Addison Airport is the busiest general aviation airport in Texas. To improve east-west traffic flow in the airport’s vicinity and highly congested Addison/North Dallas area, the North Texas Tollway Authority (NTTA) opted to construct a tunnel extending Keller Springs Road beneath the airport. The project consists of 1,990 ft of two-lane depressed roadway leading to 1,650 ft of a two-lane highway tunnel.

The tunnel was excavated with a 40 ft wide by 26 ft high cross-section in the Austin Formation, a soft limestone, with approximately 26 ft of rock cover beneath the active airport runway. Schnabel provided complete geotechnical and tunnel engineering services for the HDR-led team, including the preparation of plans, specifications, and tunnel construction cost estimates. Our geotechnical services included design and supervision of the field investigation and laboratory testing program, development of subsurface engineering properties and design criteria, development of the Geotechnical Design Summary Report (GDSR), and layout of the geotechnical instrumentation and monitoring scheme to be used during construction to monitor ground response and tunnel stability.

Our tunnel engineering services included design of both the initial and final tunnel ground support systems. The initial support scheme utilized the Sequential Excavation Method (SEM) approach based on an evaluation in the field of the ground response to excavation. The phased SEM sequences were developed and specified in conjunction with the initial support systems for each reach of tunnel to assure tunnel stability and in order to maintain safe, uninterrupted airport operations.