Effect of the Contact Angle in the Failure Pattern in Slate Under Diametral Compression

Abstract

The failure pattern in slate tested under diametral compression is strongly influenced by the contact angle created in the loaded area. A small angle is created throughout the contact between the jaws (or platens) and the disc by adopting flat platens (ASTM standard) or curved jaws (suggested method by ISRM) load configurations, inducing failures due to shear stresses in the limit of the load rim. This causes failure patterns that do not exactly match the central diameter when the material is tested with the load direction parallel and perpendicular to foliation, which contradicts the failure by pure tensile typically assumed in the test for these orientations. In the present work, a new interpretation of the failure pattern in slate samples tested with the Brazilian method is established, playing the contact angle a significant role. Brazilian tests with the loading direction along and across foliation were carried out in the laboratory, by using the load configurations of ASTM standard and the ISRM recommendation. Furthermore, an analytical study allowed the estimation of the point in the whole of the disc in which a critical stress state is firstly reached by taking into account both the foliation and the intact rock failure criteria, hence justifying the experimental failure patterns. Finally, regarding the initial failure the influence of the strength properties in order to satisfy the classic hypothesis of the Brazilian test in rocks with multiple weak planes was analysed. An appropriate interpretation of the failure pattern can be an important indicator in order to reveal the true failure mechanism of rocks; this may help to improve the characterization and prediction of the initial failure of this material, which is widely used as an industrial rock or in dam foundations, underground excavations and slope engineering.