Drilled Shaft Construction and Some NDT Surprises at the Portal North Bridge-Video

Video Transcription

Video Summary

The presentation covered construction and quality testing of drilled shafts for the Portal North Bridge’s main river piers. Thirty-two 8-foot-diameter shafts and four 10-foot shafts were installed through soft clay, dense glacial till, and into bedrock, using permanent casing, polymer support fluid, and extensive construction QA/QC.<br /><br />Despite successful load testing and clean, well-documented shafts, crosshole sonic logging (CSL) produced poor signals in the upper 25–30 feet. Retesting showed improvement over time, while coring confirmed good concrete; the likely cause was debonding of access tubes, possibly worsened by cold river water and delayed hydration. Thermal integrity results were also inconclusive because temperature changes and shaft geometry distorted interpretations. Core handling problems later affected some strength results and led to potentially unnecessary concrete removal.<br /><br />The presenters emphasized that NDT methods have environmental and interpretive limitations, especially for river shafts, and should be selected and specified accordingly. Discussion raised water-filling CSL tubes and proposed industry guidance to clarify NDT practices and specifications.

Keywords

drilled shafts

construction quality testing

crosshole sonic logging

thermal integrity testing

non-destructive testing

Portal North Bridge