The effects of commonly encountered mineral infills on shearing behaviour and shear strength development has not been extensively investigated, highlighting a need for further work. This paper compares the results obtained from a series of direct shear tests using replicates of a natural rock joint, infilled with various minerals at different thickness to amplitude ratios, to gain an understanding of the effects that different minerals have on the shear behaviour and development of shear strength of rock discontinuities. It was observed that even very thin coatings (∼0.2 mm) of mineral infill can significantly reduce peak shear strength. More interestingly, at this thickness the coating can cause either a decrease or increase in residual shear strength. A positive correlation was established between the hardness and the residual friction angle of the minerals. Increasing the thickness of the infill continues to reduce the shear strength. As the thickness approaches the average maximum amplitude of the surface, the shear strengths of the infilled joint converge to the shear strength of the infill material. For the studied surface, the critical thickness to amplitude ratio was estimated to be 1.0 to 1.2.
The development of shear strength for rock joints is influenced by a number of factors including roughness, wall compressive strength and degree of matching. Due to weathering and/or structural processes, the aperture of the joints within a rock mass can increase and in turn over time be filled with deposited material.
The presence of infill material within a joint is generally recognised to cause a reduction in the shear strength in comparison to an equivalent clean joint, and hence it can potentially influence rock mass stability (Barton 1974). It has been widely documented that the thicker the infill, the lower the joint shear strength (Goodman 1970; Kanji 1974; Ladanyi & Archambault 1977; Lama 1978; Phien-Wej et al. 1990; Papaliangas et al. 1993; De Toledo & De Freitas 1993). This occurs due to a combination of a) a reduction in the degree of surface textural interlocking due to presence of infill material within the micro roughness of the joint walls, b) a change in the friction properties of the shear surface and/or c) a reduction in the ‘effective roughness’ of the joint surface as the presence of the infill material changes its morphology (Papaliangas et al. 1993).
