Automated soil particle-size analysis using X-ray absorption

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While the hydrometer method (ASTM 422-63) has proven to be an accurate method of particle-size analysis of soil and is relatively inexpensive to conduct, it is prone to operator error as well as taking a minimum of 24 hours to conduct. Recent research has shown that an automated X-ray absorption instrument can closely approximate hydrometer test results for soils. The X-ray absorption instrument determines the change in mass of a sedimenting soil-water suspension through the absorption of X-rays while utilizing Stokes' law to determine particle sizes in the range from 300 μm to 0.1 μm. A number of hydrometer tests on three different soils were conducted and compared with particle-size analysis using an X-ray absorption instrument. Controlled testing between the hydrometer and the X-ray absorption instrument produced almost identical results for silts while tests on clay show good agreement from 75 μm through about 10 μm. However, below 10 μm the X-ray absorption test results start to show a slightly higher percent of fines than the hydrometer test, with the percent difference increasing as the particle size decreases. The increase appears to be related to the plasticity of the clay, with low plasticity clay showing an average increase of only 1 to 3 percent, while the higher plasticity clays vary from an average of 3 to 6 percent.

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ASTM Special Technical Publication