Water Tank Problems

[EddieFAC]

I have an above ground, steel, 500,000 gallon water tank (60' diameter x 24 ' high) that provides water for fire suppression, construction and potable water. The tank bears on a ring foundation system. The base plate of the tank is 1/4 steel (A36) that rests on a 4" layer of clean sand which in turn rests on compacted native material (ranges from clayey sand to sandy clay). The tank recently presented with a leak from a 3/4 inch pressure sensing line. The leak is under the tanks footprint. Water is daylighting on the exterior of the tank foundation wall . This seems a bit odd to me. (It would seem to indicate some pressure head on the water under the tank). Another concern is voids under the steel base plate left after the water washes out the clean sand that supports the base plate. Has anybody seen issues like this before. How do you verify the bearing material under the base plate? Why would the water be daylighting on the exterior of the foundation system? What else should I be worried about?

 

[bimr]

The situation that you are describing is a common occurrence when a steel storage tank starts to leak. Once the ground is saturated, the fluid will make its way horizontally.

Don't understand why you have a 3/4" line under the tank. The small line would seem to be a potential source of problems.

Leaks from steel tank structures begin at a slow rate and the volume of the leak tends to increase slowly. Contrast that to an instantaneous catastrophic leak that occurs with a plastic tank.

The slow leakage rate should allow you time to procure repair services.

 

[EddieFAC]

Thanks for the comments bimr. The 3/4 inch line is a pressure sensing line that comes out the bottom of the tank and runs back to an altitude valve. When the contractor initially installed the tank the capped the line during sand blasting of the inside. They failed to remove the cap after the operation. When we finally drew the tank down and removed the cap it was apparent the connections were not constructed correctly. I understand the water moving horizontally, but for the water to daylight on the exterior of the ring wall foundation it would seem to need some head pressure to "push" it up around the foundation wall.

 

[JStephen]

Probably just that the fill down there is not porous enough to let it flow out and about as fast as it's leaking.