Construction of Large Diameter Drilled Shafts: The 2nd Mike O’Neill Lecture-PDF

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The paper reviews construction challenges and reliability measures for large, deep drilled shafts, increasingly used for bridges and other major structures. Shafts may reach 4 m (13 ft) in diameter and 80 m (260 ft) deep, requiring specialized excavation, careful site investigation, constructible reinforcement, and controlled concrete placement.<br /><br />Excavation may require permanent or full-length temporary casing, installed with vibratory hammers, oscillators, or rotators. Reverse-circulation drilling can improve productivity at depth, while chisels and hammergrabs help remove boulders and hard rock. Geotechnical investigations should address not only design properties but also rock strength, groundwater and artesian pressures, obstructions, excavation stability, and construction sequencing. Geotechnical Baseline Reports can clarify bidding assumptions and reduce disputes over unexpected conditions.<br /><br />Reinforcing cages must be designed to ease lifting, splicing, and concrete flow. Bundled bars, adequate openings, and avoiding multiple cages can reduce congestion. Permanent steel casing may serve as structural reinforcement, improving flexural strength and confinement while reducing the need for dense transverse reinforcement.<br /><br />Underwater concrete placement requires careful control of the tremie’s initial charge and concrete levels during casing extraction to prevent water or slurry from entering the concrete. Concrete must remain workable throughout placement, pass through reinforcement, resist bleeding and segregation, and generate limited heat. Suitable mixtures may use rounded aggregate, increased sand content, water-reducing and viscosity-modifying admixtures, and supplementary cementitious materials. Placement temperature and hydration must also be managed, particularly for large pours.<br /><br />The paper’s central recommendation is to develop project-specific concrete mixtures and construction plans that address site conditions, placement duration, and shaft geometry. Close coordination among designers, contractors, and quality-assurance teams is essential to reduce construction risk and ensure shaft integrity.

Keywords

deep drilled shafts

construction challenges

bridge foundations

geotechnical investigation

permanent casing

reverse-circulation drilling

reinforcing cage design

tremie concrete placement

underwater concrete

quality assurance