RICHARDSON, TX — The Mill Creek/Peaks Branch/State Thomas Drainage Relief Tunnel (Mill Creek Tunnel) reached an important milestone, when the tunnel boring machine (TBM) broke through the shaft wall to complete the excavation phase.

The large diameter TBM, nicknamed “Big Tex,” completed a five-mile journey that included two miles of 37.5-foot-diameter excavation and three miles of 32.5-foot-diameter excavation. Big Tex was the largest hard-rock TBM in operation in the U.S. during the excavation phase.

Construction of the tunnel’s concrete liner will begin in the project's next phase. Once complete, the Mill Creek Tunnel will include two miles of 35-foot-diameter tunnel and three miles of 30-foot-diameter tunnel.

Halff led the design effort for this $320-million drainage relief project, including the centerpiece Mill Creek Tunnel, which is being constructed at depths reaching 170 feet underground. Once completed, the project will provide flood protection during the 1 percent annual chance flood (or 100-year flood) for nearly 2,200 commercial and residential properties in East Dallas.

The project team includes the owner, City of Dallas, lead design firm Halff Associates, Inc., construction manager Black & Veatch, and construction contractor Southland Mole JV.