I am designing a septic system. The area where the septic tank is located is occasionally wet (at the toe of the slope), and there is a relatively shallow periodic groundwater table. A new two compartment septic tank is to be installed in this location. The outside dimensions of the tank are 68" wide x 126" long, and 64" deep. The top of the tank is 4" thick. Walls and bottom are 3" thick. Dividing wall between first (1000 gal) and second (500 gal) compartment is 3" thick. Total weight of the tank, per manufacturer, is 11,000 lbs. Assuming the tank will be occasionally pumped for maintenance purposes, I would like to calculate the amount of additional ballast that is required at different tank depths.

One simplified approach to this would be to calculate the buoyant force, assuming that the tank is completely submerged in water, by multiplying the volume (based upon the outside dimensions of the tank) by the density of water (62.4 lbs/cu.ft.). I calculate the volume to be 317 cu.ft. and the buoyant force to be 19802 lbs. Resisting the buoyant force is the weight of the tank and the soil above it. The weight of the tank is 11,000 lbs. Dry loam has an approximate density of 80 lbs./cu.ft. and may be a fair value for this purpose, though would be higher when moist/wet. The footprint of soil above the tank is 68 x 126 / 12 /12 = 59.5 square feet. One foot of soil would therefore weigh 4,760 lbs. Equilibrium would be achieved by placing approximately 1.8 feet of soil above the tank. In many cases, this is achieved, however in cases where the tank depth is shallow, ballast may be required.

Now, here's where it gets interesting for me. The above calculations do not factor in friction along the sidewalls of the tank. A more detailed model might factor in friction. But how to model it? Further, the septic tank that is most commonly used in this area is "tapered" along the sidewalls. It comes in two pieces, a top and a bottom. The dimensions above are the dimensions at the midline of the tank (at the bottom of the top piece, for example), and the side wall tapers approximately 1-1/2" on all sides, such that the top and bottom dimensions of the tank are 65" x 123" (3" total difference). How much effect would this tapering have upon the possibility of uplift?

I would appreciate any thoughts on how to best model this scenario. I hate to specify ballast where it is not necessary, but I do not want liability either.

Thank you.