A method for calculating the safe thickness of karst in karst tunnels

Abstract

This invention discloses a method for calculating the safe thickness of karst tunnels, relating to the field of tunnel safety technology. The method includes the following steps: S1, firstly, constructing a structural model of the karst tunnel and its cavity within the karst landscape; based on the structural model, creating a safe thickness calculation model for the cavity at different locations; S2, obtaining the basic parameters within the karst landscape where the karst tunnel and cavity are located; S3, under the constructed structural model, inputting the various basic parameters from the structural model into the safe thickness calculation model to obtain the safe thickness under the structural model, and preparing a safe thickness curve based on the fundamental relationship between the safe thickness and the basic parameters; S4, introducing the actual parameters of the tunnel and cavity obtained in S2 to correct the safe thickness curve. This invention, by establishing a structural model and incorporating historical data for comparison, judges and corrects the acquired data, thereby providing guidance for actual engineering projects.

Description

Technical Field

[0001] This invention relates to the field of tunnel safety technology, and in particular to a method for calculating the safe thickness of karst in karst tunnels. Background Technology

[0002] Domestic and international incidents of water inrush disasters in karst tunnels demonstrate that this has become one of the most common, easily occurring, and destructive hazards in underground engineering construction in karst areas. However, due to the complexity and concealment of karst development and the occurrence of water-bearing and water-conducting structures, coupled with the fact that the mechanisms of tunnel water inrush and inrush involve multiple disciplines (including karstology, structural geology, hydrochemistry, groundwater dynamics, fracture mechanics, mineralogy, sedimentology, fluid mechanics, and rock hydraulics), research into the mechanical mechanisms of karst tunnel water inrush disasters is a massive undertaking, and our current understanding is still incomplete. Although numerous methods exist for detecting water bodies ahead of the tunnel face, it is still impossible to completely identify water-bearing locations such as fissures, conduits, and solution cavities hidden deep underground. The connectivity of groundwater is also extremely complex. Therefore, water inrush disasters remain one of the most significant hazards in tunnel construction, posing a tremendous threat to engineering projects.

[0003] In the early stages of tunnel excavation, it is necessary to detect whether there are cavities near the tunnel to be excavated and the pressure generated by the cavities. Based on this pressure, it can be determined whether the cavities can affect the thickness of the rock strata. Obtaining an accurate pressure-thickness curve is an important way to solve the problem of water inrush in tunnels. However, the current preparation process for calculating the safe thickness curve is relatively complicated and there are experimental data errors, which affect the accuracy of the overall curve preparation. Summary of the Invention

[0004] The purpose of this invention is to solve the problems in the prior art and to propose a method for calculating the safe thickness of karst in karst tunnels.

[0005] To achieve the above objectives, the present invention adopts the following technical solution:

[0006] A method for calculating the safe thickness of karst in karst tunnels includes the following steps:

[0007] S1. First, construct a structural model of karst tunnels and cavities within the karst. Based on the structural model, create a calculation model for the safe thickness of the cavities at different locations.

[0008] S2, obtain the basic parameters of the karst tunnel and the basic parameters of the karst cavity within the karst;

[0009] S3. Under the constructed structural model, the basic parameters in the structural model are input into the safety thickness calculation model to obtain the safety thickness under the structural model, and a safety thickness curve is prepared according to the basic relationship between the safety thickness and the basic parameters.

[0010] S4 introduces the actual parameters of the tunnel and cavity obtained in S2 to correct the safety thickness curve.

[0011] Preferably, in S1, when constructing the structural model, the upper end of the karst tunnel adopts a semi-cylindrical structure and the lower end adopts a cuboid structure, the cavity adopts an I-shaped structure, and the two ends of the I-shaped structure are arc-shaped, thereby simplifying the cavity and the karst tunnel into a flat plate shape.

[0012] Preferably, in S1, the different locations of the cavity refer to the cavity being above the karst tunnel, the cavity being below the karst tunnel, and the cavity being on both sides of the karst tunnel, so that there are at least three types of safe thickness calculation models. At the same time, the safe thickness calculation models are analyzed and calculated using quantitative analysis methods under the assumptions to establish corresponding physical and mechanical models or mathematical models. The analysis methods used include three types: the method of obtaining the stability coefficient, the Protodyakonov fracture arch theory analysis method, and the numerical simulation method.

[0013] Preferably, the finite element method, finite difference method, nonlinear finite element method, and numerical manifold method are used in the numerical simulation method. The safe thickness value is determined by applying strength reduction technology and enumeration optimization theory.

[0014] Preferably, in S2, when obtaining the basic parameters of karst tunnels and cavities, the karst cave conditions are first determined based on geological survey data. Since deep karst water is mostly in a pressurized state, it can be assumed that the cavities are full. Then, the rock mass of the tunnel surrounding rock anti-outburst safety stratum is regarded as an incomplete rock slab. Finally, the stress on the anti-outburst safety stratum is simplified to the combined effect of high water head pressure and the weight of the upper rock mass of the tunnel anti-outburst safety stratum.

