JP2007291720A - Face bolt design method and face bolt driving method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a face bolt design method and a face bolt driving method, which enable a design pitch of the face bolt to be unambiguously and immediately determined or changed at the stage of the identification of values of physical properties of natural ground. <P>SOLUTION: Numeric analysis is performed on input conditions which comprise: the strength/deformation parameters of ground, including the elastic modulus E, cohesion c and internal frictional angle ϕ of the ground; the diameter of a tunnel; the earth covering thickness of the tunnel; and the placing pitch of the face bolt. The thresholds of the stability and collapse of a mirror surface depending on the values of the strength/deformation parameters are determined for each of the driving pitches of the face bolts, and a graphic figure associated with the threshold is made. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a mirror locking bolt design method applied to a mirror locking bolt method, which is an auxiliary method for tunnel construction, and a mirror locking bolt placement method based on the design method.

  The NATM method was introduced on a trial basis in 1976, and since then it has become a major method for mountain tunnels. In recent years, the number of cases adopted in urban areas has increased due to advantages such as freedom of cross-section changes and economy. is doing. It is necessary to make full use of various auxiliary methods in order to excavate a tunnel with unconsolidated ground that should originally be constructed by the shield method or open-cut method, and with a shallow earth covering, by the mountain tunnel method. According to the tunnel standard specification [mountain construction method] and the explanation (Japan Society of Civil Engineers), the purpose of the auxiliary construction method is to "construction safety" and "preservation of the surrounding environment". In urban areas where the earth cover is shallow, it is particularly important to take measures to stabilize the face (mirror surface) as the safety of construction, and to prevent land subsidence against ground structures as the preservation of the surrounding environment.

  By the way, in recent years, the long mirror retaining bolt method has increased its construction performance due to the development of construction machines, bolt materials, injection / fixing materials, and the like. This construction method is an effective auxiliary method for both the above-described measures for stabilizing the face (mirror surface) and the ground surface settlement for ground structures. Specifically, by reinforcing the face, which is a free release surface, with a bolt, not only can the face be stabilized, but also the expansion of the slack area of the ground in front of the face can be suppressed. It can be expected to reduce internal displacement and reduce support. In addition, this construction method stabilizes the face to reduce the vertical splitting of the tunnel (dividing the tunnel excavation cross section to determine the section), and enables a larger cross section to be excavated with a large machine. Thereby, the cross-section closure of a tunnel can be performed at an early stage, and the surrounding natural ground can be stabilized.

  As mentioned above, the long mirror set bolt method is an extremely effective method as an auxiliary method at the time of tunnel construction, but on the other hand, there is also a current situation that the setting method such as the setting pitch of the mirror set bolt has not been established. is there. As for the setting method of the placement pitch and the like, it is generally determined empirically from past results, and the skilled person determines the placement pitch and the like in consideration of the properties of the natural ground.

  In addition to the empirical method described above, the earth pressure acting on the face is calculated from Murayama's formula (the following formula) and arc slip (see Fig. 8), and the bolt pitch necessary to stabilize the face is obtained. Is a so-called simple calculation design method in which the distance to the sliding surface in front of the face is calculated to obtain the bolt wrap length. More specifically, it is considered that the loose ground in front of the face resists the force that pushes the face along the slip surface by the resistance of the slip surface and the force that restrains the face, and the necessary face restraint. P in the following equation, which is a force, can be obtained. The lower second equation for obtaining P can be obtained from the lower first equation which is a balance equation of moments around the point O in FIG. Here, in past studies, the safety factor is determined in consideration of the uncertainties of bolt joints, etc., and the final required bolt strike is achieved by incorporating the safety factor that is applied when considering general arc slip. The installation pitch (placement density) is calculated.

  As another method, the maximum shear strain of the face is calculated by modeling the tunnel and the long mirror retaining bolt three-dimensionally and performing numerical analysis such as the finite element method, finite difference method, and granular material analysis method. There is a so-called numerical analysis method for obtaining a necessary bolt pitch or the like within a range not exceeding the limit maximum shear strain.

