Centrifugal Determination of Unsaturated Soil Hydraulic Properties.

The capillary pressure-saturation curve, S(P), and relative hydraulic conductivity function, K(S), are key soil hydraulic properties needed for modeling flow and transport in the vadose zone.  The K(S) function is difficult to obtain experimentally and, as a result, is generally predicted using parameters determined from experimental S(P) relationships. There is need for additional data sets, in which S(P) and K(S) are determined independently, to test the veracity of such predictions. In this NSF-DOE funded project we employed an ultra rock core centrifuge developed for the oil and gas industry (Coretest URC-628) to measure S(P) and K(S) on unconsolidated sediments from the vadose zone at Hanford, WA. By allowing the fluid saturation and capillary pressure profiles to equilibrate at a number of different angular velocities (multi-rate centrifuge experiment) the S(P) function for average conditions can be determined. The Brooks and Corey S(P) model at a physical point is then used to reconstruct the average saturation conditions. Parameters of the Brooks and Corey S(P) model are optimized by fitting modeled to measured average saturations. In a separate transient flow centrifuge experiment the fluid production over time at constant angular velocity is used to independently determine K(S); the Brooks and Corey S(P) model parameters are known and fixed while the parameters specific to the Brooks and Corey K(S) function are estimated inversely based on a numerical simulation of the transient flow problem.