Construction method of foundation pit support structure for replacing polluted soil body

Abstract

The invention relates to a construction method of a foundation pit support structure for replacing polluted soil, which comprises the following steps of: 1, carrying out refined investigation and three-dimensional modeling on a polluted site, and making a three-dimensional electronic map; secondly, pollution removal dynamic planning is conducted, soil layers with different pollution depths and underground water are partitioned through a three-dimensional electronic map, drainage wells are arranged according to the pollution depths of the different partitions, holes are formed through a casing pipe rotary drilling method, and the situation that in the drilling process, the upper layer soil body, the lower layer soil body and the underground water are subjected to layer mixing pollution is prevented; thirdly, earth excavation is accurately controlled; 4, constructing a retaining structure: constructing secant piles and a waterproof barrier outside the pit; 5, hierarchical control of a precipitation system: arranging a drainage well and an emergency precipitation well; and sixthly, staged excavation is conducted, specifically, layered excavation is conducted, and temporary bottom sealing plate construction is completed.

Description

Technical Field

[0001] The present invention belongs to the technical field of building construction, and particularly relates to a construction method for a foundation pit retaining structure for replacing contaminated soil bodies, which has functions of pollution isolation, precise removal, and digital control. Background Art

[0002] The retaining structure and construction method for contaminated sites are key points and difficulties. The traditional secant pile uses the mud slurry walling method for hole formation, which has the risk of soil layer pollution diffusion caused by mud leakage. The conventional treatment of contaminated soil relies on manual exploration to delimit the scope, and there is a large error in the treatment boundary, resulting in a large amount of uncontaminated soil being wrongly excavated, greatly increasing the treatment cost. The existing foundation pit dewatering mostly uses a mixed well group pumping method, which requires reducing both the phreatic water and the confined water at the same time. This not only causes cross-layer pollution of the aquifer but also leads to waste of underground water resources. Moreover, the current process lacks digital control means, and the effect of contaminated soil removal depends on manual sampling and detection.

[0003] Therefore, how to provide a construction method for a foundation pit retaining structure for replacing contaminated soil bodies is a technical problem that needs to be urgently solved by those skilled in the art. Summary of the Invention

[0004] Aiming at the technical problem of pollution during treatment in the traditional treatment of contaminated sites, the present invention proposes a construction method for a foundation pit retaining structure for replacing contaminated soil bodies. Through multiple anti-seepage barriers and a closed construction process, physical isolation of contaminated soil and water is achieved throughout the excavation process, solving industry problems such as cross-contamination of groundwater and secondary construction pollution in the existing technology. This method has functions of pollution isolation, precise removal, and digital control.

