Propiedades Fisicas
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Publicado: 15 de octubre de 2011
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16
INDEX PROPERTIES OF SOILS
Figure 4.18 Silt-size fraction of Mexico City clay showing poriferous whole geometric forms and fragments of diatoms.
Figure 4.20 Photomicrograph of a poriferous cellular peat particle.
Figure 4.19 Photomicrograph of fibrous Middleton peat.
display a substantial cation exchange capacity which increases with degree of humification and isstrongly influenced by the hydrogen ion concentration in the pore water. Cations such as calcium, magnesium, potassium, sodium, and also iron and aluminum, replace hydrogen at the exchange sites of organic polymolecules. The cation
exchange capacity of very fine humic substances may be as high as 1.5 to 5.0 meq/g. Soil fabrics characterized by organic coarse particles, as in fibrous peat, holda considerable amount of water because they are generally very loose, and also because organic particles are hollow and largely full of water. This is illustrated in Figs. 4.19 and 4.20 by SEM photomicrographs of a light brown fibrous peat from Middleton, Wisconsin, with natural water content and void ratio in the range of 610 to 830% and 11.1 to 14.2, respectively. In organic fine substances,water of hydration and doublelayer water are important. In general, the water-holding capacity of peats decreases with increased degree of decomposition. In a high-pH, alkaline environment, organic fine substances disperse into globular polyanions as small as 3 to 9 nm. In a low-pH, acidic environment or in high-electrolyte concentration, the polyanions coagulate to form large flocs of a more orless globular shape. Organic polyanions that pigment the surface of fine mineral particles such as clay minerals result in stable clayhumus complexes that promote loose and open fabric in organic soft clays and silts. Globular organic polyanions attach themselves, directly through hydrogen bonding or through adsorbed cations, to specific sites at the surface of minerals and thus promote flocculationand aggregation of mineral particles. Drying of organic coarse particles causes shrinkage of thin-walled tissues and collapse of cell structure and thereby decreases particle porosity and water-holding capacity. It also promotes aggregation of organic substances, whereupon organic precipitates bind mineral par-
ARTICLE 5
MECHANICAL ANALYSIS OF SOILS
17
ticles into stable aggregates.Even without drying and at moderate laboratory temperatures, the organic solid content of soil is susceptible to degradation, decomposition, dissolution, and therefore to loss. Scorching, irreversible dehydration, and oxidation of organic substances begin at about 60°C.
4.7 Practical Significance of Colloidal Properties
The foregoing sections of this article have made it clear that theproperties of the very fine soil fractions are of outstanding engineering importance. They have also indicated that an understanding of these properties has its roots in physical chemistry, colloidal chemistry, clay mineralogy, and organic chemistry. Notwithstanding the fundamental importance of these scientific disciplines, a detailed quantitative knowledge of none of them is needed in geotechnicalengineering practice, because the influence of the various scientific findings is reflected in the numerical values of the index properties and of the strength, deformation, and hydraulic properties determined by means of engineering tests and used in engineering calculations. To know the percentages of, say, illite and sodium montmorillonite in a deposit where a landslide has developed gives theengineer insight into the causes of the slide and may suggest the relative applicability of certain remedial measures, but it is the quantitative influence of the very fine soil fraction, as determined by engineering tests and analyses, that is of primary value in assessing and solving most problems in applied soil mec hanics.
Skempton, A. W. and R. D. Northey (1952). “The sensitivity of clays,”...
16
INDEX PROPERTIES OF SOILS
Figure 4.18 Silt-size fraction of Mexico City clay showing poriferous whole geometric forms and fragments of diatoms.
Figure 4.20 Photomicrograph of a poriferous cellular peat particle.
Figure 4.19 Photomicrograph of fibrous Middleton peat.
display a substantial cation exchange capacity which increases with degree of humification and isstrongly influenced by the hydrogen ion concentration in the pore water. Cations such as calcium, magnesium, potassium, sodium, and also iron and aluminum, replace hydrogen at the exchange sites of organic polymolecules. The cation
exchange capacity of very fine humic substances may be as high as 1.5 to 5.0 meq/g. Soil fabrics characterized by organic coarse particles, as in fibrous peat, holda considerable amount of water because they are generally very loose, and also because organic particles are hollow and largely full of water. This is illustrated in Figs. 4.19 and 4.20 by SEM photomicrographs of a light brown fibrous peat from Middleton, Wisconsin, with natural water content and void ratio in the range of 610 to 830% and 11.1 to 14.2, respectively. In organic fine substances,water of hydration and doublelayer water are important. In general, the water-holding capacity of peats decreases with increased degree of decomposition. In a high-pH, alkaline environment, organic fine substances disperse into globular polyanions as small as 3 to 9 nm. In a low-pH, acidic environment or in high-electrolyte concentration, the polyanions coagulate to form large flocs of a more orless globular shape. Organic polyanions that pigment the surface of fine mineral particles such as clay minerals result in stable clayhumus complexes that promote loose and open fabric in organic soft clays and silts. Globular organic polyanions attach themselves, directly through hydrogen bonding or through adsorbed cations, to specific sites at the surface of minerals and thus promote flocculationand aggregation of mineral particles. Drying of organic coarse particles causes shrinkage of thin-walled tissues and collapse of cell structure and thereby decreases particle porosity and water-holding capacity. It also promotes aggregation of organic substances, whereupon organic precipitates bind mineral par-
ARTICLE 5
MECHANICAL ANALYSIS OF SOILS
17
ticles into stable aggregates.Even without drying and at moderate laboratory temperatures, the organic solid content of soil is susceptible to degradation, decomposition, dissolution, and therefore to loss. Scorching, irreversible dehydration, and oxidation of organic substances begin at about 60°C.
4.7 Practical Significance of Colloidal Properties
The foregoing sections of this article have made it clear that theproperties of the very fine soil fractions are of outstanding engineering importance. They have also indicated that an understanding of these properties has its roots in physical chemistry, colloidal chemistry, clay mineralogy, and organic chemistry. Notwithstanding the fundamental importance of these scientific disciplines, a detailed quantitative knowledge of none of them is needed in geotechnicalengineering practice, because the influence of the various scientific findings is reflected in the numerical values of the index properties and of the strength, deformation, and hydraulic properties determined by means of engineering tests and used in engineering calculations. To know the percentages of, say, illite and sodium montmorillonite in a deposit where a landslide has developed gives theengineer insight into the causes of the slide and may suggest the relative applicability of certain remedial measures, but it is the quantitative influence of the very fine soil fraction, as determined by engineering tests and analyses, that is of primary value in assessing and solving most problems in applied soil mec hanics.
Skempton, A. W. and R. D. Northey (1952). “The sensitivity of clays,”...
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