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Pier rehab underway at Milton-Madison bridge

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By Dave Taylor

Employees and subcontractors for project contractor Walsh Construction Co. are in the process of rehabilitating the existing piers of the Milton-Madison bridge.

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Walsh project manager Charlie Gannon and Aaron Stover, an engineer with Michael Baker Jr., Inc, conducted a tour for members of the media last week during which they explained the scope of work underway to replace the aging structure with a new span on the existing piers.

“Pier-strengthening is referred to as the concrete jacket around the piers,” Gannon said.

With pier-strengthening, the first step is to create a cofferdam around the pier, according to Stover. Workers drive sheet piles into the riverbed around the pier to form a watertight seal. Water is then pumped out of the cofferdam allowing workers to get down in there and perform the pier-strengthening.

“The pier-strengthening has a lot of various components,” Stover said. “There are a number of steps: step one after they’ve got the cofferdam into place they’ll come in and de-water it and drill holes around the perimeter of the existing caissons. The caissons are the foundations that go down to bedrock and carry the weight of the bridge on top.”

The holes will be drilled down to various depths depending on what the engineers have designed.

“Once those holes have been cored out, they’ll be cleaned, a reinforcing bar will be inserted into that hole and then grout will be pumped into that hole as well,” Stover said. “The goal of that is to form a good, tight bond with that existing caisson.

“The part of the strengthening that we’re doing out here is adding all of this reinforcing around the perimeter. That steel will extend up into the next core to get a good tight bond going up. The next step will be to construct what we call the pier shaft or the pier stem, which is the portion that you’ll see coming out of the water. That’s everything from the caisson on up so that will be cast two-feet thick around the existing pier stem and then you’ll keep building on up. Eventually Walsh will construct the pier caps at the top. Those pier caps are what the final truss will rest on.”

At pier two on the Milton riverbank, Gannon described the process in more detail. Workers excavated around the pier until they reached the top of the original caisson, or foundation, of the pier. A cofferdam of steel sheeting was driven into the ground to prevent dirt and other debris from caving in on the workers.

At the top of the old caisson, “we had a company come in and core a two-inch hole five feet deep in the top of the caisson,” Gannon said. “These bars that you see sticking out were then anchored into those holes using a high-strength epoxy. That starts the reinforcing of the pier strengthening. The concrete you see poured that these bars are coming out of was poured last week all the way around and that started the pier strengthening.”

The next pour will come up to the removal line for the old pier, which is at about the top of the bars that are currently exposed, Gannon said.

“Once it’s completed and backfilled, we’ll walk away from this pier until the old bridge is removed,” he said. At that time “this pier will be removed at that line in its entirety and 13 more feet of pier stem will be poured and then the new cap.”

During the tour workers were drilling a series of foot-deep horizontal holes on a two-ft square pattern. The next step is to glue-in a sheer connector—a horizontal rod that helps tie the new concrete jacket to the old pier.

At the top of each section of rebar is a smooth section about four inches long, which is coupler-threaded “so to make the connection from this to carry that reinforcing up into the next section once the old bridge is removed,” Gannon said. “We’re going the screw rebar into the top of that and it will continue that on up.”

At that point in the process the cofferdam—the steel sheeting—will be removed, the backfill completed and the area dressed out. The pier will remain that way for the next year until removal of the old bridge span.

“Part of the process is we have to wet the old concrete for a period of 12 hours to saturate that so when you pour concrete it helps the bonding of the new to the old,” Gannon said.

Walsh Construction will be reinforcing four piers—one on the Milton riverbank and the three in the river closest to Kentucky. The river pier closest to the Indiana short will eventually be completely removed in favor of a new pier, which will be built in the floodplain on the Indiana riverbank, Gannon said.

“Between the four piers that we are reinforcing we have well over 7,000 of these horizontal holes that we have to drill and anchor the bars into,” he said. “The vertical bars you see we have to have a minimum of four inches of concrete cover around them so that gives you an idea of how big these piers are going to be.”

A cofferdam has been driven into the riverbed around pier three in the river. Caisson strengthening at pier three will begin as soon as the cofferdam is de-watered, Gannon said. That process was underway Thursday.

De-watering will expose the caisson. There are a couple of preparatory steps to be taken before reinforcement can begin.

“The first step will be to clean the face of the old pier,” he said. “We’ve already had a chance to see that it’s loaded with old marine life—mussels. Those all have to be removed. Then we’ll have to sandblast to clean the surface and to roughen it a little bit to help the new concrete stick.”

The pier will also be inspected for cracks and undergo epoxy crack injections prior to installation of the new concrete jacket.

All the while other crews continue to build a 120-ft trestle ramp from the storage and assembly yard on the Milton riverbank. The trestle will be used to preassemble the truss sections of the new bridge, which will then be floated into the river and lifted into place on temporary piers. Workmen are building a “T” section onto the end currently.