A calculation method for critical depth of deep and shallow buried loess tunnels

Abstract

The present invention is a method for calculating the critical depth of deep and shallow burial of loess tunnels. When the gravity and shear resistance of the soil body of the tunnel vault reach the limit equilibrium state: calculate the gravity of the soil body per unit length of the tunnel vault; calculate the shear resistance of the soil body of the tunnel vault; when the gravity of the soil body per unit length of the tunnel vault is equal to the shear resistance of the soil body of the tunnel vault, calculate the critical depth of deep burial and shallow burial of the tunnel; based on the critical depth of deep burial and shallow burial of the tunnel, determine whether the tunnel is deeply buried or shallowly buried, and calculate the vertical load borne by the shallow buried tunnel vault support structure. Accurately judging the depth and shallowness of the tunnel is conducive to more reasonable selection of excavation methods and support forms. At the same time, the present invention can also calculate the vertical load borne by the shallow buried loess tunnel vault support structure.

Description

Technical Field

[0001] The invention relates to the technical field of tunnels, in particular to a calculation method for critical depths of deep and shallow buried loess tunnels. Background Art

[0002] In the calculation of surrounding rock pressure and the design of supporting structures, the stress characteristics of deep buried tunnels and shallow buried tunnels are different, and the calculation modes of surrounding rock pressure of deep and shallow buried tunnels are also significantly different. Therefore, the definition standard of tunnel depth and shallowness is crucial for judging the magnitude of surrounding rock pressure on tunnel lining and designing lining structures. However, through a large number of studies, it has been found that the existing definition methods for the depth and shallowness of loess tunnels often have certain discrepancies with the actual engineering.

[0003] At present, many factors affecting surrounding rock pressure have been studied through theoretical analysis, numerical simulation and model test, and the definition of tunnel depth and shallowness has been carried out. The definition of tunnel depth and shallowness in my country mainly includes the method based on surrounding rock relaxation load theory, the method based on loose medium limit equilibrium and the empirical judgment method.

[0004] From the perspective of surrounding rock relaxation load, the maximum depth of the bearing arch cannot be defined as the boundary between deep burial and shallow burial during tunnel construction. my country's "Railway Tunnel Design Code" has given a judgment standard for the depth and shallowness of tunnels, but there will be a sudden change in load at the boundary between deep and shallow burial of tunnels, resulting in load discontinuity, which is inconsistent with the actual situation. The Pusch arch theory method calculates the boundary value of deep and shallow burial based on the height of the tunnel collapse arch, which has certain limitations.

[0005] In terms of limit equilibrium of loose media, the depth at which the relaxation of surrounding rock caused by tunnel excavation cannot reach the surface can be defined as the boundary depth of deep and shallow burial. The Terzaghi method determines the boundary of tunnel deep and shallow burial from the perspective of surrounding rock pressure, and the mechanical parameters are too simple. The Bill Bowman method believes that the depth corresponding to the maximum value of surrounding rock pressure is the boundary of tunnel deep and shallow burial, which is greatly affected by the tunnel span. The empirical judgment method is more affected by human factors in the division of tunnel deep and shallow burial, and has uncertainty.

[0006] Therefore, in the prior art, the results of determining the depth of a tunnel using various determination methods still have certain deviations from the actual situation. Summary of the invention

[0007] The present invention provides a method for calculating the critical depth of a loess tunnel, which is used to solve the technical problem that in the prior art, various judgment methods are used to determine the depth of a tunnel, which still has a certain deviation from the actual situation.

[0008] To achieve the above purpose, the present invention adopts the following technical solution: a calculation method suitable for the critical depth of deep and shallow buried loess tunnels, when the gravity and shear resistance of the tunnel vault soil reach the limit equilibrium state:

[0009] Calculating the gravity of the soil per unit length of the tunnel vault;

[0010] Calculating the shear resistance of the tunnel vault soil;

[0011] Calculate the critical depths of deep burial and shallow burial of the tunnel based on the gravity of the soil per unit length of the tunnel vault being equal to the shear resistance of the soil of the tunnel vault;

[0012] Based on the critical depth of deep burial and shallow burial of the tunnel, the deep burial or shallow burial of the tunnel is determined, and the vertical load borne by the arch support structure of the shallow burial tunnel is calculated.

[0013] Preferably, the process of calculating the gravity of the soil per unit length of the tunnel vault satisfies the following formula:

[0014] W=γhb

[0015] In the above formula: W is the gravity of the soil per unit length of the tunnel vault, γ is the weight of the tunnel surrounding rock; h is the tunnel burial depth; b is half the width of the tunnel arch span.

