Combined geophysical surveys for the characterization of a reconstructed river embankment

Abstract

The managing and monitoring of natural and artificial river levees are crucial in order to reduce the hydrological risk. As these hydraulic structures are very extensive, the typically applied techniques (e.g. geotechnical soundings and visual inspections) provide only punctual information that are generally focused on areas already recognized as troublesome. To overcome these well-known issues, non-invasive and cost-effective geophysical measurements have been proposed (and used) to supply spatially extensive data that should be integrated with direct investigations. Therefore, in this paper we present the joint use of multichannel analysis of surface waves (MASW), electrical resistivity tomography (ERT), and ground penetrating radar (GPR) to characterize a reconstructed river embankment (made of concrete and tout-venant) prone to serious leakages. We compare common lengthwise ERT profiles, performed from the levee crest, with cross-embankment and cross-river profiles, showing how relying only on the first type of surveys may lead to misinterpretations. Furthermore, we take advantage of a land-streamer for the MASW surveys and of a trans-illumination approach to improve the GPR application. The comparison between geophysical and geotechnical data helps identify the spatial extent of the volume actually invaded by grouting injections, which appears as a highly electrically conductive mean. The lack of homogeneity within this domain, highlighted by the cross-embankment ERT profiles, can be related to the seepage phenomena affecting this artificial levee. Therefore, this case study demonstrates the effectiveness of combining direct and non-invasive investigations for the characterization of river embankments.