Differentiated construction method for adjacent subway tunnel of super-deep foundation pit based on partition influence degree

Abstract

The application discloses a differential construction method for a super-deep foundation pit adjacent to a subway tunnel based on a partition influence degree, and the method comprises the following steps: a three-dimensional fluid-solid coupling finite element model containing a super-deep foundation pit, a subway tunnel adjacent to the super-deep foundation pit and a weakened joint unit of a diaphragm wall is constructed; pore water pressure, effective stress and displacement of the soil are solved by means of iterative coupling of dewatering and excavation alternately; the influence degree of each excavation partition on the tunnel is calculated; and a strong influence area, a transition influence area and a weak influence area are divided. A gradient stiffness isolation barrier is arranged between the foundation pit and the tunnel according to the partitions, and the reinforcement strength decreases successively; differential excavation is implemented in the foundation pit, and the partition layer thickness increases successively. In the whole construction process, a leakage point is identified by relying on the penetration dynamic response of the weakened joint, the leakage point is matched with a corresponding influence partition, and different grouting amounts are adopted according to the regional risk level to complete differential plugging and reinforcement. The method provides reliable technical support for the safe construction and control of underground rail transit projects adjacent to complex strata.

Description

Technical Field

[0001] This invention belongs to the field of subway tunnel construction technology, and more specifically, relates to a differentiated construction method for subway tunnels adjacent to ultra-deep foundation pits based on zoning influence. Background Technology

[0002] The scale of urban underground space development continues to expand, with numerous ultra-deep foundation pit projects located adjacent to existing operational subway tunnels. These projects often involve water-rich sand layers, and the coupling effect between groundwater seepage and foundation pit excavation unloading easily induces uneven ground deformation, leading to tunnel settlement and excessive convergence deformation, directly threatening the long-term operational safety of the subway. Current construction management methods generally employ standardized procedures for ground reinforcement and foundation pit excavation.

[0003] Dewatering excavation in water-rich strata exhibits significant fluid-structure interaction effects. Conventional numerical models cannot simultaneously reflect the small-strain deformation characteristics of the soil and the seepage effect of multi-layered confined water, nor do they incorporate the amplified impact of diaphragm wall joint leakage on stratum disturbance, making it difficult to predict the degree of disturbance in each excavation zone. Diaphragm wall joints are prone to opening and misalignment deformation due to the unloading effect of the excavation pit. There is a lack of dynamic quantitative identification methods for leakage hazards, and leakage treatment methods are often indiscriminate. Leakage in high-impact areas is not reinforced and sealed, which will continue to exacerbate tunnel structural deformation.

[0004] Existing technologies have not yet established an integrated management and control system that includes early disturbance zone prediction, mid-term gradient isolation and reinforcement and layered excavation, and graded treatment of leakage during construction. This system cannot simultaneously address the safety of subway tunnels, on-site construction efficiency, and project cost. The industry urgently needs a differentiated construction scheme based on the degree of zoning impact to systematically solve the above-mentioned engineering challenges. Summary of the Invention

[0005] This invention aims to calculate the impact of foundation pit zoning on subway tunnels and delineate risk zones using a three-dimensional fluid-structure interaction model, coupled with gradient isolation reinforcement, differentiated layered excavation, and seepage response identification of diaphragm wall joints, and to achieve differentiated and safe construction of ultra-deep foundation pits adjacent to tunnels by classifying and sealing them according to risk levels of each zone.

[0006] To address the aforementioned deficiencies or improvement needs of existing technologies, as a first aspect of this invention, the present invention provides a differentiated construction method for ultra-deep foundation pits adjacent to subway tunnels based on zoning influence, comprising: S1. Establish a three-dimensional fluid-structure coupled finite element model including an ultra-deep foundation pit, an adjacent subway tunnel, and a weakened joint unit of a diaphragm wall. Through a coupled iterative process of alternating dewatering and excavation, output the three-dimensional pore water pressure field, effective stress field, and displacement field of the soil surrounding the ultra-deep foundation pit at each construction stage. Calculate the influence of each excavation zone on the adjacent subway tunnel based on the three-dimensional pore water pressure field, effective stress field, and displacement field, and divide the ultra-deep foundation pit excavation surface into a strong influence zone, a transitional influence zone, and a weak influence zone accordingly. S2. Based on the distribution of the strong influence zone, the transitional influence zone, and the weak influence zone, differentiated isolation reinforcement is implemented between the ultra-deep foundation pit and the adjacent subway tunnel, wherein the reinforcement strength corresponding to the strong influence zone is greater than that of the transitional influence zone, and the reinforcement strength corresponding to the transitional influence zone is greater than that of the weak influence zone, forming a protective barrier with gradient stiffness. S3. Based on the distribution of the strong influence zone, the transitional influence zone and the weak influence zone, implement differentiated truncated excavation of the soil inside the ultra-deep foundation pit, wherein the layer thickness corresponding to the strong influence zone is smaller than that of the transitional influence zone, and the layer thickness corresponding to the transitional influence zone is smaller than that of the weak influence zone. S4. During the excavation and dewatering of ultra-deep foundation pits, the location of leakage points is identified based on the dynamic response of the permeability performance of the weakened joint unit. The location of the leakage points is mapped to the strong influence zone, the transitional influence zone, and the weak influence zone. Differentiated sealing and reinforcement are implemented according to the different zones where the leakage points are located.

