Water-coated sand layer foundation pit excavation method

Abstract

The invention relates to the technical field of geotechnical engineering and underground engineering construction, in particular to a water-covered sand layer foundation pit excavation method which comprises the following steps: S1, early-stage geological investigation: performing comprehensive geological investigation on a construction area by adopting drilling equipment, synchronously performing hydrogeological survey, and performing early-stage geological investigation; the stratum structure, the water sand layer thickness, the underground water level burial depth and the confined water head value are emphatically determined, and a complete investigation report is formed; a foundation pit area is divided into a plurality of independent waterproof subareas according to stratum permeability and water level distribution differences in an exploration result, key parameters need to be marked on each subarea, accurate data support is provided for subsequent subarea pressure reduction design, the problem of precipitation blind areas or excessive precipitation caused by uneven stratums is avoided, layered supporting is achieved, construction management and control are facilitated, and the construction efficiency is improved. The supporting system adopts a layered arrangement design and adapts to the layered excavation process of the foundation pit, so that segmented organization construction is facilitated, the single-time operation intensity is reduced, structural deformation of each stage can be accurately controlled, and the construction stability is improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of geotechnical engineering and underground engineering construction, in particular to a water-covered sand layer foundation pit excavation method. BACKGROUND

[0002] Water-covered sand layer is a common geological condition in urban geotechnical engineering, widely distributed in alluvial, diluvial, and marine geomorphic regions, especially in metro engineering, underground space engineering, and large urban infrastructure engineering along rivers and coasts in North China, East China, and South China. This type of stratum is mainly composed of medium-coarse sand, fine sand, and gravel sand, with characteristics of high permeability, high water saturation, loose structure, often with soft soil or silt soil layers, and the presence of confined water or confined sand layers. In recent years, domestic and foreign scholars and the engineering community have conducted a large number of research and engineering practice explorations on the construction of water-covered sand layer foundation pits, mainly focusing on the following aspects: (1) Dewatering technology: reducing groundwater level through well point dewatering and deep well pressure relief to reduce water head pressure at the bottom of the foundation pit; (2) Water stop curtain technology: including underground continuous wall, cement soil mixing wall, and high-pressure jet grouting curtain to block the infiltration of external water; (3) Optimization of support structure: commonly used structures such as steel sheet pile, SMW method pile, row pile + anchor rod, and internal support steel structure; (4) Foundation reinforcement technology: using grouting method, freezing method, and chemical reinforcement to improve foundation bearing capacity and impermeability; (5) Information-based monitoring and feedback control: implementing real-time monitoring of displacement, pore pressure, and deep horizontal displacement to ensure safety.

[0003] Although existing technologies have certain theories and engineering experience in water-covered sand layer foundation pits, they still have obvious deficiencies in the face of complex conditions such as water-rich sand layer with weak soil, mainly in: (1) Single dewatering method with limited control range. Single light well point or deep well dewatering often cannot form an effective underground water pressure relief area, which may cause local permeability imbalance and lead to sand flow or gushing.

[0004] (2) Water stop curtain difficulty in ensuring closure and water stop integrity. In multiple interlayers or strong permeable sand layers, conventional cement mixing walls are prone to joint leakage, causing curtain water leakage and reducing the safety level of the foundation pit.

[0005] (3) Insufficient stiffness of support system and weak deformation control ability. In high water head and large excavation depth conditions, traditional single-layer support structures are difficult to resist lateral soil and water combined pressure, and there is a risk of deformation exceeding the limit and instability.

[0006] (4) Lack of effective foundation heave resistance and emergency response mechanism. Sudden heave at the bottom of the foundation pit is often sudden, and conventional grouting treatment is difficult to control in time due to the lag in response, and does not have the ability to respond in real time and make local adjustments.

[0007] (5) Disconnection between monitoring and construction control. Although some projects have monitoring measures, they have not established an effective closed-loop mechanism of "monitoring-early warning-construction adjustment", and the monitoring data is difficult to form a basis for construction feedback.

