EFFECTS OF CLAY TYPE AND RATE OF WETTING ON THE MELLOWING OF COMPACTED SOILS

Soil clays differing widely in mineralogy were individually mixed with sand and silt fractions from an Alfisol. The mixtures dominated by Ca2+ were wet and dried ten times to simulate field conditions and promote microaggregation of the soils. Compacted cores were prepared at fixed water contents and tensile strengths of the cores were measured before and after a number of wetting and drying cycles, wet at either fast or slow rates. Compacted soils which were dominated by randomly interstratified minerals (RIM) exhibited higher tensile strengths before wetting and drying cycles. Tensile strengths of the compacted soils decreased with increasing number of wetting and drying cycles to different extents for fast and slow rates of wetting. The results of the equilibrium tensile strength and the mellowing ratio of the compacted soils in relation to the properties of the clay fractions, e.g. cation exchange capacity (CEC), surface area and percentage of RIM and shrink-swell potential of the soils clearly showed that the higher the shrink-swell potential the higher the aggregation and soil softening. The degree of reversibility of soil compaction by development of microcracks caused by natural processes, e.g. wetting and drying cycles, increased as the percentage of shrink-swell potential and rate of wetting increased.