[0007] Furthermore, the construction process of the three-dimensional fluid-structure interaction finite element model in S1 includes: Small strain stiffness parameters of water-rich sand layers were obtained by in-situ field tests and indoor dynamic tests, and the HSS constitutive model was calibrated accordingly. Based on the distribution of confined aquifers and hydraulic head conditions determined by the survey, seepage boundaries that change with the construction process are set in the soil layer corresponding to the model, and the pumping action of dewatering wells is used as the boundary input to establish a fluid-structure interaction analysis module with alternating dewatering and excavation. Interface elements are set between the diaphragm wall and the adjacent soil to characterize the relative sliding of the wall and soil, and the permeability of the joint element of the diaphragm wall is set to be higher than that of the main body of the wall to form a weakened joint element. Based on the calibrated HSS constitutive model, the fluid-structure interaction analysis module, the interface element, and the weakened joint element, a model is assembled to generate a three-dimensional finite element model that synchronously reflects the small strain characteristics of the soil, the seepage effect of multi-layer confined water, and the leakage characteristics of the joints of the retaining structure.

[0008] Furthermore, the fluid-structure interaction analysis module is specifically configured to alternately perform dewatering seepage calculations and excavation deformation calculations according to the construction sequence, and to perform the following coupled iterative process within each calculation step: First, the seepage field is solved: the pumping effect of the dewatering well is equivalent to the point sink boundary that varies with time, and the pore water pressure of each node in the confined aquifer is calculated based on the unsteady flow model that considers the leakage correction of the underground continuous wall joint. Secondly, the effective stress and deformation field are solved: based on the pore water pressure calculated at the current time step, the soil skeleton stress is updated based on the effective stress principle, and the updated effective stress is input into the HSS constitutive model to obtain the tangent stiffness matrix of the soil at the current time step, and then the soil displacement increment and volumetric strain are solved. Then, dynamic correction of the permeability coefficient is performed: the soil permeability coefficient is updated based on the volumetric strain. Finally, a convergence determination is made: when the maximum value of the relative change in pore water pressure calculated in two adjacent iterations is less than the preset convergence tolerance, the current time step is determined to be converged and the process is switched to the next construction stage; otherwise, the process returns to the seepage field solution step with the corrected permeability coefficient and iterates again. Through the above-mentioned coupled iterative process, the three-dimensional pore water pressure field, effective stress field, and displacement field of the soil surrounding the ultra-deep foundation pit at each construction stage are output as input data for subsequent calculation of the impact of each excavation zone of the ultra-deep foundation pit on the adjacent subway tunnel.

[0009] Furthermore, the expression for the unsteady flow model is: ; In the formula, for Horizontal distance from the dewatering well at any time Pore ​​water pressure at the location, The initial pore water pressure, for The pumping rate of the dewatering well at any given time. The aquifer's hydraulic conductivity is... For water storage rate, Theis well function, This is the weakening correction factor for the diaphragm wall joint, applied when the calculation node is located within the influence range of the weakened joint unit. Take a calibration value greater than 1.

[0010] Furthermore, the updated formula for the soil permeability coefficient is: ; In the formula, This represents the penetration coefficient updated at the current time step. The initial permeability coefficient, The volumetric strain at the current time step, The permeability-strain coupling coefficient.

[0011] Furthermore, the permeability-strain coupling coefficient The calculation methods include: Multi-stage loading permeation-compression combined tests were conducted on water-rich sand layer samples taken in situ or reconstituted in the laboratory to obtain no fewer than four sets of volumetric strains under different effective stress levels. With the corresponding steady-state permeability coefficient ; After taking the natural logarithm of each group of experimental data, linear regression was performed to establish... and The linear relationship: ; In the formula, The reference permeability coefficient under the initial effective stress state. The regression intercept; The slope of the linear relationship is used as the permeability-strain coupling coefficient. The calibration value, and when the regression determination coefficient If the value is less than 0.90, repeat the experiment until the accuracy requirement is met, then refit and determine the result. value.

[0012] Furthermore, the weakened joint unit is specifically configured as follows: Three-dimensional solid elements with a preset thickness are established at the joint positions of adjacent sections of the diaphragm wall as weakened joint elements, and an initial permeability coefficient is assigned. In each calculation step of the ultra-deep foundation pit excavation and dewatering process, the normal relative displacement and tangential relative displacement of the wall nodes on both sides of the weakened joint unit are extracted, and the equivalent opening of the joint is calculated. The permeability coefficient of the weakened joint unit is dynamically updated based on the equivalent opening of the joint, and the update formula is as follows: ; In the formula, To weaken the update penetration coefficient of the joint unit in the current time step, The initial permeability coefficient, For normal relative displacement, For tangential relative displacement, To reduce the initial equivalent thickness of the joint unit, and These are the weighting coefficients for the influence of normal opening and tangential misalignment on permeability, respectively. When the updated permeability coefficient is greater than the preset leakage threshold, the permeability performance degradation flag of the weakened joint unit is triggered, and the position coordinates of the unit are output as the location of the leakage point.

[0013] Furthermore, the calculation process for the impact on the adjacent subway tunnel in S1 is as follows: Establish a polar coordinate system with the center of the ultra-deep foundation pit excavation face as the origin, and divide the excavation face into equal parts according to angles. For each sector partition, response data of each computational node along the path from the partition boundary to the adjacent subway tunnel axis are extracted from the three-dimensional pore water pressure field, effective stress field, and displacement field. The influence of each partition on the adjacent subway tunnel is then calculated. : Based on the influence of each region The numerical values ​​are divided into partitions according to a preset first threshold and a second threshold, where... Partitions with values ​​greater than or equal to the first threshold are classified as strong influence zones. Partitions with values ​​below the first threshold and above or equal to the second threshold are designated as transitional influence zones. Partitions with values ​​below the second threshold are classified as weak influence zones.

