An approach to measure infill matric suction of irregular infilled rock joints under constant normal stiffness shearing

Rock joints infilled with sediments can strongly influence the strength of rock mass. As infilled joints often exist under unsaturated condition, this study investigated the influence of matric suction of infill on the overall joint shear strength. A novel technique that allows the direct measurement of matric suction of infill using high capacity tensiometers (HCTs) during direct shear of infilled joints under constant normal stiffness (CNS) is described here. The CNS apparatus was modified to accommodate the HCT and the procedure is explained in detail. Joint specimens were simulated by gypsum plaster using 3-D printed surface moulds, and filled with kaolin and sand mixture prepared at different water contents. Shear behaviour of both planar infilled joints and rough joints having JRCs of 8-10 and 18-20 with the ratios of infill thickness to asperity height (t/a) equal to 0.5 was investigated. Matric suction shows predominantly unimodal behaviour during shearing of both planar and rough joints, which is closely associated with the variation of unloading rate and volumetric changes of the infill material. As expected, two-peak behaviour was observed for the rough joints and both peaks increased with the increase of infill material matric suction. The results reported seem to indicate that the contribution of matric suction of the infill material on the joint peak normalised shear stress is relatively independent on the joint roughness.