Simulation of complete geobag revetment failure processes
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Geobag revetments have recently emerged as long-term riverbank protection measures in developing countries, however, their performance is still not well understood. According to previous research by Heriot-Watt University and The University of Edinburgh, the initial failure mechanisms associated with simple geobag riverbank revetments are now relatively well understood and numerical modelling has advanced to the stage where incipient failure can be simulated using Discrete Element Modelling (DEM). However, to develop the type of robust design standards needed to improve the effectiveness and durability of geobag installations, the essential next step is to develop numerical techniques to efficiently simulate the complete failure of geobag structures. In order to improve our understanding of geobag–water flow interactions and gather the data required to calibrate and validate the numerical model, a comprehensive programme of small-scale experimental tests was undertaken. Comparison of a range of different construction methods and revetment side slopes subjected to different flow loading was carried out. The results indicate that whilst failure mechanisms are highly dependent on water depth and revetment slope, the construction method had no noticeable impact and it was concluded that the dominating factor is the friction between individual geobags, which itself is dependent on bag overlap rather than specific construction method. Furthermore, flow velocity measurements taken during both the pre-failure and post-failure stages indicated that the formation of failure zones leads to a decrease in turbulence, and a subsequent stabilization of the failure process. In the second part of the research a Discrete Element Method (DEM) model was constructed using the LIGGGHTS open source software with drag and lift models applied to a multi sphere simulation of the laboratory model geobags. The validated DEM model could reproduce very well the complete failure processes of the geobag revetment, mounted on a fixed bed and also on a mobile sediment bed. Finally, it is found that the DEM model could provide more details on the performance of geobag revetment in riverbanks. Based on the results found, it can be concluded that the developed DEM model can satisfactorily simulate the complete failure of geobag revetments and hence be the basis for the development of future deign guides. Finally, recommendations on the application of DEM model for design guidelines of geobag revetment are outlined.