[0008] (6) Weak integration and systematic nature of engineering. At present, most engineering measures are single-item optimizations, lacking system integration of dewatering, support, water stopping, reinforcement and information technology. Construction safety relies on experience and there is insufficient stability redundancy.

[0009] Therefore, under complex geological conditions of water-covered sand layers, there is an urgent need for a systematic, integrated, and dynamically controllable foundation pit construction technology that can effectively isolate water and reduce pressure, enhance support rigidity, and prevent foundation heave, while also enabling multi-parameter monitoring and active intervention. This will comprehensively ensure the stability, safety, and economy of the entire foundation pit construction process. Therefore, a foundation pit excavation method for water-covered sand layers is proposed. Summary of the Invention

[0010] To address the aforementioned problems, a method for excavating foundation pits with water-covered sand layers is provided to solve the issues raised in the background section.

[0011] To address the problems of existing technologies, this invention provides a method for excavating foundation pits in water-covered sand layers, comprising the following steps: Step S1, Preliminary Geological Survey: Drilling equipment was used to conduct a comprehensive geological survey of the construction area, and a hydrogeological survey was carried out simultaneously to clarify the stratigraphic structure, the thickness of the water-sand layer, the depth of the groundwater level and the value of the confined water head, and to form a complete survey report. The specific implementation steps are as follows:

[0012] 1) Use 10 light drilling rigs to conduct profile drilling, with the drilling density controlled at 10m×10m; 2) Collect borehole water samples and perform core analysis to determine particle size, permeability, and natural density; 3) Based on the difference in confined water head and permeability, the foundation pit area is divided into 4 water-proof control blocks.

[0013] Based on the above geological information, the foundation pit area is divided into several water-proof control units or partition blocks using three-dimensional modeling or profile analysis. This serves as the spatial basis for the subsequent depressurization system and curtain wall construction, ensuring the regional adaptability of the overall dewatering, support, grouting and other systems. Based on the differences in stratum permeability and water level distribution in the survey results, the foundation pit area is divided into multiple independent water-resistant zones. Each zone needs to be labeled with key parameters (such as the thickness of the permeable layer and the pressure of the confined water) to provide accurate data support for the subsequent zone depressurization design and avoid the problem of blind spots or excessive precipitation caused by uneven strata. Step S2, Construction of a closed waterproof curtain: A closed water-tight curtain is constructed around the perimeter of the foundation pit. The curtain structure can be selected according to the geological conditions, and can be a diaphragm wall, a triple high-pressure jet grouting wall, or a cement-soil mixing wall. The curtain structure needs to be buried 1.5 to 3.0 meters below the bottom of the permeable layer to ensure that the lateral seepage path of groundwater is completely cut off. During construction, the verticality of the wall (deviation not exceeding 0.3%) and the overlap width (not less than 20cm) need to be controlled to avoid leakage channels and effectively control the effect of external hydrostatic pressure on the retaining structure. Step S3: Deployment of the active precipitation system: Within the enclosure of the waterproof curtain, multiple rows of lightweight well points or vacuum well points are evenly arranged around the perimeter of the foundation pit; The precipitation system is equipped with a central control device that integrates a pressure regulation module and operation status monitoring functions (such as flow rate and vacuum monitoring). By collecting well point operation data in real time, it automatically adjusts the pumping pressure and pumping volume to achieve dynamic control of the groundwater level. At the same time, it is equipped with a backup power supply and emergency pumping equipment to prevent the risk of water level rise caused by sudden power outages. Step S4: Layered excavation and coordinated support construction: During the layered excavation of the foundation pit, a support structure is set up simultaneously. The support structure includes an inner support and an outer support. The thickness of the inner support is 6~10cm. The inner and outer supports form a synergistic stress structure to enhance the deformation resistance and construction stability. Step S5, Emergency reinforcement measures for the bottom of the foundation pit: During the construction phase of the sand layer at the bottom of the foundation pit, pre-embed the counter-pressure grouting pipe. The grouting pipe is made of steel pipe (50~80mm in diameter), and grouting holes are opened in the pipe wall (8~10mm in diameter, 15~20cm in spacing). The depth of the grouting pipe is determined according to the soil stability calculation to ensure that the end is located in the core area of ​​the sand layer. When monitoring data shows that the bottom of the foundation pit shows signs of heave (displacement exceeding 5mm / d), sand inrush, or abnormal increase in pore water pressure (exceeding the design limit by 10%), the counter-pressure grouting system should be activated immediately to inject a fast-setting grout (such as cement-water glass dual-liquid grout, with an initial setting time controlled at 3~5min). The grout is diffused into the pores of the sand layer through the grouting pressure (0.5~1.0MPa) to form a "bottom sealing stabilization layer", which effectively suppresses the risk of bottom heave and sand inrush. Step S6: Deployment of a multi-point distributed monitoring system: A multi-point distributed monitoring system is deployed inside and outside the foundation pit to automatically monitor and alarm parameters such as groundwater level, pore water pressure, support structure deformation, soil horizontal displacement, and column internal force. Based on the monitoring data, the dewatering rhythm, support segment excavation method, or the activation of the counter-pressure grouting system monitoring point settings are adjusted in real time. Step S7, Construction Closure and Completion: Pour C30 or higher impermeable (P6) concrete base slabs with a thickness of 40~60cm and a pouring speed ≤2m / h; Backfilling is carried out 28 days after the base slab is cured, using graded sand and gravel or plain soil in layers and compacted (each layer ≤30cm, compaction degree ≥93%) to ensure structural safety and construction closed loop.