[0005] To solve the above technical problems, the present invention includes the following technical solutions: A construction method for a foundation pit retaining structure for replacing contaminated soil bodies includes the following steps: Step S1, Fine Exploration and 3D Modeling of Contaminated Sites: Horizontally scan the contaminated soil area using a ground-penetrating radar, detect the types and concentrations of pollutants using a GC-MS combined instrument, mark the pollution boundary, and import the ground-penetrating radar scan data, borehole sampling data, and pollutant information into AutoCAD Civil 3D software to generate a 3D electronic map containing the thickness of contaminated soil, permeability coefficient, and groundwater flow direction; Step S2, Dynamic Planning for Pollution Removal, carried out in two steps: Step 1, Excavation stratification and zoning of contaminated soil: According to the 3D electronic map generated in Step S1, the contaminated soil area is zoned according to different contaminated depth soil layers and groundwater, identifying the differences in contaminated depth and the changes in lithological interfaces within the contaminated soil area; taking the difference in contaminated depth greater than 2m or the location of lithological interface change as the boundary, the contaminated soil area is divided into several zones; for each zone, based on its soil lithology and stability, the optimal excavation stratification height is calculated through the CivilGPT knowledge model in combination with the finite element analysis model, and the optimal stratification height is determined by comprehensively considering the self-stabilizing ability of the soil and the efficiency of the excavation machinery; the lithological interface stratification method is adopted, and the contaminated soil is excavated and stratified according to the calculated optimal stratification height. Step 2, Control of pumping and drainage of contaminated water: For each contaminated soil zone divided in the first step of Step S2, according to the depth information of the contaminated phreatic aquifer in its 3D electronic map, dewatering wells are set in the outer edge area of the foundation pit in this zone and at the contaminated depths of different zones; the casing rotary drilling method is used to form the holes, ensuring that the casing is advanced at least 1.5m below the contaminated layer to prevent cross-layer pollution of the upper and lower soil layers and groundwater during the drilling process; the well depth of the dewatering well is set as the bottom depth of the contaminated phreatic aquifer in this zone + 1.5m; before the earth excavation, start pumping water from the dewatering wells and ensure that the groundwater level in this zone is lowered to at least 0.5m below the bottom of the contaminated layer. Step S3, Construction of retaining structure: At the designed positions around the foundation pit, the casing rotary drilling method is used to form holes to construct a ring of interlocking piles of meat and vegetables; in the contaminated soil zones outside the foundation pit, dewatering wells are set according to the second step of Step S2; at the preset anti-floating point positions at the bottom of the foundation pit, the casing rotary drilling method is used to form holes to construct anti-floating piles. Step S4, Precise control of earth excavation: Support is constructed in the foundation pit; earth excavation is carried out, and the Trimble Earthworks system is linked with the GNSS positioning of the excavator to obtain the position information of the excavator bucket in real time and compare it with the designed excavation surface in the 3D electronic map; the over-excavation alarm threshold is set at ±10cm, and an alarm is triggered when the actual excavation depth exceeds the designed value by ±10cm; stop excavating when reaching 0.3 - 0.5m above the designed elevation of the contaminated soil floor slab, and construct the cushion layer; at the same time, actively construct a water barrier outside the foundation pit, specifically: drive SP-IV type L-shaped steel sheet piles along the outer edge line of the foundation pit to form a continuous vertical partition belt; then, grouting reinforcement is carried out outside the steel sheet piles, the grouting hole spacing is 0.8m, double-fluid grout is injected, and the slurry diffusion radius is controlled ≥0.5m, so that the permeability coefficient of the stratum after grouting ≤1×10 -6 cm / s to form a horizontal water barrier.

[0006] Step S5, hierarchical control of the dewatering system: in the contaminated soil zone at the outer edge of the foundation pit, a dewatering well 15 as described in the second step of step S2 is arranged to undertake the main task of pumping out contaminated groundwater; in the non-major contaminated area or risk-critical position around the foundation pit, an additional emergency dewatering well 10 is arranged, the distance between the emergency dewatering wells is ≤25m, UPVC double-wall corrugated pipes are pre-buried in the wells, and a backup power supply system is configured to ensure that the power switching response time after power failure is ≤10s; the dewatering wells and the emergency dewatering wells together constitute a hierarchical dewatering system.

[0007] Step S6, excavate in layers and construct a temporary bottom plate: excavate in layers according to the layer height determined in the first step of step S2; after each layer of earthwork is excavated, immediately install the support corresponding to that layer and apply the designed prestressing to 75%; after the excavation reaches the designed elevation of the base and the cushion layer construction is completed, construct a temporary bottom plate on the base; the temporary bottom plate is cast in sections, with each section size ≤6m×6m, and the interval time between casting adjacent sections ≥72 hours.

[0008] Furthermore, it also includes step S7, verification of the pollution removal effect: 1. Grid sampling points are arranged on the four walls and base of the foundation pit, and a heavy metal rapid detector is used for on-site screening; cores are drilled from suspected points and sent to the laboratory, and toxicity leaching tests are carried out according to relevant standards, and toxicity leaching tests are carried out according to the national standard "Toxic leaching method of solid waste leaching - acetic buffer solution method" (HJ / T 299), thereby completing sampling and testing; 2. After the drainage well is continuously pumped for 48 hours, water samples are collected to test COD and heavy metal ion concentrations, and an acceptance certificate is issued after meeting the standards, thereby completing the groundwater acceptance.

