Laboratory Evaluation of a Proposed Nondestructive Testing System for Detecting Anomalies below Drilled Shaft Excavations-Paper

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The study evaluated a prototype nondestructive testing (NDT) system designed to detect weak zones and soil seams beneath drilled-shaft excavations, particularly in karst and weathered bedrock. Conventional inspection methods, such as coring and probing, can be invasive, time-consuming, subjective, and unable to provide a continuous picture of conditions below the shaft base.<br /><br />The system uses seismic P- and S-waves generated and recorded at the excavation base. Reflections from material boundaries are processed to estimate the location and extent of anomalies. For laboratory proof-of-concept, researchers built a cemented-sand model representing bedrock with vertical, inclined, and horizontal soil seams. They collected common-offset reflection data along multiple survey lines and produced two-dimensional images, with a three-dimensional rendering assembled from the sections.<br /><br />The images generally identified the constructed seam geometries, especially the vertical seams. S-wave results had a higher signal-to-noise ratio and often estimated seam geometry more accurately than P-wave results. P-waves tended to overestimate the depth of the shallowest seam, likely due to transducer near-field effects and overlapping reflections. The deepest horizontal seam was only weakly detected, and some unexplained reflections may have resulted from model boundaries or construction irregularities.<br /><br />The findings demonstrate that seismic reflection imaging is a promising approach for characterizing materials beneath drilled-shaft bases. However, field use will require further development to improve detection at depth and account for more complex conditions. Suggested improvements include lower-frequency, higher-energy sources; more sensitive receivers; alternative sensor arrangements and survey methods; and enhanced processing. The study establishes laboratory proof-of-concept, not yet a validated field-ready system.

Keywords

nondestructive testing

seismic reflection imaging

drilled-shaft excavation

karst detection

soil seams

P-waves

S-waves

subsurface anomaly detection

weathered bedrock

laboratory proof-of-concept