Seepage Monitoring in Embankment Dams

Thesis

Internal erosion, which is one of the major reasons for embankment dam failure, causes an increased seepage flow due to loss of fines. A seepage measuring system is therefore a vital part of an embankment dam’s monitoring system. Many existing seepage monitoring systems are not however sensitive enough to detect small changes in the seepage flow. Temperature and resistivity measurements represent two methods for seepage monitoring in embankment dams. They are able to detect effects caused by time dependent processes such as internal erosion, where the relative accuracy is more important than the absolute accuracy. Temperature can normally be easily measured in existing standpipes. Resistivity measurements are more complicated; they require a computer-based monitoring system and minor technical installations on the dam. The temperature in an embankment dam depends mainly on the temperature in the air and the water temperature in the upstream reservoir. These two temperatures vary seasonally and create temperature waves propagating through the dam. The seepage rate, and its change with time, can be evaluated from measurements repeated at regular intervals. The sensitivity of the method depends mainly on the distance between the dam crest and the measurement point, the size of the dam, the location of the standpipes, and the temperature variation in the reservoir at the inflow level. The seepage detection level of the method is about 1 ml/sm2 for a typical Swedish dam with a height of about 30 m. The detection level depends linearly on the dam height. Results from field measurements show that the method gives reasonable information concerning the condition of the dam. Zones with anomalous seepage rates have been located and seepage flow rates have been quantified. Changes in the seepage flow rate as well as the seepage pathway have also been observed. The resistivity of the ground depends mainly on the porosity, saturation and clay content. When reservoir water seeps through a dam, the pr operties of the water in the reservoir will also affect the resistivity in the dam. The resistivity of the reservoir water is temperature dependent, but it is also a function of the total dissolved solids. Both these parameters vary seasonally and this causes variations in the dam. The seepage flow can be evaluated from the resistivity data using methods similar to those employed for seepage evaluation from temperature data. The sensitivity is similar to that of the temperature method but the resolution and accuracy is lower. Zones with anomalous leakage can be located. Ground penetrating radar and borehole radar methods are based on the measurement of material dependent properties. These are less sensitive to seepage changes than flow dependent parameters. The relatively high accuracy obtained by borehole radar measurements compensates however for their lower sensitivity to porosity changes. Borehole radar based on tomographic analysis can be a valuable method for mapping areas with increased and anomalous porosity formed as a consequence of increased seepage and internal erosion.