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Originally published Thursday, March 11, 2010 at 5:05 PM

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Sound Transit to do more drilling, filling to close 9 voids in Beacon Hill

Sound Transit needs to conduct another round of drilling and filling inside Beacon Hill to completely close the nine voids discovered there last year.

Seattle Times transportation reporter

Sound Transit needs to conduct another round of drilling and filling inside Beacon Hill to completely close the nine voids discovered there last year.

These air pockets in the hill were caused by soil slides above a drilling machine, while it crawled some 160 feet below the hilltop clearing a tunnel for the Link light-rail line. When too much soil falls into the grinding teeth at the head of the machine and is whisked away by conveyor belts, that leaves gaps in the soil layers above.

Transit CEO Joni Earl called it "a miss" that officials didn't immediately recognize during construction, from 2004-09. "Had we seen and realized there was excess, we would have stopped then," she told a transit-board committee Thursday.

The voids were discovered last year when a homeowner, gardening in her yard, noticed a deep hole.

Sound Transit has spent $1.6 million since then, and injected 2,550 cubic yards of cement — about 250 truckloads. The follow-up work could cost a similar amount, officials say.

"There is no imminent danger to the public and our structure," Joe Gildner, deputy director for Link light rail and a tunneling expert, said Thursday.

But a follow-up round of scientific investigations, including test drilling from hilltop properties, will be needed.

Officials don't know precisely where they need more cement. They arrived at a 430-cubic-yard estimate by comparing what they injected last year with records of the soil volumes removed during the tunnel boring.

Voids occurred only within a 300-foot-long zone of wet sands that are especially unstable, Gildner said.

The twin tunnel tubes carry trains between Rainier Valley and Sodo. There is a deep underground station at Beacon Hill.

The one-mile Beacon Hill segment was built by Obayashi Corp., under a $280 million contract that has increased to $312 million — and further millions in claims are scheduled for arbitration.

This is despite extensive pre-drilling investigations, including a wide test shaft near where the station's elevator shaft operates now. Even before the sand slides, Sound Transit had already shifted the underground station site to reduce sand risks.

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Ahmad Fazel, light-rail director, said Sound Transit noticed discrepancies during the drilling work, but the contractors attributed that to conveyor-belt issues. "Unfortunately, we took that explanation and accepted it at the time," he told board members. Sound Transit considers void costs to be Obayashi's responsibility, he said.

Beacon Hill is part of the $2.6 billion, 16-mile rail corridor that opened last year from Westlake Center to Seattle-Tacoma International Airport, now serving roughly 16,000 riders a day.

Other bored tunnels are planned for a Capitol Hill light-rail corridor to Husky Stadium; and for an Alaskan Way Viaduct replacement through downtown Seattle.

The Brightwater sewer tunnel near the King-Snohomish county line has caused sinkholes.

Transit leaders said there are "lessons learned" from the Beacon Hill episode, to be applied to the Capitol Hill Tunnel. Voids or sinkholes are a known risk in bored-tunnel projects around the world.

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

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