Comments on “3D FE analysis of bored pile- pile cap interaction in sandy soils under axial compression- parametric study”

As always I was gratified to be cited in the recent paper “3D FE analysis of bored pile- pile cap interaction in sandy soils under axial compression- parametric study,” by Faisal I. Shalabi, Mohammad U. Saleem, Hisham J. Qureshi, Md Arifuzzaman, Kaffayatullah Khan, and Muhammad M. Rahman. It is an interesting study of the topic at hand. Some comments are in order:

• Although the citation is of Closed Form Solution of the Wave Equation for Piles, the work Improved Methods for Forward and Inverse Solution of the Wave Equation for Piles is really closer to the methodology shown in the paper, both in terms of the 3D FEA used (well, I took a shortcut and used axisymmetric 2D analysis) and to the use of Mohr-Coulomb theory for the analysis, which I discuss in An Overview of Mohr-Coulomb Failure Theory and Elasto-Perfect Plasticity with Mohr-Coulomb Failure. Mohr-Coulomb is still viable for many applications, especially with sands.
• One especially interesting aspect of this study was the inclusion of a pile cap. The problem is similar to the the one I discuss in my post When Semi-Infinite Spaces Aren’t, and When Foundations are Neither Rigid Nor Flexible, where the foundation is neither perfectly flexible relative to the soil nor perfectly rigid. Although in this study the foundation rigidity is not varied, the soil’s is, and as is the case in elastic theory as the soil becomes less rigid the relative rigidity of the foundation increases, the soil stresses relative to the foundation towards the edge of the foundation likewise increase. This Fall I plan to include that elastic theory in my discussion of mat foundations here: Foundation Design and Analysis: Shallow Foundations, Other Topics.
• I noted a drop in the shaft friction just before the toe, followed by an increase down to the toe itself. The interaction between pile, soil and shaft friction for deep foundations is a complicated one. The toe creates failure surfaces in the soil that are certainly there–and it is reasonable to assume that they affect the shaft friction near the toe as well–but they are not exactly like those generated in shallow foundations, something which has complicated toe resistance calculations for a long time. The relative uniformity of the unit toe resistance makes sense based on failure theories going back to at least Vesić’s work in the early 1970’s. One thing that bored piles do not have to consider is the effects of advancement due to impact which, as Mark Randolph’s work has shown, almost show a “leading edge” effect.

It is my opinion that 3D FEA will ultimately be our best tool for estimating the load/settlement characteristics of deep foundations–bored or driven, static or dynamic–and this paper is a step forward in that regard.