The microstructure of a slip zone from a recent landslide was investigated by combined optical microscopy (OPM) and backscattered electron microscopy (BEM) techniques. Along the toe of the slope, the slip zone consisted of two distinct segments, formed a) along a pre-existing slip zone and b) through completely decomposed granite (CDG), both of which comprise composite soils. The microstructural features were systematically captured, quantified, and analyzed using an advanced image-analysis system. It is shown that variations in density, porosity, presence of matching fragments of coarse particles, abundance of tabular aggregates and plates of kaolinite and particle alignment are significant indicators of the deformation history of the slip zone. The results indicate that multiple processes concurred in the course of deformation: compaction and dilation, cataclasis and comminution, and particle rearrangement. Such deformation features of the natural composite soils are fundamentally different from the relatively simple soils commonly used in laboratory studies. The extent of each process depends on the particle-size distribution and stress level that prevailed at time of failure. The particle movement and microstructural evolution of the slip zone could be described as particulate flow through simultaneous bulk simple shear and pure shear. Depending on their relative abundance, fine particles play an important but variable role in the development of the slip zone and the mobilization of its shear resistance to deformation. Reactivation and/or prolonged deformation evidently change the deformation behavior of the pre-existing slip zone. In general, shape-preferred orientation of particles occurs where they are obviously aligned.

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