Direct measurement of a permeability function, k(þ ) (where þ is suction), of an unsaturated soil is often tedious and time-consuming. Thus, researchers have proposed various semi-empirical predictive methods to determine k(þ ) indirectly from a soil-water characteristic curve (SWCC) or a particle size distribution. Also, stress effects on k(þ ) are generally not considered. To explore any stress effect on k(þ), a new 1m high stresscontrollable soil column is developed to measure stress-dependent SWCC (SDSWCC) and k(þ ) of soils directly. The objectives of this paper are to investigate the effects of (1) two stress-state variables (matric suction and net normal stress) and (2) a drying-wetting cycle on both SDSWCC and k(þ ) of a compacted decomposed silty clay. Each compacted soil column is subjected to an evaporation-ponding cycle under various vertical net normal stresses. The variations of pore-water pressure and volumetric water content (VWC, þw) profile with time are measured instantaneously. The k(þ) is hence determined using the Instantaneous Profile Method. At a given average vertical net normal stress, the measured k(þ ) exhibits a remarkable hysteresis loop while the k(þw) seems to be less hysteretic. The effects of the vertical net normal stress appear to be more pronounced than that of the drying-wetting cycle on k(þw). As average vertical net normal stress is increased from 4 to 78kPa, the measured permeability decreases by up to two orders of magnitude at a given VWC.
