Design charts for elastic pile shortening in the equivalent top–down load–settlement curve from a bidirectional load test

With a rigid pile assumption, the equivalent top–down load–settlement curve constructed from the results of a bidirectional or bottom–up pile load test does not fully consider the total elastic shortening when all the load components from skin-friction and end-bearing are applied downward at the pile head. In effect, the equivalent curve constructed by the original method suggested from the Osterberg test showed a much stiffer curve compared to the top–down curve. Design charts are provided in this paper from the results of a parametric study on bored piles in order to approximately evaluate the λ-factor that is used to estimate the top–down pile shortening from the bottom–up shortening due to the skin-friction component of the load. It has been shown that the λ-factor varies with the distribution of the undrained shear strength profile, pile slenderness ratio, and mobilization of the skin-friction resistance. In addition, the pile shortening due to the end-bearing load component is added to the top settlements by treating the pile as an elastic column. A modified method for the construction of the equivalent top–down load–settlement curve is presented that considers the elastic pile shortening and validated with the measured top–down load–settlement curve from pile load tests.