[0016] Preferably, the process of calculating the shear resistance of the tunnel vault soil satisfies the following formula:

[0017]

[0018] In the above formula: R is the shear resistance of the tunnel vault soil, K0 is the lateral pressure coefficient, c1 is the cohesion of the surrounding rock after considering the cohesion reduction coefficient, is the internal friction angle of the surrounding rock after considering the internal friction angle reduction coefficient, h1 is the joint development depth, h is the tunnel burial depth, and γ is the weight of the tunnel surrounding rock.

[0019] Preferably, the surrounding rock cohesion, the internal friction angle reduction coefficient, and the joint development depth influence coefficient are determined by the surrounding rock grade, loess type, and joint development conditions.

[0020] Preferably, when the gravity of the soil per unit length of the tunnel vault and the shear resistance of the soil of the tunnel vault reach a limit equilibrium, that is, W=R, the critical depth of deep burial and shallow burial of the tunnel is calculated as follows:

[0021]

[0022] In the above formula: H cris the critical depth of tunnel depth, h1 is the depth of joint development, c1 is the cohesion of surrounding rock after considering the cohesion reduction coefficient, K0 is the lateral pressure coefficient, γ is the weight of tunnel surrounding rock, is the internal friction angle of the surrounding rock after considering the internal friction angle reduction coefficient, and b is half the width of the tunnel arch span.

[0023] Preferably, the determining of the deep or shallow burial of the tunnel based on the critical depth of the deep burial and shallow burial of the tunnel, and the calculating of the vertical load borne by the arch support structure of the shallow burial tunnel include:

[0024] H is the tunnel depth, H cr is the critical depth of tunnel depth; if h>H cr , the tunnel is judged to be a deep buried tunnel; if h≤H cr It is determined to be a shallow buried tunnel. The vertical load borne by the arch support structure of the shallow buried tunnel is calculated as follows:

[0025]

[0026] In the above formula: P is the vertical load borne by the arch support structure of the shallow buried tunnel, γ is the weight of the tunnel surrounding rock, c1 is the cohesion of the surrounding rock after considering the cohesion reduction coefficient, h1 is the depth of joint development, b is the half width of the tunnel arch span, K0 is the lateral pressure coefficient, is the internal friction angle of the surrounding rock after considering the internal friction angle reduction coefficient.

[0027] Beneficial effects of the present invention:

[0028] The present invention comprehensively reflects the influence of the physical and mechanical indicators of the surrounding rock and engineering factors by comprehensively considering the influence of tunnel width, surrounding rock weight, joint development, friction strength and cohesive strength; the definition of the critical depth of the loess tunnel is more accurate, and the present invention is more concise and applicable. Accurately judging the depth of the tunnel is conducive to more reasonable selection of excavation methods and support forms. At the same time, the present invention can also calculate the vertical load borne by the arch support structure of the shallow loess tunnel. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 It is a schematic diagram of the longitudinal section of a tunnel of the method for calculating the critical depths of deep and shallow loess tunnels of the present invention;

[0030] Figure 2 It is a stress analysis diagram of soil body calculated by tunnel vault of the present invention;

[0031] Figure 3 It is an application flow chart of the present invention. DETAILED DESCRIPTION

[0032] The specific embodiments of the present invention will now be described in detail with reference to the accompanying drawings. For the purpose of clear description, many physical details will be described together in the following description. However, it should be understood that these physical details should not be used to limit the present invention. That is, in some embodiments of the present invention, these physical details are not necessary. In addition, for the purpose of simplifying the illustration, some conventional structures and components will be depicted in a simple schematic manner in the illustration.

[0033] like Figure 1-3 As shown, the present invention is a method for calculating the critical depth of loess tunnels. First, the excavation span of the tunnel is determined. At the same time, the surrounding rock density, internal friction angle and cohesion are determined based on the geological survey report and the static triaxial mechanical test data of the soil. The critical depth of the tunnel is calculated by the gravity of the soil at the top of the tunnel and the shear force of the soil reaching a limit equilibrium state. If the tunnel burial depth is greater than the critical depth, it is determined to be a deep buried tunnel; if the burial depth is less than or equal to the critical depth, it is determined to be a shallow buried tunnel. For shallow buried tunnels, the shear force offsets part of the gravity of the overlying soil, and the excess force will be borne by the support structure of the tunnel vault.

[0034] Example 1: Tangjiayuan Tunnel is a typical loess tunnel with an excavation width of about 17.25m, an excavation height of about 10.9m, a surrounding rock grade of IV, and an overlying soil layer of Old loess; surrounding rock mass is 19.8kN / m 3 , the surrounding rock cohesion is 44 kPa, the surrounding rock internal friction angle is 24.1°, and the maximum burial depth of the tunnel is 75 m.

