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

Measurements of Hydraulic Conductivity and Capillary Pressure under Unsaturated Conditions in a Laboratory Triaxial System

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.