What forces are offset by the weight of back-fill and paving on top of a tank?

The correct answer is buoyancy forces. When a tank is installed underground, it is subjected to various forces, particularly when the surrounding soil is saturated with water. Buoyancy forces arise from the fluid pressure exerted by the water in the soil surrounding the tank. These forces tend to push the tank upward due to the difference in pressure between the bottom and the top of the tank, particularly if the tank is empty or not sufficiently filled.

The weight of the back-fill and paving above the tank serves to counteract these buoyancy forces. By providing additional downward force, the weight of the materials helps to ensure that the tank remains securely in place and does not float up in response to the buoyancy. This interaction is critical in ensuring the structural integrity and safety of underground storage tanks.

Understanding the role of buoyancy forces in this context highlights the importance of proper installation practices for USTs, ensuring that they are adequately supported and secured against potential uplift.