A composite water-resisting and hierarchical support cooperative construction method for deep foundation pit

Abstract

The application relates to a composite water-resisting and hierarchical support cooperative construction method for a deep foundation pit, constructs surface water interception and drainage and an underground composite water-resisting enclosure structure, deeply integrates water-stopping function and enclosure soil-retaining function, completely blocks a seepage channel while improving the overall rigidity of the enclosure system; adopts a hierarchical support system combining upper rigid support and lower flexible support to accurately match support rigidity and deep direction soil pressure distribution, and considers both shallow deformation control and deep construction economy; establishes a three-level hydraulic control system of outer interception, middle separation and inner reduction to make the dewatering system and the water-stopping system cooperate, guarantees dry working conditions in the pit while minimizing the influence of dewatering on the surrounding environment; and realizes real-time cooperative adjustment of the dewatering rate and the support axial force through dynamic monitoring, so that the systems can cooperate and restrict each other according to the dynamic changes of the construction conditions, and the comprehensive optimization of the water-stopping effect, the deformation control capability and the construction economy of the long and large deep foundation pit near a lake is realized.

Description

Technical Field

[0001] This application relates to the field of foundation pit construction, specifically to a method for the coordinated construction of composite waterproofing and graded support for deep foundation pits. Background Technology

[0002] With the rapid development of urban underground space and the construction of transportation infrastructure, long and deep foundation pit projects near lakes and rivers are becoming increasingly common. These foundation pits are generally characterized by their large length, great depth, high groundwater level, and strong lateral recharge from lakes, resulting in high construction risks and imposing stringent requirements on foundation pit deformation control, water-stopping effect, and construction economy.

[0003] In related technologies, deep foundation pit engineering typically employs support methods such as diaphragm walls with internal bracing, bored piles with water-stop curtains and internal bracing, and pile-anchor support for construction.

[0004] However, while diaphragm walls offer good water-stopping effects, they are expensive, slow to construct, and cause serious mud pollution. In contrast, the combination of cast-in-place piles, water-stop curtains, and supports often involves separate designs for water-stopping, retaining, supporting, and dewatering systems, resulting in a lack of overall coordination. Under high water levels near lakes, gaps or seepage channels can easily form between the water-stop curtain and the retaining piles, making it impossible to effectively reduce water pressure outside the pit and potentially leading to piping accidents. Summary of the Invention

[0005] This application provides a composite waterproofing and graded support collaborative construction method for deep foundation pits, which can solve the technical problems in related technologies where the various construction systems of deep foundation pits lack overall coordination and it is difficult to simultaneously ensure the waterproofing effect, deformation control capability, and construction economy of long, deep foundation pits near lakes.

[0006] This application provides a method for the coordinated construction of composite waterproofing and graded support for deep foundation pits, which includes the following steps: The surface drainage system and the composite waterproof retaining structure around the foundation pit are constructed simultaneously to form an integrated waterproof system that coordinates the surface and underground. As the foundation pit is excavated in layers, a graded support system combining upper rigid support and lower flexible support is constructed, and an axial force compensation device is installed between the rigid support and the flexible support. The drainage system inside the construction pit, together with the surface interception and drainage system and the composite waterproof retaining structure, forms a three-level hydraulic control system. The entire construction process of the foundation pit is dynamically monitored, and the dewatering rate and support axial force are adjusted in a coordinated manner based on the monitoring data. In one embodiment, the synchronously constructed surface drainage system and the composite waterproof retaining structure around the foundation pit include: Construct intercepting ditches and retaining walls around the top of the foundation pit to form a surface drainage system; A continuous, closed water-stop curtain was constructed along the designed edge of the foundation pit; Construct retaining piles inside the water-stop curtain to create a gap between the retaining piles and the water-stop curtain. Grouting material is injected into the gap to form a pile-curtain composite waterproof layer, connecting the retaining piles and the water-stop curtain into a whole.

[0007] In one embodiment, the upper rigid support is a reinforced concrete support, which is reliably connected to the capping beam at the top of the retaining pile.

[0008] In one embodiment, the lower flexible support is a steel pipe support, and the end of the steel pipe support is provided with a hinge head, through which a pre-applied axial force is applied to the steel pipe support.

[0009] In one embodiment, the axial force compensation device is connected to the upper rigid support and the lower flexible support respectively through a steel pad box; The axial force compensation device includes a hydraulic jack or a wedge-shaped adjusting block, which can dynamically adjust the pre-applied axial force of the lower flexible support based on at least one of the monitoring data of pile body inclination, support axial force, ground surface settlement, pit top settlement or pit bottom heave, so as to achieve a smooth transition of support stiffness.

