RELATIONS BETWEEN PORE-SPACE AND HYDRAULIC-PROPERTIES IN COMPACTED BEDS OF SILTY-LOAM AGGREGATES

The aim Of this study was to better understand features of the hydrody namics of aggregated soil from experiments conducted on repacked soil samples. In the laboratory, aggregates 2 to 3 mm in diameter were prep ared using a silty-loam soil material. These aggregates were wet to a water content close to their air-entry point value, and compacted to a predetermined bulk density. Two sets of samples were studied, with me an bulk density ranging between 1.0 and 1.6 Mg . M-3. Aggregates were extracted from samples of a first set, and their bulk density and wate r retention curve were determined. Water retention curve and unsaturat ed hydraulic conductivity of samples of a second set were determined u sing an instantaneous profile method. Experimental results were interp reted with respect to a functional pore-space classification. The wate r retention properties of the aggregates extracted from the samples we re different, although differences in porosity between such aggregates were sometimes too small to be measured. This was explained by a narr owing in the constrictions of intra-aggregate pores induced by compact ion, even when this compaction was small. Discontinuities in the shape of the hydraulic properties of a loosely-compacted bed of aggregates were found. This was consistent with an independent desaturation of th e inter- and intra-aggregate pores. However, unsaturated hydraulic con ductivity of the loosely-compacted sample was affected by a small amou nt of water, which formed menisci between the aggregates close to the air entry point value of the aggregates. Soil hydraulic conductivity i ncreased with the degree of compaction for a range of mass-based water contents dryer than the water content at the air-entry point value of the aggregates. The influence of the contact surface between the aggr egates on the unsaturated hydraulic conductivity of the samples was qu antified, by assuming that water flows only through the aggregates. Th ese experimental results are related to aspects of soil hydrodynamics, which are difficult to study in situ and difficult to predict with th e existing models for the soil hydraulic properties.