5 Conclusion
This study determined the rapid reductions in shear resistance and
permeability of soft interlayers within a limited shear displacement of
20 mm. A weak dependency of the strain softening behavior on the normal
stress was found for dry mudstone granules. Under wet conditions, the
shear mode transformed from strain hardening to strain softening for
weathered mudstone granules within a limited displacement, along with a
10 to 45 times reduction in the permeability perpendicular to the shear
zone with increasing normal stress. No change in shear mode occurred in
the unweathered mudstone samples or weathered sandstone samples under
the same wet conditions.
The particle microstructure with increasing pores in at the particle
boundaries was sensitive to the water, which triggered mineral
separation and mud generation and filling throughout the specimens. With
the concomitant increment in particle roundness and uniformity, the mud
resulted in the reduction in the shear resistance and permeability by
lubricating the particle surfaces and sealing the micropores. Our
experimental results contribute to revealing the rapid strength
reduction of soft interlayers within limited amounts of shear
displacement, which may be related to landslide initiation and shallow
fault activation during groundwater infiltration.