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NAVFAC DM 7.2: Geotechnical Design in Problem Soils and Specialty Construction Methods

Posted on 4 April 2025 by Don Warrington

This week we’ll look at the first chapter of the book. The whole business of “problem soils” is not straightforward because it’s a matter of degree. Given the nature of soils vs. other engineering materials, all soils are problem soils; it’s just that some soils pose a greater problem to those of us who choose to build on, under or next to them than others, greatly so in some cases.

Most of the problem soils identified in the chapter are clays: loess, expansive soils, residual silts and clays, etc. Organic soils are included in this list, although the best way to deal with most of these is to avoid them altogether. Most of the descriptions of these soils is qualitative rather than quantitative, and that’s the weakness of the whole discussion. While general awareness of these soils and the challenges they create is useful, some quantitative description would have been useful.

An example of this is the determination of the growth of expansive soils with changing water content. Many of the studies of the volume change in these soils produce results that are either too specific or too difficult to readily implement either in an academic setting (where qualitative discussions of these soils abound) or in practice. Some of this is addressed in DM 7.1, but a more thorough approach would have been appreciated. I tackled this issue by presenting van der Merwe’s method in a more detailed fashion than usual; some more of this here would have been helpful.

The last part of the chapter deals with specialty construction methods in a page diagram reminiscent of the old driven pile diagrams from the previous edition. These are helpful because many proprietary methods don’t get the coverage in undergraduate texts that would be useful in the field. Fortunately in these cases specific references to more detailed descriptions–including design information–are given. The documents referred to are available on this site.

And a word of thanks…

I want to thank all of you who ordered the new DM 7.2 after its introduction last month. And that was quite a few of you: this has been the most successful publication launch of any book I have offered since I started doing this in 2006. Thank you so much for your support of our publications; it means a great deal for the continuation of this site.

And then there was another surprise last week in the video launching of the book from the Geo-Institute:

Notice the book cover on the splash screen? I had no idea they would do that. The first volume noted that it had been on the bookshelves of engineers for many years, and at this point the only way to acquire these books in print (AFAIK) is here.

NAVFAC DM 7.2: The Main Coordinators Present the Manual

Posted on 27 March 202527 March 2025 by Don Warrington

I’m taking a break of my own from my review of the new NAVFAC DM 7.2 manual to present the video released by the ASCE Geo-Institute on the manual from its main coordinators and authors, Dan VandenBerge of Tennessee Tech University and Mike McGuire of Lafayette College.

Their comments will definitely be helpful in my reviews in the coming weeks.

Deep Foundations, Geotechnical Engineering

NAVFAC DM 7.2: Overview and Prologue

Posted on 18 March 2025 by Don Warrington

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.

Deep Foundations, Geotechnical Engineering

It’s Official: NAVFAC DM 7.2 is Now in Print

Posted on 11 March 202511 March 2025 by Don Warrington

I’ve described this site as the “printed home of NAVFAC DM 7,” and that’s certainly been the case (along with the download home) for a long time. Now that we finally have NAVFAC DM 7.2 as replacement for the venerable NAVFAC DM 7.02, it’s time to announce that this is in print and available.

A brief table of contents of this book is as follows:

Prologue: Shear Strength for Geotechnical Design
Geotechnical Design in Problem Soils and Specialty Construction Methods
Excavations
Earthwork, Hydraulic and Underwater Fills
Analysis of Walls and Retaining Structures
Shallow Foundations
Deep Foundations
Probability and Reliability in Geotechniical Engineering

Thank you for your patience with this. In the coming weeks I’ll be doing a review of the various sections of the book. It’s an interesting and helpful revision to the work and I’m looking forward to digging into it.

The previous volume stated the following:

DM 7.1 has been on the bookshelf of many civil engineers, it has been used in many graduate and undergraduate soil mechanics classed attended by generations of geotechnical engineering students, and charts and correlations from the document have been cited in numerous textbooks and research papers.

The main reason it ended up on bookshelves is because we’ve spent the last fifteen years or so putting it out and you’ve been buying it. The “new” DM 7.1 has followed the trend; we hope you find DM 7.2 in print satisfactory as well.

Uncategorized

Analysing a Gravity Wall Using Vector Analysis

Posted on 6 January 2025 by Don Warrington

One of the reasons I was interested in teaching Statics at Lee University was because I was continually disappointed at my students’ memory of their statics. Statics is crucial in the design and analysis of geotechnical structures, and most of the problems–at the undergraduate level at least–aren’t that involved, or at least I thought they weren’t. A great deal of the problem is that geotechnical statics usually involves converting distributed loads into resultants, which Statics–and Mechanics of Materials for that matter–generally associate this with beam problems, not always the case with geotechnical problems.

Another culprit is that Statics, in the U.S. at least, is a vector proposition from the start. At the University of Tennessee at Chattanooga where I taught, it was called “Vector Statics,” which gives the game away early. (At Lee we use the same book and teach the same material, but simply title it “Statics.”)

But what if we applied a vector approach to a simple geotechnical problem? That’s what we’re going to do here with a concrete gravity wall. I will use the method outlined in my post A Simplified Method to Design Cantilever Gravity Walls. You can refer to the theory there, I will try to keep it to a minimum. The wall is pictured at the top of the post, I will reproduce it below.

Analysing Overturning

We have three forces acting on the wall:

The weight of the gravity wall itself, W_conc
The weight of the soil trapped by the heel of the wall, W_soil
The lateral force of the soil on the wall, F_h

Forces 1 and 2 are determined by computing the cross-sectional area of the concrete and soil and multiplying each by the unit weight as shown above, and then converting the result to a vector force and placing it at the centroid of the area (another Statics topic.) Instead of the “manual” approach in A Simplified Method to Design Cantilever Gravity Walls, the was was drawn in CAD and both the areas and centroids were determined automatically. You can see the magnitudes and locations of those resultants above.

