Initiation and mobility of recurring loess flowslides on the Heifangtai irrigated terrace in China: Insights from hydrogeological conditions and liquefaction criteria

Heifangtai terrace is famous in China for its irrigation-induced loess landslides. The frequently recurring loess flowslides initiated along the platform margin of the irrigated Heifangtai terrace have caused 42 fatalities and significant economic losses due to their long-runout and high-speed mobility. Yet it is still unclear if the recurring loess flowslides almost always initiate in the same places, and their initiated conditions and subsequent mobility behaviors have not been fully evaluated. To better understand such under-researched questions, we selected three typically recurring loess flowslides on the Heifangtai irrigated area. We performed multiple geophysical surveys using electrical resistivity tomography (ERT) and multichannel analysis of surface waves (MASW). We tested water content, density, and strength of loess using in-situ profile sampling. In addition, we examined the steady-state shear behaviors of saturated loess utilizing a ring shear apparatus under undrained conditions. The geophysical signatures and in-situ loess property profiles showed that hydrogeological conditions are crucial to initiating the recurring loess flowslides. The results also demonstrated that the shear liquefaction behavior of saturated loess controls the mobility after the failure of the loess flowslides. Liquefaction potential was assessed based on a chart showing the liquidity and plasticity index, pore pressure ratio, and steady-state line, which were developed as new criteria of liquefaction susceptibility assessment. The novel criteria essentially contain general information about fine-grained soil's nature, state, and behavior. These findings provide a better understanding of the dynamic mechanisms of the loess flowslides and their implications for landslide hazard mitigation.