Potable water tanks are an essential lifeline in every community. In order to ensure that the Alvarado (Calif.) Water Filtration Plant would be able to meet future water quality and capacity requirements, the city of San Diego Water Department constructed the 35 mg Earl Thomas Reservoir tank at the Alvarado Water Treatment Plant as part of a major upgrade and expansion project.
This expansion project is part of a long-term citywide capital improvement program to improve the infrastructure and services that provide San Diegans with safe drinking water. The new tank will help meet growing water demands by increasing the storage capacity of the plant to 77 mg of potable water.
Several factors were analyzed to determine the most economical, efficient and maintenance-free configuration for the city’s water storage needs. When reviewing the various tank systems available for the San Diego Water Department project, considerations included construction time and cost, reliability, aesthetics, seismic performance, security and minimal future maintenance expenses.
Another challenge the city faced was that the tank needed to be backfilled to incorporate an access road to the treatment plant on the roof, while also being aesthetically pleasing to those living in the surrounding neighborhood. After careful consideration of the long-term benefits, the planners determined that prestressed concrete would offer the highest quality, longest lasting and lowest maintenance water storage structure for their project.
The project involved the demolition and replacement of a 50-year-old, 20-mg 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 rectangular structure was replaced with a 35-mg circular, externally machine-strandwrapped, prestressed concrete tank.
Advanced techniques
The design and construction of the Earl Thomas Reservoir tank incorporated the most advanced construction and prestressing techniques in the industry. Throughout construction, the existing plant remained in operation. To expedite construction, production was maximized by simultaneously pouring the floor, walls, columns and roof sections. The reservoir 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 an 18-in. 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 by the city and helped ensure that a low-maintenance storage tank was obtained.
Measuring 1,289 ft in circumference with an inside diameter of 406 ft and a water depth of 38 ft, the tank has 27 wall sections, 241 30-in. columns, a flat roof and machine-strandwrapped prestressed concrete walls.
The individual statistics of the tank are as impressive as its size. The project utilized 20,600 cubic yards of concrete delivered by 2,280 concrete trucks and 242 miles of 3?8-in. galvanized strand.
Important ingredients
Technical advancements and liquid storage experience were important ingredients in the design and construction of this large prestressed concrete storage tank.
Working with the city of San Diego Water Department, personnel from the following firms were instrumental in making this project a success: CH2M Hill, Richard Brady & Associates, Inc. and Malcolm Pirnie, Inc., C.E. Wylie Construction Co., and DYK, Inc.
Upon completion in the summer of 2005, this tank will be the largest circular, prestressed concrete tank of its kind in the world. The tank will be buried with a road across the top to provide access to the plant. The roof of the buried tank will be ringed with hundreds of native cypress trees planted around the perimeter to help visualize the enormous scale of the underground reservoir. To simulate water, architectural glass will be installed on top for a glistening effect when driving over the tank.
The Alvarado Water Treatment Plant’s Earl Thomas Reservoir is an integral part of the city of San Diego Water Department’s long-term program to create a safe and reliable water treatment and distribution system, which will meet current and future water quality standards and capacity requirements.
