Soil gradation
Encyclopedia
Soil gradation is a classification of a coarse-grained soil
that ranks the soil based on the different particle sizes contained in the soil. 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.
Soil is graded as either well graded or poorly graded. Poorly graded soils are further divided into uniformly-graded or gap-graded soils.
Soil gradation is determined by analyzing the results of a sieve analysis
or a hydrometer analysis
.
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, Cu, and the coefficient of curvature, Cc, of the soil and comparing the calculated values with published gradation limits.
.
than well graded soils.
A uniformly graded soil is a soil that has most of its particles at about the same size. An example of a uniformly graded soil is one in which only sand of the No. 20 size is present.
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.
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
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.
The coefficient of uniformity, Cu is a crude shape parameter and is calculated using the following equation:
where D60 is the grain diameter at 60% passing, and D10 is the grain diameter at 10% passing
The coefficient of curvature, Cc is a shape parameter and is calculated using the following equation:
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.
For a gravel to be classified as well graded, the following criteria must be met:
Cu > 4 & 1 < Cc < 3
If both of these criteria are met, the gravel is classified as well graded or GW. If both of these criteria are not met, the gravel is classified as poorly graded or GP.
For a sand to be classified as well graded, the followinng criteria must be met:
Cu > 6 & 1 < Cc < 3
If both of these criteria are met, the sand is classified as well graded or SW. If both of these criteria are not met, the sand is classified as poorly graded or SP.
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.
Soil
Soil is a natural body consisting of layers of mineral constituents of variable thicknesses, which differ from the parent materials in their morphological, physical, chemical, and mineralogical characteristics...
that ranks the soil based on the different particle sizes contained in the soil. Soil gradation is an important aspect of soil mechanics
Soil mechanics
Soil mechanics is a branch of engineering 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 and particles but soil may also contain organic solids, liquids, and gasses and other...
and geotechnical engineering
Geotechnical engineering
Geotechnical engineering is the branch of civil engineering concerned with the engineering behavior of earth materials. Geotechnical engineering is important in civil engineering, but is also used by military, mining, petroleum, or any other engineering concerned with construction on or in the ground...
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.
Soil is graded as either well graded or poorly graded. Poorly graded soils are further divided into uniformly-graded or gap-graded soils.
Soil gradation is determined by analyzing the results of a sieve analysis
Sieve analysis
A sieve analysis is a practice or procedure used to assess the particle size distribution of a granular material....
or a hydrometer analysis
Hydrometer
A hydrometer is an instrument used to measure the specific gravity of liquids; that is, the ratio of the density of the liquid to the density of water....
.
The process for grading a soil is in accordance with either the Unified Soil Classification System
Unified Soil Classification System
The Unified Soil Classification System 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 represented by a two-letter symbol...
or the AASHTO Soil Classification System
AASHTO Soil Classification System
The AASHTO Soil Classification System was developed by the American Association of State Highway and Transportation Officials, and is used as a guide for the classification of soils and soil-aggregate mixtures for highway construction purposes...
. 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, Cu, and the coefficient of curvature, Cc, of the soil and comparing the calculated values with published gradation limits.
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 limitsAtterberg Limits
The Atterberg limits are a basic measure of the nature of a fine-grained soil. Depending on the water content of the soil, it may appear in four states: solid, semi-solid, plastic and liquid. In each state the consistency and behavior of a soil is different and thus so are its engineering properties...
.
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. Poorly graded soils are either uniformly graded or gap-graded. 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 liquefactionSoil liquefaction
Soil liquefaction describes a phenomenon whereby a saturated soil substantially loses strength and stiffness in response to an applied stress, usually earthquake shaking or other sudden change in stress condition, causing it to behave like a liquid....
than well graded soils.
A uniformly graded soil is a soil that has most of its particles at about the same size. An example of a uniformly graded soil is one in which only sand of the No. 20 size is present.
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 sieve analysisSieve analysis
A sieve analysis is a practice or procedure used to assess the particle size distribution of a granular material....
or a hydrometer analysis
Hydrometer
A hydrometer is an instrument used to measure the specific gravity of liquids; that is, the ratio of the density of the liquid to the density of water....
.
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
Viscosity
Viscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms , viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity...
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.
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. 20 sieve then there is 60% passing the No. 20 sieve.The coefficient of uniformity, Cu is a crude shape parameter and is calculated using the following equation:
where D60 is the grain diameter at 60% passing, and D10 is the grain diameter at 10% passing
The coefficient of curvature, Cc is a shape parameter and is calculated using the following equation:
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 Unified Soil Classification System:For a gravel to be classified as well graded, the following criteria must be met:
Cu > 4 & 1 < Cc < 3
If both of these criteria are met, the gravel is classified as well graded or GW. If both of these criteria are not met, the gravel is classified as poorly graded or GP.
For a sand to be classified as well graded, the followinng criteria must be met:
Cu > 6 & 1 < Cc < 3
If both of these criteria are met, the sand is classified as well graded or SW. If both of these criteria are not met, the sand is classified as poorly graded or SP.
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.