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Settling Convection and Grain-Size Analysis
Ph. H. Kuenen
Grain-size analysis by settling techniques presupposes free sinking of the particles. There are two phenomena that might have an adverse effect on this basic assumption. The first has been termed "hindered settling" and denotes the obstruction that settling particles experience from the upward flow around neighboring grains. This influence will not be considered here. Another phenomenon, that I have called "settling convection," signifies the current system set up by differences in density of clouds in a settling suspension. The two processes are normally combined. The action of the latter strongly varies according to conditions. Bradley's "vertical density currents" form a border case. In suspensions prepared for pipette analysis it develops lively small-scale currents. Tests are pre ented to show that the influence on the deduced size distribution is fortunately negligible and roughly equivalent to heating by 1 degree centigrade. In salt water-clay suspensions and various other mixtures of solids with liquids, the action of settling convection helps to preserve a uniform suspension below a steadily sinking interface. At the bottom there exists a rising boundary between the suspension and the deposit. This constant composition excludes the possibility of grain-size analysis in sea water by means of the settling velocity of the separate particles. A rough alternative visual method for size analysis of flocculated suspension is described.
Sedimentation balances for size analyses of silt and clay employing a shaken suspension suffer the same slight acceleration as pipette analyses. Moreover, the convection around the submerged dish forms a serious drawback to this technique.
Settling convection tends to augment the sinking velocity of sand grains released at the surface, especially in samples with a narrow range in size and also for particles of less than 0.1-0.05 mm. Hence, the usefulness of various types of sedimentation analyzers based on sample introduction from above is restricted to sizes above 50 microns. Settling convection also renders ineffective counter-current methods (elutriators) in water. Elutriation in air is inapplicable for lutum samples that have been wet, hence for most natural sediments except eolian deposits
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