Plain language summary
The shear strength and shear-induced permeability variation of soft interlayers are significant for occurrence of landslide and activation of shallow fault. Their properties are related to the shear rate and displacement, effective normal stress and water content, particle size distribution and mineral composition. Different from the hard grains as quartz, the sensitivity of mudstone to water induces the layered rock mass fracturing and softening into finer fragments. Although with a wide range of particle size distribution, rapid reduction of shear strength and permeability for the soft interlayer when encountered water has triggered catastrophic landslides disaster. How this softening behavior happening is practical significance. Here, we present that the weathered mudstone granules tend to transform into mud in wet conditions due to the increment of micropore after long-term weathering of water. Within a limited shear displacement, wet granules could trigger the reduction of shear strength and permeability, which deepens our understanding of soft interlayers in slope stability and fault activation when groundwater infiltrates.