Monitoring of tailings dams with geophysical methods
Licentiate thesis 2006:07, Luleå University of Technology
Several dam failure accidents have occurred during the last years and mine tailings dam failures are occurring at relatively high rates. Studies of past earth dam failures show embankment dam problems and failures are often related to internal erosion in one way or another. Geophysical methods have the potential of detecting internal erosion processes and anomalous seepage at an early stage of their development. The methods have been tested to monitor and investigate earth dams; however the methods have not been used very much in mine tailings dam.
The present study has been conducted to test the applicability of geophysical methods, mainly electrical resistivity and self-potential (SP), for detecting anomalous seepage through mine tailing dams and monitoring the physical condition of the dam. Field measurements of resistivity and self-potential have been performed in the Kiruna, Aitik and Kristineberg tailings dams to look for streaming potentials, inhomogeneities and time variations of electrical properties and self-potentials. SP and resistivity measurements have also been carried out with fixed electrodes in the Kiruna and Kristineberg dam at a roughly monthly interval during one year starting in November 2003 and ending in October 2004. Laboratory measurements of resistivity have been carried out on different soil samples from the tailings dams to look for eventual changes in electrical properties with change in grain size and water content.
The electric resistivity survey in the Kristineberg provides a good image of the subsurface resistivity distribution associated with filling materials and water table in the dam. The results of the electrical resistivity survey from 2004 on the Kristineberg tailings dam are fairly similar to those obtained from in 2003. The SP distribution in the dam also reveals that there are no significant changes in SP values from 2003 to 2004. The resistivity from the fixed electrodes indicates a seasonal variation in the apparent resistivity representing the freezing and thawing effect within in the dam. The SP measurements from the fixed electrode at the Kristineberg dam, shows fairly stable values during summer and more unstable during the winter probably due to change in contact resistance.
The result from the 2002 SP measurements in the Kiruna dam reveals a general pattern of positive SP values at the downstream side, which is in agreement with the expected result of streaming potentials developed over the dam core. The dam was raised during the summer 2003 and new SP measurements were repeated thereafter during the autumns of 2003 and 2004 in the same areas as for the 2002 measurements. The results from 2003 measurements deviate from 2002 measurements; with in general, more negative potentials along the downstream slope. The potential distribution obtained from the 2004 measurements is compatible with the results obtained before the raising of the dam. The SP data from the fixed electrode shows unsteady physical conditions within the dam after increasing the height of the dam. The apparent resistivity from fixed electrode survey is much influenced by the variations of the pool level of the tailings pond.
Some positive SP anomalies on the downstream slope of the IJ-dam at Aitik have been identified that could be related to the seepage through the dam. A distinct positive anomaly at the coordinate 7451330 north that continues to the toe of the downstream slope of the dam is generated from a known seepage of the water.
The laboratory measurements on soil samples from the dams reveal a decrease in resistivity as finer particles are added to the samples that contained coarser fractions. Internal erosion may thus be reflected by increase in resistivity.
This work has demonstrated the potential of using resistivity and self-potential methods for monitoring the physical condition, and the time changes in the condition of mine tailings dams.