Method for analyzing inrush failure mechanism of confined water in karst area

Abstract

The invention discloses a method for analyzing a burst failure mechanism of confined water in a karst area. The method comprises the following steps: step 1, establishing a two-dimensional seepage-stress coupling model; 2, supporting parameter setting and working condition design are carried out according to rock-soil physical and mechanical parameters; 3, non-stable seepage and foundation pit excavation simulation; and 4, performing quantitative evaluation on the inrush risk. The invention relates to the technical field of geotechnical engineering and groundwater seepage analysis, and can solve the problem of large analysis error of a karst area confined water inrush failure mechanism in the prior art.

Description

Technical Field

[0001] This invention relates to the fields of geotechnical engineering and groundwater seepage analysis, and in particular to a method for analyzing the failure mechanism of confined water inrush in karst areas. Background Technology

[0002] Foundation pit engineering plays a vital role in urban construction and infrastructure development, especially in high-rise buildings, subways, and underground space development. However, foundation pit engineering faces numerous risks, among which the sudden inrush of confined water is a significant cause of foundation pit instability and safety accidents. Research into its mechanism and optimization of dewatering and pressure-reducing measures are of great theoretical and practical significance for ensuring foundation pit stability and improving project safety.

[0003] In karst areas, the presence of karst caves further exacerbates the impact of confined water on the stability of the foundation pit. The complexity of the karst cave structure leads to uneven distribution of ground stress, making the soil at the bottom of the pit more susceptible to disturbance by confined water and prone to sudden bursting. A confined aquifer is an aquifer located between two stable impermeable layers. The water in the aquifer is under pressure, and when the overlying impermeable layer is breached, water can rise or gush out from the borehole. The hydraulic head of a confined aquifer refers to the height difference between the top surface of the confined aquifer and the static water level of the confined water; this height difference reflects the pressure of the confined water.

[0004] Traditional methods for analyzing the failure mechanism of confined water surge have the following limitations:

[0005] 1. Oversimplification of theoretical models: Traditional pressure balance theory and continuous elastic beam / plate theory ignore heterogeneous features such as karst caves and weak interlayers. Karst cave areas have a "flow around effect" (i.e., seepage water flows around the outside of the karst cave area), which leads to local seepage concentration. Their prediction error is too large, and the error can be as high as 30% or more.

[0006] 2. Poor applicability of constitutive models: The Mohr-Coulomb model overestimates the soil heave under unloading conditions, resulting in a conservative support design, and the calculation error of unloading heave is as high as 35%.

[0007] 3. Lack of seepage-stress coupling effect: The synergistic effect of dynamic changes in pore water pressure on formation displacement during precipitation is not considered, making it impossible to accurately predict the formation mechanism of the inrush channel.

[0008] Existing methods for calculating the critical head of confined water inrush in karst areas based on the cylinder criterion do not consider the spatial distribution characteristics of karst caves, leading to significant errors in karst cave region analysis. Therefore, a method for analyzing the failure mechanism of confined water inrush in karst areas is needed to address the problem of large errors in existing methods. Summary of the Invention

[0009] The purpose of this invention is to provide a method for analyzing the failure mechanism of confined water inrush in karst areas, which can solve the problem of large errors in the analysis of the failure mechanism of confined water inrush in karst areas in the prior art.

[0010] This invention is implemented as follows:

[0011] A method for analyzing the failure mechanism of confined water inrush in karst areas includes the following steps:

[0012] Step 1: Establish a two-dimensional seepage-stress coupling model;

[0013] Step 2: Set support parameters and design working conditions based on the physical and mechanical parameters of the soil and rock.

[0014] Step 3: Simulation of Unsteady Seepage and Foundation Pit Excavation;

[0015] Step 4: Quantitative assessment of surge risk.

[0016] Step 1 includes the following sub-steps:

[0017] Step 1.1: Construct a two-dimensional seepage-stress coupling model based on geological survey data, including karst caves, confined aquifers, and support structures. Karst caves are treated as permeable cavities.

