'Big Tex' to Bore 5-Mile Tunnel to Alleviate Dallas Flooding

A tunnel-boring machine named Big Tex is up for the challenge of digging a 5-mile drainage tunnel 170 feet below Dallas, Texas, to mitigate flooding and runoff

'Big Tex' to Bore 5-Mile Tunnel to Alleviate Dallas Flooding

'Big Tex' is the name given to the 230-foot long tunnel-boring machine that is embarking on a $300 million project to protect thousands of Dallas, Texas, properties from flood events.

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Dallas, Texas' storm sewers, designed and built in the 1930s, are no match for the dense neighborhoods of a 21st-century city. Despite upgrades and patches applied to accommodate a growing population and storm frequency over the years, floods were happening more frequently, and they were growing in scale. The city needed a permanent solution, so they decided to dig a giant drainage tunnel using a tunnel-boring machine (TBM) named “Big Tex.”

To say Dallas has a flooding problem is putting it mildly. In the last 15 years the city has reported five considerable flooding events in the East Dallas area home to Baylor Medical Center. The flooding caused by severe weather has caused millions in damage and threatened access to critical care when needed most. The problem has been compounded by a significant population growth which has sparked more building and the increase in runoff that typically accompanies that growth. Dallas was more than ready for a solution — they just didn’t know it would be a 5-mile tunnel 170 feet below the ground when they started planning.

100-year flood protection

The Mill Creek Tunnel Project began as three separate projects — Mill Creek, Peaks Branch and State-Thomas. That plan would have routed the mill creek system under the interstate requiring both open cut and shallow tunneling. The Peaks Branch and State-Thomas portions would have caused significant disruption to the Dallas Entertainment District and would also cross under a major freeway. Milton Brooks, senior project engineer for the city of Dallas, says it was feeling like a solution causing more problems than it was fixing.

It was during those early phases of engineering that someone suggested a deep drainage tunnel that combined sewerage overflow from several locations. Engineers got to work configuring alignments for the tunnel and studied the geology of the region to figure out how deep they would have to go to get to solid rock. Despite the scope of this much larger project, it was deemed far more cost-effective than the three separate efforts and would be faster to complete and less disruptive to the neighborhoods. In 2009, city council approved the new tunnel project dubbed the Mill Creek Tunnel.

Local officials gathered at an event with the media to introduce Big Tex to the public.
Local officials gathered at an event with the media to introduce Big Tex to the public.

The single-phase project was set up to be funded through one bond program. This eliminated the need to go back to city council and voters asking for a handout each time they were ready to begin another phase as is typical of multi-phase projects. Brooks says that while it wasn’t the largest single item ever in a bond program, it was close. By 2023, the city will have spent more than $300 million in bonds on the project, according to a 2019 report by The Dallas Morning News.

Brooks admitted that it’s challenging to get the public interested in big projects when you tell them they will not be seeing any visible activity for several years. “But they lived through that flooding in the late ’90s and early 2000s, and they wanted us to stop their houses and streets from going underwater,” says Brooks.

To get voters to approve the bond, they showed residents how the new tunnel would reduce water levels in their areas by capturing it and putting it underground. While it was a hard concept for some to grasp at first, they were receptive to whatever would reduce flooding and voted to pay for the new tunnel.

The Mill Creek Tunnel is a 5-mile underground tunnel that will provide 100-year flood protection for nearly 2,200 commercial and residential properties. The current system, in comparison, only provides a two to five-year flood protection. Ground was broken in 2018, and the project is expected to be completed by the fall of 2023.

Brooks suggests that other cities looking to do a tunneling project of this magnitude do their homework early on and set bidding criteria high and prequalify bidders whenever possible. He also recommends getting the public involved early on and keep them engaged throughout the project. “Start early on right-of-way acquisitions and subsurface easements because that may end up taking years to complete,” says Brooks.

While the groundbreaking was two years ago, the actual mining of the tunnel will begin this winter and run for about 12-18 months. Next, the tunnel will be lined with a 15-inch-thick cast-in-place concrete lining and ancillary work including surface connection will be completed. Including about $15 million of surface improvements, the entire project will cost more than $300 million. The new system is designed to handle up to 9 million gallons per minute of water and will have six intake sites.

About ‘Big ‘Tex’

But, as Brooks says, the coolest part of the project is the actual boring machine. The TBM working the mill creek project is named Big Tex because equipment that big deserves a name. The cutting head is 38 feet in diameter, and the entire machine is about 230 feet long. Pull your work truck up next to it and that pickup will look like a matchbox toy. To do the job on time, Big Tex and its crews will work 24 hours a day.

According to Ian Cahoon, vice-president of operations at The Robbins Co., the boring machine manufacturer, they understand that each job is different, and they will do what it takes to do the job. Here, that meant designing a TBM that could convert from 37.5 feet to 32.5 feet diameter midway through the 5-mile excavation.

The upstream 3.24 miles of tunnel was designed with a circular cross-section for a peak flow of 15,000 cubic feet per second. The downstream 1.76 miles was designed to allow a higher peak flow of 20,000 cubic feet per second.

Engineers initially thought that wider section would have to be expanded by a road header to create a flat invert after the TBM went through. Instead, The Robbins Co. custom built the machine to accommodate the varying diameters needed, and so the changeover can be implemented underground. This allows the project to use one machine for the entire alignment, lowering project costs and reducing the construction period.

Once drilling is underway it is expected they will clear an average of 80 feet per day, which will put the project on schedule for completion by 2023. For now, we will cross our fingers and hope that 100-year storm can hold off a little longer. 

An evening shot of the Mill Creek Tunnel project grounds.
An evening shot of the Mill Creek Tunnel project grounds.


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