Dallas Horseshoe

Since its completion in April 2017, the Dallas Horseshoe interchange has substantially improved the flow of traffic crossing the Trinity River along Interstate 30 and Interstate 35 to enter or exit the Texas city’s downtown.

The $798 million interchange replaced a notoriously complex maze of tricky interchanges, bridges and roadways in downtown Dallas known as the Mixmaster interchange.

The original intersection was state-of-the art when it first opened in 1964, but it gradually developed an unfortunate reputation as a site of frustrating bottlenecks, lengthy traffic delays and safety concerns. By the time design work began in 2013 on a complete interchange replacement, the Mixmaster was deteriorating and ill-equipped to handle the estimated 450,000 to 500,000 vehicles that pass through on a normal weekday.

The Mixmaster’s left-lane exits and odd geometry created a lot of weaving in and out of the lanes. Correcting the Mixmaster’s confusing alignments was one of the highest design priorities. The Dallas Horseshoe eliminated all left exits, providing direct connections from northbound I-35 East to westbound I-30, and eastbound I-30 to southbound I-35 East.

As the lead engineer, WSP was responsible for planning the complete removal and methodical replacement of the Mixmaster interchange, as well as the replacement of the I-30 and I-35E crossings over the Trinity River. The firm’s responsibilities included design of roadways, bridges, river crossings, retaining walls, drainage, geotechnical engineering and maintenance of traffic planning.

WSP teamed with Pegasus Link Constructors (PLC)—a joint venture of Fluor Enterprises and Balfour Beatty—for the design-build contract to create the Dallas Horseshoe on behalf of the Texas Department of Transportation (TxDOT). Construction began in January 2014.

The Dallas Horseshoe is about 80 percent elevated roadway. The project included the construction of more than 73 lane miles of new roadway, 37 conventional bridges and more than 60 retaining walls.

The Horseshoe was constructed on challenging soil and rock conditions comprised largely of alluvial soils deposited by the Trinity River, and not of high strength or stability. The PLC team performed full-scale load testing to inform the foundation design, then used test shafts with load-cell devices and a full array of displacement and stress monitoring devices to develop site-specific load-displacement curves. The geotechnical team used these curves to optimize bridge and wall foundations, for greater overall project economy and value.

Since local motorists were accustomed to traffic exiting to the left, when the alignment changed they had to be trained to look to the right for their exits. To help alleviate the confusion, after the lanes began to take shape, eight bright, eye-catching digital lane use signs were installed over each lane, which helped drivers easily identify the correct exit lanes.

The Horseshoe is located in one of the oldest sections of Dallas—a constrained area with numerous utilities. The project required the relocation of nearly 150 major utilities, including natural gas, fiber-optic communication, electric power transmission, wastewater and public water.

TxDOT provided early relocation of electric transmission lines. But interfacing with other utilities posed some unusual challenges. The City of Dallas had numerous wastewater and water lines in the area—some as large as 96 inches in diameter—so WSP endeavored to avoid relocations as much as possible. This was achieved in the design through a protect-in-place concept for the wastewater trunk line.

Several large sanitary sewer pipes constructed in the 1930s and 1940s—some of which directed storm water from downtown Dallas to the Trinity River—passed under the Horseshoe project and were expected to need replacement due to age. A detailed inspection and condition assessment program concluded that only a few systems needed replacement, and that six protected-in-place, no-touch sanitary sewer lines had decades of remaining service life. Using a special structural cap design known as the trampoline solution, new pavements were installed above these sewer lines, including bridge foundations to straddle these fragile services.