U.S. Geological Survey Toxic Substances Hydrology Program--Proceedings
of the Technical Meeting, Colorado Springs, Colorado, September 20-24, 1993,
Water-Resources Investigations Report 94-4015
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Measurements of Hydraulic Conductivity and Capillary Pressure
under Unsaturated Conditions in a Laboratory Triaxial System
by
Harold W. Olsen (U.S. Geological Survey, GD, Golden, Colo.)
Abstract
A new laboratory technique is being developed to provide media-specific
measurements of hydraulic conductivity and capillary pressure under unsaturated
conditions for modeling ground-water flow and contaminant transport in geologic
systems. The technique is designed to test undisturbed specimens in a triaxial
chamber under simulated in situ stress conditions and to provide direct
and concurrent measurements of vertical hydraulic conductivity, capillary
pressure, and the variation of these properties with degree of saturation.
Constant-flow hydraulic-conductivity measurements are conducted by infusing
and withdrawing identical flow rates at opposite ends of a specimen with
a flow pump and measuring the resultant induced head difference with a differential
pressure transducer. Additional transducers and flow pumps are used to (1)
measure the capillary pressure and effective stress, (2) control and change
the water content of the specimen during and between hydraulic conductivity
measurements, and (3) control the specimen volume. This new technique avoids
fundamental limitations of the methods commonly used in current practice,
including (1) the need for duplicate specimens and separate equipment for
laboratory measurements of hydraulic conductivity under saturated conditions
and soil water retention, (2) the lack of stress or volume control for simulating
in situ conditions in the pressure plate or tempe cell equipment generally
used to meas ure soil-water-retention curves, and (3) the need for pore-size-distribution
models to estimate the relationship between hydraulic conductivity and degree
of saturation from soil-water-retention data.
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