Method for determining seepage parameters of tunnel fault fracture zone based on double medium model

Abstract

The application discloses a tunnel fault fracture zone seepage parameter determination method based on a double medium model, which comprises the following steps: 1, according to the seepage characteristics of the tunnel water-rich fault fracture zone, a double medium model in a continuous medium model is used to determine the fracture zone seepage parameter; 2, the double medium model is modified to build a modified double medium model of the tunnel fault fracture zone seepage basic model; 3, the permeability coefficient of the modified double medium model system is determined; the method can effectively determine the seepage parameter by dividing the rock mass and the fracture into a matrix system and a fracture system respectively, the matrix system and the fracture system respectively obeying the matrix flow of the Darcy law and the non-Darcy law seepage of the Forchheimer binomial equation, establishing the double medium seepage model, and has the characteristics of good robustness of the calculation result, accurate and true description of the seepage condition of the fault fracture zone geology, early warning and prevention and control effect on the fault fracture zone geological disaster.

Description

Technical Field

[0001] This invention relates to the field of engineering surveying technology, specifically to a method for determining seepage parameters in tunnel fault fracture zones based on a dual-medium model. Background Technology

[0002] Rock mechanics engineering projects in my country, including large-scale water conservancy and hydropower development, shale gas extraction, underground water-sealed oil depot construction, geothermal resource development, carbon dioxide sequestration, and geological disposal of high-level radioactive waste, all commonly involve key technical issues such as describing the permeability characteristics of rock masses under complex geological and ecological environments, simulating the seepage process, and evaluating the seepage control effect. Seepage parameters (permeability coefficient, water storage coefficient, etc.) of rock masses in fault fracture zones are fundamental parameters for studying groundwater seepage in rock masses. Correctly and effectively determining rock permeability is crucial for quantitatively evaluating the seepage distribution law of rock masses. Indoor testing of rock permeability is currently the most widely used and technologically mature method for obtaining rock seepage parameters. It plays an irreplaceable role in obtaining rock seepage parameters and establishing and verifying theoretical models of permeability evolution. According to the experimental principle, it can be divided into three types: steady-state flow method, transient pressure pulse method, and hydraulic oscillation method. However, for low-permeability rocks, the hydraulic oscillation method has advantages that the steady-state flow method and transient pressure pulse method cannot match, offering high measurement accuracy and extremely low time consumption. It can determine the rock's water storage coefficient simultaneously with the rock's permeability coefficient.

[0003] In determining the seepage parameters of fault fracture zones in tunnels, the currently used theories generally assume that the seepage in fracture zones follows Darcy's law. However, during construction, it has been found that the seepage in fracture zones is not entirely Darcy's flow. Therefore, the results obtained by determining parameters based on Darcy's flow have a large error and cannot accurately and realistically depict the seepage situation of the fault fracture zone. Consequently, the early warning and prevention of geological disasters in fault fracture zones are ineffective, posing serious safety hazards.

[0004] Therefore, there is an urgent need to develop a method for determining the hydrogeological parameters of weakly permeable layers under non-Darcy flow conditions in order to solve the problems existing in the above-mentioned technologies. Summary of the Invention

[0005] To address the aforementioned problems, this invention aims to provide a method for determining seepage parameters in tunnel fault fracture zones based on a dual-medium model. This method divides the rock mass and fractures into a matrix system and a fracture system, respectively. The matrix system follows Darcy's law for matrix flow and a Forchheimer binomial equation for non-Darcy's law seepage flow, respectively. Based on this, the permeability coefficients of each system are calculated, and an improved dual-medium seepage model is established. This model effectively optimizes the seepage failure calculation formula, determines the seepage parameters, and studies the seepage failure situation, enabling better early warning and prevention of geological hazards in fault fracture zones. It features robust calculation results, accurate and realistic depiction of seepage conditions in fault fracture zones, and significant effectiveness in early warning and prevention of geological hazards in fault fracture zones.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0007] A method for determining seepage parameters in tunnel fault fracture zones based on a dual-medium model, including

[0008] Step 1: In the fractured fault zone of the tunnel, the seepage parameters of the fractured fault zone are determined using a dual-medium model based on the seepage characteristics of the water-rich fractured fault zone of the tunnel.

[0009] Step 2: Modify the dual-medium model and construct the basic model of seepage in the fault fracture zone of the tunnel based on the modified dual-medium model.

[0010] Step 2.1: Set up the modified dual-medium seepage model environment;

[0011] Step 2.2: Determine the permeability coefficient of the rock matrix system;

[0012] Step 2.3: Determine the permeability coefficient of the fractured system;

[0013] Step 3: Determine the permeability coefficient of the modified dual-medium model system;

[0014] The modified dual-medium flow model environment described in step 2.1 is as follows:

[0015] (1) The overall system consists of a rock matrix system and a fracture system, which overlap in space and are uniformly distributed throughout the medium;

[0016] The overall system is the fault fracture zone seepage model as a whole, the rock matrix system is the rock pore system, and the fracture system is the rock fracture system.

