Double Peaked Stress–Strain Behavior and Progressive Failure Mechanism of Encased Coal Pillars Under Uniaxial Compression

To investigate the failure mechanism of reinforced coal pillars and compare their pre- and post-reinforcement bearing capacity, a series of acoustic emission (AE)-monitored uniaxial compression tests were carried out on three types of specimens, namely coal, concrete, and concrete-encased coal. The results show that the stress–strain curve of encased coal sample shows obvious double-peaked behavior, different from encasing concrete or coal alone. This characteristic can be attributed to differences in the mechanical properties of the internal coal and encasing concrete. The encasing concrete is of a high stiffness than coal, absorbing high stress than coal during deformation. The encasing concrete has a higher elastic modulus and will take a larger share of the load before failure occurs. The internal coal with lower strength and peak strain will cause it to fail first, and the load will transfer to the stronger concrete in the surrounding. The evolution of the AE energy release and AE event locations both confirm this failure process. In addition, the strength of encased coal samples falls between coal and concrete, and the stress values of the double peak can be estimated by the presented formula, The maximum supporting capacity of the reinforced coal pillar was closely related to the radius ratio and elastic modulus ratio of the individual parts, and the corresponding bearing capacity of the reinforced coal pillar can also be calculated. This study provides a useful reference for understanding the failure mechanism of reinforced coal pillars.