A novel apparatus to determine the rock strengh parameters

This paper presents a portable apparatus, the Rock Strength Device (RSD), and the methodology to assess the strength of rock from ''partially-destructive'' scratching tests. The RSD measures the normal and tangential components of force applied to a cutter while making a groove at a constant depth on the surface of a rock specimen with sharp or blunt tool. Experiments indicate that rock cutting is associated with a 'ductile' or a 'brittle' mode of failure depending on the depth of cut. The ductile mode takes place at shallow depth of cut and is associated with plastic flow, while the brittle mode occurs above a threshold depth of cut and is characterized by the propagation of tensile crack. In ductile mode, experiments with sharp cutter show that the energy required to remove a unit volume of rock, referred to as the intrinsic specific energy e, is well correlated to the uniaxial compressive strength q. Furthermore, the friction coefficient µ mobilized along the wear flat of blunt cutter is found to be well correlated to the internal friction angle f of the rock. It is also possible to capture a thin layer of weak material or heterogeneity along the rock core.