  The empirical method is that the design specifications (material specifications, placement pitch, etc.) of the long mirror retaining bolts are left to the skill, experience, etc. of the designer, so that no one can set the design specifications. There are drawbacks. On the other hand, according to the results of comparison between the past construction results and the calculation results by the simplified calculation design method, the inventors have proved that the results by the simple calculation design method do not conform to the past results. . This is considered to be caused by the result of performing the stability calculation in the limit state using only the strength constants (adhesion force, internal friction angle) of the natural ground. Further, according to the numerical analysis method, it is possible to determine the design specification of the long mirror fixing bolt based on the accurate analysis result, but there is a disadvantage that it takes a long time for the analysis. For example, when a defective ground appears after the construction starts, changing the design conditions and performing the analysis again leads to a significant delay in the construction period.

By the way, Patent Document 1 discloses an invention related to a mirror bolt design method to which the numerical analysis method described above is applied. This design method sets the strength and deformation parameters of the natural ground, performs three-dimensional numerical analysis with the mirror surface unreinforced, determines whether the amount of mirror extrusion is within the reference value based on the excavation measurement data, When it deviates from the reference value, a mirror bolt with a predetermined pitch is set, and the three-dimensional numerical analysis is performed again to determine whether or not the mirror extrusion amount is within the reference value based on the excavation measurement data. is there. If the extrusion amount deviates from the reference value as a result of the comparison between the extrusion amount and the reference value again, the number of bolts to be installed can be increased or the bolt fixing material The strength is increased, and the same study is performed by performing the three-dimensional numerical analysis again.
JP 2003-321993 A

  According to the mirror bolt design method disclosed in Patent Document 1, by setting the reference value of the extrusion amount based on the excavation measurement data and repeatedly performing the three-dimensional numerical analysis, the specification of the mirror bolt can be achieved with high accuracy. It is thought that it can be decided. However, this reference value is obtained after actual ground excavation has started, and therefore it is unclear whether the specs of the mirror bolt can be determined at the beginning of the construction. Further, the reference value serving as a threshold value for the stability and collapse of the mirror surface cannot be clearly recognized from Patent Document 1. Furthermore, because it is a method that repeatedly performs three-dimensional numerical analysis based on the excavation data of the natural ground, the analysis will be performed for the first time when it is necessary to install a mirror bolt if a bad ground is encountered during excavation. It is inevitable that it will take a long time to determine the specifications of the mirror fixing bolt, and it will inevitably lead to a significant delay in the construction period. In tunnel construction, it is often desired to establish a design method for mirror locking bolts that does not affect the construction period, even if such unexpected situations are often encountered.

  The present invention has been made in view of the above-described problems, and sets the specifications of the mirror fixing bolt, in particular, the placement pitch thereof, with high accuracy (in accordance with the properties of the target natural ground and economically). Based on this design method, and a design method for mirror stop bolts that can set the pitch of the mirror stop bolts according to the natural ground in a simple and short time without requiring experience or skill. It is an object to provide a method for placing a mirror stop bolt.

  In order to achieve the above object, the design method of the mirror retaining bolt according to the present invention includes at least ground strength / deformation parameters including a ground elastic coefficient E, adhesive force c, and internal friction angle φ, a tunnel diameter, and a tunnel. The numerical analysis including the finite difference analysis and the finite element analysis is performed using the earth covering thickness of the mirror and the casting pitch of the mirror fixing bolt as input conditions, and the strength / deformation parameter is determined for each casting pitch of the mirror fastening bolt. The threshold value of the mirror surface stability and collapse according to the value of is determined, and a graph figure relating to the threshold value is created.