[0010] In one embodiment, the drainage system within the construction pit includes: Dewatering wells are installed within the foundation pit to form a deep well depressurization system; Drainage ditches and collection wells are set up around the perimeter of the foundation pit to form a shallow open drainage system; The deep well depressurization system and the shallow open drainage system together form a two-stage drainage network.

[0011] In one embodiment, the dynamic monitoring of the entire foundation pit construction process includes monitoring at least one of the following: Groundwater level, pile stress and tilt, support axial force, ground surface and pit top settlement, pit bottom heave.

[0012] In one embodiment, the step of coordinating the adjustment of precipitation rate and support axial force based on monitoring data includes: When the groundwater level in the pit is higher than the design water level, increase the dewatering rate of the dewatering wells in the pit; When the rate of decline of the groundwater level outside the pit exceeds the warning value, or when the surface subsidence or pit top subsidence exceeds the warning value, the dewatering rate of the dewatering wells inside the pit shall be reduced. When at least one of the monitoring data points—pile tilt, support axial force, ground settlement, pit top settlement, or pit bottom heave—exceeds the warning value, the axial force compensation value of the lower flexible support is increased through the axial force compensation device, or temporary supports are added.

[0013] In one embodiment, the construction method further includes the following steps: During the excavation of the foundation pit, wire mesh and shotcrete are applied to the space between the retaining piles and the surface of the piles to form a surface protection layer.

[0014] In one embodiment, the construction method further includes the following steps: After the main structure of the foundation pit is constructed to the level below the corresponding support elevation, the supports shall be removed in sequence from bottom to top. During the dismantling of the supports, continuous dynamic monitoring was conducted, and the axial force of the remaining supports was adjusted based on the monitoring data.

[0015] The beneficial effects of the technical solutions provided in this application include: Simultaneously constructing a surface interception and drainage system and an underground composite waterproof retaining structure deeply integrates the water-stopping function with the retaining function, completely blocking seepage channels while improving the overall rigidity of the retaining system. A graded support system combining upper rigid supports and lower flexible supports is adopted to precisely match the support rigidity with the earth pressure distribution in the depth direction, balancing shallow deformation control with deep construction economy. A three-stage hydraulic control system of external interception, middle isolation, and internal dewatering is established, enabling the dewatering system and the water-stopping system to work synergistically, ensuring dry working conditions within the pit while minimizing the impact of dewatering on the surrounding environment. Real-time coordinated adjustment of the dewatering rate and support axial force is achieved through dynamic monitoring, allowing each system to cooperate and constrain each other according to dynamic changes in construction conditions. This effectively solves the problem of lack of coordination between systems, while simultaneously achieving optimal comprehensive performance in terms of water-stopping effect, deformation control capability, and construction economy for long, deep foundation pits near lakes. Attached Figure Description

[0016] To more clearly illustrate the technical solutions in the embodiments of this application, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0017] Figure 1 This is a flowchart illustrating an embodiment of the composite waterproofing and graded support collaborative construction method for deep foundation pits in this application. Detailed Implementation

[0018] To enable those skilled in the art to better understand the present application, the technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present application, and not all embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present application.

[0019] First, some of the technical terms used in this application will be explained to help those skilled in the art understand this application.

