Rapid Field Detection of Sulfate and Organic Content in Soils: Technical Report (0-6362-1)

In recent years, the Texas Department of Transportation (TxDOT) has experienced problems chemically stabilizing moderate to high plasticity clay soils with calcium-based additives. Many of the problems are the result of soluble sulfate minerals in the soil reacting with the lime or cement added for stabilization. The occurrence of these deposits is unpredictable and often restricted to small areas. To address this problem, the researchers set about identifying a technique that provides a map showing the sulfate content of the soil over a large area to a depth of 3 to 4 ft. Two technologies were identified that provide an indirect measurement of sulfate salts (an electromagnetic device - EM-38, and a soil conductivity device - VERIS 3150). We tested these devices on three different TxDOT projects in different parts of the state that have been known for high sulfate contents. We collected soil samples at 1 ft intervals to a depth of 4 ft where the data varied. We measured the PI, moisture content, sulfate content, and organic content in each sample. We then ran multivariate statistical analyses to correlate the conductivity data collected with the VERIS 3150 to laboratory-measured soil properties. We observed that coil conductivity is related to the soil texture/clay content, moisture content, and dissolved salts (i.e., sulfate and other salt minerals). We noted that for all of the projects tested, a soil conductivity over 100 mS/m may contain problematic sulfate levels, but it may also be due to high plasticity clay soils and/or high moisture contents with other dissolved salts. What is noteworthy about this research is it provides a tool to intelligently decide where to collect soil samples in a grid pattern of analyze for problematic sulfate levels versus the current method of collecting soil samples in a grid pattern of a specified interval that may be too large and not detect problematic sulfate levels until the road explodes.