May 18, 2011

Sound Transit says it has learned from past mistakes in tunneling

The last time Sound Transit tunneled under Seattle, things did not go so well.

A construction worker, Michael Merryman, died in a 2007 accident in the Beacon Hill tunnel. Soft, sandy soils drove up the cost of designing the nearly mile-long twin tunnels fourfold.

Construction was also complex: The original construction contract was $280 million, but the final cost was $311.3 million.

Contractors are now gearing up to build the 3.15-mile twin-bore tunnels for University Link between downtown Seattle and the University of Washington, and Sound Transit officials say they're applying the lessons learned under Beacon Hill so things will go better this time.

They're being especially careful so that tunneling does not create the kind of deep voids that opened under Beacon Hill. The agency paid $476,000 to the owner of a house that had cracks in the foundation and holes in the yard, and other homeowners complained about foundation cracks. Sound Transit also paid $3.25 million to consultants and contractors to find and fill voids.

Agency officials said over excavation by the contractor, Obayashi, caused the voids and Obayashi paid for all the fixes.

Similar voids could be disastrous for the $1.9 billion U Link project. Workers will tunnel under many more houses and buildings, and under the waters of the Montlake Cut.

“That would be a bad place to over excavate I'm sort of guessing,” Sound Transit Board member Fred Butler said at last week's Capital Committee meeting.

“Yes, sir. It would be,” responded Dick Sage, Sound Transit's director of construction management.

Sage said over excavation was caused by failing to operate the tunnel-boring machine as required by the contract. Obayashi used an earth pressure tunnel-boring machine. The machine has a chamber behind the cutter head, and during tunneling the chamber was supposed to be full but was not, Sound Transit officials said. This resulted in sand and other soft soils falling in from above, creating the voids.

To avoid that, tunnel-boring machines for U Link will display real-time information along with eight hours of “face pressure sensor data.” Sage said this will allow TBM operators and inspectors to “consistently monitor” operations to ensure the machines are being operated properly.

An inspector will be in the tunnel headings at all times, and data will be displayed in the construction management team's Husky Stadium and Capitol Hill offices.

If the data shows deviations from what Sound Transit expects, Sage said “immediate action will be taken.”

Joe Gildner, Sound Transit's U Link executive project director, said both U Link contractors have given Sound Transit detailed excavation plans that the agency has reviewed with the Federal Transit Administration.

Gildner said Sound Transit's staff recognizes the risk of tunneling under the Montlake Cut. He said the contract requires Traylor Frontier-Kemper Joint Venture, the contractor building U Link between Husky Stadium and Capitol Hill, to stop at a “hold point” to check everything before tunneling under the water.

Three TBMs will be used on U Link. Two of them will tunnel the UW-to-Capitol Hill segment, and the first of those two is set to launch in the next few days. The third TBM will tunnel from Capitol Hill to downtown Seattle.

Each TBM weighs more than 1 million pounds and is longer than 300 feet, including the conveyor systems that remove spoils excavated by the 21-foot diameter cutter head.

Satellite technology will guide the machines.

The TBMs are expected to operate around the clock. Sound Transit estimates it will take Traylor Frontier–Kemper Joint Venture 12 to 18 months to complete the 2.4-mile long twin tunnels from Husky Stadium to Capitol Hill.

Tunneling between Capitol Hill and downtown is scheduled to begin in late June. JCM U-Link Joint Venture is the contractor on that three-quarter-mile-long segment.

The machine will bore from Capitol Hill to downtown, where it will be removed though a special shaft near the downtown transit tunnel. Crews will truck the TBM back to Capitol Hill, where it will start digging the second tunnel. The entire operation also is expected to last 12 to 18 months.

U Link is scheduled to open in 2016. It is expected to serve an estimated 70,000 new light rail riders daily in one of the most densely populated areas on the West Coast, according to Sound Transit.

The Seattle-area Interstate 5 corridor now operates over capacity for up to eight hours a day with some buses running up to 30 minutes late.

When light rail is completed, Sound Transit said it will take six minutes to get from Husky Stadium to downtown even though the corridor's population is projected to grow 56 percent by 2030.