Buoyancy Calcs.

[preynolds]

We have had some discussion in our office over the proper way to calculate the effect of buoyancy on a 18' deep wet well (5' dia with a 8'x8'x12" bottom slab). Such topics of discussion include: whether or not to use the submerged weight of the soil on the foundation or the dry unit weight in calculating its effect against buoyancy. Also, one guy said to take into account as the volume of water displaced as the volume above the footprint of the footing, whereas I thought the volume of displaced would be exactly that of the structure.

What would be the steps that you would use to determine if the manhole would float?

Thanks in advance

 

[ishvaaag]

I am with you in the volume of displaced water giving the buoyancy. You need to assume some watertable level.

Respect what weight to consider counteracting it, your walls and slab and whatever standing weight sure to remain on it.

For safety you will need to assume the less deeper watertable you see likely to ever occur in the life of the structure. Assume it is say upwards but close of your bottom. You may want to count the water plus solids weight downwards and stabilizing, but hydrostatic prssure also acts upwards, what means that water weigh can't be counted as stabilizing.

Even more simply, the water around the lips of your slab can't help against buoyancy, otherwise we would see more of this in vessels (the water is free to flow around), AND any submerged solid particles in the soil are themselves subject to individual buoyancy, or if you want, the sum of its weight plus the own buoyancy gives... the submerged soil weight.

Say only 1 metric ton/ m3 able to stabilize against buoyancy your well contraption.

The non submerged soil could be taken at its natural weight, since not being completely surrounded by water the hydrostatic buoyancy has not yet developed, and the whole brunt of weight is passed downwards by contact between particles. It may be however wise to consider all the displaced volume as submerged for some cases.

 

[ishvaaag]

COMMENTS TO MY POST ABOVE

Answering in a hurry does not help for accuracy. I will comment paragraph.

3) Suppress from "Assume..." to "..."stabilizing".

4) Suppress from "Even..." to "AND".

This will make I think my post more correct.

 

[JAE]

The soil on top of the foundation lips, if submerged, may not have any shear resistance and therefore is pretty useless in holding down your structure (i.e. imagine the manhole surrounded by quicksand...a boat floats in quicksand).

We always calculate the total volume of water displaced by the entire structure. This includes the footing and the walls. The counteracting weights would be the raw weight of the manhole and the amount of non-submerged soil above that is directly over the footing lip.

 

[ishvaaag]

My further comment is of the same cause but of even contrary sense to that of JAE...if the soil has standing shear strength you could even find the passive force against buoying upwards! Good evaluation should be possible, I have tables of Caquot that go to 0 deg slope. Other way to consider somewaht the shear strength is to assume the soil in the trunk of pyramid at 26 deg opening or so counting in weight.

Of course, in liquefaction of soil the strength nullifies, but you see there is some range for choice.

Mine's anyway more likely would be that of JAE, make it weigh enough to not float, which is not difficult in the kind of buildings I am likely to meet in the rest of my life. By the way, in over 400000 m2 built never met a flotation problem in any of the buildings I have had something to do with, what makes me think that where it happens it must be known problem and the solutions as well.

 

[rethornton]

I routinely check for floatation of precast manholes and vaults and typically use the buoyant weight of the column of soil over the top of the base extension to resist uplift. Such calculations are generally conservative since neither skin friction or the friction angle of the soil are considered in the analysis, both of which can add a significant factor of safety. I will occasionally take liberties with soil friction so that the soil engagement is more wedge shaped than simply a column over the lip. I only do this when higher than normal safety factors are specified. References on this subject appear to be few, however, PCA "Rectangular Concrete Tanks" has a brief but good section on buoyancy.