Evolution of the strength characteristics of briquette and raw coal containing fluid

The mechanical properties of coal containing fluid are an important factor affecting the safe mining of soft coal seams. In particular, for class III-V coal, coal and gas outbursts and other dynamic phenomena are prone to occur due to the influences of gas pressure and groundwater, which seriously threaten the safety and lives of field workers. However, briquette samples are usually used in place of raw coal in laboratory tests conducted on class III-V coal samples. Whether the research conclusions for briquette and raw coal are consistent and whether briquette coal can replace raw coal in research on strength characteristics need to be further verified. In this paper, the evolution of the strength characteristics of fluid-bearing briquette coal and raw coal is studied. The strength characteristics, instability failure characteristics, and acoustic emission characteristics of raw coal and briquette coal under uniaxial and triaxial compression are analyzed in detail. In addition, the influence of the water content and pore pressure on the strength characteristics of class III-V raw coal and briquette coal is further studied. The results show that the failure characteristic of raw coal is overall brittle failure, mainly axial splitting failure, whereas that of briquette is overall ductile failure, mainly cone-shaped continuous spalling. The strength parameters of the raw coal and briquette coal improve under confining pressure, but the internal difference in the raw coal is significantly reduced. The cohesion of the raw coal sample initially increases and then decreases with increasing water content, and the internal friction angle increases with increasing water content. In addition, it is verified that the strength, cohesion, elastic modulus, and deformation modulus of the briquette decrease with increasing pore pressure under different pore pressures, but the strength difference of the class III-V coal decreases under increasing pore pressure. Based on the abovementioned results, the strength parameters of a coal body are estimated using the Hoek-Brown (H-B) criterion. Based on a comparison of the strength parameters of the coal sample and coal body, the estimated strength parameters of the coal body are closer to the actual values on site.