Case History: Comparison of CSL Results to Physical Observations-Paper

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A 6.5-foot-diameter drilled shaft for a bridge pier showed significant Crosshole Sonic Logging (CSL) anomalies in the bottom four feet of its rock socket. Most tested tube pairs recorded substantial first-arrival-time delays and energy reductions. Follow-up coring confirmed a defect: two cores encountered little or no concrete and recovered sand, while a third did not. A camera inspection revealed a large void and exposed reinforcing steel.<br /><br />During remediation, a three-foot-diameter excavation through the shaft’s center did not expose the defect. Enlarging the excavation to four feet revealed a void extending around roughly 80% of the shaft circumference, outside the reinforcement cage and against the rock-socket wall. The observed defect closely matched the CSL results, demonstrating that CSL can detect defects located near the shaft perimeter—an area often considered difficult to assess. The case also shows that coring can confirm a defect yet miss its full extent or even fail to encounter it, depending on core location.<br /><br />The defect may have resulted from loose material at the shaft base or from the first concrete load mixing with water. The authors suggest that concrete flow during tremie placement could explain the defect’s peripheral location. They conclude that apparent CSL “false anomalies” may sometimes reflect real, peripheral poor-quality concrete that is difficult to verify by coring. However, CSL alone may not reliably establish a defect’s precise nature or extent; tomography may improve interpretation.

Keywords

Crosshole Sonic Logging

CSL anomalies

drilled shaft

bridge pier

rock socket defect

concrete void

coring confirmation

reinforcing steel exposure

tremie placement

tomography interpretation