Physical model simulation of shallow openings in jointed rock mass under static and cyclic loadings

Physical model simulations have been performed to determine the effects of depth, joint spacing and orientation on the maximum unsupported span of shallow underground openings under static and cyclic loads. Cubical and rectangular blocks of Phu Phan sandstone are arranged in a vertical test frame to simulate a two-dimensional representation of single rectangular openings in rock mass with two mutually perpendicular joint sets. Results indicate that the normalized maximum span (W/SV) rapidly increases with the normalized depth (D/SH), and tends to approach a certain limit for each joint spacing ratio, SV:SH. The maximum span increases with decreasing SV:SH ratio. Under SV = SH condition, increasing the joint angles from 0° to 45° reduces the maximum span by about 20%. At shallow depths the acceleration of 0.225 g can reduce the maximum span by up to 50%. The impact of the cyclic loads however reduces as the depth increases. The test results under both static and cyclic loadings compare reasonably well with those calculated from discrete element analyses using the UDEC code.