Analysis of retrogressive loess flowslides in Heifangtai, China

The Heifangtai terrace is located in the arid loess area in northwestern China, where a large number of landslides was induced by agricultural irrigation on the terrace, resulted in slope retreat at the terrace edge at an estimated rate of 0.024 km2 yr− 1. On the basis of field investigation and monitoring, remote sensing and aerial images, and laboratory tests, we analyzed the characteristics of slope retreat and the failure mechanism of the loess flowslides. The retreat of slope is caused by the retrogressive behavior of the loess flowslide, of which new failure develops at the backwall of the preceding one. The groundwater level variation governs the failure process and mechanism of the flowslides. The observed loess flowslides were accompanied by rapid slope deformation, high velocity, and long runout distance exhibiting liquefaction and fluidization during the movement. In the event of a loess flowslide, a small-scale drained slope failure produces a concave recession first intensifies localized groundwater flow on the slope surface while the flow is being impeded by the deposit, which produces an undrained condition with increasing localized pore-water pressure, and in turn facilitates the subsequent flowslide. Results of the isotropically consolidated undrained triaxial tests (ICU) and constant shear stress drained triaxial tests (CSD) revealed the strain-softening behavior in the saturated reconstituted loess, and they showed that the excess pore-water pressure is associated with a small axial displacement and possibly facilitated by some alternations in the loess structure at the micro-scale.