underground tank buoyancy

huangyhg

underground tank buoyancy?
hello,
i am designing an underground concrete stormwater tank.  it is rectangular and has extensions at the bottom to prevent uplift. ground water is at surface level.
what is the buoyancy force on such a tank? i can calculate the force due to the water, but what about uplift due to soil pressure?
also, what wedge angle should i assume resists uplift? some say the soil friction angle, and others say no more than 10 degrees.

tom
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if the tank is empty, the forces holding it down are the wt. of the tank and the wt. of the soil above it minus the wt. of the water in the soil. the numbers used in pipe design is .67 times unit wt. of soil. the uplift forces would be the wt. of the water volume of the tank. i wouldn't even use the 10 degree angle. saturated soil has very little friction between particles. i suppose it would be part of my safety factor. you might add a little resistance due to the pipes connecting to the tank. but if they are empty, then you have uplight on them as  well.