PORE VOLUME‐SIZE DISTRIBUTION AND SWELLING OF NATURAL SOIL AGGREGATES

Abstract

Summary Non‐polar liquid (benzene and 1,1,2,2‐tetrachloroethane) content‐energy curves for soil aggregates have been obtained by using suction plate and pressure membrane apparatus (equivalent pF 0·5‐4·3). For low relative vapour pressure measurements a temperature differential was maintained between two connected chambers, one of which contained the soil aggregates and the other liquid benzene. The results are expressed as the volume of liquid retained in relation to the logarithm of the equivalent suction in centimetres of water (equivalent pF). This procedure enables a direct comparison to be made between the pF‐water content curves and the equivalent pF‐non‐polar liquid curve. When such comparisons are made for a number of clay soils it is apparent that 70 per cent. or more of the swelling takes place before pF 3 is attained on the wetting cycle. By differentiating the non‐polar liquid‐equivalent pF curves for a number of soil materials it has been possible to obtain the pore volume‐size distribution curves. All the clay materials gave broad peaks which extend between pF 2 and 4 and a small peak near pF 5. The peak which occurs just near pF 5 in clay soils is very pronounced for a loam soil. The peak in the coarse pore range occurs irrespective of origin and therefore appears characteristic of clay materials which have been through a number of wetting and drying cycles. The presence of this broad peak indicates that normal shrinkage cannot be expected for natural clay aggregates. The coarse pores which give rise to the broad peak can be regarded as structural pores which are eliminated by compression of the clay.