Measurement and modelling of stress-dependent water permeability of collapsible loess in China

Abstract

Landslides induced by rainfall or irrigation frequently occur in the Loess Plateaus, China. The water permeability of these collapsible soils is crucial to analyse the stability of slopes in this region. To explore the stress dependency of the water permeability, i.e., kw(s) and kw(Sr) with s and Sr denoting the matric suction and degree of saturation, respectively, a modified suction-monitored triaxial apparatus was developed. Intact loess specimens were subjected to wetting from the initial suction state by adding prescribed volumes of water (in steps) at constant net isotropic stresses. On each wetting step, both deformation and pore water pressure of the specimen over time were measured. The average water permeability was determined by volumes of water added and the suction difference from the start to the end of wetting. The results suggest that the influence of the net isotropic stress on kw(s) is suction-dependent, and that for suctions larger than a specific threshold the effect is almost negligible. At low suction levels, the higher is the net isotropic stress the lower is the water permeability. The influence of net isotropic stress on kw(Sr) is significant, i.e., at a given degree of saturation, the water permeability is lower for a higher net isotropic stress. Interestingly, a unique relationship can be found between the relative water permeability and suction ratio (i.e., the ratio of the suction to the air-entry value) or degree of saturation at various net isotropic stresses. The water permeability of the collapsible loess measured at constant void ratio is poorly described by the van Genuchten–Mulaem (vG-M) equation. Thus new empirical equations for the water permeability in terms of suction or degree of saturation are proposed, in which the stress level affects the equation parameters. The proposed equations describe efficiently the water permeability of the collapsible loess on wetting as well as that of silty sands on drying at various net isotropic stresses.