Numerical modelling of slope formation with sand mining=Numerieke modellering taludvorming bij zandwinning

It is of great interest to control the shape of the slope that results during under water dredging of sandy soils. In sand mining steep slopes are usually desirable from economical point of view. The steeper the slope, however, the larger the risk of slope instability. Sometimes large, undesirable bank breaches occur, notwithstanding careful preparation and execution. Often the cause can be well explained in qualitative sense, if enough information about soil and dredging process is available. A good quantitative prediction, however, is not yet possible. Presently two types of calculation models are available to predict the shape of the slope and its stability: on the one hand models mainly based on hydrodynamic-mophological knowledge, on the other hand geotechnical models. Coupling between both types of models is not yet possible, whereas both hydrodynamic-morphological phenomena and geotechnical phenomena are relevant in case of the largest bank breaches. This problem has been discussed in a Delft Cluster meeting with experts from government, contractors, consultants, research institutes and universities. It was concluded that integration of both types of models would be desirable. This report focusses on this integration. The existing hydrodynamic-morphological models are briefly described. It is subsequently indicated how these models can be extended with geotechnical models in such a way that a prediction can be made whether a large bank breach occurs and how large it will be. The report concludes that integration of hydrodynamic and geotechnical knowledge offers great perspectives to further improve the present consultancy practise with respect to dredging in sand. A relative limited effort seems sufficient to extend an existing hydrodynamic-morphological calculation model with geotechnical models in such a way as to make it suitable to quantify the sometimes very large bank breach - that occurs if the suction pipe of a stationary suction dredger penetrates too quick or too deep - that occurs if a cutter dredger creates a too steep and deep slope: - that may occur in case of dredging in loose sand (liquefaction flow slide); - that results from a liquefaction flow slide introduced by a natural cause; - that may occur in case of dredging in sand with thin clay layers.