Laboratory Tests of a Theory of Fingering during Infiltration into Layered Soils

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Abstract
A theory has recently been presented by Hillel and Baker to explain and predict the occurrence of fingering and the fractional wetted volume during infiltration into layered soils in terms of measurable hydraulic properties. When the conductivity of a coarse‐textured sublayer at its effective water‐entry suction is greater than the rate of transmission through a finer‐textured top layer, the flow velocity increases across the interlayer plane. Thereupon, a spatially‐distributed flow field in the top layer will tend to constrict, forming spatially separated streams (fingers) in the sublayer. To test the hypothesis, ponded infiltration experiments were conducted in transparent laboratory chambers packed with air‐dry sands. The hydraulic properties of the media were determined by equilibrium capillary rise, parameter estimation, and horizontal absorption experiments. Dynamic measurements of flux and suction at the interlayer plane during infiltration revealed that the effective water‐entry suction is characteristic of the predominant pore size in the sublayer and can be predicted from its median particle size (r2 = 0.99). Predictions regarding the onset of fingering and the wetted fractional volume of fingers were both validated for the monodisperse and polydisperse sublayers studied.
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