[0014] As a technical solution of the present invention, in step S2, the curtain waterproofing structure is preferably a triple high-pressure jet grouting wall with a thickness of 60cm and a depth of 6-10m below the excavation depth, and its construction range is closed around the foundation pit to form a closed hydraulic structure.

[0015] As a technical solution of the present invention, in step S3, the wellpoint dewatering system adopts a vacuum wellpoint pump group and is equipped with an automatic liquid level control system, and its dewatering target water level is controlled at 1.5~2.0m below the bottom of the foundation pit excavation.

[0016] As a technical solution of the present invention, in step S5, the grout used in the counter-pressure grouting system is a water glass-cement composite fast-setting grout, the water glass concentration is 30%-40%, the cement grout ratio is 0.5:1-1:1, the grouting pressure is controlled between 0.3-0.5MPa, and the grouting flow rate is controlled between 3-6 L / min.

[0017] As a technical solution of the present invention, in step S4, the outer support structure includes: H-shaped steel columns (spacing 2.0~2.5m) connected to steel trusses to form a main frame, and is provided with reusable steel sheet pile wall surface; the steel sheet piles are connected by U-shaped interlocking joints to enhance the water-stopping ability.

[0018] As a technical solution of the present invention, in step S6, the monitoring system includes a wireless pore pressure sensor, a water level gauge, a laser displacement gauge, a steel support stress gauge, and a cloud data analysis platform, and has the functions of automatic alarm for exceeding limits and construction decision feedback.

[0019] As a technical solution of the present invention, the foundation pit is excavated in layers and sections, with each layer having a thickness of no more than 1.5m, and following the construction principle of "middle first, symmetrical sections" to ensure that stress release during the excavation process is balanced and controllable.

