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The effect of cut-off wall angle on seepage and uplift pressure under dams

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conference contribution
posted on 13.05.2021, 08:57 by Martin Angelov, Alireza Ahangar Asr
Seepage under dams can result in high uplift pressure experienced usually under the downstream of the dam which may lead to instability and potentially the failure of the dam. Cut-off walls are the primary solution to minimise the effects caused by the flow of water, as they extend the flow path which results in decreasing seepage, as well as uplift pressure and exit gradient. (Rice & Duncan, 2010) This study aims to research how designing a cut-off wall at an angle under a concrete dam increases its efficiency. The angles will vary from 0° to 180°, progressively increasing at an interval of 30°. Following that, two walls will be designed originating from the same point at the nose of the dam, as well as another design where the walls are under both ends of the structure. The modal analysis will be completed using the finite element geotechnical software – PLAXIS 2D, as it has proved to be extremely precise in discharge analysis specifically (Galavi, 2010). A typical impervious concrete dam will be designed in the program in a uniform soil profile with the only variable being the angle of the cut-off wall and its originating point. This is done to put the attention simply on the discharge value that is obtained from the software. The feasibility of the construction will always be taken into account, as this study intends to complete research by taking a realistic and economic approach. The results show that seepage is smallest when the cut-off wall is positioned at a 60° angle, as in this position water has to travel the furthest distance compared to the other configurations. In terms of uplift pressure, the dam experienced the least pressure when the wall is at 120°. As seepage and uplift pressure are the two main factors playing significant roles in the stability of dams – two combined configurations were introduced into the model in order to minimise both values; one being the addition of the 60° and 120° angle walls originating at both ends of the dam, and the other where the two formerly mentioned walls originated from the same point (the heel of the dam). The configuration where the two walls are constructed under the heel and the toe show a significant decrease in seepage, as well as uplift pressure at the toe. When the two walls are both constructed originating from the same point (heel) – seepage is determined to be lower compared to when only one wall is considered, however a significant reduction in uplift pressure is not present. When taking all data into account, the most feasible, economical and practical solution to decreasing seepage and uplift pressure considerably appears to be the arrangement where a 60° cut-off wall is constructed at the heel of the dam and 120 ° wall at the toe of the dam.

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