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Floating foundation for a WWTP

Floating foundation for a WWTP

I need to design a floating foundation for a WWTP. The bottom level of the mat if already set by the hydraulic design. The excavation needed is 6 meters (20 ft) deep. I need to check the limit states (failure and serviceability) of the foundation.
However, while calculating the increase in vertical stress in the soil (Δσz), the increment in vertical stress will be very low when the wet well is full; but it will be negative when the wet well is empty, since the weight of the structure is less than the weight of the soil excavated.
Then I calculated the FS against flotation is 2.5, considering the total dead weight divided by the total hydrostatic uplift force.
How can I interpret this? FS says it won't float, but what will happen when the wet well is empty and it will have a negative vertical stress difference?
Thank you!

RE: Floating foundation for a WWTP

Negative vertical stress = uplift. Your resisting force is greater than your uplift force by a factor of 2.5. Assuming your calculations are correct, you're good.

RE: Floating foundation for a WWTP

One way to provide some adjacent resistance to floating is by having the base slab extend out to "grab" some earth to help. Chances are that earth submerged density is all it will provide. Under the worst alternative for sizing that volume would be to figure on a straight upward lift using the submerged weight of that material above the lip as well as a frictional resistance on the shearing plane. Another way would be take the volume of that in the truncated cone containing that material using an assumed side angle of that truncated cone of at least 20 degrees for loose material. Dense material would be better both for density and frictional resistance. You also can consider anchors, such as grouted in "tie backs" extending down in submerged density soil. Also with a known plan for dewatering the tank, well points surrounding the tank can pump down the water table for that period. Have stand by generators. After all what sort of a plan would you have to build it in the dry anyhow? Think bout using that as you may need it later.

RE: Floating foundation for a WWTP

I agree that it all relates to side shear and/or the gamma*z over some foundation flange.

I also get it; when the tank is full, the delta-sigma is less than original.

For me, there is a big practical problem to consider - under what condition will you actually see the tank empty? To rapidly pump the waste out of the tank creates uplift on the base slab of the overall tank. So, even if there is sufficient burial to keep the tank from popping out of the ground, how will the base slab remain stable when you have full hydrostatic force acting on the bottom, but the tank has been evacuated?

Folks design relief for the slab bottom in some instances. So, how many of those is needed? What will be the inflow? How will you know if the hydrostatic pressure on the bottom is relieved at the rate of tank evacuation? These, to me, are the issues to ponder.

I'd include a piezometer in the tank backfill. That way, when you are evacuating the tank, you can track the water elevation in the free-draining backfill. I'd include enough ports in the base slab, so if you are evacuating the tank at 200 gpm, you can relieve the pore pressure on the base slab at that same pace.

Then again, you could design the base slab for all those bending moments?

Just a few things I used to think about when I was in private practice. . .


ípapß gordo ainÆt no madre flaca!

RE: Floating foundation for a WWTP

As to a one way valve in the slab to allow water in from below and supposedly also to seal out leakage, forget it. That valve will get stuck open and then you contaminate the ground water or worse yet; it is stuck closed and the tank floats. Your memory will forever see that tank sitting there a few feet higher than when built!!!

RE: Floating foundation for a WWTP

Quote (geot88)

I calculated the FS against flotation is 2.5, considering the total dead weight divided by the total hydrostatic uplift force.
...what will happen when the wet well is empty and it will have a negative vertical stress difference?

I agree with your calculation, the FS is 2.5.
IMHO, no additional concerns... for the following reasons:

To construct the foundation in the 20' deep excavation, I assume the soil will have to be dewatered and the excavation will be somewhat larger than the size of foundation. After construction is complete, the space around the wet well will be backfilled. That is the difference - you can control the properties and installation methods for the backfill. Even if you reuse the same soil that was excavated, it's backfilled properties should be different.

A good reference for flotation calculation in general is "Design Data 41, Manhole Flotation" by the American Concrete Pipe Association. idea r2d2

RE: Floating foundation for a WWTP

I assume that the FOS of 2.5 is based on the expected highest groundwater level. If so, appears that your FOS is adequate. See this old thread for your reference:

Also, you may need to check for heave and quick conditions during the excavations and add some notes about these in your drawings.

RE: Floating foundation for a WWTP


i know my question is not related to this topic.
but I'm designing a stadium with 50'000 seats that is located on a dock filled with water.
Dewatering and backfill in not an option-too expensive.
What sort of foundation would be the most suitable?


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