Foundation Seismic Retrofit of Boeing Field Control Tower

The existing control tower at King County International Airport in Seattle, Washington, was severely damaged during the Nisqually Earthquake, February 28, 2001. As a result of the damage, the control tower was immediately closed and the existing timetable for a seismic retrofit of the control tower was expedited. The existing tower and associated support structures were founded on timber piles of unknown lengths (probably extending to depths between 25 and 35 feet); subsurface explorations indicated liquefiable soils to a depth of approximately 35 feet. The foundation retrofit included compaction grouting to densify the loose, liquefiable sands beneath the existing structures and the installation of drilled shafts adjacent to the tower to support the new structural steel bracing which was added to increase the resistance of the tower to overturning during the design earthquake. With the air traffic control operations being performed in a temporary facility, the urgency of performing the retrofit led the Federal Aviation Administration to negotiate a contract directly with the general contractor. This paper addresses the method of contract delivery, compaction grouting methods and results for liquefaction mitigation, and the construction of drilled shafts adjacent to an existing structure.