Factors Affecting Water Content Determination of Soils by the Microwave Method

Abstract

Tests were run on sand, silt, clay, and organic soil samples to evaluate the effect of soil type, sample size, water content, and power output on the drying time required for accurate determination of water content by the microwave method. Three different drying methods were used for the evaluation including the American Society for Testing and Materials (ASTM) microwave method (D 4643-87), the ASTM conventional oven method (D 2216-80), and a continuous drying microwave method devised for this study. The conventional oven method was used as the standard method for comparison. The effect of sample size was evaluated by testing samples weighing 20, 40, 60, 80, 100, 150, and 200 gm, all containing 10 percent water by weight. The effect of water content was evaluated using samples weighing 75 gm and containing 5, 10, 15, 20, 30, 40, and 50 percent water by weight. Microwave ovens with power outputs of 450, 600, and 800 watts were used to evaluate the effect of microwave strength. Results indicate that accurate water content determinations can be made using soil samples weighing approximately 150 gm. Sand, silt, low plasticity clay, and slightly organic soil gave very accurate results by the microwave method when compared to the conventional oven method. High plasticity clay and highly organic soil gave erroneously high water content values. Microwave ovens with different power outputs provided consistent drying time estimates, with slightly longer drying times being necessary for the lower output ovens. Based on these results, an equation was developed which can be used to estimate the optimum drying time required for an accurate determination of water content by the microwave method.