Efficacy of Marine Electrical Resistivity Surveys to Determine Maximum Scour Depth and Scour Infill

Abstract

The maximum depth of bridge scour typically occurs during peak flood conditions when it is too dangerous to collect measurements. Often, the maximum depth is obscured by additional redeposited sediment. Applied geophysical methods are commonly used to detect the depth of scour infill and estimate maximum scour depth. With all geophysical methods, efficacy depends on site conditions and, therefore, no single method is a universal tool for assessing scour. Electrical resistivity is a near-surface geophysical measurement for geotechnical site investigations, but there has been limited use of electrical resistivity for assessing bridge scour. Previous electrical resistivity measurements have evaluated soils for their erodibility and have been used to determine the depth of unknown foundations (information required to assess scour vulnerability), as such there is the potential to further extend the method to assess bridge scour directly. Marine electrical resistivity surveys were conducted at a bridge south of San Marcos, Texas, U.S. A 0.7 m change in the riverbed surface was noted around a foundation and the marine electrical resistivity image showed a scour hole that, through post processing, extended an additional 0.5 m beneath the surface in front of the column and 0.8 m behind, filled in with loose gravel. As-built plans and grab soil samples were used to validate the resistivity interpretations. If adopted by engineers, marine electrical resistivity methods can be used to assess the bridge scour and infill, either while using electrical resistivity for other bridge assessment purposes or as another tool in a bridge inspection program.