Study of the Mechanical Behavior of Intact and Jointed Rocks in Laboratory with Particular Emphasis on Dilatancy

This paper summarizes a study carried out at the University of Vigo on the mechanical behavior of cylindrical specimens of eight different rocks, focusing on both pre- and post-peak behavior. First, more than 200 intact standard NX-sized specimens were tested under unconfined and confined conditions. Complete stress-strain curves were obtained for each test, which allowed calculating the main elastic and peak and residual strength parameters. Experimental results of the dilation angle (which showed its dependence on confining stress and plastic parameter) were fit to available mobilized dilation angle models. Trying to explore the scale effect problem, we created two series of artificially-jointed specimens of one of the previously studied rocks. These specimens featured two sets of discontinuities, one sub-vertical and another sub-horizontal set. First series (22 specimens) were cut with one sub-vertical and two sub-horizontal joints (1+2), and second series (20 specimens) were cut with two sub-vertical and three sub-horizontal joints (2+3). Results show the same trends depending on the confining stress as the intact specimens did. Some interesting trends arose when comparing these jointed specimens results with intact ones: Young's modulus and peak strength decrease with increasing jointing. Residual strength revealed to be the same for intact and jointed specimens. GSI_res (obtained when fitting the Generalized Hoek-Brown failure criterion to the residual strength results) shows dependence on confining stress for the jointed specimens, but not for the intact ones. Brittleness show a decrease when increasing jointing. Finally, dilation angle showed to be smaller in jointed specimens than in intact ones for low confining stresses, but with a decreasing gap between intact and jointed specimens as confining stress increases. The results of this experimental work are in accordance with observations at rock mass scale, which opens the door to future research on the topic in order to better understand rock mass behavior.