Physical experiment and numerical modelling of tunnel excavation in slanted upper-soft and lower-hard strata

Abstract

During the excavation of tunnels, the heading face often encounters alternating soft and hard strata. Many accidents have been reported to be closely related to the change in strata. To investigate the effect of alternating soft and hard strata on the stability of rock surrounding tunnels, physical experiments and numerical simulations were performed to simulate tunnel excavation in slanted upper-soft and lower-hard strata. The evolution laws and distribution features of stress, displacement and failure were analysed. During the process of stress redistribution, the radial stress of the surrounding rock has a step-type decrease trend with step excavation, but the evolution laws of tangential stress are closely related to lithology and the distance from the free face. The failure process of the surrounding rock under un-supported conditions began with a shearing crack in the roof that then extended further and coalesced at the interface between the hard and soft rock. Both the physical and numerical results revealed that the final failure model shows obvious asymmetrical deformation; the soft rock area experiences block dropping and roof falling, while the hard rock only shows shrinkage deformation. Therefore, excavation and support design need to pay attention to the asymmetrical deformation failure in complex rock strata, especially the large deformation of soft strata. Based on the numerical analysis results, the influence of composition of strata lithology on the failure mode were further discussed. These results can provide helpful references for the safe excavation and scientific support design of tunnels across alternating soft and hard strata.