Modeling Aggregate Internal Pressure Evolution following Immersion to Quantify Mechanisms of Structural Stability

Abstract

Identification of the key components controlling aggregate stability is important in soil structure research. The deterioration of soil aggregates during rapid wetting has often been attributed to the swelling and internal pressure buildup resulting from the compression of entrapped air by the advancing wetting front. Organic matter is known to reduce the extent of slaking, but the different modes of action have not yet been quantified. The objective of the study was to use theoretical three-dimensional models to quantify the effect of paper sludge amendment on the key processes controlling internal pressure evolution. A clay loam and a silty-clay loam were incubated for a 2-wk period with different amounts and types of paper sludge. Aggregates were then selected, air dried, and then fixed to a hypodermic needle connected to a pressure transducer, and the whole system was immersed in distilled water while images and pressure evolution were recorded. For both soils, the maximum internal pressure was lower in the sludge-amended aggregates. From the models fitted to the observed data, it appears that the addition of paper sludge resulted in an increase of the potential at the wetting front and a decrease of the near saturated hydraulic conductivity. This result suggests that sludge addition reduces pressure buildup by reducing the rate of water entry, lowering the potential at the wetting front and reducing the hydraulic conductivity of the aggregate.