Challenge
The Trinity River Main Stem Water Supply project was conceived as a drought contingency measure in 2011. A temporary pump station was initially conceived to pump treated wastewater effluent from the main stem of the Trinity River to the North Texas Municipal Water District’s (NTMWD) East Fork wetland facility. After significant rains in early 2012, the temporary project was put on hold and re-envisioned as a permanent pump station. In early 2014, design and permitting was started on the Trinity River Main Stem Pump Station, pipeline, and necessary improvements at the East Fork wetland.
In 2015, sources of raw water supply for the District Wylie Water Treatment Plant complex included Lavon Lake, tertiary-treated wastewater from the Wilson Creek Wastewater Treatment Plant, East Fork Trinity River water, and imported water originating in Lake Texoma, Lake Jim Chapman, and Lake Tawakoni. NTMWD also received water from the City of Dallas Water Utilities through a water supply contract that expired in 2016. To meet future demands in its greater Dallas metroplex service area, the District continues to develop new sources of additional water supply.
Approach
During the design process, NTMWD decided to use the construction manager at-risk (CMAR) method of delivery for this large project. Because of the size of the project and remote project location in relation to the Wylie, Texas, home office, NTMWD chose a CMAR for management and execution.
Garney was hired to construct a 100 mgd intake, pump station with horizontal split case pumps, 87,000 LF of 72-inch raw water pipeline, upgrades to existing pumping facilities with vertical turbine pumps, micro-tunnel to the Trinity River with T-screen intake, access road, bank stabilization, weir box structure, electrical, SCADA, and communications infrastructure.
Hiring a CMAR during the design process allowed design development to coincide with construction needs and methods, as well as allowing a dedicated outside team to advertise, bid, award, and manage multiple contractors and equipment suppliers without overloading NTMWD staff. The CMAR utilized several bid packages and equipment purchase packages to manage long-lead delivery items as well as the timing of multiple contractors being on-site when needed and not depending on one single contracting entity to deliver the entire project. Early equipment and material purchases provided less impact to the overall schedule by allowing manufacturing to begin prior to the contractor being awarded the construction package, and it also allowed the CMAR to time the cost and expenditure of some of the more impactful equipment to match the available funds of NTMWD. Utilizing the CMAR delivery method for this project proved beneficial to District in delivering a very large, long-term project using minimal staff and maintaining a high quality of work and efficient delivery mechanism for such a complex project.
Results
This project provides additional water sources to satisfy the increasing long-term demand of the North Texas area’s population growth, including 90 entities over 2,200 square miles. The owner, engineer, CMAR, and contractors worked together to address the challenges as they arose. Some of the notable accomplishments during these adverse conditions were:
- Adaptation of the Intake Piping Design: One of the flooding events removed approximately 30 feet of the bank from the intake location. This necessitated the installation of additional intake pipe supports in the river. The project team came up with a solution using structural steel elements that could be installed in the wet by divers in lieu of the concrete design that had to be formed and placed inside a dry cofferdam.
- Intake Tunnel Repair: The two 48-inch diameter intake pipes were installed by microtunneling into the river. One of the tunnel pipes failed in a flood during installation. The pipe separated 40 feet below ground, a sinkhole developed above the pipe, and the pipe dropped approximately 10 inches. The microtunneling contractor developed a solution to excavate a shaft over the break and raise the pipe back to grade using a pressure grouting technique from the surface. The plan was successfully executed, and the intake pipe was fully repaired.
- 72-inch Raw Water Pipeline: Most of the 14-mile raw water pipeline was installed in the flood plain. The project observed record rainfall during construction, which meant flooded trenches and difficult access. The contractor was able to strategize construction in key areas during wet and dry seasons to keep the project on schedule. In addition, the purchase timing of pipe material resulted in over $6 million in cost savings.
Despite the many challenges, Garney and their dedicated team of subcontractors persevered. The project received a Project of the Year Award in the water/environmental greater than $75 million category by the American Public Works Association Texas Chapter in 2020.
