Properties and microstructure of a natural slip zone in loose deposits of red beds, southwestern China

• The properties and microstructure of a natural slip zone were analyzed.
• The consolidation test shows that residual soils are overconsolidated.
• The shear deformation underwent two processes: dilatancy and particle alignment.

The loose deposits of red beds located in southwestern China are vulnerable to the occurrence of landslides. In this paper, a shallow progressive landslide developed in loose deposits of red beds was selected as a case study, and the physical and mechanical properties of the main slip zone were analyzed in detail. It was observed that the main slip zone of the progressive landslide is composed of fine-grained soils with substantial amounts of expansive clay particles (interlayered illite/smectite and interlayered chlorite–smectite). The one-dimensional consolidation test of the residual soils shows that the soils are overconsolidated. The shear stress versus shear displacement curves reveal that the slip zone soil has exhibited a strain softening behavior. The residual friction angle of the undisturbed slip zone soil determined by the ring shear tests is 7.2°, indicating that the residual strength of the soil is very low when in a saturated state. The microstructure of the slip zone was investigated using a combination of images captured by means of polarising microscopy (PM) and scanning electron microscope (SEM) techniques. It was found that the slip zone possessed generally softer structure with higher relative porosities than its confining materials. The microstructure of the slip zone is generally characterized by its softer structure and higher degree of alignment of fine silts, suggesting that the slip zone underwent two deformation processes (i.e. dilatancy and particle rearrangement). The global mechanical behavior of the investigated slip zone may be conceptualized as an overconsolidated clayey soil under drained conditions.