Design method for mountain tunnels using sequential excavation analysis

Abstract

To provide a design method according to sequential excavation analysis of a mountain tunnel that reflects a real phenomenon in which sprayed concrete receives a load in a variable load state and receives gradual displacement accompanying tunnel excavation, considers creep and dry shrinkage deformation at extremely early ages, and has high analysis accuracy without applying an equivalent modulus of elasticity.SOLUTION: A design method according to sequential excavation analysis of a mountain tunnel includes: a step A of conducting a multi-stage stress relaxation test of loading gradually-changing displacement on a specimen for every elapsed time according to the material age of the specimen to specify the stress-time relationship of the specimen; a step B of executing reproduction analysis of reproducing the stress-time relationship to specify dynamic characteristics according to the material age of the specimen; and a step C of executing sequential excavation analysis of a tunnel by using sprayed concrete model having dynamic characteristics according to the material age and an analysis model including a ground model around the sprayed concrete model.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a design method for mountain tunnels using sequential excavation analysis. [Background technology]

[0002] Mountain tunnels are structures in which the excavated ground itself has an arch structure that supports the space, and immediately after excavation, support structures are installed to ensure the stability and safety of the ground.These support structures mainly consist of sprayed concrete, steel support structures, and rock bolts. Among these, shotcrete is a thin, flexible support material that supports the ground. Its features include the ability to spray it onto the tunnel wall without the need for formwork and to adhere closely to the ground, the ability to quickly develop strength using an accelerator, and a material age of 5 to 10 N / mm2 at one day. 2 The strength of this material can be expected to be as high as 100%. The functions of shotcrete include axial compression resistance, shear resistance, and bending resistance, as well as a support function that distributes the loads received by these resistance functions to the ground. Here, axial compression resistance is the function of resisting axial force caused by external forces and deformation acting in the inward direction on the arch of the ground, shear resistance is the function of resisting shear force and shear displacement caused by localized collapse of the ground, and bending resistance is the function of resisting bending moment caused by localized collapse of the ground. The effects of sprayed concrete include preventing the surface from falling off, retaining small rock masses, applying internal pressure to the ground (especially in ground that is subject to large deformations or soil and sand), reinforcing weak layers and maintaining their shape, smoothing out stress distribution, and providing coverage and preventing deterioration of the ground (for example, preventing deterioration due to air or spring water).

[0003] When designing sprayed concrete, workability is taken into consideration and the appropriate construction time, thickness, strength, mix, etc. are determined based on the ground conditions. There are three design methods for sprayed concrete: empirical methods based on ground classification, methods based on similar conditions, and analytical methods. Of these, when designing shotcrete using analytical methods, design specifications are determined primarily through numerical analysis such as FEM (Finite Element Method) and FDM (Finite Difference Method), and the appropriateness of the support is evaluated by determining the stress and displacement of the surrounding ground and the load acting on the support members. Shotcrete, in particular, is a support used under special conditions where it is subjected to most of the deformation of the ground that occurs with excavation, even at an early age when changes in strength and elastic modulus are large, and because the deformation of shotcrete is the sum of elastic deformation, creep deformation, and drying shrinkage deformation, viscoelasticity must be taken into consideration. For this reason, even in numerical analysis, an equivalent elastic modulus (apparent elastic modulus) that takes viscoelasticity into account is used. However, since this equivalent elastic modulus has a significant effect on the analysis results, its setting is extremely important. In cases where the elastic modulus of the ground is small (and therefore the elastic modulus of the shotcrete is relatively large compared to the elastic modulus of the ground), setting the elastic modulus of the shotcrete (equivalent elastic modulus) becomes even more important.

[0004] Here, as a conventional method for setting the elastic modulus of shotcrete, each client organization indicates the design elastic modulus E as follows. For example, for railway tunnels, E = 3.4 kN / mm 2 (Japan Railway Construction, Transport and Technology Agency: Mountain Tunnel Design and Construction Standards and Commentary) indicates that E=4.0kN / mm for road tunnels. 2 (Japan Highway Public Corporation Testing Research Institute Tunnel Laboratory: Tunnel Numerical Analysis Manual) and all of these elastic coefficients are set based on the results of experiments conducted by Tsuchiya of the former Japanese National Railways. Tsuchiya's experiment was conducted with the aim of determining the elastic modulus for numerical analysis of sprayed concrete. The experimental method was to apply sprayed concrete to the side walls of a tunnel, and then extract cores (cylindrical cores with a circular cross section of 100 mm and a height of 200 mm) when the concrete was 12 hours old. A creep test was then carried out in which a load equal to one-third of the compressive strength was applied to the cores. Three test cases were used in which loading began when the concrete was 2, 3, and 7 days old, and the equivalent elastic modulus (elastic modulus taking into account elastic deformation, creep, and drying shrinkage deformation) was determined 28 days after loading began. The test results showed that the earlier the loading start time, the greater the creep deformation and drying shrinkage deformation, and the equivalent elastic modulus 28 days after loading was 3 to 4 kN / mm 2 It has been specified that the extent of

