Soil gradation is a classification of a coarse-grained

soil Soil, also commonly referred to as earth or dirt, is a mixture of organic matter, minerals, gases, liquids, and organisms that together support life. Some scientific definitions distinguish ''dirt'' from ''soil'' by restricting the former te ...

that ranks the soil based on the different particle sizes contained in the soil.Holtz, R. and Kovacs, W. (1981), ''An Introduction to Geotechnical Engineering'', Prentice-Hall, Inc. Soil gradation is an important aspect of

soil mechanics Soil mechanics is a branch of soil physics and applied mechanics that describes the behavior of soils. It differs from fluid mechanics and solid mechanics in the sense that soils consist of a heterogeneous mixture of fluids (usually air and wat ...

and

geotechnical engineering Geotechnical engineering is the branch of civil engineering concerned with the engineering behavior of earth materials. It uses the principles of soil mechanics and rock mechanics for the solution of its respective engineering problems. It als ...

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,ASTM D6913 - 04(2009)
.''ASTM International''. ASTM International. 1996-2009. October 13, 2009. if it is not high in clay quality. Soil is graded as either well graded or poorly graded.�

��. ''Integrated Publishing''. Integrated Publishing. 2003-2007. October 13, 2009. Soil gradation is determined by analyzing the results of a

sieve analysis A sieve analysis (or gradation test) is a practice or procedure used in civil engineering and chemical engineering to assess the particle size distribution (also called ''gradation'') of a granular material by allowing the material to pass through ...

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Sieve Analysis and Particle Analysis
”. ''Grand Solution Manual''. SJ Soft Technologies. 2008. October 13, 2009. �

��. ''Integrated Publishing''. Integrated Publishing. 2003-2007. October 13, 2009. or a hydrometer analysis. The process for grading a soil is in accordance with either the

Unified Soil Classification System The Unified Soil Classification System (USCS) is a soil classification system used in engineering and geology to describe the texture and grain size of a soil. The classification system can be applied to most unconsolidated materials, and is repres ...

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_u, and the coefficient of curvature, C_c, of the soil and comparing the calculated values with published gradation limits.Coduto, Donald P. Foundation Design Principles and Practices (2nd Edition). 02nd ed. Upper Saddle River: Prentice Hall, 2000. Print.

Soil gradations

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 The Atterberg limits are a basic measure of the critical water contents of a fine-grained soil: its shrinkage limit, plastic limit, and liquid limit. Depending on its water content, soil may appear in one of four states: solid, semi-solid, plastic ...

Well graded

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. A well-graded gravel is classified as GW, while a well-graded sand is classified as SW.

Poorly graded

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. 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 Soil liquefaction occurs when a cohesionless saturated or partially saturated soil substantially loses strength and stiffness in response to an applied stress such as shaking during an earthquake or other sudden change in stress condition, in ...

than well-graded soils. 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. 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.

Process of grading a soil

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.

Data collection

Soil gradation is determined by analyzing the results of a

or a hydrometer analysis. 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. 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. In a hydrometer analysis, a fine-grained soil sample is left to settle in a

viscous The viscosity of a fluid is a measure of its resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water. Viscosity quantifies the inter ...

fluid. This method is used based on

Stoke's Law In 1851, George Gabriel Stokes derived an expression, now known as Stokes' law, for the frictional force – also called drag force – exerted on spherical objects with very small Reynolds numbers in a viscous fluid. Stokes' law is derived by s ...

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.

Calculating the coefficients of uniformity and curvature

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. 200 sieve then there is 60% passing the No. 200 sieve. The coefficient of uniformity, C_u is a crude shape parameter and is calculated using the following equation:

C_u = \frac

where D₆₀ is the grain diameter at 60% passing, and D₁₀ is the grain diameter at 10% passing The ''coefficient of curvature'', C_c is a shape parameter and is calculated using the following equation:

C_c = \frac {D_{10} \times\ D_{60

where D₆₀ is the grain diameter at 60% passing, D₃₀ is the grain diameter at 30% passing, and D₁₀ is the grain diameter at 10% passing Once the coefficient of uniformity and the coefficient of curvature have been calculated, they must be compared to published gradation criteria.

Criteria for grading soils

The following criteria are in accordance with the

: For a gravel to be classified as well graded, the following criteria must be met: C_u > 4 & 1 < C_c < 3 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. For a sand to be classified as well graded, the following criteria must be met: C_u ≥ 6 & 1 < C_c < 3 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.

Importance

Soil gradation is very important to geotechnical engineering. 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 because there are more void spaces in a poorly graded soil. 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. When options for ground remediation techniques are being selected, the soil gradation is a controlling factor.

References

Sedimentology Granulometric analyses