By Nancy Nuttbrock, PE, DBIA-AP
The Tarrant Regional Water District and the City of Dallas Water Utilities are bringing an integrated, collaborative approach to supplying additional water to their respective service areas. The overall solution consists of 150 miles of 84 inch to 120-inch pipeline and numerous tunnels or crossings. Initially proposed as Design/Bid/Build project delivery, after 90% design the Owners decided to actively engage Contractor Teams prior to bidding. The objectives of allowing Contractor teams to offer comments on the 90% package were to help in developing accurate bids and to reduce risks involved with tunneling in challenging conditions. Shallow groundwater, crossings beneath a lake, railroad, river and multiple roadways posed specific construction risks which the Owners wanted to address proactively.
Brierley Associates was selected as tunnel engineer for Sections 19-1 and 19-2,of the 150-mile raw water pipeline between Lake Benbrook and Lake Palestine, Texas.
For the final design, Brierley Associates was engaged to design 5 tunneled crossings:
- 6,300 LF under Cedar Creek and adjacent wetlands
- 6,400 LF under Hollywood Lake
- 725 LF under Royall Blvd., adjacent railroad tracks, and an adjacent creek
- 913 LF under SH 155, numerous utilities and adjacent to a school
- 191 LF under Stream A
Brierley coordinated a supplemental geotechnical program, which was started late in the design phase. As part of the Owners-initiated collaborative process, prequalified contractor teams were able to review the geotechnical information and make recommendations to the Owners regarding additional subsurface characterization. This helped to inform Brierley’s design and reduce project risk.
Taking into consideration Contractor comments, Brierley developed a comprehensive Geotechnical Baseline Report for all 5 tunnels, specifications, cost estimates, and drawings. As these documents were developed leading into 100% design, the contractor teams were invited to provide feedback.
Currently, the Owners Project Team is evaluating Contractor Proposals, with the goal of issuing a Notice to Proceed to the successful Contractor in January 2022.
Nancy Nuttbrock is Brierley Associates’ Chief Operations Officer and leads our Design-Build practice. She has developed a well-rounded career spanning over 20 years in tunneling and underground development; engineering and regulatory expertise relative to the mining industry; high-level project management and successful leadership.