Pore-pressure generation and fluidization in a loess landslide triggered by the 1920 Haiyuan earthquake, China: A case study

• A large loess landslide was studied.
• Ground water existed on the landslide source area.
• Soil layer on the source area is liquefiable.
• Liquefaction of water-saturated loess resulted in its high mobility.
• Pore-air pressure played less role on its high mobility.

During the 1920 Haiyuan earthquake, numerous catastrophic landslides were triggered in the loess area in Northwest China. We investigated in detail a large example of these landslides, referred to as Dangjiacha landslide in this paper. This landslide originated from a slope of about 20°, and the displaced soil mass traveled about 3200 m, damming a valley. We performed a field survey and found that standing water existed in the landslide area and the loess had high porosity. We infer that it was the liquefaction of the water-saturated loess layer rather than the suspension of silt in the pore-air in the loess that caused the great mobility of this landslide. To test this inference, we performed undrained triaxial compression and ring shear tests on loess samples to examine the shear behavior of loess saturated by either air or water. The test results showed that the water-saturated loess soil was highly susceptible to flow liquefaction failure. Fast shear tests on naturally air-dried loess samples revealed that the generated pore-air pressure was small under the “undrained condition” and no significant reduction in the shear resistance was observed, implying that air entrapped in the loess was unlikely to be the main contributor to the high mobility of this large-scale landslide.