[0017] (2) The rock matrix system and the fracture system satisfy their respective seepage laws. The rock matrix system satisfies Darcy's law, while the fracture system satisfies a non-Darcy's law. The coupling between the two is achieved through the water exchange term.

[0018] (3) The rock matrix system and the fracture system each have their own porosity and permeability, and are affected by the stress state;

[0019] Step 3 describes the process of determining the permeability coefficient of the modified dual-medium model system, which includes:

[0020] Step 3.1: In the overall permeability coefficient expression, combining the Darcy flow permeability coefficient and the non-Darcy flow permeability coefficient, the overall permeability coefficient K is obtained by considering the volume fraction of the fracture domain:

[0021] (12)

[0022] In equation (12): The volume fraction of the fractured domain is represented by K; the total permeability coefficient K is the permeability coefficient of the modified model, K1 is the permeability coefficient of the rock matrix system, and K2 is the permeability coefficient of the fractured system; where:

[0023] permeability coefficient of rock matrix system

[0024]

[0025] Where k1 is the permeability of the sample, The density of water; It is the acceleration due to gravity. is the dynamic viscosity coefficient of water;

[0026] Permeability coefficient of fractured system

[0027]

[0028] Where k2 is the sample permeability, and J is the hydraulic gradient. The influence coefficient of the non-Darcy flow equation;

[0029] Step 3.2: The permeability coefficient in an isotropic medium is defined as the unit flow rate under a unit hydraulic gradient; the permeability coefficient of the model is obtained, and the seepage rate is further calculated:

[0030] (13)

[0031] In equation (13): K is the permeability coefficient; k is the permeability. Viscosity; v ’ Kinematic viscosity; ρ is the density; g is the acceleration due to gravity.

[0032] Preferably, the process of determining the permeability coefficient of the rock matrix system in step 2.2 includes...

[0033] Step 2.21: Under single-medium conditions, the rock matrix system refers to the pore system of the rock. Assume that the rock pore system satisfies Darcy's law of seepage:

[0034] (1)

[0035] In equation (1): v is the seepage velocity of water; is the dynamic viscosity coefficient of water; ρ is the density of water; p is the constant water pressure; k1 is the permeability of the sample; Here, is the Hamiltonian operator; z represents the direction of gravity; For gravitational acceleration; Hamiltonian operator in the formula for:

[0036] (2)

[0037] In equation (2): Hamiltonian operator is a vector differential operator; i is a vector in the x-axis direction of the horizontal plane; j is a vector in the y-axis direction of the horizontal plane; k is a vector in the z-axis direction of the vertical plane;

[0038] Step 2.22: Ignoring the two water pressure changes in the horizontal direction in equation (2), and only considering the vertical k direction, we obtain the following equation:

[0039] (3)

[0040] Step 2.23: Assume the hydraulic gradient is uniformly distributed throughout the model, and the water pressure is proportional to the depth, expressed as:

[0041] (4)

[0042] In equation (4): h is the height of the selected model sample;

[0043] Step 2.24: Calculate the permeability of the rock matrix system using Darcy's law formula. Substituting equations (3) and (4) into (1), the permeability of the rock matrix system is:

[0044] (5)

[0045] For Darcy flow, the permeability coefficient K1 is obtained by converting between the permeability of the rock matrix system and the permeability coefficient of the rock matrix system:

[0046] (6).

[0047] Preferably, the process of determining the permeability coefficient of the fractured system in step 2.3 includes...

[0048] Step 2.31: Assume the fractured system satisfies non-Darcy's law. The non-Darcy flow equations describing the hydraulic gradient J and seepage velocity v include exponential and quadratic forms:

[0049] For fracture flow seepage, the Forchheimer quadratic form relation is used to describe it:

[0050] (9)

[0051] In equation (9): This represents the influence coefficient of the non-Darcy flow equations. This represents the dynamic viscosity coefficient of water. This indicates the density of water. This represents the acceleration due to gravity, and v is the seepage velocity of the water.

[0052] Step 2.32: Influence coefficient of permeability k2 on non-Darcy flow equations It exhibits an exponential distribution:

[0053] (10)

[0054] In equation (10): k2 is the permeability of the fracture system;

[0055] Step 2.33: Using equations (9) and (10) and the relationship between the permeability coefficient and the permeability, the hydraulic gradient with respect to the permeability coefficient K2 for non-Darcy flow is obtained as follows:

[0056] (11).

[0057] Preferably, the dual-medium seepage model is hexahedral, consisting of a rock matrix system and a fracture system, which overlap in space and are uniformly distributed throughout the medium;

[0058] The rock matrix system satisfies Darcy's law, while the fracture system does not. The coupling between the two is achieved through the water exchange term. The rock matrix system and the fracture system each have their own porosity and permeability, and are affected by the stress state. The fracture direction is consistent with the direction of the main permeability.

[0059] Preferably, in the dual-medium seepage model,

[0060] (1) The permeability coefficient K of the modified model is

[0061]

[0062] in, K1 is the volume fraction of the fractured domain, K2 is the permeability coefficient of the rock matrix system, and K3 is the permeability coefficient of the fractured system.

