A NUMERICAL ASSESSMENT OF THE USE OF SURFACE NUCLEAR MAGNETIC RESONANCE TO DETECT INTERNAL EROSION AND PIPING IN EARTHEN EMBANKMENTS

Abstract

The United States Army Corps of Engineers’ National Inventory of Dams US Army Corps of Engineers (2012) currently (2012) identifies 26,642 dams with either high or significant hazard potential, representing over 30% of the structures in the database. Almost all of the dams in the database are of earthen construction, very few have design documents, and most are over half a century old. Levees present a similar hazard, as many were originally built by farmers in the early 1900's to keep their river-aligned farms from flooding. There are many failure modes of dams and levees: internal erosion and piping, overtopping, undercutting, and dry-side erosion. Of these failure modes, internal erosion and piping is one of the most difficult to detect, as it occurs within the structure and cannot be directly observed. Geophysical methods provide a means to assess the internal integrity of these structures. However, geophysical methods that rely on indirect relationships between material properties and water content could be unreliable in this application. For instance, there may not be a large electrical resistivity change associated with internal erosion events in every dam. Construction material and internal geometry of these structures varies greatly, and is usually unknown. One geophysical method that could directly detect the effects of internal erosion in earthen embankments under saturated conditions is surface NMR (sNMR). The method is unique in its ability to directly detect water, and also estimate the pore size and hydraulic permeability of media. For these reasons sNMR could be an invaluable tool for non-invasively detecting internal erosion in dams and levees. We present sNMR and subsurface seepage forward-modelling results from several earthen embankment scenarios. We assess the deployment of coincident and separated sNMR transmitter and receivers under changing piping conditions and static magnetic field orientations. The significant topography of the earthen embankment requires special care. Under favourable field conditions, sNMR can provide valuable information in the assessment of earthen embankment internal erosion.