Recommendations for Stabilization of High-Sulfate Soils in Texas

Over the last 15 years, the Texas Department of Transportation (TxDOT) has seen an increase in pavement failures during and immediately after construction on roads designed to last 20 years or more. The cause of many of these failures is sulfate-induced heave where an expansive mineral called ettringite is formed from a calcium-based stabilizer (lime or cement) reacting with clay and sulfate minerals (usually gypsum) in the soil. Traditionally, TxDOT has removed and replaced soils with more than 2000 ppm sulfates. Earlier in this research project, lime was identified as a plausible stabilizer in soils bearing sulfate concentrations up to 7000 ppm. This portion of the research investigates if anything can be used to stabilize soils (reduce swell and increase strength) with sulfate concentrations above 7000 ppm. Three-dimensional swell was measured on laboratory prepared specimens with sulfate concentrations of 0, 10,000, and 20,000 ppm. Twelve stabilizers were selected for the 3-D swell testing based upon positive results obtained by other researchers. Stabilizers that significantly reduced swell in the high-sulfate soils were then subjected to unconfined compressive strength testing. Three stabilizers (Clays tar 7, ground granulated blast furnace slag + lime, and class F fly ash + lime) provided significant swell reduction (10 to 12 percent) over the untreated soil; two of the stabilizers were selected for strength testing. The fly ash swell test results were obtained too late to include in strength testing. The Claystar7 showed an improvement of 41 Ib/in2 over the untreated sample for retained strength in the unconfined compressive strength after 10 days capillary rise. The ground granulated blast furnace slag showed a 79 Ib/in2 retained strength. This project showed that soils with sulfate concentrations up to 20,000 ppm can be treated in a timely manner without having to remove the high-sulfate soil and replace it with a select material