Seattle-Tacoma International Airport's two runways were only 800 feet apart. On cloudy days, arriving aircraft had to be directed to these two runways in single file.
Safety is a concern when planes are flying through clouds sideby- side in such close proximity. And the resulting number of flight delays prompted action. Sea-Tac built a new third parallel runway far enough away that two streams of aircraft would be able to approach for landing.
The reduction of flight delays meant millions of dollars in savings each year for both travelers and airlines alike. “With the new runway we won’t lose half of our operations during bad weather,” says John Rothnie, project manger for the third runway at the Port of Seattle. “The airport can remain operational in virtually all types of weather.”
Although the project was a success, construction for the new runway altered the landscape and changed the stormwater runoff characteristics drastically. Something needed to be done to prevent erosive flows from occurring in Des Moines Creek located near the end of the third runway.
The small Des Moines Creek discharges into Puget Sound just two miles away. “Washington State enforces some of the most stringent requirements for stormwater management in the country,” says Bob York, senior infrastructure engineer for the Port of Seattle, “and stormwater detention facilities are required for any new impervious surface.”
Additionally, the Port has its own plans to restore and protect surrounding sensitive areas, including wetlands. Working together, the Port of Seattle and Contech Engineered Solutions came up with a viable and costeffective approach by installing a CON/SPAN® underground detention system.
The CON/SPAN precast detention system is designed for 4.94 acre-feet of storage. The 24-foot span x 11-foot rise units were placed on 3-foot, 8-inch pedestal walls. There are three cells of varying length, including 194-, 234-, and 274-feet.
Stormwater runoff from a portion of the third runway is directed to the unit. Water is then released at a flow rate equal to the predevelopment rate. This protects the nearby creek by reducing the risk of flooding and erosion. It also traps suspended solids and reduces silt and sediment build-up.
The system’s precast arch shape provides the ability to support severe loading conditions efficiently, which lower material costs and ultimately capital costs. The system also features a 6-foot x 11-foot access opening to lower excavating equipment into the structure for maintenance.
Since this is an airport project, wildlife was a consideration. Unlike underground systems, open detention ponds have the capability to attract wildlife and threaten the safety of aircraft. “Open water has the potential to attract birds, and when you’re right next to an airport the FAA discourages that,” says York. Furthermore, a precast style detention system requires less space than a pond and is ideal for maximizing available footprint. The value of the real estate at the airport is high and there are many layout constraints.
The unique combination of precast walls and pedestal foundations of the detention system allows for an array of configurations to help meet the site-specific limitations of the airport. The arch unit is extremely versatile because it can be installed in multiple cells with, or without, staggered ends. This further allows it to be constructed in order to follow the tight area at the end of the runway.
Another benefit of precast is quality control. Rothnie says, “Something that we were impressed with was the high quality of the units. The fit and the finish of the precast was just excellent - something we really haven’t experienced before.”
Some of the cast-in-place (CIP) options that the Port of Seattle looked into utilizing would have caused installation delays well into 2007. And, such CIP systems would have cost incrementally more.
The quickness of the installation and the ability to use a precast system instead of CIP is estimated to equal over $1 million in savings. Utilizing precast units allowed the Port to get the system installed considerably ahead of schedule.
“A real advantage to doing it this year was the construction schedule - not only for this year, but for next year as well. To get a project this substantial installed before 2007 is a definite plus”, explains Rothnie.
The final stage of the project involves getting the system online, which is probably still a year away. “So far the installation has met the Port of Seattle’s expectations,” says York. “The construction has proceeded very smoothly.”
Rothnie adds that the Port of Seattle will definitely consider precast vaults for future projects. “The experience was great - both from the engineering side and the field support side.”
This particular system adheres to demanding state regulations and meets the Port’s guidelines to help protect and restore Des Moines Creek and protect Puget Sound.