Multichannel Analysis of Surface Waves and Dam Safety

Geophysical methodologies and particularly the Multichannel Analysis of Surface Waves (MASW) effectively proved their efficiency in the non-destructive testing of the dams, in the last decade, after many successful applications worldwide. The MASW method developed in the outset of this decade considerably improved the prospects and the validity of these geophysical applications. Since MASW and the other geophysical techniques do not require drilling they progressively increased their popularity significantly. The Multichannel Analysis of Surface Waves can be applied for the assessment of both earthen and concrete dams. Nevertheless, mostly cases of earthen dams can be found in the literature. The method can detect and map low shear wave velocity areas potentially associated with low cohesion zones due to differential settlement events in the core or increased seepage. The advantage of MASW is that it is not influenced by the water saturation of the interior of the dam contrary to other methods eg. p-wave tomography. Usually, a joint application of MASW with the p-wave techniques can be an optimal choice since the two methodologies can act complementary. An application of MASW on a three-dimensional structure, such as a dam, however, can actually be considered as a complicated problem since the effects of the lateral structural anomalies can strongly affect the results. For example, in an earthen dam the investigation of the core can be influenced by the presence of the shells. Therefore, the problem should be carefully examined by modeling all these the lateral anomalies with the aim to avoid a misinterpretation of the results. The effectiveness of MASW to the dam safety assessment is presented through two example applications, one at the Mornos Dam, an earthen dam responsible for the water supply of Athens, and a second one at the Marathon Dam which is a concrete dam also used for the water supply of Athens. In the case of Mornos Dam, MASW detected areas affected by the differential settlement of the dam and more specifically by the arching phenomenon in the core. Such phenomena can lead to hydraulic fracture in the core and therefore should be carefully encountered. In particular the method detected, in the central part of the core, a weak zone just below the crest and another one at depth of 45 below crest, at a place where the monitoring instruments of the dam had measured very low effective stresses. This zone follows the shape pattern of the riverbed and at the edges of the crest it is connected with two other thick tensile zones. The example from the application in the concrete dam at Marathon is focused on the interior of the dam. The data acquired there, in the tunnels, were of different character than the ones gathered on usual applications on soil environment mainly due to the high and broad frequency content and the high phase velocity values. The standard data acquisition procedure also required some modification for the triggering and recording. The analysis of the data showed that such an application could provide useful results for the testing of the concrete quality.