Macroscopic and Microscopic Characteristics of Strength Degradation of Silty Soil Improved by Regenerated Polyester Fibers under Dry–Wet Cycling

The structural stability of silt foundations, particularly sensitive to moisture content, can be severely compromised by recurring wetting and drying processes. This not only threatens the foundational integrity but also raises grave concerns about the long-term safety of major civil engineering endeavors. Addressing this critical issue, our study delves into the transformative effects of reclaimed polyester fiber on subgrade silt exposed to such environmental stressors. Through rigorous wet–dry cycle tests on this enhanced soil, we evaluate shifts in shear strength across varying confining pressures. We also dissect the interplay between average pore diameter, particle distribution, and morphology in influencing the soil’s microstructural responses to these cycles. A detailed analysis traces the structural damage timeline in the treated soil, elucidating the intertwined micro–macro dynamics driving strength reduction. Key discoveries indicate a notably non-linear trajectory of shear strength degradation, marked by distinct phases of rapid, subdued, and stabilized strength attrition. Alterations within the micropores induce a rise in both their count and size, ultimately diminishing the total volume proportion of the reinforced soil. Intriguingly, particle distribution is directly tied to the wet–dry cycle frequency, while the fractal dimension of soil particles consistently wanes. This research identifies cement hydrolysis and pore expansion as the dominant culprits behind the observed macroscopic strength degradation due to incessant wet–dry cycles. These revelations hold profound implications for risk management and infrastructural strategizing in areas dominated by silt foundations.