By Mark Rea

The North Texas Tollway Authority is teaming with the Texas Department of Transportation to complete a $31 million highway project in a rapidly growing city just north of Dallas.

Construction of the Dallas North Tollway Extension and Interchange at State Highway 121 began in early 2002 just south of Frisco, an area where TxDOT was planning improvements to SH 121.

"As we got into development of the project, working with TxDOT and its plans to expand SH 121, it quickly appeared that both of us would be trying to do construction projects in the same area at or close to the same time," said Mark Bouma, NTTA director of engineering. "The idea was developed to create a combined project, specifically at the interchange, and we designed for TxDOT the portion of SH 121 that moves through the interchange."

Houston-based Kellogg Brown & Root is the construction manager for the three-level interchange project and the Austin office of Balfour Beatty Construction Inc. is the general contractor.

The levels - which can be expanded to five in the future - include a frontage road intersection, SH 121 main lanes in the middle and tollway main lanes at the top. Had the project continued as two separate projects, it would have necessitated TxDOT constructing an additional bridge later between the completed first and third levels.

"It's rare but not that unusual for us to do work with other entities," said Bill Goodell, TxDOT Dallas district construction engineer. "Either they do the work and we pay for it or vice versa. Here in the Dallas area, for example, we have done the same type of things with (the Dallas Area Rapid Transit Authority) and NTTA on the tollway and the George Bush Turnpike.

"In this case, the tollway wanted its portion of the project completed before we were scheduled to begin. We already had two projects under way on SH 121: one on the far east side at U.S. Highway 75 and one on the far west end at Interstate 35.

"We were going to be redoing SH 121 through that area sequentially, but it was going to be awhile before we actually got to that interchange. The tollway authority proposed that they do the work and we could simply reimburse them, and that's how the collaboration came about."

Goodell said the collaboration has gone smoothly. "The tollway authority is limited in the number of staff it has, so it has a construction manager," he added. "It's KBR in this case, and we have worked quite frequently with them over the past few years, especially on the George Bush Turnpike. We have also worked with Balfour Beatty as well, so we were comfortable with that arrangement."

TxDOT also continues to perform periodic oversight inspections on the project.

Response To Growth

The project is part of the NTTA's capital improvement program as the tollway begins its initial extension into Frisco. It is located just south of the burgeoning city's new sports complex, which features newly completed minor-league baseball and ice hockey venues.

Statistics show that 60,000 to 70,000 vehicles use that SH 121 corridor daily in addition to up to 30,000 more on the Dallas North Tollway.

"We wanted to extend the tollway at least over SH 121 to give our patrons increased mobility and alleviate some of the traffic congestion in that particular area," Bouma said. "That particular part of Collin County is experiencing a tremendous amount of growth."

The project will contain more than 110,000 sq. yds. of continuously reinforced concrete pavement when completed. Noel Ibrahim, chief engineer/construction for the NTTA, said there will be 93,000 sq. yds. of 10-in. pavement on the DNT and 17,300 sq. yds. of 11-in. pavement on SH 121.

"In addition, we are building very short lanes on SH 121 and transitioning about 17,000 sq. yds. of 8-in. pavement," she said.

The project will also contain nearly 235,000 sq. ft. of bridge work although no direct-connect ramps are part of the project. "This will be a signalized intersection," Bouma said. "There are already master plans to connect SH 121 via ramps to the east of the DNT, and the schematics for those ramps have already been approved. But they are not part of this particular project."

The tollway extension and intersection features drilled-shaft construction. "We were very fortunate with this project that the rock base in the area is relatively shallow," Ibrahim said. "The piers were drilled to about 15 ft. below the surface."

Each conventional cast-in-place pier averaged in diameter between 36 in. and 48 in.
Balfour Beatty supplied its own concrete for the project from an onsite batch plant located near the new Dr Pepper/7 Up Ballpark at the Frisco Sports Complex. Dallas-based TXI has also provided concrete specialties for the project, and the Lofland Co. of Fort Worth supplied the reinforcing steel.

Earthmoving occupied a large portion of the early stages of the project as approximately 457,000 cu. yds. of materials were excavated and then used for various embankments in the extension and interchange. Some excavated material was also removed from the site and stored for future DNT extensions as well as main lane work on SH 121.

Utility relocation was part of the initial phase of construction, including electric distribution and telecommunication lines.

"Probably our biggest challenge at that time was a 24-in. water main that crossed the project at the Headquarters Drive underpass," Bouma said. The main was eventually moved approximately 600 ft. north of its original location with no interruption of water service to local customers.

Additional Features

Once the project is completed, it will boast several new design features recently adopted by the tollway authority for its entire system.

"We just implemented system-wide design guidelines, and this project will be the first to use those," Bouma said. "We are working with Balfour Beatty to change-order a lot of new design features into the project, and we believe it will be a signature project for us."

The tollway extension will feature modified traffic rails similar to TxDOT's new single-slope traffic barrier but will have a modified back face. The tollway will also feature unique steel railings on the underpass bridges and more architectural finishes with its lighting designs.

"For example, on a normal bridge truss, we're going to use large tubular steel cantilevers," Bouma said. "It is also the first project that will feature our agency's new signature look in toll plazas."

Scheduled for completion in 2004, the DNT extension will end just north of SH 121. However, plans are already under way to extend the tollway to U.S. Highway 380.
Construction on that extension is expected to begin next year.