Determination of the Length of the Rock Bolts for Tunnels with Consideration of the Nonlinear Rheological Behavior of Squeezing Rock

Abstract

An accurate model for the prediction of the rheological behavior of surrounding rocks is essential to the designing of rock bolts for tunnels under squeezing conditions. Our understanding of the state of the art suggests that the rheology of rock consists of the transient, the steady-state, and the accelerated regimes. Thus, a visco-elastic-plastic rheological model, namely the Komamura-Huang-Bingham model, was developed. The model used the Drucker-Prager yield criterion in order to consider the effects of the intermediate principal stress on the strength of the rock mass. The developed model was implemented in the framework of finite element simulations. It was validated by comparing the simulation results with the on-site monitoring data. The focus of the simulations was on the mechanical behavior of the rock bolts and the surrounding rock mechanics during the construction of the tunnel. A sensitivity analysis was performed with respect to the length of the bolts and the stress-to-strength ratio of the rock was performed. It was shown that increasing the length of the bolts up to 9 m results in a reduction in rock deformation. This critical value of the bolts’ length is approximately equal to the diameter of the investigated tunnel. A further increase in the length results in an insignificant reduction in the deformation of the surrounding rock. In the case of the Class A and B squeezing conditions suggested by Hoek, the deformation of the surrounding rock is insensitive to the length of the bolts. The elongation of the bolts does not change considerably with time, which is in contrast to the Class C and D squeezing conditions. The extent of the plastic zone is related to time and in situ stress but is independent of the length of the bolts. This is consistent with the characteristics of the nonlinear rheology of rock mass.