Volume XV Issue 4
Earthen embankments are constructed in the practical field on the basis of design procedure against shear failure analysis. The slope of earthen embankments is generally adopted based on the hand calculation based Swedish Slip Circle method. Failure of many earthen dams indicates the inadequacy of the existing hand calculation based design procedure. The hand calculation based Swedish Slip Circle method only predicts the safe slope of earthen embankments against shear failure but could not recognise overall shear behaviour within the dam body. Some studies show that the results obtained by hand calculation based gravity method may differ greatly both in magnitude and location to what is obtained by finite element based analysis. It is found by finite element modelling that for the original existing embankment a tension zone occurs within the dam body which may cause a longitudinal crack at the crest. In this context, the present paper attempts to highlight the issues related to shear failure with the example section of the original earthen embankment surrounding the Ash Pond of Bakreswar thermal power project, West Bengal, corresponding to the different depths of water at the upstream side.
The study is extended further with some modified sections to see the effect of such modified sections on shear analysis of the earthen dams.
To draw the conclusions, the failure line is drawn based on the value of apparent cohesion and the angle of shearing resistance for a large number of nodes within the body of the dam. Mohr’s circle is then drawn from the principal stresses obtained by finite element analysis for all these points. If this failure line intersects the corresponding Mohr’s circle then failure occurs. A factor of safety of 1.5 is considered to identify the safe zone within the section of the dam. The possibility of shear failure within the dam section is also parallely studied by conventional slip circle procedure corresponding to the different water depths at the upstream side of the embankment. The present study shows that the dam section which may appear to be safe by slip-circle is actually unsafe regarding the occurrence of local shear cracks within the dam body as observed from finite element based analysis.
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