Laboratory Evaluation of Organic Soil Mixing

Organic soils present a difficult challenge for roadway designers and construction due to the high compressibility of the soil structure, the often associated high water table, and the high moisture content. For other soft or loose soils (inorganic soils), stabilization via cement or similar binders (a method called soil mixing) has proven to be an effective solution. To this end, the Federal Highway Administration has published a comprehensive design manual for these techniques. Organic soils, however, are not addressed therein to a level of confidence for design, as organic soils do not follow the trends of inorganic soils. This has been attributed to the high porosity, high water content, and high levels of humic acids common to organic soils. This thesis presents the findings from a literature search, laboratory bench tests, large scale laboratory tests, and concludes with recommendations for design involving soil mixing applications in highly organic soils. Laboratory tests (bench tests) were performed to assess the effect of cementitious binder type, binder content, mixing method, organic content, and curing time on strength gain. This phase involved over 500 test where in all cases, specimens with organic content higher than approximately 10% required disproportionally more cement for the same strength gain when compared to inorganic or low organic content samples. Using the findings of the bench tests, a 1/10th scale test bed was built in which soil containing approximately 44% organics was placed and conditioned with rain water. The dimensions of the bed accommodated three side-by-side tests wherein dry and wet soil mixing were performed each on one third of the bed. The remaining third of the bed was left untreated. Load tests were then performed on the three portions of the bed where the load for a simulated roadway was placed. These loads were left in place for several weeks and monitored for movement. Results showed improvement for the treated portions relative to the untreatment with virtually identical response coming from both dry and wet methods (both used identical amounts of cement per volume). The findings of this thesis suggest that the adverse effects of organic soils can be combatted where more cement content is required to bring the water / cement ratio down to acceptable levels and even more cement is required to offset the acidity. While this has been a recurring observation of past researchers, a cement factor threshold was defined by experimental data below which no strength gain was achieved. This threshold was then defined as a cement factor offset above which the measured strengths matched well with other soil types. As a result, a recommended approach for designing soil mixing applications in organic soils was developed