[0018] Step 1.2: Set boundary conditions:

[0019] ①Seepage boundary: The two sides of the confined aquifer are set as constant water head boundaries, i.e., the first type of boundary, and the top and bottom of the confined aquifer are set as impermeable boundaries, i.e., the second type of boundary;

[0020] ②Mechanical boundary: Normal phase and lateral displacement constraints are applied to the bottom of the two-dimensional seepage-stress coupling model, and normal phase displacement constraints are applied to both sides of the two-dimensional seepage-stress coupling model to limit horizontal displacement and simulate the constraint effect of the soil around the foundation pit. The ground surface is a free deformation boundary, i.e., the third type of boundary.

[0021] The dimensions of the two-dimensional seepage-stress coupling model are 50m in height and 200m in width.

[0022] The aforementioned geotechnical physical and mechanical parameters include the density, cohesion, internal friction angle, compression modulus, anchorage bond strength, and horizontal subgrade reaction coefficient of each soil layer.

[0023] In step 2, the support structure includes retaining piles and anchor cables. The retaining piles with diameter D and spacing t are converted into plate elements using the equivalent stiffness method. The formula for calculating the thickness h of the plate element is:

[0024]

[0025] Where E is the elastic modulus of the pile, I is the moment of inertia, and Ew This is the elastic modulus of the equivalent plate element;

[0026] The anchor cable is simulated using truss elements, and the stiffness of the free section and the anchored section of the anchor cable is assigned segment by segment according to the anchoring strength of the stratum.

[0027] In step 2, the working condition design includes a first variable and a second variable. The first variable includes a confined head of 0m, a confined head of 6m, a confined head of 11m, and a confined head of 16m. The second variable includes a precipitation depth of 1m below the bottom of the pit, a precipitation depth of 2m below the bottom of the pit, a precipitation depth of 3m below the bottom of the pit, and a precipitation depth of 4m below the bottom of the pit.

[0028] In step 3, a step-by-step construction simulation is performed. The construction procedures include initial ground stress balance, layered excavation, support construction, and dewatering and pressure reduction. Water seepage in the foundation pit dewatering construction is unsteady seepage. The seepage field calculation adopts the unsteady flow equation and is solved by the finite element method.

[0029] When a confined aquifer in a saturated heterogeneous stratum extends horizontally with a relatively uniform thickness and contains first-type, second-type, and third-type boundaries, the seepage motion equation for the unsteady flow in the two-dimensional seepage-stress coupling model is as shown in equation (2):

[0030]

[0031] Where H is the groundwater head, in meters (m); Tx and Ty are the hydraulic conductivity coefficients in the x and y directions, respectively, in meters (m). 2 / day; w represents the source and sink terms, with positive values ​​for the recharge area and negative values ​​for the outflow area, and the unit is m / day; S represents the release coefficient of the confined aquifer, also known as the water storage coefficient or water storage coefficient, which is dimensionless;

[0032] Equation (2) has a definite solution given the first type of boundary conditions, the second type of boundary conditions, the third type of boundary conditions, and the initial conditions for the calculation of unstable seepage;

[0033] Among them, the first type of boundary condition Γ1 is a constant head boundary, as shown in equation (3):

[0034]

[0035] Where H1(x,y,t) is the known head of the first kind of boundary condition Γ1;

[0036] The second type of boundary condition Γ2 is a constant flow boundary, as shown in equation (4):

[0037]

[0038] Where q2(x,y,t) is the amount of water supplied to the aquifer in the Ω region through the second boundary condition Γ2, and n is the outward normal direction of the second boundary condition Γ2;

[0039] The third type of boundary condition Γ3 is a mixed boundary, as shown in equation (5):

[0040]

[0041] Where α and β are constants;

[0042] The initial conditions for calculating unsteady seepage are shown in equation (6):

[0043] H(x,y,t) t＝0 =H0(x,y,0) (x,y)∈Ω (6).

[0044] In step 4, the key indicators of sudden surge risk include: pit bottom uplift, surface settlement, seepage rate in the karst cave area, and pore water pressure gradient in the area of ​​the retaining piles in the foundation pit. The criterion for determining sudden surge risk is: when the seepage rate around the karst cave is greater than or equal to the critical value, and the pore water pressure gradient in the area of ​​the retaining piles in the foundation pit is greater than the soil impermeability strength, sudden surge failure is triggered.

[0045] The critical value is 1.5 × 10⁻⁶. -5 m / s.

