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.