Addison Airport is the busiest general aviation airport in Texas and among the busiest in the United States. Prior to this project, east-west traffic in the highly congested area was forced to go around the 368-acre site. The North Texas Tollway Authority was determined to construct a tunnel extending Keller Springs Road beneath the airport. The project consisted of 1,990 feet of depressed roadway leading to 1,650 feet of two-lane tunnel with an excavated cross section of 40 feet wide by 26 feet high. The tunnel was constructed in the soft limestone of the Austin Formation.
Schnabel provided complete geotechnical and tunnel engineering services for the HDR-led project team. The geotechnical services included the field investigation and laboratory testing program, geotechnical design summary report, and development of the instrumentation and design program to track ground response and tunnel stability during construction. Design services included preparing plans and specifications, and providing construction cost estimates.
We also designed both the initial and final tunnel ground support systems. The initial support system used the sequential excavation method, which our analysis indicated was the best approach to assure tunnel stability during construction and maintain safe, uninterrupted operations for the airport.