The Mill Creek Drainage Relief Tunnel (also known as the Mill Creek/Peaks Branch/State-Thomas Drainage Relief Tunnel) is a 5-mile underground tunnel that will provide 100-year flood protection for nearly 2,200 commercial and residential properties in the eastern area of Dallas, Texas. The current drainage system in these areas was constructed 50 to 70 years ago, and only provides a two- to five-year flood protection.
Once complete, the tunnel will carry flow of 9 million gallons per minute, providing exponential relief of flooding during large rain events. To carry this flow, part of the tunnel was constructed with a 38-ft TBM, one of the largest hard-rock TBMs to operate in North America. In a novel approach, the machine was converted mid-drive to excavate a 32.5-ft tunnel, allowing the use of one machine vs. multiple machines or enlarging a large section of the tunnel as originally envisioned.
The $206.7 million design-bid-build tunnel contract was awarded to Southland/Mole JV and given NTP in March 2018. Excavation of the tunnel began in 2020, and was completed in July 2022. Crews are in the process of concreting the tunnel in preparation for its full service date, expected in early 2026.
For its unique approach, successful excavation, and the value that it will provide the citizens of Dallas, the Mill Creek Drainage Relief Tunnel has been awarded the 2023 Tunnel Achievement Award for Project Excellence. The formal award presentation took place on Sept. 11 at the Breakthroughs in Tunneling Short Course at the University of Denver in Colorado. Accepting the award on behalf of the project team were Nick Jencopale and Quang Tran representing Southland/Mole JV.
Established in 2012, the Tunnel Achievement Award recognizes successfully completed projects that demonstrate innovation and teamwork and provide benefits to the community. The award, presented annually at the Breakthroughs in Tunneling Short Course, is presented in partnership with course organizers and TBM: Tunnel Business Magazine.
Initially, the tunnel was envisioned as a 5-mile tunnel with a 30-ft inside diameter. Because more capacity was needed at the downstream end of the tunnel, the plan was to enlarge the last 1.6 miles of tunnel by benching out the invert to create a 30-ft by 35-ft horseshoe tunnel using roadheaders and conventional methods. Southland/Mole, however, proposed the idea of using a TBM that could be converted from 38-ft OD to 32.5-ft OD within the tunnel, saving time while not increasing cost.
Changing TBM diameters is not a new concept, but changing the diameter in the middle of the bore provides unique challenges. “TBM diameter conversions have taken place in the industry, but typically they occur in an existing shaft or pre-excavated cavern,” said Jencopale, senior project manager for Southland/Mole. “In that scenario, you can utilize heavy equipment to help with the conversion process. In our case the conversion was done completely within the confines of the bore.”
A specialized Robbins main-beam, hard-rock TBM, named “Big Tex,” was designed with an adaptable cutterhead including removable spacers and adjustable bucket lips to convert to a smaller diameter. Similarly, the cutterhead support and gripper shoes included removable skins for the diameter change later. In late 2019, the machine was delivered to the site and assembled using onsite, first-time assembly under the supervision of Robbins personnel. After acceptance, the TBM was disassembled into large pieces and lowered in the shaft to begin its journey. The machine was launched on April 24, 2020.
A launch shaft was constructed at the downstream end measuring 45-ft in diameter and 120 ft deep. The shaft was built using ring beams and liner plate through the overburden and rock bolts, wire mesh and shotcrete through the rock.
Muck was removed using a continuous conveyor system, totaling 10 miles of belts by the end of the bore. The continuous conveyor fed a vertical conveyor at the shaft, which led to a radial stacker for truck loading. Southland/Mole loaded an average of about 100 trucks a day with 500 trucks a day during peak production. Advance rates average about 50 ft per day with a best day of 175 ft and a best week of approximately 600 ft.
The diameter conversion took place about 2 miles into the bore. After completion of the conversion and mining of the smaller diameter tunnel portion, the TBM broke through in the reception shaft in July 2022. The TBM was disassembled and removed so that concreting operations could begin. Lining and other finishing works are expected to take another two years. At that time, the project can begin paying dividends to the citizens of Dallas.
- Name: Mill Creek Drainage Relief Tunnel
- Location: Dallas, Texas
- Scope of Work: 5 miles of TBM bored tunnel (38-ft and 32.5-ft OD) with 15-in. thick cast-in-place final liner; 7 shafts ranging from 120 to 200 vf
- Ground: Austin Chalk
- Owner: City of Dallas
- Contractor: Southland/Mole JV
- Lead Designer: HALFF
- Tunnel Designer: COWI
- CM: Black and Veatch
- TBM Manufacturer: Robbins
- Subcontractor: Oscar Renda Constructing, Inc.
Tunnel Achievement Award Winners
Established in 2012, the Tunnel Achievement Award recognizes successfully completed projects that demonstrate innovation and teamwork and provide benefits to the community. The award is presented by the organizers of the Breakthroughs in Tunneling Short Course in conjunction with TBM: Tunnel Business Magazine.
- 2023: Mill Creek Drainage Relief Tunnel, Dallas, TX
- 2022: Kemano T2 Tunnel, Kitimat, BC
- 2021: DigIndy Tunnel System, Indianapolis, IN
- 2021: Gary Brierley, Dr. Mole Inc. (Lifetime Award)
- 2020: Regional Connector, Los Angeles, CA
- 2019: Ohio Canal Interceptor Tunnel, Akron, OH
- 2018: Northgate Link Extension, Seattle, WA
- 2017: Blue Plains Tunnel, Washington, DC
- 2016: Lake Mead Intake No. 3, Las Vegas, NV
- 2015: Bay Tunnel, San Francisco, CA
- 2014: Port of Miami Tunnel, Miami, FL
- 2013: East Side CSO, Portland, OR
- 2012: No. 7 Extension, New York, NY
- 2012: Dr. Martin Herrenknecht, Herrenknecht AG (Innovation Award)