Monitoring and evaluation of fly ash stabilization stabilized subgrade constructed by the WisDOT

Abstract

This report describes the monitoring and evaluation of a field site where Class C fly ash was used to stabilize the subgrade during construction of a rigid pavement in a portion of USH 12 near Fort Atkinson, Wisconsin. Additionally, information from a second similar project (STH 32 near Port Washington, WI) that was monitored only during construction and a third project (STH 60 near Lodi, WI) that was monitored for 8 years is reported. The following observations are made based on this investigation: 1. All of the tests consistently indicated that the stiffness and strength of the subgrade were improved significantly by fly ash stabilization at all sites. Observation during construction, however, clearly demonstrated the benefit of fly ash stabilized subgrade (FASS) because once the fly ash is mixed and compacted in a window of dry weather the FASS remained stiff in subsequent rain events. 2. It is noted several significantly different soil types were encountered in all sites and the fly ash contents and moisture contents were variable (10% and 12% and 7-14%, respectively). Resulting California bearing ratio and moduli also varied. The gain in stiffness and strength are typically 2-3 times due to fly ash stabilization. In situ stiffness measured with the soil stiffness gage and dynamic cone penetration index also illustrated that the addition of the fly ash and compaction increased the strength and stiffness appreciably. These findings suggest that fly ash stabilization of subgrade should be beneficial in terms of increasing pavement capacity and service life. 3. The data also indicates a complex relationship between base soil type, amount of fly ash, and water content. For instance, in some soils (e.g., clay) the effectiveness of stabilization decreases when the water content of the soil increases whereas in some other soils (e.g., sandy) it increases. Therefore, a careful mix design is needed for fly ash stabilization involving all potential subgrade soils. The results from all sites also confirm that fly ash stabilization results in a relatively stiffer layer irrespective of the type of soil. 4. Resilient Modulus of the field-mixed FASS is close to that of undisturbed samples of FASS obtained by thinwall tube sampler. Thus, the field-mixed FASS can be considered to be an effective method of assessing the in situ soil stiffness. 5. Moduli back-calculated from the falling weight deflectometer (FWD) test data indicated that modulus does not display significant reduction over the years. 6. Pavement distress surveys indicate that fly ash stabilized sections perform comparable to control sections stabilized with breaker run. 7. Percolation from the pavement varies seasonally. Concentration of some elements from the leachate exceeded the Wisconsin preventive action and enforcement limits. However, these concentrations are expected to fall below the limits during transport to the groundwater table.