Abstract: For the large open-pit mining, there is still a large amount of residual coal in highwall mines. However, the reasonable width of rib pillar is always a difficult problem, which has greatly restricted the high-efficiency application of highwall mining system. The working faces in highwall mining were assumed as a series of co-linear cracks with certain spacing interval. Based on theⅠ-Ⅱ composite crack model in fracture mechanics, the stress distribution at the edge of rib pillar was analyzed, combined with Hoek-Brown failure criterion and Mohr-Coulomb failure criterion. Besides, the boundary equations of damaged zone at the edge of rib pillar were established. Accordingly, the calculation formulas of damaged width of rib pillar were obtained under different failure criteria, respectively. The curve laws were presented on the basis of the theoretical formulas of damaged width of rib pillar under different failure criteria and three sets of characteristic parameters for determining different states of coal body damage. According to the curve laws under different failure criteria, the effects of independent factors, such as the width of retaining coal pillar, , the width of cavern, , burial depth, H, and the dip angle of coal seam, ?, on the damaged width of rib pillar, , were analyzed. Meanwhile, the data of field tests were substituted into the theoretical formulas under different criteria. The theoretical results and measured data showed that the mutative laws obtained by applying the Hoek-Brown failure criterion accorded with the evolutional laws of rib pillar damage of actual field better, and the application of Hoek-Brown failure criterion was more suitable for the analysis of fracture of jointed rock mass. In addition, the mechanism and process of rib pillar destabilization were described by considering the limit failure theory and the asymptotic failure theory in the stability study of coal pillar. This paper provides a new approach to calculate the reasonable width of rib pillar and the application of criteria in theoretical analysis of pillar failure, which is significant for popularizing the highwall mining technology.

Key words: highwall mining, rib pillar, fracture mechanics, damaged width, strength criterion, crack