Risk assessment approach for rockfall hazards in steeply dipping coal seams

Many examples have shown that rockfalls can occur in underground coal mines. The presence of rockfalls in the long walls of a steeply dipping coal seam (SDCS) is harmful due to the cumulative damage effects of rockfall near active mining faces. Therefore, this study aims to assess rockfall hazards in SDCS mining and propose corresponding protective measures. The steps are as follows: Step 1: The on-site coal wall spalling was investigated. Information such as the shape, size, and location of the detached block was collected. Step 2: The risk indicators Ē and ĒCOR,BE that characterize the cumulative damage effects of rockfall were proposed. Step 3: A small-scale impact test was performed in the laboratory to study the rockfall characteristics, and a sensitivity analysis for the risk indicators under different block diameters, seam inclinations, and longwall floor Schmidt hardness was conducted. Step 4: A risk assessment model for rockfall hazards was established. Rockfall risks can be divided into five degrees based on the distribution of Ē and ĒCOR,BE. Among them, the laws of Ē and ĒCOR,BE were obtained through Step 3. ΔE can be obtained under two scenes, with or without equipment, on the longwall floor. Ed is the critical energy for equipment destruction, which is related to the equipment materials. Step 5: The risk assessment method for rockfall in longwalls of an SDCS is proposed based on evaluating Ē and ĒCOR,BE. Specifically, the risk assessment model was combined with the on-site damaged hydraulic support, and then, the verification of the risk assessment method was accomplished through a comparison of the small-scale impact test in the laboratory and on-site damage measurements. Finally, three protective principles were discussed. This study provides a method for risk assessment and for determining the principles of protective systems in underground steep coal seams.