K. Vandenboer and  L. Dolphen and  A. Bezuijen,  Backward Erosion Piping through Vertically Layered Sands.   journal   = {International Journal of Geotechnical and Geological Engineering}, [online]. World Academy of Science, Engineering and Technology.
    July 2017, vol. 129(9). 766 - 769
    [viewed 25 April 2024]. Available from: https://publications.waset.org/pdf/10007839.