In this study, we focused on the effect of the degree of floc growth on the moving velocity of the interface between sediment of flocculated montmorillonite and supernatant in the semi-dilute suspension, which is characterized by the start of an extremely slow movement of interface followed by an abrupt settling and ending in consolidation. Na-montmorillonite suspension coagulated under different ionic strengths, ranging from 0.5 M to 1.5 M of NaCl, was placed in a settling cylinder. The initial height of suspension was varied from 13 cm to 50 cm. The flocculated suspension was left to settle in the cylinder after the manual mixing of end-over-end. Changes in the height of the interface between the flocculated sediment and the transparent supernatant were measured as a function of elapsed time. It was confirmed that the maximum settling velocity increased with an increase in the height of cylinder within the range of our measurement. This tendency was found to be more significantly pronounced by the growth of flocs. This result indicates the presence of a feed-forward mechanism to enhance the upward motion of fluid or downward motion of flocculated sediment, or both. That means, sedimentation is accelerated by the growth of big flocs, and the growth of big flocs is accelerated by the sedimentation. These motions will eventually induce the generation of an upward plume or channel of water. The formed plume flutters slowly with rather a large scale. We term this phenomenon the "sedimentation turbulence."

The scheme of DLVO theory and the concept of fractal structure of flocs applied to the suspension of montmorillonite have revealed out the unique nature of this clay dispersion. In this context, two major regimes are recognized. The first is the electrostatically dispersed regime. And the second is the coagulated regime. In the former, the formation of a diffusive electric double layer (EDL) characterized by reciprocal Debye length measured from the surface of the particle is distinctively important. Intrinsic viscosity with electroviscous effects and yield stress are interpreted by the steric presence of EDL. In the latter, the unit of transportation is a coagulated floc with finite cohesive strength. Sedimentation process reflecting these factors is carefully observed to recognize the turbulence generation by the formation of large flocs at the moment of gel collapse. Waiting time prior to gel collapse was found to be determined reflecting the pH-dependent charging behavior. By taking into account the effect pH-dependent charge, the DLVO based two regimes are further categorized into five. The developed tools can be extensively used for the system involved with different ionic species, pH, volume fraction and organic substances.

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