Round-Robin Testing of a Dewpoint Potentiameter versus Filter Paper to Determine Total Suction

Expansive clays exhibit high potential for volume change when changes in soil moisture occur. It has been estimated that the annual cost of damage to facilities built on expansive clay in the United States exceeds $9 billion. The use of soil suction measurements to predict potential clay volume changes has shown significant promise, yet the currently utilized method to determine soil suction, the filter paper method, has serious drawbacks. First, the test takes approximately 8 days to complete, making it less than practical for broad use by practicing engineers. Second, and more troublesome, the filter paper test is difficult to conduct successfully and the results are significantly affected by small differences in procedure, all allowed by the ASTM standard. The WP4 chilled mirror dewpoint potentiometer has been shown to provide total soil suction measurements that are consistent and repeatable within 24 hours. The WP4 device has the potential of providing a method for all geotechnical engineers to use soil suction for all project situations, thereby, reducing the cost and increasing the reliability of predicting clay soil behavior. The ASTM standard for use of the WP4 device does not specify how it should be used. The potential use of the WP4 dewpoint potentiamenter to determine total suction of expansive clay soils has been substantiated by initial testing as reported by Petry and Bryant in 2002. The project reported here provided further evaluation using a process of Round-Robin testing of specimens of three expansive clay soils at three water content-dry unit weight configurations. One soil was from Texas, one was sampled in New Mexico and the other came from Missouri. Seven geotechnical laboratories evaluated the suction characteristics of the nine specimens provided and returned their results to the UMR Geotechnical laboratory. Involved were three geotechnical companies from Texas and one from each of Colorado and New Mexico, and the geotechnical engineering laboratories at BYU and UMR. The results indicate that the relationships of total suction to soil moisture as defined by the WP4 device had significantly less variance and provided slightly more conservative values of suction, when compared to the filter paper method.