[0009] Furthermore, it also includes step S8, the connection between site delivery and development: 1. The surface of the temporary bottom plate is roughened, shear-resistant steel bars are implanted, and it is cast as a whole with the permanent raft slab; a 500m high connector is reserved on the top of the column pile, which is welded and fixed to the steel column of the building basement to complete the structural transformation; 2. Fiber grating sensor groups are embedded in the key parts of the support structure, and the key parts include retaining piles (meat-and-vegetable bite piles) and horizontal support structures; the horizontal displacement of the retaining piles and the axial force of the horizontal support are monitored in real time; the monitoring and early warning thresholds are set: the cumulative change in the horizontal displacement of the retaining piles is ≤10mm / day and the total amount does not exceed the design allowable value, and the change in the horizontal support axial force does not exceed ±10% of the design value; when the monitoring data exceeds the early warning threshold, the platform alarm is automatically triggered; the monitoring data is uploaded to the cloud platform in real time to evaluate the safety status of the foundation pit structure, thereby completing the structural safety monitoring.

[0010] Furthermore, step S1 includes: when the geological radar performs horizontal scanning, the grid spacing is ≤2m×2m, and the detection depth is at least 2m below the contaminated layer; the drilling sampling spacing is ≤10m, and the sampling depth interval is at least 0.5m.

[0011] Further, when pumping out the polluted water in step S2, it is necessary to ensure that the water level drops at least 0.5 m below the bottom of the polluted layer.

[0012] Further, in step S3, the depth of the dewatering wells is determined by adding 0.5 m to the depth of the polluted water in different areas. The outside of the pit is reinforced by grouting to block water and seal cracks. Before the earth excavation, the dewatering wells are used to pump out the polluted water 0.5 m below the bottom of the polluted water layer.

[0013] Further, in step S3, the type of the retaining piles is the alternate reinforced concrete and soil-cement piles. The holes are formed by the casing rotary drilling method. To ensure that the confined water does not need to be lowered during the excavation process, the depth of the retaining piles in different partitions is determined by the following formula: D ≥ ( γ w H w - γ s h s ) / (γ ' (1 + λ)); where D is the minimum depth of the retaining piles entering the water-resistant layer, in m; γ w is the unit weight of water, in 10 kN / m³; H_w: the height of the confined water head, in m; γ s is the weighted average unit weight of the overlying soil layer, in kN / m³; h s is the thickness of the overlying soil layer, in m; γ ' is the effective unit weight of the soil in the water-resistant layer; λ is the safety factor, and λ takes 1.1 - 1.3.

[0014] Further, in step S4, the overexcavation alarm threshold is set at ±10 cm; the polluted soil is transported by an enclosed muck truck with the hopper covered with an anti-seepage membrane, and the transportation route is isolated from the clean area; after excavating to the design elevation in layers, immediately spray a 1% polymer curing agent solution to seal the exposed soil layer to prevent the volatilization of pollutants.

[0015] Further, in step S4, the construction of the outside-pit water barrier actively on the outside of the foundation pit includes: driving SP-IV type Larssen steel sheet piles along the outside of the foundation pit to form a partition belt; the grouting uses double-fluid grout, the spacing of the grouting holes is 0.8 m, the diffusion radius ≥ 0.5 m, and the permeability coefficient of the stratum after grouting ≤ 1×10⁻ 6 cm / s.

[0016] Further, the setting of the dewatering wells in step S5 includes: the well depth of the dewatering wells = the bottom depth of the polluted phreatic aquifer + 1.5 m, and the submersible pump with frequency conversion is used, which starts and stops automatically according to the data of the real-time water level sensor; the arrangement of the emergency dewatering wells includes: the well spacing of the emergency dewatering wells ≤ 25 m, the UPVC double-wall corrugated pipe is embedded in the wells, and the standby power supply switching response time ≤ 10 s.

[0017] Compared with the prior art, the present invention has the following advantages and beneficial effects: The present invention provides a construction method for a foundation pit retaining structure for replacing polluted soil bodies. Through multiple anti-seepage barriers and a closed construction process, physical isolation of polluted soil and water is achieved throughout the excavation process, solving industry problems such as cross-contamination of groundwater and secondary construction pollution in the prior art. Aiming at the problems of fuzzy pollution boundaries and difficult quantification of treatment scopes, a three-dimensional electronic map digital treatment technology is innovated, improving the pollution treatment accuracy from the traditional meter level to the centimeter level. While ensuring the complete removal of pollution, the amount of ineffective soil treatment is reduced. By constructing a "survey - design - construction - verification" process and using three-dimensional electronic map technology, problems such as non-traceability of treatment effects and lag in emergency response in traditional processes are solved. A series of methods for foundation pit reinforcement are proposed to ensure the safety and stability of the foundation pit during the vacant period. This method fundamentally solves the technical problem of "polluting while treating" in traditional polluted site treatment. Description of the Drawings

[0018] Figures 1 to 5 It is a schematic diagram of steps S1 to S5 of the construction method for a foundation pit retaining structure for replacing polluted soil bodies in an embodiment of the present invention; Figure 6 It is a flow chart of the construction method for a foundation pit retaining structure for replacing polluted soil bodies in an embodiment of the present invention.

