By Judy Horning
Nestled in the foothills of Southern California's majestic San Gabriel Mountains, the suburban city of Rancho Cucamonga is one of the fastest growing metropolitan areas of the Inland Empire, having experienced a population growth of nearly 30 percent since 2000.
Drinking water for the surrounding multi-city area is produced for storage and distribution at several treatment plants, including the Cucamonga Valley Water District (District) Lloyd W. Michael Water Treatment Plant (LWMWTP) facility. In early 2015, the District completed upgrades to this plant.
This multi-year project was designed to enhance the treatment process to comply with new federal and state water quality standards and to provide additional reliable water supply to its customers. It was the District's largest project in its 55-year history and included an ultraviolet irradiation (UV) disinfection facility, post-filter granular activated carbon (GAC) contactors, a new 6.0-million-gallon (MG) tank, a new pump station, and relocation of three residuals ponds at the property.
The Project Team
The District contracted with the following project team: Black & Veatch, engineer/construction manager; SSC Construction, general contractor; and DN Tanks, tank designer, contractor and prestressor. The combined efforts of personnel from the District and each of these companies were instrumental in making this project a success.
Clearwell Design
The District and the project team determined that an AWWA-D110 Type-I circular, prestressed concrete tank, designed and constructed by DN Tanks, offered the optimal solution for the clearwell at this site. A Type-I prestressed concrete tank was a natural choice, as it offered high-quality, proven seismic performance and lowest maintenance with an extended service life. The tank floors, footings, walls, columns, and roof were all designed and constructed of concrete and other NSF-61 certified materials.
With an inside diameter of 161'6", service water depth of 41'7", and wall height of 44'6", the tank incorporated a 12.5-inch-thick bi-axially compressed wall and a flat slab roof. Preceding the application of the external prestressing strand and shotcrete encapsulation, the cast-in-place corewalls were constructed using over 830 cubic yards (CY) of concrete, and each wall section was poured full height, which eliminated the need for horizontal jointing. The 6-inch-thick tank floor utilized 600 CY of concrete, and the cast-in-place flat slab roof required 810 CY of concrete poured in two sections.
Critical to the operational enhancements of the treatment plant upgrade, the new 6.0-MG tank included baffle walls to meet requirements for desired chlorine contact time. By restricting and redirecting the flow of the contained water, the baffles promote a circuitous flow-through path for the treated water prior to delivery.
Seismic Considerations
Given that the tank is located in the state of California and due to the high probability that it may be subjected to high seismic forces sometime over its service life, seismic design was of particular importance. Specially anchored yet flexible seismic connections, a form of base isolation, were incorporated at the wall base and top in order to maximize the ductility of the structure under a seismic event. Additionally, integrally-cast roof tie-down connections were included to restrict upward movement of the tank roof under seismic excitation of the tank's contained water.
Accounting for hydrodynamic and hydrostatic loading, as well as differential temperature and moisture conditions, the reservoir wall was prestressed in two directions: vertically and circumferentially. The latest prestressing technology was utilized by DN Tanks for the project. The vertical prestressing design included one hundred and thirty-two (132) 1.25-inch-diameter threadbars embedded in the tank's corewall. After the corewall reached adequate strength, a hydraulic ram was threaded onto the vertical threadbars at the top of the wall and was subsequently tensioned to 137 kips of force.
External circumferential prestressing was completed by DN Tanks' specialized equipment, which continuously stressed the prestress material to a force of 14,950 pounds with a high degree of precision (± 1.5% tolerance). Further, approximately 44.7 miles of galvanized 7-wire strand was applied to the tank corewall while being continuously recorded and electronically monitored to ensure the highest quality possible. A total of 461 wraps of stranded cable were applied to provide the load resistance.
Prestressing not only ensured an economical project but also gave the structure the strength and durability to endure horizontal and vertical ground accelerations. The state-of-the-art prestressing machine allowed the tank to be constructed faster and stronger with a high degree of precision. Upon completion of prestressing, the stranded cable was encased and protected with shotcrete.
Looking Forward
With California facing its fourth year of one of the most severe droughts on record, the District has taken a proactive approach to providing a sustainable water supply. With drought conditions likely to persist, improvements to the District's LWMWTP will enhance its ability to provide reliable, high-quality water to meet current and future customer needs.
About the Author: Judy Horning is a marketing associate with over 21 years of experience researching and writing articles about water and wastewater storage tank projects for DN Tanks. DN Tanks is a specialist in the design and construction of AWWA D110 prestressed concrete tanks used for potable water, wastewater, chilled water, and other liquids.
