Nondestructive Assessment of Axial Load–Deflection Behavior of Drilled Shafts for a Suspension Bridge

Abstract

The new Río Cuarto Bridge, currently under construction in the Province of Córdoba, Argentina, consists of a 110-m long, cable-stayed main span with a prestressed concrete deck, steel pylons, and two 50-m–long side spans founded on groups of drilled shafts. The construction method, structural configuration of the superstructure, and post-tensioning sequence of the cables required a detailed characterization of the axial load behavior of the drilled shafts, both for the temporary support shafts and the foundation piers. Small-strain and working load level predictions were made during design, on the basis of conventional site investigation information and in situ geophysical testing. A series of nondestructive evaluations, coupled with nonlinear extrapolations calibrated to represent the measured small-strain range, were carried out in lieu of conventional verification of design predictions by means of more cumbersome large-strain testing. The testing program consisted on monitoring accelerations generated at the top of the shaft as a result of a small amplitude dynamic load measured by means of a dynamic force transducer. A nonlinear numerical model was then calibrated so as to reproduce the initial stiffness measured during the small-strain testing program to extrapolate the load-deflection curve into the service load range and thus define load-deflection curves of the shafts at each pier location up to service load levels. To obtain an experimental validation of the approach at the site, a conventional static load test, carried up to the service load level, was performed on a main pier shaft. Results showed a reasonable agreement between the nondestructive evaluation with nonlinear extrapolation, large-strain measurements, and design predictions for the main pier shafts, whereas some differences were observed between the design predictions and small-strain measurements at other locations, primarily as a result of as-built conditions unforeseen in the original design. Thus, the nondestructive testing program was instrumental in the verification of the as-built behavior of the shafts and allowed the development of load-deflection curves for the drilled shafts that accurately represented the behavior up to the service load level.