Third Runway

Port of Seattle

Seattle-Tacoma International Airport

Third Runway Sea-Tac Airport

17 Million Cubic Yards. Constructing a third runway at Seattle-Tacoma International Airport was an extraordinarily complex project,the sheer scale of which makes it logistically intricate to execute. The new 8,500-foot-long runway required construction of an embankment built from 17,000,000 cubic yards of compacted earth fill.

Challenges. Because the site was in an urban area, special attention had to be given to avoiding adverse impacts to local watersheds and to completing construction with the least possible impact on neighborhoods. To meet an aggressive schedule, design and initial construction needed to move forward while property acquisition was under way and before wetlands delineation was complete.

Tallest Wall in the West. Working with the Port of Seattle and a team of consultants, Hart Crowser was the principal geotechnical and hydrogeologic consultant for design, permitting, and construction of the third runway. Completing the runway involved relocating roads and utilities, and constructing three retaining walls ranging from 50 to 165 feet high to support the new embankment and minimize wetland filling. Walls of this height are rare—only one other retaining wall in the world is taller than 165 feet.

Computer Modeling. Hart Crowser developed the geotechnical basis of design for the walls and was responsible for verifying their stability. Hart Crowser also completed a site-specific probabilistic seismic hazard assessment, using a finite difference-based approach to estimate earthquake effects.

Hart Crowser used specialized drilling and in situ testing techniques to measure soil and groundwater properties and state-of-the-art computer modeling to analyze seismic performance of the embankment, slope stability, seepage, and other design issues. To evaluate construction impacts on groundwater, Hart Crowser developed customized computer models to extend the analyses beyond the capabilities of “off the shelf” software.

Complex Permitting Reviews. The Port of Seattle relied on our technical perspective in developing their construction plans and on our expertise in addressing complex permitting reviews that were part of the public oversight process.