Field tests on irrigation infiltration in thick loess

Frequent irrigation activities have triggered numerous landslide hazards along the margins of the loess platform. For a better understanding of the process of irrigation water penetrating through stratified loess sediments, a full-scale field infiltration experiment with a diameter of 20 m is conducted on the South Jingyang tableland, Shaanxi Province, China. The amount of irrigation water, volumetric water content, matric suction and pore-air pressure are monitored to reveal the infiltration process of loess sediments. The monitoring results can be drawn as follows: (1) The propagation of wetting front is more than 11 m under the ponding condition. The infiltration rate is initially high, then decreases gradually and finally approaches a constant value less than the saturated hydraulic conductivity of the soil in shallow depth. (2) The vertical fractures parallelling to the edge of the tableland in Malan loess are revealed through vertical shaft, which has an aperture of less than 15 mm. The preferential flow is observed in the preferential path of the Malan loess layer. (3) The first paleosol layer (S₁) is proved to be permeable, and a transient perched water with a waterhead less than 44 cm is developed above the lower part of S₁. (4) The pore-air in soil is compressed, and an air entrapment ahead of wetting front is observed during wetting period. The maximum pore-air pressure measured in loess sediments varies from 1.1 kPa to 4.3 kPa.