[0020] Composite waterproof retaining structure: refers to an integrated waterproof and retaining composite structure formed by connecting the surface drainage system, underground water-stop curtain, and retaining piles through grouting. It can simultaneously achieve the functions of surface runoff interception, groundwater lateral barrier, and foundation pit sidewall support. Pile-curtain composite waterproof layer: refers to the composite waterproof structure formed by injecting grouting material into the reserved gap between the waterproof curtain and the retaining piles. It not only seals the seepage channel between the piles and the curtain, but also connects the retaining piles and the waterproof curtain into a whole through the cementing effect of the grout, thus improving the waterproof performance and lateral stiffness of the structure. Graded support system: refers to an internal support system composed of upper rigid support and lower flexible support in layers. The upper part uses high-rigidity reinforced concrete support to control the deformation of the shallow part of the foundation pit, while the lower part uses steel pipe support that can be prestressed to adapt to the nonlinear increase of soil pressure in the deep part of the foundation pit, so as to achieve a reasonable match of support stiffness along the depth of the foundation pit. Axial force compensation device: refers to a device installed between rigid and flexible supports that can dynamically adjust the axial force of the supports based on real-time monitoring data. It includes hydraulic jacks or wedge-shaped adjustment blocks and is used to achieve a smooth transition between supports of different stiffness and active control of foundation pit deformation. The three-level hydraulic control system refers to the comprehensive groundwater control system formed by the coordinated action of the pit top interception and drainage system (external interception), the composite water-proof retaining structure (intermediate barrier), and the pit in-pit dewatering system (internal dewatering). It can effectively cut off the lateral recharge of groundwater on the lake side, balance the water pressure inside and outside the pit, and reduce the risk of ground subsidence in the surrounding area. Deep well depressurization system: refers to a dewatering well system installed at certain intervals in the foundation pit. By pumping water from the deep wells, the groundwater level in the pit is lowered to below the design elevation of the bottom of the pit, creating dry working conditions for the excavation of the foundation pit and the construction of the main structure. Shallow open drainage system: refers to the combined system of drainage ditches and collection wells set along both sides of the longitudinal direction of the foundation pit, used to collect and remove shallow water, construction wastewater and rainwater in the pit, and to prevent water in the pit from soaking the foundation. Steel pad box: refers to a steel transition component that connects rigid and flexible supports. One end is welded or bolted to the pre-embedded steel plate at the end of the concrete support, and the other end is connected to the axial force compensation device. It is used to uniformly transmit the axial force of the support and ensure the reliability of the connection between different types of supports. Adjustable joint: refers to a telescopic adjustable component installed at the end of the steel pipe support. It adjusts the total length of the steel pipe support by means of threads or hydraulic pressure, and is used to apply pre-loaded axial force to the steel pipe support so that the support can fit tightly against the retaining structure and effectively bear the soil pressure. This application provides a method for the coordinated construction of composite waterproofing and graded support for deep foundation pits, which can solve the technical problems in related technologies where the various construction systems of deep foundation pits lack overall coordination and it is difficult to simultaneously ensure the waterproofing effect, deformation control capability, and construction economy of long, deep foundation pits near lakes.

[0021] Firstly, such as Figure 1 As shown in the embodiment of this application, a method for the coordinated construction of composite waterproofing and graded support for deep foundation pits is provided, which includes the following steps: S100: The surface drainage system and the composite waterproof retaining structure around the foundation pit are constructed simultaneously to form an integrated waterproof system that coordinates the surface and underground. S200: As the foundation pit is excavated in layers, a graded support system combining upper rigid support and lower flexible support is constructed, and an axial force compensation device is installed between the rigid support and the flexible support. S300: The construction pit drainage system, together with the surface interception and drainage system and the composite waterproof retaining structure, forms a three-level hydraulic control system; S400: Dynamically monitor the entire process of foundation pit construction and adjust the dewatering rate and support axial force in a coordinated manner based on the monitoring data.

[0022] In this embodiment, under the condition of high water level near the lake, the foundation pit is simultaneously subjected to the dual hydraulic action of direct infiltration of surface runoff and lateral replenishment of groundwater by lake water. The traditional approach of designing the water-stopping, enclosure, support, and dewatering systems independently cannot form a complete protection chain, and is prone to problems such as water-stopping failure, uncontrolled deformation, or excessive cost. Simultaneously constructing a surface interception and drainage system and an underground composite waterproof retaining structure deeply integrates the water-stopping function with the retaining function, completely blocking seepage channels while improving the overall rigidity of the retaining system. A graded support system combining upper rigid supports and lower flexible supports is adopted to precisely match the support rigidity with the earth pressure distribution in the depth direction, balancing shallow deformation control with deep construction economy. A three-level hydraulic control system of "external interception-middle isolation-internal dewatering" is established, enabling the dewatering system and the water-stopping system to work synergistically, ensuring dry working conditions within the pit while minimizing the impact of dewatering on the surrounding environment. Real-time coordinated adjustment of the dewatering rate and support axial force is achieved through dynamic monitoring, allowing each system to cooperate and constrain each other according to the dynamic changes in construction conditions. This effectively solves the problem of lack of coordination between systems, while simultaneously achieving optimal comprehensive performance in terms of water-stopping effect, deformation control capability, and construction economy for long, deep foundation pits near lakes.

[0023] In one implementation, step S100 includes the following steps: S101: Construct intercepting ditches and retaining walls around the top of the foundation pit to form a surface drainage system; S102: A continuous, closed water-stop curtain to be constructed along the designed edge of the foundation pit; S103: Construct retaining piles inside the water-stop curtain to create a gap between the retaining piles and the water-stop curtain. S104: Inject grouting material into the gap to form a pile-curtain composite waterproof layer, so that the retaining piles and the water-stop curtain are connected as a whole.

