Large Underground Excavation Support for Ore Exploitation

"Among other components, ore body „T“ was discovered in the Bor pit copper mine at a depth of app. 500m. The ore body is relatively small: 60-65m long, 40-45m wide and up to 50m high, but with an extremely high content of copper and accompanying metals. The rock mass containing the ore body is composed of altered andesite of relatively poor physical-mechanical characteristics. The exploitation of this ore body by way of conventional mining methods was associated with high risk, cost, and potential losses of the rich ore. Due to everything specified hereinbefore, the adopted solution involved ore exploitation with complete ensuring of excavation stability, thus forming an extremely large underground opening. The excavation and supporting were performed according to the NATM principles along with applying shotcrete, 9-25 m long cables, and horizontal and vertical lattice girders. The excavation of the ore body was completed in app. 2.5 years, all the while ensuring complete stability and security of people and equipment; construction works were constantly monitored. The proposed solution for ore excavation enabled more than 95% exploitation of ore as requested by the Investor, while depletion due to waste-rock excavation was less than the allowed 5%. INTRODUCTIONThe Bor copper deposit is located in Eastern Serbia. Copper exploitation in Bor commenced as far back as 1902. A total of 26 ore bodies varying in size and valuable mineral content have been excavated so far. One of the most recent excavated ore bodies is ore body „T“, which is one of the smallest bodies by volume, but also a body with one of the largest content of copper and accompanying metals. An important fact regarding its exploitation is that it is located at a depth of app. 500 m; that its shape is irregular: 60-65 m long; 40-45 m wide and app. 50 m high; and that it lays within a rock mass of relatively poor physical-mechanical characteristics. In addition, above it, in its immediate vicinity, ore body „F“ was previously excavated and its room was refilled with backfill and water. Excavation methods common in mining practice (excavation with caving, excavation with backfilling, excavation with protective pillars) were inadequate in this case for various reasons:• The immediate vicinity of the previous ore body „F“ room could cause expansion of the caving zone also to that room, along with penetration of groundwater and flotation sludge;• Possibility of deformation expansion caused by caving as far up as the surface of the terrain, including the urban area;• Difficulties in maintaining the stability of excavation sidewalls having steep slopes and great height;• Significant loss of ore mass with a relatively high content of metals etc."