Peterson Structural Engineers: Rockwood Water Cascade Reservoir #2



The Rockwood Water People’s Utility District Cascade Reservoir #2 is a 6.0 million gallon AWWA D110 prestressed concrete reservoir with a 66ft wall height, making it the tallest wall height D110 prestressed concrete reservoir in the Northwestern United States. This reservoir serves
to provide essential storage capacity for the Cascade Groundwater Alliance: A partnership between the City of Gresham and the Rockwood Water People’s Utility District. This partnership allows both Districts to have a safe, independent water source that will preserve lower rates for consumers within each District and increases equitable access to clean water.

In addition to preserving water rate stability, the Cascade Groundwater Alliance projects serve to increase the seismic resiliency of the local water systems. The Cascade Reservoir #2 performs as a centerpiece of this resiliency. Utilizing the standards of AWWA D110 & ACI 350 design codes, PSE utilized several strategies to decrease seismic demands on the structure, and to provide the reservoir with the ductility and strength needed for the reservoir to serve the community after a code level seismic event by providing essential water storage adjacent to the planned treatment plant.

The structural design of the reservoir utilizes a flexible anchored connection between the foundation slab and the reservoir wall. This connection allows for independent movement of the wall relative to the slab during a seismic event. This connection increases the ductility of the entire system and prevents out of plane forces in the reservoir wall which could potentially cause cracking at the wall base and compromise the watertightness of the reservoir. Similar
to the base connection, the wall is connected to the roof slab through shear cans that restrain slab movement along the length of the wall, but allows for movement of the slab relative to the wall in the perpendicular direction. This connection also allows for the roof slab to expand and contract as needed with environmental temperature changes without creating out of plane pressures on the wall. Finally, PSE utilized vertical and circumferential prestressing on the wall of the reservoir to ensure that the concrete stays in compression during all potential loading conditions, preventing cracks and water loss.

The height of the wall created design challenges that PSE is distinctively qualified to address. As part of the structural design, PSE performed additional wall buckling checks using Finite Element analysis, and special detailing at the wall base connection to address the increased base shear over a small diameter. The wall height also led to unique construction challenges as the length of the wall sections needed to be restricted in order to keep the volume of concrete placed in a day to a manageable amount.
Firm: Peterson Structural Engineers

Client: Rockwood Water People’s Utility District and the City of Gresham

Project: Rockwood Water Cascade Reservoir #2

The one thing you should know: Peterson Structural Engineers designed the new concrete tank to store Rockwood Water PUD’s goal of 6 MG, match the hydraulics of the existing tank, and fit on their site with room for a future matching reservoir. This combination of requirements resulted in the tallest AWWA D110 Type 1 reservoir in the Northwest United States. Cascade Reservoir #2 is 66 feet tall, with a diameter of 126 feet. The completion of the reservoir marks a significant milestone in the efforts of the Cascade Groundwater Alliance to develop a fully independent Groundwater supply by 2026 providing safe, resilient, and affordable water to the communities served.