Resistivity investigation and monitoring for detection of internal erosion and anomalous seepage in embankment dams

Sammanfattning: Methods for monitoring seepage and detecting internal erosion are essential for the safety evaluation of embankment dams. Internal erosion is one of the major reasons for embankment dam failures, and there are several tenths of thousands of large embankment dams in the world. Internal erosion is progressing inside the dam, and is sometimes difficult to detect by conventional methods. Therefore, there is a strong need of new or improved detection methods. The resistivity method is an established method with a variety of engineering and environmental applications, but it is only occasionally tested on embankment dams. When applied for dam safety examination, the method can principally be used in two ways. Firstly, resistivity investigations at single occasions may detect spatially anomalous zones along the dam, and can be used to investigate suspected structural weaknesses. Secondly, long-term resistivity monitoring make use of the seepage-induced seasonal variation inside the embankment to detect anomalies not only in space, but also more importantly anomalies in time, by studying deviations from the time-variation pattern. Within this thesis the resistivity method has been used in practice on four Scandinavian locations, both in the form of investigation and monitoring. At Enemossen tailings dam a single investigation was carried out. At Hällby and Sädva dams an ongoing program with permanently installed monitoring systems has acquired daily data since 1996 and 2001 respectively. In addition, the method was tested under short-term monitoring conditions on a test embankment with built-in structural defects at Røssvatn. Apart from the field measurements, numerical modelling has been done in order to optimise measurements and support interpretation of the results. The experience from the field measurements demonstrates that monitoring is a more powerful approach than single investigations. The seasonal resistivity variation is evident and under favourable circumstances it may be used to evaluate the seepage. A systematic methodology has been developed for efficient monitoring and evaluation of long-term resistivity data. The monitoring instrumentation has largely proved reliable. Proper electrode installations are essential. Installing the electrodes along the top of the dam core is efficient on existing dams as it creates a current channelling effect inside the conductive dam core, which is the part of the dam with highest interest. Further development and refinement is needed to make the method more easily adaptable to the large variety of embankment dams and thereby more attractive for industrial use. Some ideas of future work are proposed. However, the efficiency of the method has been developing rapidly recently, and significant progress has been done within this thesis concerning the use of the method for dam monitoring. It is concluded that the application of the resistivity method for detection of anomalous seepage and internal erosion in embankment dams is obvious.