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. DOI: 10.1061/(ASCE)GT.1943-5606.0000560. (C) 2012 American Society of Civil Engineers.