[0046] Compared with the prior art, the present invention has the following advantages:

[0047] 1. This invention establishes a two-dimensional seepage-stress coupling model and embeds empty karst caves as permeable boundaries into the two-dimensional seepage-stress coupling model. This can reveal the phenomenon of local seepage concentration caused by the "flow around effect" in the karst cave area, and make the simulation accuracy of the seepage rate in the karst cave area as high as 90%, thereby accurately identifying high-risk areas of sudden surge.

[0048] 2. This invention improves the calculation error of unloading bulge by using the yield surface of the elliptical cap and the rounded corner plane, reducing it from 35% of the traditional constitutive model to less than 10%, which is 60% more accurate than the traditional method.

[0049] 3. This invention proposes a precipitation depth optimization algorithm based on real-time monitoring of pore water pressure, which achieves a balance between the dual objectives of "controlling uplift" and "preventing subsidence". It can support intelligent regulation of precipitation depth, reduce surface subsidence caused by excessive precipitation by more than 30%, and is applicable to stability assessment and disaster prevention of deep foundation pit projects in karst cave development areas. Attached Figure Description

[0050] Figure 1 This is a schematic diagram of the two-dimensional seepage-stress coupling model in the method for analyzing the failure mechanism of confined water in karst areas according to the present invention;

[0051] Figure 2 (a) is a vertical displacement cloud map of the strata at a confined water head of 0m in the method for analyzing the failure mechanism of confined water in karst areas according to the present invention;

[0052] Figure 2 (b) is a vertical displacement cloud map of the strata at a confined water head of 6m in the method for analyzing the failure mechanism of confined water in karst areas according to the present invention;

[0053] Figure 2 (c) is a vertical displacement cloud map of the strata at a confined water head of 11m in the method for analyzing the failure mechanism of confined water in karst areas according to the present invention;

[0054] Figure 2 (d) is a cloud map of the vertical displacement of the strata at a confined water head of 16m in the method for analyzing the failure mechanism of confined water in karst areas in this invention;

[0055] Figure 3 (a) is a cloud map of pore water pressure distribution at a confined water head of 0m (with a precipitation depth of 1m below the bottom of the pit) in the method for analyzing the failure mechanism of confined water in karst areas according to the present invention.

[0056] Figure 3 (b) is a cloud map of pore water pressure distribution at a confined water head of 6m (with a precipitation depth of 1m below the bottom of the pit) in the method for analyzing the failure mechanism of confined water in karst areas according to the present invention.

[0057] Figure 3 (c) is a cloud map of pore water pressure distribution at a confined water head of 11m (the water depth is 1m below the bottom of the pit) in the method for analyzing the failure mechanism of confined water in karst areas according to the present invention.

[0058] Figure 3 (d) is a pore water pressure distribution cloud map at a confined water head of 16m (the water depth is 1m below the bottom of the pit) in the method for analyzing the failure mechanism of confined water in karst areas according to the present invention.

[0059] Figure 4 This is a seepage rate curve of the method for analyzing the failure mechanism of confined water inrush in karst areas according to the present invention;

[0060] Figure 5 This is a schematic diagram of the stiffness method conversion of the support structure in the method for analyzing the failure mechanism of confined water inrush in karst areas according to the present invention.

[0061] In the diagram, 1 is a karst cave, 2 is a confined aquifer, 3 is a support structure, 31 is retaining piles, 32 is a slab unit, and 4 is a foundation pit. Detailed Implementation

[0062] The present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0063] A method for analyzing the failure mechanism of confined water inrush in karst areas includes the following steps:

[0064] Step 1: Establish a two-dimensional seepage-stress coupling model, as shown in the attached figure. Figure 1 As shown.

[0065] Step 1 includes the following sub-steps:

[0066] Step 1.1: Based on geological survey data, construct a two-dimensional seepage-stress coupling model including karst cave 1, confined aquifer 2 and support structure 3. The strength of the karst cave filling material is low. For the sake of conservatism, karst cave 1 is treated as a permeable cavity in the calculation, that is, it is assumed that there is no effective support force in the karst cave, and it only plays a role in weakening the overall integrity of the surrounding rock.

[0067] Preferably, the dimensions of the two-dimensional seepage-stress coupling model are 50m in height and 200m in width, covering the support structure 3 of the entire foundation pit 4 and the surrounding soil.

