The effect of taper on pile shaft bearing capacity : Finite element analysis of a tapered wooden pile in sulfate soil

Abstract: he usage of piles, especially wooden piles, have been going on for thousands of years in construction and foundation reinforcements. Sometimes this was done temporary like the Romans who used wooden piles to construct bridges in Britain, or more permanently like the piles upon which the floating city of Venice stands. The knowledge of wooden piles is old, tested and reliable, even in soft soils, but it is not complete. Wooden piles are made of trees, and trees are natural growing, not made by humans for our construction needs. This has caused trees to be varying in structural properties, and these variations are undesirable in something that needs to be as predictable as engineering. So, we found a middle ground where we could look past these variations, among them the shape of the trees such as the taper. The piles were assumed homogenous and un-tapered. Perhaps a mistake. As the world population grows our need for material increases with it and we need to be more effective with what we have. Perhaps the tapering can help us with that.  Attempts has been made to evaluate how the tapering of piles effects the stability, capacity, and settlements, but often these have been done for a very limited number of scenarios and conditions, or the tapering effect is barely treated as a footnote in whatever geotechnical method is considered. Most of the time the tapering is simply ignored. An advantage of the passage of time is the evolution of tools which can be used to help understand what was earlier not. The usage of computers and the use of the finite-element-method calculation programs like PLAXIS has made complicated, or difficult-to-execute, problems much more solvable, even without the usage of actual piles and equipment. Thus, the tapering of wooden piles could be isolated and directly tested for many different angles and the effect of it be revealed.  The theoretical tests were chosen to be located at SGI’s testing site in northern Sweden at Lampen in sulfate soil, a problematic low bearing capacity soil. None of the methods except the PLAXIS are intended for tapering wooden in such conditions and the results became polarizing either claiming tapering is definitely good or definitely bad. The FEM analysis in PLAXIS would show that the taper would not have a great effect on the bearing capacity, as long as the taper is not too high. The settlements however would be a balancing act between the penetration forces of the pile, and the build-up consolidation happening at the pile toe. An optimal tapering was thus found to be at 13 or 14 mm of diameter taper per meter of pile length, which is slightly more than the natural tapering of pine trees in southern Sweden and a bit more than pine trees in Northern Sweden. Regardless there is more settlement stability to be relied upon in the tapering of piles without losing geotechnical bearing capacity. That is, until these results can be proven true or false by actual laboratory tests.