Dilatancy of Sandstone under Unloading and High Stress Conditions

Abstract

Cylindrical samples of quartz sandstone from Chongqing Yuzui were tested using conventional triaxial compression under pre-peak unloading conditions. Conventional triaxial compression tests were carried out at confining pressures of 10, 20, 40, 60, 80, 100, 120 and 130 MPa. Correspondingly, the initial confining pressure when unloading process began was also equal to 10, 20, 40, 60, 80, 100, 120 and 130 MPa in the confining pressure reduction tests. Based on the test results, dilatant characteristics of sandstone under unloading and high stress conditions can be summarized as follows: (1) dilatant behavior of rock samples occurs in both pre-peak region and post-peak region; (2) the transition from peak strength to residual strength was accompanied by a significant increase of volumetric strain; (3) the peak of strength lags behind the peak of dilation angle; (4) the crest value of dilatant angle under unloading conditions is bigger than that under conventional triaxial compression conditions; and (5) volume increasing after the peak strength is still obvious, even when confining pressure exceeds 100 MPa, especially under unloading conditions.