1 SOIL CLASSIFICATION

2 According to their particle sizes, soils are divided into two: Coarse grained soils: Gravel » Sand Fine grained soils: Silt & » Clay

3 Cohesionless soils: Sand Silt Cohesive soil: Clay Cohesion: sticking together of like materials

4 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.

5 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 mm (U.S. No. 200 sieve) are usually analyzed by means of sieving. Soil materials finer than mm (-200 material) are analyzed by means of sedimentation of soil particles by gravity (hydrometer analysis).

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7 #4 #10 #20 #40 #100 #200 PAN SIEVE ANALYSES

8 Hydrometer Analysis

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10 General Particle Sizes ParticleAverage Diameter (mm) Average Diameter (mm) According to MIT According to ASTM Boulders  200  300 Cobbles Gravel60.0 – Sand2.0 – Silt 0.06 – Clay   MIT(Massachusetts Institute of Technology) ASTM (American Society for Testing and Materials)

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12 Classification Systems The 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.

13 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.

14 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.

15 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)

16 the soil can be: Poorly Graded or Well Graded Poorly Graded = P Well Graded = W For Cohesionless Soil (Gravel and Sand)

17 There are certain parameters used in the classification of soils. Effective size, D 10 : The size such that 10% of the particles are smaller than that size. D 60 : The size such that 60% of the particles are smaller than that size. D 30 : The size such that 30% of the particles are smaller than that size.

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19 Parameters Obtained From Grain Size Distribution Curve: 1- Uniformity Coefficient Cu (measure of the particle size range) C u = D 60 /D 10 C u < Very Uniform C u = Medium Uniform C u > Nonuniform

20 2- Coefficient of Curvature C z (C c ) (measure of the shape of the particle size curve) C z from 1 to well graded

21 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.