The idea of an underground storage tank floating seems counter-intuitive at first. How can a tank float if it’s underground? The answer has to do with buoyancy. Think of a half-filled water bottle. If you toss it into a body of water, it floats, because the air in the bottle exerts an upward force known as buoyancy. The same concept applies to underground storage tanks. If the water table reaches a high level or if flood waters reach a high flood stage, the air in the tank will start to exert an upward force. Because of the large volume of underground storage tanks, this upward force can be very strong. It can even cause the tank to break through the earth above it, and all of a sudden you’ve got an aboveground tank when you wanted an underground tank.
Fortunately, there are several effective methods that will keep a storage tank underground.
- Increased burial depth- By increasing the burial depth of the tank, you generate more downward force (by means of the weight of backfill), which counteracts the upward force of buoyancy.
- Deadman anchors- If the burial depth can’t be increased, deadman anchors can be used to anchor a tank down. Deadman anchors are made of reinforced concrete, and help provides additional stability and counteraction against buoyancy.
- Using a thicker grade paving slab- Concrete grade paving slabs are placed on the surface of the earth above the tank. Because concrete is heavier than the earth, a thicker paving slab will exert more downward force on the tank.
- Bottom hold down slab- This is essentially a paving slab that sits below the underground tank. The tank is anchored to the slab, which exerts a downward force to offset buoyancy. It also provides more stability for the tank.