Seasonal variations in the performance of driven piles in sand

This paper explores the performance of model pipe piles driven into a silica sand deposit at the Water Authority Research Station in Perth, Western Australia. The principal aim of the research was to explore pile response over a range of conditions from fully closed-ended to fully open-ended. There has been some considerable debate of the relative capacity of closed and open-ended piles driven into similar deposits of sand. In many design codes, it is customary to assume lower radial stresses (and hence shaft friction) for open-ended piles, than for solid or closed-ended piles. The base capacity is often taken as the same for both pile types, provided it can be demonstrated that the pile will ‘plug’ under static loading conditions. Potentially lower shaft capacity of open-ended piles is compensated for by the ease of installation since, in principle, the base resistance only has to be overcome around the steel annulus, as a plug of soil moves up the inside of the pile. However, in some circumstances open-ended piles may plug during driving, which leads to increased driving resistance and possible refusal before the design penetration has been achieved. This tendency may be alleviated by the use of an appropriate internal driving shoe, which provides stress relief for the soil plug.

The test results indicated a significant effect of season on the performance of the piles. The test programme included both dynamic and static testing of each pile, allowing a comparison between dynamic and static measures of capacity in tension and compression. Those tests carried out in the summer showed greater pile capacities in both tension and compression than those tests carried out in the winter. Equally the tests carried out in the summer showed a stiffer initial response than those carried out in the winter. The static test results were consistent when examined in conjunction with blow-records and the dynamic data. The test results indicated an optimum design for the driving shoe, which maximised soil plug penetration into the pile under hard driving. Of particular interest was the finding that the pile tip condition appeared to have little effect on the shaft capacity, but did influence the rate at which the end-bearing resistance was mobilised (due to compression of the internal soil plug). Thus, where the base resistance was defined at a fixed tip displacement, open-ended piles had a lower resistance than closed-ended piles. The test results are reviewed in the light of current design methods for driven piles in sand, and recommendations made for future design.