[0068] Step 1.2: Set boundary conditions:

[0069] ①Seepage boundary: The two sides of the confined aquifer 2 are set as constant head boundaries (Dirichlet condition), i.e., first type of boundary, and the top and bottom of the confined aquifer 2 are set as impermeable boundaries (Neumann condition), i.e., second type of boundary.

[0070] ②Mechanical boundary: Normal phase and lateral displacement constraints are applied to the bottom of the two-dimensional seepage-stress coupling model to ensure boundary stability during the calculation process. Normal phase displacement constraints are applied to both sides of the two-dimensional seepage-stress coupling model to limit horizontal displacement and simulate the constraint effect of the soil around the foundation pit. The surface (upper boundary) is a free deformation boundary, i.e., a third type of boundary, to allow the surface to undergo natural settlement or deformation.

[0071] By coupling seepage and stress in karst cave areas and embedding empty karst caves as permeable boundaries into a two-dimensional seepage-stress coupling model, it is possible to reveal the phenomenon of local seepage concentration caused by the "flow around the outside of the karst cave area" (i.e. seepage water flowing around the outside of the karst cave area). This can make the simulation accuracy of seepage rate in karst cave areas as high as 90%, thereby accurately identifying high-risk areas for sudden surges.

[0072] Step 2: Set support parameters and design working conditions based on the physical and mechanical parameters of the soil and rock.

[0073] Assuming the soil and rock mass conforms to the properties of an ideal elastoplastic body, the modified Mohr-Coulomb (MMC) failure criterion is used to describe the strength characteristics of the soil. The modified Mohr-Coulomb (MMC) failure criterion is a conventional analytical method in this field and will not be elaborated here. The modified Mohr-Coulomb (MMC) failure criterion can accurately simulate the yielding and failure behavior of weak surrounding rock under different stress states.

[0074] The aforementioned geotechnical physical and mechanical parameters include the density, cohesion, internal friction angle, compression modulus, anchorage bond strength, and horizontal subgrade reaction coefficient of each soil layer, as shown in Table 1.

[0075] Table 1 Geophysical and Mechanical Parameters

[0076]

[0077] The support structure 3 includes retaining piles 31 and anchor cables (not shown in the figure), please refer to the appendix. Figure 5 The retaining piles 31 with diameter D and spacing t (t << D) are converted into two-dimensional plate elements 32 using the equivalent stiffness method. The formula for calculating the thickness h of the plate element 32 is as follows:

[0078]

[0079] Where E is the elastic modulus of the pile, I is the moment of inertia, and E w This is the elastic modulus of the equivalent plate element 32.

[0080] The anchor cable is simulated using truss elements, and the stiffness of the free section and the anchored section of the anchor cable is assigned segment by segment according to the anchoring strength of the stratum.

[0081] Since the retaining piles and anchor cables are not easily affected by groundwater, their mechanical properties are assumed to remain unchanged with the water content during the calculation. The retaining piles mainly bear horizontal earth pressure and groundwater buoyancy, while the anchor cables provide additional anti-sliding and anti-pull-out forces to enhance overall stability.

[0082] The construction mechanical response of foundation pit 4 during dewatering and pressure relief excavation under different confined water head heights is mainly reflected in two aspects: different initial confined water head heights and different dewatering depths.

[0083] Please see the appendix Figure 2 (a) to (d), the described working condition design includes a first variable and a second variable, wherein the first variable includes a confined water head of 0m, a confined water head of 6m, a confined water head of 11m, and a confined water head of 16m; please refer to the appendix. Figure 3 (a)~(d), the second variable includes precipitation depth 1m below the bottom of the pit, precipitation depth 2m below the bottom of the pit, precipitation depth 3m below the bottom of the pit, and precipitation depth 4m below the bottom of the pit.

[0084] Considering the known water head height of the confined aquifer 2, the seepage boundary conditions are set as follows: the nodes on both sides of the confined aquifer 2 are set as constant water head boundaries to ensure the accuracy of water head control; the top and bottom of the confined aquifer 2 are set as impermeable boundaries, i.e., the flow rate is zero, to limit lateral seepage; in addition, to simulate the actual dewatering and pressure reduction process, the boundary condition of zero pore water pressure is set below the excavation surface of the foundation pit 4 to reflect the dynamic changes in the groundwater level during excavation and its impact on the stability of the foundation pit 4. The calculation conditions for the first and second variables are shown in Table 2:

[0085] Table 2 Calculation conditions for the first and second variables

[0086]

[0087] Through the design of different working conditions, it can support intelligent control of precipitation depth, thereby reducing surface subsidence caused by excessive precipitation by more than 30%.

