Experimental study on mechanical properties, permeability and energy characteristics of limestone from through-coal seam (TCS) tunnel

This paper focuses on analyzing the mechanical properties, permeability and energy characteristics of limestone from coal measures strata of through-coal seam (TCS) tunnel in the context of the Taoziya tunnel as the engineering background. The improved MTS815 Flex test GT rock mechanics system and acoustic emission (AE) technology were utilized for the triaxial compression with permeability test of limestone from coal measures strata of TCS tunnel. The results indicate that gas pressure and confining pressure significantly affect the mechanical properties and permeability of limestone from coal measures strata of TCS tunnel. The gas pressure promotes the development and propagation of cracks, whereas increasing confining pressure strengthens the resistance of limestone samples to crack propagation. When limestone samples transition from the initial compaction stage to the unstable cracks propagation stage, the possibility of gas leakage during tunneling in the coal measures strata should be recognized, and the stability of surrounding rock should focus on the failure stage. In the process of progressive failure of limestone samples, AE events with larger amplitude are distributed near the main macroscopic fracture zone, and the distribution of AE events closely corresponds to the macroscopic failure mode of limestone samples. Exponential functions can be used to describe the trend of initial permeability k(i), peak permeability k(p) and residual permeability k(r) decreasing with effective confining pressure sigma(e). The closure and propagation of microcracks are the fundamental reason for the corresponding relationship between the stress variation process, AE characteristics and permeability of limestone samples. Finally, the energy characteristics of limestone under triaxial compression were analyzed based on the stress-strain curves of limestone. This study may give fresh insights and necessary references to ensure the safe construction and development of TCS tunnels.