Mechanical characteristics of loess under seepage shear

In the recent years, an increasing number of loess landslides were triggered due to extreme climate. The initiation of loess landslides was related to water, including surface water and groundwater, landform, geologic structure, and other factors. Both surface water and groundwater significantly affect loess landslides. Rainfall and irrigation provide plenty of water to loess, creating surface water and groundwater. Surface water flows on the surface of a loess, infiltrating into loess at the same time. The infiltration of surface water transforms loess from an unsaturated state to a saturated state in the loess plateau. The weight of slope mass increases due to the increase in water content of loess. Therefore, the loess slope mass bears shear force and seepage stress at the same time, and the deformation of loess gradually increases with time. More attention should be paid to seepage stress during the infiltration. The fabric inside loess is damaged because of shear force and seepage stress. The presence of seepage stress makes the failure mode different from the shear mode in loess. Eventually, a loess landslide forms as the deformation exceeds the bearing capacity. In this study, the 4.29 landslide in Heifangtai was selected for the purpose of research. Based on field investigation, 60 undisturbed samples from the backwall of landslide were used to conduct triaxial tests. To simulate the shear behavior of saturated loess under seepage shear, loading rates of 0.5, 0.1, and 0.05 mm·min^-1 were used and the effect of loading rate on shear strength was identified. Moreover, water heads of 0, 1, 2, and 5 m were set to study the effect of water head on shear strength with loading rates of 0.1 mm·min^-1. The stress-strain curve shows obvious strain hardening under seepage shear. Loading rate slightly affects the stress-strain relationship of loess during the seepage shear. In contrast, an increasing water head rapidly decreases the shear stress of loess. The cohesion of loess decreases by 5.24%-63.35% due to seepage shear. Further, the strength correction formula for a loess under the seepage shear condition is obtained by fitting the existing strength index. Fitting performance is evaluated following the fitting process. An empirical equation could be used in geotechnical engineering when seepage shear is considered.