Characterization of fracture surfaces in brittle rocks

It is well known that the rock mass mechanical properties, both in terms of deformability and strength, are deeply influenced by the presence of discontinuities. Their characterization is of paramount importance in any engineering design of rock related projects. This thesis presents an experimental campaign aimed at investigating the geometrical and mechanical characteristics of fracture surfaces obtained by tensile splitting. Four different rocks have been considered: Balma Syenite, Absolute Black Gabbro, Pink Porriño Granite and Carrara marble. Prismatic samples have been tested for determining the deformability properties, by means of unconfined compression tests. Then, for each sample a tensile fracture has been induced by means of Brasilian-like tests. Ultrasonic tests have been performed on both intact and fractured specimens to evaluate the influence of the presence of the fracture on the propagation of the sonic waves. The geometrical characteristics of the induced fractures have been investigated by analyzing the roughness of a number of orthogonal profiles. Several mathematical methods have been used to quantify the roughness of the profiles. The results have been used to compare the estimation of the Joint Roughness Coefficient given by several empirical laws found in the literature. Moreover, tilt tests have been performed to evaluate the base friction angle of the different rock types. With the aim to assess the effect of the fracture process on the mineral distribution on the exposed faces, saw cut surfaces and rough fractures have been analyzed by processing their photographs.