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Addison Airport Tunnel

Addison Airport is the busiest general aviation airport in the State of Texas.  To improve east-west traffic flow in the vicinity of the airport and through this 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 two-lane highway tunnel with an excavated cross-section 40 ft wide by 26 ft high.  The tunnel was excavated in the Austin Formation, a soft limestone, with approximately 26 ft of rock cover beneath the active airport runway.

Lachel provided complete geotechnical and tunnel engineering services for the HDR-led team, including: preparation of plans, specifications, and tunnel construction cost estimates.  The 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.

Lachel’s 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.