Scale Effects Observed in Compression Testing of Stanstead Granite Including Post-peak Strength and Dilatancy

Abstract

Scale effects refer to changes in mechanical behaviour associated with the volume of material being loaded or deformed. Scale effects in rocks and rockmasses are particularly complex, as an increased volume of interest changes not only the measured behaviour of intact rock, but also results in an increased sampling of natural fractures, which can often significantly influence rockmass mechanical behaviour. To isolate one component of these effects, laboratory tests performed using different size specimens can be used (scale effects in the absence of jointing). Previous studies on scale effects based on laboratory testing have tended to focus on changes in stiffness and peak strength, particularly under unconfined conditions. In this study, data are examined from specimens tested in uniaxial and triaxial compression considering not only stiffness and peak strength, but also strength evolution and post-peak dilatancy evolution. The crack initiation and crack damage stresses are found to be scale independent, whereas the post-yield rate of cohesion loss appears to change as a function of specimen size. Trends in the dilation angle are shown to be relatively scale independent, which is consistent with prior studies that found laboratory-based dilation angle models could be used to accurately replicate data collected from sparsely fractured in situ rockmasses.