Laboratory study on the time-dependent behavior of intact and failed sandstone specimens under an unconfined condition with the relaxation test

This research reports the laboratory relaxation tests investigating the time-dependent behavior of intact and failed sandstone specimens under unconfined condition. The results indicate that the stress relaxation curves show smooth relaxation behavior in the pre-failure region and show staircase type relaxation behavior in the post-failure region. The failed specimens experienced sudden and large stress drops after a stage of smooth relaxation and as a result, more stress is relaxed from the failed specimens than the intact ones. In addition, the radial-strain-time curve during a relaxation test shows similar trend as creep. No third stage was observed for the relaxation test in the pre-failure region while in the post-failure region, the radial strain accelerates to failure in a short time after a few cycles of jump and stabilization. The sudden changes in the radial strain are likely induced by the failure propagation or shear-off of asperities along the major shear failure plane. The radial strain is much larger than the inelastic axial strain, especially in the post-failure region, demonstrating that the time-dependent deformation concentrates in radial direction once the specimen fails. Furthermore, any change in the radial strain curve coincides with the change in the stress curve during a relaxation test, and the regression analyses show that they are linearly correlated. The sensitivity of stress to radial strain depends on the failure condition of the specimens.