[0063] (2) The seepage rate k is

[0064]

[0065] In the formula: K is the permeability coefficient; k is the permeability. v is the dynamic viscosity coefficient of water; ’ Kinematic viscosity; ρ is the density; g is the acceleration due to gravity.

[0066] The beneficial effects of this invention are: This invention discloses a method for determining seepage parameters in fractured zones of tunnel faults based on a dual-medium model. Compared with the prior art, the improvement of this invention lies in:

[0067] 1. This invention designs a method for determining seepage parameters in fault fracture zones of tunnels based on a dual-medium model. This method divides the rock mass and fractures into a matrix system and a fracture system, respectively. The matrix system and fracture system respectively follow Darcy's law for matrix flow and Forchheimer's binomial equation for non-Darcy's law seepage. Based on this, the permeability coefficients of each system are calculated, the seepage failure calculation formula is optimized, and an improved dual-medium seepage model is established. Using this model, seepage parameters can be effectively determined, and seepage failure can be studied, so as to better provide early warning and prevention of geological hazards in fault fracture zones. It has the advantages of good robustness of calculation results, accurate and realistic depiction of seepage conditions in fault fracture zones, and significant effect on early warning and prevention of geological hazards in fault fracture zones.

[0068] 2. The method of studying the seepage fields of the two media separately in the model helps to depict the preferential flow in the rock mass. At the same time, the consideration of the water exchange term enables the two media to be better coupled, which can more realistically and effectively simulate the seepage problem in fractured rock mass and improve the accuracy of the description of seepage in fault fracture zone geology. Attached Figure Description

[0069] Figure 1 This is a flowchart of the method for determining seepage parameters in tunnel fault fracture zones based on a dual-medium model, according to the present invention.

[0070] Figure 2 This is a diagram of a classic dual-medium flow model.

[0071] Figure 3 This is a diagram of the modified dual-medium seepage model of the present invention.

[0072] Figure 4 This is a cross-sectional view of the fault fracture zone section of the Daliangshan Tunnel in Embodiment 2 of the present invention. Detailed Implementation

[0073] To enable those skilled in the art to better understand the technical solutions of the present invention, the technical solutions of the present invention will be further described below in conjunction with the accompanying drawings and embodiments.

[0074] Example 1: Refer to Appendix Figure 1-4 The method for determining seepage parameters in tunnel fault fracture zones based on a dual-medium model, as shown, includes...

[0075] Step 1: In the fault fracture zone of the tunnel, the dual-medium model in the continuous medium model is selected to determine the seepage parameters of the fracture zone based on the seepage characteristics of the water-rich fault fracture zone of the tunnel.

[0076] Currently, analytical models for the seepage characteristics of fractured media are mainly divided into three categories: continuous medium models, discrete fracture network models, and hybrid medium models where continuous medium and discrete fracture network coexist. Based on the geological conditions of the fault fracture zone, this embodiment selects a more suitable continuous medium model for research. The continuous medium model is based on the concept of characterization unit. Under certain equivalence principles, fractured rock masses can be divided into three different types of continuous porous media: rock mass continuum model, equivalent continuous medium model, and dual continuous medium model. Among them, the equivalent continuous medium model and the dual continuous medium model have been widely used in practice. Therefore, this embodiment uses the dual medium model for correction to determine the seepage parameters of the fracture zone.

[0077] Step 2: Modify the dual-medium model and construct the basic model of seepage in the fault fracture zone of the tunnel based on the modified dual-medium model.

[0078] Since the theory of seepage in porous media is relatively mature, this embodiment selects the dual-medium seepage model in the continuous medium model for this embodiment. The separate study of the seepage fields of the two media in the model helps to depict the preferential flow in the rock mass. At the same time, the consideration of the water exchange term enables the two media to be better coupled, which can more realistically and effectively simulate the seepage problem in fractured rock mass.

[0079] Classical dual-medium flow model, such as Figure 2 As shown, in the dual-medium seepage model, the rock mass and fractures are studied as separate media, namely the fractured medium system and the rock matrix system, respectively. The seepage of the rock matrix system obeying Darcy's law and the seepage of the fracture system not obeying Darcy's law are established. The coupling between the fracture system and the rock matrix system is represented by the water exchange term.

[0080] Dual media consist of a fracture system and a rock block pore system that is cut by the fracture. The main difference between dual media and general porous media (see porous media) is that at any point (micro-element volume) in dual media, there are two permeabilities and two porosities. Typically, the permeability of the fracture system is much greater than that of the rock block system, while the porosity is much smaller. Therefore, the fracture system acts as a fluid channel, while the rock block system acts as a fluid storage space.

[0081] Although the rock mass continuous element and the fracture continuous element in the model overlap spatially, their physical and mechanical parameters and state variables are defined separately. The seepage fields of the two continuous media are established according to Darcy's law and non-Darcy's law. The seepage fields of the two systems are calculated by balancing the water exchange between the seepage fields of the two continuous media. Compared with other continuum models, the dual continuous media model has many advantages. The separate study of the seepage fields of the two media in the model helps to depict the preferential flow in the rock mass. At the same time, the consideration of the water exchange term enables the two media to be better coupled, which can more realistically and effectively simulate the seepage problem in fractured rock masses.

