analyze the slope stability. The finite element theory and numerical software were effectively combined. The Midas geotechnical finite element analysis software was used to convert the optimal solution obtained above into: first dig 3m on the low elevation side 3, and use the Swedish strip method to calculate the slope stability (Fs≥1.3 is safe). The simulation results showed that the overall Fs=1.52, the high elevation side 4 slope foot Fs=1.41, and the low elevation side slope top Fs=1.63, which met the stability requirements and solved the problems of uneven settlement of the foundation pit and displacement of the foundation pit support under the unbalanced condition of the non-equal height difference foundation pit.

[0058] Example 3, reference Figures 1 - 3 A specific construction embodiment of the construction method of the soft soil non-conformal foundation pit structure based on fluid-solid coupling is as follows:

[0059] Step S1: Site pretreatment, slope of the upper part of the foundation pit (1:1.5 slope), mechanical excavation to the elevation +8.0m (low elevation side 3) and +4.0m (high elevation side 4), excavation depth 2m, earthwork volume 10000m 3 , completed in five days, bored piles were driven along the perimeter of the foundation pit, with a pile spacing of 1.2m, using a rotary drilling rig to drill holes, mud wall protection, and concrete poured pile body, 300 piles were constructed in ten days, and drainage channels 8 were excavated on the low elevation side 3, with a width of 2m, a depth of 1.5m, and a length of 200m. The excavation was completed in three days, and the concrete lining was completed in two days;

[0060] Step S2: Fluid-solid coupling modeling analysis: 3 groups of undisturbed soil samples were collected on site and indoor tests were conducted to determine the moisture content (35%), compression modulus (2.5 MPa), permeability coefficient (5*10 -7 m / s), the consolidation test strictly follows the GB / T50123-2019 standard and lasts for more than 24 hours. Then, a 3D model is established using COMSOL, and parameters are input to simulate three excavation schemes (synchronous excavation, 3m excavation on the low elevation side 3 first, and 5m excavation on the low elevation side 3 first). Finally, the simulation results are compared. The calculation of the complex model for a single working condition takes twelve hours. It is concluded that "3m excavation on the low elevation side 3 first" can reduce the maximum displacement of the support structure from 52mm to 37mm, and the displacement is reduced by 28%. It is determined to be the optimal solution.

[0061] Step S3: Use numerical simulation method to analyze slope stability, effectively combine finite element theory with numerical software, analyze and conclude that "digging 3m first on the low elevation side" meets the stability requirements, and determine the actual precipitation, foundation pit excavation and each support construction method;

[0062] Step S4: According to the optimal solution obtained from the fluid-structure interaction benefit balance model, a water-stop curtain 5 is set around the foundation pit, with an insertion depth of 5 m. The construction is carried out by using a three-axis mixing pile, advancing 20 m per day, with a total curtain length of 200 m, a cement content of 20%, a water-cement ratio of 1.5, and it is completed in a cumulative ten days.

[0063] Step S5: Excavation and support construction of the foundation pit 3 on the low elevation side

[0064] The first layer of excavation takes three days, excavating the top 2 m soil layer on the low elevation side 3. The mechanical excavation removes 500 m of soil per day 3 , and it is completed within three days. The rapid excavation reduces the exposure time of the soil and avoids the long-term loading deformation of the soft soil.

[0065] The construction of the first support takes seven days, binding steel bars (HRB400, diameter 25 mm) (one day) → formwork support (steel formwork) (one day) → pouring C35 concrete (one day) → curing (four days). Curing is carried out under standard curing conditions (temperature 20 ± 2 °C, humidity ≥ 95%). The strength increases with time. It is necessary to strictly follow the "Code for Construction of Concrete Structures" GB50666-2011. The support can bear the construction load only when the strength reaches more than 70% of the design value, to avoid the support cracking caused by premature excavation.

[0066] The second layer of excavation takes three days, excavating to the bottom of the second support (cumulative depth 5 m), with a layered thickness of 3 m. Avoid over-deep excavation at one time. The layered excavation controls the stress release amount each time, and each layer is spaced three days to allow the soil stress to redistribute.

[0067] The construction of the second support takes seven days, following the same process as the first support. The installation of monitoring points (one day) can be carried out synchronously during the curing period. The installation of monitoring points is parallel to the support curing, and monitoring points are arranged at the top of the pile, the deep part of the pile body, and the key parts of the support.

[0068] The third layer of excavation takes three days, excavating to the bottom of the third support (cumulative depth 8 m), and checking the displacement of the retaining pile while excavating (monitoring twice a day). High-frequency monitoring ensures the safety of excavation. If any abnormality is found, the construction can be immediately suspended (such as stopping work when the displacement rate > 5 mm / d).

[0069] The construction of the third support takes seven days, following the same process as the previous two supports. During the curing period, the pouring of the bottom cushion of the pit (C15 concrete, thickness 10 cm, completed in one day) is completed. The cushion timely seals the bottom of the pit to prevent the soft soil from heaving.

