Study of the 1920 Haiyuan earthquake-induced landslides in loess (China)

Many large-scale landslides induced by earthquakes have been reported in loess soils. They often cause catastrophes because they travel long distances at high velocity. To clarify the mechanism of these landslides, field survey was performed on the landslides triggered by the Haiyuan Earthquake (China, 1920), and a series of ring shear tests was conducted on the loess soils collected from a landslide. The field surveys revealed that most of the loess landslides triggered by the Haiyuan Earthquake occurred on concave slopes gentler than 15° with long runout distance, showing very small equivalent friction angle. Cyclic ring shear tests on saturated specimens in the undrained condition showed that when cyclic shear stress was applied to a loose loess specimen, pore pressure was built-up gradually before the failure, and after that, large pore pressure was quickly generated due to the failure of loess soil structure. Meanwhile, to investigate the mechanism that produces the high mobility of loess landslides, ring shear tests were conducted on loess specimens with large shear displacement of up to several meters under the undrained condition, and it was found that the initial normal stress has no effects, and overconsolidation ratio has little if any effects, on the apparent friction angle in the steady state for loess soils. Results of tests on soils at different saturation degrees revealed that the reduction in shear strength results more likely from the generation of pore water pressure, not of pore air pressure.