Behavior of Drilled Shafts with High-Strength Reinforcement and Casing

Abstract

Drilled shafts provide significant geotechnical resistance for support of highway bridges, and are used throughout the States of Oregon and Washington to meet their structural foundation requirements. Due to changes in construction methods and poor near-surface soils, the use of permanent steel casing for drilled shaft installation has increased. However, geotechnical design models for axial and lateral resistance of drilled shafts are largely based on soil-concrete interfaces, not soil-steel interfaces associated with large diameter steel casing. Owing to the increased understanding of our regional seismic hazards, the amount of steel reinforcement used in drilled shaft construction has increased over the past several decades, creating a new construction concern for engineers: the increased steel area results in a reduced clearance between adjacent reinforcement bars in the steel cage, such that concrete has an increased difficulty in penetrating the cage, increasing the likelihood for voids and defects within the shaft, which can lead to poor structural and geotechnical performance. The use of high-strength reinforcement steel can lead to increased clearance within the steel cage, mitigating concreting issues. The use of steel casing and the amount of steel area control the axial and lateral resistance of the shaft. However, depending on the method of construction, the steel casing may result in reduced axial load transfer to the surrounding soil. Thus existing analytical approaches need to be evaluated for modern construction methods, and new approaches developed if necessary to ensure desired performance criteria are met.