EXPERIMENTAL INVESTIGATION ON EFFECT OF INITIAL PHREATIC LINE POSITION INSIDE EARTHEN EMBANKMENT ON HEADCUT MIGRATION DURING OVERTOPPING FLOW

Abstract

 Embankment failure due to overtopping flow takes place by headcut formation and its propagation toward upstream. Vertical, nearly vertical or cliff-shaped terrain called headcut forming at landside toe of the embankment is key to cohesive embankment failure during overflow. This steep terrain or headcut triggers further erosion and propagates upstream direction until crest of the embankment is reached and breached. In this study, experiments were conducted to understand the effect of the seepage state on initial position and migration of the headcut. For this purpose, three experiment cases firstly, no seepage into the embankment, secondly and thirdly with seepage and phreatic line exit point on downstream/landside slope at 1/3 and 1/6 of embankment height, h, from toe level respectively were conducted in laboratory. From the experimental results, it was found that the higher the exit point of phreatic line on the downstream slope, the higher the initial occurrence position of the headcut and higher the migration speed. When the experimental results were applied to a previous predictive formula regarding the headcut location, a large error occurred between the measured value and the predicted value for the case where initial phreatic line exit point was higher. In order to solve this problem, modification was made to address the effect of the seepage state in its predictive formula. As a result, for 1/6h permeation case, the predicted value for the position of the headcut almost agreed with the measured value. However, for 1/3h permeation case, it was confirmed that the predicted value of the headcut position was greatly underestimated compared to the measured one.