[0082] Similarly, the dual-medium model also has some problems. Although treating the fracture network as a continuous medium is beneficial for research and calculation, there are cases where the fracture network does not have typical unit cells or the typical unit cells are too large. In these cases, it cannot be regarded as a continuous medium, so the use of the model has certain applicability. In addition, the exchange coefficient of the water exchange term between rock mass pore water and fracture water in the dual-continuous medium model is relatively difficult to determine, which also has a certain impact on the model's simulation.

[0083] The rock masses studied often possess complex geological environments and inherent properties. For example, rock masses in fault fracture zones are not only geologically complex but also highly water-rich and anisotropic. The relatively weak rock mass strength leads to extremely complex coupling between seepage and stress. Therefore, in order to make the dual-medium seepage model more applicable to water-rich fault fracture zones, it is necessary not only to consider the seepage of complex soils and the exchange flow between soil and fractures, but also to determine whether the seepage of soils and fractures satisfies the traditional Darcy's law. Further research is needed to investigate this.

[0084] Based on the classic dual-medium seepage model and considering the characteristics of fault fracture zones, this embodiment proposes a modification to the dual-medium seepage model. It is assumed that seepage in rock pores satisfies Darcy's law, but seepage in fractures satisfies Forchheimer's non-Darcy seepage. The linear combination of the two ultimately constitutes the overall seepage model. On this basis, a more suitable method for calculating permeability coefficients for fault fracture zones is proposed, making it more consistent with the geological characteristics of fault fracture zones and providing a theoretical basis for numerical simulation.

[0085] Step 2.1: The modified dual-medium seepage model makes the following assumptions:

[0086] (1) The overall system consists of two parts: the rock matrix system and the fracture system. The two overlap in space and are evenly distributed throughout the medium.

[0087] (2) The rock matrix system and the fracture system satisfy their respective seepage laws. The rock matrix system satisfies Darcy's law, while the fracture system satisfies a non-Darcy's law. The coupling between the two is achieved through the water exchange term.

[0088] (3) The rock matrix system and the fracture system each have their own porosity and permeability, and are affected by the stress state;

[0089] The modified dual-medium seepage model is mainly hexahedral, consisting of a rock matrix system and a fracture system. The rock matrix system and the fracture system each satisfy their respective seepage laws, and the two combine to produce an effect. Furthermore, the fracture direction is consistent with the direction of the main permeability.

[0090] The modified model assumes that the fault fracture zone is a continuous medium consisting only of a rock matrix system and a fracture system. Seepage exists in both systems and satisfies different seepage laws. The two systems are closely combined and uniformly distributed throughout the medium. During the seepage process, there is a water exchange term between the rock matrix system and the fracture system. The seepage volume of the two systems depends on the properties of their respective systems and the volume proportion of the fracture domain in the entire fault fracture zone, as determined by the actual development of geological fractures.

[0091] The model assumes that seepage in the rock matrix system follows Darcy's law, while seepage in the fracture system follows Forchheimer's law, which is not Darcy's law. Figure 3 In the modified dual-medium seepage model shown, i m i represents the seepage flow rate per unit size area of ​​the rock matrix system. f This represents the seepage flow rate per unit size area of ​​a fractured system. This represents the water exchange term between the rock matrix system and the fracture system; therefore, the parameters of the calculation model need to be modified based on the parameters of the classic dual-medium model to meet the relevant calculation requirements of this model.

[0092] Step 2.2: Determine the permeability coefficient of the rock matrix system

[0093] Step 2.21: Under single-medium conditions, the rock matrix system mainly refers to the pore system of the rock. Assume the rock pore system satisfies Darcy's law of permeability:

[0094] (1)

[0095] In equation (1): v is the seepage velocity of water; is the dynamic viscosity coefficient of water; ρ is the density of water; p is the constant water pressure; k1 is the permeability of the sample; Here, is the Hamiltonian operator; z represents the direction of gravity; For gravitational acceleration; Hamiltonian operator in the formula for:

[0096] (2)

[0097] In equation (2): Hamiltonian operator is a vector differential operator; i is a vector in the x-axis direction of the horizontal plane; j is a vector in the y-axis direction of the horizontal plane; k is a vector in the z-axis direction of the vertical plane;

[0098] Step 2.22: For the calculation of the rock matrix system, the model mainly considers the constant water pressure in the vertical direction, so the two water pressure changes in the horizontal direction in equation (2) are ignored. Therefore, only the vertical k direction needs to be considered, and the equation can be obtained as follows:

[0099] (3)

[0100] Step 2.23: For ease of calculation, the hydraulic gradient is assumed to be uniformly distributed throughout the model, and the water pressure is proportional to the depth, which can be expressed by the formula:

[0101] (4)

[0102] In equation (4): h is the height of the selected model sample;

[0103] Step 2.24: After determining the Hamiltonian operator and basic parameters such as water pressure, the permeability of the rock matrix system is calculated using Darcy's law formula. Substituting equations (3) and (4) into (1), the formula for calculating the permeability of the rock matrix system is as follows:

