Compressive and Tensile Shaft Resistance of Nondisplacement Piles in Sand

This paper presents the results of a series of tensile-compressive (TC) and compressive-tensile (CT) static load tests performed on instrumented model piles preinstalled in silica sand samples prepared in a half-cylindrical calibration chamber with viewing windows along its symmetry plane. Images of the model pile and the surrounding soil were captured during axial loading through the observation windows using digital cameras and were subsequently analyzed using the digital image correlation (DIC) technique to generate the displacement and strain fields in the sand domain. The shaft resistances mobilized during the experiments were obtained using load cells placed at the head and the base of the model piles. The results obtained from the load tests revealed that reversal of loading direction substantially reduces the unit shaft resistance. This response was attributed to a drop in the radial strain exhibited by the soil elements surrounding the model pile shaft, which could be explained by a misalignment of the principal axes of the stress and fabric tensors resulting from load reversal. The results also indicated that the tensile-to-compressive shaft resistance ratio of fresh preinstalled piles is always nearly one.