The design method of the mirror retaining bolt of the present invention is to set (fix) the mirror mounting bolt placement pitch (or placement density (lines / m ² )) and the tunnel covering thickness arbitrarily, and to improve the strength and deformation of the ground. By performing numerical analysis using the parameters and tunnel diameter as variables, the threshold values for mirror surface stability and collapse according to the strength and deformation parameter values are determined, and the finally calculated threshold values are connected. This is a method of creating a graph figure regarding the ground properties and the placement pitch with respect to the cover thickness of a certain tunnel.

  In the present invention, the elastic coefficient E, the adhesive force c, the internal friction angle φ, etc., which are the deformation parameters of the ground, are variously changed with respect to the arbitrarily set pitch (for example, 1.5 m pitch) of the mirror fixing bolt. Thus, various graphic figures of various variations are created, and this is created for each of various pitches of the mirror fixing bolts (for example, 1 m pitch, 1.2 m pitch, 1.8 m pitch, etc.). For example, the setting pitch of the mirror fixing bolt is set to 1.5 m and the elastic modulus E is set to 150 Mpa, and numerical analysis is performed using the adhesive force c and the internal friction angle φ as parameters under the set conditions. Here, it is preferable that the numerical analysis is performed based on a finite difference analysis, a finite element analysis, or the like after three-dimensional modeling of the tunnel and the mirror fixing bolt.

  The threshold values for the stability and collapse of the mirror surface when the adhesive force c and the internal friction angle φ are used as parameters are obtained by numerical analysis, and a graph figure is created based on these threshold values. The elastic modulus E is reset to another 100 Mpa while the placement pitch remains 1.5 m, and the same numerical analysis is executed to create a graph figure based on the threshold value. Further, in addition to obtaining a graphic figure in the case of a separate elastic modulus E, a graphic figure in a plurality of elastic coefficients E is similarly obtained in a separate placement pitch. In addition, it is preferable that this elastic modulus E is efficiently selected from the value assumed by the object ground, a typical value, etc.

  According to the design method of the mirror retaining bolt of the present invention, at the stage where the physical property values (strength / deformation parameters described above) of the ground where the tunnel is actually constructed are obtained, It is possible to uniquely determine the setting pitch of the mirror bolt by simply comparing the physical property values. This determination does not require any skill or experience. In addition, even when a bad ground is encountered during the excavation stage in tunnel construction, it is possible to uniquely determine the pitch of the mirror-stop bolts by simply comparing the physical property values against the graph figure already created. Or, it is possible to change the setting pitch of the mirror fixing bolt on an occasional basis.

  Further, according to another embodiment of the design method of the mirror fixing bolt according to the present invention, in the numerical analysis, the setting pitch of the mirror fixing bolt is set to a predetermined value, and the strength and deformation parameters of the ground are set. First, set a small arbitrary value and perform numerical analysis based on the three-dimensional finite difference method, calculate the maximum shear strain of the mirror surface by the numerical analysis, and limit based on the strength and deformation parameters of the set ground The corresponding threshold is determined based on the condition that the maximum shear strain and the maximum shear strain are equal, and the corresponding threshold is sequentially determined by performing numerical analysis while sequentially setting the strength and deformation parameters to a large value, By connecting the determined threshold values, a graph figure relating to the threshold value at an arbitrary placement pitch of the mirror fixing bolt is created.

  The present invention relates to an embodiment of a design method. The maximum shear strain of a mirror surface is calculated by numerical analysis based on a finite difference method, and the maximum limit shear strain based on the strength and deformation parameters of the ground and the maximum shear strain are calculated. The threshold value is determined based on the condition that the strain becomes equal. As this graph figure, the vertical axis of the graph is c / (γD) and the horizontal axis is φ, and the threshold line is graphed for each pitch of the mirror fixing bolt and for each elastic coefficient of the ground. There are other forms.

  Here, D is the diameter of the tunnel, and this value is also used as a parameter in the design method of the present invention. In addition to the above, it is also possible to use the cover thickness, the lateral pressure coefficient, etc. as parameters.

