Fig. 9 Shear mode and variation in permeability of mudstone granules in the process of weathering and wetting. a) Variation in the shear mode with the action of weathering and wetting. b) Dry specimen containing debris in the shear band. c) Weathered mudstone granules changed changing into mud when encountering water.
As shown in Fig. 6 and Fig. 10, the fine particle contents were different between the weathered and unweathered samples and between the dry and wet conditions. According to the ESEM images, the loose parts of the particle boundaries tend to separate the clay minerals from each other and transform them into mud when exposed to water (Fig. 9 a). Therefore, mud wrapped the particles and served as lubrication, which obviously decreased the shear resistance more than the debris generated under dry conditions. This facilitated the shear behavior of granular specimens transitioning into strain softening within a limited shear displacement. Thus, the softening effect of water on the damaged part decreased the requirement of a large shear displacement to reach the residual strength. Although the role of water was considered in many hard particle shear tests (Agung et al., 2004; Fukuoka et al., 2007), the water provided interparticle lubrication or bore the pore pressure rather than induced strain softening of the particles. Therefore, the strain softening behavior of the soft interlayer was closely connected to the combination of the water softening and weathering intensity to the mudstone grains (Chandler, 1969; Ma et al., 2019). Moreover, the roundness and uniformity of the granules increased with the weathering intensity, which corresponded to the low friction mechanism presented by Mair et al. (2002), who found that spherical glass beads show a lower resistance than angular quartz grains do when no particle crushing occurs.

4.2 Rapid reduction in the permeability

The permeability of the loose medium was controlled by the void ratio, which can vary with the particle size distribution and mineralogy, confining stress, shear displacement, and flow path (Crawford et al., 2008; Dewhurst et al., 1996; Faoro et al., 2009; Feia et al., 2016; Ikari and Saffer, 2012; Kimura et al., 2019; Reece et al., 2012; Zhang et al., 1999). From the dry condition to the wet condition, an increasingly strong relationship between the finer particle content and the normal stress after shearing was found, as shown in Fig. 10, which was verified in other studies (Feia et al., 2016; Kimura et al., 2020; Tanikawa et al., 2012). Regarding the differences in the permeability and finer particle content between the dry and wet samples, apparent crushing in the shear band occurred after the peak stress or a large shear displacement (Ikari and Saffer, 2012; Kimura et al., 2018; Uehara and Takahashi, 2014). More importantly, although these differences were also found for fault gouges with clay components (Tanikawa et al., 2012), there was a lack of discussion of the relationship between the increase in weathering intensity and finer particle generation, where our results showed a positive relationship (Fig. 10). With the increasing content of clay in the fault gauge, the permeability reduction was more prominent than that observed for quartz grains within a small shear strain (Crawford et al., 2008). Therefore, the permeability of the soft interlayer after shearing was closely related to the weathering intensity and water content within a limited shear displacement. Different from the crushing behavior concentrated in the shear band observed among the hard grains (Fukuoka et al., 2007), the weathered particles tended to transform into mud and fill the pores of the entire specimen under wet conditions, as described in Fig. 9 c and d. Mud blocking of the pore throats not only decreased the macropore content but also increased the compactness, so the vertical height of the wet specimens were greater than those of the corresponding dry specimens in Fig. 6. Similar to the results of the particle size distribution analysis in Fig. 6, more finer particles were found along the two sides of the shear bands in the weathered specimens under wet conditions. Thus, the variation in the permeability of the sheared soft interlayer was not only controlled by shear crushing in the shear band but also influenced by the argillization of the whole specimen. In addition, the permeability anisotropy between the directions parallel and perpendicular to the shear band was attributed to the difference in the finer particle distributions and rearrangement of clay minerals (Dewhurst et al., 1996; Zhang et al., 1999). Correspondingly, the slight variation in our results of the finer particle content in the vertical direction complicates this anisotropy, which is worthy of further investigation in future work.