Soil weight on footings (yes or no)?

[TonyES]

I'm just wondering what other engineers do for footing design when the governing factor is the allowable bearing pressure. Do you include the weight of the soil that bears directly above the footing or do you ignore it? My company includes it and has included it for years but we did a survey of local engineers and not all of them do, and then they have smaller footings. We saw a part in the code that says the soil/footings is "permissible" to be used in calculating the soil bearing pressure. Does this mean we don't have to include it?
thanks for the feedback.

 

[msquared48]

I include it, especially, and always, for retaining wall footing design.

Mike McCann
MMC Engineering

 

[Galambos]

Tony, check your soils reports and see if the term "net allowable soil bearing pressure" is being used, which most do.

"net" refers to the loads applied to the footing without regard to the soil weight.

there should be a discussion of this in your geotech textbook if you want to look into this further.

 

[TonyES]

Mike: I definetely include it too for retaining walls, but what about restrained walls?
Galambos, most of the residential homes don't include a soils report, the city gives us an allowable, but I'll check out how the city terms it, Thanks.

 

[nickky]

I agree with Galambos. I should add that when there is no soil report available then you have too check the reference (i.e. code) table to see if the given (suggested) values are "net" or not.

Vanik Abedian,
Kartez Engineering, Inc.

 

[JAE]

Keep in mind that the "net" allowable bearing pressure you get from the geotech reports may be based upon the soil height that was existing originally at the site, prior to moving ground around for the building pad.

So you may have 4 feet of dirt over your footing, but the original grade was 3 feet above the footing. In that case you'd include 1 foot of surcharge soil in addition to your footing loads when using net allowable bearing pressures, but you'd neglect the 3 feet since that was "already there".

 

[civilperson]

Ask your geotech, some do and some don't. The various interpretations of "net allowable" can have concrete foundations assumed to be the same weight as the soil removed.

 

[TonyES]

How about when i don't have a geotech report and the city says to use 1500 allowable bearing pressure, if my footing is 10' below grade can I increase that 1500 because of overburden pressure or ignore the 10' of soil instead? what do you guys think about that?
thanks

 

[civilperson]

If the soil weighs 95 pcf, the soil ten feet deep can hold 950 plus 1500 psf, (2450 psf). Most codes give greater pressures for depths greater than 2 feet. Why are you not getting a geotech report? Too smart or too stupid?

 

[msucog]

get a geotech (even if it's a simplified evaluation/exploration)...effectively doubling the design bearing pressure can have incredibly different results. heck, even simply "assuming" 1500 psf could have adverse consequences on the structure (regardless of who you think is so-called saying it's ok).

 

[csd72]

civilperson,

The philosophy you describe is one that is used by many engineers throughout the world. As for most rules of thumb there are some exceptions to the rule.

1. I have actually encountered soils with a strong layer underlain by a weaker soil layer. It was so much weaker that I had some helical pile contractors refusing to tender in case they broke through to it.

2. The reason why the bearing pressure gets greater as you get deeper is because you are removing the soil over. So if you increase the pressures as indicated, you also have to include all loads from above including the weight of the soil.

 

[BigH]

The foundation forum has had numerous discussions on what is meant by bearing pressure and you might be interested in searching for them. They have been pretty good and complete.