Tunnel face stability in heavily fractured rock masses that follow the Hoek-Brown failure criterion

A tunnel face may collapse if the support pressure is lower than a limit value called the 'critical' or 'collapse' pressure. In this work, an advanced rotational failure mechanism is developed to compute, in the context of limit analysis, the collapse pressure for tunnel faces in fractured rock masses characterized by the Hoek-Brown non-linear failure criterion. The non-linearity introduces the need for additional assumptions about the distribution of normal stresses along the slip surface, which translate into new parameters in the limit analysis optimization problem. A numerical 3D finite difference code is employed to identify adequate approximations of the distribution of normal stresses along the failure surface, with results showing that linear stress distributions along the failure surface are needed to obtain improved results in the case of weaker rock masses. Test-cases are employed to validate the new mechanism with the three-dimensional numerical model. Results show that critical pressures computed with limit analysis are very similar to those obtained with the numerical model, and that the failure mechanisms obtained in the limit analysis approach are also very similar to those obtained in small scale model tests and with the numerical simulations. The limit analysis approach based on the new failure mechanism is significantly more computationally efficient than the 3D numerical approach, providing fast, yet accurate, estimates of critical pressures for tunnel face stability in weak and fractured rock masses. The methodology has been further employed to develop simple design charts that provide the face collapse pressure of tunnels within a wide variety of practical situations. (C) 2013 Elsevier Ltd. All rights reserved.