Compression testing and analysis of drilled concrete tapered piles in cohesive-frictional soil

When pile driving is difficult and (or) economically not viable, cast-in-place piles are indispensable. Tapered piles, which have top cross-sections larger than the bottom cross-sections, have the potential for substantial advantages over conventional straight-sided piles. This paper investigates the construction and performance of innovative drilled concrete tapered piles. A full-scale pile load-testing program was conducted to evaluate the axial compressive capacity of drilled concrete tapered piles in frictional soil. One straight and three tapered augers were designed and manufactured to produce six piles. The piles that were constructed and tested included four tapered and two straight piles. The testing results showed that tapered piles with a taper angle varying between 0.95 degrees and 1.91 degrees had a load carrying capacity up to 50% higher than the straight-sided piles with equal volume. It should be noted, however, that the experimental results are site specific. Moreover, an analytical expression was developed to evaluate the pile taper effect on its shaft capacity in terms of a taper coefficient, K-t. The value of K-t evaluated from the analytical expression compared well with experimentally measured values. Finally, a two dimensional nonlinear finite element analysis was conducted to simulate the load test conditions, and it proved to be successful.