Comprehensive research on the failure evolution of the floor in upper mining of deep and thick coal seam

The Group A coal seam represents one of the most important coal deposits of the Huainan coalfield. It poses a series of dynamic geohazards during mining operations due to its thickness, stratification, and depth location. One of the major issues is the susceptibility of the coal floor to failure under the combined action of mining and pressurized water. At greater depths, the stress becomes concentrated at the tip of the primary fractures under the influence of the high surrounding pressure. The exploitation of coal has caused new fissures to develop along the primary fissures, with energy regrouping occurring at the tips of the new fissures. To better understand the rock deformation and failure of the coal floor, uniaxial loading strain tests were performed on a standard rock specimen based on the Brillouin optical frequency domain analysis (BOFDA) to capture the deformation characteristics on the surface of the rock specimen with high accuracy. During loading, the strain of the fiber optic at peak strength was equal to 4281 με. To further study the evolution of the failure of the coal floor rock stratum during mining, electrodes, and distributed fiber optic were placed in the coal floor monitoring borehole of the coal seam to record the changes in the current and strain. The monitoring results indicate that during the initial stage, when the working face was close to the monitoring borehole, the abutment pressure of the surrounding rock mass gradually increased, the strain increased, and the current value steadily rose. During the middle stage of the monitoring process, the working face was located above the monitoring hole, the upper part of the hole was under compressive strain, the current value increased, the bottom of the hole was under tensile strain, and the electrode current showed fluctuating values. At the late stage of the monitoring process, the working face was located in proximity to the monitoring hole and the mudstone layer. The current value dropped below 1 mA, and the peak tensile strain reached 8589 με, which was followed by an abrupt change, showing the characteristics of rock failure. Comprehensive diagnosis indicated that the maximum floor failure depth was at the 21.0 m mark. During mining, the electrical current and the strain values had a positive correlation when the rock at the bottom was not failure. However, these values suddenly changed after the rock failed. The obtained results can serve as guidance for borehole multi-field monitoring in mining engineering.