Estimation of Shaft-Base Initial Stiffness and Ultimate Resistance of O-Cell Piles

Abstract

Pile foundations are often used to transfer axial loads through shaft and base into deep soil strata in which the pile–soil response is nonlinear and difficult to predict. The testing of large bored piles by using conventional kentledge is difficult and uneconomical. Instead of testing with Osterberg (O)-cell is being used in recent times, in which the load application is bidirectional. The upward load application is against the gravity and is resisted only by the shaft and in the downward direction is resisted by both shaft and base resistances for O-cell placed at some depth from top. In this study, a simple approach is proposed to analyze the load versus displacement curves obtained for both upward and downward responses. Ultimate resistances and initial stiffnesses of shaft and base are estimated from the downward load–movement curve and ultimate resistance and initial stiffness for the shaft response only is obtained from the upward load versus movement curve by Chin’s method assuming nonlinear hyperbolic responses for both. The proposed method is applied to a few available O-cell test data. Initial shaft stiffnesses, ultimate shaft resistances and percentage variations for the two sets of results for both directions were obtained and compared. This study helps the designers to predict the O-cell pile responses better and validates the initial design.KeywordsInitial stiffnessNonlinear hyperbolic responseO-cellPile–soil responseUltimate resistance