[0088] Appendix Figure 2 (a) to (d) are displacement distribution cloud maps under different confined water heads (0m, 6m, 11m, 16m), from the attached... Figure 2 It can be seen that the heave deformation at the bottom of the excavation pit gradually increases with the increase of the confined water head, especially under high confined water head conditions (11m and 16m), the vertical displacement in the central area of ​​excavation pit 4 is significantly enhanced. This phenomenon may be related to the buoyancy effect of the confined water. As the water head increases, the upward force on the foundation soil increases, leading to a more obvious heave at the bottom of excavation pit 4. At the same time, the horizontal displacement of the excavation pit sidewalls also increases with the increase of the confined water head, indicating that the excavation pit retaining structure is subjected to greater lateral water pressure, which may cause further development of lateral deformation of the soil. Furthermore, due to the presence of karst caves, the displacement distribution characteristics of the strata have changed significantly. The heave above the karst caves is larger than that without karst caves, indicating that the local effect of confined water is more concentrated at the karst caves. The soil overlying the karst caves, lacking effective restraint, is more significantly affected by the buoyancy effect of the confined water, thus leading to an enhancement of local heave. Meanwhile, as the confined water head increases, the gradient of the uplift deformation above the karst cave gradually increases, and the area of ​​dense contour lines expands towards the edge of the pit, which may be related to the change in the seepage field at the karst cave.

[0089] When the confined hydraulic head is high, karst caves may become concentrated areas of seepage, leading to increased local permeability and exacerbating soil deformation. Furthermore, the uplift area at the bottom of the excavation pit and above the karst caves gradually expands with increasing hydraulic head, indicating that the action of confined water not only affects local areas but may also influence soil deformation over a wider area through the transmission of pore water pressure. Displacement changes near the pit sidewalls also show a certain trend: displacement is smaller at low hydraulic heads, and lateral deformation intensifies with increasing hydraulic head, possibly related to changes in the hydraulic gradient within the pit. Under higher confined hydraulic heads, the groundwater potential energy increases, resulting in stronger seepage driving forces and further development of pit slope deformation.

[0090] Appendix Figure 3 Images (a) to (d) show the pore water pressure distribution cloud maps at a depth of 1m below the bottom of the foundation pit during precipitation. Figure 3 It can be seen that the excavation of the foundation pit significantly altered the distribution characteristics of pore water pressure, especially under the influence of karst caves at the bottom of the pit, where the local pressure gradient changed markedly. In the unexcavated area, the pore water pressure was generally distributed in layers, gradually decreasing from bottom to top. After excavation, anomalies appeared in the pore water pressure at the bottom and surrounding areas. The presence of karst caves caused a decrease in the pore water pressure of the soil layer above them, forming a pressure disturbance zone in the surrounding area. This may be due to the lack of soil support inside the karst caves, which increased the mobility of groundwater in this area, thus altering the distribution pattern of pore water pressure. In addition, the pore water pressure on both sides of the foundation pit was significantly higher than that at the bottom, indicating a tendency for groundwater to seep towards the bottom, which may exacerbate the risk of heave deformation or instability at the bottom of the pit.

[0091] Step 3: Simulation of unsteady seepage and excavation of foundation pit 4.

[0092] The GTS NX software was used for step-by-step construction simulation. The construction procedures included initial ground stress balancing, layered excavation, support construction, and dewatering and pressure reduction. In the GTS NX program, the node head height can be set by adding a total head (i.e., the pipe head, including position head and pressure head). During seepage analysis, the position head of each element is calculated based on the origin of the global coordinate system; that is, the height of a node relative to the origin is its position head. When applying the total head to a node, the pressure head can be determined by calculating the difference between the total head and the position head. Therefore, this method can effectively apply the confined water head.

[0093] Water seepage refers to the flow of water in a porous medium, and its flow properties are determined by the soil, rock, and the fluid itself. Water seepage in foundation pit dewatering is unsteady seepage, and the seepage field is calculated using unsteady flow equations and solved using the finite element method.

