Rock Mass Failure and Remedial Measures Implemented When Excavating Ultra-Deep Shaft Stations

ABSTRACT: This paper presents several geotechnical challenges experienced while excavating two shaft stations at the depths of 2490 m and 2550 m in highly stressed grounds, along with engineering solutions that effectively reduce exposure and risk associated with the excavations. Proactive measures were completed ahead of beginning the 2490 shaft station excavation; however, adverse conditions further influenced by a nearby fault led to significant rock mass failure and bowling of the station floor. These unanticipated conditions resulted in project delays and increased risk for the operators working at the face. Based on detailed field observations made while excavating the 2490 shaft station, insights from instrumentation and microseismic monitoring data, the methodology for excavating the 2550 shaft station was modified to mitigate the risk of ground failure. Engineering-based modifications introduced for the 2550 station included reducing the size of the initial shaft station blast, and pre-sinking and supporting the shaft below the station floor before the excavation of the shaft station. These modifications and increased distance from the fault resulted in better control of the ground conditions, leading to a 50% reduction in seismic activity and rockburst risk, and no bowling of the station floor. The outcomes not only resulted in cost savings but also provided valuable insights into the geotechnical intricacies and excavation strategies essential for safe mining operations at extreme depths. 1. INTRODUCTION In the underground mining industry, many operations use shafts to access orebodies. A shaft is a vertical or inclined excavation sunk from the surface or underground. When a shaft is sunk from underground, it is called an internal winze. Operations that utilize shaft infrastructure require the construction of shaft stations. A shaft and its connecting shaft stations are critical infrastructures that provide access for personnel, equipment, material, and the transportation of ore to the surface. (McCarthy & Livingstone, 1993) determined that a shaft is a more viable option over a haulage ramp when the depth is greater than 650 m and the production approaches 1.2 million tonnes per year. To access deep ore deposits, a ramp is inherently slower and requires the removal of more waste rock than shaft sinking.