Controlling mechanisms of rainfall intensity on erosion and progressive instability of unsaturated loess slopes in heifangtai: insights from integrated experimentation and coupled numerical simulation

Integrated physical experiments and numerical simulations quantitatively elucidate erosion mechanisms of loess slopes under rainfall intensities of 20–60 mm/d, representing moderate to torrential events. Controlled laboratory scouring tests on Heifangtai loess samples demonstrate that hydro-mechanical processes driving slope failure are critically controlled by both rainfall intensity and slope geometry. Key findings reveal: (1) slope erosion progresses sequentially through splash erosion, rill development, and collapse, with transition time decreasing by over 30% under 60 mm/d compared to 20 mm/d; (2) cumulative sediment yield exhibits a strong positive correlation with slope inclination (30°–60°), where a 45° slope produces up to 25% more erosion than a 30° slope under identical rainfall; (3) numerical simulations quantify stability degradation, showing the safety factor drops to 2.84 after 24 h of rainfall at 60 mm/d. A critical divergence emerged at 40 mm/d, where the numerical safety factor initially exceeded the analytical solution but fell below it after 15 h, with a maximum discrepancy of 0.06, indicating simplified analytical methods may underestimate risks during prolonged moderate-intensity rainfall. This study provides quantitative insights and a validated modeling framework for geo-hazard assessment and slope mitigation strategies in loess regions under increasing rainfall extremes.