[0070] The excavation of the bottom of the pit takes two days, excavating to the design elevation (cumulative depth 10 m), leaving 20 cm for manual bottom cleaning, and the pouring of the cushion is completed within one day. Manual bottom cleaning avoids mechanical disturbance of the original soil, and the rapid sealing of the cushion controls the deformation of the bottom of the pit.

[0071] Step S6: Excavation and soil reinforcement of the foundation pit on the high elevation side 4,

[0072] Excavation of the foundation pit on the high elevation side 4: Excavate in layers (3m per layer). After each layer is excavated, wait for two days. Wait until the monitoring data is stable before continuing to excavate deeper. It will be completed in a cumulative of fifteen days (five layers * three days per layer);

[0073] Reinforcement of the passive area 7 inside the pit: Use high-pressure jet grouting piles for construction. Equip two pile drivers and construct ten piles per day (pile diameter 800mm, pile length 13m). The reinforcement range is 3 rows longitudinally * 2 rows transversely, and it will be completed in a cumulative of ten days;

[0074] Reinforcement of the active area 6 behind the piles: Synchronously carry out the construction of jet grouting piles behind the piles. The lifting speed during the construction of jet grouting piles needs to be controlled (the specification requires ≤20cm / min). Construct eight piles per day (pile diameter 800mm, pile length 15m), and it will be completed in a cumulative of twelve days;

[0075] Step S7: Monitor the whole process (synchronous with the construction),

[0076] Axial force and horizontal displacement: Monitor twice a day during excavation and once a day during the support curing period, and control the cumulative displacement ≤30mm;

[0077] Deep displacement and axial force: Monitor once before and after each layer of excavation. When abnormal (displacement rate > 5mm / d or axial force mutation > 10%), increase the monitoring frequency to once every four hours to ensure the safety of the support structure;

[0078] It is necessary to reserve 4 - 6 hours from data collection to analysis and decision-making (such as the daily report cycle issued by the third-party monitoring unit) to ensure that the problem-solving measures are implemented before the deformation gets out of control, meeting the requirements of the Technical Standard for Monitoring of Building Foundation Pit Engineering GB50497 - 2019.

[0079] Finally, it should be noted that the above are only the preferred embodiments of the present invention and are not used to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, for those skilled in the art, they can still modify the technical solutions recorded in the foregoing embodiments, or perform equivalent replacements for some of the technical features. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A soft soil non-uniform height foundation pit structure based on fluid-structure interaction, comprising a foundation pit, a support module, a reinforcement module, a drainage module and monitoring points, characterized in that: The foundation pit is divided into a low elevation side (3) and a high elevation side (4); the support module includes support piles (1) and an internal support system (2). The support piles (1) are arranged along the perimeter of the foundation pit to bear the lateral earth pressure; the internal support system (2) includes corner braces (2-1) and cross braces (2-2). The corner braces (2-1) are set at the top corners of the foundation pit, and the cross braces (2-2) are set at the center of the foundation pit; The reinforcement module includes in-pit passive zone reinforcement (7) and post-pile active zone reinforcement (6). The reinforcement area of the in-pit passive zone reinforcement (7) is the bottom of the inner perimeter of the foundation pit, and the reinforcement area of the post-pile active zone reinforcement (6) is the active zone soil in the middle of the low elevation side (3) and the high elevation side (4); The drainage module includes a cut-off curtain (5) and a drainage channel (8). The drainage channel (8) is set on the low elevation side (3) for surface drainage, and the cut-off curtain (5) is set around the perimeter of the foundation pit to block the seepage of groundwater; The monitoring points include deep displacement monitoring points (9), axial force and horizontal displacement monitoring points (10), and axial force measuring points (11) to achieve comprehensive monitoring of the deformation and internal forces of the foundation pit support module.

2. The soft soil non-uniform height foundation pit structure based on fluid-structure interaction according to claim 1, characterized in that: It also includes a fluid-solid coupling model. The fluid-solid coupling model establishes a coupling model through COMSOL to analyze the interaction between the seepage field and the stress field, and control the deformation of the foundation pit and the influence of groundwater.

3. A soft soil non-uniform height foundation pit structure based on fluid-structure interaction according to claim 2, characterized in that: After the fluid-solid coupling model is constructed, the numerical simulation method is used for slope stability analysis, effectively combining the finite element theory with numerical software, and using Midas geotechnical finite element analysis software.

4. A soft soil non-equal-height foundation pit structure based on fluid-structure interaction according to claim 1, characterized in that: The deep displacement monitoring points (9) select the middle part of the support piles (1) as the monitoring points, the axial force and horizontal displacement monitoring points (10) select the top of the support piles (1) as the monitoring points, and the axial force measuring points (11) select the stress concentration area of the cross braces (2-2) and the shear concentration area of the corner braces (2-1) as the monitoring points according to the layout form of the internal support system (2).

