Macropores and hydraulic conductivity in a swelling soil.

Abstract

The volume of macropores filled with water in a near-saturated swelling clay soil has been measured and related to the hydraulic conductivity. To measure the macropore volume, it has been assumed that only water absorbed by the soil matrix (or peds) contributes to swelling, that this swelling is 'normal', and hence the volume of water held in macropores (termed 'structural' or interpedal water) does not contribute to swelling. This volume of water is thus estimated from the difference between water absorption by the soil and the normal swelling, provided that previous drying had not reached the shrinkage limit. The hydraulic conductivity was measured and found to be exponentially related to the volume of structural water, at least for a degree of saturation greater than 0.90. The relative proportions of the three types of macropore - planes, channels and vughs - were evaluated from thin sections. The profile depth interval for which hydraulic conductivity was found to be greatest, coincides with the depth at which channels and interconnected vughs show maxima.