[0094] When the confined aquifer 2 of the saturated heterogeneous stratum extends horizontally with a relatively uniform thickness and has first-type, second-type, and third-type boundaries, the seepage motion equation of the unsteady flow in the two-dimensional seepage-stress coupling model is as shown in equation (2):

[0095]

[0096] Where H is the groundwater head (in meters); Tx and Ty are the hydraulic conductivity in the x and y directions, respectively (in meters). 2 / day); w is the source and sink term, with positive for the recharge area and negative for the outflow area (unit: m / day); S is the release coefficient of the confined aquifer 2, also known as the water storage coefficient or water storage coefficient, which is dimensionless.

[0097] Equation (2) has a definite solution given the first type of boundary conditions, the second type of boundary conditions, the third type of boundary conditions, and the initial conditions for the calculation of unstable seepage.

[0098] Among them, the first type of boundary condition Γ1 (Dirichlet boundary condition) is a constant head boundary, as shown in equation (3):

[0099]

[0100] Where H1(x,y,t) is the known head of the first kind of boundary condition Γ1.

[0101] The second type of boundary condition Γ2 (Neuman boundary condition) is a constant flow boundary, as shown in equation (4):

[0102]

[0103] Where q2(x,y,t) is the amount of water supplied to the aquifer in the Ω region through the second boundary condition Γ2, and n is the outward normal direction of the second boundary condition Γ2.

[0104] Given initial conditions: Water level distribution at various points within the seepage zone at a selected time (usually t=0):

[0105] H(x,y,z,t)| t =0=H0(x,y,z), (x,y,z)∈Ω region, where H0 is the known water level distribution in Ω region, and Ω region is the seepage region (solution region) composed of source, sink and 0 potential line.

[0106] The third type of boundary condition Γ3 is a mixed boundary, as shown in equation (5):

[0107]

[0108] Where α and β are constants.

[0109] The initial conditions for calculating unsteady seepage are shown in equation (6):

[0110] H(x,y,t) t＝0 =H0(x,y,0) (x,y)∈Ω (6).

[0111] Seepage rate curves under different confined water heads are attached. Figure 4 As shown, from the appendix Figure 4 It can be seen that the closer the seepage rate is to the retaining pile 31 on the inner side of the support structure 3, the greater the seepage rate.

[0112] From the attached Figure 4 It can be seen that as the confined water head gradually increases from 0m to 16m, the overall seepage rate between the two pile bottoms significantly increases, and a more concentrated high-value area appears in the region above the retaining piles and karst caves. This reflects the intensifying effect of the confined water head on the groundwater dynamics: the higher the water head, the greater the hydraulic gradient, and the easier it is for the seepage rate to accumulate in a local area. At the same time, the presence of the karst caves disrupts the originally relatively uniform seepage channels, causing a significant "bypass" phenomenon of groundwater above the karst caves, leading to a further increase in the seepage rate in this area, exhibiting a localized surge. As the confined water head increases, the concentrated seepage effect above the karst caves becomes more pronounced, potentially posing a threat to the safety of the foundation pit.

[0113] Through simulation of unsteady seepage and excavation of foundation pit 4, the accuracy of the seepage rate simulation in the karst cave area reaches 90%, which can accurately identify high-risk areas of sudden surge.

[0114] The above numerical model requires the absence of any external boundaries. Essentially, boundary conditions characterize the influence of the external environment on the model region, while also enhancing the modeler's understanding of water flow or solute transport in the study area. Boundary conditions represent the recharge and discharge relationships between the groundwater system and surrounding water systems. Boundaries appear at the edges of the simulation region and at the locations of source and sink terms, such as rivers, wells, leaking reservoirs, and pollutant leakage areas. Common natural boundaries include surface water body boundaries, fault boundaries, pumping (injection) wells, rock mass contact boundaries, and watersheds, as shown in the table below. Sometimes, due to model size limitations, the groundwater system under investigation may not have the aforementioned obvious natural boundaries, requiring the delineation of artificial boundaries based on specific circumstances. In the generalized system, these boundaries will supply or remove groundwater from the system in various ways, thereby controlling the groundwater system.

