| A water conveyance tunnel is quite difficult to design and construct as it is deep buried and its geologic conditions is complicated and varies along the long tunnel line. The silty mudstone section is the deepest segment of the tunnel, buried depth of which is in the range of 1650 m to 2320 m with high in situ stress, as a result, the section is confronted with severe rock-burst problem. In addition, the mechanical behavior of rock under high in-situ stress environment is totally different from that under routine conditions, and it has been proved by laboratory tests that silty mudstone can show obvious rheological effect under high stress. It is not easy to ensure the stability of surrounding rock and the safety of supporting structure affected by long-term rheology, which general speaking, need to do some specialized research. The research work of this paper has been carried out aiming at two prominent issues, namely the creep behavior of rock and rock-burst problem of the tunnel section. The main work and achievements of this paper are as follows:(1) A nonlinear creep model in accordance with the creep properties of silty mudstone in the project area was established.Through the studying of fifteen common rheological models and the comparative analysis of rheological characteristics of these models and the indoor creep test curve of the silty mudstone samples, Burgers models was chosen to establish the nonlinear creep model of silty mudstone. Creep equation of Burgers was embedded into the function library of Origin through its UDF(user-defined-function) function in order to figure out the rheological parameters of the equation, and eventually, these parameters were figured out by using the Levenberg-Marquardt(L-M) method.(2) Based on the secondary development platform of ABAQUS, the user material subroutine of improved Burgers model was compiled successfully with FORTRAN.Solution formulas of creep component in improved Burgers model were deduced in great details and the programming block diagram for the model was put forward. Then based on the secondary development platform of ABAQUS, the user material subroutine of improved Burgers model was compiled successfully with FORTRAN. Indoor uniaxial and triaxial tests were simulated with the developed Umat subroutine. Numerical results show good agreement with the experimental results, which can proved the correctness and effectiveness of the developed Umat subroutine.(3) On the basis of taking full account of rock rheological effect, proper supporting time of the silty mudstone was determined and it is proposed that second lining should be constructed about 150 days after the excavation of the tunnel.Creep process of silty mudstone tunnel section on the condition of no supporting after excavation was analyzed with the developed Umat subroutine, and it is founded that the initial deformation of surrounding rock after tunnel excavation accounts for 40.4%, 46.0% and 51.19% of the final stable deformation, for the tunnel section with a buried depth of 1700 m, 2000 m and 2320 m respectively. And the creep deformation mainly occurs in the first six months. In addition, a comparative study on tunnel deformation, plastic deformation and stress change rule of segment structure under different supporting time(90 days, 120 days and 150 days after the tunnel excavation). Then it is proposed that second lining should be constructed about 150 days after the excavation of the tunnel.(4) Several stress-strength ratio criterion and strength-stress ratio criterion of rock burst prediction were used to evaluate the rock burst possibility and rock burst levels of silty mudstone tunnel segment comprehensively.Method recommended by standard, Russense Coefficient Method and Gu-Tao Criterion were used to evaluate the rock burst possibility and rock burst levels of silty mudstone tunnel segment comprehensively and results of all criterion show that it is very likely to have strong rock burst problem in that segment and the rock burst may be accompanied by severe spalling and other serious damage to the surrounding rock.(5) Taking the brittle fracture of granite in Mine-by test tunnel of underground laboratory in Canada as an example, five constitutive models were used to simulate the brittle failure zone of the surrounding rock as to compare their fitness in predicting brittle failure. In the meanwhile, mechanism of brittle fracture of rock was revealed and brittle failure zone of the silty mudstone tunnel segment was simulated.Elastic model, ideal elastoplastic, CWFS model, CSFH model as well as m-0 criterion based on Hoek-Brown model were used to simulate the brittle failure zone of the surrounding rock in mine-by test tunnel as to compare their fitness in predicting brittle failure. It is founded that models which can consider the mechanical properties degradation of rock during its breakage process, namely the CWFS model, CSFH model and m-0 criterion can simulate the brittle failure process of hard rock rather well. And the CWFS model is the best among them. And in the end, brittle failure zone of silty mudstone tunnel segment was simulated with CWFS model and m-0 criterion. |
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