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.