Specimen Slenderness and the Influence of Joint Orientation on the Uniaxial Compressive Strength of Singly Jointed Rock

Limitations on the joint orientations for which a joint is free to fail exclusively by sliding (without the need for rupture of intact material), for a given specimen height-to-diameter ratio ($L/D$), can influence the results obtained from uniaxial compressive strength (UCS) testing of jointed rock. For example, a jointed specimen with $L/D=2$ can only possibly fail exclusively by sliding for joint angles up to $\sim 63°$ from the plane perpendicular to the axial direction. For joints greater than $\sim 63°$, intact material rupture is required to accommodate failure and UCS is likely to be overestimated. Experimental work was performed to determine the UCS of jointed rock containing various joint orientations, for sample sets with $L/D=2$ and 4. Only joint orientations that allowed joint formats for which failure could occur by sliding on the joint for the $L/D=4$ sample set (i.e., up to 75°) were considered. For joint orientations greater than $\sim 63°$, the $L/D=2$ sample set produced significantly higher UCS values than the $L/D=4$ sample set. Additionally, specimens with joint orientations in excess of 63° displayed failure mechanisms involving intact-material rupture for the $L/D=2$ sample set. Only sliding failure was observed for the same tests on the $L/D=4$ sample set. Care must be taken to ensure methodologies adopted for UCS testing of jointed rock do not influence the mechanisms by which failure can occur. In some cases, strict adherence to methods set out in popular testing standards may need to be reconsidered.