Soil Weight on Basement Wall Footing

[Mullky]

When designing a 2-foot wide basement wall footing that is supporting 12" CMU, is the weight of the soil above the 6" strip of footing that extends into the retained soil area accounted for when calculating bearing pressure?

For example, a 10-foot tall basement wall supporting 10-feet of unbalanced fill would have an additional vertical load of roughly 10-ft * 0.5-ft * 120 PCF = 600 PLF.

Thanks, Mullky

 

[atrizzy]

This is how I typically consider it.

 

[Mullky]

Agreed. However, what about a 20-foot tall wall? In this case there is an additional 1,200 PLF plus the load from 2-floors and a roof. If we widen the footing to 30", then it needs to be 36" wide for 2,000 PSF allowable bearing. If we go to 36", then it needs to be 40" wide, until we get to a 48" wide footing. Is it ever practical or safe to assume that the weight of the soil spreads "away" from the wall like with a vertical point load acting on soil?

 

[JAE]

If you are using NET bearing capacity - remember that you've removed half of the earth above this footing (inside of basement) and the bearing pressure of the soil on the outside lip of the footing has typically had that earth already on it since dinosaurs.

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[atrizzy]

I agree with JAE here, if you're 20' below original grade, chances are your bearing capacity is pretty high, higher than the difference this soil can make.

 

[Mullky]

Thanks guys, [ feel a lot better about our assumptions. The house is on the side of a mountain. What I decided to do was use 2,500 psf allowable for the front (uphill) wall due to how deep into residual soil the footing will be. Then 2,000 ever where else. This will be stated in our design assumptions.

 

[KootK]

It's an interesting question though and I've considered it often. On most projects, I try to get the geotech to specifically state that our bearing capacities are net. At least half of them seem to have no idea what I'm talking about or why it would matter. In your particular case, I'd say that it's quite likely that you'd get more than a vertical column of soil weight would predict rather than less. Often I see geotech reports that say "you get this much at this depth". In my mind that doesn't make it any less net but in their minds it sometimes seem to.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[JohnnnyBoy]

Mullky, something to consider before specifying different bearing capacities for connected sections of your footing. When bearing capacities are normally obtained it is done through allowable settlement, usually 25mm. In your case, the real settlement will be higher in the 2500psf allowable than the 2000psf allowable causing differential settlement. With foundations, the differential settlement is the main factor that causes cracks and failures more often than uniform settlement.

JB