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.