Failure Mechanisms Around Shallow Tunnels In Brittle Rock

As part of an ongoing study on the mechanical characteristics of the damaged rock zone (DRZ) around a tunnel boundary and its influence on the overall behaviour of the near-field host rock, a numerical study was conducted to investigate the dominant rock mass failure mechanisms around shallow tunnels located in a body of brittle rock mass. Preliminary studies have clearly indicated that, the rock mass behaviour was largely dictated by the different yield mechanisms involved. These mechanisms appear to be multifarious due to the complex state of the induced stresses around the tunnel boundary at shallow depth. Some of these yield mechanisms are often difficult to capture using the traditional yield criterion such as the Mohr- Coulomb. In the study reported in this paper a series of computer simulations were performed using typical in-situ stresses and rock mass conditions generally encountered in shallow tunnelling projects in Sweden. The presence of a disturbed/damaged rock zone with finite thickness was also added into the models. Due to the nature of the simulations the FLAC2D code was selected and used. The values for the input parameters were estimated using a systematic procedure. To prevent premature tensile yielding due to transient loading traction was applied during tunnel excavation. The results show that, the dominant yield mechanism occurring around the shallow tunnel in brittle rock is the one of tensile nature. Compressive mechanisms do occur but seems to be less critical to overall performance of the tunnel. However, they do influence the overall behaviour of the rock mass response. The results further indicated that, the ‘true’ locality of instability could be predicted from elastic analyses, by observing if all the major principle stresses are in tension in that locality. The shear strain increment plots of the model in plastic mode further supported this observation.