Elevated Pore-Water Pressure Regulating Dynamic Liquefaction of a Flow-Like Landslide in Loess

At 23:59 (UTC + 8) on 18 December 2023, an earthquake of Ms 6.2 struck Jishishan County in Gansu Province, China, and triggered a large-scale, flow-like loess landslide in Zhongchuan Town, resulting in some 20 deaths. Originated from relatively gentle terrain, the loess flow displayed high mobility with a run-out distance of 3,200 m, suggesting that pore-water may play a critical role in the mobility of Zhongchuan flowslide. Following onsite investigations and soil sampling, we replicated the initiation process of the flowslide through dynamic back pressure direct shear tests under a constant shear stress condition. Two types of tests were conducted on saturated loess samples: elevated back pressure tests to simulate instability induced by high pore-water pressure, and dynamic loading tests to examine the evolution of pore-water pressure under seismic loading conditions. The experimental results, supported by microscopic analysis, indicate that elevated pore-water pressure is the key factor driving the progressive transformation of shear displacement from accelerated motion to instantaneous runaway. Meanwhile, dynamic loading substantially amplifies the generation of excess pore-water pressure. Moreover, the initial pore-water pressure was found to be a critical factor in both the initiation and high mobility of the Zhongchuan flowslide. These experiments quantitatively capture the in situ evolution of pore-water pressure throughout the liquefaction process, providing a physically based framework for understanding the mechanisms of loess landslides.