An extensive direct shear test program has been conducted on regular saw-tooth artificial joint samples under constant normal load (CNL) conditions. The analysis of shear data reveals that shear stiffness (kss = dτ/du) and dilation (ψ = dv/du), where τ and v are the shear stress and vertical displacement respectively, are not constants throughout the evolution of shear stress. Rather, it is clear that both the variables change non-linearly with shear displacements (u) and can be approximated by two-parameter hyperbolic function with respect to u. These parameters are estimated using regression analysis using the experimental data. From the functions kss (u) and ψ(u), a shear displacement exists at which the basic friction angle occurs. Also, it is found that this displacement occurs where contraction ends and dilation begins. Based on that, the dilatant behavior and evolution of peak shear strength can be described leading up to the determination of dilation angle at the peak stress.
Joint can be defined as a line of break from geological formation along which there is no observable deformation (Muralha et al., 2014). In most of the literature, for regular saw-tooth shaped artificial samples under CNL boundary condition, shear behavior is mostly characterized by peak shear strength which depends on the normal load and dilation angle for a given basic friction angle (Budi et al., 2014; Haberfield & Johnston, 1994; Ladanyi & Archambault, 1969; Yang & Chiang, 2000; Zhu et al., 2019). A few studies have been conducted to understand the evolution of the mobilized friction angle while shearing, i.e. with shear displacement (Hoek & Brown, 1997; Barton, Bandis & Bakhtar, 1985; Bai et al., 2010). In other words, understanding the development of shear resistance of a joint surface with shear displacement may reveal the dilatant behavior as well as the variation of shear stiffness. In most of the applications, shear stiffness and dilation are considered to be the constant throughout the evolution of shear stress until the peak strength. From laboratory experiment results, it is clear that they are not constant rather vary nonlinearly with shear displacements even at the initial stages of loading. Therefore, from the relationships between shear stiffness/dilation angle and shear displacement, it may be possible to develop models of the peak shear strength and mobilized friction angle depending on shear displacement.
