Chapter 2 - The Mineralogy and the Physicochemical Properties of Cohesive Sediments

Abstract

This chapter reveals the mineralogy and the physicochemical properties of cohesive sediments. In hydraulic engineering, the term cohesive sediment implies a mixture of silt and clay with settling diameter less than 50 μm and as small as a fraction of 1 μm with various degrees of organic matter. The same term applies to such mixtures containing a substantial percentage of sand, provided that they still display cohesive properties. Two of the most common properties of clay masses subjected to various degrees of consolidation are plasticityand cohesion. The first is the property of a clay mass to undergo substantial plastic deformation under stress and within a certain range of water content without breaking. The Atterberg limits are used as a measure of this property. Cohesion is the ability of a clay sample to withstand a finite shear stress within its mass without confinement. The plastic and cohesive properties of fine sediments are due to that part of the soil mass that is fine enough and of specific area sufficiently large for the surface physicochemical forces to become dominant. The size of these particles, also known as colloids, varies from a few micrometers to a small fraction of 1 μm, and they normally have the shape of little flat plates or needles or laths, depending on their mineralogical composition. Modern electron microscopy, X-ray diffraction, and differential thermal analysis have revealed that clays are composed essentially of one or more members of a small group of clay minerals. These minerals have a predominantly crystalline arrangement as a result of the directional covalent bonds. Clay minerals are silicates of aluminum and/or magnesium and iron.