Vallecitos Water District is building 40 million-gallon reservoir in San Marcos

By Linda Lou

Near the top of Twin Oaks Valley Road, workers are completing what is believed to be the largest concrete water tank of its type in the world.


The interior of possibly the world’s largest pre-stressed concrete water tank dwarfed a worker in Twin Oaks Valley, San Marcos. When the tank is complete, the roof will be held up by 256 columns.

Not many people know about the construction feat in progress on a hill north of San Marcos. Once the 40 million-gallon tank is completed, filled with water and submerged under 3 feet of dirt this fall, those who never saw it will miss out on its massive size.

To get an idea: Imagine the infield of Qualcomm Stadium when the Padres played there, and look up to the Plaza Level seats 40 fee. above the field.

In reality, the tank is actually a little larger, with a diameter of 432 feet, said Dennis Lamb, Vallecitos Water District’s director of engineering and operations.

Tank facts A massive water tank is being built by the Vallecitos Water District. Capacity: 40 million gallons

Dimensions: 40 feet high, 432 feet in diameter

Materials used: 22,950 cubic yards of concrete, 575,000 pounds of steel

Cost: $21 million

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