  In the design method disclosed in Patent Document 1 described above, an increase in the amount of casting is examined depending on whether or not the mirror locking bolt yields. If the mirror surface does not exceed the limit strain even if the bolt yields, the mirror surface There is no collapse. Therefore, it can be said that a more direct evaluation method is to compare the mirror surface strain with its limit strain, rather than the presence or absence of the yield of the mirror retaining bolt. Therefore, in the present invention, the stability / collapse is determined based on the distortion of the mirror surface, and the line that defines the stability / collapse for the set bolt pitch and the elastic modulus E is determined.

  Further, in another embodiment of the design method of the mirror bolt according to the present invention, the numerical analysis is a step analysis corresponding to excavation based on a three-dimensional finite difference method, and the input has already been performed based on the step analysis. It is characterized in that the timing and the lap length of the next casting mirror locking bolt to be wrapped on the remaining length of the mirror fastening bolt in the ground are determined.

  By performing step analysis, for example, the maximum strain on the mirror surface when the remaining length of the mirror retaining bolt in the ground is 5 m to 0 m is calculated, and an area where this value exceeds the maximum limit strain appears or is limited A step in which the region exceeding the maximum strain becomes a predetermined ratio or more with respect to the entire mirror surface can be obtained. For example, when it can be determined that the mirror surface is collapsed in the step when the remaining length is 2 m, while the remaining length is 3 m in the previous excavation step, It is possible to make a construction plan such as placing (for example, 3 m lap).

  According to the design method of the present invention, it is possible to accurately set the timing and lap length of the mirror locking bolts that need to be replaced according to excavation in addition to the pitch of the mirror locking bolts. Become.

  Further, according to the present invention, the method of setting the mirror stop bolt is to calculate the graph figure in advance by the mirror stop bolt design method, the ground strength / deformation parameters obtained, the planned tunnel diameter and From the tunnel covering thickness and the setting pitch on the mirror surface of the mirror fixing bolt that is arbitrarily set, the setting pitch for stabilizing the mirror surface is set based on the graph figure, and the mirror fixing bolt is set. Features.

  By creating graph figures corresponding to various pitches of mirror-stop bolts and ground strength / deformation parameters in advance, the pitch of mirror-stop bolts can be set immediately when the physical properties of the ground are specified. Can be determined. In addition, it is possible to change the setting pitch of the mirror fixing bolt on an occasional basis. In addition, the actual installation of the mirror set bolt is performed using a known installation machine such as a hydraulic jumbo.

  As can be understood from the above description, according to the mirror locking bolt design method and mirror locking bolt driving method of the present invention, highly accurate numerical analysis is performed, and various mirror locking bolt mounting pitches and grounds are obtained. By creating a graph figure corresponding to the strength / deformation parameter in advance, the design pitch of the mirror bolt can be determined or changed uniquely and immediately at the stage where the physical property value of the natural ground is specified.

  Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a longitudinal sectional view showing an outline of a long mirror fixing method, FIG. 2 is a schematic view showing a pitch of bolts on a mirror surface, and FIG. 2a is a diagram in the case of a 1.2 m pitch. FIG. 2 b shows a diagram in the case of a 1.8 m pitch. FIG. 3 is a flow chart for investigating the setting pitch of the mirror fixing bolt, FIG. 4 is a graph figure for the casting pitch of the three cases, and FIG. 5 is a diagram comparing the construction results with the graph figure. Yes. FIG. 6 shows a flow chart for examining the wrap length of the mirror fixing bolt, and FIG. 7 shows a diagram showing an embodiment of the step analysis result corresponding to the excavation step.