The lateral force of the soil is computed using Rankine’s theory. The first thing is to determine the working internal friction angle of the soil by applying the Shear Mobilisation Factor SMF. Assuming an SMF of 2/3, that friction angle changes from the 30 degree one shown above to a 21.05 degree one, which is applied to the formula for Rankine active pressures for level backfill,

$k_h = \frac{1-sin\phi}{1+sin\phi}$ (1)

Doing that results in a k_h = .471. The force on the wall is then determined by the formula

$F_h = \frac{k_h\gamma_{soil}H^2}{2}$ (2)

The division by two reflects the fact that soil effective stress (and thus lateral earth pressure) increases linearly with depth (like a fluid,) creating a triangular distribution (yet another concept from Statics.)

At this point there the resisting forces R and T are not defined. The forces themselves are easily computed by summing forces in the x and y directions. Doing this, we have

$R = -W_{conc}-W_{soil}$ (3a)
$T = -F_{h}$ (3b)

The location of F–along the surface of the footing–is evident. The location of R is not; it is some distance x from the toe (Point “A”) of the footing. We can obtain x by summing moments around Point “A,” and with a vector method that means taking cross products of the moment arms with the forces.

Converting both the moment arms r and the forces to vector notation yields the following:

Concrete Weight: r = 2.358 i + 4.049 j, W_con = −4.35 j
Soil Weight: r = 5.576 i + 8.264 j, W_soil = −6.30 j
Lateral Earth Pressure: r = 8 i + 4 j, F_h = −4.073261616 i
Vertical Footing Force: r = x i, R = 10.65 j (Equation (3a))

The force T does not enter into this because its line of action runs through Point “A,” thus its moment is zero as its moment arm is zero.

The cross product moments around the toe (Point “A” in the drawing) are as follows:

Concrete Weight: $\left [\begin {array}{ccc} i&j&k\\{\medskip} 2.358& 4.049&0 \\{\medskip}0&- 4.35&0\end {array}\right ] = -10.257 k$
Soil Weight: $\left [\begin {array}{ccc} i&j&k\\{\medskip} 5.576& 8.264&0\\{\medskip}0&- 6.3&0\end {array}\right ] = -35.129k$
Lateral Earth Pressure: $\left [\begin {array}{ccc} i&j&k\\{\medskip}8&4&0\\{\medskip}- 4.073&0&0\end {array}\right ] = 16.293k$
Vertical Footing Force: $\left [\begin {array}{ccc} i&j&k\\{\medskip}x&0&0\\{\medskip}0& 10.65&0\end {array}\right ] = 10.65 x k$

Summing these moments,

$29.093\,k+ 10.65\,xk=0$ (4)

Solving yields x = 2.732′. At this point we need to determine whether this is an acceptable location or not for the force. The goal is for the pressure to be positive (downward) along the entire surface of the footing. There are two ways of determining this:

Computing the pressures on each end of the footing, as is done in A Simplified Method to Design Cantilever Gravity Walls.
Determining whether the force is in the “middle third” of the foundation, as is discussed in the Soils and Foundations Reference Manual.

We will do the latter. The middle third of this foundation falls between 2.67′ < x < 5.33′, so the vertical footing force is within the middle third (barely.) As I noted in A Simplified Method to Design Cantilever Gravity Walls, “In this case we make a common assumption that, as long as the resultant force of the wall is within the kern and there are no negative pressures on the base, overturning will not be experienced. It is certainly possible to do an explicit overturning analysis to check this result.”

Analysing Sliding

With the lack of keys or deep foundations, the only lateral resistance to sliding is the friction force T. We computed that force based on Equation (3b,) but in reality that force cannot exceed–and there should be a factor of safety in that inequality–the frictional force possible, which is defined by the equation

$T_{max} = \mu R$ (5)

in which case

$FS_{Sliding} = \frac{T_{max}}{T}$ (6)

Equation (5) is written in “mechanical engineers format.” Geotechnical engineers understand all too well the concept of a friction angle. In my post Explaining the Relationship Between the Coefficient and the Angle of Friction I relate the two from a non-geotech standpoint; we can turn Equation (5) into a more “geotech-friendly” form by noting that

$\mu = \tan\phi$ (7)

Let us assume that the value of $\phi$ is the same under the wall as next to the wall, and let us also assume that the friction angle between the base and the soil is the same as the friction angle of the soil overall, as was done in A Simplified Method to Design Cantilever Gravity Walls. That being the case, $\mu = \tan(30) = 0.577$ . Substituting into Equation (5,) $T_{max} = 0.577 \times 10.65 = 6.15\,kips$ . The factor of safety from Equation (6) is thus $FS_{Sliding} = \frac{6.15}{4.073} = 1.51$ , which is barely over the minimum criterion for usual loads given in A Simplified Method to Design Cantilever Gravity Walls.

Observations

The use of vectors for this problem is overkill from a computational standpoint. It also requires locating the centroid/CG of the two regions in both the x- and y-directions, although with using CAD this is trivial. On the other hand doing it using vectors is more “bullet proof” in that the student is not required to “think” but just “plug and chug” without having to identify lines of action and perpendicular moment arms.
The fact that the word “barely” appears in both analyses should inspire some additional conservatism in the design. The simplest way to improve the situation would be to move the heel to the right, which would shift the resisting forces away from the toe (and thus increase their resisting moment) and also put the footing force resultant deeper into the middle third.
Both bearing capacity and settlement of the wall’s foundation, the methodology for which are discussed in A Simplified Method to Design Cantilever Gravity Walls, are beyond the scope of this post. Also beyond the scope of this post is the structural design of the wall and of course the global stability of the wall as well.