[0035] The calculation criteria for deep and shallow tunnels by different methods are shown in Table 1:

[0036] Table 1 Demarcation standards for deep and shallow burials calculated by different methods

[0037] method Highway Tunnel Design Specifications Railway tunnel design specifications Terzagyl formula Demarcation standard / m 40.1 40.5 38.4

[0038] An on-site investigation was conducted on the surface cracks of the Tangjiayuan Loess Tunnel, and the results are as follows: the maximum burial depth of the tunnel is 75m. About 300m after entering the Tangjiayuan Tunnel, two longitudinal cracks appeared on the plateau. The surrounding houses experienced overall uneven settlement, and serious cracks appeared on the walls of the houses. The longitudinal cracks were basically parallel to the central axis of the tunnel.

[0039] The above survey results show that the longitudinal cracks in the Tangjiayuan Tunnel are basically parallel to the central axis of the tunnel, and obvious cracks are also produced in the maximum buried depth area of ​​the tunnel. In summary, the critical depth of deep and shallow burial of the Tangjiayuan Tunnel is greater than 75m. This indirectly proves that the deep and shallow burial limits of the Tangjiayuan Tunnel calculated according to the existing theoretical formula are conservative.

[0040] The calculation process of the critical depth of the Tangjiayuan Tunnel using the method of the present invention is as follows:

[0041] Table 2 Influence coefficients of surrounding rock physical parameters

[0042]

[0043] 1. According to the surrounding rock grade, loess type, joint development degree and engineering geological conditions, select the cohesion and internal friction angle reduction coefficients and the joint development depth influence coefficients through Table 2; calculate the surrounding rock cohesion and internal friction angle after considering the reduction coefficients.

[0044]

[0045] c1=c×K c =44×0.8=35.2kPa

[0046] 2. According to the influence coefficient of joint development depth in Table 2, calculate the joint development depth;

[0047] h1=h×K h =75×0.85=63.75m

[0048] 3. Find the depth of the boundary between deep and shallow burial:

[0049]

[0050] H cr =85m

[0051] The maximum burial depth of this tunnel is 75m, which is less than the critical depth of 85m calculated by the present invention. It is judged to be a shallow buried tunnel, which is consistent with the results of the on-site investigation.

[0052] Case Verification 2: Xinzhuangling Tunnel is located in the loess ridge, and the surface is covered by a thick layer of loess. The tunnel excavation width is 13.2m, the surrounding rock grade is Grade V, and the stratum lithology can be divided into two layers from top to bottom. New loess, layer thickness 4.6 ~ 40m, thicker at the top of the mountain, thinner at the entrance and exit of the tunnel, with developed joints; the lower layer is Old loess, the tunnel calculation section and test section are both in the lower layer, the maximum burial depth of the tunnel is 82m, and the soil parameters are shown in Table 3.

[0053] Table 3 Soil parameters of Xinzhuangling Tunnel

[0054]

[0055] The following table 4 shows the standards for calculating the boundary between deep and shallow tunnels using different methods:

[0056] Table 4 Demarcation criteria of deep and shallow burial calculated by different methods

[0057] method Highway Tunnel Design Specifications Railway tunnel design specifications Terzagyl formula Demarcation standard / m 65.2 40 59.5

[0058] The buried depth of the test section of Xinzhuangling Tunnel is 82m, which is greater than the demarcation standard calculated by the three methods, and should be classified as a deep buried tunnel. However, when the tunnel construction passed through the test section, two longitudinal cracks appeared on the surface along the direction of the tunnel. Undoubtedly, according to the above-mentioned deep and shallow buried demarcation standards, it cannot be classified as a shallow buried tunnel.

[0059] The calculation process of the critical depth of the Xinzhuangling Tunnel by the method of the present invention is as follows:

[0060] 1. According to the surrounding rock grade, loess type, joint development degree and engineering geological conditions, select the cohesion and internal friction angle reduction coefficients and the joint development depth influence coefficients through Table 2; calculate the surrounding rock cohesion and internal friction angle after considering the reduction coefficients.

[0061]

[0062] c1=c×K c =(28.1×0.488+36.06×0.512)×0.8=25.74kPa

[0063] 2. According to the influence coefficient of joint development depth in Table 2, calculate the joint development depth;

[0064] h1=h×K h =82×0.9=73.8m

[0065] 3. Find the depth of the boundary between deep and shallow burial:

[0066]

[0067] H cr =83.3m.

[0068] The maximum buried depth of this tunnel is 82m, which is less than the critical depth of 83.3m calculated by the present invention. It is judged to be a shallow buried tunnel, which is consistent with the results of the on-site investigation and once again verifies the accuracy of the calculation of the present invention.

[0069] Case verification 3: The data of field investigation and research on Hanguguan, Yanxiang, Taicun, Tongluochuan tunnels of Zhengxi Passenger Dedicated Line are shown in Table 5. According to the investigation, longitudinal cracks are often seen on the surface of the shallow buried area of ​​the tunnel entrance, and obvious settlement grooves appear, while no cracks are seen on the surface of other sections with greater burial depth, and no arch collapse is seen in the tunnel.

