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.