A mechanical insight into the triggering mechanism of frequently occurred landslides along the contact between loess and red clay

The triggering mechanism and movement evolution of loess-red clay landslides, which occurred frequently along the contact between the loess and red clay on the Loess Plateau, are closely related to the mechanical properties of the contact surface. This work presents an experimental investigation on loess, clay and loess-red clay interlaminar (LRCI) samples obtained from a typical loess-red clay landslide in northern part of Shaanxi province of China, using a series of ring shear tests, microscopic observation and scanning electron microscopy tests, in an attempt to explore the mechanical behavior of loess, clay and LRCI samples with variation in moisture content, normal stress and shear rate. The results revealed that for all specimens, both the peak shear strength [Formula: see text] and the residual shear strength [Formula: see text] decreased with increasing moisture content, among which, moisture content has the greatest influence on the [Formula: see text] and [Formula: see text] of red clay, followed by the LRCI specimen, and the loess specimen is least affected by moisture content. Meanwhile, exponential functions describing the correlations between shear strength and moisture content of LRCI, red clay and loess specimens were proposed. Furthermore, the macroscopic morphological characteristics and the microstructure of shear surface obtained from the LRCI specimens showed that a localized water accumulation was built up within the shear surface as the water content increases to some extent, and a high degree of liquefaction developed within shear surface when the moisture content reached to the saturate degree. The microstructural observation on LRCI specimen suggested that the shear surface became smoother and the larger percentage of small-sized pores was observed with moisture content. Accordingly, the built-up excess pore water pressure during shearing is difficult to be dissipated due to a close structure of small-sized pores. Due to the low permeability, high pore-water pressure built up within the shear zone and the increase in the fine particle content, the LRCI soils with a high saturation degree shows the potential for the localized liquefaction within shear zone, which further provides a scientific explanation for the triggering mechanism of loess-red clay landslides with high-speed and long- run out.