Although time-dependent deformation of geomaterials underpins slope-failure prediction models, the influence of strain rate on shearing strength and deformation behavior of loess remains unclear. The consolidated undrained (CU) and drained (CD) triaxial testing elucidated the impact of strain rate (0.005-0.3 mm/min) on strength envelopes, deformation moduli, pore pressures, and dilatancy characteristics of unsaturated and quasi-saturated loess. Under drained conditions with a controlled matric suction of 50 kPa, increasing strain rates from 0.005 mm/min to 0.011 mm/min induced decreases in failure deviatoric stress (qf), initial deformation modulus (Ei), and cohesion (c), while friction angles remained unaffected. Specimens displayed initial contractive volumetric strains transitioning to dilation across varying confining pressures. Higher rates diminished contractive volumetric strains and drainage volumes, indicating reduced densification and strength in the shear zone. Under undrained conditions, both unsaturated and quasi-saturated (pore pressure coefficient B = 0.75) loess exhibited deteriorating mechanical properties with increasing rates from 0.03 mm/min to 0.3 mm/min. For unsaturated loess, reduced contractive volumetric strains at higher rates manifested relatively looser structures in the pre- peak stress phase. The strength decrement in quasi-saturated loess arose from elevated excess porewater pressures diminishing effective stresses. Negative porewater pressures emerged in quasi-saturated loess at lower confining pressures and strain rates. Compared to previous studies, the qf and Ei exhibited rate sensitivity below threshold values before attaining minima with marginal subsequent influence. The underlying mechanism mirrors the transition from creep to accelerated deformation phase of landslides. (c) 2025 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/ 4.0/).

To investigate the freeze-thaw resistance of hydroxypropyl methylcellulose (HPMC)-modified loess, the study analyzed the effects of HPMC dosage and the number of freeze-thaw cycles on the shear strength of modified loess through triaxial shear tests. The results indicated that the peak stress of modified loess exhibited a tendency to increase and then decrease with the increase of the dosage. The optimal dosage of HPMC was 0.5 %. When the confining pressure were 100kPa and 200kPa, the stress-strain relationship curves of modified loess with optimal dosage after freeze-thaw cycles exhibited a weak hardening behavior; at confining pressures of 300kPa and 400kPa, the stress-strain curves exhibited weak softening behavior. As the number of freeze-thaw cycles increased, the peak stress and shear strength indices of the optimal dosage of modified loess exhibited fluctuations. with the lowest values observed after five freeze-thaw cycles. Based on the peak stress, an anti-freezethaw modification effect parameter was proposed, which exhibited a positive correlation with the number of freeze-thaw cycles. The anti-freeze-thaw modification effect parameters for modified loess with optimal dosage were all greater than 1, indicating that the addition of HPMC significantly enhances the freeze-thaw resistance of seasonally frozen soil. HPMC functions both within the soil and at the air-water interface, forming hydrogen bonds, three-dimensional network structures, and flocculated agglomerates, thereby enhancing the strength and freeze-thaw resistance of the soil.