Delivering a Large Diameter Gravity Flow Pipeline through the Construction Manager At-Risk Delivery Method

The Sister Grove Regional Water Resource Recovery Facility (Sister Grove project) is a new wastewater treatment plant being constructed to accommodate the continued regional growth in North Texas Regional Water District’s (NTMWD’s) service area. Garney Construction is the Construction Manager At-Risk (CMAR) responsible for the entirety of the treatment plant and outfall pipeline construction, which includes preconstruction design services, coordination, scheduling, estimating, management, oversight, portions of construction, and cost management. The outfall pipeline was designed as a 96-in. gravity system that extends roughly 4 mi from the treatment facility to the discharge point. The project is in a geologic region where solid rock exists just feet below the surface. There are constructability challenges associated with large diameter pipeline installation in solid rock coupled with elevations of the pipeline requiring excavations in excess of 60 ft deep. The CMAR worked with the district’s engineering and operations teams along with the design team to optimize the construction methods to deliver this project. Using the CMAR’s experience, it was determined that a combination of trenchless installations and a traditional open-cut installation would be the best options for construction. The project’s scope ultimately included 9,453 linear feet (LF) of large diameter tunneling completed using a 123-in. tunnel boring machine and 12,071 LF of open-cut installation within mass excavations, vertical shoring, and employing both aggregate and controlled low strength material (CLSM) embedment around the pipeline. The CMAR delivery method engages a contractor early in the design phase unlike that of the traditional design-bid-build contracting method. For the Sister Grove project, the CMAR’s early engagement in the design process allowed for constructability reviews and suggestions, participation and input in public engagement and relations, construction sequencing to optimize schedule and efficiency, and development and procurement of the scope of work and material supply bid packages. In an environment where pricing on projects remains volatile, the CMAR’s early engagement afforded NTMWD the ability to secure pricing on materials, execute construction contracts, and procure long-lead items prior to the final design. This engagement ultimately provided NTMWD with the ability to overcome constraints in schedule and procurement that commonly exist without the involvement of a CMAR.