[0020] As a technical solution of the present invention, in step S2, the underground continuous wall is suitable for foundation pits with a large depth, exceeding 15m and complex geology, and has the advantages of strong impermeability and high rigidity. Triple high-pressure jet grouting wall: suitable for sand and gravel layers, forming a continuous waterproof layer by high-pressure jetting of cement grout; Cement-soil mixing wall: suitable for clay and silt layers, with low cost and high construction efficiency. As a technical solution of the present invention, in step S3, the lightweight well point is suitable for scenarios with shallow water depth, less than 6m, and the vacuum well point is suitable for scenarios with larger water depth, 6~12m. The well depth needs to be more than 2m greater than the designed excavation depth to ensure that the groundwater level is stable at 0.5~1m below the excavation surface after dewatering, and to avoid water inrush and quicksand during the excavation process. As a technical solution of the present invention, in step S4, the inner support structure is shotcrete + anchor or soil nail support, the shotcrete layer thickness is 6~10cm, the anchor parameters are φ22-25mm steel bars, length 3.0m, spacing 1.0~1.5m, so as to achieve the function of quickly closing the fresh excavation face and preventing the shallow soil from loosening and initial sliding deformation.

[0021] The advantages of this invention compared to the prior art are: 1. Adaptable to complex strata and risk control: For complex working conditions such as highly permeable sand layers, uneven strata and weak interlayers, the system can effectively reduce the risks of quicksand, piping and bottom heave of the foundation pit during the construction stage through accurate zoning surveys in the early stage, closed water-proof curtains to block lateral seepage, and active dewatering system to dynamically control water level. 2. Energy saving and efficiency improvement, enhanced safety redundancy: Through the synergistic effect of waterproof curtain and layered support, the overall precipitation depth requirement is reduced, and energy consumption is reduced to achieve energy saving and environmental protection; at the same time, the stress state of the foundation can be adjusted in an emergency by relying on the pre-embedded grouting system, flexibly improving the foundation safety level and enhancing construction safety redundancy. 3. Layered support facilitates construction management. The support system adopts a layered layout design, which is compatible with the layered excavation process of the foundation pit. This facilitates segmented construction, reduces the intensity of single operations, and enables precise control of structural deformation at each stage, thereby improving construction stability. Attached Figure Description

[0022] Fig. 1 This is a structural schematic diagram of a method for excavating a foundation pit in a water-covered sand layer.

[0023] Fig. 2 This is a flowchart illustrating the operation of a method for excavating a foundation pit in a water-covered sandy layer. Detailed Implementation

[0024] To further understand the features, technical means, and specific objectives and functions achieved by the present invention, the present invention will be described in further detail below with reference to the accompanying drawings and specific embodiments.

[0025] See Figs. 1-2 As shown, a method for excavating a foundation pit in a water-covered sandy layer includes the following steps: Step S1, Preliminary Geological Survey: Drilling equipment was used to conduct a comprehensive geological survey of the construction area, and a hydrogeological survey was carried out simultaneously to clarify the stratigraphic structure, the thickness of the water-sand layer, the depth of the groundwater level and the value of the confined water head, and to form a complete survey report. Based on the differences in stratum permeability and water level distribution in the survey results, the foundation pit area is divided into multiple independent water-resistant zones. Each zone needs to be labeled with key parameters (such as the thickness of the permeable layer and the pressure of the confined water) to provide accurate data support for the subsequent zone depressurization design and avoid the problem of blind spots or excessive precipitation caused by uneven strata. Step S2, Construction of a closed waterproof curtain: A closed waterproof curtain is constructed around the perimeter of the foundation pit. The curtain structure can be selected according to the geological conditions, and can be a diaphragm wall, a triple high-pressure jet grouting wall, or a cement-soil mixing wall.

[0026] The specific implementation steps are as follows: 1) Use a triple high-pressure jet grouting equipment to drive jet grouting piles along the outer ring of the foundation pit. The pile diameter is 600mm and the overlap is 20%. 2) The curtain depth is designed to be 6.5m below the bottom of the excavation, penetrating the bottom boundary of the permeable sand layer; 3) Reinforced piles are installed at corners and turning points to form a closed and continuous water-stopping barrier.

[0027] The curtain structure needs to be buried 1.5 to 3.0 meters below the bottom of the permeable layer to ensure that the lateral seepage path of groundwater is completely cut off. During construction, the verticality of the wall (deviation not exceeding 0.3%) and the overlap width (not less than 20cm) need to be controlled to avoid leakage channels and effectively control the effect of external hydrostatic pressure on the retaining structure.