[0024] In this embodiment, when traditional cast-in-place piles and cutoff walls are constructed separately, a construction gap inevitably exists between them due to the influence of construction accuracy and ground disturbance. This gap, under the influence of high water head near the lake, will form a continuous seepage channel along the wall, which is a high-risk area for piping and water inrush accidents in the foundation pit. By leaving a uniform gap between the cutoff wall and the retaining piles and performing pressure grouting, the cement grout fully fills the gap under pressure and penetrates into the tiny pores of the structures on both sides, forming a continuous and dense cemented interface, completely cutting off the seepage path between the piles and the curtain. At the same time, the cementing effect of the grouting material connects the originally independent retaining piles and cutoff wall into an integral load-bearing unit, thereby simultaneously improving the lateral bearing capacity of the combined structure and enabling it to more effectively resist the combined action of external soil and water pressure.

[0025] In one implementation, step S200 includes the following steps: S201: The upper rigid support is made of reinforced concrete, and the reinforced concrete support is connected to the capping beam at the top of the retaining pile.

[0026] In this embodiment, the shallow soil layers of the foundation pit are mostly miscellaneous fill and silty clay with low shear strength. After excavation, these layers are prone to significant horizontal displacement under earth pressure, and this displacement is directly transmitted to the ground surface, having the most significant impact on surrounding buildings, pipelines, and other environmentally sensitive areas. Reinforced concrete supports are used as the upper support, possessing lateral stiffness far exceeding that of steel supports, effectively limiting horizontal deformation in the shallow part of the foundation pit. The reinforced concrete supports are integrally cast with the capping beam to form a rigid connection, evenly distributing the support reaction force to each retaining pile, avoiding localized stress concentration that could lead to pile breakage or tilting, and ensuring that the deformation at the top of the foundation pit is strictly controlled within the design limits.

[0027] In one implementation, step S200 includes the following steps: S202: The lower flexible support is a steel pipe support, and the end of the steel pipe support is provided with a hinge head, through which a pre-applied axial force is applied to the steel pipe support.

[0028] In this embodiment, the soil pressure in the deep soil layer of the foundation pit increases significantly and non-linearly with depth, and the construction period of the deep layer has a decisive impact on the overall project progress. The disadvantage of the long curing period of concrete supports is greatly amplified. Using steel pipe supports as the lower support allows for quick adjustment of the support length and application of pre-applied axial force through the end hinges, thus offsetting part of the soil pressure in advance and reducing the deflection deformation of the retaining piles. Steel pipe supports are characterized by high standardization, fast installation speed, and reusability, which can significantly shorten the construction period of the lower support and reduce the cost of engineering materials. At the same time, steel pipe supports are easy to dismantle and will not interfere with the subsequent construction of the main structure, which can significantly improve the overall construction efficiency.

[0029] In one implementation, step S200 includes the following steps: S203: The axial force compensation device is connected to the upper rigid support and the lower flexible support respectively through a steel pad box; The axial force compensation device includes a hydraulic jack or a wedge-shaped adjusting block, which can dynamically adjust the pre-applied axial force of the lower flexible support based on at least one of the monitoring data of pile body inclination, support axial force, ground surface settlement, pit top settlement or pit bottom heave, so as to achieve a smooth transition of support stiffness.

[0030] In this embodiment, the stiffness of the reinforced concrete support and the steel pipe support differs by orders of magnitude. Direct connection would cause a significant abrupt change in stiffness at the joint, leading to a sharp increase in deformation of the retaining pile at that location, and even causing support connection failure. Simultaneously, factors such as soil creep and dewatering consolidation during construction would cause the axial force of the support to gradually decrease, affecting the long-term stability of the support system. Using a steel pad box to achieve a transition connection between supports of different stiffnesses allows the axial force of the support to be evenly transferred to adjacent components, avoiding localized stress concentration. The axial force compensation device can dynamically adjust the axial force of the lower flexible support based on real-time monitoring of the foundation pit's stress and deformation data, ensuring a smooth transition of support stiffness along the depth of the foundation pit and timely compensation for axial force loss caused by soil deformation, ensuring that the support system can always provide sufficient reaction force to resist earth pressure.

[0031] In one implementation, step S300 includes the following steps: S301: Dewatering wells are installed in the foundation pit to form a deep well depressurization system; S302: Drainage ditches and sump pits are set up around the perimeter of the foundation pit to form a shallow open drainage system; S303: The deep well depressurization system and the shallow open drainage system together form a two-level drainage network.

[0032] In this embodiment, the construction of a deep foundation pit near a lake requires addressing both deep groundwater control and shallow water drainage simultaneously. A single drainage method cannot meet the drainage requirements at different depths. A two-stage drainage network combining a deep well depressurization system and a shallow open drainage system is adopted. The deep well depressurization system can quickly lower the deep groundwater level below the design elevation of the pit bottom through deep well pumping, ensuring that the excavation surface of the foundation pit remains dry. The shallow open drainage system can promptly collect shallow water, construction wastewater, and rainwater within the pit and pump them out to the outside through collection wells. The two-stage drainage network works in tandem to comprehensively cover the drainage needs at different depths within the foundation pit, creating a continuously dry working environment for foundation pit excavation and main structure construction, and preventing the foundation soil from being soaked in water, thus reducing its bearing capacity.