[0014] Furthermore, the degree of influence of each zone on adjacent subway tunnels The calculation process is as follows: ; in, For the first The influence of each partition ; For the first The total number of computing nodes on the path corresponding to each partition; For the first On the path of the partition, the first The pore water pressure drop at each node, The initial pore water pressure; This represents the vertical effective stress increment at the node. This represents the initial effective vertical stress. This represents the magnitude of the displacement vector at that node. For the first The horizontal distance from the boundary of each zone to the axis of the adjacent subway tunnel; For the first The actual distance from each node to the axis of the adjacent subway tunnel.

[0015] Furthermore, the differentiated sealing and reinforcement method in S4 is as follows: When the leakage point is located in the strong influence zone or the transitional influence zone, a first grouting volume is used for sealing and reinforcement. When the leakage point is located in the weak influence zone, a second grouting volume is used for sealing and reinforcement. The first grouting volume is greater than the second grouting volume.

[0016] In summary, compared with the prior art, the above-described technical solutions conceived by this invention can achieve the following beneficial effects: 1. The present invention provides a differentiated construction method for ultra-deep foundation pits adjacent to subway tunnels based on the influence of zoning. This method constructs a three-dimensional fluid-structure interaction finite element model including weakened joint elements of diaphragm walls, and introduces a calibrated HSS constitutive model to perform coupled iterative calculations of dewatering and excavation. The method outputs the pore water pressure field, effective stress field, and displacement field of the soil at each construction stage. Based on the data from these three fields, the influence of each excavation zone on the tunnel is calculated. Strong, transitional, and weak influence zones are defined according to thresholds. Quantitative values ​​replace empirical judgments of zoning risks, distinguishing the degree of disturbance to the subway tunnel from different excavation areas, and providing a unified and reliable data basis for subsequent differentiated construction measures.

[0017] 2. The differentiated construction method for ultra-deep foundation pits adjacent to subway tunnels based on zoning influence degrees of this invention involves setting up gradient stiffness isolation barriers according to the divided influence zones, and simultaneously implementing differentiated excavation of the foundation pit soil. Strong influence zones require higher reinforcement strength and thinner excavation layers, while weak influence zones require lower reinforcement strength and thicker excavation layers. External gradient isolation and internal refined excavation form a dual disturbance control system, reducing the disturbance and deformation of the subway tunnel caused by foundation pit dewatering and excavation, while avoiding material waste and extended construction periods caused by uniform high-standard construction across the entire area, thus balancing tunnel protection effectiveness and on-site construction economy.

[0018] 3. The differentiated construction method for ultra-deep foundation pits adjacent to subway tunnels based on zoning influence of this invention identifies leakage points by dynamically updating the permeability coefficient of weakened joint units, and maps the leakage locations to pre-defined influence zones for graded sealing. When the leakage point is in a strong or transitional influence zone, a larger grouting volume is used for reinforcement; when it is in a weak influence zone, the grouting volume is reduced. The sealing treatment intensity is matched according to the regional disturbance risk, and the seepage path in high-risk areas is specifically cut off to prevent continuous leakage from inducing ground subsidence. This constructs a dynamic graded treatment process for leakage throughout the entire excavation and dewatering process. Attached Figure Description

[0019] Figure 1 This is a flowchart of a differentiated construction method for ultra-deep foundation pits adjacent to subway tunnels based on zoning influence degree, according to an embodiment of the present invention. Figure 2 This is a finite element model of the railway core area according to an embodiment of the present invention; Figure 3 This is a schematic diagram of the foundation pit support system model according to an embodiment of the present invention. Detailed Implementation

[0020] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other.

[0021] Example 1 Please refer to Figure 1 This embodiment 1 provides a differentiated construction method for ultra-deep foundation pits adjacent to subway tunnels based on zoning influence, including: S1. Establish a three-dimensional fluid-structure coupled finite element model including an ultra-deep foundation pit, an adjacent subway tunnel, and a weakened joint unit of a diaphragm wall. Through a coupled iterative process of alternating dewatering and excavation, output the three-dimensional pore water pressure field, effective stress field, and displacement field of the soil surrounding the ultra-deep foundation pit at each construction stage. Calculate the influence of each excavation zone on the adjacent subway tunnel based on the three-dimensional pore water pressure field, effective stress field, and displacement field, and divide the ultra-deep foundation pit excavation surface into a strong influence zone, a transitional influence zone, and a weak influence zone accordingly. S2. Based on the distribution of the strong influence zone, the transitional influence zone, and the weak influence zone, differentiated isolation reinforcement is implemented between the ultra-deep foundation pit and the adjacent subway tunnel, wherein the reinforcement strength corresponding to the strong influence zone is greater than that of the transitional influence zone, and the reinforcement strength corresponding to the transitional influence zone is greater than that of the weak influence zone, forming a protective barrier with gradient stiffness. S3. Based on the distribution of the strong influence zone, the transitional influence zone and the weak influence zone, implement differentiated truncated excavation of the soil inside the ultra-deep foundation pit, wherein the layer thickness corresponding to the strong influence zone is smaller than that of the transitional influence zone, and the layer thickness corresponding to the transitional influence zone is smaller than that of the weak influence zone. S4. During the excavation and dewatering of ultra-deep foundation pits, the location of leakage points is identified based on the dynamic response of the permeability performance of the weakened joint unit. The location of the leakage points is mapped to the strong influence zone, the transitional influence zone, and the weak influence zone. Differentiated sealing and reinforcement are implemented according to the different zones where the leakage points are located.

[0022] Please refer to Figure 2 as well as Figure 3 This embodiment 1 further elaborates on the above steps.