[0015] Preferably, the basic parameters of the karst tunnel and the cavity include at least the diameter, cohesion, internal friction angle, burial depth, unit weight, height, and Poisson's ratio of the rock mass of the karst tunnel, as well as the tensile strength, height-to-span ratio, shear strength, maximum shear stress, and maximum normal stress at the depth of the cavity.

[0016] Preferably, in S3, when outputting the safe thickness value under the corresponding parameters, the minimum safe distance of historical karst tunnels is obtained. The obtained value is compared with the historical value under various influencing factors to determine whether the obtained value is within the basic range and to determine the correctness of the value. Finally, the correct safe thickness value is retained, and then the actual parameters are introduced for calculation. After another judgment, a curve of safe thickness and corresponding actual parameters is prepared.

[0017] Compared with the prior art, the present invention has the following advantages:

[0018] 1. This invention establishes a structural model and a safety thickness calculation model within that model. Based on different location conditions and the acquired tunnel and cavity foundation parameters, it calculates the required safety thickness. Furthermore, it incorporates historical data for comparison and makes judgments and corrections to the acquired data, thereby determining the correct calculation model value and providing guidance for actual engineering projects. Attached Figure Description

[0019] Figure 1 This is a flowchart illustrating the structural process of a method for calculating the safe thickness of karst tunnels proposed in this invention.

[0020] Figure 2 This is a structural diagram showing the location of the tunnel and the karst cavity in the karst tunnel safety thickness calculation method proposed in this invention. Detailed Implementation

[0021] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0022] In the description of this invention, it should be understood that the terms "upper", "lower", "front", "rear", "left", "right", "top", "bottom", "inner", "outer", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention.

[0023] The shapes of the rock strata between the tunnel and the cave vary greatly. However, when the cave diameter is not smaller than the maximum size of the tunnel, the cave shape approximates a large hall shape, and the rock strata between the cave and the tunnel can be simplified to a flat plate shape for analysis. For ease of analysis, it is assumed that the cave size is not smaller than the tunnel size, and the tunnel diameter is not less than 5m (Song Zhanping, 2006). The basic conditions of the rock strata between the tunnel and the cave are as follows:

[0024] (1) Karst caves are formed over a long geological period. Before tunnel construction, the stress in the surrounding rock is in a stable equilibrium state. During the construction and operation of the tunnel project, the karst cave morphology does not change.

[0025] (2) Without considering the arching effect of the rock strata between the cave and the tunnel, the rock strata between the cave and the tunnel are flat;

[0026] (3) The rock strata between the cave and the tunnel are a homogeneous and continuous isotropic body; the rock mass can withstand both compressive and tensile stresses:

[0027] (4) The load during tunnel construction can be regarded as a concentrated load or a local vertical uniformly distributed load on the rock slab surface.

[0028] Based on the above conditions, when the size of the karst cavity is not less than the maximum size of the tunnel or the karst cave intersects the tunnel at a large angle, the size of the karst cave on the tunnel's cross-section is greater than the tunnel span. Under the influence of the rock strata's own weight and external human loads, the rock strata mechanical model between the karst cave and the tunnel can be regarded as an elastic plate simply supported at both ends or fixed on all four sides. Considering the most unfavorable conditions, the failure of the rock strata is regarded as the instability of a thin plate. Therefore, the minimum safe thickness of the roof slab of the karst tunnel is the minimum stable thickness of the thin rock slab.

[0029] Reference Figure 1 A method for calculating the safe thickness of karst in karst tunnels includes the following steps:

[0030] S1. First, construct a structural model of karst tunnels and cavities within the karst. Based on the structural model, create a calculation model for the safe thickness of the cavities at different locations.

[0031] S2, obtain the basic parameters of the karst tunnel and the basic parameters of the karst cavity within the karst;

[0032] S3. Under the constructed structural model, the basic parameters in the structural model are input into the safety thickness calculation model to obtain the safety thickness under the structural model, and a safety thickness curve is prepared according to the basic relationship between the safety thickness and the basic parameters.

[0033] S4 introduces the actual parameters of the tunnel and cavity obtained in S2 to correct the safety thickness curve.

[0034] Reference Figure 2In S1, when constructing the structural model, the upper end of the karst tunnel adopts a semi-cylindrical structure, and the lower end adopts a cuboid structure. The cavity adopts an I-shaped structure, and the two ends of the I-shaped structure are arc-shaped, thus simplifying the cavity and the karst tunnel into a flat plate. In S1, the cavity is located in different positions, namely above the karst tunnel, below the karst tunnel, and on both sides of the karst tunnel, so that there are at least three types of safe thickness calculation models. At the same time, the safe thickness calculation models are analyzed and calculated using quantitative analysis methods under the assumptions to establish corresponding physical and mechanical models or mathematical models. The analysis methods used include three types: the method for obtaining the stability coefficient, the Protodyakonov fracture arch theory analysis method, and the numerical simulation method. In the numerical simulation method, the finite element method, the finite difference method, and the application of nonlinear finite element and numerical manifold methods are used. Through the application of strength reduction technology and enumeration optimization theory, the safe thickness value is determined. Quantitative analysis methods are mostly based on numerical simulation data, combined with orthogonal experiments and multiple regression analysis. They can effectively overcome the dependence of semi-quantitative analysis methods on engineering experience, thereby avoiding the inapplicability of mechanical structure models and facilitating a comprehensive consideration of the interaction and influence of multiple factors.

