Dynamic Compression Behavior of Coal Under Different Initial Gas Pressures

Coal seams are often affected by gas effects in addition to static and dynamic superimposed loads in the process of coal mining. Understanding the mechanical properties and energy dissipation of impacted coal under different initial gas pressures is extremely important. Therefore, dynamic compression experimentation of coal samples was conducted using a self-developed observable combined dynamic and static loading test system of gas-bearing coal. Furthermore, based on the crack volume increment, the crack energy dissipation density (CEDD) was defined to measure the energy required for a unit volume of cracks under different gas pressures, and the following conclusions were drawn: the deformation of gas-bearing coal can be divided into the elastic stage, elastic‒plastic stage, plastic stage and failure stage, and the dynamic strength and secant modulus decrease with increasing initial gas pressure. The impacted coal contains annular parallel cracks that run through the cross section and axial splitting/thoroughgoing cracks, and the two forms of cracks become more obvious with increasing initial gas pressure. The reflected energy of gas-bearing coal increases and the transmitted energy, dissipated energy and energy dissipation ratio decrease with increasing gas pressure. In addition, the CEDD decreases with increasing gas pressure because free gas and desorbed gas participate in external expansion work at the moment of impact, which results in a decrease in the CEDD required for gas-bearing coal failure. These conclusions enrich the basic theory of the dynamic coal–rock–gas disaster induction mechanism and can provide theoretical support for the monitoring, early warning and prevention technology of dynamic disasters in composites.