[0019] In the figure, 1 - polluted soil area, 2 - local polluted soil excavation area, 3 - polluted phreatic aquifer, 4 - unpolluted phreatic aquifer, 5 - aquiclude, 6 - confined aquifer, 7 - cushion, 8 - support, 9 - temporary sealed bottom slab, 10 - emergency dewatering well, 11 - column, 12 - anti-floating pile, 13 - column pile, 14 - grouting reinforcement area, 15 - dewatering well, 16 - meat and vegetarian interlocking pile. Detailed Embodiments

[0020] The following further elaborates in detail on a construction method for a foundation pit retaining structure for replacing polluted soil bodies provided by the present invention in combination with the accompanying drawings and specific embodiments. According to the following description, the advantages and features of the present invention will be clearer.

[0021] Embodiment The following combines Figures 1 to 6 , and details the construction method for a foundation pit retaining structure for replacing polluted soil bodies of the present invention.

[0022] Please continue to refer to Figures 1 to 6 , a construction method for a foundation pit retaining structure for replacing polluted soil, comprising the following steps: Step S1, refined exploration and 3D modeling of the polluted site: Horizontally scan the polluted soil area 1 using a ground-penetrating radar, detect the types and concentrations of pollutants using a GC-MS combined instrument, mark the pollution boundary, and import the ground-penetrating radar scan data, borehole sampling data, and pollutant information into the AutoCAD Civil 3D software to generate a 3D electronic map containing the thickness of the polluted soil, permeability coefficient, and groundwater flow direction; Step S2, dynamic planning for pollution removal, carried out in two steps: The first step, excavation of polluted soil in layers and zones: According to the 3D electronic map generated in Step S1, identify the differences in pollution depth and changes in lithological interfaces within the polluted soil area; taking the difference in pollution depth greater than 2m or the change in lithological interface as the boundary, divide the polluted soil area into several zones; for each zone, based on its soil lithology and stability, calculate the optimal excavation layer height through the CivilGPT knowledge model and in combination with the midas GTS NX finite element analysis model, and the optimal layer height is determined by comprehensively considering the self-stabilizing ability of the soil and the efficiency of the excavation machinery; adopt the lithological interface layering method and carry out the excavation of the polluted soil in layers according to the calculated optimal layer height; Please refer to Figure 2 , the specific polluted soil zones include: the local polluted soil excavation area 2, the polluted phreatic aquifer 3, the unpolluted phreatic aquifer 4, the aquitard 5, and the confined aquifer 6.

[0023] The second step, control of pumping and drainage of polluted water: For each polluted soil zone divided in the first step of Step S2, according to the depth information of the polluted phreatic aquifer in its 3D electronic map, set dewatering wells in the outer edge area of the foundation pit of this zone; the dewatering wells are drilled using the casing rotary drilling method to ensure that the casing follows at least 1.5m below the polluted layer to prevent cross-layer pollution of the upper and lower soil layers and groundwater during the drilling process, and also ensure that all the polluted soil is dug out; the well depth of the dewatering well 15 is set to the bottom depth of the polluted phreatic aquifer in this zone +1.5m; before the earth excavation, start pumping water from the dewatering wells and ensure that the groundwater level in this zone drops at least 0.5m below the bottom of the polluted layer.

