The most important rock mechanical factors at the ?yhgsalmi mine affecting stope design are the strong, stiff orebody surrounded by a weak country rock and a high, 30 70 MPa, horizontal stress field. Due to the low unit value of the ore and the high mill capacity, open stoping is applied at the mine. Low dilution is essential for the economic survival of the mine but at the same time the stope size should be as large as possible to obtain high productivity and low unit costs. In practice, quite extensive rock reinforcement and stress control measures are found viable if open stoping can be used instead of conventional cut-and-fill. In stope design, numerical modeling is today a frequently used tool but a lot of input data is needed for the modeling. Material properties are composed from laboratory tests and back-calculations using the results of displacement measurements. The FLAC-program using a finite difference method is used for numerical modeling. There are three main methods employed for stress control at Pyhasalmi mine: sequential stoping technique, yielding pillars and destressing using slot stopes. Rock bolting, shotcreting and cable bolting are widely used both in drifting and stope reinforcement. Special cable bolt design layout, parallel or "mandolin" cable bolting has been found effective because it can tolerate large rock displacements.
The Pyhgsalmi mine is situated in central Finland, about 470 km north of Helsinki. Annual mine production is 1 000 000 t. The mill produces 30 000 t copper concentrate, 50 000 t zinc concentrate and 630 000 t pyrite concentrate per annum. The orebody is steeply dipping and its length is 300 500 m. The orebody width is 6 40 m. The orebody is known today to a depth of 900 m. Geologically the orebody is associated with volcanic rocks.
The ore itself is strong and very stiff, but during the ore formation process, the country rocks next to the ore altered to sericite quartzite and mica rocks, which are very weak and schistose. The thickness of altered zone is about 10 30 m at both sides of the orebody. Beyound of the altered zone are leptites. The strength and stiffnes of the different rock types vary considerably (Table 1). The high horizontal stress field is another problem in mining. The major principal stress is horizontal and almost perpendicular to the orebody. The magnitude of the horizontal stress varies from 30 MPa in the near surface part of the mine to 70 MPa at depth.