Abstract
This study investigated the settling behavior of Na-montmorillonite suspensions under the field of gravity. Na-montmorillonite suspensions, whose initial solid ratios φ0 ranged from 2.0 to 3.0%, were placed on deionized water in the settling tube 20cm in total height. Then, solid ratio profiles of suspension columns were measured at various elapsed times until 30 days.
The settling characteristics of clay suspensions were examined by comparison to an ideal Newtonian viscous flow model. But, in this experiment, Na-montmorillonite suspensions behaved as a Bingham plastic flow, and remained in the upper part of settling tube. These results indicated that suspension layer was supported by some force, against the gravitational force.
Therefore, we estimated the yield stresses τ0 and maximum shear stresses τmax of Na-montmorillonite suspensions, and compared them numerically. The yield stresses τ0 could be calculated, using experimental relationship between τ0 and q50, obtained by means of steady shear flow test. They showed remarkably high value for dilute suspensions, ranging from 18.0 to 338.8 Pa.
Then, shear stresses in suspension column took maximum at the boundary between suspensions and the settling tube wall, and these values could be deduced by applying the Michaels and Bolger model to the hypothetical thin unit layer of suspensions. Neglecting the pressure difference of liquid and solids between top and bottom of the unit layer, we could obtain the maximum shear stresses τmax at most 8 Pa, which applied lateral side of column upward. Hence, shear stresses were considerably less than yield stresses τ0. Maximum shear stresses obviously resulted from the frictional force between suspensions and tube wall. Thus, uplifting of suspensions during 30 days could be ascribed to the frictional force, so settling behavior could be observed only just at the interface between suspensions and deionized water.
