STRENGTH PREDICTIONS FOR JOINTED ROCKS IN CONFINED AND UNCONFINED STATES

Most of the rotational approaches to the design of structures on or in a rock mass are based on the strength response of the rock mass. Realizing this important aspect, the present investigation was undertaken to understand the strength response of jointed rocks. The objective was achieved by simulating joints in intact isotropic rock cores in the laboratory. Three materials, namely, plaster of Paris, Jamrani sandstone and Agra sandstone were selected. The intact specimens of these materials provided a wide range of compressive strength (sigma(ci) = 11.3-110 MN/m2). A special technique was devised to develop joints varying in number and inclination. In all, about 250 uniaxial compressive strength (UCS) tests and 1300 triaxial tests on jointed and intact specimens of these materials were conducted. Based on this extensive experimentation, a joint factor J(f), has been evolved to account for the number of joints per metre length, inclination of the sliding joint and the shear strength along this joint. This factor is found to be uniquely related to the ratio of compressive strength of jointed rock to that of the intact rock irrespective of the type of rock. A strength criterion for jointed rocks is proposed and the parameters defining this criterion can be evolved simply by knowing the joint factor, compressive strength of intact rock and triaxial strength of intact specimens at two convenient confining pressures. The empirical relations developed have been verified with similar data for other jointed rocks and model materials.