[0023] (1) Modeling and partitioning For the construction protection of ultra-deep foundation pits adjacent to subway tunnels, a three-dimensional fluid-structure interaction finite element model is needed, which includes the ultra-deep foundation pit, the adjacent subway tunnel, and the weakened joint unit of the diaphragm wall. Due to the large variation in soil stress path caused by the excavation of ultra-deep foundation pits, and the extreme sensitivity of the adjacent subway tunnel to small soil strains, conventional constitutive models are insufficient to reasonably describe the actual stiffness response of the soil during excavation and unloading. Therefore, in-situ field tests and indoor dynamic experiments are used to obtain the small-strain stiffness parameters of the water-rich sand layer, and the HSS constitutive model is calibrated accordingly. This allows the model to reflect the high stiffness of the soil within a small strain range and its attenuation as strain increases.

[0024] Based on the distribution of confined aquifers and hydraulic head conditions determined by the survey, a seepage boundary that changes with the construction process is set in the soil layer corresponding to the model. The pumping action of dewatering wells is used as the boundary input to establish a fluid-structure interaction analysis module for alternating dewatering and excavation, so as to realistically reproduce the actual working conditions in which dewatering and excavation alternate and influence each other.

[0025] Considering that the joints of the diaphragm wall are the weak points of the waterproofing of the retaining structure, in actual construction, due to factors such as the quality of the trench connection and the cold joint of the concrete pouring, the impermeability of the joint is significantly lower than that of the main wall. Under the uneven deformation caused by precipitation and excavation, it is easy to open or misalign, resulting in deterioration of the permeability performance. Therefore, an interface unit is set between the diaphragm wall and the adjacent soil to characterize the relative sliding of the wall and soil, and the permeability performance of the joint unit of the diaphragm wall is set to be higher than that of the main wall to form a weakened joint unit.

[0026] The calibrated HSS constitutive model, fluid-structure interaction analysis module, interface element and weakened joint element are assembled as a whole to generate a three-dimensional finite element model that synchronously reflects the small strain characteristics of soil, the seepage effect of multi-layer confined water and the leakage characteristics of the joints of the retaining structure.

[0027] After the model is established, dewatering seepage calculations and excavation deformation calculations are performed alternately according to the construction sequence, with a coupled iterative process executed within each calculation step. Since dewatering wells create a cone of water head drawdown centered on the well in the aquifer, the dewatering effect is equivalent to a time-varying point sink boundary. Based on an unsteady flow model considering leakage correction at diaphragm wall joints, the pore water pressure at each node within the confined aquifer is calculated. The expression for this unsteady flow model is: In the formula, for Horizontal distance from the dewatering well at any time Pore ​​water pressure at the location, The initial pore water pressure, for The pumping rate of the dewatering well at any given time. The aquifer's hydraulic conductivity is... For water storage rate, Theis well function, This is the correction factor for weakening the joints in the diaphragm wall. When the calculated node is within the influence range of the weakened joint element, the increased permeability at the joint causes the head drop at nearby nodes to be faster than in the case without joints. A calibration value greater than 1 is used to reflect the accelerating effect of joint leakage on the surrounding seepage field.

[0028] After obtaining the pore water pressure distribution at the current time step, the soil skeleton stress is updated based on the effective stress principle. The updated effective stress is then input into the HSS constitutive model to obtain the tangent stiffness matrix of the soil at the current time step, and then the soil displacement increment and volumetric strain are solved.

[0029] Because soil undergoes volumetric compression or expansion during stress adjustment, its pore structure changes accordingly, and the permeability coefficient also changes. Therefore, it is necessary to dynamically correct the soil permeability coefficient based on volumetric strain, and the updated formula is as follows: In the formula This represents the penetration coefficient updated at the current time step. The initial permeability coefficient, The volumetric strain at the current time step, The permeability-strain coupling coefficient.

[0030] The above permeability-strain coupling coefficient The value of the permeability coefficient directly affects the rationality of the correction in fluid-structure interaction iteration. Different soil layers have different sensitivities to strain rate under different stress states, and relying solely on empirical values ​​can easily lead to deviations. Therefore, calibration through experiments is necessary. Multi-stage loading permeability-compression combined tests were conducted on water-rich sand samples taken in situ or reconstituted in the laboratory. After the samples were consolidated under initial effective stress, the steady-state permeability coefficient was measured as the benchmark value. Then, the effective stress is increased step by step, and the corresponding volumetric strain is recorded after consolidation is completed at each pressure level. And the steady-state permeability coefficient was measured. Obtain at least four sets of data under different effective stress levels.

[0031] After taking the natural logarithm of each group of experimental data, linear regression was performed to establish... and The linear relationship: ,in, The regression intercept is used. The slope of the fitted line is taken as... The calibration value of the regression determination coefficient If the value is less than 0.90, additional tests should be conducted until the accuracy requirements are met, and then the fit should be re-evaluated. Simultaneously, the permeability changes of the weakened joint unit should be monitored within each calculation step.

[0032] Due to the uneven deformation caused by foundation pit excavation and dewatering, the joint will experience normal opening and tangential misalignment. The change in the joint opening directly determines its permeability. Therefore, the normal relative displacement of the wall nodes on both sides of each weakened joint unit is extracted. and tangential relative displacement The permeability coefficient of the weakened joint unit is dynamically updated according to the following formula. In the formula, To weaken the update penetration coefficient of the joint unit in the current time step, The initial permeability coefficient, To reduce the initial equivalent thickness of the joint unit, and These are the weighting coefficients for the influence of normal opening and tangential misalignment on permeability, respectively. The formula uses a cubic relationship, which is based on the cubic proportionality between fracture flow rate and fracture aperture in fluid mechanics, i.e., the cubic law, and can reasonably reflect the amplification effect of increasing joint aperture on permeability.

