Abstract
Loess deposits in arid and semiarid area which makes it typical unsaturated. Around a single particle of loess, there are single layer absorbed water, multi-layer of absorbed water, strong bonded water, weak bonded water and capillary water from particle surface to outer in order. The soil water content vs matric suction, namely SWCC , is a basic tool to character the soil unsaturated properties. The soil-water interaction significantly reflects on its macro behaviors.
    Especially, the unsaturated loess reveals some typical behaviors in contrast to saturated loess. First is about preferential flow. Loess in field characterizes by rich of large pores and vertical fissures, so many investigators confirm that loess is dominated by preferential flow.  However, in unsaturated state, water flows into finer pores first, then into larger in order.  The pores and fissures in loess will be obstacles rather than paths for water flow, which is in reverse to the saturated loess. So, preferential flow in loess is a false statement (Li et al, 2022). Second, in the stability assessment model of shallow loess landslide induced by rainfall, some researchers supposed that the pore water pressure above the wetting front is positive, because they think the zone is saturated under the rainwater infiltration. However, even saturated, the zone still has a negative pore pressure, which is equivalent to the air-entry suction value on SWCC. It is conceptual false to introduce a positive pressure in the model. It is essential to understand the loess behavior with the principle of unsaturated  soil mechanics.
 

Loess， unsaturated soil， matric suction