Enhancement of the crack and erosion resistance of silty clay under freeze–thaw cycles: Synergistic effect of sisal fiber–fly ash

Freeze–thaw (F–T) cycle-induced cracking in silty clays poses a significant risk to engineering stability. Although the individual addition of fly ash (FA) or sisal fiber (SF) provides partial solutions, their simultaneous application may result in a synergistic effect to compensate for their respective shortcomings. In this study, the effects of SF and FA on the mechanical properties, crack resistance, water retention, and erosion resistance of improved soil were systematically investigated through unconfined compressive strength (UCS) tests, crack evolution analysis, simulated rainfall erosion tests, and microscopic characterization (laser particle size analysis and nitrogen adsorption). The results reveal that the volumetric stability of FA particles significantly inhibits cracking in soil after F–T cycles. However, FA contributes only slightly to soil strength and erosion resistance. SF, on the other hand, plays a substantial role in increasing both soil strength and erosion resistance. The synergy between FA and SF results in the simultaneous increase in crack resistance, erosion resistance, and strength. FA improves the aggregate stability during F–T cycles, whereas SF reinforces the bonds between these aggregates. A comprehensive evaluation of the improved soil during F–T cycles using the entropy weight-TOPSIS method reveal that the combination of 10% FA + 18 mm SF performed the best, achieving a 246% higher composite score than the unmodified soil did. With respect to this optimal combination, compared with the unmodified soil, the SF–FA-improved soil exhibits a 30% reduction in the average crack width, a 30% reduction in the erosion rate, and a 46% increase in strength. The findings of this study provide a scientific basis for the design of soil improvement in disaster mitigation engineering in seasonally frozen soil regions.