[0104] (5)

[0105] For Darcy flow, by converting between the permeability of the rock matrix system and the permeability coefficient of the rock matrix system, the expression for the permeability coefficient K1 can be obtained as follows:

[0106] (6);

[0107] Step 2.3: Determine the permeability coefficient of the fractured system

[0108] Step 2.31: In constructing the model, it is assumed that the fractured system satisfies the non-Darcy law. The non-Darcy flow equations describing the hydraulic gradient J and seepage velocity v mainly have two forms:

[0109] (1) Quadratic form:

[0110] (7)

[0111] In equation (7), A, B, and C are of arbitrary form, v is the seepage velocity, and M is a constant;

[0112] The quadratic non-Darcy seepage equation is the squared form of the seepage equation. It is usually solved by the Imay flow model and the average Navier-Stokes equation. In steady flow, the Forchheimer empirical formula can only be obtained when the Reynolds number is large.

[0113] (2) Exponential type:

[0114] (8)

[0115] While exponential formulas are simple in form, they lack theoretical derivation, whereas quadratic formulas have clear physical meanings for each term and are theoretically derived. Therefore, the Forchheimer formula has wider applicability. For fracture flow seepage, the Forchheimer quadratic relationship is more reasonable, namely:

[0116] (9)

[0117] In equation (9): Indicates the influence coefficient of the non-Darcy flow equations;

[0118] Step 2.32: Based on comparative analysis of data, existing studies have fitted the permeability coefficient and influence coefficient, and proposed the relationship between permeability k2 and the influence coefficient of the non-Darcy flow equation. The relationship exhibits an exponential distribution:

[0119] (10)

[0120] In equation (10): k2 is the permeability of the fracture system;

[0121] Step 2.33: For non-Darcy flow, the permeability coefficient is no longer a constant, but a quantity affected by the hydraulic gradient; from equations (9), (10) and the relationship between permeability coefficient and permeability, the formula for the influence of the hydraulic gradient on the permeability coefficient K2 of non-Darcy flow can be obtained as follows:

[0122] (11);

[0123] Step 3: Determine the permeability coefficient of the modified two-medium model system

[0124] Based on the modified dual-medium seepage model, the model consists of a rock matrix system and a fracture system. The rock matrix system and the fracture system each satisfy their own seepage laws. The rock matrix system satisfies Darcy's law, while the fracture system satisfies a non-Darcy's law. The rock matrix system and the fracture system each have their own porosity and permeability. The permeability of the model is determined by the linear combination of the permeability of the two parts.

[0125] For water-rich fault fracture zones, it is assumed that the fault fracture zone is composed of a fracture system and a matrix system, which overlap and are evenly distributed throughout the rock mass. The fracture domain can be represented by volume fraction based on the distribution and development of fractures.

[0126] Step 3.1: In the overall permeability coefficient expression, combining the Darcy flow permeability coefficient and the non-Darcy flow permeability coefficient, the overall permeability coefficient K is expressed as follows through the volume fraction of the fracture domain:

[0127] (12)

[0128] In equation (12): Represents the volume fraction of the fractured domain;

[0129] Based on the development of fractures, the volume fraction of the fracture domain The value is typically taken as 0.025 to 0.200, and the total permeability coefficient K is the permeability coefficient of the modified model;

[0130] Step 3.2: In isotropic media, the permeability coefficient is defined as the unit flow rate under a unit hydraulic gradient, representing the ease with which fluid passes through the porous medium. By obtaining the permeability coefficient of the model, the seepage rate can be further calculated using the following formula:

[0131] (13)

[0132] In equation (13): K is the permeability coefficient, with units of m / d or cm / s; k is the permeability. v is the dynamic viscosity coefficient of water; ’ Kinematic viscosity; ρ is the density; g is the acceleration due to gravity;

[0133] The seepage coefficient and seepage rate of the fault fracture zone in the tunnel can be determined based on the above formulas. This can be used to study seepage damage and to better predict and prevent geological disasters in the fault fracture zone.

[0134] Example 2: Unlike Example 1, this example is designed to verify the effectiveness of the method described in Example 1.

[0135] Step 4: Verify this method based on the measured data from the engineering project and obtain its seepage parameters (verify the modified version based on the measured data from the engineering project).

[0136] The invention is verified through an engineering example of the Daliangshan Tunnel section of the Leshan-Xichang Expressway, between chainages K72+050 and K80+500. The strata traversed in this project are mainly sandstone, shale, and limestone, with a small amount of basalt. These are relatively soft surrounding rocks, with the rock mass exhibiting a thin- to medium-thick layered structure, local carbonaceous enrichment, occasional coal seams, and poor interlayer bonding. The tunnel passes through F1 and T1 railway lines before reaching the K80+500 section. 1f T 1j+2L T 3xj The tunnel is located in a large fault zone consisting of F2 and F3. A geological profile of this section is shown in the attached diagram. Figure 4 As shown; the engineering geological conditions of the fault and fault zone it traverses are as follows:

[0137] (1) F1 fault on the west wing of the Siyamo overturned anticline: The rocks in the fracture zone and footwall of the F1 fault have a fractured to pulverized structure and are extremely soft rocks. The rock mass is between fractured and extremely fractured and has a loose structure. The surface width of the fracture zone is about 40-60m, and the dominant dip is 30°~45°, ∠40°~60°. The tunnel axis intersects the fault at a large angle. The tunnel is buried at a large depth, with a maximum burial depth of 1014m and a general burial depth of 950~1000m. When the tunnel passes through this section, the stability of the tunnel roof and sidewalls is poor, which may cause large deformation and large-scale collapse. The groundwater mainly seeps out in the form of dripping water and rain, with local streams of water flow and sudden water inrush.

