Determination of Hydraulic Properties of Unsaturated Soils Based on Nonequilibrium Multistep Outflow Experiments

In this paper, a new procedure for rapidly determining the hydraulic properties of unsaturated soils is developed. The proposed procedure is used to characterize the evolution of the instant degree of saturation in an unsaturated soil during a multistep outflow experiment. By introducing the concept of capillary relaxation, a theoretical model is developed and adopted to describe the dynamic hydraulic properties of unsaturated soils under constant-volume conditions. A series of nonequilibrium multistep outflow experiments are performed on various types of unsaturated soils. It is shown that the evolution of the instant degree of saturation or the outflow mass generally follows an exponential law. Both experimental results and theoretical simulations illustrate that flow instability and water blockage can occur in the soil during multistep outflow experiments. The theoretical predictions are compared to the experimental results, showing that the proposed procedure can be effectively used to determine the hydraulic properties of unsaturated soils. Because the outflow test is performed under nonequilibrium flow conditions, the experimental time for measuring the hydraulic properties is much shorter than those needed for conventional equilibrium procedures. (C) 2016 American Society of Civil Engineers.