Category: Deep Foundations

This is the first of periodic (hopefully not sporadic) articles on the new NAVFAC DM 7.2. In this post I’m going to make some general observations on the book and look at its “Prologue” on shear strength for geotechnical design.

Of the two classic NAVFAC DM 7 volumes, the second–Foundations and Earth Structures–was the “longest in the tooth” largely due to advances in construction technology, and needed upgrading the most. The result of the effort is a long book (721 pages as opposed to 578 in DM 7.1) but one the need for which is greater than 7.1. It also includes some new sections, including one on probabilistic design (LRFD for short) that has been a major change in the way foundations are designed over the last half century.

I’ll get to that later but in the meanwhile here are some general observations:

• As was the case with DM 7.1, the graphics are greatly improved, and are up to the standards of their civilian counterparts from the FHWA (which have also been an inspiration to the content of this work.) This is good news, not only for the readers of this book but for those publications which use the graphics in their own books. One of the major upgrades to my and Lee Schroeder’s Soils in Construction was using the FHWA’s better graphics; we had no budget at all for illustrations. It’s always needful to be profitable in business, but textbooks have become cash cows (especially with the major publishers and textbooks) with most of the effort going to places like mastering (which has issues of its own) and not on graphics. For books like Soils in Construction, the graphics will be very helpful. One thing that I held back on was replacing every table and graphic in my Soil Mechanics slides with the new DM 7.1 replacements; I found the older ones, although a lot better looking, to show up better on a screen.
• Pursuant to that, any textbook out there needs to be reviewed in reference to both of these volumes, not only for the graphics but to the content. How soon these changes will appear in textbooks depends upon the publisher; I wouldn’t hold my breath, as many textbooks represent “rearranging deck chairs on the Titanic” rather than moving things forward. In the case of this field, we have an advantage, because…
• Although DM 7.2 documents many advances in geotechnical engineering, one thing that strikes someone who has followed the history of the profession is struck once again by how conservative this business is. Both volumes get into software solutions but the main audience of both is the geotech who uses formulas (straightforward and otherwise) to achieve their design analysis. This is not to say that this approach is without merit; “black box” technologies are never a good thing from an understanding approach, and can be dangerous when applied indiscriminately. But the nonlinear nature of geotechnical engineering makes some kind of numerical solution (as opposed to a closed form solution) a necessity to deal with the problems geotechnical engineers face. A solution “duo” where closed form solutions (empirical to varying degrees) and numerical ones are used together is the best, and although that’s not the way it’s presented in DM 7.2 it’s a useful guide to put the two together.

Prologue: Shear Strength for Geotechnical Design

The first section of the book isn’t a chapter strictly speaking but is better described as an excursus on the topic of shear strength. My guess (I haven’t as of now discussed the details of this book with its editor) is that it was added due to feedback from DM 7.1; it really belongs in that document.

One of those “conservative” things about both documents is their sticking with Mohr-Coulomb as the “go-to” failure theory in soil mechanics. I discuss this in more depth in My Response to Rodrigo Salgado’s “Forks in the road: decisions that have shaped and will shape the teaching and practice of geotechnical engineering” and an announcement but the bottom line is that, even with all of the alternative failure models that have been developed (such as Cam Clay) none is as widely applicable across the spectrum of soil types as Mohr-Coulomb, and this doesn’t look to change for the foreseeable future.

The prologue chiefly deals with two topics: non-linear failure envelopes and the applicability of different testing methods to different soil conditions. Non-linear failure envelopes have been understood to exist for a long time and get some coverage in the old DM 7 but in this case some additional quantification of these is presented, especially as they relate to “y-intercept” issues of cohesion in various soils when a purely linear failure envelope is used. Application of different testing methods to different soil conditions (including the composition and the drainage state) is helpful; many texts get bogged down on this topic and it’s sometimes hard to figure out how to use the information. In this document the presentation is more concise.

Although it’s probably beyond the authors’ scope on this prologue, there are two topics which could use some further discussion in both of these volumes.

The first is the simple question: what is failure? For most engineered materials the first form of failure considered is yield failure, which is the transition from elastic/path independent behaviour to plastic/path dependent behaviour. With soils how successful this is depends in part upon how elastic the soil is before failure and how sharp the transition is across the failure envelope. There are other ways of dealing with the non-linearity of soils. A hyperbolic model, for example, posits that there really is no failure point; the slope of the line progressively flattens with increasing shear stress. Some soils are amenable to a specific type of modelling and some aren’t. Defining failure also depends upon the application as well.

The second is the issue of dilatancy, which isn’t discussed much in either volume. As detailed in my response to Salgado, this doesn’t get the coverage–or quantification–it deserves in most geotechnical literature, and is one of those things that geotechnical engineers need to be made more aware of. DM 7 was and is a “state of the practice” document, and hopefully by the time our government gets around to revising it again it will take its rightful place in this applied science.