[0024] Step S3, construction of retaining structure: at the designed position around the foundation pit, use the casing rotary drilling method to drill holes and construct a circle of meat-vegetable bite piles 16; in the contaminated soil zone outside the foundation pit, set up a drainage well 15 according to the second step of step S2; at the preset anti-floating point position at the bottom of the foundation pit, use the casing rotary drilling method to drill holes and construct anti-floating piles 12; Step S4, precise control of earthwork excavation: construct support 8 in the foundation pit; carry out earthwork excavation, use Trimble The Earthworks system is linked to the excavator's GNSS positioning to obtain the excavator bucket position information in real time and compare it with the designed excavation surface in the three-dimensional electronic map; the over-excavation alarm threshold is set to ±10cm, and the alarm is triggered when the actual excavation depth exceeds the design value by ±10cm; excavation is stopped after excavation reaches 0.3~0.5m above the design elevation of the contaminated soil bottom plate, and the concrete cushion layer 7 is immediately constructed; at the same time, an external water barrier is actively constructed outside the foundation pit, specifically: SP-IV type Larsen steel sheet piles are driven along the outer edge of the foundation pit to form a continuous vertical partition belt; then grouting reinforcement is carried out on the outside of the steel sheet piles, the spacing between grouting holes is 0.8m, and double liquid slurry is injected to control the slurry diffusion radius to be ≥0.5m, so that the permeability coefficient of the stratum after grouting is ≤1×10 -6 cm / s, forming a horizontal aquiclude.

[0025] Step S5, hierarchical control of the dewatering system: in the contaminated soil zone at the outer edge of the foundation pit, a dewatering well 15 as described in the second step of step S2 is arranged to undertake the main task of pumping out contaminated groundwater; in the non-major contaminated area or risk-critical position around the foundation pit, an additional emergency dewatering well 10 is arranged, the distance between the emergency dewatering wells is ≤25m, UPVC double-wall corrugated pipes are pre-buried in the wells, and a backup power supply system is configured to ensure that the power switching response time after power failure is ≤10s; the dewatering wells and the emergency dewatering wells together constitute a hierarchical dewatering system.

[0026] Step S6, layered excavation and construction of temporary bottom plate 9: perform layered excavation according to the layered height determined in the first step of step S2; after each layer of earthwork is excavated, immediately install the support corresponding to that layer and apply the designed prestressing to 75%; after the excavation reaches the designed elevation of the base and the cushion layer construction is completed, construct a temporary bottom plate on the base; the temporary bottom plate is cast in batches according to the partitions, with each block size ≤6m×6m, and the interval time between casting adjacent blocks ≥72 hours.

[0027] In this embodiment, more preferably, it further includes step S7, verification of pollution removal effect: First, arrange grid sampling points on the four walls and the base of the foundation pit, and conduct on-site screening using a rapid heavy metal detector; drill cores from suspected points and send them to the laboratory for toxicity leaching tests according to the national standard "Leaching Method for Toxicity of Solid Wastes - Acetic Acid Buffer Solution Method" (HJ / T 299), so as to complete sampling and detection; Second, after the dewatering wells continuously pump water for 48 hours, collect water samples to detect the COD and heavy metal ion concentrations, and issue an acceptance certificate after passing the standards, so as to complete the groundwater acceptance.

[0028] In this embodiment, more preferably, it further includes step S8, connection between site delivery and development: First, roughen the surface of the temporary sealing floor slab 9, implant shear-resistant steel bars, and pour it into an integral body with the permanent raft slab; reserve a 500mm-high connector at the top of the column pile 13 and weld it to the steel column of the building basement to complete the structural transformation; Second, embed a fiber Bragg grating sensor group at the key parts of the support structure, and the key parts include retaining piles, such as selecting the alternate cast-in-place pile 16 and the horizontal support structure; monitor the horizontal displacement of the retaining pile and the axial force of the horizontal support in real time; set the monitoring warning threshold: the cumulative change in the horizontal displacement of the retaining pile ≤ 10mm / day and the total amount does not exceed the design allowable value, and the change range of the axial force of the horizontal support does not exceed ±10% of the design value; when the monitoring data exceeds the warning threshold, automatically trigger the platform alarm; the monitoring data is uploaded to the cloud platform in real time for evaluating the safety status of the foundation pit structure, so as to complete the structural safety monitoring.

[0029] In this embodiment, more preferably, step S1 includes: when the geological radar performs horizontal scanning, the grid spacing ≤ 2m×2m, and the detection depth reaches at least 2m below the pollution layer; the drilling sampling spacing ≤ 10m, and the sampling depth interval is at least 0.5m.

