Depth of Foundation for Bearing Capacity Analysis

[amstat]

I am looking for more detailed information on depth of foundation as used in the bearing capacity equation. It was generally defined by Terzaghi as the distance between the base of the footing and the ground surface. Many other sources do not give much more detail.

In some circumstances, are there any "Equivalent Depths" that should be used in the bearing capacity formula as opposed to the actual depth from the ground surface to the base of the footing? Examples of what I mean are:

1. Should we use the same distance for depth in the equation in cases where the concrete footing is poured within forms in the bottom of an excavation, remove forms, then backfill on the sides of the footing -- as opposed to placing the concrete directly in a trench with the earth used as forms?

2. What about foundations on fill material such as newly placed and compacted fill? I imagine settlement would govern in most cases, but should we use an "equivalent depth" in this case? What about the depth for a foundation within an established embankment fill?

3. Lastly, the depth used in the equation for a basement footing should be the distance from the base of the footing to the basement floor. Could an incresed "equivalent depth" be used due to the confinement of the concrete floor?

Any information would be appreciated.

 

[Mccoy]

amstat,
I concur the topic is not discussed as it should be, since the bearing formula is very sensitive upon the footing depth.
I'm not aware of any use of equivalent depths.
Sure, to different conditions would apply different models:
The case of concrete poured in trenches is the simpler, since the only aspect to be evaluated should be change in density and possibly parameters of the superficial soil, IF surface layer has not been already excavated.
In the case of forms at the bottom of the excavation, I always assume (and write it) that the backfill has been well constipated.
No doubt here is a confinement action from the basement floor. In this case by the minimum energy principle I would consider the side of the footing where there is soil up to the surface; if not I would be conservative and neglect this effect. A quantification could be tempted, treating the concrete basement as a rock layer, but I doubt it is worth it.
All considerations should be done having well in mind the shape of the wedge and sliding surface below the foundation.

 

[GeoPaveTraffic]

I have never seen any type of equivalent depth used when designing footings. Since most footing designs are limited by settlement, not bearing, this is not a question that effects most engineers. However, below are my methods of dealing with the specific situations that you bring up.

1. The depth of footing is not effected by the use of forms, assuming that the fill is compacted.

2. The fact that the material is fill is not important, however, the strength and density of the material is important.

3. The footing depth should be measured from the top of the basement slab. If you want to be precise, you could increase the unit weight used in the bearing capacity equation to account for the fact that some of the material is soil, some rock, and some concrete.

 

[fuchsjr]

You might find the following site of interest. Does not address equivalent depth.

http://www.union.edu/PUBLIC/CERDEPT/FACULTY/GHALY/FOUNDATION/