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.