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STH 100 (Brown Deer Road) Reconstruction Fly Ash Stabilization - Milwaukee County, Wisconsin

STH 100 (Brown Deer Road) Reconstruction Fly Ash StabilizationUnique things were happening this summer on the $5.6 million, 2-mile, multi-staged construction of State Highway 100 (Brown Deer Road) in Milwaukee County, Wisconsin. An interesting aspect of the project was the use of fly ash for subgrade stabilization. The on-site reuse and recycling of fly ash resulted in significant time and cost savings, as well as environmental benefits.

Fly ash is the finely divided reside that results from the combustion of pulverized coal and is transported from the combustion channel by exhaust gases. Fly ash is produced by coal-fired electric and steam generating plants; in this case, the WE-Energies Oak Creek Power Plant.

Here’s how the fly ash was used. One of the Wisconsin Department of Transportation’s (WisDOT) conventional means to establish a new subgrade for highways requires removal of existing soil for placing a 16-inch deep layer of breaker run material.Removal of soil and import of breaker run material was substituted on the Highway 100 project by “cultivating” and compacting fly ash into the site’s existing clay soil.This 12-inch fly ash subgrade stabilization layer of existing soil was then covered by a 6-inch layer of base aggregate material before placing the 8-inch concrete pavement.

While the fly ash stabilization process is not new, it has not been used extensively. Certain types of soils and the right setting are needed. The use of fly ash is ideal when integrated with clay soils (such as those prevalent along the Highway 100 project), or sandy or silty soils. Projects with underground utilities are not conducive to fly ash stabilization because of the pulverizing that takes place. The process works best on prolonged stretches of rural roadway where there are no driveway or crosswalk access issues. Fly ash stabilization requires a 24- to 48-hour window during which the roadway is completely restricted to all traffic. The process is cost-prohibitive if only a small section of roadway can be closed at one time. On the Highway 100 project, WisDOT was able to shift all traffic to one side for about two miles of roadway.

Early Analysis Saves Money
Historically, fly ash stabilization has been added as an extra to a highway construction contract after the project has already been bid and the subgrade has been deemed unacceptable for the roadway. Fly ash stabilization is seldom bid as part of the construction project.

But things transpired differently on the Highway 100 project. At the project’s onset, WisDOT evaluated the feasibility of using fly ash stabilization by reviewing the soil boring in detail and analyzing the performance of the previous pavement.

WisDOT’s decision to proceed with the fly ash stabilization yielded a significant cost savings of approximately $631,000. Conventional removal of the soil and placement of the breaker run material would have cost approximately $940,000 (~$10.30/SY) in the project areas where fly ash was used instead.The bid price for fly ash subgrade stabilization used along a 1.5-mile segment of this 2-mile, six-lane highway reconstruction project was $309,000 (~$3.40/SY).

To date WisDOT has used fly ash stabilization on a half dozen or dozen projects statewide and is considering expanding the use of fly ash stabilization on future highway projects. Andy Zimmer, Southeast Region Geotechnical Engineer, WisDOT, said, “The process works well, is cost-effective and makes good environmental sense as we are reusing a coal combustion byproduct that otherwise would end up in a landfill.”

R.A. Smith National's Role
R.A. Smith National provided construction services to WisDOT that included overseeing all aspects of the project to ensure a successful completion. Operations included grading, fly ash stabilization, concrete pavement, concrete curb and gutter, concrete sidewalk, decorative concrete sidewalk, storm sewer, traffic signals, pavement marking, traffic control, permanent signing, erosion control, and restoration.



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