So, on the actual PE test, if they specify that you should use the Meyerhof and Vesic table, will they also specify which N sub gamma to use between the two?(to ktulu or jreg).
I will offer my 'theoretical' take on this problem but I would like ktulu to offer his insights since he has tested for the geotechnical exam whereas I am completing a M.S. in Civil Eng with emphasis in Geotechnical Engineering.
First, a brief discussion of the problem as presented.
Warning ... lots of theory!
The shallow foundation equation as first presented by Terzaghi (1943) yielded an approximation to the bearing capacity equation based on general shear failure. In simpler terms, Terzaghi evaluated failure conditions along known planes based on shear and developed a general relationship based on the THEORY. Fundamental assumptions (applicability) is based on level strip footings placed on (near) ground surface where the depth of the footing is less than the minimum width of the footing. Additionally, the footing is assumed to be in plastic equilibrium with defined failure surfaces that follow a theoretical pattern (boussinesq distribution).
The Meyerhof Model came along and included in the solution correction factors for eccentricity, load inclination, and foundation depth. This solution also varies from the basic Terzaghi solution in that the influence of shear strength of soil ABOVE the base of the foundation is INCLUDED. This means that beneficial effects of the foundation depth (
e.g. surcharge) is included in the analysis. The failure plane analysis is slightly more complex as the soil is still in plastic equilibrium but has a log spiral failure surface that includes shear above the base of the foundation. Ngamma is based on upsilon = 45 + phi/2.
The Vesic Model closely follows the Meyerhoff and Hansen Models but incorporates analysis for Ngamma from Caquot and Kerisel (1953) to address the assumption that local shear failure leads to lower bound estimates of ultimate bearing capacity. It is the local shear that becomes the driver in the analysis.
So ... where does this all lead you?
In practical terms, Practioners of geotechnical analysis will typically evaluate the effects under the different shallow bearing capacity models and may even combine the results of some of the models in order to better understand how the bearing capacity will be affected by the loading.
In terms of the PE exam, I do not believe you will be given enough information in any given problem that will allow you to distinguish when to apply the Meyerhoff Model vs. the Vesic Model in terms of the correction factor. Therefore, it will be necessary for the correction factor to be provided in the problem statement as a 'known' value rather than asking you to derive it based on other information.
That is my take from my own perspective. :2cents:
JR