Response of drilled shafts with minor flaws to axial and lateral loads

摘要

The use of drilled shafts as foundations for bridges and other structures has greatly increased in recent years due to the advent of routine non-destructive evaluation (NDE) methods. The availability of the NDE methods has given engineers and contractors the false impression that any defects that may have been produced during construction due to problems in concreting, drilling, casing, slurry and rebar cage placement can be identified and repaired before the bridge is opened to traffic. In reality, there is a lower limit on the size of defects that can be detected by current NDE techniques, which recent studies have identified it to be roughly between 10 and 20 percent of the cross-sectional area of the shaft. In this study, eleven 1/3-scale concrete shafts with minor void flaws were tested in the lab to determine the effects of minor void flaws on the stiffness, structural capacity, mode of failure, cracking pattern and ductility of concrete shafts, without the confining soil. Three different loadings were considered for the shafts: (a) pure flexure, (b) pure axial compression, and (c) combined axial compression and flexure. The size of the void was taken equal to 15 percent of the cross-sectional area of the shaft. The shape of the void resembled a wedge with the outer arc length and length of the void along the centerline of the shaft being both variables. The results of the lab tests showed that void flaws that penetrate the concrete core of a shaft are more critical than those that are located within the concrete cover. Further, the presence of a void affects the ductility of a shaft subjected to axial compression much more than it affects the axial strength. Computational methods that are based on strain compatibility and force equilibrium can predict the strength of a defective shaft with reasonable accuracy. Ultimately, the study will be helpful in developing rationally based structural resistance factors for drilled shafts that account for possible presence of minor void flaws.