Stability analysis of tunnel face reinforced by bolts in pure cohesive soils

The bolt reinforcement technique is an efficient method to improve the stability of tunnel faces in weak layers, but the design of bolt reinforcement in practical engineering is still based on experience. The reinforcement mechanism and the quantitative calculation of the reinforcement effect are subjected to further investigation. To investigate the stability of tunnel faces reinforced by bolts in purely cohesive soilsin the framework of upper-bound limit analysis theorem, an advanced three-dimensional (3D) failure mechanism was proposed by combining a modified 3D homogenization method related to bolt-soil interaction mechanism and the continuous velocity field concept. Due to the complex calculation of velocity, the 3D failure model was discretized and the work rate equations were established in a discretization form. Then the strength reduction method and bisection method were adopted to solve the equations, and the factor of safety was obtained. Numerical simulations were conducted to validate the proposed mechanism, which showed a good agreement. Finally, the effects of bolt length and bolt density on tunnel face stability were investigated based on this failure mechanism. Results show that the bolt reinforcement is not satisfied enough to ensure the tunnel face stability in purely cohesive soils, and for a better reinforcement effect, other reinforcement techniques (such as pipe reinforcement or ground grouting reinforcement) are also necessary as a supplement. In general, the proposed mechanism can properly evaluate the stability of bolts-reinforced tunnel faces in purely cohesive soils, which can provide reference to practical design and construction. © 2022, Central South University Press. All rights reserved.