[0033] In one implementation, step S400 includes the following steps: S401: The dynamic monitoring of the entire construction process of the foundation pit includes monitoring at least one of the following: groundwater level, pile stress and inclination, support axial force, ground surface and pit top settlement, and pit bottom heave.

[0034] In this embodiment, the safety status of the foundation pit is jointly determined by hydraulic conditions, stress state, and deformation characteristics. Monitoring a single parameter cannot fully reflect the actual working conditions of the foundation pit, and it is easy to miss or misjudge abnormalities. By simultaneously monitoring multiple key parameters such as groundwater level, pile stress and inclination, support axial force, surface and pit top settlement, and pit bottom heave, the real-time status of the foundation pit can be comprehensively grasped from three dimensions: hydraulic, stress, and deformation. Different monitoring parameters can verify and complement each other, significantly improving the accuracy and timeliness of abnormality identification, and providing comprehensive and reliable data support for subsequent adjustments to construction parameters.

[0035] In one implementation, step S400 includes the following steps: S402: When the groundwater level in the pit is higher than the design water level, increase the dewatering rate of the dewatering wells in the pit; S403: When the rate of decline of the groundwater level outside the pit exceeds the warning value, or when the surface subsidence or pit top subsidence exceeds the warning value, reduce the dewatering rate of the dewatering wells inside the pit. S404: When at least one of the monitoring data of pile body tilt, support axial force, ground surface settlement, pit top settlement or pit bottom heave exceeds the warning value, the axial force compensation value of the lower flexible support shall be increased by the axial force compensation device, or temporary support shall be added.

[0036] In this embodiment, dewatering and support are two interconnected and coupled systems. Dewatering alters the effective stress of the soil, thus affecting the earth pressure acting on the retaining structure. Conversely, the deformation of the support structure alters the groundwater seepage field. Adjusting only one system cannot achieve optimal control. Differentiated adjustment measures are implemented based on the anomaly types reflected by different monitoring parameters: increasing the dewatering rate to ensure construction safety when the water level inside the pit is too high; reducing the dewatering rate to protect the surrounding environment when the water level outside the pit drops too quickly or surface subsidence is excessive; and increasing the support axial force or adding temporary supports to suppress deformation when the pit deforms or the support axial force exceeds limits. Through this precise and coordinated adjustment mechanism, the construction safety of the pit itself can be ensured while minimizing the adverse impact of construction activities on the surrounding environment.

[0037] In one embodiment, the construction method further includes the following steps: S500: During the excavation of the foundation pit, wire mesh and shotcrete are installed between the retaining piles and on the surface of the piles to form a surface protection layer.

[0038] In this embodiment, after the foundation pit is excavated, the soil between the retaining piles is directly exposed to the external environment and is susceptible to weathering, rainwater erosion, and groundwater seepage. This makes it prone to spalling and collapse, especially in soft soil layers. Damage to the soil between the piles can cause the retaining piles to lose lateral restraint, leading to overall instability of the retaining structure. By applying wire mesh and shotcrete to the piles and their surfaces, a continuous rigid protective surface can be formed between the piles, effectively restraining the soil between them and preventing spalling and collapse. At the same time, the shotcrete layer can block local seepage, preventing water from eroding the soil between the piles and ensuring the integrity and stability of the foundation pit sidewalls.

[0039] In one embodiment, the construction method further includes S600, which includes the following steps: S601: After the main structure of the foundation pit is constructed to the level below the corresponding support elevation, the supports shall be removed in order from bottom to top. S602: During the support removal process, continuous dynamic monitoring is carried out, and the axial force of the remaining supports is adjusted according to the monitoring data.

[0040] In this embodiment, the support removal is a gradual unloading process. Each time a support is removed, the soil pressure originally borne by that support is redistributed to the remaining supports and the completed main structure. Improper removal sequence or excessively rapid unloading can lead to a sudden increase in foundation pit deformation, potentially causing safety accidents. Adopting a bottom-up support removal sequence allows the completed main structure to bear part of the soil pressure, ensuring the continuity of stress on the foundation pit during support removal. Continuous dynamic monitoring during the removal process allows for timely understanding of changes in foundation pit deformation and stress caused by support unloading. Adjusting the axial force of the remaining supports balances the impact of unloading, preventing sudden changes in foundation pit deformation due to abrupt support removal and ensuring construction safety during the support removal phase.

