The city of Syracuse, N.Y., recently finished renovation of its Westcott Reservoir by constructing twin, 32 million-gallon concrete water storage tanks. The $40 million project allowed storage at the disused open basin to be restored, adding 64 million gallons of more secure storage to the city’s drinking water system while meeting current quality standards for drinking water. At 375 feet in diameter and 43 feet tall, these tanks are the largest prestressed concrete tanks east of the Rocky Mountains, and among the largest in the world. The city of Syracuse chose CH2M HILL to provide engineering design and construction services and DYK Inc. to complete tank construction.
The Westcott Reservoir is located within the western edge of the city limits, adjacent to the town of Geddes and the village of Solvay. Constructed in 1930, the original 110 million-gallon uncovered Westcott Reservoir balanced storage in the city’s supply transmission system between the source, Skaneateles Lake, and the city. The original reservoir faithfully provided storage for the city’s drinking water supply for nearly 70 years. However, a failing liner and piping prompted the city to close the reservoir in 1999 and rely exclusively on its remaining uncovered Woodland Reservoir.
The primary driver for rehabilitating the Westcott Reservoir was to reinstate storage at the site, providing the storage redundancy and operational flexibility required in the city’s water system. An engineering evaluation concluded that prestressed concrete storage tanks were the most cost-effective solution when compared with repairing and covering the reservoir. Prestressed concrete tanks met the requirements of providing increased water quality, addressed security needs, and complied with the U.S. Environmental Protection Agency (EPA) Long Term 2 Enhanced Surface Water Treatment Rules of 2006 (LT2 rule). In addition, the prestressed concrete tanks will reduce maintenance and are designed with a lifespan to provide the city with clean drinking water into the next century.
Funding for the $40 million project was a significant concern for the city. However, the city was able to obtain a federal matching funds grant through the EPA, as well as a subsidized loan through the New York State Environmental Facilities Corp. In addition, the city received an American Recovery and Reinvestment Act grant to develop two renewable energy projects at the site.
Construction on the tanks began during the late summer of 2008. To keep the project on schedule, construction progressed through the winter of 2008. Despite substantial snowfall and cold temperatures, significant progress was made during the winter months. This required soil to be thawed prior to construction and concrete to be heated for several days after pours to ensure proper curing. The first tank was prestressed in the summer of 2009 and completed by its scheduled completion date of December 2009. Work on the second tank finished during the fall of 2010.
The two, 32 million-gallon covered tanks were built concurrently within the existing Westcott Reservoir’s earthen footprint. Prior to construction of the tanks, the earthen berm was breached to give trucks access to the site, and the concrete liner and stone base from the existing basin were removed. No material was allowed to leave the site; the concrete and stone were crushed and utilized to construct a berm along the new access road.
Once the aggregate base was compacted, a 6-inch-thick floor was cast in 16 sections with PVC waterstops spanning every floor joint. Walls were cast in place, full height, in 48-foot-long increments for a total of 25 wall sections. These walls are 22 inches thick at the base and taper to 9 inches at the roof. After curing, the walls were vertically post-tensioned via high-strength steel threadbars placed every 3.4 feet and located within the wall. This vertical post-tensioning prevents cracks from developing in the wall due to vertical bending, allowing for a coating-free and maintenance-free tank that will last for generations to come. The 9-inch-thick concrete roof was cast in nine sections and slopes up to the center of the tank at 1.5 percent to facilitate drainage. The roof is supported by 208 concrete columns with diameters of 24 inches. The floor of the tank also is sloped at 1.5 percent to match the roof and allow for columns of equal length. Nearly 12,000 yards of concrete were used in the construction of each tank.
In addition to the vertical prestressing, the tanks were circumferentially prestressed. This was accomplished with four layers of three-eights-inch-diameter seven-wire galvanized strand. This strand was spirally wrapped under 14,950 pounds of permanent tension around the tank, providing force to counter the design liquid loads as well as 200 psi of additional compression. Continuous electronic recording through DYK Inc.’s specialty prestressing equipment provided verification of the tension of all 222 miles of prestressing strand placed on each tank. In addition, the circumferential wrapping of the tank incorporated galvanized prestressing, a new feature for the area but already commonly used in the western United States. This will offer a further layer of environmental protection to the core of the tank structure and provides further confidence in its longevity.
During design and development of this project, Matthew Driscoll, the then-mayor of Syracuse, championed sustainability in the city and encouraged projects to include green aspects. For the Westcott Reservoir Rehabilitation Project, a twofold renewable energy approach was implemented. First, a 50-kW photovoltaic solar system was installed on the roof of one of the tanks. Second, a 56-kW hydroturbine system was installed on the inlet to the tanks. This turbine will generate electricity from excess pressure in water entering the tanks. The energy generated onsite will exceed the energy needed to operate the site and allow the city to sell energy back to the grid.
The Westcott Reservoir Rehabilitation Project is the first of its kind in New York state and set a precedent in the area for uncovered reservoirs meeting the new LT2 rule. Many area consulting engineers attended tours of the job site and technical seminars on the project for professional development. In addition, Mayor Driscoll hosted a public open house. The public was invited to visit the site on a Saturday when construction was shut down. More than 1,300 residents of the city and surrounding communities attended and were able to walk onto the job site to see the project during a middle phase of construction.
This project reflects the efficient use of available space while guaranteeing minimal disturbance to the surrounding community, which was important to the city. Since the tanks are located inside the existing reservoir basin, most nearby residents are not able to see the tanks. In addition, the curvature of the maintenance road and new earthen berm provide a visual barrier to the road and tanks.
The tanks now provide the 140,000 residents in the city of Syracuse with safe and reliable drinking water. Additionally, the tanks helped bring the city’s drinking water system into compliance with new regulations, while also enhancing water quality and improving security because of the protection offered by closed storage tanks.
David Bain is a business development engineer, Eastern Region, for DYK Inc. He can be contacted at 410-200-8707 or firstname.lastname@example.org.