[0030] In this embodiment, more preferably, when pumping and draining the polluted water in step S2, it is necessary to ensure that the water level drops at least 0.5m below the bottom of the pollution layer.

[0031] In this embodiment, more preferably, the depth of the dewatering wells in step S3 is based on the depth of the polluted water in different areas + 0.5m. The grouting reinforcement method is used outside the pit to block water and seal the cracks, forming a grouting reinforcement area 14; before the earth excavation, use the dewatering wells to pump out the polluted water 0.5m below the bottom of the polluted water.

[0032] Particularly, the unpolluted phreatic aquifer 4 is provided with column piles 13 and anti-floating piles 12. There is a column 11 above the column pile 13. The function of the anti-floating pile 12 is to offset the groundwater buoyancy, adapt to the dynamic change of the water level, and disperse the concentrated action of the buoyancy on the foundation pit floor slab.

[0033] In this embodiment, more preferably, in step S3, the retaining piles are of the type of alternating plain and reinforced concrete piles 16, and the holes are formed by the casing rotary drilling method. To ensure that the confined water does not need to be lowered during the excavation process, the depths of the retaining piles in different zones are determined by the following formula: D ≥ ( γ w H w - γ s h s ) / (γ ' (1 + λ)); where D is the minimum depth of the retaining pile entering the water - resistant layer, in m; γ w γ_w is the unit weight of water, in 10 kN / m³; H_w is the confined water head height, in m; γ s γ is the weighted average unit weight of the overlying soil layer, in kN / m³; h s h is the thickness of the overlying soil layer, in m; γ ' γ' is the effective unit weight of the water - resistant layer soil; λ is the safety factor, and λ takes values from 1.1 to 1.3.

[0034] In this embodiment, more preferably, in step S4, the over - excavation alarm threshold is set to ±10 cm; the polluted soil is transported by enclosed muck trucks with the truck bed covered with anti - seepage membranes, and the transportation route is isolated from the clean area; after excavation to the design elevation in layers, a 1% polymer curing agent solution is immediately sprayed to seal the exposed soil layer to prevent pollutant volatilization.

[0035] In this embodiment, more preferably, in step S4, the construction of the external - pit water - proof barrier outside the foundation pit actively includes: driving SP - IV type Larssen steel sheet piles along the outside of the foundation pit to form a partition zone; the grouting uses double - liquid grout, the grouting hole spacing is 0.8 m, the diffusion radius ≥ 0.5 m, and the permeability coefficient of the stratum after grouting ≤ 1×10⁻ 6 cm / s.

[0036] In this embodiment, more preferably, in step S5, the arrangement of dewatering wells includes: the well depth of the dewatering wells = the bottom depth of the polluted phreatic aquifer + 1.5 m, and the submersible pumps used are variable - frequency submersible pumps, which are automatically started and stopped according to the data of the real - time water - level sensors; the arrangement of emergency dewatering wells includes: the well spacing of the emergency dewatering wells ≤ 25 m, UPVC double - wall corrugated pipes are embedded in the wells, and the response time of the standby power supply switch ≤ 10 s.

[0037] In this embodiment, more preferably, in step S6, the layered excavation control includes: the excavation thickness of each layer is carried out according to the completed design plan. After excavation, steel supports are installed in a timely manner, and the preloading force is applied to 75% of the design value; the monitoring frequency of the support axial force is ≥ 1 time / 8h, and an acoustic-optical alarm is triggered when the over-limit value (±10%) is reached. The construction of the temporary bottom slab 9 includes: determining the thickness of the partitioned temporary bottom slab according to the construction large model, with the block size ≤ 6m × 6m and the skip interval ≥ 72 hours; the concrete pouring temperature is controlled at 5~30°C, and the surface is covered with a waterproof geomembrane during the curing period.

[0038] Specifically, for the construction of the bottom slab, an emergency dewatering well 10 is set up, and the assessment of the contaminated soil cleaning is carried out in combination with the three-dimensional electronic map. During the period when the foundation pit is vacant, the foundation pit needs to be regularly evaluated for safety to ensure the stability of the supporting structure, surrounding buildings and underground pipelines.

[0039] The above examples are the preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. The above embodiments only represent one implementation manner of the present invention, and its description is relatively specific and detailed, but it cannot be understood as a limitation on the scope of the invention patent. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present invention, several modifications and improvements can still be made, and these all belong to the protection scope of the present invention. Therefore, the protection scope of the invention patent shall be subject to the appended claims.

