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SOIL CLASSIFICATION

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According to their particle sizes, soils are divided into two:

• Coarse grained soils: Gravel» Sand

• Fine grained soils: Silt &» Clay

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Cohesionless soils:• Sand • Silt

Cohesive soil:• Clay

Cohesion: sticking together of like materials

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Plasticity of Fine-Grained Soils

Plasticity is an important characteristic in Fine-Grained Soils.

Plasticity: is the ability of a soil to undergo unrecoverable deformation at constant volume without cracking and crumbling.

Plasticity is due to the presence of clay minerals or organic material.

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PARTICLE SIZE DISTRIBUTION

It is a screening process in which coarse fractions of soil are separated by means of series of sieves.

• Particle sizes larger than 0.074 mm (U.S. No. 200 sieve) are usually analyzed by means of sieving.

• Soil materials finer than 0.074 mm (-200 material) are analyzed by means of

sedimentation of soil particles by gravity (hydrometer analysis).

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#4

                                            

#10

#20

#40

#100

#200

PAN

SIEVE ANALYSES

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Hydrometer Analysis

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Hydrometer Analysis

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General Particle Sizes Particle Average Diameter (mm) Average Diameter (mm)

According to MIT According to ASTM

Boulders 200 300

Cobbles 200-60 300-75

Gravel 60.0 – 2.0 75- 4.75

Sand 2.0 – 0.06 4.75- 0.075

Silt 0.06 – 0.002 0.075- 0.002

Clay 0.002 0.002

MIT(Massachusetts Institute of Technology)

ASTM (American Society for Testing and Materials)

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Classification SystemsThe most widely used classification systems

are:

• the American Association of State Highway and Transportation Officials (AASHTO) and

• the Unified Soil Classification System (USCS).

In this class, we will mainly discuss the USCS system that is used by engineering consulting companies and soil-testing laboratories.

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Well graded soil

Well graded: a coarse grained soil is described as well graded if there is no excess of particles in any size range and if no intermediate sizes are lacking. Almost all particle sizes are present.

In general a well-graded soil is represented by a smooth concave distribution curve.

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Poorly graded soil

• UNIFORM SOIL: A high proportion of the particles have sizes within narrow limits.

• GAP GRADED SOIL: Particles of both large and small sizes are present but with a relatively low proportion of particles of intermediate size.

Example: • mixture of coarse sand and clay with no silt.• mixture of gravel and silt with no sand.

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A - a poorly-graded medium SAND B - a well-graded GRAVEL-SAND (i.e. equal amounts of gravel and sand) C - a gap-graded COBBLES-SAND D - a sandy SILT E - a typical silty CLAY (e.g. London clay, Oxford clay)

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the soil can be:

• Poorly Graded or • Well Graded

Poorly Graded = P Well Graded = W

For Cohesionless Soil (Gravel and Sand)

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There are certain parameters used in the classification of soils.

• Effective size, D10: The size such that 10% of the particles are smaller than that size.

• D60: The size such that 60% of the particles are smaller than that size.

• D30: The size such that 30% of the particles are smaller than that size.

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Parameters Obtained From Grain Size Distribution Curve:

1- Uniformity Coefficient Cu (measure of the particle size range)

Cu = D60/D10

• Cu < 5 ----- Very Uniform

• Cu = 5 ----- Medium Uniform

• Cu > 5 ----- Nonuniform

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2- Coefficient of Curvature Cz (Cc)

• (measure of the shape of the particle size curve)

Cz from 1 to 3 ------- well graded

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Unified Soil Classification System (USCS)

•Note:  The amount of Fines in a soil sample is the percent by weight that passes a number 200 sieve.