Macroscopic mechanical response and microscopic action mechanism of hydrogel–modified Yili loess in seasonal frozen region

Abstract Background To address the significant geotechnical engineering challenge of stability degradation in loess slopes within the Yili River Valley region induced by seasonal freeze–thaw (F–T) cycles, the study innovatively introduces polymer hydrogel (dosage of 3%) as the modified material. Undisturbed loess from Ili, Xinjiang, was used as the test material, and hydrogel-treated loess specimens with initial water contents of 14.2%, 18.2%, and 22.2% were prepared. F–T cycling tests, conventional triaxial shear tests, scanning electron microscopy (SEM), nuclear magnetic resonance (NMR) tests, and Fourier-transform infrared spectroscopy (FTIR) analyses were conducted. These tests were used to quantitatively reveal the evolution of the mechanical properties and pore structure of the treated soil at macro- and microscales, and to clarify the intrinsic mechanism underlying the F–T resistance of hydrogel-treated loess. Results Hydrogel treatment significantly altered the stress–strain behavior of loess. The soil response changed from brittle strain-softening behavior in untreated loess to ductile strain-hardening behavior in hydrogel-treated loess, accompanied by a pronounced F–T strengthening effect. After 20 F–T cycles, the shear strength of the treated soil did not decrease; instead, it increased from 269 to 491 kPa, representing an increase of 82.5%. Microstructural tests indicate that the three-dimensional network structure of the hydrogel can buffer frost-heave stress and restrict detrimental pore coarsening. In the treated group, the pore fractal dimension changed only slightly, with a variation of -0.003; the proportion of macropores increased only marginally by 2.495 percentage points, and the mean pore diameter increased by 0.52 μm. In contrast, the pore fractal dimension of the control group increased by 0.0078, while the proportion of macropores increased to 4.06% and the mean pore diameter increased by 0.80 μm. These results demonstrate that hydrogel incorporation effectively inhibits F–T induced pore coarsening and pore expansion. Fourier-transform infrared spectroscopy further shows that hydrogen-bond interactions in the treated soil were markedly enhanced after F–T cycling, which is the primary reason for the increase in strength after F–T cycles. Conclusions The research results not only clarify the micro-scale mechanism of hydrogel improvement but also demonstrate a provides a novel from traditional ‘damage resistance’ to innovative ‘strength enhancement’ for loess in seasonal frozen regions, providing important theoretical and practical guidance for slope stability prediction and ecological restoration.