In December 2019, the City of Dallas unveiled the largest hard rock TBM ever to bore in the United States. The 11.6 m (38.1 ft) diameter Robbins Main Beam TBM will excavate the 8 km (5 mile) Mill Creek Drainage Relief tunnel, and its size is not its only distinction. The adaptable machine will change size partway through the bore, to a more compact 9.9 m (32.5 ft).
The unique Robbins TBM will be used to dig a tunnel designed to provide 100-year flood protection for east and southeast Dallas, areas affected in recent years by severe storms. The tunnel will protect 2,200 commercial and residential properties, including Baylor Medical Center. The current drainage system in these areas was constructed 50 to 70 years ago, and only provides two to five years of flood protection. “The completion of the TBM assembly marks a major milestone in the Mill Creek Tunnel Project,” said Council Member Lee Kleinman, chair of the Transportation and Infrastructure Committee for the City of Dallas. “I’m thrilled to see this type of engineering marvel happening right here in Dallas.”
The dual-diameter aspect of the Robbins TBM will be a first-of-its-kind conversion process. The contractor, Southland/Mole Joint Venture (SMJV), will make the conversion underground about 2.8 km (1.8 mile) into the bore. The two diameters are needed as the upstream section of the tunnel is designed with a circular cross section and peak flow rate of 42 m3/sec (15,000 ft3/sec), while the downstream portion has a higher peak flow of 565 m3/sec (20,000 ft3/sec) and was initially designed as a horseshoe cross section. Using the TBM for the entire tunnel is less time consuming and costly. “Robbins and SMJV are working closely to create the safest and most efficient sequence for completing this conversion within the limits of the bore. The City of Dallas (Owner) and our Project Team are very excited to embark on this unique challenge,” said Nick Jencopale, Project Manager for Southland Holdings.
The Robbins TBM, named “Big Tex” with permission of the State Fair of Texas, has been designed with a specialized cutterhead including removable spacers and adjustable bucket lips to convert to a smaller diameter. The TBM will first complete its 11.6 m (38.1 ft) diameter section of the alignment, then back up about 21 m (75 ft) to a transition area for the conversion, which is expected to take six to eight weeks.
As the TBM bores, it will pass through Austin Chalk between 12 to 30 MPa (1,800 to 4,400 psi) at depths from 31 to 46 m (100 to 150 ft) below the city. The route is potentially gassy, so probe drilling is mandatory throughout the project. Crews will utilize ground support including eight 3.9 m (13 ft) long rock bolts every 1.5 m (5 ft) with wire mesh and channel straps as needed. The finished tunnel will be lined with a 380 mm (15 in.) thick cast-in-place concrete lining.
“Big Tex will work 24 hours a day to excavate the tunnel with crews ranging in size depending on activities,” said Rachel Sackett, marketing and communications director for Southland Holdings. Based on previous work through similar geology, the project team expects TBM excavation to progress rapidly to an average of 25 m (80 ft) per day, allowing the project to be completed on schedule in 2023.