5. A soft soil non-uniform height foundation pit structure based on fluid-structure interaction according to claim 1, characterized in that: The insertion depth of the cut-off curtain (5) is determined according to the fluid-solid coupling model, and its control effect on the deformation of the foundation pit is verified through numerical simulation.

6. A soft soil non-uniform height foundation pit structure based on fluid-structure interaction according to claim 1, characterized in that: The in-pit passive zone reinforcement (7) uses high-pressure jet grouting piles for grid-shaped reinforcement to improve the strength of the passive soil, and the post-pile active zone reinforcement (6) uses high-pressure jet grouting piles for reinforcement to reduce the stress difference on both sides.

7. A soft soil non-uniform height foundation pit structure based on fluid-structure interaction according to claim 1, characterized in that: The internal support system (2) balances the asymmetric loads on both sides by optimizing the support spacing and cross-sectional dimensions.

8. A construction method for a soft soil non-uniform height foundation pit structure based on fluid-structure interaction, comprising the soft soil non-uniform height foundation pit structure based on fluid-structure interaction according to any one of claims 1 to 7, characterized in that: It includes the following steps: Step S1: Site pretreatment. Slope the upper part of the foundation pit, use mechanical excavation, drive the support piles (1) along the perimeter of the foundation pit, and excavate the drainage channel (8) on the low elevation side (3) of the foundation pit; Step S2: Fluid-solid coupling modeling and analysis. First, collect soil samples on site and complete indoor geotechnical tests. Then, use COMSOL to establish a three-dimensional model, input parameters and simulate the excavation plan. Finally, compare the simulation results to determine the optimal plan. The geotechnical tests strictly follow the standard procedures to ensure the accuracy of the parameters. Multiple plan simulations are fully calculated to avoid mistakes caused by rough data; Step S3: Conduct slope stability analysis using numerical simulation method, effectively combine the finite element theory with numerical software, conduct stability evaluation to obtain the main factors affecting the deformation of the foundation pit with unequal elevation differences, and determine the construction methods for foundation pit excavation and each support; Step S4: Set up a water-stop curtain (5) around the foundation pit according to the optimal plan simulated by the fluid-solid coupling model; Step S5: Asymmetric excavation and support construction; First, excavate the foundation pit on the low elevation side (3), construct three supports in sequence, and then excavate the foundation pit on the high elevation side (4) to restrain the displacement inside the high elevation side (4) pit and the displacement outside the low elevation side (3) pit; Step S6: Soil reinforcement; Reinforcement of the passive area inside the pit (7): Use high-pressure jet grouting pile construction; Reinforcement of the active area behind the pile (6): Synchronously conduct jet grouting pile construction behind the pile; Step S7: Monitor the whole process (synchronously with construction); Axial force and horizontal displacement: Monitor twice a day during excavation and once a day during the support curing period; Deep displacement and axial force: Monitor once before and after each layer of excavation. When abnormal, increase the frequency to once every four hours.

9. The construction method of a soft soil non-uniform height foundation pit structure based on fluid-structure interaction according to claim 8, characterized in that: The construction in Step S5 includes the following: First, conduct the excavation and support construction of the foundation pit on the low elevation side (3); First layer excavation: Excavate the soil on the low elevation side (3). Use mechanical excavation and quickly excavate to reduce the exposure time of the soil, and avoid the long-term loading deformation of the soft soil; First support construction: Tie steel bars → formwork support → pour C35 concrete → cure. The concrete curing needs at least 7 days (the strength reaches more than 70% under standard curing conditions) to ensure that the support has bearing capacity; Second layer excavation: Excavate to the bottom of the second support. Avoid over-excavation at one time. Conduct layered excavation to control the stress release amount each time, and let the soil stress redistribute every three days between each layer; Second support construction: Follow the same process as the first support. During the curing period, the installation of monitoring points can be carried out synchronously. The installation of monitoring points is parallel to the support curing. Arrange monitoring points at the top of the pile, the deep part of the pile body, and the key parts of the support; Third layer excavation: Excavate to the bottom of the third support. Check the displacement of the retaining pile while excavating. Conduct high-frequency monitoring to ensure the safety of excavation. If any abnormality is found, the construction can be immediately suspended; Third support construction: Follow the same process as the previous two supports. During the curing period, complete the pouring of the bottom cushion of the pit. The cushion promptly seals the bottom of the pit to prevent the soft soil from heaving; Bottom excavation of the pit: Excavate to the design elevation, reserve 20 cm for manual bottom cleaning, and complete the pouring of the cushion. Manual bottom cleaning avoids mechanical disturbance of the original soil, and the cushion quickly seals to control the bottom deformation of the pit; Then, conduct the excavation of the foundation pit on the high elevation side (4); Layered excavation: Wait for two days after each layer of excavation, and continue to dig down after the monitoring data is stable.

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