Post-installation residual stresses in preformed piles jacked into granular soils

The working performance of a preformed pile may be significantly affected by the residual pile stress generated in the pile installation process. A field-test program is conducted in this study to investigate the load transfer behavior of two instrumented steel H-piles jacked into granular deposits during installation. Significant post-installation residual stresses are found to be locked in the test piles. The depth of neutral point for the residual shaft friction increases with increasing pile penetration but their ratios tend to be an invariable. Besides the well-known elasticity coefficient, the factors affecting the residual point resistance include the jacking force and the penetration rate. The average residual negative friction accumulates as the number of jacking strokes increases. Nevertheless, the unit residual negative friction at a given horizon shows a trend of degradation, which can be attributed to the presence of friction fatigue during pile installation. The residual pile stress is responsible for the variation in the stress state of the soils surrounding the pile shaft and beneath the pile base prior to loading service. This indicates that the pile-soil system will follow an unanticipated stress path toward axial loading failure. The data interpretation methods routinely adopted in the static and O-cell loading tests are going to be challenged in case that the influence of post-installation residual stress is involved. Also, the post-installation residual stress imposes positive effect on the uplift capacity of pile and increases the complexity of the plugging behavior of open-ended pipe piles. Assuming that the distribution of residual shaft friction resembles a folded line, a semi-empirical framework is proposed to estimate the residual pile stress at any depth from the knowledge of residual point resistance.