1 Introduction

The development of soft interlayers in sandwich strata and fault gouges is a crucial geological structure related to landslide and shallow fault activation (Christaras, 1997; Eberhardt et al., 2005; Ma et al., 2019; Panek et al., 2011; Zou et al., 2017). Extensive slope failure cases in southwest China (Li et al., 2020; Tang et al., 2015; Xu et al., 2016) and other regions of the world (Christaras, 1997; Eberhardt et al., 2005; Panek et al., 2011) have been observed to be controlled by a argillic soft interlayer. Smectite-clay gouge and mudstone are widely found at different depths in fault zone (Cuisiat and Skurtveit, 2010; Ikari et al., 2009; Morrow et al., 2015). In general, the shear band crushing induced low resistance was relevant to the particle shape and size distribution (Feia et al., 2016; Mair et al., 2002), lithology, water content (Crawford et al., 2008; Ma et al., 2019; Zhang et al., 1999), shear speed and effective normal stress (Fukuoka et al., 2007; Kimura et al., 2018; Lupini et al., 1981). In ring shear tests, samples of hard grains required a large displacement and high normal stress to reach their residual strengths (Fukuoka et al., 2007; Kimura et al., 2019). For a soft interlayer consisting of clay grains, Ma et al. (2019) revealed the dependency of the transformation from strain hardening to strain softening on the water content. Due to the sensitivity of clay minerals to water, the shear behavior of mudstone granules considering the effect of weathering is of engineering significance (Chandler, 1969). Thus, different in sensibility to weathering between clay minerals and quartz, the relationships of the shear behavior of mudstone granules to shear conditions, such as the water content, shear displacement, and normal stress, have remained enigmatic when considering the influence of water weathering.
The shear-induced reduction in permeability contributed by crushing band with a few millimeters thickness has been studied through particle size analysis, high-speed imaging, and field emission scanning electron microscopy (Agung et al., 2004; Cuisiat and Skurtveit, 2010; Fukuoka et al., 2007; Kimura et al., 2019; Kimura et al., 2020; Kimura et al., 2018). The ring shear test results (Kimura et al., 2020; Kimura et al., 2018) demonstrate the negative effect of shear rate and effective normal stress (within 10 MPa) on silica sand permeability. However, the permeability of fault gouges under tens of megapascals of normal stress has no significant relationship with shear speed (Tanikawa et al., 2012). At the microscale, blockage of the pore throat by fine particles and rearrangement of the flaky minerals was attributed to the variation in effective porosity and permeability (Dewhurst et al., 1996; Schneider et al., 2011; Zhang et al., 1999). Although the permeability reduction property of quartz grains after shearing explains the fluid distribution of faults and sealing properties of sediment (Kimura et al., 2019; Tanikawa et al., 2012), it seems difficult to extend to soft interlayers composed of clay rock due to their distinct mineralogy and shear behavior (Crawford et al., 2008; Ma et al., 2019; Zhang et al., 1999).
Thus, mudstone granules from red sandwich strata in a landslide were prepared to research the shear behavior and permeability of soft interlayers under different weathering intensities through dry-wet cycling. To clarify the weathering effect on the specimens, the change in particle shape and formation of micropores with dry-wet cycle was discussed in terms of their influence on shear behavior and permeability. The results suggested that the increment of particle micropore promoted clay mineral separation and transformation into mud, which filled the specimen pores, inducing a rapid reduction in the shear resistance and permeability of the soft interlayer.