lateral earth pressure

[christi999]

I am designing a circular concrete manure storage tank, 100 ft dia x 16 ft high in a floodplain. For bearing capacity considerations, it needs to be buried 3 ft. The 3 ft of backfill will be 5" minus crushed. 100-year flood level is an additional 2 ft above natural ground.

In designing for buoyancy control, I want to take credit for the overturning moment of the horizontal pressures on the outside tank wall. How do I calculate the combined resultant against the tank wall of the active soil pressure + the water?

(I find all kinds of references for when the water table is at some point within the soil prism, none when the water is above soil level). Thanks for any pointers!

 

[JasonG]

Did you consider calculating it as though the WT was at the ground surface and include the water above the surface as a surcharge (2'x62.4 pcf = 125 psf surcharge)?

 

[christi999]

Good idea - don't know why I didn't think of that! Thanks

 

[ElGreco]

I agree, u should think of the water as an additional surcharge to calculate lateral stresses. However, it is important to mention the soil type on which your foundation is based. For example you mention a crush. If it is possible to encounter any cracks in the top 3 m. of the soil you should think of water getting into these cracks and cause tension. Hence, an additional lateral force acts on your foundation...Thats exaggerating however, thats why i say you should check the soil type and properties...Tension could ruin your foundation in some soils

 

[Focht3]

In designing for buoyancy control, I want to take credit for the overturning moment of the horizontal pressures on the outside tank wall.

Generally speaking, this is a very bad idea. Because the soil's fabric is susceptible to foundation movements, the lateral pressures may or may not act as you expect. Soils are very weak in tension. You are much better off using the dead load of the soil above the perimeter footing and tank weight to offset the buoyant force.


Draw a free body diagram of the tank and foundation, including the earth directly above the perimeter footing (if any.) Then ask yourself how a lateral force on the perimeter of the tank can withstand the (vertical) buoyant forces acting on the tank. They can't.



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See faq158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"