Transition in rate-dependent shear response of rock-like joints due to clay-infill: strength, dilation, stick-slip, and roughness analysis

This study investigates the transition in rate-dependent shear response of rock-like joints due to clay-infill. The model material was used to prepare discontinuity samples, and Kaolin clay was used as the infill. A total of 64 direct shear tests were performed on prepared samples with varying displacement rates (0.05-40 mm/min), normal stress (0.5-4 MPa), and infill thicknesses (t/a = 0.5 to t/a = 1.5). The effect on strength, dilation, and joint surface roughness under different shearing conditions is investigated. The changes in joint roughness profile post-shearing are quantified via a 3D laser scanner, with its data processed via the Delaunay triangulation algorithm implemented in MATLAB. The quantified changes in roughness are used to understand the possible micro-mechanisms during joint shearing. The unfilled joints exhibited velocity-strengthening behaviour, i.e., strength increases with rate, with its extent depending upon the normal stress. A transition in the rate-dependent nature of the joints, i.e., velocity strengthening to velocity-weakening, was noticed with the introduction of the infill. The rate-dependency of infilled joints showed minimal dependence on the normal stress and the infill thickness. The volumetric behaviour of both unfilled and infilled joints was rate-dependent. The unfilled joints were, in general, dilatative in nature, with higher dilation observed at low rates and low normal stress. The infilled joints exhibited a compressive nature in general, with higher compression observed at low rates and high infill thickness. Further, the stick-slip behaviour was exhibited by both unfilled and infilled joints at lower rates, with high-amplitude events observed for unfilled joints. These observations were validated and explained via detailed roughness analyses.