2015 APWA Sacramento Chapter Project of the Year Award
Pre-Stressed Concrete Tanks Replace Hypalon-Lined Reservoirs
Gateway Pacific Contractors and Bennett Engineering Services formed a design-build team to replace two Nevada Irrigation District reservoirs. The existing, Hypalon-lined and covered reservoirs provided the main storage and emergency reserve for the Elizabeth George Water Treatment Plant. Together, they held up to 16.9 million gallons of treated water. Constructed in 1992, the Hypalon covers had deteriorated badly, requiring frequent repairs. Faced with ongoing expense and safety issues, the district decided to replace the reservoirs with pre-stressed concrete tanks.
Tanks Accommodate 20 Years of Growth
The Gateway-Bennett team conducted an economic and hydraulic evaluation and determined the optimal solution was to build two pre-stressed DN Tank tanks to hold a combined 10.5 million gallons while providing a designated area for a future third tank. The design enables the district to increase the total site capacity to accommodate 20 years of growth. The project phasing maintained operation of one reservoir while the other was replaced and brought into operation before the second reservoir was replaced.
This design-build project included 10, 50, and 90 percent design reviews, geotechnical evaluations, topographic survey, visual impact analysis, environmental mitigation, permitting, cost estimating, construction scheduling, and as-built drawing preparation. The team worked diligently at the beginning of the project to size the pipeline, establish horizontal locations for the tanks, and vertical limits of the tank’s storage.
Hydraulic Challenge Initiates Innovative Solution
A significant constraint in the design of the two tanks was the limited hydraulic grade from the water treatment plant to the downstream tank. NID wanted the maximum amount of storage possible. Therefore, to increase the volume of water in each tank, the design team developed an innovative solution that allowed for less freeboard and higher operating water levels, which resulted in thousands of gallons of additional storage.
Delivery Ahead of Schedule
The design-build team also made sure there was substantial time available for quality control and constructability review. Once construction began, the amicable relationship between the contractor and design team allowed the project to be delivered ahead of schedule and within budget.
“The design-build team really listened to our concerns and was very responsive. Using the design-build delivery process functioned exceptionally well—the project was on time and on budget.”
Gary King, PE, Ph.D, Engineering Manager, Nevada Irrigation District