Evolution of Shear-Zone Structure in Undrained Ring-Shear Tests

Undrained monotonic torque-controlled tests were conducted on fine-grained silica sand to study the shear-deformation process in granular materials by using a ring-shear apparatus. Shear-zone structures at various stages in the undrained shear tests were observed during a series of tests in which the experiments were terminated at different shear displacement. For undisturbed samples, the shear zone was only developed during the post-failure stage and its thickness increased with progressed shearing. First the shear surfaces had undulating and asymmetric structures; later they gradually became smooth and parallel to the shearing direction. During this process, pore water pressure was generated, and the effective friction angle decreased correspondingly. Generally, the shear zone could be divided into three parts: the compacted core, the adjacent zone above the core, and the adjacent zone below the core. Grain-size analysis on the sample from the shear zone revealed that grain crushing occurred during each stage and the extent of grain crushing differed for different shear stages. An interesting phenomenon occurred during the steady-state deformation where the coarse and fine particles within the shear zone segregated during motions and a parallel orientation structure developed. These results are helpful for understanding the mechanism of progressive failure in granular material as well as the rapid landslide with long runout study.