Numerical Study of the Mechanical Behavior of Nonpersistent Jointed Rock Masses

Estimating the mechanical properties of nonpersistent jointed rock masses is one of the most challenging problems in practical rock engineering due to the complex interaction of rock joints and intact-rock bridges. In this paper, the effect of joint geometrical parameters of nonpersistent rock mass on uniaxial compressive strength (UCS) and the deformation modulus was studied by using the discrete-element particle flow code PFC3D. In this numerical approach, the intact material is represented by an assembly of spherical particles bonded together at their contact points, and the joint interface is explicitly simulated by slip surfaces that are applied at contacts between the particles lying on the opposite sides of the joint interface. The failure process is simulated by the breakage of bonds between particles. A previous study of the authors has shown that this approach is capable of reproducing the mechanical behavior of nonpersistent jointed rock masses by a comparative study against physical experiments. In this study, the effect of joint geometrical parameters, including joint-orientation angle, spacing, persistency degree, step angle, and aperture, on the UCS sigma cm and the deformation modulus Em was studied. To reduce the number of experiments and also gain a sound understanding of the effect of each parameter, experiments were designed using a well-established experiment design technique. Five failure modes were observed in the experiments, including intact failure, step path, block rotation, semiblock generation, and planar failure. It was found that the initiation and propagation of tensile cracks have a controlling effect on the observed failure modes. The sigma cm and Em of jointed samples were analyzed using analysis of variance. It was found that joint-orientation angle, spacing, and persistency have a significant effect on both sigma cm and Em and that joint-orientation angle has the greatest effect on the mechanical behavior of a rock mass.