Finding the right pile design is a key element in replacing the Sakonnet River Bridge
Source of Article:http://www.projo.com/news/content/bz_sakonnet_bridge_piles_02-08-09_MMD78TI_v15.3f29bed.html
By Bruce Landis, Journal Staff Writer
Crane operator Damon Hamilton lifts a hydraulic hammer onto the steel pile to be driven into the ground for the new Sakonnet River Bridge.
The badly-rusted bridge has been subject to weight limits on and off for more than a decade — the present rule bans large trucks. It has also absorbed millions of dollars in fixes and is now in line for $2.5 million more in “high-priority repairs.”
The bridge is a critical part of the regional highway system. It carries Routes 24 and 138 between Portsmouth and Tiverton and links Newport and Aquidneck Island to Fall River, Boston and the rest of Southeastern Massachusetts. Built in 1956 to replace a stone bridge, it carried about 10,000 vehicles per day in the early 1960s. Now, upward of 40,000 vehicles cross it daily.
Dates when the DOT has said it would begin construction of a new bridge have come and gone for years. There have been repeated battles over how to pay for it — tolls or no tolls? — but there is no new bridge.
Now the project is apparently on the move. It passed a key milestone last month, when the DOT opened contractors’ bids. The DOT says it wants to award the contract in April and then start construction.
What took so long?
A big problem, DOT officials say, was to make sure that they knew how to hold the new bridge up.
The subject of their anxiety will be invisible once the bridge is built — the piles that will be driven into the river bottom and river banks that will support the concrete piers holding up the superstructure.
Unfortunately, the soil at the site, immediately south of the existing bridge, is silty and difficult to build in. If the engineers get it wrong, there could be lengthy and expensive construction delays to redesign the foundations.
Farhoumand said the collapse of one shaft during testing illustrated the kind of things that he said he was thinking about when he insisted on doing extensive probing before starting construction.
He said that the DOT had to find out what kinds of piles to drive, how many, and how deep. The agency has spent three years and more than $5 million satisfying itself that it has the right combination.
The testing program ended last year. It had grown from one contract, in 2006, costing $2.2 million, to two, the second starting in 2007 and costing $3.2 million. It included numerous engineers and companies specializing in piles, pile driving and foundation design. That cost is in addition to the Cardi Corp.’s $163.7-million low bid to build the new bridge itself.
Piles are shafts of solid material driven into the ground to support bridges or other structures when the ground itself is unsuitable. The piles can be driven down to bedrock, or to “refusal,” where trying to drive them further would damage them. When they aren’t supported by their ends resting on something solid, they are supported by the friction between their sides and the ground around them.
The DOT looked at three types: drill shafts, where holes are bored and filled with reinforced concrete; H-piles, which are steel beams whose cross-section looks like that letter and are driven into the ground; and pipe piles, also driven into the ground on end.
The engineers tested drill shafts first, but on Aug. 30, 2006, Farhoumand said, a test shaft collapsed before it could be filled with concrete. That raised two serious concerns.
First, he said, a botched pile can make putting a replacement in the same place impossible. That could mean redesigning the bridge, at huge expense, in mid-construction.
“You only get one chance,” Farhoumand said.
Second, he said, a failed drill shaft for the new bridge might undermine the piles holding up the existing bridge nearby.
Trying to find a better approach, in the summer of 2007 the DOT’s consultants installed a giant pile driver near the beach on Acquidneck Island next to the existing bridge. After driving a pile, they would test its load-bearing capacity by mounting powerful jacks on top of it. Jacking upward against a framework of heavily-anchored steel beams let them put as much as 4 million pounds of downward pressure on the pile.
It was a major project — people in the industry said it was the biggest load-testing program in New England. There was a delay while the pile driver, or “hammer,” was built in the Netherlands. The crane supporting the pile driver and the piles themselves were prominent, if temporary, landmarks, sticking up higher than the bridge’s deck and right next to it.
An advantage of pipe piles is that, unlike solid piles, they generate friction on their inside surfaces as well as their outsides. The DOT’s consultants tried a refinement of the pipe pile — a steel disk, with a hole in the middle, fastened across the inside of the pipe, part way up.
The plan was to retain the interior friction while preventing the pile from sinking deeper into the ground by blocking soil from rising all the way up the inside. It worked, increasing the piles’ weight-bearing capacity.
After the testing, the DOT decided to hold up the new bridge’s eight piers with a combination of 457 H-piles, ranging in length from 76 to 119 feet, and 30 pipe piles, from 206 to 256 feet. If they were lined up end to end, they would go for more than 11 miles.
With luck, after they are driven, the DOT engineers will never see or
hear about them again.