SEEPAGE AND STRAIN MONITORING IN EMBANKMENT DAMS USING DISTRIBUTED SENSING IN OPTICAL FIBRES-THEORETICAL BACKGROUND AND EXPERIENCES FROM SOME INSTALLATIONS IN SWEDEN

摘要

Temperature measurement is probably the most sensitive geophysical method to detect and quantify seepage flow. The best application of the method is long term monitoring, where slow and small seepage flow changes can be detected. The sensibility is in the order of some 10-5m3/(s,m) for normal dams, but may be lower or higher depending on where the temperature is measured and local conditions. Temperature measurements in embankment dams started initially in Germany in 1957. Further improvements of the evaluation methodology were made in Sweden during the 1990-ties, when the fundamentals of thermal processes were studied. However, the acceptance and a general use of the method have been slow, but today it is getting more and more accepted. It has during the last five years been used as a standard tool at embankment dam investigations and monitoring in Sweden, and also in Germany. Until today, HydroResearch and Sensornet have installed optical fibres for monitoring of seepage and/or movements in more than 20 dams, dykes and tailing dams at 16 different dam sites in Sweden. Different installation techniques have been developed depending on the dam. These include embedding of the fibre inside the dam (at new constructions), in the crest during raising of a dam, within new toe berms at upgrading work, just downstream of the dam toe or vertically in standpipes. The experiences from field the measurements show that optical fibres installed along a dam toe will provide possibilities to both locate anomalous seepage areas and to estimate the seepage flow after short time investigations (normally some days). Longer measurements or repeated measurements will improve the detection capability and the accuracy of the seepage flow evaluations, and are therefore recommended.