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Prediction of Infiltration of Water into Aggregated Clay Soil Samples¹

ABSTRACT

The physical properties—stability, water retention, diffusivity and conductivity—relevant to the study of infiltration into aggregated media were measured for four aggregate sizes: 2.38–2.00 mm; 1.168–0.840 mm; 0.417–0.295 mm; 0.105–0.053 mm referred to in the study as sizes 1, 2, 3, and 4. There were < 15, 5, and 4% aggregate breakdown of sizes 1, 2, and 3 during infiltration. These media were considered stable to infiltration. The large volume changes (swelling and shrinking) which result from wetting and drying aggregates of sizes 4 and the nature of the hysteresis curves suggest that aggregate rear-rangement rather than breakdown may be dominant. The release of water in two distinct steps and the several thousand-fold decrease in conductivity at the moisture content of these two steps, particularly for sizes 1 and 2 suggest that these media can be described as well-aggregated, i.e. having a discontinuity of pore sizes.

The hysteresis in moisture retention, equilibrium moisture retention curves, and the changes in moisture retention with time were measured for confined and unconfined samples of the aggregates. Wetting and drying diffusivities and conductivities of confined and unconfined samples were also measured and used in the prediction of horizontal and vertical infiltration under zero and small negative pressures into columns of each of the four aggregate sizes using the diffusion equation. Diffusivities and conductivities were larger on wetting than on drying and generally larger in unconfined than in confined samples.

Horizontal and vertical infiltration were reasonably well predicted when water infiltrated under negative pressure and the diffusivities and conductivities used were calculated from infiltration profiles developed under the same water tension. Predictions of infiltration under zero pressure were generally not as successful.

For these media, the values of water tension, diffusivity and conductivity at any water content depend on the rate of wetting. The values to be used in the prediction of infiltration must therefore be measured for times of wetting which correspond to the duration of infiltration.

The classical diffusion equation can be used to predict infiltration into aggregated claysoils if the correct diffusivities and conductivities are used.

¹ Contribution from the Dept. of Soil Science, Macdonald College of McGill Univ. Montreal, Quebec, Canada. Part of the work submitted as a Ph.D. thesis by the senior author. This study was supported by a Grant in Aid of Research from the National Research Council, Canada.

² Graduate Research Student and Professor of Soil Science. The senior author is now Lecturer in Soil Science, Univ. of the West Indies, St. Augustine, Trinidad.

Received for publication January 4, 1974. Accepted for publication December 10, 1974.