Loess slope toe degradation as a result of capillary water dynamics

The fragmented topography of the Chinese Loess Plateau hosts tens of thousands of loess slopes, whose failures critically threaten linear infrastructure safety, including pipelines, highways, railways, etc. Through integrated field surveys of 102 loess slopes, in-situ geotechnical investigations, and long-term in-situ moisture monitoring, this study identifies capillary water dynamics as the predominant cause for loess slope toe degradation, which is one of the main precursors of loess slope failures. Key findings reveal that water accumulation at slope–foot junctions triggers upward and inward capillary migration, forming a zone of elevated moisture (18 %–35 %) that expands both during and for hours post-rainfall. This zone exhibits a scalene triangular cross-section (vertical height: horizontal depth ≈ 1.6), with maximum capillary migration height and depth correlating linearly with rainfall amount. Subsequent evaporation gradually restores baseline soil moisture of this zone. The capillary-driven wetting-drying cycles within loess slope toe area induce dissolution and transport of soluble salts (mainly Na₂CO₃ and Na₂SO₄), and formation of stratified salt crust via evaporative reprecipitation (mainly CaSO₄·2H₂O and Mg(OH)₂). This mechanism degrades soil fabric and reduces soil strength. The spatial extent of the softening zone with low strength identified by cone penetration tests overlaps with the integrated zones of high moisture concentration and high salt concentration. Superimposed weathering processes (e.g., thermal fluctuations) induce cyclic shrink–swell deformation, and initiate surface spalling, promoting concave undercutting along loess slope toe area. Partial toe suspension creates cantilevered slope segments. Such slope geometry triggers viscoplastic deformation within the degraded zones, causing stress redistribution in the slope, which in turn promotes slope deformation. Under cyclic deformation–stress interplay, a failure surface is ultimately formed. This mechanistic understanding highlights the necessity to implement hydraulic isolation techniques at loess slope toe as targeted stabilization measure, particularly under climate change-induced precipitation intensification.