Direct Shear Tests on Opalinus Clayshale for Natural Fracture Characterization

TitleDirect Shear Tests on Opalinus Clayshale for Natural Fracture Characterization
Publication TypeConference Proceedings
Year of Conference2021
AuthorsArzuaga, I, Einstein, HH, Germaine, J
Conference Name55th US Rock Mechanics/Geomechanics Symposium
PublisherAmerican Rock Mechanics Association
Conference LocationHouston, TX

One of the parameters that govern the interaction between hydraulic fractures (HF) and natural fractures (NF) is the coefficient of friction between the two NF-surfaces (Gu and Weng, 2010). Therefore, the characterization of natural fractures is important when analyzing this type of interaction in both numerical and experimental analyses. In this study, we conducted a set of Direct Shear Tests on Opalinus Clayshale specimens employing a shear box to obtain the coefficient of friction and cohesion of the sliding surfaces. Two conditions were tested: vertical and horizontal bedding planes, i.e. perpendicular and parallel to the fracture surface. Each condition was analyzed at different polishing levels of the natural fracture’s surfaces, using sandpapers with average particle diameters ranging from 201 μm (N80) to 25.8 μm (N600). Also, two horizontal displacement rates were applied: slow and fast. We observed that the coefficient of friction in Opalinus Clayshale varies between 0.1 and 0.65 depending on the degree of polishing. The displacement rate practically does not affect the coefficient of friction but the bedding planes’ direction does. Specimens with the same polishing grade have a higher coefficient of friction for the vertical bedding planes. Moreover, in this configuration, higher surface wear is observed in the layers of the Opalinus Clayshale with higher resistance (light layers). This can be explained because lower resistance layers (darker ones) have more material removed during the polishing process, exposing the higher resistance layers.