Providing Ballast over a Septic Tank or Pump Chamber - resist buoyancy

[ALTHORP]

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.

 

[cvg]

do a search on this site, this subject has been debated numerous times

http://www.eng-tips.com/search.cfm?...yancy&t=&h=&searchSite=2&searchArea=1&d=1&s=1

 

[ALTHORP]

Thank you, cvg. I had done a quick search before posting, but must have used too specific search terms, as I came up with nothing. The article from the American Concrete Pipe Association entitled "Design Data 41 Manhole Flotation", which was referenced in another thread, provided very helpful information on this subject.

 

[oldestguy]

Be aware that some state environmental agencies have specific requirements for flood proofing a septic tank. In getting a permit, you may be faced with that.

 

[chicopee]

I would not consider surcharge nor wall friction as there is a possibility that when the tank is placed in the ground, you have a big hole with water in the excavation which means more equipment for drainage and time delay. There are means to anchor the tank or to make it heavier; use your search engine for that.

 

[eea]

Curious overview question, does the septic tank installation meet local DPH code if ground water inundation is a factor? I haven't had to directly deal with this yet, but I understand that leachate fields are being more scrutinized under recent code revisions for seasonal high ground water conditions. I'm not advocating that the uplift shouldn't be checked, but maybe another possible factor here.

I have an old package lift-station design handbook from the mid-70s that steps though the uplift considerations you are after. I think that was by Fairbanks-Morris, I'll check my shelf at the office. Tapered walls, oversized base, all are addressed - similar to the aforementioned ACPA Design Data #41.

 

[BUGGAR]

Very good point, eea.

Maybe the tank effluent is being pumped up to an above ground mound system?

Bob