Soil Weight for Footing Bearing & Overturning

[JR55]

As far as I understand, the weight of the soil above the footing is included in the footing overturning and sliding calculations, and neglected in the bearing calculations when a net allowable bearing pressure is given.

When calculating the bearing stress, (P+W)/B[1+6e/B], the soil weight would then not be included in the "P+W". However, because the soil weight is included in the resisting moment for overturning, is it typically included in Mr when using Mr-Mo to calculate e?

Logically, I can see that the weight would act to resist footing overturning and at the same time not add additional weight to the soil, but it feels strange to both include the weight and not include it within the same equation. What is the standard approach here?

 

[DoubleStud]

Lets say you dig foundation 50 ft deep. At that depth, the soil already experienced 50 ft of soil. If we assume soil is 110 pcf, that is 5500 psf! How are we supposed to design a foundation when the geotech give us 4000 psf allowable bearing pressure?

Now if you build the foundation on surface, then add soil on top of it then technically the soil on surface never supported this soil before. You have to add that soil weight for this scenario.

Is that what you were asking?

 

[JR55]

Not quite, I understand that any existing soil should be neglected in terms of the weight, but I’m wondering if the same weight would still contribute to the resisting moment when calculating the eccentricity in the bearing stress equation?

 

[DoubleStud]

I think so. I count the soil above heel and toe on my spreadsheet. What is the P & W on your post?

 

[JR55]

Vertical reaction (P) plus weight of concrete, soil if considered, and stored bulk material (W)

 

[DaveAtkins]

What I do is always include the weight of the soil, but when checking against net bearing pressure, subtract out the existing overburden.

DaveAtkins

 

[JR55]

So it sounds like for bearing checks it would be reasonable to use:

e =(Mr-Mo)/P where Mr includes the existing overburden, but the P has the existing overburden subtracted out.

 

[RWW0002]

Many times the allowable bearing pressure is given as a "net allowable". Therefore it is justifiable for allowable pressure to be increased with depth by the weight of the soil being removed.

This will essentially be similar to the approach outlined above (ignoring the net effect of weight of soil above for bearing calculations) but will be more consistent if you would like to take advantage of this same soil weight for overturning and sliding calculations.. This make sense?

 

[azcats]

I'm not a fan of removing overburden for bearing pressure calculations unless the overburden is negligible or there's little overturning. It penalizes the foundation too much, IMO.

If given net allowable bearing pressure by a geo, I'll increase my design allowable by the overburden at B.O. foundation and design from there.

For context, I'm typically dealing with huge overturning moments and often using overburden atop the foundation to resist OT and bearing pressure usually controls foundation size.