Microstructural constraints on geotechnical properties of Malan Loess: A case study from Zhaojiaan landslide in Shaanxi province, China

The microstructure of Malan Loess (a late Pleistocene loess deposit in China) dominates its macroscopic geotechnical properties. When subject to inundating or humidifying conditions, the distinct metastable structure of the Malan Loess is sensitive to collapse; thus, its geotechnical properties undergo change. The results of triaxial and uni-axial compression tests under the humidifying conditions on undisturbed loess samples demonstrate that its geotechnical properties of the top loess were dominated by metastable structure, while the basal loess were more influenced by inter-particles cementation bond. Based on analyses from scanning electron microscopy images, change of the loess pore structure can be primarily attributed to the ratio of metastable pores (> 5 μm). Accordingly, an end-member model is proposed to illustrate how varying in the size and distributions of very large pores (> 200 μm), large pores (50-200 μm), medium pores (5-50 μm), and small pores (< 5 μm) successively change the undisturbed loess microstructure. Rising groundwater level along the base of the Malan Loess weakens the inter-particles bond, and can lead to a chain reaction where progression of pore collapse results in elevated pore pressures and loss of shear strength. This mechanism is used to explain the periodic accelerated movement of the Zhaojiaan landslide complex in northern Shaanxi, China.