DALLAS TX:Swank L’il Toll Tunnel Opens under Airport

These guys haven’t been employing lobbyists in Washington DC or the state capital Austin to squeeze ‘pork’ out of the governmental system or big bucks out of taxpayers somewhere else in the country. In the best western tradition they’ve said: “We’ll do it ourselves. We’ll cough up a couple of quarters a trip for the extra amenity and convenience we’ll get.”

The Addison Airport Toll Tunnel will provide commuters and business people and moms and dads and delivery trucks and a lot of other diverse traffic a straight shot east-west Richardson to Carrollton along the Keller Springs Road and between the Keller Springs Rd interchange of the Dallas North Tollway and points west. And it will take a few big rigs off local streets. A Wilbur Smith traffic and revenue study estimated that 12.5k veh/day on opening (and 21k veh/day in 2010) would pay 50c/trip to save up to 2 miles (3km) and 9mins with a couple of signalized intersections jogging north or south. That amounts to tolls of $2.3m/yr on opening (and $3.8m/yr in 2010) plus some extra revenue on the N Dallas Tollway itself ($445k/yr on opening and $800k in 2020.)

On that basis this $15m project ($13m for the tunnel itself and $2m for the toll plaza and approach roads) will be self-financing. That “swank” party down under the runway at Addison will rightly celebrate a good road project that will improve the quality of local life without slipping a hand into other people’s pockets via the tax collector.

Project manager Mark Bouma of the North Texas Tollway Authority, Bill Leech engineering supervisor of Brown & Root, and Dan Hubenak of Zachary Monterrey the tunnel contractor and their staffs completed the tunnel to contract price in 16 months. By contrast the permitting, studies and financing took 7 years!

The tunnel construction was not uneventful. A ten ton rock fell from the roof two-thirds through tunneling. And before they had built a retaining wall they had quite a rockslide on one approach ramp - 600 cub yds of rock that came rumbling down uninvited, blocking the entrance to the tunnel. That set them back a few weeks. Fortunately no one was hurt in either of the incidents.

The airport is a busy one with just the single runway so the tunnel had to be mined underneath it. Exploiting what is often called the New Austrian tunneling method they used quite basic machinery, a single roadheader with a grinding head that grinds the rock up at the face and sends gravelly material onto a conveyor belt into waiting dump trucks. That cuts out the “heading” or top of the tunnel section. A jumbo with a rotary drill was used to insert rock bolts into the rock to put it into compression and stabilize it, the number and depth of the anchors being calculated for a ‘just-enough’ support, along with stiff quick drying concrete (shotcrete) ‘shot’ onto the walls. The shotcrete was reinforced not with conventional rebar but with economical shards of steel called fiber reinforcement. A D9 dozer with a rear ripper then tore out ‘benches’ deepening the tunnel progressively downwards, as wheel loaders put the stuff in trucks which drove it out the tunnel.

As usually happens in tunneling they ran into some underground water and had to intensify their anchoring and provide drainage systems and pumps. The water was at what they call a reverse fault in the rock. Then there was waterproof lining, reinforcement, horseshoe shaped concrete finishing, and roadway pavement, small side walkways, lighting, and suspended jetfans for ventilation.

Volume of heavy trucks is expected to be small, so the owner and operator North Texas Tollway Authority has decided to forgo vehicle classification and different rates. All vehicles will pay the same 50c at the 6 toll lane plaza at the western end of the To keep down staffing costs there is a single central toll booth with windows each side for one or two collectors looking after manual payers. The middle lane each way will have a coin machine lanes and the right lane will be for electronic tolling.

“Swing on down” is their slogan, both for the opening ceremony and for patrons. (Contact Mark Bouma NTTA 972 267 0465)