[0028] The specific implementation steps are as follows:

[0029] 1) Set up 3 rows of vacuum well points around the foundation pit, with a spacing of 1.0m and a well depth of 9.0m; 2) Configure two sets of vacuum pump units and install frequency converters to achieve dynamic water level adjustment; 3) Start pumping to reduce pressure and control the groundwater level to 2.0m below the bottom of the pit within 48 hours and keep it stable. Step S3: Deployment of the active precipitation system: Within the enclosure of the waterproof curtain, multiple rows of lightweight well points or vacuum well points are evenly arranged around the perimeter of the foundation pit; The precipitation system is equipped with a central control device that integrates a pressure regulation module and operation status monitoring functions (such as flow rate and vacuum monitoring). By collecting well point operation data in real time, it automatically adjusts the pumping pressure and pumping volume to achieve dynamic control of the groundwater level. At the same time, it is equipped with a backup power supply and emergency pumping equipment to prevent the risk of water level rise caused by sudden power outages. Step S4: Layered excavation and coordinated support construction: During the layered excavation of the foundation pit, a support structure is set up simultaneously. The support structure includes an inner support and an outer support. The thickness of the inner support is 6~10cm. The inner and outer supports form a synergistic stress structure, which enhances the resistance to deformation and the stability of construction. The specific implementation steps are as follows: 1) The excavation is carried out in 7 layers using the "middle first + symmetrical partitioning" method, with each layer being 1.0m thick; 2) After each layer is excavated, sprayed concrete should be applied immediately to seal the area, with a thickness of 8cm. 3) Install soil nails simultaneously (φ25 steel bars, L=2.0m, @1.2m); 4) The outer support is provided with H-shaped steel columns (H300×300×10×15) at a spacing of 2.2m, and horizontally connected with steel sheet piles (U-shaped interlocking connection). 5) The cross bracing truss adopts a combination of I-beams and steel tie rods to ensure the overall lateral force resistance. Step S5, Emergency reinforcement measures for the bottom of the foundation pit: During the construction phase of the sand layer at the bottom of the foundation pit, pre-embed the counter-pressure grouting pipe. The grouting pipe is made of steel pipe (50~80mm in diameter), and grouting holes are opened in the pipe wall (8~10mm in diameter, 15~20cm in spacing). The depth of the grouting pipe is determined according to the soil stability calculation to ensure that the end is located in the core area of ​​the sand layer. When monitoring data shows that the bottom of the foundation pit shows signs of heave (displacement exceeding 5mm / d), sand inrush, or abnormal increase in pore water pressure (exceeding the design limit by 10%), the counter-pressure grouting system should be activated immediately to inject a fast-setting grout (such as cement-water glass dual-liquid grout, with an initial setting time controlled at 3~5min). The grout is diffused into the pores of the sand layer through the grouting pressure (0.5~1.0MPa) to form a "bottom sealing stabilization layer", which effectively suppresses the risk of bottom heave and sand inrush. Step S6: Deployment of a multi-point distributed monitoring system: A multi-point distributed monitoring system is deployed inside and outside the foundation pit to automatically monitor and alarm parameters such as groundwater level, pore water pressure, support structure deformation, soil horizontal displacement, and column internal force. Based on the monitoring data, the dewatering rhythm, support segment excavation method, or the activation of the counter-pressure grouting system monitoring point settings are adjusted in real time. The specific implementation steps are as follows: 1) Set up 8 pore pressure gauges, 4 water level gauges, 6 displacement gauges, and 4 steel support stress gauges; 2) All sensor signals are connected to the wireless acquisition gateway and uploaded to the cloud platform; 3) Set alarms for monitoring data exceeding limits: pore pressure increment > 25 kPa, column axial force > 20% variation, etc. 4) When the limit is exceeded, the automatic adjustment of the precipitation rate, the construction sequence of the support structure segments, or the activation of the grouting system will be triggered.

