Category: Civil Engineering

Now we get to another topic of intense interest: deep foundations. No topic in this book has advanced more than this one. When the original was published, driven piles were still the most common deep foundations. As much as we hate to admit it, that’s no longer the case.

But something else has happened along the way: most of the advances in the technology have been promoted and advanced (from a documentation standpoint at least) by the FHWA. Most of the chapter is a summary of those documents, and all of them (except for this one and helical piles, where a commercial book was referenced) are on this site. The summary is a reasonable one (and one which, hopefully, will inspire some textbook revisions) but there are a few points that need to be made.

Bearing Capacity vs. Settlement

Most engineering failure criteria in geotechnical engineering outside of lateral structures are based on what’s been traditionally called a “bearing capacity vs. settlement” paradigm. In current parlance (especially when considering LRFD, which is coming up) that referred to as “strength limit state vs. service limit state.” In NAVFAC DM 7.2: Shallow Foundations we saw both in evidence; which one predominated depended upon the configuration of the foundation and the nature of the soil.

NAVFAC DM 7.2 applies this paradigm to deep foundations as well. However, there is a “minority” school (Bengt Fellenius being its most vocal advocate) who believe that deep foundations basically don’t fail in bearing capacity but in excessive settlement. While structurally that may not be the case, geotechnically it’s hard to argue with this idea if one thinks about it long enough. Although, for example, classical bearing capacity equations have been applied to the pile toe, failure there really isn’t the same as shallow foundations due to the significant overburden. When we add the effects of shaft friction, and we look at the load-settlement curve we get out of a static load test (actual or simulated) we find that somewhere along the curve there is a “failure” point, determination of which depends upon the settlement limitations of the application and how we define “failure” along that curve (which is not univocal in geotechnical engineering.)

To get to the point where the ultimate load for a deep foundation is determined from predicted settlement, however, is going to take a major shift in how settlement is computed. NAVFAC DM 7.2 recognises the fact that the best way to estimate axial settlement is the t-z method and does not really offer a closed form, back of the envelope method to estimate them (for driven piles at least; drilled shafts get a different treatment.) The most straightforward method I’m aware of–Vesić’s Method of Estimating the Settlement of Driven Piles and Drilled Shafts–was in the previous book but has gone by the wayside. Further complicating things is the fact that many practitioners have used the bearing capacity/strength methods to estimate the ultimate resistances for the t-z method!

The situation we have on this topic is manifestly unsatisfactory but, until computer methods gain wider acceptance–and the wisdom in how to use them correctly–and we obtain more confidence, I suppose we’re stuck with the current paradigm.

Alpha and Beta Methods

This is another one of those “controversial topics” but NAVFAC DM 7.2 pretty much sticks with the current practice of alpha methods for clay soils and beta methods for sands. I’ve spent a great deal of time on this topic on this website in articles such as Shaft Friction for Driven Piles in Clay: Alpha or Beta Methods? To be fair, as is the case with the FHWA’s Soils and Foundations Reference Manual, Fellenius’ beta method for all types of soils is featured. I am more optimistic that this will be resolved in favour of the beta methods than I am with the settlement issue, but things move slowly in this business.

Lateral Loads and Settlements

For the last 30+ years it has been recognised that the p-y methods are the best for longer, laterally loaded piles. (An example of their application can be found in Driven Pile Design: Lateral Loads on Piles.) These, of course, require computer software, which these days is proprietary. An interesting development in the late 1990’s was the CLM 2.0 method, which featured a spreadsheet simplification for obtaining a solution. (I used it for many years in my teaching.) This study, however, shows shortcomings of the CLM method, and the authors of this part of NAVFAC DM 7.2 would have done well to consider this document in their deliberations.

Wave Mechanics

As someone who started out calling this site the “Wave Equation Page for Piling” this topic is of interest. Since this does require a computer solution (except perhaps for the Case Method,) the section on the subject is a good qualitative overview of the topic. In the wake of my Improved Methods for Forward and Inverse Solution of the Wave Equation for Piles I am seeing interest in advancing this technology, and am looking forward to overviews like this in the future.