Evaluation of progressive damage of discontinuity asperities due to shearing by means photogrammetric survey

It is well known that the mechanical behaviour of rock discontinuities strongly influences the stability of slopes and fractured rock walls. With this end, particular attention must be paid to the analysis of the roughness of natural discontinuities, which represents a peculiar geometric feature strongly influencing their shear strength. The paper describes an experimental procedure carried out at laboratory scale on natural rock discontinuities to measure the shear strength and the roughness of their surfaces to analyse the progressive damage of the asperities during shearing process. The direct shear tests along discontinuities were coupled to photogrammetric surveys of the surfaces carried out before the tests (natural surfaces), after the first cycle and at the end of the last cycle. This allowed the reconstruction of the digital surface models of the intact and degraded surfaces. Through analytical procedures, the data obtained were processed to obtain geometric descriptors and adequately estimate the Joint Roughness Coefficient (JRC), analysing several profiles extracted along the direction in which the mechanical tests were conducted. The comparison between the experimental results and the roughness surface direct measure showed that discontinuities, even at the small scale, have an inhomogeneous roughness and that discontinuity degree of damage is a progressive process influenced by the state of confinement applied during the tests.