[0005] In conventional methods for determining the equivalent elastic modulus of sprayed concrete, the elastic modulus based on the above-mentioned Tsuchiya experiment has generally been used, but there are various inherent problems with this conventional method for determining the equivalent elastic modulus. The first issue is that the modulus of elasticity is determined by a load-controlled creep test. In reality, shotcrete is subjected to a displacement-loading load as a reaction force when attempting to restrain the deformation of the ground caused by excavation, so it is difficult to say that a load-controlled creep test reflects actual phenomena. The second issue is that creep and drying shrinkage deformation at very early ages (e.g., less than 24 hours) are not taken into account. The age at which creep and drying shrinkage deformation are most significant is thought to be less than 24 hours (less than one day), but this behavior is not reflected in the evaluation. The third issue is that the behavior of the ground during excavation is not taken into consideration. In reality, the shotcrete is subjected to gradual displacement during tunnel excavation, but this behavior of the ground is not simulated in constant load creep tests.

[0006] For these reasons, when designing shotcrete using analytical methods, it is desirable to have a method for determining the equivalent elastic modulus of shotcrete that reflects the actual phenomenon in which shotcrete is subjected to loads in a displacement-loading manner and is further subjected to gradual displacements associated with tunnel excavation, and that takes into account creep and drying shrinkage deformation at very early ages.

[0007] However, the design method based on the setting of the equivalent elastic modulus has the following additional problems. In other words, the problem is that it is not possible to uniquely determine an appropriate equivalent elastic modulus within the same cross section of a tunnel. Because the amount of strain applied to the shotcrete varies from part to part, even within the same cross section of a tunnel, due to the influence of the tunnel's excavation shape, ground stress, construction method, etc., it is necessary to determine the equivalent elastic modulus for each part in order to improve the accuracy of the analysis. For this reason, it is difficult to say that highly accurate analysis results can be obtained using conventional design methods in which a uniform equivalent elastic modulus is set within the cross section of a tunnel.

[0008] Patent Document 1 proposes a method for estimating the static modulus of elasticity of early-age shotcrete from which cores cannot be extracted for uniaxial compression tests. This estimation method includes: (a) a preliminary test step in which a pullout test or a needle penetration test is performed as a preliminary test on a sample of early-age shotcrete from which cores cannot be extracted to determine the relationship between the age and the converted compressive strength; (b) a main test step in which a core is extracted from the shotcrete from which cores can be extracted and a main uniaxial compression test is performed to determine the relationship between the age and the uniaxial compressive strength and the relationship between the age and the static modulus of elasticity; (c) an approximation curve setting step in which an approximation curve is set that approximates the relationship between the converted compressive strength and the static modulus of elasticity of the sample; and (d) a static modulus of elasticity estimation step in which the static modulus of elasticity of the sample is estimated from the converted compressive strength of the sample based on the approximation curve. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Japanese Patent Application Laid-Open No. 2013-72738 Summary of the Invention [Problem to be solved by the invention]

[0010] The method for estimating the static elastic modulus of early-age sprayed concrete described in Patent Document 1 also assumes that a uniform elastic modulus is set for the tunnel, and therefore the accuracy of the analysis using the set elastic modulus remains questionable.

[0011] The purpose of the present invention is to provide a design method for mountain tunnels using sequential excavation analysis, which reflects actual phenomena in which shotcrete is subjected to loads in a displacement loading manner and is further subjected to gradual displacements associated with tunnel excavation, takes into account creep and drying shrinkage deformation at very early ages, and can obtain highly accurate analytical results without applying an equivalent elastic modulus. [Means for solving the problem]

[0012] In order to achieve the above object, one aspect of the design method for mountain tunnels by sequential excavation analysis according to the present invention is as follows: A design method for mountain tunnels using sequential excavation analysis, taking into account the stiffness over time of shotcrete applied to the tunnel wall, A step A includes carrying out a multi-stage stress relaxation test on a specimen of the sprayed concrete, in which a displacement that changes stepwise over time according to the age of the specimen is applied, and identifying the stress-time relationship of the specimen; A step B of performing a reproduction analysis to reproduce the stress-time relationship and identifying mechanical properties according to the age of the specimen; The method is characterized by having a step C in which a sequential excavation analysis of the tunnel is carried out using an analytical model including a sprayed concrete model having mechanical properties according to the age of the material and a surrounding ground model.

