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| [[Image:Soil Gradations.JPG|thumb|Soil Gradation Categories]]
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| '''Soil gradation''' is a classification of a coarse-grained [[soil]] that ranks the soil based on the different particle sizes contained in the soil.<ref name="c">Holtz, R. and Kovacs, W. (1981), ''An Introduction to Geotechnical Engineering'', Prentice-Hall, Inc. ISBN 0-13-484394-0</ref> Soil gradation is an important aspect of [[soil mechanics]] and [[geotechnical engineering]] because it is an indicator of other engineering properties such as compressibility, shear strength, and hydraulic conductivity. In a design, the gradation of the in situ or on site soil often controls the design and ground water drainage of the site. A poorly graded soil will have better drainage than a well graded soil.<ref name="a">"[http://www.astm.org/Standards/D6913.htm ASTM D6913 - 04(2009)]".''ASTM International''. ASTM International. 1996-2009. October 13, 2009.</ref>
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| Soil is graded as either well graded or poorly graded. Poorly graded soils are further divided into uniformly-graded or gap-graded soils.<ref name="b">“[http://www.tpub.com/content/engine/14081/css/14081_454.htm Soil Gradation]”. ''Integrated Publishing''. Integrated Publishing. 2003-2007. October 13, 2009.</ref>
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| Soil gradation is determined by analyzing the results of a [[sieve analysis]]
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| <ref name="d">“[http://www.aboutcivil.com/Sieve-analysis-and-soil-classification.html Sieve Analysis and Particle Analysis]”. ''Grand Solution Manual''. SJ Soft Technologies. 2008. October 13, 2009.</ref>
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| <ref name="e">“[http://www.tpub.com/content/engine/14080/css/14080_127.htm Sieve Analysis]”. ''Integrated Publishing''. Integrated Publishing. 2003-2007. October 13, 2009.</ref> or a [[hydrometer|hydrometer analysis]].<ref name="c"/>
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| The process for grading a soil is in accordance with either the [[Unified Soil Classification System]] or the [[AASHTO Soil Classification System]]. Gradation of a soil is determined by reading the grain size distribution curve produced from the results of laboratory tests on the soil. Gradation of a soil can also be determined by calculating the coefficient of uniformity, C<sub>u</sub>, and the coefficient of curvature, C<sub>c</sub>, of the soil and comparing the calculated values with published gradation limits.<ref name="c"/><ref name="f">Coduto, Donald P. Foundation Design Principles and Practices (2nd Edition). 02nd ed. Upper Saddle River: Prentice Hall, 2000. Print. ISBN 0-13-589706-8</ref>
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| == Soil Gradations ==
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| Soil gradation is a classification of the particle size distribution of a soil. Coarse-grained soils, mainly gravels or sands, are graded as either well graded or poorly graded. Poorly graded soils are further divided into uniformly-graded or gap-graded soils. Fine-grained soils, mainly silts and clays, are classified according to their [[Atterberg limits]].<ref name="c"/><ref name="f"/>
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| === Well Graded ===
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| A ''well graded'' soil is a soil that contains particles of a wide range of sizes and has a good representation of all sizes from the No. 4 to No. 200 sieves.<ref name="b"/> A well graded gravel is classified as GW while a well graded sand is classified as SW.<ref name="c"/>
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| === Poorly Graded ===
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| A ''poorly graded'' soil is a soil that does not have a good representation of all sizes of particles from the No. 4 to No. 200 sieve. Poorly graded soils are either uniformly graded or gap-graded.<ref name="b"/> A poorly graded gravel is classified as GP while a poorly graded sand is classified as SP. Poorly graded soils are more susceptible to [[soil liquefaction]] than well graded soils.<ref name="c"/>
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| A ''uniformly graded'' soil is a soil that has most of its particles at about the same size.<ref name="c"/> An example of a uniformly graded soil is one in which only sand of the No. 20 size is present.<ref name="b"/>
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| A ''gap-graded'' soil is a soil that has an excess or deficiency of certain particle sizes or a soil that has at least one particle size missing.<ref name="c"/><ref name="b"/> An example of a gap-graded soil is one in which sand of the No. 10 and No. 40 sizes are missing, and all the other sizes are present.<ref name="b"/>
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| === Process of Grading a Soil ===
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| The process of grading a soil is in accordance with either the Unified Soil Classification System or the AASHTO Soil Classification System. The steps in grading a soil are data collection, calculating coefficients of uniformity and curvature, and grading the soil based on the grading criteria given in the used soil classification system.<ref name="c"/>
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| ==== Data Collection ====
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| Soil gradation is determined by analyzing the results of a [[sieve analysis]] or a [[hydrometer|hydrometer analysis]].<ref name="d"/><ref name="e"/>
