Deformation and kinematics of vertically jointed rock layers in underground openings

Layered and jointed rock masses are common in underground environments. Honest stability analysis of such rock masses requires the application of discrete element methods, since other methods of analysis are either continuous by nature (FEM), or limited to two block interaction (Voussoir model). In this paper the stability of jointed rock layers is studied using DDA. Two different configurations are studied: (1) single layer-immediate roof, (2) multi-layered roof. The influence of joint spacing and shear strength is investigated and different failure modes are discussed; the kinematics and evolution of deformation are studied in depth. For a single layer configuration it is shown that stable arching is achieved when the shear resistance along joints is sufficient to inhibit inter-block shear and to induce block rotation at the abutments. For a multi-layered configuration it is found that the transition from unstable multi-layer deflection due to vertical load transfer, to stable arching is marked by reduction of differential deflections and homogenization of displacement within the stack of layers. The numeric solution is compared with the semi-analytical Voussoir solution, which for the given rock mass structure and mechanical properties, is found to be un-conservative.