Influence of rock strength on the mechanical behavior and P-velocity evolution of coal-rock combination specimen

To research the influence of rock strength on the mechanical behavior, AE characteristics and P-velocity evolution of coal-rock combination specimen, six kinds of combination specimens with different rock strengths are chosen to carry out the ultrasonic wave and uniaxial compression tests. During the loading process, AE signals are monitored. Then, a new method for calculating the P-velocity is given and the relationship between the P-velocity and mechanical parameters of coal-rock combination body is discussed. Results show that: (1) As the rock strength of the combination specimen increases, its peak compressive strength increases first and then turns constant while its elastic modulus keeps increasing; meanwhile, the failure model transfers from combination specimen failure to coal dynamic failure. (2) AE energy rate changes from multi-peak to single-peak near the peak stress of combination specimen as increasing the rock strength, which indicates that the released energy of combination body greatly increases in one time. (3) P-velocity of combination specimen is between the rock and coal, but is closer to coal. P-velocity of combination specimen experience an increasing-steady trend with the increase of rock strength. (4) A new method using P-velocity of rock and coal to obtain P-velocity of combination specimen is proposed, which is in good agreement with the test results. Additionally, we found that there exists a linear relationship between the P-velocity of coal-rock combination specimen and the elastic modulus. But no distinct relationship can be determined between the P-velocity of coal-rock combination specimen and the peak stress. This method provides a new idea for predicting the capacity of energy storage of coal-rock combination structure in the filed practice.