Comparative analysis of static uplift pile load tests under different loading conditions

For the static uplift pile load tests, the ideal tension condition is that only the tensile load is applied on the pile head. However, in the conventional uplift tests the reaction system can influence the soil surrounding piles. Also, the Osterberg cell test is also different from the ideal pushing condition where only the compressive force is applied on the pile toe, because the Osterber cell can influence the soil surrounding pile via contact soil or reaction pile beneath the pile toe. In this paper, the field pile load tests and FEM simulation are conducted to investigate the pile load tests under different loading conditions. It is found that the capacity of pile in the conventional uplift tests is closed to that in the Osterberg cell test, while the stiffness in the former is higher than that in the later. The Osterberg cell test, the ideal pushing test and ideal uplift test give similar results, which are regarded to be consistent with the characteristics of load transfer and settlement performance of real single tension piles. Therefore, the conventional uplift tests overestimate the stiffness of pile. It is due to the reaction system which transfers the reaction load to the soil surrounding the upper part of pile. On the one hand, the skin friction and the stiffness of the upper part of pile are increased. On the other hand, the reaction load increases the pile-soil relative displacement. For a given settlement of pile head, the practical tension tests can mobilize larger skin friction than the ideal tension tests and hence increases the stiffness of pile as well.