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

Paper

Pdf Summary

The paper documents construction and quality assurance for 32 large-diameter drilled shafts supporting the four main river piers of New Jersey’s Portal North Bridge. Each shaft was 8 feet in diameter, up to 160 feet long, permanently cased into competent bedrock, and extended into a rock socket. A vibration-free oscillator installed the casing, avoiding recorded settlement of the adjacent century-old bridge. A demonstration shaft load test exceeded design resistance targets and supported the construction approach.<br /><br />Construction verification included sonic caliper measurements, rock-socket video inspection, and Shaft Inspection Device (SID) checks of base cleanliness. Post-construction testing used Crosshole Sonic Logging (CSL), tomography, and Thermal Integrity Profiling (TIP). Results were frequently inconclusive near the tops of river shafts. The authors attributed CSL signal loss in the upper 2–5 feet mainly to access-tube debonding, likely worsened by cold river water. Because tomography relies on CSL data, it could not resolve anomalies caused by debonding. TIP interpretations were also unreliable: flowing water, tidal exposure, soil and rock properties, and shaft geometry affected heat loss, producing inaccurate estimates of shaft radius and concrete cover.<br /><br />Coring of 13 shafts produced visually sound concrete and generally confirmed that anomalous CSL results did not indicate defects. However, some Pier 1E cores showed low strengths that the authors considered suspect because of additional handling, grinding, and inadequate end preparation. The results illustrate that coring, too, can be compromised by sampling and laboratory procedures.<br /><br />The authors recommend selecting tests for the specific construction conditions and recognizing each method’s limitations, rather than applying redundant testing to every production shaft. More extensive verification may be most valuable on test shafts or where construction methods change; shallow coring can help investigate upper-shaft CSL anomalies. They also caution that QA programs can add substantial cost and time without necessarily improving acceptance decisions.

Keywords

drilled shafts

construction quality assurance

Portal North Bridge

Crosshole Sonic Logging

Thermal Integrity Profiling

rock socket

sonic caliper

shaft inspection device

coring

bridge pier foundations