Experimental Studies on Sedimentation Mechanism and Sediment Formation of Clay Materials

Sedimentation mechanism and sediment formation characteristics are examined by use of dilute clay-water mixtures. Direct measurement of the distribution of water content in the mixture is successfully used in the examination. According to the results, the process of soil sedimentation in general comprises three stages. In the first stage, no settling takes place, but flocculation yields floes. In the second stage, the floes gradually settle and form a layer of sediment, which undergoes consolidation and reduction of water content. The boundary between the upper settling zone and the sediment is the birth place of new sediment. While the sediment grows, the settling zone becomes thinner and finally vanishes. In the last stage, all of the sediment thus formed undergoes self-weight consolidation and finally approaches an equilibrium state. The water content value at which a mixture changes into sediment is not uniquely determined, but varies corresponding to the mixture’s initial water content; the two values are close to each other. That is, soil sediment is not formed at a water content peculiar to the material concerned. This remarkable finding is a reflection of the fact that, even for a clay, countless compression curves exist under very low effective stresses.