[0041] Secondly, this application provides a specific application of the above method in a lakeside tunnel foundation pit project. The project is 680m long, 18.6m wide, and 32.7m deep at its deepest point. The site strata, from top to bottom, are: ① miscellaneous fill, ② silty clay, ③ fine sand, ④ gravel, ⑤ strongly weathered mudstone, and ⑥ moderately weathered mudstone. The groundwater is strongly recharged laterally by the lake, resulting in high construction risks.

[0042] The construction steps in this embodiment are as follows: Step 1: Construction preparation and construction of the surface drainage system Level the site and excavate a drainage ditch with a cross-section of 0.4m × 0.4m at a distance of 1.0m from the opening line on both sides of the foundation pit. The ditch should be constructed using brick masonry or cast-in-place concrete, with a 0.3% longitudinal slope at the bottom leading to the municipal rainwater pipe. The retaining wall should be constructed using C20 concrete, with a height of 0.4m, a width of 0.3m, and a top surface 0.3m above the ground.

[0043] In this embodiment, by setting up a water interception ditch and a water retaining wall at the top of the foundation pit, rainwater and surface runoff within the site area can be effectively intercepted, preventing them from directly flowing into the foundation pit or laterally infiltrating to replenish groundwater, cutting off the surface water replenishment path from the source, and reducing the hydraulic load on the underground water-stopping structure.

[0044] Step 2: Construction of Composite Waterproof Layer The water-stop curtain was constructed using double-row φ800@600 jet grouting piles with an interlocking depth of 200mm and a pile length of 33m, with the pile bottom penetrating 1m into the mudstone. After the jet grouting piles were completed, φ1000@1200 bored cast-in-place piles were constructed 15cm inside the curtain as retaining piles, with a pile length of 35m and concrete strength grade C35. Three days after the cast-in-place piles were completed, grout was injected into the gap between the piles and the curtain through φ25 grouting pipes pre-embedded on the side of the piles. The grouting pressure was 0.5MPa, and cement grout with a water-cement ratio of 0.8 was used. The grouting volume was controlled at 20L per meter of gap until grout returned to the borehole opening.

[0045] In this embodiment, a continuous and closed water-stop curtain is formed by double-row interlocking jet grouting piles, which can effectively block the lateral seepage of groundwater. Pressure grouting is performed in the gap between the piles and the curtain to completely seal the seepage channels formed by the construction gap. At the same time, the cast-in-place piles and jet grouting piles are bonded together with cement grout to form an integral load-bearing structure, which significantly improves the lateral stiffness and water-stopping performance of the retaining system and effectively avoids the risk of piping and water inrush under the action of high water head near the lake.

[0046] Step 3: Construction of the cap beam and the first layer of concrete support Excavate to the first support bottom elevation (2.5m below ground level), construct the capping beam and the first concrete support; the capping beam is 1.2m wide and 0.8m high, with a concrete strength grade of C35; the first concrete support has a cross-sectional dimension of 800mm×1000mm, a spacing of 8m, and is integrally cast and connected to the capping beam, and cured for 7 days.

[0047] In this embodiment, high-rigidity reinforced concrete supports are used in the shallow part of the foundation pit, which can effectively control the horizontal displacement of the top of the foundation pit and prevent excessive deformation of the shallow soft soil layer, thus preventing the surrounding ground from settling. The concrete supports are cast integrally with the cap beam, ensuring that the support reaction force can be evenly transmitted to each retaining pile, avoiding local stress concentration.

[0048] Step 4: Layered excavation and graded support construction Excavation was carried out in layers until the bottom elevation of the second support (-10.5m). The second layer of concrete support was constructed, with the same cross-sectional dimensions and spacing as the first layer. Excavation continued in layers until the bottom elevation of the third support (-19.0m). The third layer of steel support was installed, using φ609×16mm steel pipes spaced 4m horizontally, with a prestress of 800kN. Excavation continued in layers until the bottom elevation of the fourth support (-27.0m). The fourth layer of steel support was installed, with the same specifications as the third layer, and a prestress of 1200kN was applied. Each steel support was connected to the upper concrete support via a steel pad box. The steel pad box was 1.2m long and 0.5m wide, welded from Q345 steel plates. One end was bolted to the pre-embedded steel plate of the concrete support, and the other end was equipped with a hydraulic jack (100mm stroke, rated pressure 2000kN), which was connected to the movable head of the steel support. Based on monitoring data, the axial force was adjusted daily, with an adjustment range not exceeding ±10%.

