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Summary The phenomena related to the suction of sand were studied by means of small-scale tests. The tests clearly showed that the quantity of sand that is dredged depends mainly on soil characteristics as grain size, permeability and relative density. The influence of grain size and permeability could be characterized by a typical feature: the ”wall velocity”. It is possible to relate the translation velocity of the suction tube, the angle of repose of the material, the slope of the suction pit, this ”wall velocity” and the quantity of sand that is flowing towards the suction tube. If the translation velocity of the suction tube is equal to the ”wall velocity”, the slope of the suction pit is vertical and the production of the sand pit is maximal. During the process steep slopes are maintained. The reason for the phenomenon is the property of shear-dilatancy of the sand. Due to the approach of the steep sides of the pit shear stresses increase which cause a volume increase of the sand if the initial density was relatively high. This volume increase causes an inflow of water and a decrease in pore pressure. The decrease in pore pressure increases the effective normal stress and therefore the resistance against failure of the soil. The capacity of the suction pit by the process described here is not sufficient in many cases. The dredge operator then tries to bring the soil to failure by undermining the slopes. After this failure the soil is loosely packed so that it can be dredged more easily. This method increases, of course, the risks of the operation.
Summary The phenomena related to the suction of sand were studied by means of small-scale tests. The tests clearly showed that the quantity of sand that is dredged depends mainly on soil characteristics as grain size, permeability and relative density. The influence of grain size and permeability could be characterized by a typical feature: the ”wall velocity”. It is possible to relate the translation velocity of the suction tube, the angle of repose of the material, the slope of the suction pit, this ”wall velocity” and the quantity of sand that is flowing towards the suction tube. If the translation velocity of the suction tube is equal to the ”wall velocity”, the slope of the suction pit is vertical and the production of the sand pit is maximal. During the process steep slopes are maintained. The reason for the phenomenon is the property of shear-dilatancy of the sand. Due to the approach of the steep sides of the pit shear stresses increase which cause a volume increase of the sand if the initial density was relatively high. This volume increase causes an inflow of water and a decrease in pore pressure. The decrease in pore pressure increases the effective normal stress and therefore the resistance against failure of the soil. The capacity of the suction pit by the process described here is not sufficient in many cases. The dredge operator then tries to bring the soil to failure by undermining the slopes. After this failure the soil is loosely packed so that it can be dredged more easily. This method increases, of course, the risks of the operation.
Suction of sand
Breusers, H. N. C. (author)
1974
Article (Journal)
English
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