F CEE 120-Attbrg lmts and class

F CEE 120-Attbrg lmts and class - Classification properties...

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Classification properties of soils (continued) and soil classifications Attterberg limits and consistency indices Soil classification 1.6 Attterberg limits and consistency indices As explained above, particle size and the distribution of different sizes in a volume of soil have some effect on the soil’s engineering behavior, in particular the behavior of clean coarse-grained soils such as clean gravels and sands. The soil classifications therefore consider particle size and the distribution of different sizes in a volume of soil. However, if a soil contains just a moderate amount of fine particles such as silts and clays, its behavior will be dominated by these fines and their water content. It has been already strongly emphasized (many times over) that behavior of clays is greatly affected by the presence of water. Accordingly, classification and characterization of fine grained soils must take into account the effect of water and the water content. When a chunk of relatively stiff clay having a low water content is mixed and remolded in a dish with increasing amount of water it is becoming softer and softer. It first becomes plastic and then when mixed with more water very soft and liquid like. The paramount importance of water in making the fine grained soils plastic was discussed as early as the end of eighteen century. In these early investigations it was realized that two factors are necessary to obtain a plastic soil, water as the fluid between the particles and small platy particles with large relative surface. For example, Casagrande (1932) wrote that Voigt in 1897 advanced the hypothesis that plasticity and other important properties of clay are caused by the presence of scalelike, platy particles. Atterberg, who was a Swedish chemist and soil scientist (see 1
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Geotechnique Vol. III, Number 1, March 1952, pp. 17-19), first presented the reliable evidence of this hypothesis (Atterberg, 1913). Some other scientists later independently arrived at the same conclusion. Terzaghi (1925; 1931), for example, observed that clays have a much lower permeability than very fine sands with the same equivalent grain diameter (what Terzaghi meant by very fine sands was probably what we call silt), and concluded that the volume of a clay particle must be much smaller than that of a bulky grain of the same size, which to him was an indication that clay particles are of scalelike, platy shape. Casagrande (1932) also stated that Goldschmidt (1926) found a satisfactory explanation for the action of platy particles in making the soil plastic. Goldscshmit showed that the surfaces of platy particles are charged with electricity which affects the arrangement of the bipolar molecules in a liquid like water. When the clay powder is mixed with a liquid consisting of molecules which are not distinctly bipolar (e. g. carbon tetrachloride), the mixture does not acquire plasticity. In other words, if not for the water (H 2 O) being the pore fluid, clayey saturated soil would not necessarily be a plastic
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This note was uploaded on 11/08/2010 for the course CEE 120 taught by Professor Vucetic during the Fall '10 term at UCLA.

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F CEE 120-Attbrg lmts and class - Classification properties...

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