Dilation in granite during servo-controlled triaxial strength tests

We investigated the stress–strain response of three different intact granitic rocks. To do this, a press with servo-controlled loading was modified to control the confining pressure in triaxial tests and to measure the volume of hydraulic fluid displaced from Hoek's triaxial cell so that this volume could be related to volumetric strain in the rock sample. A series of unconfined and confined compressive tests were performed on the rock samples and results were plotted, analysed and interpreted regarding the most relevant parameters, including elastic, strength and post-failure parameters, with special attention paid to the dilation angle. Our main conclusions refer to the capability to investigate consistent post-failure rock properties by means of servo-controlled loading set-ups, which we plan to improve in the future. The dilation angle of granite was captured, with all the tested granites showing similar behaviour trends that fit reasonably well with recently developed theories on plastic shear-strain and confinement-stress-dependent dilation and that are analogous to those observed for other hard rocks.

► A servo-controlled press was prepared to control volumetric strain on triaxial compressive tests.
► A series of unconfined and confined compressive tests were performed on three types of granite rock samples.
► We investigated the stress–strain response particularly focusing dilation.
► The plastic shear-strain and confinement-stress-dependent dilation angle of granite was captured and quantified.