[0138] (2) The sandstone and mudstone section of the western limb of the Siyamo overturned anticline (T 1f The main strata in this section are the Lower Triassic Feixianguan Formation, and the rock mass is mainly composed of gravelly sandstone, siltstone, sandstone, and silty mudstone, moderately to slightly weathered. The gravelly sandstone and ordinary sandstone are mainly medium-thickly layered, while the mudstone and silty mudstone are mainly thin- to medium-thickly layered. The western wing of the Siyiamo overturned anticline is monoclinic, dipping eastward, with an overturning attitude of 35°~60° and ∠40°~60°. The maximum burial depth of the tunnel through this section is 950m, with a general burial depth of 850~900m. Due to tectonic influence, the surface rock mass is relatively fragmented. Statistics on joints and fissures at the surface rock mass outcrops show: J V =10~15, K V =0.35~0.55. In-situ acoustic wave testing and sampling chamber analysis results at this location of the deep hole indicate that:

[0139] The natural compressive strength of the silty mudstone is 7.29 MPa, classifying it as a soft rock; its natural density is 2.53 g / cm³. 3The rock mass acoustic wave statistical value is 2.7 km / s, and the rock block acoustic wave statistical value is 3.62 km / s, corresponding to Kv=0.556. The rock mass is relatively broken to relatively intact; the moderately weathered silty mudstone has BQ=250.87 and [BQ]=210.87.

[0140] The statistical value of the saturated compressive strength of the sandstone is 23.79 MPa, which is considered a relatively soft rock; the statistical value of its saturated density is 2.63 g / cm³. 3 The rock mass acoustic wave statistical value is 3.05 km / s, and the rock block acoustic wave statistical value is 3.97 km / s, corresponding to Kv=0.59. The rock mass is relatively broken to relatively intact; the moderately weathered sandstone has BQ=308.87 and [BQ]=258.87.

[0141] (3) Carbonate rock section on the western limb of the Siyamo overturned anticline (T 1j+2L The strata in this section are mainly the Middle Triassic Leikoupo Formation and Jialingjiang Formation. The rock mass is mainly composed of dolomite, dolomitic limestone, limestone, and marl. Among them, the dolomitic limestone, dolomite, and limestone are all medium-thick layered structures, while the marl is mainly thin-layered. The strata are overturned at 45°~70° and ∠40°~66°. The maximum burial depth in this section is 838m, and the general burial depth is 790~820m. Due to the tectonic structure, the surface rock mass is mainly fractured, and some rock masses are relatively fractured. The joint and fracture statistics of the surface rock mass outcrops show: J V =13~20, K V =0.35~0.55;

[0142] The statistical value of the saturated compressive strength of this section of limestone is 49.79 MPa, classifying it as hard rock; the statistical value of its saturated density is 2.66 g / cm³. 3 The acoustic velocity of limestone rock masses is generally 3.5 km / s to 3.82 km / s, while that of rock blocks is generally 4.67 km / s to 5.21 km / s, corresponding to Kv = 0.50 to 0.57. Moderately weathered limestone has BQ = 381.87 and [BQ] = 321.87.

[0143] (4) The coal-bearing strata of the western wing of the Siyamo overturned anticline in the Xujiahe section (T 3xj The main strata are the Upper Triassic Xujiahe Formation, with lithology consisting of silty mudstone, siltstone, sandstone, shale, carbonaceous shale, carbonaceous mudstone, and coal seams. The siltstone is thin- to medium-thick-bedded, while the siltstone and sandstone are predominantly medium-thick to thick-bedded, with a small amount exhibiting extremely thick-bedded structures. The carbonaceous shale, carbonaceous mudstone, and coal seams are thin- to platy. The strata are overturned at 40°–65° and ∠40°–62°. The maximum burial depth of the tunnel traversing this section is 812m, with typical burial depths ranging from 666 to 716m. Due to tectonic influences, the rock mass is fractured to relatively fractured. Statistics on joints and fractures at surface rock outcrops show: J V=13~20, K V =0.35~0.55;

[0144] According to the statistical results, the natural compressive strength of silty mudstone is 8.31 MPa, which is classified as soft rock, with Kv = 0.47~0.55, BQ = 237.43, and [BQ] = 207.4; the saturated compressive strength of sandstone is 30.41 MPa, which is also classified as soft rock, with Kv = 0.50~0.59, BQ = 316.87, and [BQ] = 276.2.

