The location of significant volumes of the lower level reserves close to an open pit which produces the adjacent Niorkpakhk deposit considerably complicates underground mining of the Oleniy Ruchey rockburst-hazardous deposit. The progress of underground excavations into the under-pit space is permitted if the undermined rock thickness and open pit walls remain stable. 3D numerical studies of stress-strain state have indicated particularities in deformation such as increase in undermined thickness stability under high tectonic stresses. The parameters of safe mining of temporary inactive reserves were scientifically substantiated. The boundaries of maximal development of underground mining were determined. The stability of overlying rocks and open pit walls is maintained by forming an advance of overlying sublevels relative underlying ones under 45° angle and providing the monitoring for the undermined rock mass. The mining method in a protective pillar the authors propose allows increasing reserves of the first mining stage by 76.9% and avoiding decrease in production volumes when mining operations terminate.
The Oleniy Ruchey apatite-nepheline deposit is located at the Khibiny rock massif, north-western Russia. Nowadays the deposit is being mined by both the open pit and the underground mine. Acting tectonic stresses typical for all the Khibiny deposits [1] and rock proneness to brittle fracture determine the geomechanical state of the deposit’s rock mass. One of the factors restraining underground mining progress is a close location of underground reserves to the adjacent Niorkpakhk open pit. Due to this fact, the considerable part of the Oleniy Ruchey reserves is conserved in a protective pillar (figure 1).
The current instructions [2] use a simplified approach to determine parameters and construction of protective pillars. This approach ignores secondary compression of the rock mass typical for the deposits under study. "Management Instructions of overlying rocks caving" [3] also disregards a factor of high tectonic stresses in the part of construction of protective pillars. The boundaries of the protective pillars are determined by planes under an angle of 75° (or 70° in oxidized zones). The production of underground reserves of the Khibiny apatite deposits with ore and host rock caving mining methods has shown the difficulty to achieve total caving of undermined rocks in analogous conditions. For example, the Apatitovy Tsirk deposit’s ore body thickness is several-fold larger and initial excavation depth is smaller by 100 and more meters. The total caving of undermined rocks took place there while constructing a stoping excavation of 1 km length.