  FIG. 1 is a longitudinal sectional view showing an outline of a long mirror retaining bolt method. In this construction method, long bolts B, B,... Are driven by a hydraulic jumbo H or the like in front of the mirror surface G1 substantially perpendicularly to the face surface (mirror surface G1) of the target ground G, and the mirror surface G1 is a free surface. It is a construction method that reinforces before drilling. These long mirror retaining bolts B, B,... Are cut by excavation, and when the remaining length becomes shorter, the long mirror retaining bolts for the next shift are applied by overlapping. The overlap length (wrap length) can be set to about 3 to 6 m, but the wrap length is determined by the physical properties of the natural ground, the construction cycle, and the like. Further, the long mirror fixing bolt is elongated while joining a plurality of short bolts having a length of about 3 m with a plurality of screws. Here, the long length means, for example, a bolt length of 10 m or more, and the mirror surface front can be reinforced by about 1D (D: tunnel diameter) when constructing a road tunnel or a railway tunnel. This is a bolt length that can be expected to produce an axial force in response to the preceding displacement generated at, and to suppress the deformation of the mirror surface.

  FIG. 2 is a schematic view showing the pitch of bolts on the mirror surface. FIG. 2a is a schematic diagram when the pitch of the long bolts B, B,... Is 1.2 m with respect to a 10 m diameter tunnel cross section (mirror surface G1), and FIG. FIG. 5 is a schematic diagram when the placement pitch is 1.8 m. In addition to this, a suitable pitch can be selected depending on the physical properties and workability of the ground, such as 1.5 m and 2 m.

  Next, based on the flow chart for investigating the setting pitch of the mirror fixing bolt shown in FIG. 3, a method for calculating the natural ground physical property value at which the mirror surface at an arbitrarily set pitch is stable will be described.

  First, the physical property value of the target natural ground is set (step S10). Examples of the physical property values of the natural ground include ground deformation parameters such as elastic coefficient: E, adhesive strength: c, internal friction angle: φ, and other ground strength parameters.

  First, the pitch of the long bolts is fixed to an arbitrary value such as a 1.5 m pitch, and a three-dimensional model of natural ground and bolts is created (step S11). Furthermore, the physical property value of the ground can be set, for example, by fixing the elastic coefficient to an arbitrary value such as 150 MPa, fixing the internal friction angle to 10 °, and changing the adhesive force while changing the adhesive force (three-dimensional excavation analysis (three-dimensional Step analysis corresponding to the excavation step is performed (step S12).

  As analysis software used for this three-dimensional excavation analysis, for example, commercially available FLAC3D (Fast Lagrangian Analysis of Continuum in Three Dimensions, Itasca Consulting Group, Inc., trade name) can be applied.

  The distribution of the maximum shear strain: γ on the mirror surface is calculated by the three-dimensional excavation analysis (step S13).

On the other hand, after setting the physical properties of the natural ground, in a separate route, the limit maximum shear strain: γ _c is calculated in consideration of a predetermined safety factor using the limit strain (lower limit value) at the mirror surface (step S14). .

The calculated maximum shear strain: γ and the limit maximum shear strain: γ _c are compared (step S15). If γ <γ _c , it can be determined that the mirror surface is stable (step S16), and the length is arbitrarily fixed. It can be determined that the pitch of the long bolt is appropriate in terms of the pitch of the long bolt, the elastic coefficient of the ground, the internal friction angle of the ground, and the set adhesive strength of the ground.

On the other hand, if γ> γ _c , it can be determined that the mirror surface is collapsed (step S17), and it can be determined that the driving pitch of the long bolt is inappropriate (the driving density is too low) under the set conditions. In this case, do not change the pitch of the long bolt, and change the ground parameters until the mirror surface is stabilized at this pitch (set the strength and deformation parameters to relatively large values). Repeat again and repeat the three-dimensional excavation analysis and comparison determination steps. For example, the internal friction angle is increased from 10 ° to 15 °, recalculation is performed, and comparison determination is performed.

  The above examination flow is similarly performed by changing the elastic coefficient of the ground at an arbitrarily fixed long bolt placement pitch, and further, these examinations are also conducted at a separate long bolt placement pitch. By performing the above, it is possible to obtain analysis results under various conditions using at least the pitch of the long bolt, the elastic coefficient of the ground, the internal friction angle, and the adhesive force as parameters. By adding the tunnel diameter to this parameter, more practical analysis results can be obtained in consideration of the tunnel size. In executing the examination flow, it is preferable to set the ground strength / deformation parameter values as small as possible, perform the initial analysis, and sequentially change the values to large values.

