Thermal properties of swelling clay soils

Abstract

The thermal properties of swelling clay soils do not appear to have been previously studied, even though they should be different from those of non-swelling soils because of changes in pore size and structure with water content. We measured the thermal diffusivity of small discs cut from cores of a highly swelling black earth (Pellustert) using the pulse method, common in materials science but not previously applied to soils. Thermal conductivity was calculated from thermal diffusivity and heat capacity per unit volume. The behaviour of these thermal properties was indeed different from those reported for nonswelling soils. The conductivity and diffusivity at low water contents were several times higher, increased much less with water content, and eventually decreased when the soil became saturated and swelled. The heat capacity per unit volume increased more slowly with water content because of the swelling. Over the agriculturally important suction range from 10 kPa to 1.5 MPa (0.1 to 15 bar), the conductivity decreased with increasing water content, a behaviour opposite to that of non-swelling soils. The behaviour could be predicted by three theoretical models, each with two parameters estimated from the data. The first parameter, the conductivity of soil solids, was common to all models, and estimated values ranged from 2.2 to 2.6 Wm-' K-l. The model of de Vries, commonly used in soil science, was applied with the soil solids matrix as the continuous medium and fitted the data particularly well. The second parameter in this model was a shape factor for small pores. Its estimated value corresponded with spheroidal pores with a diameter-to-height ratio of 16, which is reasonable in a swelling clay soil. The de Vries model was used to calculate the conductivity of bulk soil in the vertical direction, assuming that 20% of soil shrinkage appeared as isolated, randomly distributed cracks between aggregates. The conductivity was substantially less than that of the aggregates at lower water contents, but differed little for wetter soil at suctions below 1.5 MPa. In contrast, calculated values of conductivity were much lower when the de Vries model was applied with air as the continuous medium to a cultivated soil structure such as might be found in a seedbed.