How do you calculate the weight of load needed on top of underground water tanks so that it will not float

Question

Example you have a cube tank (1 cubic meter) and needed to bury the tank. How much weight needed on top so that it will not float in case there is flood.

Answer 1

Draw a free body diagram:

F_B (force of buoyancy) is the weight of the earth displaced by the tank

F_G (force of gravity) is the weight of the tank

F_L (force of the load) is the weight of the earth you put on top of the tank.

These weights are calculated by using densities and volumes. The weight of the tank would have to include both the weight of the water and the weight of the empty tank. You would need to know the density of the earth being displaced and also of the load.

When F_G + F_L = F_B there is an equilibrium. A greater F_L than that required for equilibrium would give you a margin of safety.