Rate Effects on Shear Deformation of Rough Limestone Discontinuities

The relationship between slip velocity, resistance to shear, and geometrical evolution of initially rough limestone surfaces is investigated in servo-controlled direct shear experiments. We focus on shear stress evolution throughout the deformation process, as function of the shear rates applied. Direct shear tests are performed on tensional fractures positioned in a perfectly mating configuration, using a hydraulic, closed-loop, servo-controlled system. The discontinuities are sheared to prescribed displacement target of 10 mm under a constant normal stress of 5 MPa while shear stress and dilation are recorded under controlled shear rates between 0.1 and 50 µ/s. Surface geometry is measured using laser profilometer before and after tests, and roughness evolution is statistically quantified. We find that, before peak shear stress is attained, the shear stiffness increases with slip velocity, manifesting asperity strain hardening during this largely elastic deformation stage. Peak shear stress and stress drop, however, are both inversely proportional to velocity, indicating velocity weakening coupled with rate-dependent healing of the interfaces. The final roughness of the sheared surfaces increases moderately with increasing slip velocity indicating enhanced fragmentation and brittle deformation with increasing shear rate.