In-situ and laboratory investigations of stress-dependent permeability function and SDSWCC from an unsaturated soil slope

Permeability function of an unsaturated soil, k(?), where ? is suction, is a vital hydrogeological property that governs seepage in various geotechnical problems. Owing to considerably long test duration, direct measurement of k(?) is often avoided if at all possible. Instead, numerous semi-empirical predictive equations have been developed to determine k(?) indirectly. However, effects of drying-wetting history and net normal stress are not generally considered, casting doubts on the validity of some semi-empirical predictive equations. In this paper, stress-dependent k(?) and stress-dependent soil-water characteristic curve (SDSWCC) of a decomposed silty clay are investigated under both field and laboratory conditions. To measure effects of drying and wetting on k(?) directly, an in-situ one-dimensional (1D) permeability test was carried out using the instantaneous profile method on a saprolitic hillslope in Hong Kong. In the laboratory, a new 1D stress-controllable soil column was developed to determine stress-dependent k(?) and SDSWCC on block samples taken from the same hillslope. Effects of drying-wetting cycle(s) and net normal stress on measured stress-dependent k(?)s and SDSWCCs are explored and analysed. By comparing measured and predicted k(?)s, the predictability of some existing semi-empirical equations is evaluated.