June 27, 2002
Driven piles make stadium strong
By MARTIN PAGE
Shannon & Wilson
What’s beefier than Ryan Leaf, sturdier than Jerramy Stevens, and able to withstand more battering than Maurice Morris?
The foundations supporting the new Seahawks Stadium.
Officially called Seahawks Stadium, the landmark facility is located over loose fill and old Seattle mud flats, the worst possible soils in earthquake country. Designing it to survive a seismic jolt, on a fast-track schedule, with an affordable budget was an ambitious undertaking.
Building on borrowed soil
The stadium site was originally a tidal marshland crisscrossed by dozens of railroad trestles and elevated saw mills. It was filled around the turn of the century when Seattle’s hills were leveled and sluiced down into the mud flats. Manmade fill up to 40 feet deep was dumped at the site. Soils are therefore soft and prone to liquefaction — not exactly an ideal location for a massive structure like a stadium.
Earthquake engineering is a constantly evolving science and engineers weren’t about to rely merely on data and analysis for reassurance about seismic risks. They tapped an earthquake engineering research team at the University of Washington, where professors and graduate students were developing cutting-edge computer modeling software for projects exactly like this one.
Called DYNOPILE analysis, the UW program performed a dynamic pile-soil-interaction analysis, estimating lateral movement and pile bending moments during design-level earthquakes. The evaluation indicated significant differential ground deformation where the soft soils of the old mud flats met the dense, glacial soils below. By modeling these earthquake-induced deformations, the engineers were able to design cost-effective pile foundations with confidence that they will withstand a major earthquake.
Piles can be good
Four different foundation types were considered for the stadium: drilled piers, concrete piles, auger-cast piles and closed-end pipe piles filled with concrete. The winner was pipe piles, selected for being relatively affordable, quick to install and a known entity, since pipe piles also hold up the baseball stadium across the street.
Closed-end pipe piles are giant steel tubes, in this case between 20 and 24 inches in diameter and up to 90 feet long. They’re sealed at the bottom with a thick steel plate to prevent soil from entering, and are rammed into the earth with diesel-powered driving hammers. Once in place, they’re filled with reinforced concrete for added strength.
Clearly, piles must be placed strategically and driven to precise lengths to limit costs. To create comprehensive specifications, engineers performed extensive geologic studies involving soil borings. Then they prepared subsurface cross sections, drawing something like an underground map of soil conditions. The result was an accurate location of competent glacially overridden soils that could support high-capacity piles. Subsurface soil mapping helped the owner obtain realistic bids from potential pile-driving contractors
Another wise move was installing preproduction test piles. Not only did this confirm the bearing capacity of piles, but it also gave contractors greater certainty when preparing bids, resulting in overall lower costs. As is almost always the case, the more you spend up front in testing, the more you can decrease contractor risk and, therefore, reduce project estimates. Test pile driving therefore provided significant benefits to the project.
Close to 1,750 piles, each carrying about 300 tons, were driven into dense, glacial soils at the stadium. They support those huge white roof trusses spanning an astounding 720 feet. Another 500 piles support the new exhibition hall and parking garage.
Tale of two stadiums
We learned while constructing the baseball stadium that piles easily become damaged during hammering. They can hit a boulder and crumple at the tip or simply buckle due to the tremendous forces exerted during driving. So we specified a higher strength of steel pipe for piles at the football stadium. This resulted in a much better survival rate.
Still, every pile had to be visually inspected after placement for quality assurance. Field engineers used ladders and hydraulic lifts to reach the top of driven piles, sometimes 20 feet in the air, and ran a halogen light down the center. Once their integrity was confirmed, the piles could be filled with concrete. Only a handful of piles had to be yanked out of the ground and replaced.
In many key areas, however, the two stadiums were vastly different. Safeco Field was built below grade to achieve aesthetic goals. Seahawks Stadium, on the other hand, is at grade, minimizing the need for extensive excavations and reducing the likelihood of encountering hazardous materials or groundwater. This saved the owners a great deal in design, construction and potential remediation costs.
Also, the football stadium was able to recycle most of the on-site fill material, namely the rubble of the former Kingdome. By crushing, sifting and then recycling hunks of the old structure, owners were able to save between 10 percent and 20 percent of the cost of bringing in new fill.
Keeping neighbors happy
Noise is always a factor during major construction in an urban area. It’s much more so in cases like the new stadium, where condos and businesses line one side of the project. To consider neighbors’ needs, piles were driven only during business hours, and a sound deflector wall was carried along beside the driving hammer. This helped shelter both homes and businesses along Occidental Avenue.
Neighbors, owners and everyone navigating the streets of Seattle wanted the stadium to be completed as soon as possible. It will, in fact, open right on schedule, something of a miracle considering the track record of many civic endeavors.
Seahawks Stadium is a magnificent facility that all residents of the state can take pride in. The teamwork it illustrates easily rivals a Super Bowl or World Cup performance. Best of all will be its longevity. You can trust a geotechnical engineer on this. This one is engineered to last forever.
Martin Page, is the project manager for the stadium project and principal geotechnical engineer with Shannon & Wilson, a geotechnical and environmental consulting firm in Seattle.
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