[0070] On-site surveys can determine the depth of a tunnel through surface cracks. Shallow tunnels cannot form natural arches, and the stress redistribution caused by tunnel excavation will spread to the ground surface, causing tunnel damage in the form of overall collapse or roof fall; deep tunnels can form stable natural arches, and the stress redistribution caused by tunnel excavation will not spread to the ground surface, causing tunnel damage in the form of local collapse at the bottom of the soil column. Therefore, whether there are surface cracks is the key to determining the depth of a tunnel.

[0071] Table 6 is a table of parameters for each tunnel. Various methods are used to define the critical depths of the above tunnels, and the results vary greatly, as shown in Table 7.

[0072] Table 5 Statistics of surface crack investigation

[0073]

[0074] Table 6 Tunnel parameters

[0075]

[0076]

[0077] Table 7 Calculation results of critical depth of tunnel depth

[0078]

[0079] Table 8 The results of the present invention on the determination of tunnel depth

[0080]

[0081] The tunnel burial depth is defined by the method of the present invention, as shown in Table 8. According to the actual survey results, the surface cracks of Hanguguan Tunnel appear at a burial depth of about 40m, indicating that the critical depth of the deep and shallow burial of Hanguguan Tunnel is greater than 40m, and the critical depth calculated by the present invention is 52.85m, which is consistent with the actual situation. The calculation results of the deep and shallow burial of Taicun Tunnel, Yanxiang Tunnel, and Tongluochuan Tunnel are also consistent with the actual survey results, which once again proves the accuracy of this method.

[0082] The above description is only a specific embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent substitution, improvement, etc. made within the spirit and principle of the present invention should be included in the scope of the claims of the present invention.

Claims

1. A method for calculating the critical depth of loess tunnels, characterized by: When the gravity and shear force of the tunnel vault soil reach the ultimate equilibrium state: Calculating the gravity of the soil per unit length of the tunnel vault; Calculating the shear resistance of the tunnel vault soil; Calculate the critical depths of deep burial and shallow burial of the tunnel based on the gravity of the soil per unit length of the tunnel vault being equal to the shear resistance of the soil of the tunnel vault; Based on the critical depth of the tunnel deep burial and shallow burial, determine whether the tunnel is deep burial or shallow burial, Calculate the vertical loads borne by the arch support structure of the shallow tunnel; The process of calculating the gravity of the soil per unit length of the tunnel vault satisfies the following formula: ， In the above formula: is the gravity of the soil per unit length of the tunnel vault, is the weight of the tunnel surrounding rock; The depth of the tunnel; It is half the width of the tunnel arch span; The process of calculating the shear resistance of the tunnel vault soil satisfies the following formula: ， In the above formula: is the shear resistance of the tunnel crown soil, is the lateral pressure coefficient, In order to consider the cohesion of the surrounding rock after the cohesion reduction coefficient is taken into account, The internal friction angle of the surrounding rock after considering the internal friction angle reduction coefficient is: is the depth of joint development, Tunnel depth ， The surrounding rock mass of the tunnel is heavy; When the gravity of the soil per unit length of the tunnel vault and the shear force of the soil of the tunnel vault reach the ultimate equilibrium, that is, , the critical depth of deep and shallow tunnel is calculated as follows: ， In the above formula: is the critical depth of tunnel depth, is the depth of joint development, In order to consider the cohesion of the surrounding rock after the cohesion reduction coefficient is taken into account, is the lateral pressure coefficient, The surrounding rock of the tunnel is heavy. The internal friction angle of the surrounding rock after considering the internal friction angle reduction coefficient is: It is half the width of the tunnel arch span.

2. The calculation method according to claim 1, characterized in that: The surrounding rock cohesion, the internal friction angle reduction coefficient, and the joint development depth influence coefficient are determined by the surrounding rock grade, loess type, and joint development conditions.

3. The method according to claim 1, characterized in that The method of determining whether the tunnel is deeply buried or shallowly buried based on the critical depth of the tunnel deeply buried and shallowly buried, and calculating the vertical load borne by the arch support structure of the shallowly buried tunnel includes: The tunnel depth, is the critical depth of tunnel depth; if , judge that the tunnel is a deep buried tunnel; if It is determined to be a shallow buried tunnel. The vertical load borne by the arch support structure of the shallow buried tunnel is calculated as follows: ， In the above formula: The vertical load borne by the arch support structure of the shallow tunnel. The surrounding rock of the tunnel is heavy. In order to consider the cohesion of the surrounding rock after the cohesion reduction coefficient is taken into account, is the depth of joint development, is half the width of the tunnel arch span, is the lateral pressure coefficient, is the internal friction angle of the surrounding rock after considering the internal friction angle reduction coefficient.

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