Hydrostatic uplift pressure on a spread footer 2

[cad0211]

I am looking at a possible foundation design on a site where the worst case water table is 4 ft below grade.

Our spread footer is deeper than this depth so we are concerned about uplift from the water.

What area do you use to apply the 62.4 pcf of water pressure to the foundation? I was assuming the surface area of the footer but what about depth?

Thanks.

 

[fattdad]

Just use effective stress. Water pressure on the base of the footing also operates on the top of the footing too, eh?

So, the fluctuating wster table influences the effective weight (buoyant weight) of the concrete and the soil.

f-d

ípapß gordo ainÆt no madre flaca!

 

[MotorCity]

As long as your concrete weighs more than 62.4 pcf, you won't have uplift on your footing due to hydrostatic pressure.

Think of it this way: if you drop a block of concrete (i.e. the footing) into a pool of water, does it float (uplift)?

 

[oldestguy]

One thing that submerging concrete in water does is it's effect of the weight to develop bearing capacity. It is the buoyancy effect of submergence.

 

[SlideRuleEra]

...the worst case water table is 4 ft below grade.
What area do you use to apply the 62.4 pcf of water pressure to the foundation?

Flotation is calculated based on the volume (not area) of concrete that is submerged. For this problem, only the volume of concrete that is 4 ft, or more, below grade is affected by buoyancy.

http://www.SlideRuleEra.net

 

[jdonville]

Also be careful that your bearing capacity analysis was performed with the water level assumed for this limiting condition.

 

[PEinc]

You did not mention what type of structure you are designing. Check that there is no hydroststic uplift on any floor slabs. An individual footing may not float but a mat or a floor slab might.

http://www.PeirceEngineering.com

 

[cad0211]

"Also be careful that your bearing capacity analysis was performed with the water level assumed for this limiting condition."

Sorry could you explain a little more what this means? The water has an effect on the allowable bearing pressure? I spoke to the geotechnical engineer on this project and he said it wouldn't have an effect.

Thanks!

 

[Ron]

Terminology Police here....it is a footing, not a footer.

In addition to SRE's always great advice, if your footing is fully submerged below the water table, any soil overburden that is also below the water table will have a reduced unit weight from bouyancy of the soil (as f-d noted).