Mitigating the Effects of Organics in Stabilized Soils: Technical Report (0-5540-1)

The Texas Department of Transportation has reported difficulty stabilizing soils bearing high and low concentrations of organic matter with lime. Problems include: the stabilizer disappearing over time, difficulty measuring organic matter (ignition oven), and rough pavement due to poor subgrade support. The researchers wanted to identify a good test to measure organ matter in soils and identify the problematic fraction of organic matter that causes problems with lime stabilization. Secondly, we wanted to determine what mechanism or mechanisms were responsible for organic matter interfering with lime stabilization. We developed a simple technique to measure the humic acid content of organic matter using a UV-Vis spectrophotometer. We also constructed hundreds of manufactured soils consisting of a humic acid standard mixed with different clay mineral standards, lime, and quartz sand. These soils were used to measure changes in engineering properties as well as monitor chemical changes to elucidate factors controlling reaction mechanisms. From the manufactured soils, we determined that humic acid does interfere with lime stabilization by inhibiting the formation of Calcium Silicate Hydrate reaction products between the lime and clay minerals. This results in reduced unconfined compressive strength values for stabilized cores. The concentration of humic acid that inhibits the formation of pozzolanic reaction products is around 1 percent. Calcareous soils containing organic matter appear to respond better to lime stabilization than acid soils. This study was the first to provide direct evidence of the effects of organic matter on soil stabilization.