Quantitative evaluation of roughness with a 2D digital instrument

Roughness of discontinuities, when clean and unfilled, has a significant impact on rock joint mechanical behaviour and, as a consequence, of rock masses. The need to solve ambitious rock mechanics problems encouraged research to study shear mechanism acting at the joint level and to produce sound models to reproduce it. In this framework, roughness measurement allows to gather information about shear strength of rock joints at an acceptable cost. A brief summary of published approaches and parameters for quantitative roughness evaluation is presented. Then, measures taken on rock samples extracted in a test site placed in the nearby of Esino Lario, 60 km north of Milan, are shown and discussed. The device used on-site to obtain digital replicas of profiles is a laser-camera profilometer, developed in the Department of Mechanical Engineering of the Politecnico di Milano. This device, combining a digital camera and a laser source, uses the principle of laser triangulation to extract a maximum of 746 points/profile. Numerical coordinates obtained by rock joint profiles are submitted to a Matlab® script which computes geometric parameters. Statistical analysis of gathered data was used to define a minimum number of measures that have to be used to determine roughness of a joint set.