When you’ve taught any course for as long as I taught Soil Mechanics and Foundation Design and Analysis, you’ll sometimes get inquiries from your students about topics that you either didn’t cover or they might have missed. (Many comforted themselves during the course with the thought that “I’ll never go into geotech,” only to have it be their first job.) This topic came from a student from Florida (which is, of course of special interest for a long list of reasons, esp. since I grew up there) who saw on a geotech report about bearing pressure on a shallow foundation (in this case, a pool) citing the net bearing pressure on the foundation.
The topic applies to both bearing capacity and settlement of shallow foundations. The simplest illustration of this I could find is at the top of the post; it comes from the presentation of Schmertmann’s Method of settlement estimation from the Soils and Foundations Reference Manual, and I discussed it in Foundation Design and Analysis: Shallow Foundations, Settlement.
Using the notation above, the pressure p is the gross bearing pressure on the foundation. The pressure p0 is the effective stress at the base of the foundation. The difference between the two is Δp, which is the net bearing pressure on the foundation.
So how is this relevant to foundation design? In the case of Schmertmann’s Method, it is a part of the method. For bearing capacity, the Soils and Foundations Reference Manual has the following to say:
The net ultimate bearing pressure is the difference between the gross ultimate bearing pressure and the pressure that existed due to the ground surcharge at the bearing depth before the footing was constructed, q (= γaDf). The net ultimate bearing pressure can thus be computed by subtracting the ground surcharge (q) from Equation 8-6:
qult net = qult – q (8-14)
qult net = cNcscbc + q (Nq−1) Cwq sqbqdq + 0.5γBfNγCwγsγbγ (8-15)The structural designer will typically include the self-weight of the concrete footing and the backfill over the footing (approximately equal to γaDf) in the loads that contribute to the applied bearing stress. Therefore, if the geotechnical engineer computes and reports a net ultimate bearing pressure, the effect of the surcharge directly over the footing area is counted twice. Reporting an allowable bearing capacity computed from a net ultimate bearing pressure is conservative and generally not recommended provided that a suitable factor of safety is maintained against bearing capacity failure. If the geotechnical engineer chooses to report an allowable bearing capacity computed from a net ultimate bearing pressure, this fact should be clearly stated in the foundation report.
Bearing capacity theory is explained in Chapter 11 of Soils in Construction.
In general, the ultimate capacity is the more conservative option to use. Combined with the difficulties associated with bearing capacity analysis, I decided not to emphasise net bearing capacity in my teaching. Additionally, for a small pool like the one we had in Palm Beach, bearing capacity failure is remotely possible. For a large (say, Olympic) size pool, we’re dealing with something approaching a mat foundation, and these generally fail in settlement, as noted in Foundation Design and Analysis: Shallow Foundations, Other Topics.
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