[0033] When the updated permeability coefficient is greater than the preset leakage threshold, it indicates that the joint has developed into an actual leakage channel, triggering the permeability performance degradation flag of the weakened joint unit, and outputting the position coordinates of the unit as the leakage point location.

[0034] After completing the permeability coefficient correction, the pore water pressure of each node calculated in two adjacent iterations is compared. When the maximum value of the relative change in pore water pressure in all nodes is less than the preset convergence tolerance, the current time step is determined to be converged and the process is switched to the next construction stage. Otherwise, the process returns to the seepage field solution step with the corrected permeability coefficient and iterates again. This process is repeated until each time step is converged. Finally, the three-dimensional pore water pressure field, effective stress field, and displacement field of the soil around the ultra-deep foundation pit under each construction stage are output.

[0035] After the model calculations are completed and the three-dimensional pore water pressure field, effective stress field, and displacement field for each construction stage are output, it is necessary to assess the impact of each excavation section of the ultra-deep foundation pit on the adjacent subway tunnel. A polar coordinate system is established with the center of the ultra-deep foundation pit excavation face as the origin, and the excavation face is divided into equal sections according to angles. The system divides the data into several sectors. For each sector, it extracts the response data of each computational node along the path from the sector boundary to the axis of the adjacent subway tunnel from the 3D field data, and calculates the influence of each sector on the adjacent subway tunnel. .

[0036] Impact The calculation formula is: ,in, For the first The influence of each partition ; For the first The total number of computing nodes on the path corresponding to each partition; For the first On the path of the partition, the first The pore water pressure drop at each node, The initial pore water pressure; This represents the vertical effective stress increment at the node. This represents the initial effective vertical stress. This represents the magnitude of the displacement vector at that node. For the first The horizontal distance from the boundary of each zone to the axis of the adjacent subway tunnel; For the first The calculation method comprehensively assesses the impact on the tunnel from three dimensions: the decrease in pore water pressure reflects the change in the effective stress state of the soil around the tunnel due to the head loss caused by precipitation; the increase in vertical effective stress reflects the stress redistribution caused by excavation unloading; and the displacement vector modulus directly reflects the effect of soil deformation on the tunnel. Furthermore, it uses a distance-inverse weighted method to reflect the physical fact that the closer the soil is to the tunnel, the greater its contribution to the response.

[0037] Based on the influence of each region The relative magnitude of the values ​​is divided into zones according to a preset first threshold and a second threshold, thus determining the degree of influence. Partitions with values ​​greater than or equal to the first threshold are classified as strong influence zones. The area between the second threshold and the first threshold is designated as the transitional influence zone. Areas below the second threshold are classified as weak influence zones.

[0038] (2) Differentiated isolation reinforcement Based on the planar distribution of the strong impact zone, transitional impact zone, and weak impact zone between the foundation pit and the subway tunnel, the implementation scope of isolation and reinforcement is delineated in the soil between the outer side of the foundation pit retaining structure and the adjacent subway tunnel. The reinforcement area is arranged along the entire length of the outer side of the retaining structure closest to the subway tunnel, and the planar coverage includes the retaining structure segments corresponding to the strong impact zone and the transitional impact zone, extending a certain distance at both ends to ensure that the reinforcement range completely covers the sections significantly affected by the foundation pit construction. The reinforcement method uses triaxial cement-soil mixing piles to form an isolation curtain. By adjusting the number of rows, pile length, pile spacing, and cement content of the mixing piles in different zones, the reinforcement strength is differentiated.

[0039] In the section corresponding to the strong impact zone, the isolation curtain adopts double-row triple-axis cement-soil mixing piles. The pile length extends to a certain depth below the top surface of the lower relative aquitard layer to cut off the hydraulic connection of the aquifer. The double-row piles are arranged in an overlapping and interlocking manner with a larger overlap thickness and a higher cement content, so that the curtain in this section has a higher seepage resistance and overall rigidity, so as to block the water head drop caused by precipitation from being transmitted to one side of the tunnel and provide lateral restraint.

[0040] In the section corresponding to the transitional influence zone, the isolation curtain adopts a single row of three-axis cement-soil mixing piles, with the pile length also extending to the lower relative waterproof layer. The cement content and the overlap thickness of adjacent piles are appropriately reduced compared to double-row mixing piles, so as to control the project input under the premise of meeting the basic deformation control and seepage prevention requirements.

[0041] In the section corresponding to the weak influence zone, no continuous isolation curtain is set up. Instead, single-row triaxial cement-soil mixing piles are arranged at intervals only at the locations of local weak interlayers or lenses with high permeability revealed by the exploration. The pile length and cement content are further reduced.

[0042] To avoid abrupt changes in stiffness at the reinforced boundary that could lead to shear slippage or uneven settlement of the soil along the interface, the isolation curtains between adjacent affected zones are arranged in a smooth transition in plan. At the boundary between the strong affected zone and the transitional affected zone, the double-row mixing piles gradually transition to single-row mixing piles, with the pile spacing gradually changing from dense to sparse within the transition section, and the cement content decreasing synchronously. At the boundary between the transitional affected zone and the weak affected zone, the continuous single-row mixing piles gradually transition to an intermittent arrangement, with the pile spacing gradually increasing.

