Post-dynamic behaviors of the saturated loess in the Zhongchuan slide triggered by the Jishishan M6.2 earthquake

In 2023, a M6.2 occurred earthquake in Jishishan County triggered a large-scale saturated loess liquefaction flowslide disaster in Zhongchuan Township, Minhe County, Qinghai Province. To study the shear characteristics of saturated loess after liquefaction, undisturbed loess samples were prepared and saturated in the laboratory. A combination of cyclic shear-monotonic shear tests was conducted to investigate the mechanical properties of Zhongchuan saturated loess after liquefaction, and the effects of reconsolidation ratio, excess pore water pressure ratio, monotonic shear rate, and surrounding pressure on the stress–strain curves and pore water pressure of post-earthquake saturated loess were analyzed. Monotonic shear tests on saturated loess under different surrounding pressures before and after cyclic shear were carried out to compare and analyze the effects of cyclic shear on the deformation, pore water pressure ratio, and stress path of loess simulating different depths. The apparent viscosity of saturated loess under different conditions was calculated based on the test results, and the flow characteristics of post-earthquake saturated loess were analyzed to discuss the mechanism of the Zhongchuan saturated loess flow slide induced by the Jishi Mountain earthquake. The research results indicate that the saturated loess after liquefaction under cyclic shear exhibits a slight increase in shear stress, minor changes in pore water pressure, and a slight decrease followed by a continuous increase in mean effective stress under sustained monotonic shear. The reconsolidation ratio, cyclic shear excess pore water pressure ratio, and consolidation pressure significantly affect the shear characteristics of liquefied loess. The larger the reconsolidation ratio, the smaller the cyclic shear excess pore water pressure ratio, and the greater the consolidation pressure, the greater the peak stress during monotonic shear of post-earthquake loess, the smaller the excess pore water pressure, and the greater the apparent viscosity during shear. The monotonic shear rate has a definite impact on the shear behavior of liquefied saturated loess; the greater the shear rate, the smaller the peak strength, the smaller the apparent viscosity during shear, and the better the flowability of the soil. During the Jishi Mountain earthquake, the saturated loess at a depth of 8–12 m in Zhongchuan Township underwent extensive complete liquefaction. The liquefied soil, under the continuous action of the earthquake, produced high-speed flow. Intense shear action further thinned the liquefied soil, resulting in large-scale, long-distance debris flow disasters.