A New Non-Destructive Method to Measure Scour Depth and the Depth of Unknown Foundations

E. J. Mercado, North American Geotechnical Co.
M. W. O'Neill, Dept. of Civil and Environmental Engineering, University of Houston

A seismic technique based on refraction wave technology has successfully detected both the thickness of a mud-filled scour zone and the length of a model pier.

A 24 inch diameter, 17 feet long model pier was constructed in a pit that could be filled and emptied with water. Adjacent to the pier (24 inches away) a 4 inch diameter, 17 foot deep hole was drilled and lined with PVC pipe. A refraction wave was generated in the pier by striking the top of the pier with a hammer. The refraction wave was recorded at one foot intervals in the PVC pipe for both sources. The pit was emptied and a 4 foot deep scour zone was hand-dug around the pier, then filled with soft mud typical of the area (Gulf Coast sediments). The refraction experiment was repeated, and the data analyzed.

The recorded refraction data clearly shows the presence of the mud-filled scour zone because the mud acts as major acoustic energy adsorber compared to energy transmission through competent soil. By analysis of the received energy as a function of depth, the bottom of the scour zone is determined with an accuracy of one foot.

The refraction wave changes to a diffraction mode of travel geometry below the bottom of the pier. By analyzing the first arrival times of the seismic energy as a function of depth, the change from refraction to diffraction path geometry is readily detected and determines the depth to the bottom of the pier to one foot.