Proper mechanical model selection is critical in tunnel support design and stability analysis, especially to reflect the creep and strain-softening behavior of soft rock. We present a coupled nonlinear Burgers strain-softening (NBSS) model and numerical calculation method to investigate the coupled effects of creep and strain-softening of soft rock tunnels. The nonlinear elastic-viscous model is used to simulate the steady creep behavior of mudstone, and the nonlinear viscoplastic strain-softening model is used to simulate the accelerated creep behavior and post-peak strength attenuation behavior. The experimental results show that the viscoplastic parameters and post-peak softening parameters of mudstone are highly sensitive to confining pressure and exhibit nonlinear characteristics. The accelerated creep curve obtained by the numerical calculation is consistent with the experiments, which verifies the model reliability. We use the NBSS and nonlinear Burgers Mohr-Coulomb (NBMC) models to calculate the plastic zone distribution characteristics and deformation law. The distribution of the plastic zone calculated by the NBSS model is larger with more localized fractures. The NBSS model is useful for studying the evolution of stress and displacement fields of complex surrounding rock mass, which provides important theoretical guidelines for the support design and stability analysis of soft rock tunnel engineering.
The complex mechanical properties of deep surrounding rocks during excavation and unloading have always been a concern in engineering communities. Based on deep roadway engineering, the mechanical properties of granite strain-softening were investigated by laboratory tests. An exponential relationship between granite peak softening modulus and confining pressure was obtained using a nonlinear fitting method. A strain-softening model was developed based on plasticity theory for granite which took into account the nonlinear dilatancy angle and confining pressure. A mathematical model was developed using FLAC3D as platform. By developing a numerical model for a deep roadway, the local fracture characteristics of deep surrounding rocks under strain-softening conditions were evaluated. It has been found that the postpeak failure of granite had a tendency for brittle-ductile transformation. Under high confining pressure conditions, granite exhibited brittle failure characteristics during the postpeak period, and the postpeak softening modulus was decreased with the increase of confining pressure. From FLAC3D numerical calculations, it was found that the numerical models of different mesh densities had basically the same characteristic curves of surrounding rocks, which showed that the local cracking phenomenon had little effect on calculation results when the convergence constraint method was applied for the calculation of the stability of supporting structures and surrounding rocks. It was seen from the numerical simulation analyses of surrounding rock local fracture properties in deep roadways that plastic shear strain appeared in the local areas of roadway vault and arch foot, which basically coincided with the damage location and depth of surrounding roadway rocks.
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