[0030] Step S7, Construction Closure and Completion: Pour C30 or higher impermeable (P6) concrete base slabs with a thickness of 40~60cm and a pouring speed ≤2m / h; Backfilling is carried out 28 days after the base slab is cured, using graded sand and gravel or plain soil in layers and compacted (each layer ≤30cm, compaction degree ≥93%) to ensure structural safety and construction closed loop.

[0031] The specific implementation steps are as follows:

[0032] 1) Concrete base slab with a thickness of 400mm and double-layered, bidirectional reinforcement; 2) After the concrete curing is completed, the counter-supports are removed in stages, and graded crushed stone is backfilled simultaneously; 3) Check whether the monitoring system data is stabilizing and confirm the safety of the structural closed loop; 4) Complete the construction closed loop.

[0033] In this embodiment, in step S2, the curtain waterproofing structure is preferably a triple high-pressure jet grouting wall with a thickness of 60cm and a depth of 6-10m below the excavation depth, and its construction range is closed around the foundation pit to form a closed hydraulic structure.

[0034] In this embodiment, in step S3, the wellpoint dewatering system adopts a vacuum wellpoint pump group and is equipped with an automatic liquid level control system, and its dewatering target water level is controlled at 1.5~2.0m below the bottom of the foundation pit excavation.

[0035] In this embodiment, in step S5, the grout used in the counter-pressure grouting system is a water glass-cement composite fast-setting grout with a water glass concentration of 30%-40%, a cement grout ratio of 0.5:1-1:1, an grouting pressure controlled between 0.3-0.5 MPa, and an grouting flow rate controlled between 3-6 L / min.

[0036] In this embodiment, in step S4, the outer support structure includes: H-shaped steel columns (spaced 2.0~2.5m) connected to a steel truss to form the main frame, and a reusable steel sheet pile wall is provided; the steel sheet piles are connected by U-shaped interlocking joints to enhance the water-stopping ability.

[0037] In this embodiment, in step S6, the monitoring system includes a wireless pore pressure sensor, a water level gauge, a laser displacement gauge, a steel support stress gauge, and a cloud data analysis platform, and has functions for automatic alarm for exceeding limits and construction decision feedback.

[0038] In this embodiment, the foundation pit is excavated in layers and sections, with each layer having a thickness of no more than 1.5m. The construction follows the principle of "middle first, symmetrical sections" to ensure that stress release is balanced and controllable during the excavation process.

[0039] In this embodiment, in step S2, the underground continuous wall is suitable for foundation pits with a large depth, exceeding 15m, and complex geology, and has the advantages of strong impermeability and high rigidity. Triple high-pressure jet grouting wall: suitable for sand and gravel layers, forming a continuous waterproof layer by high-pressure jetting of cement grout; Cement-soil mixing wall: suitable for clay and silt layers, with low cost and high construction efficiency. In this embodiment, the lightweight well point in step S3 is suitable for scenarios with shallow water depth, less than 6m, while the vacuum well point is suitable for scenarios with larger water depth, 6~12m. The well depth needs to be more than 2m greater than the designed excavation depth to ensure that the groundwater level is stable at 0.5~1m below the excavation surface after dewatering, and to avoid water inrush and quicksand during the excavation process. In this embodiment, in step S4, the inner support structure is shotcrete + anchor bolt or soil nail support. The shotcrete layer thickness is 6~10cm, and the anchor bolt parameters are φ22-25mm steel bars with a length of 3.0m and a spacing of 1.0~1.5m, so as to achieve the function of quickly closing the fresh excavation face and preventing the shallow soil from loosening and initial sliding deformation.

