Buoyancy Uplift on footing

[ztengguy]

I am trying to understand buoyancy uplift on a footing.

For example, If I have a pedestal wall footing, say 2'-0" deep, pedestal wall 8'-0" (10'-0) total, and the water level is at the top of the wall, Am I safe to say I can reduce the bearing pressure on the soil by 62.4*10'-0', or that value is the uplift buoyancy force?

I have this comment from a geotech, I am trying to understand it a bit more

Even if it is assumed that the groundwater level is the same on both sides of the structures (equalized via weep holes), buoyancy and uplift will significantly impact stability. The earth pressures for portions of the backfill soil below the ground water level should be based on effective stress, using submerged unit weights. The weights of soil and concrete below the ground water level should be estimated using submerged unit weights, or, as an alternative, hydrostatic uplift force on the bottoms of the footing should be included.

It seems that he is saying either add the water fluid pressure behind as a lateral force, or the hydrostatic uplift force.

Any comments?

 

[hokie66]

Not sure I completely understand your question, but hydrostatic pressure acts in all directions.

Lateral pressure is the sum of the soil pressure using the net density (dry density minus water density) and hydrostatic pressure.

As to hydrostatic uplift, the buoyancy force is the volume displaced x density of water.

 

[KootK]

Dirt will tend float a bit when submerged in water, just like anything else. If the dirt is denser than water, then it won't actually lift off; rather, it will just be effectively "lightened". This is just Archimedes stuff that you learned in high school. The buoyant force is equal to the equivalent weight of the water displaced. While this sounds like a good deal in terms of lateral pressures, there's no free lunch. You need to consider the worst case of the following:

1) Lateral earth pressure without the buoyancy effect and without hydrostatic pressure and;
2) Lateral earth pressure with the buoyancy effect and with the hydrostatic pressure.

Since k_water = 1.0, k_soil < 1.0, and soil generally weighs more than water, case number two will usually exceed case number one (soil without the water). No free lunch.

The buoyancy effect will reduce the effective bearing stress at the underside of your foundation and reduce sliding resistance. I believe that is what your geotech is getting at with regards to stability.

Long story short, it's HYDROSTATIC LATERAL <AND> HYDROSTATIC UPLIFT.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[ztengguy]

Thanks guys, Thats what I thought, I was confused about the geotech comment to do submerged weights (lateral) or the uplift. We are doing lateral and uplift.

 

[paddingtongreen]

You must mean that the water is at the top of the wall on both sides or it would be a dam.

The water in the soil below the surface supports the water above so the thin layer of water under the footing is pushing upward the the pressure of 10' of water. If you have a hollow wall and it, and the dirt above the footing weigh less than that, your wall may float upward.

Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin

 

[ztengguy]

So why is it all pushing up on my wall, and not just 'equalizing' by pushing outwards below the footing, and up around the front?

As a short term thing (water in front of wall runs away, soil saturated behind) I do have a dam.

I end up with 10' of water pressure pushing upward and outward on my wall, thus the wall stability to be way off, and sliding be almost impossible to control.

 

[BAretired]

Confusing description of problem. Sketch please.

BA

 

[cvg]

this has been discussed at length on the forum. suggest reading a few of the threads below.

http://www.eng-tips.com/search.cfm?q=bouyancy&action=search

basics are:

the concrete wall displaces water equal to the submerged volume of the wall. water weighs approximately 64 pcf so that is a force pushing up on the bottom of your footing. in addition, soil loads are based on the unit weight of the soil and the friction angle. both will change when you saturate with water.

worst case loading is generally when water is higher on one side of a wall, especially during flooding or construction phase this can cause overturning or sliding which might cause your structure to move.

 

[BAretired]

Any water above the footing pushes down, partially compensating for the upward pressure from below.

BA

 

[dhengr]

Ztengguy:
To the extent of the buoyancy affect, the footing gets lighter, in terms of resisting uplift or providing lateral frictional resistance. When you immerse things in water they get lighter by 62.4 lbs./cu.ft. But, obviously, a solid block of conc. doesn’t usually float either, (150 lbs./cu.ft. less 62.4) is still a positive weight.

 

[ztengguy]

Here is my sketch. We cant come to agreement what the uplift would be at the bottom of the footing.

 

[ztengguy]

crap, here is my attachment.

 

[BAretired]

What prevents the water from leaking out on the right hand side? Where is grade? Is the soil permeable under the footing?

BA

 

[ztengguy]

Thats sort of my thoughts too. Basic Buoyancy actually gives me a 'uplift' force of the volume of the wall multiplied by the water weight. That is the buoyancy force. As for the water head force, I am not sure its 100% calculable easily. As you said, would it more go under and around the footing, or try and push up the footing.

We went with the buoyant uplift force on the footing, and it still made the footing bigger.

 

[BAretired]

It depends on the soil. If the footing and wall rest on impervious clay (see attached) then there is no leakage and no uplift due to buoyancy. The wall and clay act like a bowl containing water.

If the material labeled "IMPERVIOUS CLAY" is changed to granular material, there is uplift but there is also flow under the footing and possible erosion as well. In that case, there will be upward pressure which will vary from maximum on the left to minimum on the right depending on the permeability of the soil.

BA