Disturbance of mining operations to a deep underground workshop

Abstract

An underground workshop was located at a depth of 880 m in a metal mine, Sweden. A few years after its excavation, the workshop was subjected to a certain extent of damage. A number of fractures appeared on the walls of the workshop. Shotcrete separated from the rock surface and even fell down to the floor. The mine worried about what happened and wondered how the mining operations underneath would affect the workshop. A further question was if the workshop should be closed and a new one should be excavated. Those questions therefore concerned not only with the matter of safety but also of economy. An investigation of the instability in the workshop was conducted in 2000. It was carried out through geological surveys, in situ deformation measurements and also numerical modelling. The study revealed that the problem of instability in the workshop was associated with the mining operations at the level of the workshop even though the mine stopes were over 90-m away from the workshop. It was also assessed that the mining operations underneath would not disturb the workshop and thus the workshop could continue its service as usual. The mining operations underneath was finished in 2003. The results of a follow-up programme confirmed the assessments made in the study. Measurements of rock deformation showed that the workshop was stable during the whole period of mining underneath. Two lessons are learned from this case: (1) permanent underground infrastructures should not be placed in geological contact zones even if they are not necessarily weak zones; (2) the horizontal extent of the disturbance zone around an opening at depth can be very large. For the case in question, the horizontal stretch of the disturbance zone around the mine stope was approximately in the same order as the height of the stope.