[0049] In this embodiment, two concrete supports are used at the top to provide high rigidity to control shallow deformation, while two steel supports are used at the bottom to adapt to the nonlinear increase of deep soil pressure, achieving a precise match between support stiffness and soil pressure distribution. A smooth transition between supports of different stiffnesses is achieved through steel pad boxes and hydraulic jacks, and the axial force of the steel supports is dynamically adjusted according to monitoring data, which timely compensates for the axial force loss caused by soil creep and precipitation consolidation, effectively controlling the deformation development in the deep part of the foundation pit.

[0050] Step 5: Netting and shotcreting for protection After each layer of excavation (approximately 2-3m), immediately install φ6@200×200 steel mesh between the piles and on the surface of the pile body, and spray C20 fine aggregate concrete with a thickness of 80mm.

[0051] In this embodiment, by excavating in layers and applying mesh and shotcrete protection in layers, the exposed soil between piles can be sealed and protected in a timely manner, preventing it from being eroded by weathering, rainwater erosion and groundwater seepage, thus ensuring the integrity and stability of the foundation pit sidewall.

[0052] Step Six: Construction of the Dewatering and Drainage System in the Pit Dewatering wells were installed 3m on each side of the centerline of the foundation pit, with a spacing of 15m, for a total of 92 wells. The wells were 38m deep, with the bottom of the wells penetrating 3m into the mudstone. Each well was equipped with a submersible pump with a flow rate of 50m³ / h. Drainage ditches were set up along both sides of the foundation pit, and a water collection well (1.2m×1.2m×1.5m) was set up every 40m. Each water collection well was equipped with two 110kW centrifugal pumps (one for standby) to pump the water accumulated in the pit to an off-site sedimentation tank.

[0053] In this embodiment, the deep well depressurization system rapidly lowers the deep groundwater level in the pit, while the shallow open drainage system promptly removes accumulated water, construction wastewater, and rainwater from the pit. The two-stage drainage network works in tandem to create continuously dry working conditions for the excavation of the foundation pit and the construction of the main structure. At the same time, because the composite waterproof retaining structure effectively blocks the lateral replenishment of lake water, dewatering in the pit only needs to lower the groundwater level within a limited area inside the foundation pit, significantly reducing the impact of dewatering on the surrounding environment.

[0054] Step 7: Dynamic monitoring and feedback adjustment Monitoring points were set up around and inside the foundation pit, including 16 water level observation holes, 20 pile inclinometer tubes, 40 support axial force gauges (10 for each support), and 50 surface settlement points. The warning values ​​were set as follows: 30mm horizontal displacement of the pile top, 25mm surface settlement, and 80% over the design value of the support axial force. During construction, the dewatering rate and the axial force of the steel support were adjusted in real time based on the daily monitoring data.

[0055] In this embodiment, by simultaneously monitoring multiple key parameters, the real-time stress and deformation state of the foundation pit was fully understood. Dynamic adjustments were made based on the monitoring data, enabling coordinated control of dewatering and support, timely response to abnormal situations during construction, and ensuring the safety and stability of the foundation pit construction. During construction, the maximum displacement of the pile top was 22mm, the surface settlement was 18mm, the axial force fluctuation of the support was within ±8%, and the daily seepage rate in the pit was 25m³ / d, all meeting the design requirements.

[0056] Step 8: Support Removal and Main Structure Construction After the foundation pit bottom slab and side walls are constructed to below the fourth steel support, the steel supports and concrete supports are removed in sequence from bottom to top. During the support removal process, dynamic monitoring is continuously carried out, and the axial force of the remaining supports is adjusted according to the monitoring data. At the same time, some dewatering wells are retained until the foundation pit is backfilled.

[0057] In this embodiment, a bottom-up support removal sequence is adopted, utilizing the existing main structure to bear part of the soil pressure, thus ensuring the continuity of the foundation pit's stress during the support removal process. Through continuous monitoring and axial force adjustment, sudden changes in foundation pit deformation caused by support unloading are effectively avoided, ensuring construction safety during the support removal stage.

[0058] After construction using the above method in this embodiment, compared with projects using all-steel support + conventional water-stopping scheme under similar geological conditions, the pile top displacement was reduced by 54.2%, the daily seepage in the pit was reduced by 79.2%, the total project cost was reduced by 26.5%, and the construction period was shortened by 15%, achieving significant economic and social benefits.

[0059] In the description of this application, it should be noted that the terms "upper," "lower," etc., indicating the orientation or positional relationship are based on the orientation or positional relationship shown in the accompanying drawings, and are only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this application. Unless otherwise expressly specified and limited, the terms "installed," "connected," and "linked" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication between two elements. For those skilled in the art, the specific meaning of the above terms in this application can be understood according to the specific circumstances.

