Soil Particle Size Analysis Using X-Ray Absorption

Abstract

The hydrometer method is the standard method of grain size analysis used in geotechnical engineering. Although the hydrometer method provides accurate grain size distributions and is relatively easy to conduct, it takes a minimum of 2 days to complete and is subject to operator error. In studies where small-magnitude changes or more rapid results are required, an alternative method to hydrometer testing is to use an automated particle size analysis instrument employing X-ray absorption. This technique passes a finely collated X-ray beam through a suspension of settling particle in a fluid. Because the intensity of the X-ray is directly related to the percentage mass of soil in a suspension, Stokes' law can be used to calculate the grain size distribution of a soil assuming an equivalent particle diameter for the soil grains. X-ray absorption has been found to produce accurate grain size distributions in the 75 μm to 1 μm size range when sample preparation adheres to AASHTO T88-90 specifications and suspension concentrations are approximately 2 percent by volume. Testing for particles sizes down to 1 μm takes approximately 20 min per sample. Technical concerns remaining involve obtaining representative samples of the soil in the 75 μm, to 1 μm size range suspension for testing. One method being evaluated is injection flow analysis, which is an inexpensive method of obtaining representative samples used with a variety of inorganic, industrial, and environmental materials in which direct sampling of fluid is required.