resisting bouyancy uplif

huangyhg

resisting bouyancy uplift?
i have a concrete tank that will be installed 20' below the water table.  
soil surcharge bearing on an extension of the bottom slab (beyond the outside face of the walls) will be used to resist uplift. if i only use a vertical column directly over the extension the extension width req'd seems large to me.  aci 350 mentions (very briefly near the front) an "angle" creating a wedge of soil that can be included in the surcharge. does anyone have any advice as to how i can determine this angle?  any references, advise or experience is greatly appreciated.
thanks in advance!
dont forget to check your bouyant soil weight for this application.
you probably need to investigate coulomb's theory, all good geotechnics/soil mechanics textbooks will have an explanation.
i am not a geotechnical engineer so it may pay to post on that thread as well.
for sandy soil we used 25-30 degrees (internal friction angle). no problems yet.
thanks.  bouyant soil is being considered.
i have reviewed coulombs theory and it is helping.
first, the angle to be used depends on the soil type you have.  in my opinion, no more than 10 - 12 degrees can be used.  you can find some help in this item in electrical eng. handbooks, related with foundations of high tension transmission lines tower.
portland cement association publication "rectangular concrete tanks" has a description of this method. i agree that no more than 10 -12 degrees should be used, nor should any more than that be necessary to acheive a reasonable safety factor against floatation.
thanks everyone for you time and suggestions.
when we designing clarifier for wwtp we use clow pressure relief values to  releive the pressrue on the bottom slab and bouyancy
naval facilities design manual 7.02, september 1986, page 7.2-171 recommends using soil wedge within 30 degrees from vertical for cohesive soil, 20 degrees for granular soil.