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Current Findings Every year large amounts of coal are burned in electrical power plants in the United States for production of electricity. Air pollution controls at these plants produce large amounts of fly ash that are disposed in landfills at considerable cost. In recent years, however, environmental regulations in many states have promoted reuse of fly ash, and many other industrial by products. Fly ash has found several applications within civil engineering, such as a partial replacement for Portland cement in concrete, as an embankment or fill material, as a stabilizing agent for soft subgrades and aggregates in road bases, as a component in flowable fill, and as a mineral filler in asphalt paving mixtures. Soft subgrade soils are a common problem for highway construction in Wisconsin. The typical approach for soft subgrade remediation consists of removal of the soft soil, and replacement with a thick layer of crushed rock. The high cost of this approach has prompted the construction industry to evaluate alternative methods of constructing highways on soft subgrades. One approach is to use fly ash to stabilize the soft subgrade in place. Evaluating fly ash as a subgrade stabilizer for Wisconsin soils was the objective of this study. A laboratory testing program was conducted to evaluate how different fly ashes can improve the engineering properties of several soft subgrade soils from Wisconsin. The soils were selected based on several factors such as geography, composition, and soil texture. Three soils and three fly ashes were selected for the study. The soil samples were prepared with fly ash contents (0, 10 and 18%), and compacted 7% wet of optimum water content to simulate a soft and wet condition often observed in the field. Three tests were performed to evaluate how adding fly ash affected the mechanical properties of the soils: California Bearing Ratio tests, resilient modulus tests, and unconfined compressive strength tests. The soils selected represent poor subgrade conditions with CBRs ranging between 0.4 and 6. A substantial increase in the CBR was achieved when the soils were mixed with fly ash. Soil-fly ash mixtures prepared with a low-plasticity clay at 10 and 18% fly ash has CBRs values in the range of 11 to 46. Soil-fly ash mixtures prepared with a high-plasticity clay with 10 and 18% fly ash had CBRs in the range of 9 to 31 after 7 days of curing. The resilient modulus also increased significantly by adding fly ash. The resilient modulus of the soil-fly ash mixtures with 18% fly ash were comparable to resilient moduli of soil specimens compacted at optimum water content and maximum dry density. The test results show the effectiveness of the fly ash for stabilization of poor subgrade soils. The results can be used for development of design guidelines that will help pavement engineers make better selections of the different layer thicknesses in pavement structures. More research is currently being performed to more thoroughly define the resilient modulus and unconfined compressive strength of compacted soil-fly ash mixtures.
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