Damage accumulation during triaxial creep of darley dale sandstone from pore volumometry and acoustic emission

Abstract

We performed triaxial creep tests on water-saturated samples of Darley Dale sandstone to investigate the effect of pressure on the process of time-dependent brittle deformation under all-round compression. Axial strain, acoustic emission (AE) output and pore volume change were monitored continuously during each test. We observed the three classical creep regimes (primary, steady-state and tertiary). The level of applied differential stress has a crucial effect on the creep rate and on the time-to-failure; from 30 minutes at 90% of the short-term strength to almost one day at 80%. For the experiments performed at the lower levels of stress, the duration of the primary creep phase increases, while the acoustic emission level during the steady-state phase decreases dramatically. The variations of axial strain and differential pore-fluid volume are more regular when the tests are conducted at stresses closer to the strength of the material. AE measurements suggest that the final stage of the deformation occurs for similar levels of cumulative events and cumulative AE energy, regardless of stress level. The same comment can be made for the pore-fluid volumometry results. This suggests that the final stage that leads to failure occurs for almost the same level of damage in all samples.