Characteristics and Computational Model of In-Situ Water-Induced Collapse Deformation in Loess Sites from Bottom to Top

Groundwater fluctuation-induced collapse of deep loess poses a critical threat to the long-term safety of deep-buried metro tunnels. A field sand-well immersion test was conducted along a metro line in Xi'an, utilizing a water-level control system to simulate wetting-induced deformation under constant overburden pressure with bottom-up infiltration. A new collapsibility model was established considering the hydromechanical path. Results show that moisture migration exhibits an inverted-funnel pattern, with saturation front angles of approximately 70° within 2 m, decreasing to 55° at 2~6 m, and increasing to 85° beyond 6 m. During immersion, deformation progressed through three stages: collapse governed by structural degradation, rebound controlled by cavity-induced unloading (positively correlated with water level), and compression resulting from residual structure failure (negatively correlated with water level). Post-immersion consolidation developed from shallow to deep and from the exterior inward. A "wetting-unloading" hydromechanical model was established, showing a relative error of only 8.85% between calculated and measured values at<italic>η</italic>₁ = 0.8 with actual unloading ratios. This study provides a reference for collapsibility evaluation of deep loess foundations in fluctuating groundwater regions.