Geoengineers Shoring combination of soldier piles, soil nails and tieback anchors for future.

Soldier piles, soil nails and tieback anchors saved the day—and nearly $1.5 million over a pipe-bracing shoring alternative—allowing the excavation for a 39-story residential condominium in Seattle to progress without disturbing the too-close-for-comfort neighbors.

The biggest shoring headache for the Olive 8 project was along the site’s 240-ft-long west side, bordered by a 16-ft-wide alley and the 34-story Qwest building, which extends 50 ft below grade. Uninterruptible power and communication utilities run under the alley.

Geoengineers 39-story residential tower kept the neighbors happy.

None of the usual methods of shoring the 76-ft-deep excavation would work. Vacating the alley or installing tiebacks into Qwest’s underground garage were not options, says David G. Winter, president of the project’s local geotechnical consultant, GeoEngineers. Pipe bracing across the excavation was viable but not economical. It would probably have sunk the entire project, currently seven stories, due to its $2-million cost, he says.

The challenge was to support the excavation externally and limit deflections of the shoring wall to 1 in., to meet the city’s requirements.

Matt Smith, GeoEngineers’ project manager, dreamed up a hybrid system of closely spaced soldier piles, 86 ft deep, with tightly spaced soil nails in the walls’ upper portion and steeply inclined tieback anchors in the lower portion, under Qwest.

Winter says the three methods in combination have never been used in Seattle. GeoEngineers, which worked with local project structural consultant KPFF Consulting Engineers, used finite element modeling to design the system.

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Geoengineers Shoring combination of soldier piles, soil nails and tieback anchors for future.

Shoring work started in April 2006 and took five months. There weren’t too many challenges related to the system. “The first row of soil nails was the most nerve-wracking because of the risk of hitting the fiber-optic cables,” says Greg Domgaard, superintendent for the shoring subcontractor, DBM Contractors Inc., Federal Way, Wash.

Drilling through some unexpected, abandoned tieback anchors from the Qwest building slowed the work. But there was no overall delay because DBM brought in a second drill rig.

Measured deflections were 3⁄4 in. or less at all locations along the Qwest wall, instead of 1 in. or just under, as predicted. There were also no deflection problems with the neighbors, says Winter. “This validates that finite element modeling is reliable for complex shoring designs,” he adds.