Analyzing the Effect of Bolt Spacing on Coal Rib Stability Through Numerical Simulation

Abstract

ABSTRACT: Rib falls pose a major safety risk in underground coal mines, and there is currently no standardized approach for rib support design in the U.S. coal mines. To tackle this issue, a standalone application, Design of Rib Support (DORS), was developed for rib stability assessment and rib support design under development load. The last step of the rib support design in DORS was to calculate the bolt spacing from the recommended rib support density, bolt grade and number of bolts. Notably, the change of bolt grade and the number of bolts significantly affects bolt spacing, but it remains unclear how varying bolt spacing impacts the supported rib factor of safety (RibFOS). In this study, numerical simulations were conducted to analyze the influence of bolt spacing on coal rib stability. Initially, plane-strain 3DEC models were used to estimate the RibFOS of unsupported solid coal ribs, serving as a baseline for rib stability evaluation. Subsequently, 3D models were conducted for supported solid coal pillar ribs. A parametric study was conducted to investigate the influence of bolt spacing on the supported RibFOS under different conditions, such as varying overburden depths and bolt lengths. The simulation results show a general trend of reduced supported RibFOS with the increase in bolt spacing, and the trend can be further affected by varying conditions. The variation of supported RibFOS with bolt spacing can be quantified by normalizing the supported RibFOS to its values with a bolt spacing of 0.3 m. Finally, when the rib support density was maintained by varying bolt grade and bolt spacing, the influence of bolt spacing can still be observed in the results. Thus, it is essential to consider this bolt spacing effect in rib support design. 1. INTRODUCTION Coal pillars are intentionally retained during mining to support overlying rock strata and prevent mine roof collapse and play a crucial role in maintaining the structural integrity of underground coal mines. When coal is extracted from a seam, the overburden load and/or abutment load tend to transfer and redistribute to the remaining pillars, leading to excessive pillar load. Coal ribs are the walls of coal pillars with free surfaces. Without rib support, the coal mass has low or no confinement, leading to lower strength and less constraint for deformation. The occurrence of rib falls presents severe safety risks, encompassing injuries, fatalities, and equipment damage. According to the Mine Safety and Health Administration (MSHA), statistics during the 10-year period between 2010 and 2019, rib-fall accidents contributed to 45.2% of the ground-fall fatalities (Rashed et al. 2022). Thus, rib falls pose a major safety hazard in U.S. underground coal mines.