Seismic Stability of 3D Tunnel Face considering Tensile Strength Cut-Off

Classical Mohr-Coulomb (M-C) criteria is typically applied to assess the soil strength in the analysis of tunnel face stability, in which the tensile strength is expressed as the extension of the compressive portion. However, the actual tensile strength of soils is quite unreliable. Therefore,the classical Mohr-Coulomb criterion is modified by involving the concept of tension cut-off, in which a nonlinear portion is introduced to express the tensile strength. Based on this, this paper presents a kinematic approach of limit analysis to estimate the face stability of tunnels with tensile strength cut-off subject to seismic excitations. An effective 3D rotational failure mechanism with consideration of tension cut-off is established. Based on the 3D failure mechanism, an explicit expression of the critical face pressure can be deduced by equating the external work rate and internal energy dissipated rate. The most critical face pressure is sought through an optimization program. A comparison between the existing researches and this work is made to verify the proposed approach. A parameter analysis and a series of design charts are presented. The results show that considering the presence of soil cohesion has a beneficial effect on the stability of the tunnel face and the presence of tension cut-off results in a more conservative outcome. The influence of tension cut-off increases with the increase of soil cohesion and is more noticeable at a small tensile strength cut-off coefficient. In some cases, the tunnel face stability may be mis-evaluated without considering tension cut-off, threatening engineering safety. In addition, the presence of seismic force will reduce the impact of tension cut-off on the analysis of tunnel face stability. This work provides a useful method to assess the face stability of 3D seismic tunnels under the impact of tensile strength cut-off, and the findings can serve as a reference tool in the seismic design of tunnel engineering.