NAVFAC DM 7.2: Analysis of Walls and Retaining Structures, Part II: The Rest of the Story

Now that we’ve taken a look at NAVFAC DM 7.2: Analysis of Walls and Retaining Structures, Part I: Will the Real Rankine Theory Please Stand Up?, in this post we’ll look at the rest of the chapter.

Water and Surface Loading Effects

Both of these topics get expanded–and welcome–coverage. Water pressure loads are important and needed the attention, especially for those of who teach–or have taught–these at the undergraduate level. With surface loading, a chart for rectangular loads has been included, similar in concept to the Fadum charts. The traditional Boussinesq (Flamant should be included, per Verruijt) with Terzaghi modifications, but these really need another look (the tests to confirm them date back to Spangler in the 1930’s.) Also extensively treated are compaction loads; although compaction adversely affects the permeability of the backfill, it is unavoidable in many cases.

Earthquake Loads

It’s a clear sign of the conservatism of the industry that, for all of the earthquake research that has taken place since the 1960’s, the method that NAVFAC DM 7.2 chose to feature is the Mononobe-Okabe method, which is a century old. I think the basic problem is that it converts a dynamic problem into a static one, which increases civil engineers’ comfort level with the method. A more thorough treatment of the method is here, but this is yet again another topic where, although NAVFAC DM 7.2 has chosen to reflect current practice, it’s time to more forward.

Rigid Retaining Walls

This is another topic that gets a nice upgrade (with much better graphics,) but I would have included the Corps of Engineers’ (this is an interservice document, after all) method for marine gravity walls.

Although I suspect that it was a political decision to include it, I think it’s time to ditch the Terzaghi “low walls” method of analysis. At the time it was a nice, quick method for engineers armed with slide rules to design walls, but given the computational power–and the relative simplicity of the problem–I think it’s time to move on from this too.

MSE Walls

These have advanced a great deal since the older document; however, the complexity of designing these walls inspired the authors (who had covered many of these issues in NAVFAC DM 7.1) to “punt” to the FHWA’s offerings on the subject. These can be found on our page Mechanically Stabilised Earth (MSE) Walls.

Sheet Piles and Other Flexible Retaining Structures

As the co-author/editor of Sheet Pile Design by Pile Buck, I need to point out that many advances have been made in the reference materials on sheet pile design since US Steel’s Steel Sheet Piling Design Manual. The principal author of US Steel’s manual was Harry Lindahl, who went on to author the Pile Buck Steel Sheet Pile Design Manual (an immediate successor to US Steel’s manual) and began work on Sheet Pile Design by Pile Buck, a work which was interrupted by his untimely death and which I had the privilege to finish.

With that out of the way, we can proceed as follows:

The treatment of anchored walls includes the introduction of what the Corps refers to as a shear mobilisation factor (SMF,) but which NAVFAC DM 7.2 refers to as a safety factor. The two are, strictly speaking, not the same. The SMF is featured in A Simplified Method to Design Cantilever Gravity Walls. The factor of safety in sheet piling design is generally a straight reduction of the passive earth pressure coefficient, which would include both frictional and cohesive portions of the soil strength (the Corps’ method includes that but it isn’t included in NAVFAC DM 7.2.)
Rowe’s Moment Reduction method gets overhauled graphics, but it should be pointed out that it is necessitated by the reality of soil-structure interaction (SSI,) which is better handled by a software solution. A simple implementation of this can be found in Analyzing Sheet Pile Walls with SPW 2006 (although I don’t recommend it for commercial use, it’s good to see how such software works and for academic use.) The problem with software like this–and this is common with many numerical methods–is that the results of the software given the same data are not exactly the same with different software packages and numerical methods. Software like SPW911 (more about that in a moment) are replications of “closed form” methods which should give the same results following the same procedures, but do not take some of the complexities of actual application into account. That leads to the procedure itself, with issues such as…
I am not sure why NAVFAC DM 7.2 is averse to the use of the “simplified method” for cantilever walls, the conventional method is retained as normative. This has been in SPW911 for many years and used elsewhere with success, but there are American practitioners which have stuck with the conventional method.
The chart solutions given in NAVFAC DM 7.2 are a carryover from the old book; except for academic use, I don’t think they’re very useful. OTOH I don’t see regular design of sheet pile walls without software with some hand checks; as the anchored bulkhead design scenarios show, the calculations can become very complex very quickly. That’s true whether you use “hand solution automations” such as SPW911 or more sophisticated software.
The braced cuts coverage gives more modern research into the whole issue of lateral pressure distribution but does not address some of the structural issues which I discuss in my post Getting to the Bottom of Terzaghi and Peck’s Lateral Earth Pressures for Braced Cuts.
Cellular cofferdams get a broad overview, but the complexity of the topics prohibits much more than that. We deal with this in Sheet Pile Design by Pile Buck.

Overall the treatment of these walls is an improvement over what we had before. Some of the issues are the function of the editorial decisions of the authors, but many are issues in the sheet pile community that themselves have not been resolved, as was the case with the lateral earth pressures.