Assessment and mitigation of Crosshole Sonic Logging Anomalies in Large Diameter Bored Piles-Paper

Paper

Pdf Summary

The paper examines crosshole sonic logging (CSL) anomalies in large-diameter, rock-socketed bored piles built for a river bridge. Anomalies—often near pile toes or at depths associated with interruptions or changes in tremie concrete placement—raised concerns about concrete quality, rock-socket bond, pile stiffness, and load redistribution across the foundations.<br /><br />The designers assessed the risk by modelling affected piles as having reduced stiffness and capacity, then iteratively checking how loads redistributed through each pile group. Of 85 original piles, 41 were down-rated and 13 were treated as having no effective rock socket. This analysis showed that only five additional piles were needed to manage the potential foundation risk, avoiding more extensive investigation and repairs.<br /><br />Excavation of a pile with a shallow CSL anomaly revealed significant concrete inclusions around the reinforcement and in the cover zone. The authors warn that coring could recover sound concrete from the pile core and miss such defects, potentially leading to acceptance of a structurally compromised pile. In a separate instrumented load test, a pile with a Class B anomaly cracked and bent at a load well below its calculated capacity; the defect may have severely reduced reinforcement effectiveness and bending resistance.<br /><br />Repeated CSL tests also showed that readings and classifications can change with pile age. First-arrival times generally decreased as concrete cured, while a Class C anomaly in one test later appeared less severe. The authors recommend considering curing time and temperature when scheduling tests; on this project, results generally stabilised after 12–14 days.<br /><br />Overall, CSL anomalies should be treated as evidence of potential defects, not judged by numerical thresholds alone. Interpretation should incorporate construction and concreting records, and investigation methods such as coring should be used cautiously. Rigorous control of tremie placement and debris at the pile base is also essential.

Keywords

crosshole sonic logging

CSL anomalies

rock-socketed bored piles

pile foundation risk

tremie concrete placement

load redistribution

pile stiffness

pile toe defects

concrete inclusions

pile load testing