[0013] According to this aspect, a multi-stage stress relaxation test is conducted in which a stepwise-changing displacement corresponding to the age of a shotcrete specimen is applied, thereby identifying the stress-time relationship, and a reproduction analysis is conducted to reproduce the identified stress-time relationship, thereby identifying the mechanical properties corresponding to the age of the specimen. A sequential excavation analysis of a tunnel is then conducted using an analytical model including a shotcrete model having mechanical properties corresponding to the age of the specimen and a surrounding ground model, thereby eliminating the need to set an equivalent elastic modulus, reflecting actual phenomena such as stepwise displacements caused by tunnel excavation, and taking into account creep and drying shrinkage deformation at very early ages, resulting in a design method using sequential excavation analysis that can obtain highly accurate analytical results. In Process C, a step-by-step tunnel excavation analysis is carried out using an analytical model that includes a shotcrete model and a surrounding ground model, which reflects the fact that both the ground and the shotcrete (support) are subjected to the gradual displacement (excavation release force) that accompanies tunnel excavation. This leads to a rational shotcrete design in which the actual displacement (or loading strain) that accompanies excavation is properly evaluated, compared to, for example, a load strain analysis in which only the ground is displaced. Furthermore, according to Process C, since it is a sequential excavation analysis in which load strain is applied to both the ground and the sprayed concrete, it is possible to obtain analysis results that correspond to the actual phenomenon in which the load strain differs at each part of the tunnel cross section (the equivalent elastic modulus differs at each part) with a single analysis. Here, the "multi-stage stress relaxation test" in Process A is a test in which a fixed displacement is applied to a test specimen of predetermined specifications and the change in stress over time is measured, and the test fully reflects creep and drying shrinkage deformation at very early ages according to the tunnel construction steps (time intervals). In this multi-stage stress relaxation test, in order to reproduce the behavior of the ground during excavation, the same test specimen is repeatedly subjected to a constant rate of displacement and held at a loading displacement that substantially corresponds to the time intervals of the construction steps.

[0014] In addition, in the reproduction analysis in process B, numerical analysis methods such as FEM and FDM are used to perform an analysis to reproduce the stress-time relationship of the test specimen obtained in process A. For example, to achieve this reproduction (fitting), the mechanical properties, which are the viscoelastic parameters of the sprayed concrete, are identified.

[0015] In addition, when carrying out the sequential excavation analysis of the tunnel in Process C, the construction conditions and ground conditions at a specific construction site are set based on a certain construction plan, and the sequential excavation analysis of the tunnel is carried out under the various set conditions, making it possible to design a mountain tunnel that reflects the construction conditions and ground conditions at a specific construction site.

[0016] Another aspect of the method for designing mountain tunnels by sequential excavation analysis according to the present invention is to: The method is characterized in that the elapsed time from when the displacement loading starts in the step A is set to a time equivalent to the tunnel excavation cycle time.

[0017] According to this aspect, by setting the start time of displacement loading in step A to a time equivalent to the tunnel excavation cycle time, it is possible to realize a test that fully reflects creep and drying shrinkage deformation at an extremely early age. For example, if the tunnel excavation speed is 5 hours / m, the start time of displacement loading in step A can be set to 5 hours of this excavation cycle, and further, the 5 hours of the excavation cycle can be set to the time interval (corresponding to the elapsed time according to the age of the test specimen) at which the loading displacement is changed stepwise in step A.

[0018] In another aspect of the method for designing a mountain tunnel by sequential excavation analysis according to the present invention, The mechanical property in the step B is a viscoelastic property.

[0019] According to this embodiment, since the mechanical properties in process B are viscoelastic properties, it is possible to simulate with high reproducibility that the deformation of the sprayed concrete is the sum of elastic deformation, creep deformation, and drying shrinkage deformation.

[0020] In another aspect of the method for designing a mountain tunnel by sequential excavation analysis according to the present invention, In the reproduction analysis in the step B, a viscoelastic model expressing the stress-time relationship is used as a mechanical model, the viscoelastic model is composed of spring elements that represent elastic behavior and dashpot elements that represent viscous behavior; The step B is characterized in that the viscosity coefficient of the dashpot element is adjusted to fit the mechanical characteristics to the stress-time relationship of the specimen.

