Modeling hard rock failure induced by structural planes around deep circular tunnels

Construction of tunnels is often associated with fault or structural features that could affect tunnel stability during the construction phase and service life. Therefore, failure characteristics of hard rock around circular tunnels induced by pre-existing structural features under high in-situ stresses have been the subject of various studies. In the current study, a combined finite element approach, namely ELFEN, has been used for better reflection of entire failure process (including crack initiation, propagation and coalescence) and intrinsic properties of hard rock mass, thus rock heterogeneity, around circular tunnels during excavation unloading process. Parametric analysis which consider the dip angle, location (exposure or not), frictional coefficient of structural planes and lateral pressure coefficient was conducted in detail to reveal the mechanical responses of circular tunnel induced by structural plane under unloading condition. Numerical results indicate that the failure intensity of rock tunnel is a function of both dip angles and frictional coefficients of structural planes. The most critical dip angles of structural planes leading to failure in tunnel rocks largely depend on the frictional coefficient. Also, the results indicate that the released strain energy for the case of exposed structural plane is higher than those not intersecting the tunnel, leading to more violent rock failure for the former. With the increase of the lateral pressure coefficient, the failure intensity and damage extent around the tunnel is aggravated, especially for the roof and floor of the tunnel. Rock failure can be categorized as slabbing failure near excavation boundary and shear slip failure, controlled by structural plane. Progressive slabbing failure induced by excavation unloading may activate internal structural planes and the extensive release of energy caused by shear and slip failure may in turn further induce the slabbing failure. Rockburst is more prone to be triggered under such condition.