Buried Fuel Tank 1

[Dov]

Ten 50,000-liter diesel fuel tanks need to be buried in a large pit. How does one design the dead-weight concrete slab to which the tanks are tied? Do you have to design for the worst case scenario of the pit filling up with rain water, in which case the specific weights of the earth above the tank and the slab itself change when submerged in water (and the bouyant force of the tank becomes a serious factor to consider)? Or do you design for a dry environment, without any consideration of bouyant forces? There is presently no drainage out of the pit. The only change of water swelling up inside the pit is from rain run-off.
Thanks for your help.

 

[wiktor]

I don’t think that you can pour the concrete slab heavy enough to resist the buoyancy of the 50-m3 empty tanks. The more practical solution could be to design effective drainage system for the pit, to prevent the flooding. Otherwise the buoyancy has to be accounted for, and that’s a very large force. I recall having seen empty water storage tanks popping from the ground during heavy rainstorm.
The critical issue will be the permeability of the soil surrounding the tank(s), and possibility of the water table level rising above the bottom of the tanks. Once you will estimate the levels, check the buoyancy against weight of the tank w/ surrounding soil, insulation, and concrete slab. Depending on the accuracy of your assessments use safety factors 1.3 to 1.5.

 

[jackyyau]

If the backfill is insufficient to counter-act the buoyancy, you design tension anchor to anchor the base slab of the tank.

For the element design of the base slab, perimeter wall and top slab, bear in mind to account all the possible load combinations by the inward underground water and soil pressures, outward fuel pressure and vertical (both upward on base and down on top slabs). The following combinations for horizontal balanced case are possible:

Max Vertical + Max Horizontal
Max Vertical + Min Horizontal
Min Vertical + Max Horizontal

In addition, the horizontal pressures on opposite sides can be different (unbalanced case).