[0021] According to this aspect, the viscoelastic model, which is the mechanical model used in the reproduction analysis, is composed of spring elements that represent elastic behavior and dashpot elements that represent viscous behavior. By adjusting the viscosity coefficient of the dashpot elements, the mechanical characteristics of the mechanical model are fitted to the stress-time relationship of the test specimen, thereby making it possible to efficiently create a mechanical model that simulates the mechanical characteristics of the test specimen with as high accuracy as possible. Here, the viscoelastic model composed of a spring element and a dashpot element can take various forms, such as a model in which the spring element and dashpot element are connected in series, or a model in which a first spring element and a dashpot element are arranged in parallel and these are then arranged in series with a second spring element.

[0022] In another aspect of the method for designing a mountain tunnel by sequential excavation analysis according to the present invention, Further comprising step D of conducting a compressive strength test on the specimen; The elastic modulus for each material age of the specimen, determined by the compressive strength test, is applied to the spring element in step B.

[0023] According to this embodiment, by applying the elastic coefficients for each material age determined by compressive strength tests on the test specimen to the spring elements of the viscoelastic model in the reproduction analysis of process B, a mechanical model can be created in which the mechanical properties of the test specimen are simulated with as high accuracy as possible.

[0024] In another aspect of the method for designing a mountain tunnel by sequential excavation analysis according to the present invention, In the multi-stage stress relaxation test in the step A, a constant displacement is maintained for a certain elapsed time, and the constant displacement is changed according to the elapsed time, The change in displacement is performed instantaneously.

[0025] According to this aspect, in the multi-stage stress relaxation test of step A, a constant displacement is maintained for a certain elapsed time, and then the constant displacement is changed according to the elapsed time, thereby simulating the phenomenon in which the loaded displacement changes stepwise for each tunnel excavation speed (excavation cycle), in which displacement is loaded on shotcrete according to actual tunnel construction. Furthermore, by changing the displacement instantaneously, it is possible to eliminate the viscoelastic effects during the loading process. Here, "instantly changing the displacement" means that the displacement changes at a certain time without taking any time, but it may also include the displacement changing occurring in a short period of time (a few seconds compared to the elapsed time of several hours for an excavation cycle). [Effects of the Invention]

[0026] The design method of mountain tunnels using sequential excavation analysis of the present invention reflects actual phenomena in which shotcrete is subjected to loads in a displacement loading manner and is further subjected to gradual displacements associated with tunnel excavation, takes into account creep and drying shrinkage deformation at very early ages, and can provide highly accurate analysis results without applying an equivalent elastic modulus. [Brief explanation of the drawings]

[0027] [Figure 1] 1 is a flowchart showing an example of a design method for a mountain tunnel by sequential excavation analysis according to an embodiment. [Figure 2]1A and 1B are diagrams illustrating a general multi-stage stress relaxation test, in which FIG. 1A is a diagram showing an example of the displacement-time relationship, and FIG. 1B is a diagram showing an example of the stress-time relationship. [Figure 3] FIG. 10 is a diagram showing an example of the displacement-time relationship in a multi-stage stress relaxation test in step A. [Figure 4] FIG. 1 is a diagram showing an example of the stress-time relationship of a test specimen identified by a multi-stage stress relaxation test in step A. [Figure 5] FIG. 10 is a diagram showing the elastic modulus of the specimens at each age, determined by the compressive strength test in step D. [Figure 6] 10A and 10B are diagrams showing various models used in the reproduction analysis in the B process, in which FIG. 10A is a diagram showing an example of an analytical model, and FIG. 10B is a diagram showing an example of a mechanical model. [Figure 7] FIG. 10 is a diagram showing the displacement-time relationship, explaining the analysis steps in the reproduction analysis in the B process. [Figure 8] FIG. 10 is a diagram showing a stress-time relationship for explaining an example of a reproduction analysis result in the B process. [Figure 9] This is an example of a table summarizing the results of sequential tunnel excavation analysis in Process C, along with the age and mechanical properties of the sprayed concrete model for each elapsed time (excavation step) after the tunnel excavation. [Figure 10A] This is a cross-sectional view of the tunnel to be analyzed. [Figure 10B] FIG. 1 is a diagram illustrating an analytical model. [Figure 11] FIG. 10 is a diagram showing the change over time in the elastic modulus of shotcrete in the analysis. [Figure 12A] FIG. 10 is a deformed view showing the analysis results. [Figure 12B] This is a stress diagram of ultra-high strength shotcrete from the analysis results. [Figure 12C] Among the analysis results, this is a stress diagram for normal strength shotcrete. [Figure 13] This is a diagram showing the stress-strain relationship of sprayed concrete at the top of the tunnel. DETAILED DESCRIPTION OF THE INVENTION

[0028] Hereinafter, a method for designing mountain tunnels using sequential excavation analysis according to an embodiment will be described with reference to the accompanying drawings. Note that in this specification and the drawings, substantially identical components may be designated by the same reference numerals to avoid redundant explanation.