[0060] It should be noted that in this application, relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0061] The above description is merely a specific embodiment of this application, enabling those skilled in the art to understand or implement this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features claimed herein.

Claims

1. A method for the coordinated construction of composite waterproofing and graded support for deep foundation pits, characterized in that, It includes the following steps: The surface drainage system and the composite waterproof retaining structure around the foundation pit are constructed simultaneously to form an integrated waterproof system that coordinates the surface and underground. As the foundation pit is excavated in layers, a graded support system combining upper rigid support and lower flexible support is constructed, and an axial force compensation device is installed between the rigid support and the flexible support. The drainage system inside the construction pit, together with the surface interception and drainage system and the composite waterproof retaining structure, forms a three-level hydraulic control system. The entire process of foundation pit construction is dynamically monitored, and the dewatering rate and support axial force are adjusted in a coordinated manner based on the monitoring data.

2. The method for coordinated construction of composite waterproofing and graded support for deep foundation pits as described in claim 1, characterized in that, The synchronous construction surface drainage system and the composite waterproof retaining structure around the foundation pit include: Construct intercepting ditches and retaining walls around the top of the foundation pit to form a surface drainage system; A continuous, closed water-stop curtain was constructed along the designed boundary of the foundation pit; Construct retaining piles inside the water-stop curtain to create a gap between the retaining piles and the water-stop curtain. Grouting material is injected into the gap to form a pile-curtain composite waterproof layer, connecting the retaining piles and the water-stop curtain into a whole.

3. The method for coordinated construction of composite waterproofing and graded support for deep foundation pits as described in claim 1, characterized in that, The upper rigid support is made of reinforced concrete, and the reinforced concrete support is connected to the capping beam at the top of the retaining pile.

4. The method for coordinated construction of composite waterproofing and graded support for deep foundation pits as described in claim 1, characterized in that, The lower flexible support is a steel pipe support, and the end of the steel pipe support is provided with a hinge head, through which a pre-applied axial force is applied to the steel pipe support.

5. The method for coordinated construction of composite waterproofing and graded support for deep foundation pits as described in claim 1, characterized in that, The axial force compensation device is connected to the upper rigid support and the lower flexible support respectively through a steel pad box; The axial force compensation device includes a hydraulic jack or a wedge-shaped adjusting block, which can dynamically adjust the pre-applied axial force of the lower flexible support based on at least one of the monitoring data of pile body inclination, support axial force, ground surface settlement, pit top settlement or pit bottom heave, so as to achieve a smooth transition of support stiffness.

6. The method for coordinated construction of composite waterproofing and graded support for deep foundation pits as described in claim 1, characterized in that, The drainage system within the construction pit includes: Dewatering wells are installed within the foundation pit to form a deep well depressurization system; Drainage ditches and sump pits are set up around the perimeter of the foundation pit to form a shallow open drainage system; The deep well depressurization system and the shallow open drainage system together form a two-stage drainage network.

7. The method for coordinated construction of composite waterproofing and graded support for deep foundation pits as described in claim 1, characterized in that, The dynamic monitoring of the entire foundation pit construction process includes monitoring at least one of the following: Groundwater level, pile stress and tilt, support axial force, ground surface and pit top settlement, pit bottom heave.

8. The method for coordinated construction of composite waterproofing and graded support for deep foundation pits as described in claim 1, characterized in that, The method of coordinating the adjustment of precipitation rate and support axial force based on monitoring data includes: When the groundwater level in the pit is higher than the design water level, increase the dewatering rate of the dewatering wells in the pit; When the rate of decline of the groundwater level outside the pit exceeds the warning value, or when the surface subsidence or pit top subsidence exceeds the warning value, the dewatering rate of the dewatering wells inside the pit shall be reduced. When at least one of the monitoring data points—pile tilt, support axial force, ground settlement, pit top settlement, or pit bottom heave—exceeds the warning value, the axial force compensation value of the lower flexible support is increased through the axial force compensation device, or temporary supports are added.

9. The method for coordinated construction of composite waterproofing and graded support for deep foundation pits as described in claim 1, characterized in that, The construction method also includes the following steps: During the excavation of the foundation pit, wire mesh and shotcrete are applied to the space between the retaining piles and the surface of the piles to form a surface protection layer.

10. The method for coordinated construction of composite waterproofing and graded support for deep foundation pits as described in claim 1, characterized in that, The construction method also includes the following steps: After the main structure of the foundation pit is constructed to the level below the corresponding support elevation, the supports shall be removed in sequence from bottom to top. During the dismantling of the supports, continuous dynamic monitoring was conducted, and the axial force of the remaining supports was adjusted based on the monitoring data.

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