Piles Support Success of Highway Upgrade at Milwaukee's Marquette

By Mike Larson, Editor | September 28, 2010

The Marquette. The largest transportation construction project in state history, it is the extensive $810-million project in the heart of Milwaukee so well known it requires only a single name, like Cher, Prince or Pele.

This massive highway upgrade really comprises five distinct projects: the Clybourn Street project, the north leg, the south leg, the west leg, and the core.

The hub of the Marquette project is its "core," where spokes of highway from all directions converge in the heart of downtown Milwaukee.

Stacked five levels high and packed with a tight weave of crisscrossing roadways, overpasses and ramps, the core is a complex crossroad that features some of the system's widest overpasses and highest concentrations of ramps.

To tackle the core's web of interchanges, sometimes called the spaghetti bowl, three Wisconsin contractors formed the "Marquette Constructors" joint venture:

Edward Kraemer & Sons, Inc., Plain; Lunda Construction Co., Black River Falls; and Zenith Tech, Inc., Waukesha, are the general contractors.

CTE Engineers, Inc., Milwaukee, is the state's lead on-site representative inspecting the work.

The core's overpasses will sit solidly atop a forest of some 5,000 concrete-filled steel piles driven deep into the earth. Laid end to end, all these supporting piles would stretch 70 miles, or nearly to Madison.

The project's largest and deepest piles are being driven to support the overpasses near the intersection of N. Sixth and Clybourn streets.

These 16-inch-diameter piles, made of half-inch-thick steel and measuring up to 130 feet (13 stories) deep, will support the interchange's heaviest sections, which are up to 130 feet long and up to six lanes wide.

The steady thump, thump, thump of pile hammers is now common in downtown Milwaukee as the project drives smoothly ahead.

But the pile driving portion of this project really began in 2003, when 89 test piles were hammered into the earth across the site in order to tell engineers exactly what soil conditions they would be working with.

The test piles were driven to a specific resistance, measured by the number of hammer blows needed to sink a piling 1 foot.

When resistance rose to the specified number of blows per foot, the piles were left until the water that had been squeezed out of the ground during pile driving could disperse and the ground disturbed by the driving could reach maximum firmness.

The ground's grasp on the test piles was then measured, and the results used to guide engineers as they designed the roadways and supports.

Though this method required a bit more investment up front, it actually saved about $2 million by permitting a more efficient design.

The process for driving the piles starts with a backhoe excavating a large trench that will eventually encase massive concrete footings poured on top of the piles. The piles are then accurately positioned using a state-of-the-art satellite system.

Once the trench is cut, it's time for a lattice-boom crawler crane equipped with a pile driving hammer and guides called "leads" to go to work. The leads — actually a single, square, lattice frame — hang from the crane's boom and provide support and guidance for the pile and pile-driving hammer.

The hammer is essentially a hollow steel tube, closed at the bottom and having a several-thousand-pound metal piston inside. The piston for the largest hammer used on the core project weighs more than 7 tons.

The closed bottom of the tube sits atop the pile, held in alignment by the leads. Diesel or kerosene power blasts the piston up the tube, and gravity pulls it back down to smack a blow that drives the pile into the ground.

The process repeats perhaps 30 times a minute, the pile moving inches with each blow. Markings on the side of the pile tell how far it has traveled.

In the case of the Marquette core project, the piles come in sections that measure up to 70 feet long and weigh up to 4,900 pounds.

When the first section of pile has been nearly buried in the ground, another section is welded, or "spliced," on top of it and the driving continues.

When the pile has been driven to the specified resistance, it is cut to the correct height, and the process begins anew for the next pile.

When all the piles for one pier have been driven and cut off, they are filled with concrete and some have vertical rebar attached to them.

After that, they are encased in a cast-concrete footing that forms a solid foundation for the overpass's vertical pier, cap and roadway.

In addition to the footings, the vertical piers and the horizontal caps that support the overpasses are all cast-in-place concrete.

And all have rebar-mesh cages inside to add strength and structural integrity. All of the rebar in the pier footing, vertical column and horizontal cap is attached to the rebar from the adjacent section to form one integral rebar frame inside the concrete.

Many of the footings at Sixth and Clybourn are 14 feet or more square, 5 feet high and contain 40 or more cubic yards of concrete.

The vertical columns poured atop the footing can be single or double, as required by the width of roadway they will support. Their height varies depending on the contours of the land, the clearance required, and the specified slope of the roadway.

Across the top of the column or columns rests the cast-in-place cap, which supports the beams that support the roadway.

Straight sections of overpass use prestressed concrete "I" girders 70 inches deep and up to 130 feet long. Sitting side by side on two piers and tied to each other by diaphragms, up to seven girders support a placed concrete roadbed and side parapet guards.

Curved sections of road use a single fabricated steel "tub girder" instead of multiple precast concrete girders. This method simplifies construction of the curved segments because the single-piece steel tub can be fabricated with the required banking built in.

When the single-piece tub is set atop its supporting piers, there is no need to try to adjust several individual pieces to create banking. It's built in, ready for placing of the concrete roadbed.

The system is working smoothly, with several piers continuously in process. On average, a team constructs a complete pier about every four to eight weeks.

When the entire project is complete, Wisconsin's busiest traffic hub will offer new efficiency with a magnificent and beautiful new system of highways.

Unseen, buried in the ground beneath it, will sit the solid piling foundation that makes it all possible.

Editor's note: Information for this story was graciously provided by Steven Maxwell, P.E., geo-technical/soils, Wisconsin Department of Transportation (WisDOT); Ryan Luck, P.E., project manager— core, WisDOT; Tom Hessling, P.E., project manager for CTE (construction inspection consultants); and Brian Manthey, communications officer, WisDOT. Helpful insight was also provided by Brian R. League of Collins Engineers, Inc.