Suction of sand

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.

Résumé

Les phénomènes concernant la succion de sable ont été étudiés au moyen de tests en petite échelle. Les résultats ont montrés que la quantité de sable que l’on obtient depend pricipalement de la dimension des grains, de la perméabilité et de la densité relative. L’influence de la dimension des grains et de la perméabilité a pu être caractérisé par un phénomène typique: la ”vitesse de paroi”.

C’est possible de relater cette ”vitesse de paroi” et les autres phénomènes qui se produisent pendant le processus de succion. Ainsi on a pu déterminer quelles conditions doivent être remplies pour obtenir une production maximale.

Quelques phénomènes de succion sont traités par l’auteur à peu près en détail et il donne quelques informations étroitement liées à la pratique.