With 75 percent of the $45-million, three-story Pier Wisconsin project in Milwaukee being constructed over Lake Michigan, much of the project will be built on piles embedded in the lake bottom.

GPD¦Gilbane's Milwaukee office is the construction manager on the job.

To keep the fast-track project on schedule, contractors needed to get the footings and foundations in and be ready to pour structural concrete over the winter.

The excavation for the underground parking structure and below-grade aquarium extends right up to the steel seawall, and that, combined with silty soils used to fill the site in the 1950s — created challenges for contractors charged with dewatering the site.

The team from Griffin Dewatering Corporation used a newly developed system designed to screen out silt and reduce turbidity of discharge water.

"This is a highly unusual project because it is all artificial fill and dates back to the mid-1950s when it was installed. There were a lot of unknowns," said Tom Minihan, Griffin Dewatering Corporation vice president and regional manager

Soils turned out to be relatively consistent.

"This whole pier out here was built in 1958. They brought in the sheet piling and filled with dredge from over on the Michigan side of the lake. It was a lot of pretty clean fill over the existing glaze," said Andy Wigman, GPD¦Gilbane project manager.

With the fine silt on-site, Minihan and Griffin's licensed driller Tom Abbring installed a system the company developed and refined over the last two years. Rather than traditional screening, the system relies on a permeable geotextile manufactured by American Wick Drain, Monroe, N.C.

"It is similar to passive soil stabilization materials," Minihan said of the fabric. "We are just now completing a deal that would give us exclusivity with the manufacturer in order to protect our proprietary system."

Griffin is using the fabric-based filtering system with automatic vacuum sumps on an underdrain system that goes around the south and eastern perimeter.

"Instead of screen we use a geotextile material with a polypropylene core," Minihan said. "We havea new valving system that we designed — and a high air capacity vacuum system."

Minihan said he had the idea for the system 15 years ago, but hadn't had an opportunity until the Pier Wisconsin project.

The fine silt along the eastern shore of Lake Michigan was a factor in implementing the system, as were water quality requirements from the Wisconsin Department of Natural Resources (DNR).

"Regulatory pressures were significant. Since we were discharging into the lake, there were close controls on water quality. We were closely involved with the DNR in implementing this system on the site," he said.

The dewatering system was put to the test in August and again in September when Edgerton Contractors — excavating for the below-grade portions of the project — had to remove tiebacks that secured the seawall to the soil.

"We came up with some challenges of removing the old seawall tieback system. We had to cut the tieback, but to do that we first had to remove the bolt that secured it to the seawall. There was just a series of bolt holes all the way along and they plugged them up with a washer and a gasket.

"Water was coming in almost as fast as we could dewater it. Everyone on-site was involved. Edgerton subcontracted Gillen Construction to cut the ties and patch the holes," Wigman said.

Minihan said their proprietary system came through — and performed better than vertical drains would have.

"The lower zone has been dewatered," Minihan said of the clay material below the level of the seawall. "The water — as it comes through the sheeting — perches on top of the clay and saturates the silty material. At this point, we have automatic vacuum sumps that are roughly every 50 feet around the perimeter. The influx of water doesn't impact the system that much."

A conventional dewatering system wouldn't have worked at the site, according to Minihan.

"We put in a couple of exploratory wells and they did not work at all. The water will not drain vertically from the top zone to the bottom zone. To demonstrate this to Gilbane we drilled two wells but they were drilled dry. You drill 36-inch holes and the water would not drain," he said.

As crews prepared to pull the last of the tiebacks this fall, crews from Gillen Construction were driving piles on the site and in the lake.

The scope is that we have over 600 piles that we are installing on the project," Gillen Project Manager Tom Ritzer said. "Of those 600 piles, a third of them are in the water to support the overhanging portion of the building."

The size of the job, and driving pilings into the lake are some of the challenges on the project.

Gillen's 40-foot by 80-foot Harbor Builder barge with a 100-ton Manitowoc crane has been used on the job.

"Some of the marine piles — the top of the piles will be below the water level eventually. Those that are driven on land for the parking structure and the building — that's pretty conventional except they are high-capacity piles," Ritzer said.

Plans call for structural concrete work to be done during the winter.

"The building structure is mild steel up to the grade with a matt slab in the basement below the water table. Above grade it is all post-tensioned slabs. It's three stories," Wigman said.