Μελέτη της μακροχρόνιας παραμόρφωσης του φράγματος Κρεμαστών με βάση ανάλυση γεωδαιτικών δεδομένων και μεταβολών στάθμης ταμιευτήρα

Abstract

Monitoring has proved to be crucial for the safety of dams. Geodetic methods play an important role on this. The aim of geodetic monitoring is the detection of any change on the dam geometry as well as any displacements of its foundations. Despite the fact that dams are systematically monitored for the last decades, long-term monitoring records and their analyses are extremely rare in the literature. The majority of available data cover only the period of the first filling or a few years later (< 10 years). The aim of this study was the analysis of the geodetic monitoring record of Kremasta Dam, one of the highest earthfill dams in Europe (160m high and 456m long) for the first time after its construction. The available data cover a period of > 35 years (1966 – 2003) and consist of the horizontal deflections and vertical displacements of 25 control stations established on the crest and the body of the dam relative to reference points on stable ground as well as the reservoir level fluctuations and the rainfall height at the dam area. Maximum horizontal deflection was equal to 30cm while maximum vertical displacements were up to 77cm. The accuracy of the data was found to be better than 1mm for horizontal deflections and 1.3mm for vertical displacements. Analysis in the time domain revealed that displacements can be described by a 4th degree polynomial and have a tendency of stabilization. The amplitude of displacements is normal compared to the age of the dam. On the other hand, a part of the downstream slope was found to move systematically upstream. This phenomenon is possibly due to leakage that can be up to 200lt/sec. Spectral analysis using Fourier Transforms and Lomb Periodogram was applied in order to investigate the effects of reservoir level fluctuations and rainfall on the behaviour of Kremasta dam. A dominant period of 1 year was found present in reservoir level and rainfall timeseries while no periodicity was detected in the values of displacements. Time-frequency analysis using Weighted Wavelet –Z Transforms revealed that there is a relationship between some of the peaks of the obtained spectrogram and specific events e.g. the operation of spillways in 1996. A high-pass filter in combination with linear correlation method was applied in order to define the critical values for (1) reservoir level elevation, (2) rate of reservoir level fluctuations and (3) rainfall rate above which the settlement rate of the crest increases significantly. These thresholds are equal to 270m, 1.3m/month and 130mm/month respectively. Statistical analysis of the crest settlements of > 40 earthfill dams (up to 30 years old) with central clay core revealed that crest settlements of up to 0.8 – 1% of dam height can be considered to be normal. In case of Kremasta Dam this percentage is up to 0.48% which is within safety limits. Kremasta Dam is > 40 years old and thus the continuation of monitoring its displacements is suggested in order to ensure the dam’s safety. Geodesy Laboratory of Patras University and Public Power Corporation are working on the design of a new geodetic monitoring system of Kremasta Dam based on modern instruments like GPS. Results of this study, the first study of the long-term behaviour of Kremasta Dam, could also be used in the development of a 3-D model for prediction of the dam’s displacements and the behaviour of the clay core under high water levels.