[0145] The F2 fault on the western limb of the Si'amo overturned anticline: The main strata are the Upper Triassic Xujiahe Formation, with a small amount of P. 2β The basalt F2 fault is approximately 80-100m wide at the surface, with a dominant dip of 30°-45° and ∠50°-70°. The tunnel axis intersects the fault at a large angle. The fault is mainly composed of fractured rock with a relatively small amount of siltstone. Its structure is primarily loose or fragmented, and the rock and soil are extremely broken to fractured. The tunnel is located at a considerable depth, and the stability of the tunnel roof and sidewalls is relatively poor. This section has a large burial depth, with a maximum depth of 824m and a typical burial depth of 680-780m.

[0146] Based on the survey and test results and the physical and mechanical parameters of the rock in different strata of this tunnel section according to the "Specifications for Design of Highway Tunnels" (JTG3370.1-2018), Table 1 shows the following:

[0147] Table 1: Rock Mass Indicators

[0148]

[0149] The groundwater in this section of the tunnel is mainly fissure water generated by the pores of the bedrock. There may be streams of water in the sandstone and fissure development zone and the fractured rock mass zone, which have a certain degree of pressure resistance. When the relatively impermeable layers such as silty mudstone and carbonaceous shale are broken through and relatively aquifers such as sandstone and siltstone are entered, the water inflow will suddenly increase. Locally, there may be sudden water inflow at the chainage. The construction is high-risk and the safety of the construction needs to be analyzed.

[0150] In this model, at the fault fracture zone, based on the characteristics of dual-medium seepage, Darcy flow calculation is used for the rock matrix system, and the permeability coefficient calculation formula is shown in Equation (6). For Darcy flow, the permeability coefficient is only related to the pore size of the medium and the properties of the fluid, and is independent of the hydraulic gradient. The expression is:

[0151] (6)

[0152] Because of its high seepage velocity, fractured systems do not satisfy Darcy's law of flow. However, due to the high velocity, the fluid flow follows the Forchheimer binomial equation. Therefore, the formula for calculating the permeability coefficient of fractured systems is:

[0153] (11)

[0154] Combining the Darcy flow permeability coefficient and the non-Darcy flow permeability coefficient, the total permeability coefficient is expressed as the volume fraction of the fracture domain:

[0155] (12)

[0156] Based on the obtained permeability coefficient calculation formula, the permeability model parameters of each stratum in the Daliangshan Tunnel were calculated.

[0157] Unlike continuous porous media, rocks are also affected by physicochemical properties such as osmotic pressure, stress state, groundwater and temperature. Therefore, their permeability coefficient is usually not constant; however, in model studies, it is usually assumed to be constant for the convenience of simulation and calculation.

[0158] The rock mass density, compressive strength, Poisson's ratio, internal friction angle, cohesion, and porosity were selected to study seepage variations. Groundwater temperature is generally 15-17°C. According to the standard, the dynamic viscosity coefficient is taken as 1.115 × 10⁻⁶ at 16°C. -6 kPa·s, i.e., the dynamic viscosity coefficient v = 1.115 × 10⁻⁶ kPa·s. -6 kPa·s;

[0159] The total permeability coefficient K is the permeability coefficient of the modified model. From the permeability coefficient of the model, the seepage rate can be further calculated using the following formula:

[0160] (13)

[0161] Based on actual measurements and calculations using permeability coefficient and permeability conversion, the permeability coefficient and permeability parameters for each location are shown in Table 2 below:

[0162] Table 2: Permeability Coefficient and Permeability

[0163]

[0164] The calculated seepage parameters are consistent with the actual engineering conditions. Further numerical simulation based on the seepage parameters can more accurately and realistically depict the seepage situation of the fault fracture zone. Furthermore, based on the seepage parameters, the early warning and prevention of geological disasters in the fault fracture zone during tunnel construction in this area is very effective.

[0165] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed. The scope of protection of this invention is defined by the appended claims and their equivalents.

