Assessment and Mitigation of Crosshole Sonic Logging Anomalies in Large Diameter Bored Piles

When the results of integrity testing of pile foundations identify anomalies in the readings, assessment by the designer is required in order to determine if the geotechnical and/or structural performance of the pile is compromised and if remediation measures are required. This paper presents a case history where cross hole sonic logging (CSL) was used on large diameter rock socketed bored piles constructed with tremie concrete methods and where significant anomalies were identified with CSL and then subsequently investigated by excavating around the pile to the anomaly level with significant inclusions within the concrete being observed. The potential for common methods for investigating CSL anomalies to accept piles with defects is discussed. The risks to the performance of bored pile foundations through incorrect identification of the defects of this nature is illustrated through results from a fully instrumented load test pile exhibiting similar CSL anomalies where structural failure observed at a load significantly lower than the design capacity. The time dependant nature of CSL data and discussion on when to undertake these tests after concreting a pile is also included and illustrated through a series of repeat tests 1, 7, 10, 50 and 80 days after casting.