Failure Mechanisms of Shear Zones in an Undisturbed Residual Soil by SEM Image Analysis

The mechanisms of soil failure on a microscopic scale are still not fully understood. Many soil behaviour models characterize a soil failure at the macroscopic level as single, continuous plane. Although on a microscopic scale, failure planes have a complex structure and are considered shear zones. This work aims to contribute to the microscopic analysis of the shear zones in residual soils. It describes the structures observed at the shear zones and correlates the microstructures to the macroscopic behaviour of a gneiss residual soil. The analyses interconnect the microscopic view with the stress–strain behaviour, stress path, and critical state line of the tested soil. The studied shear zone was generated in a laboratory during CAU triaxial tests. Images were obtained using a backscattered electron detector in a scanning electron microscope and subsequently subjected to digital image analysis. The variation in porosity and the degree of alignment of grains and lumps along the shear zones were evaluated. Smaller shear zones that make up a larger shear zone were also found. From the results, it was concluded that there is a strong influence of the strain level and effective confining stresses on the structure and geometry of the shear zones.