The Seattle Times. Winner of Eight Pulitzer Prizes.


Thu, Oct 30, 2014

WEATHER | TRAFFIC

VIEW SECTIONS

Home


Updated Monday, November 5, 2012 at 10:56 AM

520 pontoon flaws not fatal, says state DOT

By Mike Lindblom
Seattle Times transportation reporter

Despite the discovery of cracks in the first batch of pontoons produced for a new Highway 520 floating bridge, state Secretary of Transportation Paula Hammond said Friday she expects all six to be made watertight.

That includes the critical east endpiece, known as Pontoon W, which left the casting basin on Grays Harbor near Aberdeen on July 30 while water seeped through an exterior wall.

"I have no reason to believe we are going to reject pontoons, even W," Hammond said.

Casting is under way on the second batch of pontoons, which will be attached to the first set as builders on Lake Washington assemble the world's longest floating bridge, at 7,710 feet. Traffic is scheduled to use the new floating segment by early 2015.

In followup inspections this week, by engineers for contractor Kiewit and the state Department of Transportation (DOT), Hammond said it appeared seeping had stopped on W — perhaps the wall is still curing and "apparently the concrete continues to heal."

There are also cracks in the undersides of some other pontoons, which must be sealed.

Mike Cotten, a DOT design-build manager, said last week there are both superficial cracks and "structural" cracks, defined as those more than six-thousandths of an inch thick.

Hammond said the public should have confidence in the DOT, which she said has so far delivered 80 percent of $16 billion in projects approved in 2003-05.

"The thing I wish people would remember is we don't accept product until it's right, and that this project will be safe and live the expected life."

Gov. Chris Gregoire has requested the state reconvene the project's expert review panel, which diagnosed several problems in August.

To reject a large pontoon like W would be an extreme move, risking delays or added cost. But keeping flawed pontoons would increase maintenance or shorten the bridge's 75-year design life.

Even keeping them all, there might be costs. The DOT will negotiate with the partnership of Kiewit-General-Manson over who is responsible for what repairs or delays. Three months were lost this spring when an interior corner crumbled during post-tensioning, when steel was pulled at high pressure through a conduit to compress and strengthen the concrete box.

A total of 77 pontoons are being cast in Aberdeen and Tacoma to form a six-lane floating structure. The first set includes three of the standard, 360-foot-long highway pontoons; two smaller supplemental pontoons attached to the sides to add buoyancy; and the big endpiece W.

The cracks seem puzzling since Washington state claims experience making the world's four longest floating bridges, including the Lacey V. Murrow and Homer M. Hadley Interstate 90 bridges and the Hood Canal Bridge.

Pontoons have cracked in years past, even during construction, said review-panel Chairman John Reilly.

"The simple answer is, WashDOT's [WSDOT] been sealing these cracks on other bridges for decades," said Reilly, an international adviser on concrete, tunnels and cost estimating. Explanations are complex and involve the performance of concrete, he said.

Early in the project, the state researched mixes and built a concrete-mixing plant on site in Aberdeen.

Dave Ziegler, principal engineer for the DOT at Grays Harbor, said in a Popular Mechanics feature that $2.8 million in tests, including a fly-ash mixture, had virtually eliminated cracking from a test pontoon.

But later, the expert panel found that flaws in the concrete mixture, combined with the pontoon design and high pressure, led to shrinkage and cracks.

A state inspector found "weeping" at Pontoon W where water was getting in. Workers grouted the cracks but that didn't succeed, said Hammond. In other places, state video shows ballast water leaking between interior cell walls, a less dire problem.

The decision to release the pontoons from the basin, and fix them later, was in part motivated by a desire to make room for the second batch, Hammond said

The night the pontoons floated out, Ziegler and Hammond didn't mention the W cracks. Ziegler said there would be underwater inspections and patching if needed, before their trip to Seattle.

Kiewit-General-Manson changed strategy — choosing to tow them and make followup repairs in Seattle instead of leaving them exposed to saltwater and tides. The team got permission from insurer Lloyd's [of London] after it determined the pontoons would make the trip, DOT officials said.

Another inspection report is due in about a week.

Mike Lindblom: 206-515-5631 or mlindblom@seattletimes.com.


MARK HARRISON / THE SEATTLE TIMES
Concrete pontoons to be used in the construction of the new Highway 520 bridge are staged in a group on Lake Washington just north of the present bridge.




SECTIONS

Top News arrow

Latest News arrow

Local arrow

Nation & World arrow

Business & Technology arrow

Editorial & Opinion arrow

Sports arrow

Entertainment arrow

Living arrow

Travel & Outdoors arrow

Obituaries arrow


CLASSIFIEDS

Jobs arrow

Autos arrow

Homes & Rentals arrow

More Classifieds arrow