Gross allowable bearing capacity of soil 1

Some of my foundation design books talk about "gross" and "net" allowable bearing pressures. The net allowable pressure equals the gross allowable pressure minus the surcharge pressure from soil, etc.

However, I also have a question I have about this concept. Using this logic, the lower you place the footing, the lower the net bearing capacity of the footing is because some of the capacity is used up by the soil on top of the footing. Now, I'm not a geotechnical expert (and I don't even play one on TV), but it seems like the capacity should not decrease (or should even be better) because of the confining effect of the surrounding soil. Can any dirt expert shed some light on this?

Neither am I and expert Taro, but when you determine some bearing capacity at some depth, it usually gets related to tip bearing (fr friction/adhesion there is separate computation). Whatever the case, you will be always including the dead loads in your structure...or the evaluation of teh bearing capacity to our purposes is (in what I have seen) always net, against of course weight of the foundation (and any -even any soil load loading above the footing, for shear resistance zero, the more prudent would exact) needs also be counted. That accounting this way leads to a diminishing law of bearing capacity at some depth I esteem rare; that it diminish its usefulness in the amount of the weight of the loading soil, I think if not necessarily correct at least conservative.

As I recall from my classes in foundations and soil mechanics (and a few years of practice in the field), the term "allowable bearing capacity" is normally meant to be a "net" value, that is the increased load that the soil can safely bear, above that already loaded in place by the soil. For practical purposes this is usually taken as the building load imparted by the structure, without adjusting for soil over the foundation. In homogeneous soil, the gross, as well as the net allowable bearing capacity does increase with depth below ground surface, as surmised by TARO. (May not be true for cetain soil conditions where limited by settlement or a weak soil layer). But geotechnical reports don't usually provide a range of capacities for various depths, but should state a minimum footing depth applicable for the stated capacity. If not stated otherwise you should be able to consider this value a net capacity that would not have to be decreased to account for soil over a footing, so don't use a reduced value for greater depths. And if you could use the additional capacity, ask the geotechnical engineer to provide you a higher allowable bearing capacity for deeper footings.

Hope this helps some!
Carl

From the above discussion we encounter terms that are only partially understood but, are used daily in design. Most discussions center on the soil shearing strength, which is usually substantially enhanced with confinement. Depth is usually good. Soil Consolidation is normally a secondary concern in the books, but in some areas may be the governing factor, most of the time.

An important point is that bearing capacity taken from in-situ testing (SPT, CPT, Pressure cell etc.) begins as Gross Bearing, as the soil trength considerations are direction addressed. Initially, the soil overburden (and in some cases, existing foundation loads) is considered.

Consolidometer (settlement) testing begins as Net Bearing, as settlement limitations are addressed. Remember that settlement is caused by Net increases in applied load.

There are many references but only a few that provide easily understood definitions, along with discussion which illuminates the issue and presents potential pitfalls.

B.K. Hough, Basic Soils Engineering, 2nd Edition, 1969, provides a good discussion in chap. 9, without going into how the actual bearing values are obtained.
A.S.C.E., Bearing Capaity of Soils, Technical Engineering & Design Guide,()US Corps of Engrs., 1993 provides a useful set of definitions, good discussion, methods of computation and a good bibliography.

C.G.S., Canadian Foundation Engineering Manual, 3rd Edition, 1992. Chaps 8, 9 & 10 directly addresses the question.
Fellenius, Basics of Foundation Design, 2nd Edition, 1999, provides discussion which is to the point.

Sorry for the length.