The effect of height of sample and confinement on the moisture characteristic of an aggregated swelling clay soil

Abstract

The pore pressure/moisture content relationship of 0.5-1.0 mm aggregates of Narrabri soil, initially 1.66 cm high, was examined at 1/3 cm height intervals by means of dual source gamma ray scanning, for confined and unconfined conditions. In the unconfined sample, absorption and desorption of water to or from the interaggregate pore space occurred at less negative pore water pressures than in the confined sample. In the latter, the swelling aggregates deformed into the inter-aggregate pore space during absorption and reduced the size of the pores. In the unconfined sample, absorption into the inter-aggregate pore space occurred at less negative water pressures with increasing height in the sample. During absorption, bulk density decreased with height in the sample, and bulk density at all points within the sample decreased until pore water pressures were between -20 to - 10 cmH2O. At pore water pressures closer to zero, bulk density increased as the sample compacted under its own weight. During desorption, bulk density increased at all heights as the pore water pressures decreased from zero to - 8 cmH2O, because of isotropic stress imparted by the hanging water column. During this phase normal shrinkage occurred, therefore overburden potentials, and hence moisture potentials, could be calculated. Between - 10 and - 50 cmH2O, the inter-aggregate pore space drained rapidly, and no further increase in bulk density was detectable and hence moisture potential equates to manometric pressure. It was found that the derived moisture characteristics (Ø versus è) are different for different heights in the aggregate mass. This result is of consequence to the theory of hydrostatics in swelling media. In the unconfined sample, no differences in the pore water pressure/moisture content relationships occurred with height during absorption or desorption, and bulk density remained constant throughout. The application of these results to the behaviour of swelling soils in the field is briefly discussed. The use of dual source gamma ray scanning seems an essential tool for further investigations.