[0040] When used, it is suitable for complex strata and controls risks. For complex working conditions such as highly permeable sand layers, uneven strata and weak interlayers, it can effectively reduce the risks of quicksand, piping and bottom heave of foundation pit during the construction stage through accurate zoning survey in the early stage, closed water-proof curtain to block lateral seepage, and active dewatering system to dynamically control water level. Energy saving and efficiency improvement, enhanced safety redundancy: Through the synergistic effect of waterproof curtain and layered support, the overall precipitation depth requirement is reduced, and energy consumption is reduced to achieve energy saving and environmental protection; at the same time, the stress state of the foundation can be adjusted in an emergency by relying on the pre-embedded grouting system, flexibly improving the foundation safety level and enhancing construction safety redundancy. Layered support facilitates construction management. The support system adopts a layered layout design, which is adapted to the layered excavation process of the foundation pit. This not only facilitates segmented construction and reduces the intensity of single operations, but also enables precise control of structural deformation at each stage, thereby improving construction stability.

[0041] The above embodiments only illustrate one or more implementations of the present invention, and their descriptions are relatively specific and detailed, but they should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention. Therefore, the protection scope of the present invention should be determined by the appended claims.

Claims

1. A method for excavating a foundation pit covered with water-sand layer, characterized in that, Includes the following steps: Step S1, Preliminary Geological Survey: Drilling equipment was used to conduct a comprehensive geological survey of the construction area, and a hydrogeological survey was carried out simultaneously to clarify the stratigraphic structure, the thickness of the water-sand layer, the depth of the groundwater level and the value of the confined water head, and to form a complete survey report. Based on the differences in stratum permeability and water level distribution in the survey results, the foundation pit area is divided into multiple independent water-resistant zones. Key parameters need to be marked for each zone to provide accurate data support for subsequent zone depressurization design and avoid water-blind spots or excessive watering caused by uneven strata. Step S2, Construction of a closed waterproof curtain: A closed water-tight curtain is constructed around the perimeter of the foundation pit. The curtain structure can be selected according to the geological conditions, and can be a diaphragm wall, a triple high-pressure jet grouting wall, or a cement-soil mixing wall. The curtain structure needs to be buried 1.5 to 3.0 meters below the bottom of the permeable layer to ensure that the lateral seepage path of groundwater is completely cut off. During construction, the verticality of the wall and the overlap width need to be controlled to avoid the appearance of seepage channels and effectively control the effect of the hydrostatic pressure outside the foundation pit on the retaining structure. Step S3: Deployment of the active precipitation system: Within the enclosure of the waterproof curtain, multiple rows of lightweight well points or vacuum well points are evenly arranged around the perimeter of the foundation pit; The precipitation system is equipped with a central control device that integrates a pressure regulation module and operation status monitoring functions (such as flow rate and vacuum monitoring). By collecting well point operation data in real time, it automatically adjusts the pumping pressure and pumping volume to achieve dynamic control of the groundwater level. At the same time, it is equipped with a backup power supply and emergency pumping equipment to prevent the risk of water level rise caused by sudden power outages. Step S4: Layered excavation and coordinated support construction: During the layered excavation of the foundation pit, a support structure is set up simultaneously. The support structure includes an inner support and an outer support. The thickness of the inner support is 6~10cm. The inner and outer supports form a synergistic stress structure to enhance the deformation resistance and construction stability. Step S5, Emergency reinforcement measures for the bottom of the foundation pit: During the construction phase of the sand layer at the bottom of the foundation pit, a counter-pressure grouting pipe is pre-embedded. The grouting pipe is made of steel pipe, and grouting holes are opened in the pipe wall. The depth of the grouting pipe is determined according to the soil stability calculation to ensure that the end is located in the core area of ​​the sand layer. When monitoring data shows signs of bulging or sand inrush at the bottom of the foundation pit, or an abnormal increase in pore water pressure, the counter-pressure grouting system is immediately activated to inject fast-setting grout. The grout is diffused into the pores of the sand layer through the grouting pressure to form a "bottom sealing stabilizing layer", which effectively suppresses the risk of bottom bulging and sand inrush. Step S6: Deployment of a multi-point distributed monitoring system: A multi-point distributed monitoring system is deployed inside and outside the foundation pit to automatically monitor and alarm parameters such as groundwater level, pore water pressure, support structure deformation, soil horizontal displacement, and column internal force. Based on the monitoring data, the dewatering rhythm, support segment excavation method, or the activation of the counter-pressure grouting system monitoring point settings are adjusted in real time. Step S7, Construction Closure and Completion: Pour C30 or higher impermeable (P6) concrete base slabs with a thickness of 40~60cm and a pouring speed ≤2m / h; Backfilling is carried out 28 days after the foundation slab is cured, using graded sand and gravel or plain soil in layers and compacted to ensure structural safety and a closed-loop construction process.