[0115] Natural boundaries include: low-permeability geological bodies, rivers, water-blocking faults, lakes, surface watersheds, reservoirs, groundwater watersheds, wetlands, swamps, groundwater evaporation, ditches, streams, pumping / injection wells, springs, recharge, and others.

[0116] Artificial Boundaries: Based on the distribution of aquifers and impermeable layers, geological structures, groundwater flow characteristics at the boundaries, and the hydraulic connection between groundwater and surface water, the boundaries of the simulation area can generally be generalized into three types: a type of boundary with a given groundwater level (head), a type of boundary with a given lateral runoff, and a type of boundary with a given relationship between groundwater lateral flow and level. Among these boundaries, the most controlling is the constant head boundary, meaning that regardless of changes in groundwater within the system, a fixed head value is maintained at the boundary, and groundwater flow can be provided or removed indefinitely. Large rivers, lakes, and oceans are often defined as constant head boundaries. The least controlling boundary is the constant flow boundary, meaning that this boundary always provides or removes groundwater at a given rate. Pumping wells, rainfall infiltration, and watersheds are often defined as constant flow boundaries. Other types of boundaries are combinations or variations of these two types. The most common mathematical expressions for the three types of boundaries (where h represents the head, n represents the outward normal direction of the boundary, and c is a constant) are:

[0117] Boundary name: Class I boundary (constant head), commonly known as Dirichlet boundary, mathematical expression h(x,y,z,t) = constant.

[0118] Boundary name: Type II boundary (constant flow), commonly known as Newman boundary, mathematical expression dh(x,y,z,t) / dn = constant.

[0119] Boundary name: Type III boundary (mixed boundary), commonly known as Cauchy boundary, mathematical expression dh / dn+c*h=constant.

[0120] Step 4: Quantitative assessment of surge risk.

[0121] Key indicators for sudden surge risk include: pit bottom uplift, surface settlement, seepage rate in the karst cave area, and pore water pressure gradient in the area next to the retaining piles within the foundation pit.

[0122] The criterion for determining the risk of sudden inrush is: when the seepage rate around cave 1 is ≥ the critical value (1.5 × 10⁻⁶). -5 When the pore water pressure gradient in the area of ​​the retaining piles in the foundation pit exceeds the soil's impermeability, a sudden surge failure is triggered.

[0123] The impermeability of soil is determined by the properties of the soil.

[0124] This invention is based on an algorithm that optimizes precipitation depth by real-time monitoring of pore water pressure, achieving a balance between the dual objectives of "controlling uplift (i.e., controlling the amount of uplift at the bottom of the pit)" and "preventing subsidence (i.e. preventing surface subsidence)".

[0125] The above are merely preferred embodiments of the present invention and are not intended to limit the scope of protection of the invention. Therefore, any modifications, equivalent substitutions, improvements, etc., 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 method for analyzing the failure mechanism of confined water inrush in karst areas, characterized by: Includes the following steps: Step 1: Establish a two-dimensional seepage-stress coupling model; Step 2: Set support parameters and design working conditions based on the physical and mechanical parameters of the soil and rock. Step 3: Simulation of unsteady seepage and excavation of foundation pit (4); Step 4: Quantitative assessment of surge risk.

2. The method for analyzing the failure mechanism of confined water inrush in karst areas according to claim 1, characterized in that: Step 1 includes the following sub-steps: Step 1.1: Based on geological survey data, construct a two-dimensional seepage-stress coupling model including karst caves (1), confined aquifers (2) and support structures (3), with karst caves (1) treated as permeable cavities; Step 1.2: Set boundary conditions: ①Seepage boundary: The two sides of the confined aquifer (2) are set as constant water head boundaries, i.e., the first type of boundary, and the top and bottom of the confined aquifer (2) are set as impermeable boundaries, i.e., the second type of boundary; ②Mechanical boundary: Normal phase and lateral displacement constraints are applied to the bottom of the two-dimensional seepage-stress coupling model, and normal phase displacement constraints are applied to both sides of the two-dimensional seepage-stress coupling model to limit horizontal displacement and simulate the constraint effect of the soil around the foundation pit. The ground surface is a free deformation boundary, i.e., the third type of boundary.

