Analysis of Raise Boring with Grouting as an Optimal Method for Ore Pass Construction in Incompetent Rock Mass—A Case Study

The construction of ore pass systems in underground mines is a high-risk activity, especially in an environment with incompetent rock mass. This study aims to investigate the optimal method for ore pass construction in incompetent rock masses. We evaluated the conventional and raise boring (RB) methods based on safety, efficiency, excavation control, and ground support for ore pass construction. We also performed a stability analysis using analytical Q-raise (Q_R method) and kinematic analysis methods for ore pass construction with a Raise Borer before and after grout injection of the rock mass. As a case study, an ore pass (diameter, 3 m; depth, 100 m) within an incompetent rock mass was considered to gain further insight. The rock mass was characterized according to the classification methods Q Barton, rock quality designation (RQD), rock mass rating (RMR), and geological strength index (GSI). The grout intensity number (GIN) method of grout injection is used. The safety factor (<1.075) obtained before injection was lower than the acceptance criteria in all sections of the rock mass. However, grout injection before Raise Borer excavation resulted in a rock mass safety factor greater than 1.5. Using RB without pre-grouting in the case study indicated that the maximum unsupported diameter (MUSD) of the ore pass was less than the required 3 m. On the contrary, an MUSD of the rock mass post-grouting was equal to or larger than 3 m at all depths.