Earth pressure to resist overturning in shallow footings

[Daves10]

I just had a conversation with an engineer who designs shallow moment resisting footings using passive earth pressure to help restrain overturning. The footings are for pre-fab buildings without a slab and the tops of footings are at grade. Rather than just increasing the footing size to eventually balance the overturning moments, he includes some passive resistance from soil against the sides of the 2'-6" footing. I have used passive pressure to resist sliding but using the pressure against the footing sides to resist overturning of an isolated footing is contrary to what I would call normal practice.
Does anyone have experience using this method?
David G. Smith
Smith Associates
Gainesville, GA

 

[bootlegend]

I have checked this for various footers before and it usually is a very small resistance force compared to the sliding load, and with only 2'-6" depth I'd bet the resisting moment contribution is even smaller. I always try to avoid using the passive pressure on shallow footings unless in a real pinch and I can be sure there will be no excavation or scour around the foundation in the future.

 

[msquared48]

I would not do that, for with the first attempt at rotational movement, the soil is compacted, a gap develops, and the resistance decreases, in fact, does not re-engage until the initial rotational deflection is exceeded... Bad idea - works on paper, but not in reality.

Also, unless the concrete was poured directly against the soil, requiring no backfill, the use of any passive pressure would not be allowed by code.

Mike McCann, PE, SE

 

[ztengguy]

How much size of the footing can he save doing that? Sometimes as engineers we are made to feel that we are buying the concrete or steel ourselves, so we will do what it takes to save a bit.

 

[JedClampett]

There's probably no Geotechnical Report on this, because most of them are pretty restrictive on using passive pressure on shallow foundations for all the reasons noted above.
And as ztengguy says, slab on grade concrete is one of the cheapest items in a project, pretty much just costing the delivered price of concrete ($110 per cu. yd. here), so using concrete to ballast a footing is pretty inexpensive.

 

[BUGGAR]

My landscape contractor just dug up a foot of soil around my house and put in top soil so I could landscape the yard. If I had this guy's foundation, I'd (he'd) be in trouble.

Most Geotechs I know ignore the top 12 inches of what they call "fluff", a sophisticated term if I ever heard one. Or "duff" if you're in a forest.

Bob