Diameter Change – USA’s Largest Hard Rock TBM Undergoes Planned In-Tunnel Conversion

The largest hard rock TBM ever to bore in the USA, an 11.6 m diameter Robbins Main Beam TBM, had a big task ahead. The unique machine was designed to undergo a planned diameter change mid-way through its bore of the Mill Creek Drainage Relief Tunnel. The final machine size? A more compact 9.9 meters.

The drainage relief tunnel in Dallas, Texas, USA is an important project, as the tunnel will provide 100-year flood protection for east and southeast areas of the city--locations 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 dual-diameter aspect of the Robbins TBM is a first-of-its-kind conversion process, in that the process is taking place underground about...

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