Long-term shear strength weakening of soft interlayers due to low-permeability

Soft interlayers are distiuctive geological units that serve as semi-permeable layers that regulate water flow and chemical transport in the subsurface. However, their hydrogeolocial characteristics are still poorly understood due to its unique sedimentary features. Duo to the low-permeability of Permian Maokou soft interlayers, water contents remain invariable and cause continuous weakening effect to its shear strength. Series of ring-shear creep tests are conducted to study the effect of water contents on ring-shear rheological properties. According to the experimental results, the instantaneous shear strain modulus is closely affected by water content, normal stress, and duration of test. Besides, the long-term shear strength and residual shear strength are close to each other when the normal stress is relatively small. But the strength reduction factor from the residual shear strength to the long-term shear strength should be considered under high normal stresses. Furthermore, the long-term cohesion and friction angle are obtained by fitting the long-term shear strength values under different normal stresses. An exponential function relation exists between the two long-term shear strength parameters and water contents. Finally, based on the analysis of the SEM (scanning electron microscope) results, it can be concluded that the effect of water content on rheology properties is mainly caused by lubrication, expansion of particles of samples in water. Micro-holes and cracks on failure surfaces also exhibit ductile failure characteristics.