Shear failure process of rectangular tunnel: Physical experimental test and numerical back-analysis

Abstract

Shear failure is a typical failure mode in underground engineering and has resulted in a large number of instability issues. To study the shear failure mechanism, physical shearing tests for rectangular tunnel of sandstone, including single rectangular tunnel (SRT) and double rectangular tunnels (DRT) specimens, were carried out under three normal stress states. During the shearing tests, the internal failure process and the external cracks evolution of tunnel specimens were recorded by video camera, and the acoustic emission (AE) events of specimens were also monitored for exposing the mesoscopic failure characteristics. In addition, the discrete element method was also applied to analyze the corresponding shear experiment of rectangular-tunnel specimens. The tested results showed that the shear strength, shear modulus of the samples were positively correlated with the normal stress. For example, the shear strength, and shear modulus of SRT increased from 5.18 MPa to 8.70 MPa, and 1.02GPa to 1.88GPa, respectively when the normal stress increased from 2.0 MPa to 5.0 MPa. More importantly, the shear failure of rectangular tunnel model can be generally divided into four stages: (i) Shearing compression stage characterized by loading contacting and compressing; (ii) Shearing micro-crack propagation stage dominated by inner particles ejection of tunnel, tensile crack initiation and shear crack propagation of specimen; (iii) Shearing fracture stage dominated by the specimen’s crack propagation, coalescence, penetration, and corresponding rock ejection; (iv) Shearing friction stage dominated by shear damage of tunnel specimen. Corresponding back analysis by numerical simulation and scanning electron microscope results indicated that the shear failure of rectangular-tunnel specimen can be classified as tensile-shear mixed failure mode. The research results provide a new cures for shear prevention and support design of rectangular tunnels in underground engineering.