Mechanical Behavior of Intact and Increasingly Jointed Marble Laboratory Specimens

For many civil and mining engineering structures, rock is the primary construction medium. At engineering scales, the material of interest is the “rockmass,” which contains sections of intact rock separated by a network of pre-existing joints (i.e. fractures). The mechanical behavior of a rockmass is defined by attributes of its intact rock (e.g. strength, stiffness, dilatancy), in addition to attributes of the pre-existing joints (e.g. orientation, surface condition, persistence). Testing representative rockmass specimens in the laboratory is uncommon because of its difficulty due to the required scale. The present study uses laboratory-scale rockmass analog specimens to study the effects that the pre-existing joints have on rockmass mechanical behavior. Smooth joints were sawed into decimeter-scale cylindrical specimens of Carrara marble. Specimens with two joint sets and two different degrees of jointing were tested. Specimens were loaded in compression using a triaxial apparatus under varying levels of confining pressure, and mechanical attributes were calculated including peak strength, residual strength, and pre-peak damage threshold parameters. At the higher end of confining pressures tested (12 MPa), the peak strength of the Carrara marble specimens was not influenced by the presence of pre-existing joints. These results were compared to the results of previous jointed specimen testing on the more brittle Blanco Mera granite. Notably, the drops in peak strength with increasing degrees of jointing were smaller for Carrara Marble than for Blanco Mera granite, which suggests that the Generalized Hoek–Brown failure criterion may not be appropriate to estimate rockmass strength for less brittle rocks.