[0043] After the isolation curtains in each zone are completed, a protective barrier with gradient stiffness is formed between the foundation pit retaining structure and the subway tunnel. Its overall stiffness, impermeability and lateral restraint capacity decrease in a stepwise manner from the strong influence zone to the weak influence zone, which matches the spatial attenuation law of the degree of disturbance to the surrounding soil caused by the foundation pit construction.

[0044] (3) Differentiated cut-off excavation Before excavation inside the ultra-deep foundation pit, the excavation surface is divided into excavation blocks on the plane according to the determined plane distribution range of the strong influence zone, the transitional influence zone and the weak influence zone. A continuous soil isolation zone is set between each block along the direction perpendicular to the adjacent subway tunnel, so that the soil inside the foundation pit is cut off into several relatively independent excavation units on the plane by the isolation zone.

[0045] For the excavation blocks corresponding to the high-impact zone, since this area is closest to the adjacent subway tunnel and the preliminary assessment showed that the disturbance to the tunnel was the most significant, the spacing of the cut-off isolation zone was minimized, resulting in smaller excavation unit areas within the high-impact zone. Vertical excavation was carried out using the minimum layer thickness within each excavation unit. Before each layer of excavation, the corresponding dewatering wells were activated in advance to lower the water level to a certain depth below the excavation surface of that layer. Excavation began only after the water level stabilized. The smaller cut-off unit area, combined with the smaller layer thickness, kept the amount of soil unloading and stress release caused by each excavation step at a low level. The deformation increment of the retaining structure was released gradually rather than all at once, thereby reducing the risk of excessive additional deformation to the subway tunnel due to sudden stress changes.

[0046] For the excavation blocks corresponding to the transitional impact zone, the spacing of the cut-off isolation zones is between that of the strong and weak impact zones. The excavation unit area is moderate, and the layer thickness is greater than that of the strong impact zone but less than that of the weak impact zone. The impact of this area on the subway tunnel is moderate. Using a medium cut-off spacing and a medium layer thickness can achieve a balance between controlling deformation and improving soil removal efficiency. This avoids tunnel response exceeding limits due to excessive single-step unloading, and also avoids unnecessary extension of the construction period due to overly dense zoning.

[0047] For the excavation blocks corresponding to the low-impact zone, the spacing of the cut-off isolation strips is the largest, the excavation unit area is larger, and the layer thickness is also the greatest. This area is far from the subway tunnel, and the preliminary assessment showed that the disturbance to the tunnel was within an acceptable range. The larger cut-off units and thicker layers reduced the number of excavation layers, the number of support erections, and the number of isolation strips, thereby improving the soil removal efficiency and construction progress while ensuring safety.

[0048] The excavation sequence of each excavation unit within the same floor follows the principle of excavating from far to near, that is, first excavating the units corresponding to the weak influence zone, then excavating the units corresponding to the transitional influence zone, and finally excavating the units corresponding to the strong influence zone. This excavation sequence allows the deformation of the foundation pit retaining structure to be released gradually from the side away from the subway tunnel towards the tunnel side, avoiding the impact disturbance to the tunnel caused by the concentrated release of soil stress on the side of the strong influence zone.

[0049] The isolation strips between different zones are excavated only after the foundation slab of the adjacent excavation unit has been poured and reached a certain strength. The reserved isolation strip soil provides temporary lateral support to the excavated area using its own weight and lateral restraint, reducing the free deformation of the retaining structure before the foundation slab is closed. After the foundation slabs of the adjacent units have formed overall rigidity, the isolation strip soil is removed and the foundation slab at that location is poured, so that the deformation at the cut-off location is controlled under the constraints of the existing structure.

[0050] (4) Differentiated leakage sealing During the excavation and dewatering of ultra-deep foundation pits, the joints of the diaphragm walls in the foundation pit retaining structure are potential weak points prone to leakage. In the aforementioned steps, a weakened joint unit is pre-embedded at the diaphragm wall joint. This weakened joint unit is equipped with a seepage channel and a pressure sensor. The seepage channel runs through the thickness direction of the weakened joint unit, and the pressure sensors are arranged at intervals along the seepage channel to monitor water pressure changes at different locations within the seepage channel.

[0051] When groundwater outside the foundation pit seeps into the pit through minute defects at the joints of the diaphragm wall under the influence of water head difference, the seepage water enters the seepage channel of the weakened joint unit. Pressure sensors within the seepage channel respond to changes in water pressure and collect water pressure data in real time. Since the seepage water flows along the channel after entering from one side, the pressure sensor near the seepage inlet measures a higher water pressure value, while the pressure sensor farther from the seepage inlet measures a lower water pressure value. The water pressure distribution along the seepage channel shows a decreasing trend from the seepage inlet towards both sides. Based on the water pressure values ​​collected by each pressure sensor and their distribution gradient along the channel, the location corresponding to the highest water pressure is determined, and this location is used as the identification result of the seepage point.

[0052] After identifying the location of the leakage point, based on the construction drawings of the foundation pit retaining structure and the planar distribution range of the strong influence zone, transitional influence zone and weak influence zone determined in the previous steps, the location coordinates of the leakage point are compared with the planar boundaries of the above-mentioned zones to determine the influence zone to which the leakage point belongs.

[0053] When the leakage point is located in a strong influence zone or a transitional influence zone, the impact of the drop in water level outside the pit and the increase in effective soil stress caused by the leakage on the deformation of the tunnel is significant due to the proximity of these areas to the adjacent subway tunnel. Therefore, the leakage point is sealed and reinforced using the first grouting volume. The specific operation involves drilling grouting holes inside the retaining structure corresponding to the leakage point. The grouting holes penetrate the wall thickness of the retaining structure to reach behind the weakened joint unit, and a two-component grout is injected into the leakage channel and surrounding soil through a grouting pipe.

