Mechanical Responses in Rocks with Different Lithologies Under Mining Loading–unloading: An Insight by Energy Damage and Ultrasonic Characterization

Underground mining disasters are generally caused by the deterioration in mechanical properties of coal and overlying rock exposed to mining loading–unloading, while their differential mechanical deterioration and catastrophe mechanisms are not well understood. Therefore, a series of uniaxial loading–unloading tests with ultrasonic monitoring are performed on coal and roof sandstone, while the mechanical responses are investigated using energy-based damage analysis and ultrasonic characterization method. The sandstone yields a greater strength and deformation modulus and experiences a brittle failure, while the coal possesses a larger Poisson’s ratio and undergoes a ductile failure. A higher (lower) crack initiation (damage) ratio in coal reflects its faster crack propagation than sandstone. Energy evolution analysis shows that coal has a poorer elastic energy storage capacity, but its energy dissipation is severer. The energy-type catastrophe criteria for coal and sandstone are derived based on an elastic energy ratio. A new damage variable is defined by the cumulative dissipated energy and total input energy, and the damage energy release rate is introduced to develop a damage evolution model under uniaxial loading–unloading condition, from which it is indicated that the coal experiences a greater damage degree, but is less dependent on energy release rate. Furthermore, the differential energy controlling mechanisms of instability between coal and sandstone are revealed, i.e., the coal is controlled by pre-peak energy dissipation and post-peak energy release, while the sandstone is dominated by post-peak rapid energy release. Meanwhile, different wave velocity evolution patterns are generalized, where the denser sandstone yields a higher stress sensitivity of wave velocity. The structural degradation process is quantitatively characterized by using a combination of dynamic elastic parameters, intactness index, and disturbance damage factor. The structural deterioration occurs in the unloading and final loading failure stages, where coal shows a progressive pattern, while sandstone possesses a transient pattern. Finally, the quantitative relationships between wave velocity and stress, energy and damage are established.