Environmental effects on the hydro-mechanical behaviour of Ashfield shale

This Thesis presents the results of a comprehensive experimental research aimed at studying the influence of environmental effects, as those caused by cyclic variations in relative humidity (RH) and temperature, on the behaviour of Ashfield shale, a low porosity clayey rock from the Sydney Basin. The experimental program focused on tracking the progressive degradation of engineering properties such as rock stiffness, rock permeability, rock compressibility, rock shear strength and rock tensile strength. Five main aspects were evaluated in this research: (i) the influence of the number of RH cycles, N, (ii) the influence of the amplitude of the RH cycle, ∆RH, (iii) the effects of the stress level, (iv) the effects of low-temperature changes caused by freezing/thawing cycles and (v) the effects of using either liquid water or water vapour during wetting paths on rock degradation. A new high-pressure isotropic apparatus was designed and constructed in this research to induce and track hydraulic degradation of clayey rocks under controlled laboratory conditions. In addition, non-conventional experimental techniques such as bender elements transducers as well as gas pulse tests were implemented in this research to monitor variations in rock stiffness and rock permeability. Consequences of the aforementioned environmental effects on rock microstructure were studied via Mercury Intrusion Porosimetry (MIP) tests as well as Scanning Electron Microscopy with Energy Dispersive Spectrometer (SEM-EDS) analysis. The experimental results showed that the exposure of Ashfield shale to environmental factors such as relative humidity cycling (either via vapour or liquid transfer), freezing-thawing cycles (at saturated and unsaturated conditions) and changes in the confining state, led to a progressive degradation of its hydro-mechanical properties. The interaction of the rock with liquid transfer caused larger degradation compared to tests in which saturation was achieved via vapour transfer. The application of low temperature cycles (freezing/thawing) proved to be the most harmful degradation factor. Rock degradation is strongly influenced by the stress level. Degradation of the shear strength reduced with increasing the vertical stress at which RH cycles were applied. Stress relief after the exposure of RH cycles was also observed to play an important role on the degradation of the shear strength in Ashfield shale. A remarkable reduction in rock shear strength, rock tensile strength, rock stiffness as well as an important increase in rock compressibility and rock permeability was clearly observed in the laboratory tests carried out in this research. This was accompanied by a progressive accumulation of irreversible volumetric strains which seemed to follow the framework of behaviour described by Pineda et al. (2014).