Determination of the hydraulic characteristics of unsaturated soils using a centrifuge permeameter

A new experimental approach to determine the hydraulic characteristics of unsaturated soils using a centrifuge permeameter was developed in this study. Specifically, the centrifuge permeameter is used to determine the water retention curve (WRC), which quantifies the energy required to retain water in the soil pores during wetting and drying, and the hydraulic conductivity function (K-function), which quantifies the soil's change in impedance to water flow as it becomes unsaturated. An aim of this study is the promotion of using experimentally-derived hydraulic characteristics in engineering practice. Accordingly, the goals behind development of the centrifuge permeameter were a reasonable testing time, measurement of all variables relevant to water flow in unsaturated soils, and a methodology allowing straightforward interpretation of experimental data to determine the hydraulic characteristics. Development of the centrifuge permeameter was guided by lessons learned from an evaluation of previous characterization approaches. Specifically, issues such as the use of steady-state or transient water flow, boundary condition effects, and the use of instrumentation were evaluated in conventional tests to better develop the centrifuge permeameter. Steady-state infiltration of water through a soil specimen instrumented with tensiometers to measure matric suction and time domain reflectometry to infer moisture content was found to be the most reliable means of characterization. Steady-state water flow permits straightforward, repeatable interpretation of instrumentation results, boundary conditions, and flow data to determine the hydraulic characteristics. Centrifugation is employed to decrease the time required to reach steady-state water flow through a soil specimen by imposing a centripetal acceleration on the infiltrating water. The water infiltration rate and centripetal acceleration can be independently controlled in the centrifuge permeameter in order to reach different target hydraulic conductivity values. Continuous, in-flight measurement of the variables relevant to hydraulic characterization is possible through an on-board data acquisition system. The experimental component of this study is focused on validation of the centrifuge permeameter and verification of the hydraulic characteristics obtained using this approach. Simultaneous determination of the WRC and K-function for a clay of low plasticity was found to be possible in less than a week using the centrifuge permeameter, whereas several months were required in conventional tests. Consistent measurements of hydraulic conductivity were obtained using this approach, and little hysteresis was observed in the hydraulic characteristics. Additional experiments were performed to evaluate the validity of different assumptions required to interpret the experimental data and different issues in centrifuge testing. Two major assumptions required in previous centrifuge permeameter approaches were evaluated using the instrumentation available in the centrifuge permeameter. During steady-state water flow in the centrifuge, the suction and moisture content were found to be relatively uniform along the longitudinal axis of the permeameter, and the outflow boundary was found to have a negligible influence on the suction profile. Settlement under the increased body forces in the centrifuge were found to be negligible for the soil investigated in this study. The hydraulic characteristics were found to be sensitive to the calibration of the transducers and sensors used to infer the water pressure and moisture content during centrifugation. Overall, the expeditious, direct determination of the hydraulic characteristics of unsaturated soils was successfully achieved using centrifuge technology. Accordingly, the centrifuge permeameter approach helps promote the use hydraulic characteristics of unsaturated soils in geotechnical engineering design.