Performance of multi-stage Triaxial tests on rock deformation

Abstract

Knowledge of the mechanical response of rocks under different loading conditions is vital for rock engineering applications. Triaxial testing simulates in-situ stress conditions to evaluate the mechanical behavior of rocks. Conventional single-stage triaxial testing requires multiple samples to develop a whole failure envelope whereas multi-stage testing gives the same results using a single sample. This study determines the peak and residual strengths of Kimachi sandstone, a medium hard clastic rock, under different confining stresses through both testing methods. Peak strengths were measured at zero and non-zero pore pressures. Residual strengths in the multi-stage test were estimated by two methods: the Descending Pc and Ascending Pc approaches. The Descending Pc approach is started at a higher confining pressure, whereas the Ascending Pc method is started on low pressure. Results showed peak strengths in multi-stage testing were closely similar to those in single-stage tests, which demonstrated reliability of the approach. Minor differences in multi-stage outcomes are attributed to accumulated microcrack development. The residual strength outcomes were different because of the different deformation behavior under two approaches. Overall, multi-stage triaxial testing is shown to be a dependable and convenient method of assessing mechanical response of medium-hard rocks like Kimachi sandstone.