Project Description

The Addison Airport Tunnel Project is part of the Dallas North Tollway System and extends Keller Springs Road under the Addison Airport property, the busiest general aviation airport in the State of Texas . The project consists of 1,990 lineal feet of two-lane depressed roadway and 1,650 lineal feet of two-lane highway tunnel. The host material for the tunnel is the Austin Formation, a soft limestone. The excavated tunnel cross section is 40 feet wide by 26 feet high and the amount of rock cover varies from a minimum of 10 feet to a maximum of 26 feet.

Scope of Services

Working on a team lead by HDR Engineering, Lachel & Associates, Inc. provided complete geotechnical services, tunnel design, plans, specifications and cost estimating services for the project. The geotechnical services included the design and supervision of the field boring and laboratory testing program, interpretation of the data, development of subsurface engineering properties, development of design criteria based on subsurface data, design of construction phase instrumentation program and the development of the Geotechnical Design Summary Report (GDSR).

As part of its tunnel design services, Lachel not only designed the initial and final support systems based on the evaluation of the ground response, but also developed and specified phased excavation sequences in conjunction with the initial support systems for each reach of tunnel to assure tunnel stability in order to maintain safe, uninterrupted airport operations throughout the life of the tunnel project. Critical tunnel sections were modeled using FLAC, a finite difference code, to confirm the adequacy of the support systems, the stability of the excavations and to predict the ground movement and stresses in the rock mass during each excavation phase.