Variations in hydraulic properties of collapsible loess exposed to wetting and shearing

Loess landslides induced by rainfall or irrigation frequently occur in the Loess Plateaus, China, and the stress paths followed are similar to water infiltration at a constant shear stress. Hydraulic properties of collapsible loess are key information for capturing the physical causes of those landslides. The compelling need requires deep understanding variations in hydraulic properties of collapsible loess exposed to wetting and shearing involved, including soil–water retention curve (SWRC) and unsaturated permeability k w ( s ) or k w ( S r ) with s and S r denoting the suction and degree of saturation. Wetting in steps and permeability tests at constant shear stresses were conducted to understand the SWRC and the k w behaviours of the collapsible loess. The results demonstrated that the evolution of measured SWRC during wetting was closely dependent on shear stress, ascribing to the variation in the air-entry value associated with change in void ratio caused by wetting–shearing coupling acting mainly. The shear acting leaded to an obvious evolution of the void ratio of the loess and thus of its saturated permeability, and an approximate log-linear relationship was observed. The loess was wetted, resulting in a decrease in suction or an increase in saturation, accompanied by an increase in k w . The effect of shear acting on k w ( S r ) was monotonous; this was not the case for k w ( s ), highly dependent on suction level. For suction levels lower than a specific threshold, a significant decrease in the magnitude of k w began to appear due to shear acting, while above this value there was insignificantly affected. Regardless of shear stress levels, the relative permeability of the loess can be represented solely by a function of either normalized suction (i.e. the ratio of the suction to the air-entry value) or degree of saturation. The closed-form empirical equations were proposed to characterize SWRC and k w of the loess considering influences of wetting and shear acting and thus to further calculate of transient unsaturated flow problems more conveniently, in which the shear stress level affected the equation parameters directly. There was a good comparison between the calculated and measured data of the loess and other clays that establishes the rationality and validity of the proposed equations.