Mill Creek Drainage Tunnel TBM unveiled

22 January 2020

Texas – The City of Dallas unveiled the 11.6m-diameter hard rock machine that will bore the 8km-long Mill Creek Drainage Relief Tunnel. The machine, manufactured by Robbins, will change diameter at one point along the bore and continue at a diameter of 9.9m.

The contractor, a joint venture of Southland and Mole (SMJV) will reduce the diameter some 2.8km into the drive. The TBM, named “Big Tex”, has been designed with a cutterhead that includes removable spacers and adjustable bucket lips to convert to a smaller diameter. The TBM will first complete its 11.6m diameter section of the alignment, then back up about 21m to a transition area for the conversion, which is expected to take six to eight weeks.

A spokesperson for the manufacturer said, “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 2.8km 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.”

The bore passes through 12-30MPa Austin Chalk at depths of 31-46m. The alignment has the potential to encounter gas, so probe drilling has been mandated. Eight 3.9m-long rock bolts will be installed every 5m, with wire mesh and channel straps as needed. The final lining will be a 380mm-thick cast-in-place concrete lining.

Based on work through similar geology, the project team expects TBM excavation to progress at an average of 25m per day, and is targeting project completion in 2023.

The 11.6 m Robbins Main Beam TBM is the largest hard rock TBM ever to be used in the US
A ceremony was held in December 2019 to mark the impending launch of the TBM
The 8km Mill Creek Drainage Relief tunnel will provide 100-year flood protection for east and southeast Dallas, areas affected in recent years by severe storms
The machine will change size partway through the bore, to a more compact 9.9m