[0029] [Design method for mountain tunnels using sequential excavation analysis according to the embodiment] An example of a design method for mountain tunnels using sequential excavation analysis according to an embodiment will be described with reference to FIGS. 1 to 9. FIG. 1 is a flowchart illustrating an example of a design method for mountain tunnels using sequential excavation analysis according to an embodiment. FIG. 3 is a diagram illustrating an example of the displacement-time relationship in a multistage stress relaxation test in step A, and FIG. 4 is a diagram illustrating an example of the stress-time relationship of a test specimen determined by the multistage stress relaxation test in step A. FIG. 5 is a diagram illustrating the elastic modulus of the test specimen for each material age determined by a compressive strength test in step D. FIG. 6 is a diagram illustrating various models used in the reproduction analysis in step B, where FIG. 6(a) is a diagram illustrating an example of an analytical model, and FIG. 6(b) is a diagram illustrating an example of a mechanical model. FIG. 7 is a diagram illustrating the displacement-time relationship, explaining the analysis steps in the reproduction analysis in step B, and FIG. 8 is a diagram illustrating the stress-time relationship, explaining an example of the reproduction analysis results in step B. Furthermore, Figure 9 is an example of a table summarizing the results of the sequential excavation analysis of the tunnel in Process C, along with the age and mechanical properties of the sprayed concrete model for each elapsed time (excavation step) after the tunnel was excavated.

[0030] As shown in Figure 1, the design method for mountain tunnels using sequential excavation analysis includes a multi-stage stress relaxation test (step S100, process A), a reproduction analysis (step S102, process B), a compressive strength test (step S104, process D), and a sequential excavation analysis of the tunnel (step S106, process C).

[0031] In the multi-stage stress relaxation test (step S100), a specimen is prepared with the specified specifications (specified mix) to be used in the construction of a planned mountain tunnel. This specimen is, for example, a cylindrical specimen with a circular cross section of φ100 mm and a height of about 200 mm.

[0032] A constant displacement is applied to this specimen, and the change in stress over time is measured. As shown in Figure 2(a), which shows an overview of a typical multi-stage stress relaxation test, in this test, in order to reproduce the behavior of the ground during excavation, the same specimen is used, and a constant rate of displacement and loading displacement are maintained for each construction step (excavation step), and this is repeated over multiple construction steps.

[0033] The multi-stage stress relaxation test identifies the stress-time relationship of the shotcrete for each construction step, as shown in Figure 2(b). As is clear from this stress-time relationship, the stress of the shotcrete increases linearly when a constant rate of displacement loading is applied, and when the loaded displacement is maintained, the shotcrete exhibits viscoelastic behavior in which stress is released in a curved manner, and this behavior is demonstrated across multiple construction steps.

[0034] In the construction method shown in the example, test specimens were prepared using two types of mixes A and B shown in Figure 8. The mix table is shown in Table 1 below.

[0035] [Table 1]

[0036] Mix A is equivalent to ultra-high strength shotcrete, and Mix B is equivalent to conventional high strength shotcrete. Ultra-high strength shotcrete has a compressive strength of 100 N / mm at 28 days. 2 Since the application of shotcrete using this mixture is thought to be an effective alternative to conventional measures, test specimens of Mix A are being prepared.

[0037] As shown in FIG. 3, in the multi-stage stress relaxation test in step A of the illustrated example, a constant displacement is maintained for a certain elapsed time, and this constant displacement is changed according to the elapsed time.

[0038] Here, the elapsed time for starting to load the displacement in step A is set to a time equivalent to the tunnel excavation cycle time. In the illustrated example, the tunnel excavation speed is 4 hours / m as the construction cycle, and a predetermined displacement is loaded onto the test specimen after the first construction cycle of 4 hours has elapsed, and these 4 hours are used as the time interval (displacement holding period) for gradually changing the loaded displacement.

[0039] Furthermore, when moving to the next construction cycle, the displacement loading is carried out instantaneously (loading time is as short as one to several seconds).

[0040] In this way, by setting the time corresponding to the construction cycle as the start time of loading displacement on the test specimen and the time corresponding to the tunnel excavation cycle time as the time interval for gradually changing the loading displacement, it is possible to realize tests that fully reflect the creep and drying shrinkage deformation of very early material ages according to the time interval of the tunnel construction steps.

