Underwater Concrete in Drilled Shafts: The Key Issues and Case Histories-PDF

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Underwater concrete placement in drilled shafts is technically demanding, but high-quality results are achievable with suitable mix design, reliable logistics, careful placement, and rigorous quality control. Failures commonly arise from cement washout, segregation, trapped water or debris, poor consolidation, and loss of concrete workability.<br /><br />Underwater concrete must be flowable enough to pass through reinforcing cages and self-compact, while remaining cohesive enough to resist segregation and washout. It must also retain these properties throughout delivery and placement and achieve adequate in-place strength. Mix proportions, aggregate size and grading, cementitious materials, and admixtures all affect performance. Clear spacing between reinforcing bars should be generous enough to allow concrete to flow freely.<br /><br />Placement should be continuous and controlled. Tremie placement generally presents fewer risks than pumping because gravity feed allows better control of flow, whereas pump pressure can disturb placed concrete or cause segregation. Planning must account for concrete supply, transport time, production capacity, and potential delays. Quality control should include checking shaft cleanliness, tremie integrity and seal, concrete workability, placement rate, tremie embedment, concrete levels, and the relationship between delivered and measured concrete volumes. Non-destructive testing can help verify shaft integrity.<br /><br />Two case histories illustrate common problems. In a California railway project, many shafts developed voids and grooves around reinforcing bars because the concrete lost flowability before placement was complete. Field testing revealed rapid slump loss, and the mix lacked measures to maintain workability. In a bridge project, withdrawing a temporary casing suddenly enlarged the concrete pour, allowing concrete to mix with trapped slurry, soil, and debris. About 87 percent of shafts required extensive remediation.<br /><br />The cases show that mix performance must be verified under actual field conditions and that changes in shaft geometry or casing procedures require special planning. Maintaining a positive concrete head, withdrawing casings slowly through vulnerable zones, and using a highly flowable, cohesive mix can reduce defects, delays, and cost overruns.
Keywords
underwater concrete
drilled shafts
tremie placement
mix design
segregation resistance
cement washout
quality control
workability
temporary casing
shaft integrity