4 Discussion and mechanism analysis

4.1 Mechanism of the shear mode variation

The transformation of the shear mode of a granular specimen is important to describe the shear mechanism of a soft interlayer; in our results, this transformation was influenced by the weathering intensity and water condition. No significant residual strength was revealed for mudstone_0 or other specimens under dry conditions, which was similar to the ring shear results of soft interlayers at low water contents or high normal stresses obtained by Ma et al. (2019). The strain hardening of weathered specimens suggested that the crushing of particles was insufficient with limited shear displacement and concentrated in the middle part of the specimens, as shown in Fig. 6. The results of Alberti et al. (2019), Fukuoka et al. (2007) and Riaz et al. (2019) suggested that the shear resistance of quartz grains is almost constant in the initial stage, within a small displacement. Cuisiat and Skurtveit (2010) stated that the similar strain hardening behaviors of layered sand and clay in the normal stress of 0.7 MPa. Thus, the dry specimens showed a shear behavior similar to that of specimens of quartz grains, with no apparent dependency on the normal stress within a limited shear displacement (Agung et al., 2004; Faoro et al., 2009; Fukuoka et al., 2007; Kimura et al., 2020; Mair et al., 2002; Tanikawa et al., 2012). According to the description of the turbulent mode in residual shear presented by Lupini et al. (1981), particles slide along the contact surfaces, which induced the damaged part of weathered particle crushing into debris under both shear and compression (Fig. 9 a). With a limited shear displacement, the debris with a strength equal to that of the rock is not crushed enough to attain a residual strength in dry conditions.