Claims

1. A construction method for a foundation pit retaining structure for replacing contaminated soil, characterized in that, The steps include: Step S1, detailed investigation and three-dimensional modeling of the contaminated site: Use geological radar to horizontally scan the contaminated soil area, use GC-MS combined instrument to detect the type and concentration of pollutants, mark the contaminated boundary, import the geological radar scanning data, drilling sampling data and pollutant information into AutoCAD Civil 3D software, and generate a three-dimensional electronic map including the thickness of the contaminated soil, permeability coefficient and groundwater flow direction; Step S2: Dynamic planning of pollution removal is carried out in two steps: The first step is to excavate the contaminated soil in layers and zones: Based on the three-dimensional electronic map, identify the difference in contamination depth and the change in rock interface in the contaminated soil area; divide the contaminated soil area into several zones based on the difference in contamination depth greater than 2m or the change in rock interface; for each zone, calculate the optimal excavation layer height based on its soil lithology and stability through the CivilGPT knowledge model combined with the finite element analysis model; Using the lithology interface stratification method, excavating and stratifying the contaminated soil according to the optimal stratification height obtained by the calculation; The second step is to control the drainage of contaminated water: for each contaminated soil zone, according to the depth information of the contaminated water layer in its three-dimensional electronic map, a drainage well is set up in the outer edge area of ​​the foundation pit of the zone; the drainage well is drilled by the casing rotary drilling method to ensure that the casing is followed to at least 1.5m below the contaminated layer to prevent the upper and lower layers of soil and groundwater from being polluted during the drilling process; the depth of the drainage well is set to the bottom depth of the contaminated water layer in the zone + 1.5m; before excavation, start pumping water from the drainage well and ensure that the groundwater level in the zone drops to at least 0.5m below the bottom of the contaminated layer; Step S3, construction of retaining structure: at the designed position around the foundation pit, use casing rotary drilling method to drill holes, and construct a circle of meat-vegetable interlocking piles; in the contaminated soil zone outside the foundation pit, set up drainage wells according to the second step in step S2; at the preset anti-floating point position at the bottom of the foundation pit, use casing rotary drilling method to drill holes, and construct anti-floating piles; Step S4, precise control of earthwork excavation: construct support in the foundation pit; carry out earthwork excavation, use Trimble Earthworks system to link with the excavator GNSS positioning device, obtain the excavator bucket position information in real time and compare it with the designed excavation surface in the three-dimensional electronic map; set the over-excavation alarm threshold to ±10cm, and trigger the alarm when the actual excavation depth exceeds the design value by ±10cm; stop excavation when the distance to the design elevation of the contaminated soil bottom plate is 0.3~0.5m above, and immediately construct the concrete cushion layer; at the same time, actively construct the outer pit water barrier outside the foundation pit; Step S5, hierarchical control of the precipitation system: a drainage well is set up in the contaminated soil zone at the outer edge of the foundation pit to undertake the main task of pumping out the contaminated groundwater; in the non-major contaminated area or risk-critical location around the foundation pit, an additional emergency precipitation well is arranged, the distance between the emergency precipitation wells is ≤25m, UPVC double-wall corrugated pipes are pre-buried in the wells, and a backup power supply system is configured to ensure that the power switching response time after power failure is ≤10s; the drainage well and the emergency precipitation well together constitute a hierarchical precipitation system; Step S6, excavate in layers and construct a temporary bottom plate: excavate in layers according to the layer height determined in the first step of step S2; after each layer of earthwork is excavated, immediately install the support corresponding to that layer and apply the designed prestressing to 75%; after the excavation reaches the designed elevation of the base and the cushion layer construction is completed, construct a temporary bottom plate on the base; the temporary bottom plate is cast in batches according to the partitions, with each block size ≤6m × 6m, and the interval time between casting adjacent blocks ≥72 hours.

2. The construction method according to claim 1, characterized in that, It also includes step S7, verification of the pollution removal effect:

1. Grid sampling points are arranged on the four walls and base of the foundation pit, and a heavy metal rapid detector is used for on-site screening; cores are drilled from suspected points and sent to the laboratory for toxicity leaching tests, thereby completing sampling and testing; 2. After the drainage well has been continuously pumped for 48 hours, water samples are collected to test COD and heavy metal ion concentrations. After meeting the standards, an acceptance certificate is issued, thereby completing the groundwater acceptance.

