Resistivity monitoring for internal erosion detection at Hallby embankment dam

Internal erosion is one of the major reasons for embankment dam failures. Despite this, the knowledge of the temporal development of internal erosion in full scale structures is limited. Results from a research field monitoring program by daily resistivity measurements in Sweden clearly show long term changes in resistivity in the embankment dam. The results indicate that resistivity monitoring can detect development of internal erosion at an early stage, as resistivity variation is caused by changes in the material as well as changes in the seepage flow. The method can be used where intrusive investigations are normally avoided. Embankment dams are generally constructed with a low permeable core, one or more surrounding filters and support fill. In practise, an electrode layout along the dam crest, or preferably electrodes buried in the top of the core, is the most sensible installation for resistivity monitoring on existing dams. It is important to pass the current through the conductive core, which is the main target of investigation. Moreover, the installation must be done in a way that allows high accuracy measurements. Monitoring equipment as well as data handling and monitoring routines must also be able to handle the small resistivity variations that occur in the dam. If so, methods of analysing and comparing the seasonal resistivity variation schemes in the reservoir and inside the dam body are capable of identifying areas with anomalous seepage, and in some cases also internal erosion. Hallby was the first Swedish embankment to get a permanently installed monitoring system intended for resistivity measurements. Daily measurements started to take place already in late 1996. This paper includes examples of long term time series from Hallby along with some evaluation and interpretation techniques used when analysing such monitoring data. Increasing resistivity and larger variations have been noticed in a particular zone in the left embankment.