foundation support

[JackRit]

I'm interested in opinions about how much excavating around a residential strip foundation can be done without compromising support, assuming non-problematic fairly stiff clay soil and no danger of frost. The consensus in the residential construction industry seems to be that a 45 degree pyramid running down from the bottom edges of the footing is all the soil that participates in the support, and the rest can be safely removed. The Uniform Building Code requires two feet of overburden on the outside of a strip foundation but only 8" on the inside, perhaps only out of consideration for the possibility of frost heave. Any thoughts?

 

[BigH]

There is a nice figure in the Ontario Bridge code for this. Sadly, it is in some pile at the office - but why don't you do a quick and dirty estimation using a footing on a slope to see about the allowable bearing - Bowles' method or Meyerhof's. If I find the OBC, I'll advise.

 

[eric1037]

There is the old 2:1 method of determining stress increase below a foundation. It essentially says that the soil "feels" the load of the foundation on a 2 vertical to 1 horizontal pyramid below the bottom edge of the footing.

 

[BigH]

eric1037 - but that is for stresses causing settlements - you should look at the bearing slip lines too.

 

[eric1037]

BigH - You are correct. According to Terzaghi's bearing capacity theory, the angle of the slip plane failure from horizontal should be 45 -(phi/2). For a clay, phi should be relatively low. Therefore, 45 should be conservative. Or am I not looking at it correctly?

It's too bad we can't post sketches. It's hard to explain in words.

 

[PEinc]

You may need to check more than bearing. You may need to check stability of the slope with the footing surcharge at the top of the slope.

 

[GeoPaveTraffic]

JackRit,

I agree with the above posts. If the cut you are making near the footing is shallow, a few feet, then the 2:1 (45 degree) method works very well in good soils as you describ. As the cut gets deeper, you have to start worring about slope stability as PEinc indicated in the post just above this one.

You should also consider the value and sensitivity of the structure to settlement. Your post says it is a residential footing, so it is likely that these are not big factors, but they can be in some cases.

 

[BigH]

PEinc - that's what the Bowles and Meyerhof methods actually are - footings on/adjacent to slopes.

 

[Elmilo]

I think I might have a similar Situation. Now im not an Engineer, but i have a crawl space of 6 feet and then it gradually slopes up to decrease my crawl space to about 5 feet. Yes, it feels like a basement, but since it sits on a gentle slope it is still a crawl space. I want to turn this space into a basement, but i realize as i was digging out the dirt and making room for a 7 foot clearance with a 4" cement slab, that my footings were being exposed and naturally it did not have 12" depth of soil in the interior. So my question is, can i pour a new Footing side by side to match the existing footing? Its obvious im not an engineer, but its just blowing my mind. Any advice would be greatly appreciated. I think i have made my footings weak at this stage and have stopped the digging.

 

[cap4000]

eric1037

I believe what you are describing is when NO soil is removed from the footing sides. Once the soil is excavated its a different set of Terzaghi line of action formulas for horizontal pressures such as against sheet piling,retaining walls, underpinning situations and open cuts. I use 2 typical ratios depending on the type of soils. 2H & 1V-soft material or clay, 1H & 1V for good sands as a quick and dirty evaluation. Most good soils can go as far out as 2V & 1H.