Construction of Large Drilled Shafts-Article

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The paper reviews construction considerations for large, deep drilled shafts used primarily in major bridge projects. These foundations can exceed 2.5–3 m in diameter and 60 m in depth, creating significant challenges in excavation, reinforcement placement, underwater concreting, and quality assurance.<br /><br />Excavation methods may include permanent casing, full-length temporary casing installed with oscillators or rotators, reverse-circulation drilling, and percussion tools such as chisels and hammergrabs. Permanent casing can improve constructability and may contribute to shaft capacity. Slurry selection and exposure time also matter, particularly where mineral slurry could affect side resistance.<br /><br />Geotechnical investigations should address construction needs as well as design parameters, including rock strength and characteristics, groundwater and artesian conditions, obstructions, and effects of construction sequencing on nearby structures. A Geotechnical Baseline Report can clarify expected conditions for bidding and help manage differing-site-condition disputes.<br /><br />Reinforcement cages should be designed for lifting, splicing, and concrete flow. Bundling bars, avoiding multiple cages, and using permanent steel liners structurally can reduce congestion and improve constructability.<br /><br />Underwater concrete placement requires careful control of the tremie, initial concrete delivery, and concrete levels during casing extraction to prevent water or slurry from breaching the concrete seal. Mixtures must remain workable throughout placement, flow through reinforcement, resist bleeding and segregation, and generate limited heat. Suitable aggregates, admixtures, and temperature control can help meet these requirements.<br /><br />The paper’s central recommendation is to treat drilled-shaft concrete as a project-specific high-performance material, with mix designs and construction procedures tailored to the shaft’s dimensions, reinforcement, ground conditions, and placement duration. Reliable results depend on integrating design, construction planning, geotechnical investigation, and quality assurance.

Keywords

drilled shafts

bridge foundations

deep excavation

temporary casing

reverse-circulation drilling

geotechnical investigation

reinforcement cages

tremie concrete

underwater concreting

quality assurance