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Updated Saturday, December 1, 2012 at 02:46 PM

Divers map cracks in pontoon for new 520 bridge

By Mike Lindblom
Times transportation reporter

About 24 feet below the surface of Lake Washington, diver Jon Leite wiped away air bubbles to get an intimate view of the underside of Pontoon W, the huge east endpiece of the new Highway 520 bridge.

He perceived a 2-foot-long line near an edge. Using a transparent ruler, the shape of a credit card, he measured the width: 0.012 inch, about as thick as two dollar bills.

In a shack on a barge, engineer Fariborz Vossoughi watched the dive on a live camera feed and marked the flaw on a grid map.

Friday morning's search was part of the effort by Kiewit-General and the state Department of Transportation to identify cracks that occurred during construction this summer in a Grays Harbor casting basin where most of the pontoons are being built. Many of the cracks were filled with epoxy sealant then, but some had spread to the underside of the pontoons and could not be reached.

Some cracking is normal, but a harsh internal audit by DOT found mistakes in concrete-drying temperatures for the first batch of six pontoons. There were also extreme, uneven stresses on certain walls. Those areas were reinforced, and the design has been changed.

Kiewit-General says it's carrying out rigorous quality inspections to ensure the new bridge is solid. Kiewit hired Oakland, Calif.-based Ben Gerwick marine-engineering firm to devise a repair plan.

As of midday Friday, divers had found 15 cracks at the south side of the end pontoon.

About half the cracks are wider than the DOT standard of 0.006 inch. Experience with I-90 has shown that a crack has to be at least 0.016 inch for water to seep in, said Neil Hawkins, member of an expert-review panel monitoring the project. Recent inspections found droplets at just one site in a different pontoon, said panel chair John Reilly.

Reilly said the pontoons are fully capable of withstanding traffic and lake forces, and they are watertight enough to eliminate the risk of capsizing. More review is needed to predict whether they'll require extra maintenance.

"I'm personally not worried about the cracks. Once you seal it, it's stable," he said.

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


KEN LAMBERT / THE SEATTLE TIMES
Seen from Medina, looking west: The first five concrete columns have been built atop the east-end pontoon of the new Highway 520 bridge. On Friday, beneath the surface, a diver was searching for cracks in that pontoon.




KEN LAMBERT / THE SEATTLE TIMES
Diver Jon Leite takes a break from inspecting a pontoon for the new Highway 520 bridge. As of midday Friday, divers had found 15 cracks at the south side of the end pontoon.




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