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Testing Seismic Techniques for Imaging Structural Defects in Engineered Drilled Shafts – A Case Study

There is an urgent need for more reliable, yet quick and economical techniques for assessing structural integrity and unknown depth of bridge foundation elements. This paper describes seismic investigation of drilled shafts at two test sites - one in sand, and one in clay soil - at the National Geotechnical Experimentation Site (NGES) near the campus of Texas A&M University. Constructed in 1990 for NDT investigation, the drilled shafts were of different lengths and containing planted or otherwise known construction-related defects, and likely unknown structural anomalies. The investigation compared the results of UltraSeismic Profiling (USP), and Shaft Reflector Tracing (SRT - using migration imaging). Seismic waves were introduced at the above-ground surface portion of each shaft by two synchronized seismic swept-frequency sources applied at different orientations to enhance different wave modes. A hammer strike was used for comparison. Three components of seismic waves were recorded by an array of accelerometers mounted on surveyed shaft surface, and by a string of hydrophones placed in a hole cored down the center of the shaft. The major findings of the investigation were: 1) different techniques and different wave modes appear to emphasize different structural defects; 2) a swept frequency source provided superior results as compared to non-repeatable impulsive hammer strike; 3) steel rebar cage appears to influence the survey results; and, 4) tube waves from water-filled cored holes appear useful for detecting structural anomalies intersecting these holes.



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