To minimize the footprint of the tanks, the smaller diameter contact tank was built inside the larger diameter clearwell.
Click here to enlarge imageThe long alliance the city has had with the Contra Costa Water District (CCWD) resulted in an agreement that established a long-term water supply, treatment and storage solution for both organizations and their customers. This arrangement represented an exemplary model of regional partnership through communal facilities.
CCWD and the city analyzed several options to provide water treatment for a portion of Brentwood that was included in CCWD’s boundaries. It was determined that the most cost effective and efficient solution was to use shared facilities between the two. The city purchased treatment capacity in the existing Randall-Bold Water Treatment Plant (RBWTP) and then constructed a scaled-down water treatment facility adjacent to the RBWTP located in Oakley, CA.
After consulting with the city, CCWD hired the engineering firm Camp, Dresser & McKee (CDM) to prepare the design and specifications for the Brentwood WTP. Monterey Mechanical was awarded the construction contract and DYK Inc., El Cajon, CA, was hired as the tank builder and prestressor.
Tank-Within-a-Tank
After careful consideration of the long-term benefits, the city and CCWD determined that prestressed concrete would offer the highest quality, longest lasting and lowest maintenance water storage structure for this project. Based on the amount of treated water going through the proposed plant, up to 12 mgd, it was determined that a 1.17 million-gallon (MG) clearwell and a 0.33 MG contact tank were needed to meet water quality requirements.
The project team proposed a new idea to minimize the footprint of the tanks -- integrate the smaller diameter contact tank inside the larger diameter clearwell. In effect, the outside tank has the dimensions of a 1.5 MG prestressed concrete tank. The contact tank was designed with three concrete baffle walls that served the purpose of allowing increased contact time of the water. The design facilitated additional contact time in the clearwell by incorporating a radial baffle wall between the inside wall and outside wall at the overflow point from the contact tank into the clearwell.
DYK was awarded the contract to design and build this unique structure. The floor, footings, columns, walls and roof were all designed and constructed with concrete. In order to avoid horizontal joints, all of the walls were poured full height through openings in the sides of the wall forms. The inner tank was designed with an inside diameter of 49 feet, a water depth of 24 feet, and a 12-inch-thick circular wall. The outer tank was designed with an inside diameter of 106 feet, a water depth of 24 feet, and a 10-inch-thick wall.
Specialized seismic connections were incorporated into the outer wall base details, similar to the base isolation system in essential buildings, to account for the high seismic forces anticipated over the service life of the tank. The seismic connections were designed to allow for maximum ductility under a seismic event to ensure the structure would continue to serve the city should the tank undergo horizontal and vertical ground accelerations.
The outer tank corewall was both circumferentially and vertically prestressed by DYK. The outer tank corewall was post-tensioned vertically with 1¼" diameter high-strength threadbars and subsequently grouted with epoxy. The tank wall was circumferentially post-tensioned by DYK’s unique strandwrapping machine which applied the desired force to the 3/8” diameter galvanized strand. This machine continuously and electronically monitored the applied stressing force as it was applied. By keeping the corewall in compression along with the independent connections, a long-life, water-tight structure was ensured.
After strandwrapping, the strand was encapsulated with several coats of machine-applied concrete. The machine used to apply the strandwrapping and shotcreting is considered to be the world’s most technologically advanced, fully automated machine and ensured that the necessary quality control was consistently maintained.
Exemplary Model
The project exemplifies the perfect model of collaboration between two municipalities resulting in a long-term solution for providing a reliable source of water while ensuring efficient use of available facilities and resources. The “tank-within-tank” provided economical use of land by incorporating two tanks on the same footprint. The optimal result of shared facilities for production of treated water creates significant revenues for the Contra Costa Water District as well as cost savings for the City of Brentwood and its customers.
