ABSTRACT
The most important rock mechanical factors at the Pyhäsalmi 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 Pyhäsalmi 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.
