Effect of Base Geometry on the Resistance of Model Piles in Sand

Abstract

Because only limited research has been done on base geometry effects on pile base resistance, design methods used in practice for full-displacement, partial-displacement, and nondisplacement piles in sand do not consider these effects on base resistance calculations. In this paper, the effect of base geometry on pile resistance is studied by performing a series of tests in a half-cylindrical calibration chamber that allows observation of the sand domain through the chamber symmetry plane during installation and loading. Two model piles, one with a flat base and one with a conical base with an apex angle of 60°, were installed and load-tested in dense and medium-dense sand samples. Digital images taken during the loading of the model piles were processed using the digital image correlation (DIC) technique. Detailed displacement and strain fields obtained with the DIC technique and base resistance measurements demonstrate that the addition of a conical base to a pile changes the deformation pattern in the sand below the base during loading. Furthermore, the base resistances of both the jacked and preinstalled model piles with a conical base were less than those for the piles with a flat base for the same conditions, with the corresponding ratio ranging from 0.64 to 0.78 at relative displacement levels less than or equal to 10% of the diameter B of the model pile and from 0.70 to 0.84 at greater relative displacements. The results have implications also for the use of cone resistance from the cone penetration test (CPT) in the pile base resistance estimation.