Wetting-Induced Collapse of Crushed Sedimentary Rocks Under High Stress

This study investigates the evolution of the porosity of crushed sedimentary rock masses in one-dimensional compression for dry and inundated conditions with wetting-induced collapse at different stress levels in the context of underground pumped hydro energy storage (UPHES) facilities, where the collapsed underground void of a disused mine serves as the lower reservoir. Three fine-grained rock lithologies typically present in underground mine roofs, indurated mudstone, interbedded sandstone-mudstone, and massive siltstone with unconfined compressive strength (UCS) values in the range of 25 to 157 MPa, were subjected to constant strain rate compression tests up to a stress of 12 MPa, with different specimens of each lithology inundated at stresses of 50 kPa, 2 MPa, 5 MPa or 12 MPa. The results show that the weaker the rock, the greater the reduction in porosity under stress. The porosity reduction upon collapse is independent of the magnitude of the applied stress for the stress range considered in the tests, but its magnitude depends on the material. It is also observed that the porosity of the crushed rock samples in dry conditions under the maximum stress level (i.e., 12 MPa) increases linearly with the UCS on a semi-logarithmic scale. The stress-strain curves were scaled to account for the size of the rock boulders forming a real goaf, which is usually found in disused underground mines. It was found that the change in porosity upon collapse is not scale-dependent and, hence, can be satisfactorily characterised at the laboratory scale, provided that the particle size distribution is adequate.