2. The method for excavating a foundation pit with a water-covered sand layer according to claim 1, characterized in that, In step S2, the curtain waterproofing structure is preferably a triple high-pressure jet grouting wall with a thickness of 60cm and a depth of 6-10m below the excavation depth, and its construction range is closed around the foundation pit to form a closed hydraulic structure.

3. The method for excavating a foundation pit with a water-covered sand layer according to claim 1, characterized in that, In step S3, the wellpoint dewatering system uses a vacuum wellpoint pump set and is equipped with an automatic liquid level control system. The target water level for dewatering is controlled at 1.5~2.0m below the bottom of the foundation pit excavation.

4. The method for excavating a foundation pit with a water-covered sand layer according to claim 1, characterized in that, In step S5, the grout used in the counter-pressure grouting system is a water glass-cement composite fast-setting grout with a water glass concentration of 30%-40%, a cement grout ratio of 0.5:1-1:1, an grouting pressure controlled between 0.3-0.5 MPa, and an grouting flow rate controlled between 3-6 L / min.

5. The method for excavating a foundation pit with a water-covered sand layer according to claim 1, characterized in that, In step S4, the outer support structure includes: H-shaped steel columns (spaced 2.0~2.5m apart) connected to steel trusses to form the main frame, and a reusable steel sheet pile wall surface; the steel sheet piles are connected by U-shaped interlocking joints to enhance the water-stopping ability.

6. The method for excavating a foundation pit with a water-covered sand layer according to claim 1, characterized in that, In step S6, the monitoring system includes a wireless pore pressure sensor, a water level gauge, a laser displacement gauge, a steel support stress gauge, and a cloud data analysis platform, and has functions for automatic alarm when exceeding limits and construction decision feedback.

7. The method for excavating a foundation pit with a water-covered sand layer according to claim 1, characterized in that, The foundation pit is excavated in layers and sections, with each layer having a thickness of no more than 1.5m. The construction follows the principle of "middle first, symmetrical sections" to ensure that stress release is balanced and controllable during the excavation process.

8. The method for excavating a foundation pit with a water-covered sand layer according to claim 1, characterized in that, In step S2, the diaphragm wall is suitable for foundation pits with a large depth, exceeding 15m, and complex geology, and has the advantages of strong impermeability and high rigidity. Triple high-pressure jet grouting wall: suitable for sand and gravel layers, forming a continuous waterproof layer by high-pressure jetting of cement grout; Cement-soil mixing wall: suitable for clay and silt layers, with low cost and high construction efficiency.

9. The method for excavating a foundation pit with a water-covered sand layer according to claim 1, characterized in that, In step S4, the inner support structure is shotcrete + anchor bolt or soil nail support. The shotcrete layer thickness is 6~10cm, and the anchor bolt parameters are φ22-25mm steel bars, 3.0m in length, and 1.0~1.5m in spacing, so as to achieve the function of quickly closing the fresh excavation face and preventing the shallow soil from loosening and initial sliding deformation.

10. The method for excavating a foundation pit with a water-covered sand layer according to claim 1, characterized in that, In step S3, the lightweight well point is suitable for scenarios with shallow water depth, less than 6m, while the vacuum well point is suitable for scenarios with larger water depth, 6~12m. The well depth needs to be more than 2m greater than the designed excavation depth to ensure that the groundwater level is stable at 0.5~1m below the excavation surface after dewatering, and to avoid water inrush and quicksand during the excavation process.

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