BREACH FORMATION ON NON-COHESIVE HOMOGENEOUS EMBANKMENT EXPOSED TO THROUGH AND OVER FLOWS

摘要

This paper analyzes the breach formation mechanism and stability of embankment dam made up of non-cohesive material subjected to through and overtopping flow using small scaled physical model in a rectangular flume. For the purpose of attenuating the lack of knowledge about the breaching mechanism on rock fill dam, this paper concentrated mostly on what happen on the downstream part of the embankment when embankment dam is subjected to through flow and/or overtopping flow. Homogeneous small scaled embankments were constructed in flumes using two different types of uniform non-cohesive materials. This study has investigated systematically the embankment models made up of non cohesive material by varying dam height from 0.3, 0.45 to 0.6 m, mean grain size from 5.5 to 10.2 mm and embankment slope angle of 1:1.5, 1:2 and 1:3 in a 0.61 m wide and 0.75 m high flume. These slope angles have been selected based on the typical dimensions of the existing embankment dams in the world. Two fundamental different failure modes have been observed dominating the failure mechanism which is erosion/scour and mass sliding. The safety factor for the rockfill dam against overtopping and or through flow can be determine by assessing the safety factor against mass sliding and scouring mechanism, the observed main failure mechanisms. The effect of changing material type, dam geometry and construction methods (compaction) on the breaching process and final breach widths were investigated. The present findings describe embankment erosion as evolves from primarily vertical to predominantly lateral in nature controlled by three identified discharges causing initial particle movement, provoking first mass sliding and discharges that induces the beginning of the breaching process. The experimental results also describe the breach final shape, the ratio between breach parameters. The plans of the breach longitudinal profile confirmed the hourglass shape where analysis of the observed breach profile demonstrated that the breach inlet flow operate in a similar manner as in a minimum energy loss structure. The findings can be utilized for prediction of erosion and flooding occurring as result of embankment failure. In this study the dam height has been observed to have very high influence on the stability. Design equation has been proposed which incorporated the embankment dam height parameter without considering the effect of slope for the tested range of slopes.