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K. Vandenboer and  L. Dolphen and  A. Bezuijen,  Backward Erosion Piping through Vertically Layered Sands.   journal   = {International Journal of Environmental, Chemical, Ecological, Geological and Geophysical Engineering}, [online]. World Academy of Science, Engineering and Technology. September 2017, vol. 129(9). 740 - 743[viewed 21 November 2017]. Available from: http://waset.org/publications/10007839.