Abstract

Soils under loaded conditions may have different shrinkage behaviour from that of load-free soils. In this study, we applied two kinds of mechanical stress (s) on repacked homogeneous soil samples: transient and constant stresses, simulating the traffic load during tillage and the overburden pressure, respectively. Three transient stresses were applied on the soil surface with 150, 400 and 1400 kPa, while the constant stresses ranged from 1.8, 3.8, 5.5, to 7.3 kPa. We hypothesized that the two stresses play different roles in soil shrinkage behaviour as depicted by void ratio (e) and moisture ratio, as compared with load-free soil. Thus, our aim was to build up the relationship between e, and s. For a swelling soil, total pores can be divided into rigid and non-rigid components according to their swelling and shrinkage capacity relative to soil moisture. The non-rigid pores compacted by the transient stress can be regained in the subsequent wetting at load-free conditions, whereas the compacted rigid pores do not recover. The reduction in rigid pores does not alter the soil pore shrinkage capacity. The shrinkage curves of transiently-loaded soils are therefore parallel to each other with an identical coefficient of linear extensibility (COLE) and the same shrinkage slope, although their structural shrinkage phase narrows with an increase of stress. However, the constant stress compresses non-rigid pores readily through suppressing their swelling capacity during wetting as well as compacting rigid pores. If the change of rigid pores is negligible, the shrinkage curves of constantly-loaded soils converge at the zero shrinkage or the dry-end point with the load-free soil shrinkage. If the reductions of rigid and non-rigid pores are both considered, the soil shrinkage combines the part of parallel shrinkage derived from the reduced rigid pores and the intersected shrinkage resulted from the altered non-rigid pores. On the basis of different shrinkage behaviours resulting from the two mechanical stresses, we propose numerical formulae to illustrate a series of curves for the e-s relationship. The different changes in rigid and non-rigid pores cause soil water release differently. © 2009 British Society of Soil Science.