American Underground-Construction Association's Featured Project

Industry News

Each month AUA will feature an outstanding Underground Project currently under construction, featuring its unique aspects in terms of technology, location, function, etc. Contact AUA to nominate projects.

This month's Featured Underground:

Addison Airport Tunnel Project

New toll road tunnel will ease Dallas traffic congestion

Starting in September 1997, tunneling crews have been constructing a two-lane vehicular toll tunnel under the Addison Airport, located in the Dallas Texas metropolitan area. When complete, in early 1999, the tunnel will provide a convenient link under the airport to relieve traffic congestion in the northern metropolitan area.

Brown & Root, Inc. is functioning as the construction manager for the owner, the North Texas Tollway Authority (NTTA). Brown & Root also coordinates and liases with the Addison and Carrollton local governments, Addison Airport of Texas, Inc. - the airport operator, and the Federal Aviation Authority. The Addison Airport is the third busiest general aviation airport in the United States.

The $13 million construction contract awarded to H. B. Zachry Company includes the tunnel, portal approaches, retaining walls, mechanical and electrical work, and associated construction. Lachel & Associates, a subconsultant for HDR Engineering, is the tunnel designer. Zachry/Monterey, J.V. has a subcontract to excavate, support, geomembrane, concrete and equip the tunnel.

Addison Airport Tunnel Project

The 1,590-foot long tunnel was excavated to a horseshoe configuration 39-ft wide by 28-ft high. The entire tunnel was driven in Austin Chalk, a soft limestone (2,400psi) rock. The top heading and bench method was used for tunnel excavation. A Mitsui roadheader excavated the 14-ft high top heading in one pass. A modified Vermeer trenching machine with a 12-ft wide milling head excavated the 14-ft high bench in ten 3-ft high passes. The ground cover above the tunnel crown is 10-ft at the west portal, 18-ft at the east portal and 28-ft under the airport runway.

The majority of the top heading was supported with No. 7 tensioned rock bolts on 5-ft centers and 3-in. thick steel fiber reinforced shotcrete (NATM). For approximately 60-ft 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 4-ft centers, 6x6-in.-4x4-gauge welded wire fabric, and 6-in. thick steel fiber reinforced shotcrete.

A -in. thick drainage fabric and 80-mil thick PVC waterproofing geomembrane was attached to the shotcrete lining. A 12-in. thick reinforced concrete lining was placed against the geomembrane. The finished tunnel concrete dimensions are 36-ft wide at spring line and 22-ft high at the crown.

On June 24, 1997, NTTA gave H. B. Zachry a notice-to-proceed. On January 29, 1998 Zachry/Monterey "holed-thru" the tunnel's top heading. After the first 80-ft, excavation of the top heading averaged over 15-ft/work day. On April 1st the tunnel subcontractor finished removing the bench. Bench removal averaged approximately 1000-cy/work day. On August 19th Zachry/Monterey finished placing 53 concrete tunnel arch sections. The arch sections were planned to be placed at a rate of 30-ft/day (130cy/day), in a five (5) day work week. The tunnel subcontractor made the 53 arch placements in 65 work days. As of mid-August, the prime contractor has nearly finished the approaches precast retaining walls and has finished placing concrete for the tunnel cut-and-cover section. H. B. Zachry now estimates to complete the Project in 18-months, by January 1998.

Bill Leech is the Resident Engineer for Brown & Root. Jim Gardner is the Project Manager for H. B. Zachry. J. D. Martin is the Project Manager for Zachry/Monterey. For more information contact, Thomas L. Kelley, Brown & Root's Construction Manager, at 972-267-0444.

Los Angeles Metro Red Line - Santa Monica Mountains Tunnels

The Central Artery/Tunnel Project in Boston, Massachusetts. The Central Artery, a six-lane elevated road built in the 1950s to handle about 75,000 vehicles a day, is now jammed with over 190,000. A few years ago, with the specter of 14-15 hour a day traffic jams looming in our region's future, the Massachusetts Highway Department began construction of the Central Artery/Tunnel Project.