Tracking changes in the specific storage of overburden rock during longwall coal mining

Abstract

The hydraulic properties of geologic formations, such as specific storage and hydraulic conductivity, are known to vary spatially due to formation heterogeneity; however, they may also change temporally if the formation undergoes mechanical disturbance such as occurs during mining. Characterising changes in hydraulic properties is important for groundwater systems that are disturbed by mining. Conventional hydrogeological investigations rely on literature derived values or limited programs of field testing (e.g. pumping test) to define specific storage; but these approaches do not consider the change in compressibility that may occur due to mechanical disturbance. This paper presents, for the first time, direct in situ measurements of the changes in compressibility, and consequently specific storage, as a result of mechanical disturbance within geologic formations overlying underground mining operations. The in situ measurements of compressibility are derived in this study from the pore-pressure response to barometric loading or strains induced by earth tides. Even prior to disturbance, the compressibility obtained from in situ measurements was found to be an order of magnitude lower than that measured on core samples by unconfined strength test in the laboratory. The differences in compressibility for the intact formation are likely due to sample disturbance and the differences in strain level imposed by the field and laboratory methods. The increase in compressibility of the overburden rock varied with the location of the monitoring site relative to longwall extraction and the level of strain the formation sustained. The study found that quantifying the changes in compressibility, and consequently specific storage, as a result of mine-induced disturbance is critical to our understanding of subsidence, the extent of depressurization, and the impact that mining may have on regional groundwater flow systems.