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| In a sieve analysis, a coarse-grained soil sample is shaken through a series of woven-wire square-mesh sieves. Each sieve has successively smaller openings so particles larger than the size of each sieve are retained on the sieve.<ref name="d"/><ref name="e"/> The percentage of each soil size is measured by weighing the amount retained on each sieve and comparing the weight to the total weight of the sample. The results of a sieve analysis are plotted as a grain size distribution curve, which is then analyzed to determine the soil gradation of the particular soil.<ref name="c"/>
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| In a hydrometer analysis, a fine-grained soil sample is left to settle in a [[viscosity|viscous]] fluid. This method is used based on [[Stoke's Law]] which relates terminal velocity of fall of a particle in a viscous fluid to the grain diameter and density of the grain in suspension.Grain diameter is calculated from a known distance and time of the fall of the particle. This is used to classify fine-grained soils.<ref name="c"/>
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| ==== Calculating the Coefficients of Uniformity and Curvature ====
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| Calculating the coefficients of uniformity and curvature requires grain diameters. The grain diameter can be found for each percent of the soil passing a particular sieve. This means that if 40% of the sample is retained on the No. 20 sieve then there is 60% passing the No. 20 sieve.<ref name="c"/>
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| The ''coefficient of uniformity'', C<sub>u</sub> is a crude shape parameter and is calculated using the following equation:
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| <math>C_u = \frac {D_{60}}{D_{10}}</math>
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| where D<sub>60</sub> is the grain diameter at 60% passing, and D<sub>10</sub> is the grain diameter at 10% passing
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| The ''coefficient of curvature'', C<sub>c</sub> is a shape parameter and is calculated using the following equation:
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| <math>C_c = \frac {(D_{30})^2}{D_{10} \times\ D_{60}}</math>
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| where D<sub>60</sub> is the grain diameter at 60% passing, D<sub>30</sub> is the grain diameter at 30% passing, and D<sub>10</sub> is the grain diameter at 10% passing
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| Once the coefficient of uniformity and the coefficient of curvature have been calculated, they must be compared to published gradation criteria.<ref name="c"/>
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| ==== Criteria for Grading Soils ====
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| The following criteria are in accordance with the Unified Soil Classification System:
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| For a gravel to be classified as well graded, the following criteria must be met:
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| C<sub>u</sub> > 4 & 1 < C<sub>c</sub> < 3
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| If both of these criteria are not met, the gravel is classified as poorly graded or GP. If both of these criteria are met, the gravel is classified as well graded or GW.
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| For a sand to be classified as well graded, the following criteria must be met:
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| C<sub>u</sub> ≥ 6 & 1 < C<sub>c</sub> < 3
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| If both of these criteria are not met, the sand is classified as poorly graded or SP. If both of these criteria are met, the sand is classified as well graded or SW.<ref name="c"/>
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| === Importance ===
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| Soil gradation is very important to geotechnical engineering. It is an indicator of other engineering properties such as compressibility, shear strength, and hydraulic conductivity.
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| In a design, the gradation of the in situ or on site soil often controls the design and ground water drainage of the site. A poorly graded soil will have better drainage than a well graded soil because there are more void spaces in a poorly graded soil.
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| When a fill material is being selected for a project such as a highway embankment or earthen dam, the soil gradation is considered. A well graded soil is able to be compacted more than a poorly graded soil. These types of projects may also have gradation requirements that must be met before the soil to be used is accepted.
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| When options for ground remediation techniques are being selected, the soil gradation is a controlling factor.<ref name="a"/>
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| == References ==
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| <references/>
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| [[Category:Sedimentology]]
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| [[Category:Geotechnical engineering]]
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| [[Category:Soil]]
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I would like to introduce myself to you, I am Andrew and my wife doesn't like it at all. Credit authorising is how he makes money. Her family life in Ohio but her spouse desires them to move. She is truly fond of caving but she doesn't have the time lately.
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