[0054] The initial grouting volume is set to a relatively large value. During the grouting process, the grouting pressure is controlled to ensure that the grout fully fills the seepage channel and diffuses into the soil within a certain range outside the retaining structure, forming a large-volume grout consolidation body. This seals the seepage channel while simultaneously reinforcing the soil outside the retaining structure, restoring its impermeability. The water pressure response of the pressure sensor within the weakened joint unit is continuously monitored during grouting. When the water pressure in the seepage channel returns to its stable value before dewatering and no longer shows a continuous downward trend, the sealing is considered complete, and grouting is stopped.

[0055] When the leakage point is located in a low-impact zone, the impact of water level changes outside the tunnel caused by the leakage on tunnel deformation is relatively small because the area is far from the subway tunnel. A second grouting volume is used to seal and reinforce the leakage point. The grouting operation is the same as described above, but the second grouting volume is smaller than the first, resulting in a correspondingly smaller diffusion range of the grout solidification body. The primary purpose is to seal the leakage channel, reducing grouting material consumption and construction time while meeting seepage prevention requirements.

[0056] In the event of multiple leakage points occurring simultaneously within the same construction section, the leakage points will be sealed in sequence according to their priority zone. Leakage points in the high-impact zone will be dealt with first, followed by those in the transitional-impact zone, and finally those in the low-impact zone, ensuring that leakage channels with a significant impact on the subway tunnel are sealed first.

[0057] Those skilled in the art will readily understand that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A differentiated construction method for ultra-deep foundation pits adjacent to subway tunnels based on zoning influence, characterized in that, include: S1. Establish a three-dimensional fluid-structure coupled finite element model including an ultra-deep foundation pit, an adjacent subway tunnel, and a weakened joint unit of a diaphragm wall. Through a coupled iterative process of alternating dewatering and excavation, output the three-dimensional pore water pressure field, effective stress field, and displacement field of the soil surrounding the ultra-deep foundation pit at each construction stage. Calculate the influence of each excavation zone on the adjacent subway tunnel based on the three-dimensional pore water pressure field, effective stress field, and displacement field, and divide the ultra-deep foundation pit excavation surface into a strong influence zone, a transitional influence zone, and a weak influence zone accordingly. S2. Based on the distribution of the strong influence zone, the transitional influence zone, and the weak influence zone, differentiated isolation reinforcement is implemented between the ultra-deep foundation pit and the adjacent subway tunnel, wherein the reinforcement strength corresponding to the strong influence zone is greater than that of the transitional influence zone, and the reinforcement strength corresponding to the transitional influence zone is greater than that of the weak influence zone, forming a protective barrier with gradient stiffness. S3. Based on the distribution of the strong influence zone, the transitional influence zone and the weak influence zone, implement differentiated truncated excavation of the soil inside the ultra-deep foundation pit, wherein the layer thickness corresponding to the strong influence zone is smaller than that of the transitional influence zone, and the layer thickness corresponding to the transitional influence zone is smaller than that of the weak influence zone. S4. During the excavation and dewatering of ultra-deep foundation pits, the location of leakage points is identified based on the dynamic response of the permeability performance of the weakened joint unit. The location of the leakage points is mapped to the strong influence zone, the transitional influence zone, and the weak influence zone. Differentiated sealing and reinforcement are implemented according to the different zones where the leakage points are located.

2. The differentiated construction method for ultra-deep foundation pit adjacent to subway tunnels based on zoning influence degree as described in claim 1, characterized in that, The construction process of the three-dimensional fluid-structure interaction finite element model in S1 includes: Small strain stiffness parameters of water-rich sand layers were obtained by in-situ field tests and indoor dynamic tests, and the HSS constitutive model was calibrated accordingly. Based on the distribution of confined aquifers and hydraulic head conditions determined by the survey, seepage boundaries that change with the construction process are set in the soil layer corresponding to the model, and the pumping action of dewatering wells is used as the boundary input to establish a fluid-structure interaction analysis module with alternating dewatering and excavation. Interface elements are set between the diaphragm wall and the adjacent soil to characterize the relative sliding of the wall and soil, and the permeability of the joint element of the diaphragm wall is set to be higher than that of the main body of the wall to form a weakened joint element. Based on the calibrated HSS constitutive model, the fluid-structure interaction analysis module, the interface element, and the weakened joint element, a model is assembled to generate a three-dimensional finite element model that synchronously reflects the small strain characteristics of the soil, the seepage effect of multi-layer confined water, and the leakage characteristics of the joints of the retaining structure.

3. The differentiated construction method for ultra-deep foundation pit adjacent to subway tunnels based on zoning influence degree as described in claim 2, characterized in that, The fluid-structure interaction analysis module is specifically configured to alternately perform dewatering seepage calculations and excavation deformation calculations according to the construction sequence, and to perform the following coupling iteration process within each calculation step: First, the seepage field is solved: the pumping effect of the dewatering well is equivalent to the point sink boundary that varies with time, and the pore water pressure of each node in the confined aquifer is calculated based on the unsteady flow model that considers the leakage correction of the underground continuous wall joint. Secondly, the effective stress and deformation field are solved: based on the pore water pressure calculated at the current time step, the soil skeleton stress is updated based on the effective stress principle, and the updated effective stress is input into the HSS constitutive model to obtain the tangent stiffness matrix of the soil at the current time step, and then the soil displacement increment and volumetric strain are solved. Then, dynamic correction of the permeability coefficient is performed: the soil permeability coefficient is updated based on the volumetric strain. Finally, a convergence determination is made: when the maximum value of the relative change in pore water pressure calculated in two adjacent iterations is less than the preset convergence tolerance, the current time step is determined to be converged and the process is switched to the next construction stage; otherwise, the process returns to the seepage field solution step with the corrected permeability coefficient and iterates again. Through the above-mentioned coupled iterative process, the three-dimensional pore water pressure field, effective stress field, and displacement field of the soil surrounding the ultra-deep foundation pit at each construction stage are output as input data for subsequent calculation of the impact of each excavation zone of the ultra-deep foundation pit on the adjacent subway tunnel.

