San Diego’s mega-tank reaches completion
Increasing demands on both quality and capacity at the Alvarado Water Filtration Plant in the US city of San Diego, California, meant constructing the world's largest circular, prestressed concrete storage tank. Judy Horning of US tank manufacturer, DYK, describes the challenge of meeting not only the highest standards for potable water storage but also of making the tank earthquake resistant.
In order to ensure that the Alvarado Water Filtration Plant would be able to meet future water quality and capacity requirements, the City of San Diego Water Department, in California, USA, has constructed the 132,000m3 Earl Thomas Tank as part of a major upgrade and expansion project. The project is part of a long-term city-wide capital improvement programme to improve the infrastructure and services that provide San Diegans with safe drinking water.
The tank is the largest circular, externally machine-strandwrapped, prestressed concrete tank of its kind in the world. The new tank will help meet increased water demands by increasing the storage capacity of the plant to 291,000m3 of potable water.
Several factors were analysed to determine the most efficient, economical, and maintenance-free configuration for the city’s water storage needs. Numerous factors were considered, including: reliability, aesthetics, construction time and costs, seismic performance, security, and minimal future maintenance expenses.
After careful consideration of the long-term benefits, it was determined that prestressed concrete would offer the highest quality, longest lasting and lowest maintenance water storage structure. The project involved the demolition and replacement of a 50-year-old, 76,000m3 rectangular water reservoir.
This reservoir was constructed in 1950 and did not meet current design standards, especially current seismic criteria. In addition, the structure had deteriorated to a point that made its repair economically unfeasible.
The design and construction of the Earl Thomas Tank incorporated the most advanced construction and prestressing techniques in the industry. To expedite construction, production was maximized by simultaneously pouring the floor, walls, columns and roof sections.
The tank walls were cast-in-place, vertically post-tensioned with Dywidag high-strength threadbars, externally machine-wrapped with an electro-servo controlled prestressing apparatus and covered with a 457mm thick, two-way, flat-slab concrete roof. Galvanized circumferential prestressing material, epoxy grouting of the vertical threadbars and automated shotcrete application provided the high level of quality control required, and helped ensure that a low-maintenance storage tank was obtained.
Due to the large hydrostatic and hydrodynamic forces applied to the tank wall, a tapered-wall design was used. The wall measures 965mm thick at the base and tapers to 305mm thick at the top. The walls utilised a 5000 psi concrete mix and were cast in 14.6m widths using engineered wall forms with special openings to limit the free fall of concrete. Each wall section was poured full height eliminating horizontal jointing.
The tank was also designed to withstand large earthquakes. Specialized seismic connections, similar to base isolation joints used by lifeline building structures, were used at the top and base of the wall.
The tank wall concrete was cast over neoprene rubber bearing pads to allow movement in the radial direction during fill and draw cycles and during a seismic event. The wall to roof joint was similar to the base connection and also allows radial movement of the wall relative to the roof but restricts the movement during an earthquake.
The tank’s prestressed concrete walls place the concrete in biaxial compression and eliminates vertical bending and subsequent cracking, making the structure liquid-tight.
The tank roof was designated to accommodate vehicular traffic so that the access road to the plant could be incorporated across the top to further maximise the plant’s use of available land. A tall and slender concrete ‘art column’ was constructed on top of the roof, extending 12m up, to symbolise the depth of the tank below.
The roof of the buried tank will be ringed with palm trees planted around the perimeter to help visualise the enormous scale of the underground tank. To simulate water, an architectural glass covering will be installed on top of the roof for a glistening effect when driving over the tank.
The tank’s statistics are as impressive as its size, measuring a circumference of 393m with an inside diameter of 124m and a water depth of 11.5m. The tank has 27 wall sections, 241 columns of 762mm width, and a 457mm-thick, two-way, flat-slab, concrete roof designed for a 30.0 psf live load, H20 truck, and 610mm of soil. The project utilised 390km of 10mm galvanized strand and 15,750m3 of concrete, delivered by 2,280 trucks.
The visual impact of the structure was another important concern of the project. A residential neighborhood overlooks the tank and the plant itself is designated as a historic landmark.
The tank was completed and commissioned in December 2005. Working with the City of San Diego Water Department were personnel from the following firms: Richard Brady & Associates and Malcolm Pirnie (consulting engineers), CH2M Hill (construction manager), CE Wylie Construction (general contractor) and DYK Incorporated (tank contractor).
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