[0035] In S2, when obtaining the basic parameters of karst tunnels and cavities, the karst cave conditions are first determined based on geological survey data. Since deep karst water is mostly under pressure, it can be assumed that the cavities are filled. Then, the rock mass of the tunnel's surrounding rock for outburst prevention is considered as an incomplete rock slab. Finally, the stress on the outburst prevention rock slab is simplified to the combined effect of high water head pressure and the weight of the rock mass above the tunnel's outburst prevention rock slab. The basic parameters of karst tunnels and cavities include at least the tunnel's diameter, cohesion, internal friction angle, burial depth, unit weight, height, and Poisson's ratio, as well as the tensile strength, height-to-span ratio, shear strength, maximum shear stress, and maximum normal stress at the cavity's depth. Because karst tunnels operate under the complex environment of these influencing factors, the predictive mathematical model for the safe thickness of the cave roof must simultaneously consider the combined effects of these factors.

[0036] In S3, when outputting the safe thickness value under the corresponding parameters, the minimum safe distance of historical karst tunnels is obtained. The obtained value is compared with the historical value under various influencing factors to determine whether the obtained value is within the basic range and to determine the correctness of the value. Finally, the correct safe thickness value is retained, and then the actual parameters are introduced for calculation. After another judgment, a curve of safe thickness versus the corresponding actual parameters is prepared.

[0037] For example, 50-100 sets of data can be measured to create a curve showing the relationship between model data and safety thickness, and a new curve can be determined after corrections are introduced. In practice, the number of sets of data to be measured is changed according to the actual measurement environment.

[0038] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A karst tunnel karst safety thickness calculation method, characterized in that, The method comprises the following steps: S1, first, a structural model of the karst tunnel and the karst cave in the karst is constructed, and a safety thickness calculation model based on different positions of the karst cave is prepared according to the structural model; S2, basic parameters of the karst tunnel in the karst and basic parameters of the karst cave in the karst are obtained; S3, in the constructed structural model, the safety thickness calculation model is input according to each basic parameter in the structural model, the safety thickness under the structural model is obtained, and a safety thickness curve is prepared according to the basic relationship between the safety thickness and the basic parameters, S4, the actual parameters of the tunnel and the karst cave obtained in S2 are introduced to correct the safety thickness curve; In S1, when the structural model is constructed, a semi-cylindrical structure is adopted at the upper end of the karst tunnel, a cuboid structure is adopted at the lower end of the karst tunnel, and a I-shaped structure is adopted for the karst cave, and the two ends of the I-shaped structure are arc-shaped, so that the karst cave and the karst tunnel are simplified into a flat plate shape; In S1, the different positions of the karst cave refer to the karst cave being above the karst tunnel, below the karst tunnel, and on both sides of the karst tunnel, so that at least three safety thickness calculation models are prepared, and the safety thickness calculation model adopts a quantitative analysis method to analyze and calculate under the assumption of establishing a corresponding physical and mechanical model or a mathematical model, and the analysis method includes three kinds of difficult-to-obtain stability coefficient method, Prusse's broken arch theory analysis method, and numerical simulation method; In the numerical simulation method, the finite element method and the finite difference method are adopted, and the nonlinear finite element and the numerical flow method are applied, and the safety thickness value is determined through the application of the strength reduction technology and the enumeration optimization theory; In S2, when the basic parameters of the karst tunnel and the karst cave are obtained, first, the karst cave situation is determined according to the geological survey data, and the deep karst water is assumed to be in a pressure-bearing state, then the rock mass of the tunnel surrounding rock burst prevention safety rock layer is regarded as an incomplete rock plate, and finally the stress of the burst prevention safety rock layer is simplified as the joint action of high water head pressure and the gravity of the rock mass above the tunnel burst prevention safety rock layer; The basic parameters of the karst tunnel and the karst cave at least include the diameter of the karst tunnel, the cohesion, the internal friction angle, the buried depth, the unit weight, the height, the rock mass Poisson's ratio, and the tensile strength of the karst cave at the depth, the height-span ratio, the shear strength, the maximum shear stress, and the maximum normal stress; In S3, when the safety thickness value under the corresponding parameters is output, the minimum safety distance of the historical karst tunnel is obtained, the obtained value is compared with the historical value under various influencing factors, whether the obtained value is within the basic range is judged, the correctness of the value is judged, the correct safety thickness value is reserved, and then the actual parameters are introduced for calculation, after re-judgment, the safety thickness curve under the corresponding actual parameters is prepared.

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