Abstract: Landslides frequently occur in northwestern China,where failure surfaces often traverse Quaternary loess and extend into the overlying Tertiary Pliocene red clay. To better understand the failure mechanisms of loess-red clay composite landslides and to evaluate the effects of water,salt,and seasonal freeze-thaw cycles on slope stability,this study monitored the energy signals during shear failure at the loess-red clay interface using acoustic emission techniques during direct shear tests. Longitudinal wave velocity and low-field nuclear magnetic resonance(NMR)measurements were also analyzed. The results indicate that:(1)Water content and sodium sulfate concentration significantly affect the longitudinal wave velocities of loess-red clay soils. When the water content is below 17%,the wave velocity of soil samples first decreases and then increases with increasing water content. When the water content exceeds 17%,the freeze-thaw temperature shows a negative correlation with the increase in wave velocity. At sodium sulfate concentrations below 1%,wave velocity initially increases and then decreases with increasing concentration,whereas above 1%,it consistently decreases. (2)After multiple freeze-thaw cycles,the percentage of small pores in loess-red clay samples first decreases and then increases with rising water content,the proportion of medium pores first increases and then decreases,and the percentage of large pores remains largely unchanged. (3)Based on direct shear test results,the shear-slip models can be classified into three types: interface slip type,cohesive slip type,and obstruction type.