Design considerations for an underground room in a hard rock subjected to a high horizontal stress field at Rana Gruber, Norway

Rana Gruber AS is an iron mining company in the North of Norway, and it operates Kvannevann mine 30 km east of Mo i Rana. The Kvannevann mine is located in a foliated gneiss host rock, with an ore body about 70 m wide and more than 300 m deep. The mine has been in operation for many years using sublevel stoping, and is now changing to sublevel caving. Experience from past mining activity in the infrastructure preparation for the new mining method indicates that the mine is located in a hard, brittle rock mass with high horizontal stresses. Stress measurements have been made from time to time. The measurement results indicate a major principal stress of 20 MPa perpendiculars to the strike of the ore, and a minor principal stress of 10 MPa parallel to the strike of the ore, which is 10–15 times higher than the theoretical vertical stress caused by gravity at the measured location. In addition to the high horizontal stress, lessons learned regarding failure and rock support during the underground excavation need to be considered for designing and excavating a new canteen room (B × H = 9 × 8 m) at the mine. Numerical modelling is utilised to be able to include all of the adverse conditions for consideration.