[0041] In addition, by changing the displacement instantaneously, the viscoelastic effects during the loading process can be eliminated.

[0042] The multi-stage stress relaxation test in process A (step S100) identifies the stress-time relationship of the test specimen, an example of which is shown in FIG.

[0043] In Process B (Step S102), a reproduction analysis is carried out to reproduce the stress-time relationship and identify the mechanical properties according to the age of the specimen. This reproduction analysis is an analysis to reproduce the stress-time relationship of the specimen obtained in Process A using a numerical analysis method such as FEM or FDM (for example, the finite difference method code FLAC3D), and identifies the mechanical properties, which are the viscoelastic parameters of the shotcrete, to achieve a highly accurate reproduction (fitting).

[0044] For this reproduction analysis, a compressive strength test is conducted on the specimen in process C (step S106), and the elastic modulus of the specimen for each age determined by the compressive strength test is calculated. This compressive strength test complies with JIS regulations (JIS A1108:2018 Concrete compressive strength test method).

[0045] Figure 5 shows an example of a graph of the logarithmic relationship between the elastic modulus (secant elastic modulus, Young's modulus) and material age (hours) for each mix, based on the test results for mixes A and B. Figure 5 shows that good approximations are obtained as logarithmic relationships for both mixes A and B.

[0046] For the reproduction analysis, we created an analytical model shown as an example in Figure 6(a) and a mechanical model shown as an example in Figure 6(b). The analytical model shown in Figure 6(a) has the same shape and dimensions as the test specimen.

[0047] On the other hand, the mechanical model shown in Figure 6(b) is a viscoelastic model, and is composed of spring elements that express elastic behavior and dashpot elements that express viscous behavior. The viscoelastic model shown in the figure is a model in which a first spring element (elastic coefficient: Ek) and a dashpot element (viscous coefficient: η) are arranged in parallel, and these are arranged in series with a second spring element (elastic coefficient: Es). Note that there are various forms of viscoelastic models composed of spring elements and dashpot elements. Here, the stress of the spring element (elastic component): σ s can be expressed as Eε (ε: strain), and the stress of the dashpot element (viscous component): σ d can be expressed as ηdε / dt (dε / dt: time derivative of strain).

[0048] In the reproduction analysis, a viscoelastic model, which is an analytical model, is created in a computer, and the elastic coefficients for each material age determined in the compressive strength test are applied to the elastic coefficient of this viscoelastic model.Then, by changing the viscosity coefficient η, which is a dashpot element, as a parameter, a mechanical model (viscoelastic model) is created that reproduces (fits) with high accuracy the stress-time relationship of the specimen obtained in Process A.

[0049] In the reproduction analysis, the boundary conditions of the mechanical model (viscoelastic model) are a complete constraint on the bottom surface and a horizontal displacement constraint on the top surface. s =E k Then, the loading process and displacement holding process similar to the multi-stage stress relaxation test are repeated at each analysis step (corresponding to the construction step), and E is calculated from the elastic modulus for each material age (here, every 4 hours, which is the time interval for the construction step) obtained in process C at each analysis step. s (=E k Therefore, when repeating loading and displacement hold five times as shown in Figure 7, the input value of the elastic modulus is changed five times to reproduce the manifestation of stiffness in the concrete.

[0050] To determine the viscosity coefficient, a value that matches the results of the multi-stage stress relaxation test is determined by parametric analysis. In the example shown, the viscosity coefficient of the fitted viscoelastic model is set to 50 GPa·h (h: time) for Blend A and 10 GPa·h for Blend B.

[0051] In the reproduction analysis, as shown as an example in Figure 7, in the analysis step corresponding to the four-hour interval, which is the construction step, loading is applied to the mechanical model while changing the displacement, and the displacement is kept constant.

[0052] An example of the reproduction analysis results shown in Figure 8 compares the measured values ​​and analytical values ​​for the test specimen of blend A.

[0053] From Figure 8, it can be seen that in the case of the viscoelastic model, the analytical values ​​and the measured values ​​show relatively good agreement at all stages of displacement loading.

[0054] From the above, it can be seen that in order to analytically express the behavior of shotcrete, it is preferable to assume a viscoelastic model such as that shown in Figure 6(b).In addition, it is also considered appropriate to treat it as an elastic body (elastic model) as in the general method, and then use an equivalent elastic modulus that takes viscous behavior into consideration.