3. The method for analyzing the failure mechanism of confined water inrush in karst areas according to claim 1 or 2, characterized in that: The dimensions of the two-dimensional seepage-stress coupling model are 50m in height and 200m in width.

4. The method for analyzing the failure mechanism of confined water inrush in karst areas according to claim 1, characterized in that: The aforementioned geotechnical physical and mechanical parameters include the density, cohesion, internal friction angle, compression modulus, anchorage bond strength, and horizontal subgrade reaction coefficient of each soil layer.

5. The method for analyzing the failure mechanism of confined water inrush in karst areas according to claim 1, characterized in that: In step 2, the support structure (3) includes retaining piles (31) and anchor cables. The retaining piles (31) with diameter D and spacing t are converted into plate elements (32) using the equivalent stiffness method. The formula for calculating the thickness h of the plate element (32) is as follows: Where E is the elastic modulus of the pile, I is the moment of inertia, and E w The elastic modulus of the equivalent plate element (32); The anchor cable is simulated using truss elements, and the stiffness of the free section and the anchored section of the anchor cable is assigned segment by segment according to the anchoring strength of the stratum.

6. The method for analyzing the failure mechanism of confined water inrush in karst areas according to claim 1, characterized in that: In step 2, the working condition design includes a first variable and a second variable. The first variable includes a confined head of 0m, a confined head of 6m, a confined head of 11m, and a confined head of 16m. The second variable includes a precipitation depth of 1m below the bottom of the pit, a precipitation depth of 2m below the bottom of the pit, a precipitation depth of 3m below the bottom of the pit, and a precipitation depth of 4m below the bottom of the pit.

7. The method for analyzing the failure mechanism of confined water inrush in karst areas according to claim 1, characterized in that: In step 3, a step-by-step construction simulation is performed. The construction procedures include initial ground stress balance, layered excavation, support construction, and dewatering and pressure reduction. Water seepage in the foundation pit dewatering construction is unsteady seepage. The seepage field calculation adopts the unsteady flow equation and is solved by the finite element method.

8. The method for analyzing the failure mechanism of confined water inrush in karst areas according to claim 7, characterized in that: When the confined aquifer (2) of the saturated heterogeneous stratum extends horizontally with a relatively uniform thickness and has first-class, second-class, and third-class boundaries, the seepage motion equation of the unsteady flow in the two-dimensional seepage-stress coupling model is as shown in equation (2): Where H is the groundwater head, in meters (m); Tx and Ty are the hydraulic conductivity coefficients in the x and y directions, respectively, in meters (m). 2 / day; w is the source and sink term, with positive for the recharge area and negative for the outflow area, and the unit is m / day; S is the release coefficient of the confined aquifer (2), also known as the water storage coefficient or water storage coefficient, which is dimensionless; Equation (2) has a definite solution given the first type of boundary conditions, the second type of boundary conditions, the third type of boundary conditions, and the initial conditions for the calculation of unstable seepage; Among them, the first type of boundary condition Γ1 is a constant head boundary, as shown in equation (3): Where H1(x,y,t) is the known head of the first kind of boundary condition Γ1; The second type of boundary condition Γ2 is a constant flow boundary, as shown in equation (4): Where q2(x,y,t) is the amount of water supplied to the aquifer in the Ω region through the second boundary condition Γ2, and n is the outward normal direction of the second boundary condition Γ2; The third type of boundary condition Γ3 is a mixed boundary, as shown in equation (5): Where α and β are constants; The initial conditions for calculating unsteady seepage are shown in equation (6): H(x,y,t) t＝0 =H0(x,y,0) (x,y)∈Ω (6)。 9. The method for analyzing the failure mechanism of confined water inrush in karst areas according to claim 1, characterized in that: In step 4, the key indicators of sudden surge risk include: the amount of bottom heave, the amount of surface settlement, the seepage rate in the karst cave area, and the pore water pressure gradient in the area of ​​the retaining piles in the foundation pit. The criterion for determining the sudden surge risk is: when the seepage rate around the karst cave (1) is ≥ the critical value, and the pore water pressure gradient in the area of ​​the retaining piles in the foundation pit is > the soil impermeability strength, sudden surge failure is triggered.

10. The method for analyzing the failure mechanism of confined water inrush in karst areas according to claim 9, characterized in that: The critical value is 1.5 × 10⁻⁶. -5 m / s.

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