ABSTRACT:

Rana Gruber is an iron mine in the North of Norway. It is located in a foliated gneiss host rock, and the ore body is about 70 m wide and more than 300 m deep. The ore starts from the surface and continues to go down for more than 300 m deep. Due to this situation, mining cost could be reduced by changing the mining technique from sublevel stoping to sub-level caving. Many underground works have been considered such as field tunnel, transport tunnels, drilling tunnels, shafts and a crusher hall. However, this paper is limited to discuss rock excavation and support of the crusher hall only. The hall is 14 m wide and 23 m high, and need to be designed to accommodate the high horizontal stress and in an “elastic behaviour” rock mass. Numerical models are built for the purposes. This is an updated version of a paper published in Oslo [1].

1. INTRODUCTION

Rana Gruber is an iron mine, located about 30 km east of Mo I Rana, owned by Leonard Nilsen & Sons. The mine has been in activity for more than 200 years and open pit mining, sublevel stoping have been used. However, a new ore body of about 70 m wide, 1 km long and going more than 300 m deep requires a different excavation method. Due to deep development and thin body, it is more economic to apply the sub-level caving technique and the owner has decided to go for this option. To accommodate the sub-level caving mining method, number of underground works need to be planned. The structures are including transport tunnels, field tunnels, drilling tunnels, shafts and crusher hall (Figure 1). The whole system is complex, and many issues need to be solved. However, this paper is limited to some rock mechanic issues, excavation procedure and support considerations for the crusher hall only. The crusher hall has a large dimension, and the largest cross section is 14 m wide and 23 m high. This hall is located about 300 m below the surface and about 120 m distance from the ore body excavation, so it may be in the affected stress area. In addition to that, the whole area is subjected to a high horizontal stress. Many rock bursting incidents in the excavating infrastructure tunnel suggests that the planned crusher hall should be carefully studied.

2. HIGH HORIZONTAL STRESS AND “ELASTIC BEHAVIOUR” ROCK MASS

At the time of this study, more than 4 km of the infrastructure tunnel has been excavated. The tunnel is started at ground surface of about 420 m.a.s.l and already reached to elevation of 123 m.a.s.l. Many observed rock bursts along the tunnel convincingly demonstrate the high horizontal stress problem. In many locations, where bolts are not sufficient, rock is popping out from wall bringing along with them the fibre reinforced shotcrete. Rock bursting is also experienced at the tunnel face, which requires shotcreting instead of leaving it unsupported.

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