Loess and red clay, as special soils, are widely distributed in northwest and southwest China. Their strength significantly decreases under water action, especially under rainfall, making them prone to landslides. Loess landslides typically exhibit flow-slide characteristics, while red clay landslides only show sliding behavior. To investigate their failure mechanisms under water action, this study conducted ring shear tests on saturated loess and red clay from Heifangtai, Yongjing County, Gansu Province, under normal stresses of 150 kPa, 200 kPa and 250 kPa, followed by Scanning Electron Microscopy experiments. Results show: (1) Under undrained conditions, loess exhibited rapid pore pressure increase, effective stress reduction, and strain softening, with pore pressure ratio ≥ 1 indicating liquefaction tendency; red clay showed rapid pore pressure decrease, effective stress increase, and strain hardening, with negative pore pressure ratio and no liquefaction tendency. Under drained conditions, both soils showed slight shear strength reduction. (2) During shearing, loess demonstrated increased vertical displacement with contractive behavior, while red clay showed decreased vertical displacement with dilative behavior. After-shearing, loess exhibited a loose, porous structure, whereas red clay displayed a dense structure with smaller pores and finer particles. (3) The loose structure of loess facilitates liquefaction under water, with extremely low residual strength leading to flow-type landslides; the strain hardening of red clay maintains its dense structure with closely packed particles and higher residual strength, resulting in sliding-type landslides. This study clarifies the mechanistic differences between loess and red clay landslides, providing theoretical insights for landslide prevention.
