Measurements of Drying and Wetting Permeability Functions Using a New Stress-Controllable Soil Column

Direct measurement of a permeability function, $k(\psi )$ (where $\psi$ = suction), of an unsaturated soil is often tedious and time-consuming. Thus, researchers have proposed various semiempirical predictive methods to determine $k(\psi )$ indirectly from a soil-water characteristic curve (SWCC) or a particle-size distribution. Also, stress effects on $k(\psi )$ are generally not considered. To explore any stress effect on $k(\psi )$ , a new 1-m-high stress-controllable soil column is developed to measure stress-dependent SWCC (SDSWCC) and $k(\psi )$ 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 $k(\psi )$ 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, ${\theta }_w$ ) profiles with time are measured instantaneously. The $k(\psi )$ is hence determined by using the instantaneous profile method. At a given average vertical net normal stress, the measured $k(\psi )$ exhibits a remarkable hysteresis loop, whereas the $k({\theta }_w)$ 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 $k({\theta }_w)$ . As average vertical net normal stress is increased from 4 to 78 kPa, the measured permeability decreases by up to two orders of magnitude at a given VWC.