Large-scale infiltration experiments into unsaturated stratified loess sediments: Monitoring and modeling

Two large-scale field experiments were conducted to track water flow through unsaturated stratified loess deposits. In the experiments, a trench was flooded with water, and water infiltration was allowed until full saturation of the sediment column, to a depth of 20 m, was achieved. The water penetrated through a sequence of alternating silty-sand and sandy-clay loess deposits. The changes in water content over time were monitored at 28 points beneath the trench, using time domain reflectometry (TDR) probes placed in four boreholes. Detailed records were obtained from a 21-day-period of wetting, followed by a 3-month-period of drying, and finally followed by a second 14-day-period of re-wetting. These processes were simulated using a two-dimensional numerical code that solves the flow equation. The model was calibrated using PEST. The simulations demonstrate that the propagation of the wetting front is hampered due to alternating silty-sand and sandy-clay loess layers. Moreover, wetting front propagation is further hampered by the extremely low values of the initial, unsaturated, hydraulic conductivity; thereby increasing the water content within the onion-shaped wetted zone up to full saturation. Numerical simulations indicate that above-hydrostatic pressure is developed within intermediate saturated layers, enhancing wetting front propagation.