[0055] In the sequential excavation analysis of the tunnel in process C (step S106) (for example, three-dimensional sequential excavation analysis), the wall strain and the strain of the shotcrete are calculated for each elapsed time of tunnel excavation under the conditions of installation of the shotcrete (shoring), reflecting the fact that the shotcrete is quickly applied after the tunnel is excavated (unlined excavation of the tunnel).In this sequential excavation analysis, the construction conditions and ground conditions at a specific construction site are set based on the construction plan, and the sequential excavation analysis is carried out under the various set conditions.

[0056] Here, the construction conditions include the excavation form (full-section excavation, half-section excavation, etc.), the cross-sectional shape and dimensions of the tunnel, and the time interval between construction steps (excavation speed, etc.).One example of ground conditions and construction conditions is a 10m diameter tunnel constructed in elastic ground with an initial ground pressure of 4.0MPa (equivalent to 200m of soil cover), a deformation coefficient of 1.0GPa, and a Poisson's ratio of 0.3, with a construction step of 4 hours / m (displacement retention period).

[0057] In the example of the results of sequential tunnel excavation analysis shown in Figure 9, the wall strain ε1', ε2', ..., and the age and mechanical properties of the shotcrete model are displayed in a table for each elapsed time (excavation step, construction step) s1, s2, ... after the excavation of the tunnel. Here, the material ages t1, t2, ... and the elastic modulus (E s =E k ) is the elastic modulus for each age of the specimen, as determined in the compressive strength test in step C.

[0058] In the table shown in Figure 9, material age t1 corresponds to excavation step s1, the viscosity coefficient of the viscoelastic model at the time of fitting used in the reproduction analysis corresponding to excavation step s1 is ηt1, the wall strain by the sequential excavation analysis corresponding to excavation step s1 is ε1', the sprayed concrete strain is ε1, and the stress is σ1. A similar correspondence exists for material ages t2 and onwards.

[0059] The design method using incremental excavation analysis of mountain tunnels shown in the figure reflects actual phenomena in which shotcrete is subjected to loads in a displacement loading manner and is further subjected to gradual displacements associated with tunnel excavation, takes into account creep and drying shrinkage deformation at very early ages, and can obtain highly accurate analytical results without applying equivalent elastic moduli.

[0060] [Validity evaluation and verification of 3D sequential excavation analysis and its results] The inventors have actually implemented the design method for mountain tunnels using sequential excavation analysis according to the embodiment and obtained analytical results. In mountain tunnel construction where weak ground and high ground pressure act, there is a concern that large displacement and destruction of support structures may occur. Therefore, countermeasures such as the pilot tunnel advance method and the use of double support structures have been adopted. However, the actual strength at 28 days is 100 N / mm in uniaxial compressive strength. 2 Since ultra-high strength shotcrete that can reach tensile strength has been developed, applying this ultra-high strength shotcrete to shotcrete is thought to be a possible alternative to conventional countermeasures. However, since analytical methods for designing support structures using ultra-high strength shotcrete have not yet been established, in this analysis, ultra-high strength shotcrete was installed in a small cross-section tunnel, and the natural ground properties were identified by inverse analysis from the measurement data obtained there. The validity of the analytical method was evaluated by comparing the measurement data with the results of a three-dimensional sequential excavation analysis that applied deformation properties such as stiffness and creep characteristics according to the age of the ultra-high strength shotcrete.

[0061] <Construction of ultra-high-strength shotcrete in an actual tunnel> The cross section of the tunnel where ultra-high strength shotcrete was used is shown in Figure 10A. The geology is granodiorite, and the excavation cross section is about 20 m 2 The tunnel is a small cross section with an earth covering of approximately 400m. Measurements of displacement inside the tunnel and effective stress of the shotcrete were carried out in the center of the section where ultra-high strength shotcrete was applied. In addition, measurements of the effective stress of the shotcrete, which has a design standard strength of 18N / mm 2 Measurements were also taken in the normal strength shotcrete section.

[0062] <Analysis conditions for 3D sequential excavation analysis> The three-dimensional incremental excavation analysis was performed using the finite difference code FLAC3D.ver6. The analytical model diagram is shown in Figure 10B, the soil properties are listed in Table 2, and the support properties are listed in Table 3. The initial stress was assumed to be equivalent to a 400m soil cover, and the soil properties and lateral pressure coefficient of 0.7 were identified through prior back analysis. The viscoelastic model shown in Figure 6(b) was used for the shotcrete, and the stiffness development was simulated using the approximate curve shown in Figure 11. The viscoelastic parameters were based on previous studies, Refs. 1 and 2. Reference 1 is "A Study on an Analysis Method Applying the Deformation Characteristics of Ultra-High-Strength Shotcrete," Tunnel Engineering Reports, Vol. 31, I-31, 2021. Reference 2 is "A Study on the Support Effect of Tunnel Shotcrete," 63rd Annual Academic Conference of the Japan Society of Civil Engineers, Division 3, pp. 641-642, 2008.

