Effect of freeze-thaw cycles on the strength behaviour of recompacted loess in true triaxial tests

Freeze-thaw induced change in the strength behaviour of the underlying loess is an important cause of the instability of artificially excavated and natural loess slopes. The loess is mostly in a true triaxial stress state, which was rarely considered in previous strength tests. This study explored the effect of freeze-thaw cycles on the strength behaviour of recompacted loess under true triaxial stress and its relationship with the meso-structure by carrying out freeze-thaw tests, true triaxial tests and scanning electron microscope (SEM) tests. Freeze-thaw cycles exhibit little effect on the shape of the stress-strain curves of recompacted loess, while more significant changes were observed with different b-values. The shear strength first decreases with freeze-thaw cycles and then increases to a constant after five cycles, while it continues to decrease with the b-value. For strength parameters, the cohesion decreases after freeze-thaw and tends to stabilize after five cycles while the internal friction angle is less affected. As the b-value grows, the cohesion first grows and then declines, with the peak value at b = 0.5 while the internal friction angle varies within a narrow range (28.11° - 32.53°). The effect of freeze-thaw on the shape of the failure surface for recompacted loess on the π plane is negligible but pronounced on the size of the failure surface. SEM images and the specimen size imply that the structure of recompacted loess tends to be loose after freeze-thaw cycles, corresponding to a greater pore area ratio and the decrease in the cohesion. Moreover, both the orientation and circularity of soil particles show limited changes, which reveals the insignificant change in the internal friction angle with freeze-thaw