  FIG. 4 shows a graph figure regarding the placement pitch of the three cases obtained based on the repetitive flow of FIG. 3 described above. In this figure, the elastic coefficient of the ground is set to an arbitrary value, and a different graph figure is created with a separate elastic coefficient.

  In FIG. 4, X1 is a mirror surface stability / disintegration threshold line when the placement pitch is 1.2 m, Y1 is a mirror surface stability / disintegration threshold line when the placement pitch is 1.5 m, and Z1 is The mirror surface stability / collapse threshold lines when the placement pitch is 1.8 m are shown. In each line, the lower left region is a region where the mirror surface collapses, and the upper right region is a region where the mirror surface is stable.

  The graph figure shown in FIG. 4 is created for each of various placement pitches and for each of various elastic coefficients at each placement pitch, so that the diameter of the tunnel to be constructed and the tunnel covering thickness of the tunnel can be determined. Once the strength and deformation parameters of the target ground have been determined, the optimum placement pitch can be set simply by reading the position of the graph figure (c / γD, φ). It becomes.

  Next, the results of the analysis conducted by the inventors and the like show the results of verifying the appropriateness / inappropriateness of the actual construction results in the case of the simple calculation design method described above and the study flow shown in FIG. As shown in FIG.

  In the figure, X2 is a threshold line when the simplified calculation design method (safety factor is 1.2), Y2 is a threshold line when the elastic modulus is 150 MPa in the examination flow of the present invention, and Z2 is the present invention. The threshold line when the elastic modulus is 100 MPa in the examination flow, and W2 indicates the threshold line when the elastic modulus is 50 MPa in the examination flow of the present invention. In addition, points P1 to P5 are points in which construction results in the case where the elastic modulus is 10, 13, 68, 126, and 50 MPa are plotted in order.

  From the figure, it can be seen that the graph figure by the simplified calculation design method is greatly different from the construction result, and that all the construction results are denser than the bolt placement pitch obtained by this simple calculation design method. In addition, the study results of this method identified that the effect of the difference in elastic modulus almost disappeared when the internal friction angle increased, and conversely, when the internal friction angle was small, the difference in the analysis result due to the elastic coefficient appeared markedly. It was. In sandy lands with low adhesive strength and a large internal friction angle, the impact of the elastic modulus on the mirror surface stability is specified to be small. Further, the larger the elastic coefficient, the more stable the mirror surface.

  Next, the examination flow of the wrap length of the mirror fixing bolt will be described based on FIG. In this examination, steps S20 to S27 shown in FIG. 6 are the same steps as steps S10 to S17 in FIG. That is, by performing the three-dimensional excavation step analysis shown in FIG. 3, the stability or collapse of the mirror surface is determined for each step. Therefore, the timing of the next shift placement in step S28 based on the step analysis result. Can be determined. For example, the step where the region that collapses on the mirror surface appears is specified, and the next shift placement is set one step before this.

  The remaining length of the long bolt in the step before the step where the collapse region appears or the length considering the safety factor can be set as the lap length. In order to set the wrap length in consideration of the safety factor, it is necessary to place a next shift at a step 2 or 3 steps before the step where the collapse region appears.

  FIG. 7 illustrates the mirror surface stability / disintegration at each step (each remaining length) when excavation analysis based on three-dimensional finite difference analysis is executed. This analysis focuses on the maximum shear strain distribution of the mirror surface when long bolts are installed up to a mirror surface of 42m in length and the remaining length becomes short. When the internal friction angle is 20 °, the elastic modulus of the ground Is set to 50 MPa, and the placement pitch is set to 1.5 m. In the figure, the hatched portion is the collapse region. In this analysis result, when the remaining length reaches 1 m, the strain region suddenly extends over a wide range, so that the lap length can be set to about 3 m from this analysis result.

