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.
Rock mass discontinuities such as rock joints or fractures control the failure of rock mass. The normal stress acting on the discontinuity strongly influences shear strength. In deep excavations the surrounding rock mass resists displacement and limits the dilatation. In near surface excavations, the normal load is constant and there is no normal stiffness. Typically two boundary conditions are used: either the near-surface Constant Normal Load (CNL) or the deep Constant Normal Stiffness (CNS) (Figure 1). Poturovic (2015) examined this effect in his Master's Thesis and a synopsis of the results was published as a conference paper Poturovic et al. (2015). Poturovic et al. (2015) argue that friction angle and dilatation are key factors in determining the shear strength. AI high normal loads of 8 MPa or more the dilatation potential is similar between the methods. The results are consistent with earlier results by Grasselli & Egger 2003, where the dilatation potential was shown to be completely suppressed at normal stress of 20 % of the uniaxial compressive stress.