4. The differentiated construction method for ultra-deep foundation pit adjacent to subway tunnels based on zoning influence degree as described in claim 3, characterized in that, The expression for the unsteady flow model is: ； In the formula, for Horizontal distance from the dewatering well at any time Pore ​​water pressure at the location, The initial pore water pressure, for The pumping rate of the dewatering well at any given time. The aquifer's hydraulic conductivity is... For water storage rate, Theis well function, This is the weakening correction factor for the diaphragm wall joint, applied when the calculation node is located within the influence range of the weakened joint unit. Take a calibration value greater than 1.

5. A differentiated construction method for ultra-deep foundation pit adjacent to subway tunnels based on zoning influence degree, as described in claim 3, is characterized in that... The update formula for the soil permeability coefficient is: ； In the formula, This represents the penetration coefficient updated at the current time step. The initial permeability coefficient, The volumetric strain at the current time step, The permeability-strain coupling coefficient.

6. A differentiated construction method for ultra-deep foundation pit adjacent to subway tunnels based on zoning influence degree, as described in claim 5, is characterized in that... The permeability-strain coupling coefficient The calculation methods include: Multi-stage loading permeation-compression combined tests were conducted on water-rich sand layer samples taken in situ or reconstituted in the laboratory to obtain no fewer than four sets of volumetric strains under different effective stress levels. With the corresponding steady-state permeability coefficient ; After taking the natural logarithm of each group of experimental data, linear regression was performed to establish... and The linear relationship: ； In the formula, The reference permeability coefficient under the initial effective stress state. The regression intercept; The slope of the linear relationship is used as the permeability-strain coupling coefficient. The calibration value, and when the regression determination coefficient If the value is less than 0.90, repeat the experiment until the accuracy requirement is met, then refit and determine the result. value.

7. A differentiated construction method for ultra-deep foundation pit adjacent to subway tunnels based on zoning influence degree, as described in claim 2, is characterized in that... The weakened joint unit is specifically configured as follows: Three-dimensional solid elements with a preset thickness are established at the joint positions of adjacent sections of the diaphragm wall as weakened joint elements, and an initial permeability coefficient is assigned. In each calculation step of the ultra-deep foundation pit excavation and dewatering process, the normal relative displacement and tangential relative displacement of the wall nodes on both sides of the weakened joint unit are extracted, and the equivalent opening of the joint is calculated. The permeability coefficient of the weakened joint unit is dynamically updated based on the equivalent opening of the joint, and the update formula is as follows: ； In the formula, To weaken the update penetration coefficient of the joint unit in the current time step, The initial permeability coefficient, For normal relative displacement, For tangential relative displacement, To reduce the initial equivalent thickness of the joint unit, and These are the weighting coefficients for the influence of normal opening and tangential misalignment on permeability, respectively. When the updated permeability coefficient is greater than the preset leakage threshold, the permeability performance degradation flag of the weakened joint unit is triggered, and the position coordinates of the unit are output as the location of the leakage point.

8. A differentiated construction method for ultra-deep foundation pit adjacent to subway tunnels based on zoning influence degree, as described in claim 1, is characterized in that... The calculation process for the impact on the adjacent subway tunnel in S1 is as follows: Establish a polar coordinate system with the center of the ultra-deep foundation pit excavation face as the origin, and divide the excavation face into equal parts according to angles. For each sector partition, response data of each computational node along the path from the partition boundary to the adjacent subway tunnel axis are extracted from the three-dimensional pore water pressure field, effective stress field, and displacement field. The influence of each partition on the adjacent subway tunnel is then calculated. : Based on the influence of each region The numerical values ​​are divided into partitions according to a preset first threshold and a second threshold, where... Partitions with values ​​greater than or equal to the first threshold are classified as strong influence zones. Partitions with values ​​below the first threshold and above or equal to the second threshold are designated as transitional influence zones. Partitions with values ​​below the second threshold are classified as weak influence zones.

9. A differentiated construction method for ultra-deep foundation pit adjacent to subway tunnels based on zoning influence degree, as described in claim 8, is characterized in that... The impact of each zone on adjacent subway tunnels The calculation process is as follows: ； in, For the first The influence of each partition ; For the first The total number of computing nodes on the path corresponding to each partition; For the first On the path of the partition, the first The pore water pressure drop at each node, The initial pore water pressure; This represents the vertical effective stress increment at the node. This represents the initial effective vertical stress. This represents the magnitude of the displacement vector at that node. For the first The horizontal distance from the boundary of each zone to the axis of the adjacent subway tunnel; For the first The actual distance from each node to the axis of the adjacent subway tunnel.

10. A differentiated construction method for ultra-deep foundation pit adjacent to subway tunnels based on zoning influence degree, as described in claim 1, is characterized in that... The differentiated sealing and reinforcement method in S4 is as follows: When the leakage point is located in the strong influence zone or the transitional influence zone, a first grouting volume is used for sealing and reinforcement. When the leakage point is located in the weak influence zone, a second grouting volume is used for sealing and reinforcement. The first grouting volume is greater than the second grouting volume.

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