Sedimented layer compressibility

Figure 7.28 Zone sedimentation in a suspension, (a) The various zones are delineated, showing the clear layer at the top of the suspension and the sedimented layer at the bottom immediately above this layer is a region in which the particles are crowded and begin to be compressed to form sediment (b) The height of the interface between the clarified zone and the suspension as a function of time.

Ever since the origin of life on Earth approximately 3 billion years ago, carbon compounds have been accumulating on the ocean floors as generations of organisms expired and drifted down to form layer upon layer of rich sediment. Some of that sediment, when compressed at just the right temperature and depth, transformed into the hydrocarbon... 

At small values of p (extremely diluted suspension) the sedimentation occurs quickly, then p increases at distances close to the bottom of the container, and j decreases almost down to zero near the sediment layer. The other feature is the upward propagation of the compression wave (increase in p), which slows down the process of sedimentation. In order to prevent this process in practice, it is necessary to remove the solid phase formed at the bottom of the container. It is possible to select such removal rate that the upward velocity of the compression wave be equal to zero. In this case the flux j consists of two contributions - the sedimentation and convection fluxes. The dependence of the total flux on p in this case is shown in Fig. 8.12 [42]. 

Among the properties measured here, the settling rate is mainly a measure of the size of the floes and in later stages the compressibility of floes and floe networks, and the supernatant clarity is a measure of the size distribution of floes and size dependent capture of the particles and floes by the polymer. The sediment volume and the pulp viscosity on the other hand, are direct measures, not only of floe size and structure but also of adsorbed polymer layers. It is to be noted in this regard that it is this latter aspect which makes it possible to estimate the thickness of adsorbed polymer layers by measuring the viscosity of the medium and the suspension in the presence of polymers (20,21). This combination of effects is another reason one cannot always expect correlation between various flocculation responses. 

Chert is another organic marine sediment, less common than carbonate rocks, but found in huge deposits in some parts of the world. It initially consists of the skeletons of billions of tiny, single-celled animals called radiolaria. These skeletons are composed of microcrystalline quartz or chalcedony (Si02). Dense layers of this material accumulate on the ocean floor, where they are buried and compressed over time. The term chert is sometimes also applied to any compact, very fine-grained siliceous sediment that has resulted from precipitation or consolidation of silica gel. There may be chert lenses or very thin layers within other types of sediments, such as limestone. 

Coal is a sediment that consists of the remains of plants that have been buried, compressed, and dehydrated over time. Coal varies widely in composition and often contains clay or silt, but it is usually composed of at least 75% carbonaceous material. It is theorized that most coal formed in ancient swamps, where there was a huge biomass of plant material. Dead plants falling into an anaerobic, aquatic environment does not rot but accumulates in layers, sometimes for millions of years. 

The Taconic mountains rose 480 million years ago, wrinkled under pressure like the hood of a wrecked car, from Maryland to the Gaspe Peninsula of Quebec. The compressed rocks from this mountain-building event are exposed in the Taconic Range of New York, and in eastern Pennsylvania. But by 410 million years ago the peaks that had towered over the east coast had been eroded away, and the sea washed over their exposed roots and covered them with level deposits of limestone. As these mountains wore down, the resulting sediment filled a shallow sea basin running from New York southward to Alabama, in layers up to 1,000 ft (300 m) thick. 

Whereas the theoretical half-width of the sample layer as loaded is 1 mm, after 25 min sedimentation at unit gravity in the absence of an electrical field it is only 0.36 mm due to the compressing effect of medium viscosity and density (Figure 16). It is this band-width that determines resolving power (Section 1II.3). 

The thickness of the steric layers attached to colloidal particles can be measured by a variety of techniques viscometry (Doroszkowski and Lambourne, 1968 Fleer e/a/., 1972), sedimentation coefficient measurements (Garvey et ai, 1974), photon correlation spectroscopy (Garvey et al., 1976), surface balance measurements (Doroszkowski and Lambourne, 1971 1973), compression cell measurements (Cairns et al., 1976) and even electrophoretic measurements (Homola and Robertson, 1975). 

The rate of plant debris accumulation required to produce a coal seam must be greater than the rate of decay. Once a thick layer of plant debris is formed, it must be buried by sediments such as mud or sand. These are typically washed into the swamp by a flooding river where the increasing weight of the mud and/or sand material compresses the plant debris and aids in its transformation into coal. 

The presence of electrolytes in suspension leads to the compression diffuse particle layer, the particles lose their stability, which facilitates rapid coagulation and the formation of a sediment. 

All of the fossil fuels in existence today were created millions of years ago when aquatic plants and animals were buried and compressed by layers of sediment at the bottoms of swamps and seas. Over time this organic matter was converted by bacterial decay and pressure to petroleum (oil), gas, and coal. Figure 6.16 gives the percentages of the total energy consumed in the United States from various sources. Fossil fuels account for over 80% of the total. 

Streefer, V. F., ed. 1961. Handbook of Fluid Dynamics. New York McGraw-Hill. A classic handbook on fluid dynamics wifh confributions from distinguished experts. Written for engineers and scientists in the field. Deals wifh bofh fundamenfal concepts and applications. Covers fluid flow (one-dimensional, ideal, laminar, compressible, two phase, open channel, stratified), turbulence, boundary layers, sedimentation, turbomachinery, fluid transients, and magnetohydrodynamics. Includes many formulas, equations, tables, graphs, and illustrations. Each chapter has a bibliography and the volume has subject and author indexes. 

The major advantage of the seismic refraction method is that the derived sound velocity of an individual layer correlates very well to the compactness of the sediment or to the Unconfined Compressive Strength rock strength (UCS). However, whereas the UCS value is a rock strength property the seismic sound velocity is more a rock mass characteristic that may strongly be affected by joints, fractures and other discontinuities. These properties in combination with the UCS determine the dredgeability of rock. 

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