New technologies for time-dependent failure modeling of discontinuities in rock containing intact rock bridges

Rock bridges are sections of an intact rock along otherwise through-going discontinuities, and the rock bridges play an important role in the geotechnical stability of underground excavations, surface slopes of mines, dams, foundations and other earth retaining structures. Numerical and analytical techniques for predicting the time-dependent degradation and failure of the rock bridges have been previously developed, and the results show a high sensitivity to the area percentage, shape, and distribution of the rock bridges along a discontinuity. Even a rock bridge percentage less than 1% can result in the stabilization of an otherwise unstable block in the roof of an underground excavation. This paper describes a three-dimensional (3D) numerical modeling technique that has been developed to model the time-dependent failure of rock bridges. It utilizes a fracture mechanics formulation with modes I subcritical crack growth, integrated into the Abaqus 3D finite element model. Since the constitutive model of the subcritical crack growth is not available in Abaqus material libraries, user subroutine (usdfld) code is developed to incorporate the effect of the subcritical crack growth on the potential failure of the rock bridges. This methodology is applied to model time-dependent rockfall in the Kartchner Caverns, and the numerical simulation results are compared with the analytical closed-form solutions. The purpose of this research is to improve our understanding of the time-dependence in fractured rock masses. Applications include surface and underground mining, highway and rail slopes, tunnels and other underground excavations, dam and bridge foundations, and other engineered and natural rock structures.