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Finding a firm footing
By THOMAS M. GURTOWSKI
The Safeco Field site would never have received a Most Valuable Place (MVP) award based on its geology. The new home for the Seattle Mariners is built on fill over a former tide flat.
The loose, cohesionless fill and underling tide flat deposits combined with a high groundwater table would result in soil liquefying and becoming like quicksand in a strong earthquake. Liquefaction can cause any heavy structure not adequately supported to crack and sink.
The ballpark was also constructed over one of the city's major sewer lines, precluding the possibility of a standard foundation approach. In addition, the proposed field elevation was below the groundwater table, creating obvious drainage issues in the rainy Pacific Northwest.
With this less-than-stellar starting lineup of issues, Safeco Field might seem like a geotechnical consultant's worst nightmare. Yet, Shannon & Wilson was able to find opportunities among the many challenges. A stable foundation system was recommended that was capable of supporting the structure and baseball fans for many generations to come.
High-capacity pile foundation
The biggest engineering feat was figuring out how to drive a reasonable number of piles deep enough to hold up the stadium, its 10,000-ton roof, and the adjacent parking garage. Piles are a cost-effective means of supporting buildings in soft alluvial deposits and are typically used in the tide flat areas surrounding Puget Sound. The piles at Safeco Field, however, were designed to support an unprecedented load of 300 tons each and had to be driven as deep as 90 feet to penetrate very dense competent glacial deposits.
This capacity was near the load limit these piles could carry without being overstressed. Furthermore, tests showed that the stiff, heavy-walled piles could be driven 80 feet through softer soil, and then an additional 10 feet into very dense glacial deposits, without damage.
Another benefit of the pipe piles was a stiff foundation system that would resist high lateral forces during an earthquake. Based on these results, a total of 1,407 high-capacity piles were installed to support the stadium and retractable roof. Another 445 piles now serve as a foundation for the ballpark's garage.
Selecting the right size of diesel hammer to drive the piles sufficiently into the glacial soils was another complication. Too big a hammer and the piles would buckle. Too small, and the piles would not penetrate deep enough to develop the required uplift resistance. Shannon & Wilson's specifications proved to be successful because all 1,852 piles were driven to the required capacities.
Side-stepping the sewer line
Right through Safeco Field runs the Elliott Bay Interceptor, a sewer main 96 inches in diameter, that serves most of downtown Seattle. Building over active sewer lines, especially large, critical, full ones such as the Elliott Bay Interceptor, is rarely attempted. The danger of accidental damage from construction and related vibrations is simply too great.
Original plans for the ballpark called for moving the sewer. But an engineering analysis showed it would be possible to put foundation piles within 3 feet of the interceptor, still protect the line from vibration, and avoid the expense of relocation. Piles were very carefully positioned near the sewer line, and surrounded with a casing to mitigate the risk of damage from vibration. Additional casings were placed directly on the sewer and tiny sensors measured vibration and relative displacement. Although nearly 285 piles were driven within 30 feet of the interceptor, the line suffered no damage during construction.
Keeping the field dry
After the fans have gone through either entrance on First Avenue South, they will walk down and experience a ballfield in the tradition of great baseball stadiums, one that is intimate and positioned at a lower level than the surrounding streets. In fact, the field is 6 feet lower than street level. Dugouts, batting cages and VIP areas behind home plate are another 2 to 3 feet below the infield. Thus, most areas are well beneath the groundwater table. Without a reliable dewatering system, the Mariners would have been playing water polo instead of baseball.
Engineering expertise struck out the problem. Aquifer testing evaluated soil permeability and determined the direction and gradient of groundwater flow. Then, we designed a permanent dewatering system consisting of a series of deep subdrain trenches. The beauty of the system is that it is passive, or gravity-powered, so that water needs to be pumped from the system at only a few key locations. The system collects groundwater 24 hours a day, 365 days a year, ensuring that players' cleats and fans' sneakers alike will stay dry.
Conserving groundwater for irrigation
Water collected from the ballpark's dewatering system would normally have gone unused, straight into Puget Sound. Why not use it to irrigate the grass? Recycling the groundwater is a cost-effective solution and reduces the ballpark's demand on Seattle's water system. This will be especially important during summer months, when the region's water resources are often scarce, but public water supply and irrigation needs are highest. The Public Facilities District, which manages Safeco Field, approached the Department of Ecology, and with Shannon & Wilson's and Perkins Coie's assistance was successful in obtaining a water right permit for this novel usage.
If all of these issues were not enough, the near-surface soil at the future Safeco Field turned out to be saturated by the record rainfalls we received during the construction period this winter. Any dump truck driver foolish enough to try to cross the field would have sunk out of sight into muck, some of which contained contamination from past uses. The clock was ticking, with barely enough weeks remaining to place the turf in time for the start of baseball season.
Hauling out more soil than necessary did not make sense, given the costs of transporting wet wastes to a special landfill. Another option was to dig out the dirt and put in sand and gravel, a very time-consuming and expensive endeavor. Instead, we recommended treating the wet, soft soil with cement. This kept the soil on site, but stabilized it enough to make it workable. Some areas required the additional procedure of excavating wet soil with large track-mounted backhoes, spreading it on a treated subgrade for mixing with cement, and then compacting it back into place. The relatively impermeable treated soil that resulted made it possible to proceed with infield construction in January-February, before spring temperatures and dryer weather conditions arrived for dazzling green grass on Opening Day.
Performing under pressure
Making sure that the various pieces of the Safeco Field puzzle all came together smoothly and within such a tight time frame was a significant challenge. Field investigations, recommendations, pile testing and production pile driving were all completed within just 14 months, meeting the fast-track schedule for this high-profile project.
Thomas M. Gurtowski is a vice president of Shannon & Wilson, a Seattle-based geotechnical and environmental consulting firm.
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