3. The construction method according to claim 2, characterized in that, It also includes step S8, site delivery and development connection:

1. The surface of the temporary bottom plate is roughened, shear steel bars are implanted, and it is cast as a whole with the permanent raft slab; a 500mm high connector is reserved on the top of the column pile, and it is welded and fixed to the steel column of the building basement to complete the structural transformation; 2. The fiber grating sensor group is embedded in the designed part of the support structure, and the designed part includes the retaining pile and the horizontal support; the horizontal displacement of the retaining pile and the axial force of the horizontal support are monitored in real time; Set monitoring and early warning thresholds: the cumulative change in horizontal displacement of retaining piles is ≤10mm / day and the total amount does not exceed the design allowable value, and the change in horizontal support axial force does not exceed ±10% of the design value; when the monitoring data exceeds the early warning threshold, the platform alarm is automatically triggered; the monitoring data is uploaded to the cloud platform in real time to evaluate the safety status of the foundation pit structure, thereby completing structural safety monitoring.

4. The construction method according to claim 1, wherein The step S1 includes: when the geological radar performs horizontal scanning, the grid spacing is ≤2m×2m, and the detection depth is at least 2m below the contaminated layer; the drilling sampling spacing is ≤10m, and the sampling depth interval is at least 0.5m.

5. The construction method according to claim 1, characterized in that, When the polluted water is pumped out in step S2, it is necessary to ensure that the water level drops to at least 0.5 m below the bottom of the pollution layer.

6. The construction method according to claim 1, wherein In step S3, the depth of the drainage well is +0.5m according to the depth of polluted water in different areas. Grouting reinforcement is used outside the pit to block water and seal cracks. Before earth excavation, the drainage well is used to pump out the polluted water 0.5m below the bottom of the polluted water.

7. The construction method according to claim 1, characterized in that, In step S3, the retaining pile type is a meat-vegetable interlocking pile, and the casing rotary drilling method is used to form the hole. To ensure that the excavation process does not require the release of pressure water, the depth of the retaining piles in different partitions is determined by the following formula: D ≥ ( γ w H w - γ s h s ) / (γ ' (1 + λ)); Wherein, D is the minimum depth of the retaining pile entering the water - resistant layer, with the unit of m; γ w is the unit weight of water, with the unit of 10 kN / m³; H w : the confined water head height, with the unit of m; γ s is the weighted average unit weight of the overlying soil layer, with the unit of kN / m³; h s is the thickness of the overlying soil layer, with the unit of m; γ ' is the effective unit weight of the water - resistant layer soil; λ is the safety factor, and λ takes values from 1.1 to 1.

3.

8. The construction method according to claim 1, characterized in that, In step S4, the over-excavation alarm threshold is set to ±10 cm; the contaminated soil is transported by a closed dump truck, the truck bed is covered with an anti-seepage membrane, and the transportation path is isolated from the clean area; after the layered excavation reaches the designed elevation, 1% polymer curing agent solution is immediately sprayed to seal the exposed soil layer to prevent the volatilization of pollutants.

9. The construction method according to claim 1, characterized in that In the step S4, the construction of the external impervious barrier outside the foundation pit actively includes: driving SP-IV type Larssen steel sheet piles along the outside of the foundation pit to form a partition zone; the grouting uses double-fluid grout, the spacing of the grouting holes is 0.8 m, the diffusion radius is ≥ 0.5 m, and the permeability coefficient of the stratum after grouting is ≤ 1×10⁻ 6 cm / s.

10. The construction method according to claim 1, characterized in that, The setting of dewatering wells in step S5 includes: the well depth of the dewatering well = the bottom depth of the polluted phreatic aquifer + 1.5 m, and the submersible pump with frequency conversion is used, which starts and stops automatically according to the data of the real-time water level sensor; the arrangement of emergency precipitation wells includes: the well spacing of the emergency precipitation wells ≤ 25 m, UPVC double-wall corrugated pipes are embedded in the wells, and the standby power supply switching response time ≤ 10 s.

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