Experimental Study on Failure Mechanisms of Composite Bank in the Jingjiang Reach
In order to explore the erosion mechanisms of composite riverbanks,a series of flume experiments about the erosion process of Upper Jingjiang Reach (U JR) and Lower Jingjiang Reach (L JR) were conducted using prototype soils of Jingjiang Reach.Through analyzing the experimental results,bank erosion process and mechanism,the deposition,decomposition and transport of the failed block were analyzed.Firstly,the results indicate that planar sliding failure represents the most frequent bank erosion in UJR flume experiment,which the vertical tension cracks will appear firstly on the top of bank and then the failed block will slide along a slip failure plane.Secondly,the main bank failure types are shear failure,beam failure and tensile failure in LJR flume experiment.The mechanical conditions of cantilever failure are that the width of overhanging block is bigger than the critical value and the weight moment of overhanging block is greater than the resistance moment of cohesive layer,which produce rotational motion around the neutral axis toward the direction of river channel.Therefore,the mechanical equilibrium principle of overhanging cohesive layer is the erosion mechanism of bank failure in the LJR.Finally,based on the results of experiment,most failure block of cohesive soil upper bank may deposit on the toe of the bank.They will remain until be broken down or entrained by the flow.The failed block,in turn,may temporarily protect the toe of the bank from erosion.The deposition form of failed block is a triangle,which the slope is approximately equal to the underwater stable slope of bank or underwater repose angle of sediment.The deposition volume of failed block is mainly related to the collapsed volume and the ratio between them is from 0.38 to 0.74,which can provide a basis of actual calculation.