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

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Title: Determination of the hydraulic characteristics of unsaturated soils using a centrifuge permeameter
Author: McCartney, John Scott, 1979-
Abstract: 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 .
URI: http : / /hdl .handle .net /2152 /3365
Date: 2008-08-28

Citation

Determination of the hydraulic characteristics of unsaturated soils using a centrifuge permeameter. Doctoral dissertation, The University of Texas at Austin. Available electronically from http : / /hdl .handle .net /2152 /3365 .

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