9-Foot (2.75m) Diameter Drilled Shafts at Cranston Viaduct: Design, Load Testing, and Construction

Analytical and empirical design methods are available to determine side shear and tip resistances of large diameter drilled shafts in cohesionless soils. The replacement of Cranston Viaduct in Rhode Island provided a good opportunity to test these design methods and compare it with a full-scale load test in the field. The foundation support for the replacement Viaduct consists of nine-foot (2.75m) diameter drilled shafts. As required by the Federal Highway Administration design guidelines, nine-foot (2.75m) diameter trial shaft was drilled at the site to establish the proper construction procedures and to verify design assumptions. The construction of the trial shaft and the production shafts for the southbound of the viaduct has already been completed. The paper will present a description of the unique design and construction methods adopted for the foundations. In addition, the paper will present the results of the Osterberg load test performed on the nine-foot diameter trial shaft and will compare side shear resistance and end bearing values obtained from the test to the values computed during design. The design equations for drilled shafts in cohesionless soils, adopted by the Federal Highway Administration and AASHTO, proved to provide a good design tool in this project and compared very well with actual field measurements.