[0063] [Table 2]

[0064] [Table 3]

[0065] <Analysis results> First, we will compare the measurement results with the analysis results. Figures 12A to 12C show a comparison of the measurement results at a position where the face is far enough away (5D or more) and the 3D sequential excavation analysis results. Here, for the purpose of comparing the behavior with the viscoelastic model, we also show the results when the mechanical model of the shotcrete is an elastic body that does not take creep characteristics into account.

[0066] A comparison of the measurement results and the analysis results showed that the deformation of the ultra-high strength shotcrete cross section was generally consistent, and the differences due to the mechanical model were minimal. Furthermore, the viscoelastic model showed a relatively good agreement with the measurement results for the shotcrete stress, but the elastic model generated greater stress than the viscoelastic model. This indicates that the viscoelastic model is able to reproduce the stress relaxation tendency of the shotcrete, and that not taking creep characteristics into account may result in an overestimation of the shotcrete stress.

[0067] Next, we will consider the validity of the analysis method. The stress-strain relationship of the shotcrete at the tunnel crown obtained from the three-dimensional sequential excavation analysis is shown in Figure 13. The value obtained by dividing the maximum value of the shotcrete stress by the cumulative strain is considered to be the equivalent elastic modulus, and is shown in the figure.

[0068] In the case of the viscoelastic model, the equivalent elastic modulus is 5.9 GPa for ultra-high strength shotcrete and 2.7 GPa for normal strength shotcrete. The equivalent elastic modulus for normal strength shotcrete is similar to the value given in the Japan Highway Public Corporation's Tunnel Numerical Analysis Manual, 2002, confirming the validity of the value of the equivalent elastic modulus for ultra-high strength shotcrete.

[0069] However, considering the above-mentioned previous references 1 and 2, it is believed that the equivalent elastic modulus of shotcrete cannot be uniquely determined, and that it is desirable to set it appropriately based on various conditions such as the natural ground properties, initial stress, construction method, etc. Therefore, an analytical method such as that used in this study, which takes into account deformation characteristics such as stiffness changes and creep characteristics according to material age, is in harmony with the measurement results and is believed to be a method that can streamline support design.

[0070] It should be noted that the present invention is not limited to the configurations shown here, and other embodiments may be possible in which other components are combined with the configurations described in the above embodiments. In this regard, the present invention can be modified within the scope of the present invention, and can be appropriately determined depending on the application form. [Explanation of symbols]

[0071] S100:A Project S102:B Project S104:D Project S106:C Project

Claims

1. A design method for mountain tunnels using sequential excavation analysis, taking into account the stiffness over time of shotcrete applied to the tunnel wall, A step A includes carrying out a multi-stage stress relaxation test on a test piece of the sprayed concrete, in which a displacement that changes stepwise over time according to the age of the test piece is applied, and identifying the stress-time relationship of the test piece; A process B includes performing a reproduction analysis to reproduce the stress-time relationship and identifying mechanical properties according to the age of the specimen; and a step C of carrying out a sequential excavation analysis of a tunnel using an analytical model including a shotcrete model having mechanical properties according to the age of the material and a ground model in the vicinity thereof; The mechanical property in the step B is a viscoelastic property, In the multi-stage stress relaxation test in the step A, a constant displacement is maintained for a certain elapsed time, and the constant displacement is changed according to the elapsed time, A design method for mountain tunnels using sequential excavation analysis, characterized in that the displacement change is performed instantaneously.

2. 2. A design method for mountain tunnels using sequential excavation analysis as described in claim 1, characterized in that the elapsed time at which displacement loading begins in step A is set to a time equivalent to the tunnel excavation cycle time.

3. In the reproduction analysis in the step B, a viscoelastic model expressing the stress-time relationship is used as a mechanical model, the viscoelastic model is composed of spring elements that represent elastic behavior and dashpot elements that represent viscous behavior; The design method for mountain tunnels using sequential excavation analysis as described in claim 1 or 2, characterized in that in step B, the viscosity coefficient of the dashpot element is adjusted to fit the mechanical characteristics to the stress-time relationship of the test specimen.

4. Further comprising step D of conducting a compressive strength test on the specimen; A design method for mountain tunnels using sequential excavation analysis as described in claim 3, characterized in that the elastic coefficients for each age of the test specimen identified by the compressive strength test are applied to the spring elements in step B.

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