Effects of dry-wet cycles on compacted loess: from macroscopic to microscopic investigation

Abstract

To investigate the response mechanism of mechanical properties for compacted loess to dry-wet cycles, multi-scale tests including direct shear, consolidation compression, crack quantification, SEM, NMR and XRD were designed and performed on samples from 0 to 9 cycles. The results indicate that both the shear strength and overall compressibility deteriorate substantially after the first dry-wet cycle and stabilize after five cycles. Meanwhile, mesoscopic cracks appear on the loess surface during the initiation, propagation and equilibrium phases, accompanied by a growth in the crack ratio, as well as a more chaotic crack distribution with cycles. The penetration of the pore space during repeated dry-wet leads to a decrease in the volume of micropores and small pores and an increase in that of mesopores and macropores. Additionally, particle rounding and orientation are slightly enhanced and the soil microstructure becomes more fragmented. Furthermore, the relative content of the primary minerals has grown insignificantly after performing the dry-wet cycles, while the clay minerals and soluble salts content has decreased inconsiderably. The deterioration of the macroscopic mechanical properties of compacted loess can be considered as the comprehensive manifestation of irreversible fatigue damage to its microscopic and mesoscopic structures induced by dry-wet cycles.