Study On The Evolution Law Of Tunnel Rock Mass Parameters And Engineering Application

During excavation of tunnel, the processes of stress redistribution can produce the effect on the stiffness and strength parameters in the host rock, which reflected in the changing of the rock mass deformation and strength characteristics, and directly related to the stability of the host rock. In this paper, the evolution law of the mechanical parameters was studied by adopting laboratory test, theoretical analysis, numerical simulation and field test. The main research results as follows:(1) According to the triaxial tests of sandstone and granite, which were carried out under six different confining pressure 5,10,15,20,30 and 40 MPa, so that, the deformation, strength and failure mode were analyzed. After determining crack initiaton stress, fracture penetration stress and peak stress with different confining pressure, the strength parameters corresponding to characteristic stresses was obtained, therefore, the variation law of the strength parameters is clear and definite.(2) Based on the Mohr-Coulomb failure criterion, the strength parameters evolution law of sandstone and granite were discussed by setting the plastic shear strain as the internal variable. The Weibull and Lognormal functions could be used to describe the peak strength and residual strength parameters, as well as the evolution law. The frictional angle firstly increases and then decreases with the increase of plastic shear strain. The developed constitutive model was programmed in FLAC3D numerical simulation software and the evolution law of strength parameters was called to simulate triaxial laboratory test. The rock strain hardening before rock peak and strain softening are captured well in the simulations. By comparing the numerical results and real test data, the average relative errors of peak strength and residual strength are respectively 4.14%and 11.42%.(3) The stiffness of rock masses degrades due to the unloading and dynamic disturbances during tunnel excavation. Through drilling and testing of the core samples in the disturbed host rock, the distribution of elastic modulus was analyzed. Based on degree and scale of disturbance, a method of hole distance elastic modulus was developed, and a calculation formula of the disturbance factor of host rock mass was presented in the form of normalization and integration to evaluate the elastic stiffness degradation. The disturbance factor which was obtained by the elastic modulus method was verified well with that of the conventional acoustic method, and, the elastic modulus of the disturbed rock mass was corrected from a new approach.(4) Through analyzing the strength evolution model and stiffness degradation of host rock, the elastic-plastic analytical solution for the plane problem of circular tunnel is obtained by considering confining pressure and dilation. Taking Ji’an diversion tunnel (circular section) as an example, and using the MATLAB program which is made for the elastic-plastic analytical solution, the effects of support force on the deformation and plastic zone in host rock was studied. And the radius of plastic zone is exponentially dependent on the supporting force. In the meantime, the plastic areas increases linearly, while the convergence displacement exponentially, with the elastic modulus degradation of rock mass which is major reason to lead the large deformation of host rock. A comparison was made between the revised and the original analytical solution. And the rationality of the stiffness and strength evolution law was verified, by comparison of experiment data and revised calculations. The results show that the modified value is closer to that of test.(5) Based on the stiffness and strength evolution of host rock, the displacement field, stress field, plastic zone and elastic strain energy of host rock are predicted by numerical simulation for the excavation of Ji’an diversion tunnel (arched section). The changes of principal stress and release of displacement in the whole process of excavation are in the range of [-D, D] (D is tunnel diameter). The relative error is 8.63% between the displacement and the monitoring value, and simultaneously, the rationality and reliability of the law is verified, which can describe the evolution of rock mass mechanics parameters.