Addison Airport Toll Tunnel, Dallas, TX, USA

Following seven years of permitting and studies, the construction of the Addison Airport tunnel started in September 1997. The tunnel is a two-lane vehicular toll tunnel under Addison Airport, located in the Dallas metropolitan area, Texas, US. The construction was completed within 16 months and by January 1999 the tunnel was open to traffic. The tunnel is 1,650ft long and relieves traffic congestion in the northern metropolitan area between the Dallas North Tollway and Interstate 35E. The North Texas Tollway Authority (NTTA) commissioned the $15m project ($13m for the tunnel and $2m for the approach roads and toll plaza). All vehicles pay a %0.50 toll at the six lane toll plaza regardless of size (vehicles can use the TollTag electronic system).

TOLL TUNNEL

Built for the North Texas Tollway Authority, the toll tunnel enables vehicular traffic to travel under rather than around the airport, saving time and traffic congestion. The old route used Belt Line Road and Trinity Mills Road, which would become heavily congested. The tunnel reduces the journey time by nearly ten minutes.

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, and among the busiest in the USA. The tunnel was processing more than 15,000 toll transactions per day by 2000. Studies have indicated that there were 12,500 vehicles per day upon opening and that by 2010 the tunnel will be used by 21,000 vehicles per day providing some $3.8m per year in tolls.

PROJECT MAKE-UP

The work carried out by the lead contractors, HB Zachry, included the tunnel, portal approaches, retaining walls, mechanical and electrical work, and associated construction. The project consisted of 1,990ft of two-lane depressed roadway and 1,650ft of two-lane highway tunnel. The tunnel was excavated to a horseshoe configuration 40ft-wide by 28ft-high. The ground cover above the tunnel crown is 10ft at the west portal, 18ft at the east portal and 28ft under the airport runway.

Zachry's team completed the project using specialised tunnelling equipment in varying terrain, soils and climates, and successfully fulfilled both OSHA and MSHA guideline requirements. The top heading and bench method was used for tunnel excavation. A Mitsui roadheader excavated the 14ft-high top heading in one pass. A modified Vermeer trenching machine with a 12ft-wide milling head excavated the 14ft-high bench in 10ft-high passes. The host material for the tunnel was Austin Formation Chalk, a soft limestone (2,400psi) rock.

The majority of the top heading was supported with No. 7 tensioned rock bolts on 5ft centres and 3in-thick steel fibre reinforced shotcrete (NATM). For approximately 60ft inside the west portal and in difficult ground, the tunnel was driven using the multi-drift top heading method. The portals and difficult ground sections were supported with No. 7 fully grouted rebar spiles, No. 7 tensioned rock bolts on 4ft centres, 6in x 6in - 4x4 gauge welded wire fabric, and 6in-thick steel fibre reinforced shotcrete.

The geotechnical services included the design and supervision of the field boring and laboratory testing programme, 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 Geotechnical Design Summary Report (GDSR).

As part of the tunnel design services, the subcontractors designed the initial and final support systems based on the evaluation of the ground response. As well as this, Zachry's project team developed and specified phased excavation sequences in conjunction with the initial support systems for each reach of tunnel. This was done to ensure tunnel stability for safe maintenance and uninterrupted airport operability throughout the life of the tunnel project. Critical tunnel sections were modelled using FLAC, a finite difference code, to confirm the adequacy of the support systems, the stability of the excavations and also to predict the ground movement and stresses in the rock mass during each excavation phase. There were two rock falls during construction. The first occurred two thirds of the way through tunnelling and involved a 10t rock fall from the roof and the second was a rockslide on one of the approach ramps involving 600 yd³ of rock.

LEAD CONTRACTORS

Brown & Root, Inc. functioned as the construction manager for the project in conjunction with Bill Leech Engineering. The $13m construction contract was awarded to HB Zachry Company who was also involved in the Zachry/Monterey joint venture that had a subcontract to excavate, support, geomembrane, concrete and equip the tunnel. Working on a team led by HDR Engineering, LACHEL & Associates provided complete geotechnical services, tunnel design, plans, specifications and cost estimating services for the project. AGUIRRE were the project managers for the six-lane toll plaza and supplied architectural design, electrical engineering and mechanical engineering services.