NAVFAC DM 7.2: Shallow Foundations

This week we’ll turn in NAVFAC DM 7.2 to shallow foundations. It’s a well worn path and NAVFAC DM 7.2 does a good job covering it, but there are a points that are worth making. (Settlement for these foundations is covered in NAVFAC DM 7.1.)

Upper and Lower Bound Plasticity

It’s mentioned in both NAVFAC DM 7.1 and NAVFAC DM 7.2 but not really explained: the whole concept of upper and lower bound plasticity. Until recently most American textbooks avoided the subject; I used Verruijt’s coverage of the subject, which had problems of its own but gave a reasonable introduction to the subject. NAVFAC DM 7.2 refers to Terzaghi’s bearing capacity method (which really needs to be retired from use) as an “upper bound” method, but the truth is that all of the bearing capacity methods mentioned–Terzaghi, Brinch-Hansen, Vesić (more about that shortly) and Meyerhof–are all upper bound methods. For a credible lower bound method which is good to illustrate the concept with bearing capacity, take a look at my post Lower and Upper Bound Solutions for Bearing Capacity.

Geotechnical Eccentricity

For both square/rectangular and circular foundations, this gets nice coverage, with illustrations, in NAVFAC DM 7.2. It’s a subject my students wrestled with, especially when juxtaposed with “middle third” types of distributions (and that’s covered too,) and this will be a help.

Vesić’s Method

If there’s one serious lacuna in the presentation on bearing capacity, it’s the lack of coverage of Vesić’s Method. That’s because the FHWA, for better or worse, has adopted it (or a modification of same) as their principal recommended method for bearing capacity.

Groundwater and Layered Stratigraphies for Bearing Capacity

The coverage of both of these topics is extensive and welcome. The groundwater part is basically the same method as the FHWA uses; the layered part is an advance over this or any other method I’ve seen. One question hangs over the festivities: what’s the best way to put them together?

Shallow Foundations on Slopes

This represents a major advance over the old book. My students found Meyerhof’s method hard to use (I did too) and the tabular alternatives given are a welcome break.

Mat Foundations

This is always a difficult topic because, at the end of the day, a computer solution is necessary for a realistic analysis. In the interim NAVFAC DM 7.2 furnishes a method which hopefully will be helpful for preliminary or verification work. One topic that isn’t consistently treated is whether a foundation should be analysed as rigid or flexible in the first place. NAVFAC DM 7.1 was very helpful when I put together When Semi-Infinite Spaces Aren’t, and When Foundations are Neither Rigid Nor Flexible, and a similar approach here would have been helpful.

The rest of the chapter focuses on drainage of shallow foundations and rock and soil anchors. The latter edges into deep foundations, which will be our next topic.