DRYING, CRACKING, AND SUBSIDENCE OF A CLAY SOIL IN A LYSIMETER

The relation between changes in water content and swelling and shrinkage processes was studied by exposing an undisturbed heavy clay soil in a lysimeter to evaporation at controlled conditions in the laboratory during a period of 82 days. Changes in water content were measured with tensiometers and by weighing the lysimeter. Swelling and shrinkage were determined by measuring the surface subsidence. The loss of water from the clay soil amounted to 45 mm, 40% less than the loss of water from a comparable silty soil lysimeter. Drying of the clay soil was restricted to the top 15 cm of the soil. As much as 67% of the water loss originated from the top 7.5 cm of the soil. Simultaneous shrinkage in the clay soil resulted in a three-dimensional decrease in volume of 34 mm, consisting of a crack volume of 22 mm and a surface subsidence of 12 mm. The clay soil exhibited the successive occurrence of structural shrinkage, isotropic normal shrinkage, isotropic residual shrinkage, and isotropic normal shrinkage again. The occurrence of normal and residual shrinkage could be predicted by the water content changes in the soil and the shrinkage characteristics of soil aggregates. Water loss in the structural shrinkage phase occurred from interaggregate pores and could therefore only be qualified from the lysimeter experiment.