Analysis of raise boring with grouting as an optimal method for ore pass construction in incompetent rock mass — A case study

This study aims to investigate the optimal method for ore pass construction in incompetent rock masses. We evaluated the conventional and raise boring methods based on safety, efficiency, excavation control and ground support for ore pass construction. A stability analysis using analytical Q-raise (QR method) and kinematic analysis methods for ore pass construction with a raise borer before and after grout injection of the rock mass was performed. As a case study, an ore pass (diameter of 3 m, depth of 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 was used. The safety factor of less than 1.075 obtained before injection was lower than the acceptance criteria. However, grout injection before raise borer excavation resulted in a safety factor greater than 1.5. Using a raise borer without pre-grouting indicated that the maximum unsupported diameter (MUSD) of the ore pass was less than the required 3 m and equal to or larger than 3 m post-grouting. Background An ore pass construction with a raise borer incorporates the use of equipment that performs the following tasks: (1) drilling of a pilot hole from one level to another and (2) reaming of the pilot hole to a desired section without using explosives to create the vertical tunnel. The use of grout injections during the construction process is typically viewed as a contingency measure. Grout injections can improve the strength of the rock mass and establish an impermeable zone around the periphery of the tunnel.An ore pass construction in rock mass of low geotechnical quality constitutes significant challenges in terms of safety, efficiency, excavation control and ground supports. Several projects have used the raise borer method without an exhaustive analysis of the ground conditions from the prefeasibility study to construction, causing challenges in later stages. The most recent and relevant work for raise borer performance evaluation involves the use of the McCracken and Stacey stability and risk assessment methods with out-of-date empirical stability charts [1]. Penney et al. [2] concluded that without a basic assessment of risks associated with the stability of a formation, the performance of a raise borer could not be determined with reliability. This is a limitation found in current studies. Penney et al. [2] recommended coupling empirical analysis for raise borer assessments with numerical analysis, a strategy that informs the basis of our contribution to literature. Methodology This paper proposes and evaluates the use of a raise borer with pre-injection grouting as a method of ore pass construction in incompetent rock masses. Empirical and analytical models were used for stability analysis before and after the injection of the rock mass with grout when excavating with a raise borer.Specifically, we (1) present a detailed analysis of feasible methods for ore pass construction in weak rock masses, (2) perform rock mass classification before and after grout injection, adjusting for raise boring as a construction method, (3) perform stability analysis using empirical and analytical methods before and after construction using a raise borer and (4) provide a framework for the construction of an ore pass in weak rock masses. Results and discussion Stability analysis using empirical and analytical methods shows that the MUSD of the rock mass pre-grouting is less than the 3-m diameter of the ore pass. However, both methods show improved rock mass stability post-grouting, with safety factors greater than the acceptance criteria and MUSD equal to or exceeding 3 m (Fig. 1). The rock mass was stable in all directions of the wedge formation post-grout injection. The safety factor per meter of ore pass depth was calculated. The factor pre-grouting ranged from 0.93 to 1.07, which is below the acceptable criterion (1.5). For the grouted rock mass, the factor improved to a range of 1.5 to 1.745 (Fig. 2). Conclusions The study evaluated the construction of an ore pass pre- and post-injection grouting using the raise borer method in an incompetent rock mass. The case study was of an ore pass construction project, 3 m in diameter and 100 m in depth. The raise borer with pre-grouting was determined to be the optimal method for ore pass construction in rock mass of low quality. A stability analysis indicated that the maximum unsupported area of the ore pass was less than 3 m pre-grouting and greater than 3 m post-grouting at all depths. Additionally, the safety factor before grouting was less than 1.075. The analysis confirmed that the grouted rock mass achieved a safety factor greater than the acceptance criterion (1.5) in all directions. Selected references Cajahuanca Sánchez J (2019). Influencia del sistema Raise Boring en la ventilación de la Zona I-B NV 4530 de la Veta Alexia de la Unidad Minera Arcata - Cía Minera Ares S.AC (Influence of the Raise Boring system on the ventilation of Zone I-B NV 4530 of the Alexia Vein of the Arcata Mining Unit – Cía Minera Ares S.AC), [Thesis, Universidad Nacional del Centro Del Perú] Penney AR, Stephenson RM, Pascoe MJ (2018). Raise bore stability and risk assessment empirical database update. In Proceedings of The Fourth Australasian Ground Control in Mining Conference, Sydney, Australia November

Analysis of raise boring with grouting as an optimal method for ore pass construction in incompetent rock mass — A case study

Authored by: Cluber Rojas, Angelina Anani, Eduardo Cordova, Wedam Nyaaba, Sefiu O. Adewuyi and Edward Wellman

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