Rainfall-induced failures in red clay slopes are common, yet the coupled influence of soil structure degradation and rainfall temporal patterns on slope hydromechanical behavior remains poorly understood. This study advances the understanding by investigating a cut slope failure in Yunnan through integrated field monitoring, laboratory testing, and numerical modeling. Key advancements include: (1) elucidating the coupled effect of structure degradation on both shear strength reduction and hydraulic conductivity alteration; (2) systematically quantifying the impact of rainfall temporal patterns beyond total rainfall; and (3) providing a mechanistic explanation for the critical role of early-peak rainfall. Mechanical and hydrological parameters were obtained from intact and remolded samples, with soil-water retention estimated via pedotransfer functions. A hydro-mechanical finite element model of the slope was constructed and calibrated using recorded rainfall, displacement data and failure surface. Six simulation scenarios were designed by combining three strength conditions (intact at natural water content, intact at saturation, remolded at natural water content) with two hydraulic conductivity values (intact vs. remolded). Additionally, four synthetic rainfall patterns, including uniform, peak-increasing, peak-decaying and bell-shaped rainfall, were simulated to evaluate their influence on pore water pressure development and slope stability. Results show remolding reduced hydraulic conductivity 4.7-fold, slowing wetting front advance and increasing shallow pore water pressure. Intact soil facilitated deeper drainage, elevating pressure near the soil-rock interface. Strength reduction induced by structure degradation (water saturating and remolding) enlarged the slope deformation zone by 1.5 times under same hydraulic conductivity. Simulations using saturated intact strength best matched field observations. The results from this specific slope indicate that strength parameters primarily control stability, while permeability affects deformation depth. Simulations considering different rainfall patterns indicate that slope stability depends more critically on the temporal distribution of rainfall intensity than on the total amount. Overall, peak-decaying rainfall led to the most rapid rise in pore water pressure, earliest instability and lowest failure rainfall threshold, whereas peak-increasing rainfall showed the opposite trends. Our findings outline a practical framework for assessing red clay slope stability during rainfall. This framework recommends using saturated intact strength parameters in stability analysis. It highlights the important influence of rainfall temporal patterns, especially those with an early peak, on failure timing and rainfall threshold.

Long-term water immersion can substantially alter the hydro-mechanical behavior of water-sensitive geomaterials used in highway subgrades. This study investigated the responses of highly weathered phyllite (HWP) and 2% cement-improved HWP (CIHWP) using a full-scale subgrade model subjected to prolonged water immersion and intermittent static surcharge loading. The degree of compaction, California bearing ratio (CBR), and dynamic deformation modulus ($E_{vd}$) were measured during construction. After construction, earth pressure, pore water pressure, volumetric water content, and layered settlement were monitored. Both materials met the required compaction criteria. However, CIHWP exhibited better compactability and higher CBR and $E_{vd}$ values. Following long-term immersion, water content increased across approximately 88% of the HWP profile, while the saturated area accounted for approximately 28% of the profile. Progressive wetting induced aggregate softening, stress redistribution, and increased deformation, with the final surface settlement reaching 9.3 mm under the applied static surcharge. In contrast, only approximately 12% of the CIHWP profile showed moisture changes, and its final settlement was 6.5 mm. These findings provide full-scale experimental evidence for understanding and improving the hydro-mechanical performance of water-sensitive geomaterials in transportation infrastructure, with direct implications for applied geotechnical engineering and subgrade stabilization.

Reliable characterization of freeze–thaw-induced structural change is essential for evaluating compacted soils used in cold-region engineering, where repeated freezing and thawing can alter both surface integrity and internal pore structure. This study combines digital imaging, quantitative crack analysis, and X-ray computed tomography to characterize structural changes in compacted expansive soil subjected to 0, 1, 4, 7, and 10 freeze–thaw cycles. Specimens with initial water contents of 18% and 25% were observed during freezing and after thawing, while the post-thaw three-dimensional pore structure of the 18% specimen was reconstructed from CT images. The 18% surface specimen exhibited no continuous crack network despite evident internal pore restructuring, whereas the 25% specimen showed pronounced crack opening during freezing and partial closure after thawing. With increasing cycles, the crack pattern evolved from a dominant wide crack to a branched network of narrower cracks and then became less continuous. CT-resolvable porosity increased from 23.0% initially to 26.7% after four cycles and then remained nearly stable, whereas connected porosity increased to 25.4% after ten cycles. These complementary observations indicate that post-thaw surface appearance alone may underestimate freeze–thaw-induced structural disturbance and highlight the importance of moisture condition and early-cycle evolution in cold-region engineering.