  As described above, according to the design method of the present invention, analysis is performed by changing the tunnel diameter, ground strength and deformation parameters in various ways, and the mirror surface stability / collapse threshold line is graphed for each setting case. This makes it possible to immediately set the bolting pitch when the tunnel diameter and ground physical properties at the site are known. Therefore, by creating a graph figure by this design method and applying a construction method in which bolts are placed based on this graph figure, a bad ground appears after the start of tunnel construction, and the design must be changed. Even in the case where the robot is placed, it is possible to immediately change the placement pitch based on the graph figure.

  The embodiment of the present invention has been described in detail with reference to the drawings. However, the specific configuration is not limited to this embodiment, and there are design changes and the like without departing from the gist of the present invention. They are also included in the present invention.

The longitudinal section which showed the outline of the long mirror stop bolt construction method. It is the schematic diagram which showed the casting pitch of the mirror surface volt | bolt, Comprising: (a) is a figure in the case of 1.2 m pitch, (b) is a figure in the case of 1.8 m pitch. The examination flow figure of the setting pitch of a mirror stop bolt. Graph figure at placement pitch of 3 cases. The figure which compared construction results and a graph figure. The examination flow figure of the wrap length of a mirror stop bolt. The figure which showed one Embodiment of the step analysis result according to an excavation step. The model figure which showed the balance of the face applied with the conventional simple calculation design method.

Explanation of symbols

  G ... Ground mountain, G1 ... Mirror surface, B ... Mirror stop bolt, H ... Hydraulic jumbo

Claims (5)

  At least the ground strength and deformation parameters including the ground elastic modulus E, adhesive strength c, and internal friction angle φ, tunnel diameter, tunnel covering thickness, and pitch of bolts for mirror fixing bolts are input conditions. Numerical analysis including finite difference analysis and finite element analysis is performed, and a threshold value of stability and collapse of the mirror surface corresponding to the value of the strength / deformation parameter is determined for each placement pitch of the mirror fixing bolt, and the threshold value A method for designing a mirror-stop bolt, characterized in that a graph figure is created.   In the numerical analysis, a numerical value based on a three-dimensional finite difference method is set by first setting a small arbitrary value as the strength / deformation parameter of the ground by setting the placement pitch of the mirror fixing bolt to a predetermined value. The analysis is performed, the maximum shear strain of the mirror surface is calculated by the numerical analysis, and the threshold value corresponding to the limit maximum shear strain based on the set strength / deformation parameters of the ground and the maximum shear strain corresponding to each other is calculated. The numerical value analysis is performed while sequentially setting the strength / deformation parameters to large values, and the corresponding threshold values are sequentially determined. The design method of the mirror bolt according to claim 1, wherein a graph figure relating to the threshold value is created.   The numerical analysis is a step analysis according to excavation based on a three-dimensional finite difference method, and based on the step analysis, the mirror lock of the next placement to be wrapped on the remaining length portion of the ground stop bolt already input 3. The method for designing a mirror-stop bolt according to claim 2, wherein the timing of bolt placement and the lap length are determined.   The vertical axis of the graph figure is c / (γD), the horizontal axis is φ, and threshold lines are graphed for each pitch of the mirror fixing bolts and for each elastic coefficient of the ground. The design method of the mirror stop bolt in any one of Claims 1-3. Here, c is the adhesive strength of the ground, γ is the unit volume weight of the ground, D is the diameter of the tunnel, and φ is the internal friction angle of the ground.   The graph figure is calculated in advance by the mirror bolt design method according to any one of claims 1 to 4, and the obtained ground strength / deformation parameters, planned tunnel diameter, and tunnel soil are calculated. The mirror is characterized in that, based on the graph figure, a mirror pitch is set by setting a pitch at which the mirror surface is stable from a covering thickness and a pitch at which the mirror bolt is arbitrarily set on the mirror surface. How to place a set bolt.

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