Claims

1. A method for determining seepage parameters in tunnel fault fracture zones based on a dual-medium model, characterized in that: include Step 1: In the fractured fault zone of the tunnel, the seepage parameters of the fractured fault zone are determined using a dual-medium model based on the seepage characteristics of the water-rich fractured fault zone of the tunnel. Step 2: Modify the dual-medium model and construct the basic model of seepage in the fault fracture zone of the tunnel based on the modified dual-medium model. Step 2.1: Set up the modified dual-medium seepage model environment; Step 2.2: Determine the permeability coefficient of the rock matrix system; Step 2.3: Determine the permeability coefficient of the fractured system; Step 3: Determine the permeability coefficient of the modified dual-medium model system; The modified dual-medium flow model environment described in step 2.1 is as follows: (1) The overall system consists of a rock matrix system and a fracture system, which overlap in space and are uniformly distributed throughout the medium; The overall system is the fault fracture zone seepage model as a whole, the rock matrix system is the rock pore system, and the fracture system is the rock fracture system. (2) The rock matrix system and the fracture system satisfy their respective seepage laws. The rock matrix system satisfies Darcy's law, while the fracture system satisfies a non-Darcy's law. The coupling between the two is achieved through the water exchange term. (3) The rock matrix system and the fracture system each have their own porosity and permeability, and are affected by the stress state; Step 3 describes the process of determining the permeability coefficient of the modified dual-medium model system, which includes: Step 3.1: In the overall permeability coefficient expression, combining the Darcy flow permeability coefficient and the non-Darcy flow permeability coefficient, the overall permeability coefficient K is obtained by considering the volume fraction of the fracture domain: (12) In equation (12): The volume fraction of the fractured domain is represented by K; the total permeability coefficient K is the permeability coefficient of the modified model, K1 is the permeability coefficient of the rock matrix system, and K2 is the permeability coefficient of the fractured system; where: permeability coefficient of rock matrix system ； Where k1 is the permeability of the sample, The density of water; It is the acceleration due to gravity. is the dynamic viscosity coefficient of water; Permeability coefficient of fractured system ； Where k2 is the sample permeability, and J is the hydraulic gradient. The influence coefficient of the non-Darcy flow equation; Step 3.2: The permeability coefficient in an isotropic medium is defined as the unit flow rate under a unit hydraulic gradient; the permeability coefficient of the model is obtained, and the seepage rate is further calculated: (13) In equation (13): K is the permeability coefficient; k is the permeability. Viscosity; v ’ Kinematic viscosity; ρ is the density; g is the acceleration due to gravity.

2. The method for determining seepage parameters in tunnel fault fracture zones based on a dual-medium model according to claim 1, characterized in that: Step 2.2, the process of determining the permeability coefficient of the rock matrix system, includes... Step 2.21: Under single-medium conditions, the rock matrix system refers to the pore system of the rock. Assume that the rock pore system satisfies Darcy's law of seepage: (1) In equation (1): v is the seepage velocity of water; is the dynamic viscosity coefficient of water; ρ is the density of water; p is the constant water pressure; k1 is the permeability of the sample; Here, is the Hamiltonian operator; z represents the direction of gravity; For gravitational acceleration; Hamiltonian operator in the formula for: (2) In equation (2): Hamiltonian operator is a vector differential operator; i is a vector in the x-axis direction of the horizontal plane; j is a vector in the y-axis direction of the horizontal plane; k is a vector in the z-axis direction of the vertical plane; Step 2.22: Ignoring the two water pressure changes in the horizontal direction in equation (2), and only considering the vertical k direction, we obtain the following equation: (3) Step 2.23: Assume the hydraulic gradient is uniformly distributed throughout the model, and the water pressure is proportional to the depth, expressed as: (4) In equation (4): h is the height of the selected model sample; Step 2.24: Calculate the permeability of the rock matrix system using Darcy's law formula. Substituting equations (3) and (4) into (1), the permeability of the rock matrix system is: (5) For Darcy flow, the permeability coefficient K1 is obtained by converting between the permeability of the rock matrix system and the permeability coefficient of the rock matrix system: (6)。 3. The method for determining seepage parameters in tunnel fault fracture zones based on a dual-medium model according to claim 1, characterized in that: Step 2.3 describes the process of determining the permeability coefficient of the fractured system, which includes... Step 2.31: Assume the fractured system satisfies non-Darcy's law. The non-Darcy flow equations describing the hydraulic gradient J and seepage velocity v include exponential and quadratic forms: For fracture flow seepage, the Forchheimer quadratic form relation is used to describe it: (9) In equation (9): This represents the influence coefficient of the non-Darcy flow equations. This represents the dynamic viscosity coefficient of water. This indicates the density of water. This represents the acceleration due to gravity, and v is the seepage velocity of the water. Step 2.32: Influence coefficient of permeability k2 on non-Darcy flow equations It exhibits an exponential distribution: (10) In equation (10): k2 is the permeability of the fracture system; Step 2.33: Using equations (9) and (10) and the relationship between the permeability coefficient and the permeability, the hydraulic gradient with respect to the permeability coefficient K2 for non-Darcy flow is obtained as follows: (11)。 4. The modified dual-medium seepage model established using the method according to claim 1, characterized in that: The dual-medium seepage model is hexahedral and consists of two parts: a rock matrix system and a fracture system. The two parts overlap in space and are uniformly distributed throughout the medium. The rock matrix system satisfies Darcy's law, while the fracture system does not. The coupling between the two is achieved through the water exchange term. The rock matrix system and the fracture system each have their own porosity and permeability, and are affected by the stress state. The fracture direction is consistent with the direction of the main permeability.

5. The modified dual-medium seepage model according to claim 1, characterized in that: In the dual-medium seepage model, (1) The permeability coefficient K of the modified model is ； in, K1 is the volume fraction of the fractured domain, K2 is the permeability coefficient of the rock matrix system, and K3 is the permeability coefficient of the fractured system. (2) The seepage rate k is ； In the formula: K is the permeability coefficient; k is the permeability. v is the dynamic viscosity coefficient of water; ’ Kinematic viscosity; ρ is the density; g is the acceleration due to gravity.

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