When R.A. Smith National began designing the reconstruction of Drexel Avenue in Oak Creek, the engineers didn’t anticipate an opportunity to employ a technique never used before on a Wisconsin roadway.
The section of Drexel Avenue between S. 27th Street and S. 13th Street was in very poor condition and was constantly inundated with floodwaters. The road was nearly impassable during heavy storms. As a result, the City of Oak Creek identified Drexel Avenue to be upgraded from a two-lane rural cross section to a four-lane urban cross section with a median and turn lanes. R.A. Smith National was retained to design the new road with a strong emphasis on controlling the flooding and developing a storm water retention system.
However, two years into the design of the project, the City of Oak Creek decided that the road was no longer going to stay in its original location. Instead, the centerline of the road was going to be moved 40 feet south to avoid disturbing Milwaukee County park lands (Falk Park), as well as the Butlers Garter Snake habitat that was on the north side of the road. This changed everything. The team was no longer designing the reconstruction of an existing road but rather, creating a new road that crossed through wetlands that had very poor soil conditions.
R. A. Smith National engineers performed an alternatives analysis of nine different cross section options. This level of effort was necessary to develop a design that would accommodate the city’s anticipated traffic volumes and meet the WDNRs requirements for minimal wetland disturbance.
Because of the poor bearing capacity of the wetland soil, the excavation to remove the poor soil was anticipated to be 20 to 25 feet deep through the wetland section. An excavation of that depth would have resulted in substantial construction costs because of sheet piling needed to support the excavation walls, the amount of fill needed to restore the excavation to grade and safety concerns of construction staff and equipment – all this without being absolutely sure that the base would be stable for the long term.
At this point a collaboration meeting was held with city engineers, R.A. Smith National engineers and Midwest Engineering, the geotechnical consultant on the project. The purpose of the meeting was to develop a solution for a problem that none of the engineers had experienced before at this level. After several meetings, R.A. Smith National provided the city with three alternatives: build a bridge over the wetland area, excavate 20 to 25 feet into the ground or, as suggested by Midwest Engineering, use a Rammed Aggregate Pier® (RAP) system to reinforce the soils in place.
A decision was made to consult with Steve Weyda, P.E., senior engineer of Ground Improvement Engineering, who designs RAP systems. Weyda indicated that RAP systems had never been used for shallow embankment road base reinforcement in Wisconsin, but he was confident they could cost effectively design a reinforcement system in conjunction with a load transfer platform to improve the surrounding ground.
Weyda stated, “The Rammed Aggregate Pier System was ideal for the organic soil conditions on this project. The benefits of the system include reinforcing the extremely unstable soils; transforming the subgrade into a stiffer composite mass; and providing a conduit for drainage through the open-graded aggregate used to construct the piers to expedite consolidation (or settlement) time.”
After further research by all parties and additional collaboration meetings, the City of Oak Creek agreed on the use of the RAP system as the preferred solution. With consultation from Ground Improvement Engineering, the final design included the installation of 3,600 piers through the wetland section.
Piers Require Four-Step Process
The piers were created through a four-step process. The first step was to drill 30-inch diameter holes, ranging from five to 20-feet deep. The second step was to fill the hole with crushed aggregate and compact the hole with direct ramming energy. Ramming took place with a high-energy beveled tamper that both densified the aggregate and forced the aggregate laterally into the sidewalls of the hole. This action increases the lateral stress in surrounding soil, further stiffening the stabilized composite soil mass. The result of RAP installation is a significant strengthening and stiffening of subsurface soils that then support the embankment above.
Once the aggregate was built up to ground level, the third step involved constructing a Load Transfer Platform (LTP) to minimize differential settlement and to aid in stress distribution to the RAP system below. The LTP consisted of 12 inches of aggregate base, sandwiched between several layers of geogrid. The fourth step was to surcharge the embankment with crushed aggregate to a height of two feet above finished grade to expedite settlement before constructing the road.
Elevations on settlement plates were gathered daily to plot out the magnitude of settlement over time and determine when primary consolidation had subsided. Once the subgrade has stabilized, construction of the concrete road will begin. The reconstruction of Drexel Avenue is anticipated to be complete in 2012.
The use of the RAP system on Drexel Avenue may transform the way Wisconsin civil engineers look at poor soil. The soil doesn’t have to make or break a roadway. Now there is an operative and practical alternative to build on poor soils using RAP systems.
Contact Tim Barbeau, P.E., RLS, senior project manager, at 262-317-3307 to learn more about this innovative project.