Use of mineralogical analysis in geotechnical assessment of rock strata for coal mining

Abstract

The fundamental geotechnical properties of sedimentary rocks from the coal-bearing Sydney Basin of New South Wales, Australia, including density, porosity, water absorption and moisture content, as well as compressive strength, have been related to the mineralogy of the materials as determined by quantitative X-ray diffraction of powdered rock samples and processing by the Rietveld-based Siroquant technique. The results show, for example, that the overall proportions of clay minerals in the rocks, as well as the types of clay minerals present, affect properties such as water absorption and moisture content. The presence of clay minerals also adds to the dry strength of silica-cemented quartz sandstones; however, the strength of finer-grained mudrocks is inversely related to the total percentage of clay minerals. Correlations have been established between the deterioration or slaking behaviour of mudrocks in water and their clay mineral content, based on either simple immersion tests or on more sophisticated slake durability index determinations. These relationships may be of significance in understanding the longer-term behaviour of rock strata when exposed in different types of mining operations. The propensity of different rocks to ignite methane in underground mines by frictional effects has also been shown to depend on the mineralogy of the rock materials, whether determined by point counting of thin sections or by Rietveld-based XRD analysis. Abundant quartz, feldspar, and rock fragments in sandstones, for example, increase the likelihood of frictional ignitions, whereas clay minerals and (especially) carbonates in the rock reduce the frictional ignition potential. Pyrite in the rocks, if present, also increases the ignition risk; pyrite undergoes exothermic oxidation at high temperatures whereas other minerals respond by simple heating due to rock-on-rock or pick-on-rock friction processes.