Study on the mechanical properties and microstructural evolution of loess under different wet-dry and freeze-thaw coupled cycling paths

Owing to its significant water sensitivity, the mechanical properties of loes degrade after repeated wet-dry and freeze-thaw cycles, significantly harming earthwork stability. To investigate the macroscopic mechanical properties and microstructural evolution of remoulded loess under different cyclic paths, cyclic tests with wet-dry-freeze-thaw and freeze-thaw-wet-dry sequences were carried out on samples with the initial water contents of 5 %, 10 %, 15 % and 20 %, respectively. Based on these tests, direct shear tests and scanning electron microscopy (SEM) were conducted on the remoulded loess. We discuss the mechanism of cyclic action combined with the results of the decoupling analysis. The results show that (1) the shear strength deterioration of remoulded loess under different cycling paths is caused mainly by cohesion decay, and the angle of internal friction has little effect. (2) With increasing cycling time, the attenuation amplitude of cohesion decreased, whereas the degradation of cohesion D(mn)gradually increased in a hyperbolic pattern. (3) Correspondingly, with increasing number of cycles, small and medium-sized pores gradually transformed into large pores, whereas the micropore content changed only slightly. Moreover, the pore orientation increased, the average pore diameter increased, and the pore morphology tended to be complex. (4) The dissolution of cement in dry-wet cycles and the generation of frost heave and shrinkage stress in freeze-thaw cycles are the main origins of the different mechanisms of these two cycles. (5) The degree of damage also varies because of the different cycle sequences. The mutual promotion effect f damage caused by wet-dry-freeze-thaw cycles is weaker than that of freeze-thaw-wet-dry cycles. (6) The conversion factor A is introduced to realize the mutual conversion of cohesion attenuation D-mn and total area growth value K-mn under different numbers of cycle and initial water contents.