Small and large strain assessment of axial load deflection behavior of as-built drilled shafts for a cable-stayed bridge

FEDERICO PINTO; CARLOS GERBAUDO; CARLOS A. PRATO

Ecole Centrale de Nantes, Nantes, Francia

Congreso; NDTCE?09, Non-Destructive Testing in Civil Engineering; 2009

Laboratoire Central des Ponts et Chaussées

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 prestressed concrete deck, steel pylons, and two 50-m long side spans founded on groups of drilled shafts. The construction method and structural configuration are such that an accurate characterization of the foundation movements under service loads was warranted at the design stage. In order to determine the axial load-deflection behavior of the as-built drilled shafts, a pile load testing program was developed. The test program consisted of both small strain dynamic measurements and a large strain (static) load test carried slightly above service load level. Small strain tests consisted on monitoring accelerations generated at the top of the shaft due to a small amplitude dynamic load, measured by means of a dynamic force transducer. Measurements were repeated several times for different impacts and a coherency function was determined by means of spectral analysis in order to identify the frequency range where results showed high signal to noise ratios. Results from small strain tests were used in order to calibrate the initial stiffness of a nonlinear numerical model capable of extrapolating the load deflection curve into the service load range. Design estimates and results derived by means of the nonlinear model were compared with deflections measured during the static load test performed at a main pier location. Results showed a favorable comparison with design predictions, as well as the accuracy of the nonlinear extrapolation of low-strain results in order to predict deflections at the service load level. By means of comparing measured initial stiffness vs. design predictions, small strain tests were also instrumental in verifying the as-built behavior of the shafts, as well as the integrity of these structural elements.