A new approach for measuring the shear strength of cleated coal

The shear strength of coal cleats is a key input parameter for many engineering aspects of coal seam gas extraction, such as the selection of the equivalent circulating density (ECD) for drilling and the analysis of borehole stability. The focus of existing studies has been on determining coal shear strength using intact coal samples, which is then downgraded to estimate bulk coal strength by considering the cleat characteristics such as density and roughness. This approach is considered to be acceptable when trying to understand coal bulk behavior but is less representative of the actual coal failure mechanism, which is typically dominated by the cleat strength. In this work, a new approach was applied to directly measure both the peak and residual shear strength of coal cleats for two different types of coal from the Bowen Basin, Australia. The results show a wide distribution of peak shear strength, residual shear strength, cohesion, and friction angle for the tested coal samples. An exponential correlation was also observed between peak shear strength and the cleat aperture. The findings of this work represent a useful and important data set of the shear strength of coal cleats, which would benefit almost all stages of coal seam gas extraction and geotechnical aspects of coal mining.