Determination of joint mechanical parameters for stability analysis in low stress open pit mines

For low stress mining and civil engineering projects, slope stability is an essential part of safety and financial considerations. While large-scale stability can be modelled using equivalent rock mass properties, at smaller scale the local variations become significant and failure along the fracture planes is possible. The relevant boundary condition for low stress conditions is the Constant Normal Load (CNL), which allows for dilatation to occur. For deep mines the corresponding condition is the Constant Normal Stiffness, which restricts the dilatation. When dilatation is supressed the normal load is increased. This may lead to shearing of the asperities. If both the vertical and horizontal displacement are recorded during CNL testing, the dilatation may be calculated and numerically removed to provide a CNS estimate. In this paper three rock joint samples from the Siilinjärvi open pit mine were tested in a CNL shear box using displacement resetting at three different low normal loads. Profilometer and photogrammetry were used to measure the roughness of the surfaces. The results show poor match between the expected behaviour and observations. One possible cause is the reduced matedness often associated with natural near-surface rock joints. The potential weaknesses of the method are discussed and future research topics are suggested.