Soils and their Classification

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Soils and their Classification

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Soils - What are they?

• Particulate materials
• - Sedimentary origins (usually)
• - Residual
• Wide range of particle sizes
• - larger particles quartz, feldspar
• - very small particles clay minerals
• Voids between particles

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Need for Simple Classification

• Usually soil on site has to be used.
• Soils differ from other engineering materials in
  that one has little control over their properties
• Extent and properties of the soil have to be
  determined
• Cheap and simple tests are required to give an
  indication of engineering properties, e.g.
  stiffness, strength, for preliminary design
• The classification must use core samples
  obtained from the ground. This information is
  often supplemented by in-situ tests such as cone
  penetration tests.

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Classification based on Particle Size

• Particle size is used because it is related to
  mineralogy
• e.g. very small particles usually contain clay
  minerals
• Broad Classification
• Coarse grained soils
• sands, gravels - visible to naked eye

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Classification based on Particle Size

• Particle size is used because it is related to
  mineralogy
• e.g. very small particles usually contain clay
  minerals
• Broad Classification
• Coarse grained soils
• sands, gravels - visible to naked eye
• Fine grained soils
• silts, clays, organic soils

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Procedure for grain size determination

• Sieving - used for particles gt 75 mm
• Hydrometer test - used for smaller particles
• Analysis based on Stokes Law, velocity
  proportional to diameter

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Procedure for grain size determination

• Sieving - used for particles gt 75 mm
• Hydrometer test - used for smaller particles
• Analysis based on Stokes Law, velocity
  proportional to diameter
• Figure 1 Schematic diagram of hydrometer test

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Procedure for grain size determination

• Sieving - used for particles gt 75 mm
• Hydrometer test - used for smaller particles
• Analysis based on Stokes Law, velocity
  proportional to diameter
• Figure 1 Schematic diagram of hydrometer test

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Grading curves
W Well graded
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Grading curves
W Well graded U Uniform
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Grading curves
W Well graded U Uniform P Poorly graded
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Grading curves
W Well graded U Uniform P Poorly graded C Well
graded with some clay
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Grading curves
W Well graded U Uniform P Poorly graded C Well
graded with some clay F Well graded with an
excess of fines
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Simple Classification

• In general soils contain a wide range of particle
  sizes
• Some means of describing the characteristics of
  soils with different proportions of
  sand/silt/clay is required.

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Simple Classification

• In general soils contain a wide range of particle
  sizes
• Some means of describing the characteristics of
  soils with different proportions of
  sand/silt/clay is required.

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Simple Classification

• In general soils contain a wide range of particle
  sizes
• Some means of describing the characteristics of
  soils with different proportions of
  sand/silt/clay is required.
• Note the importance of clay - Finest 20 control
  behaviour

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Example equal amounts sand/silt/clay
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Example equal amounts sand/silt/clay
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Example equal amounts sand/silt/clay
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Atterberg Limits

• Particle size is not that useful for fine grained
  soils

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Atterberg Limits

• Particle size is not that useful for fine grained
  soils
• Figure 4 Moisture content versus volume relation
  during drying

SL
LL
PL
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Atterberg Limits

• Particle size is not that useful for fine grained
  soils
• Figure 4 Moisture content versus volume relation
  during drying
• SL - Shrinkage Limit
• PL - Plastic Limit
• LL - Liquid limit

SL
LL
PL
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Atterberg Limits

• SL - Shrinkage Limit
• PL - Plastic Limit
• LL - Liquid limit

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Atterberg Limits

• SL - Shrinkage Limit
• PL - Plastic Limit
• LL - Liquid limit
• Plasticity Index LL - PL PI or Ip

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Atterberg Limits

• SL - Shrinkage Limit
• PL - Plastic Limit
• LL - Liquid limit
• Plasticity Index LL - PL PI or Ip
• Liquidity Index (m - PL)/Ip LI

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Classification Systems

• Used to determine the suitability of different
  soils
• Used to develop correlations with useful soil
  properties
• Special Purpose (Local) Systems
• e.g. PRA system of AAHSO
• 1. Well graded sand or gravel may include fines
• 2. Sands and Gravels with excess fines
• 3. Fine sands
• 4. Low compressibility silts
• 5. High compressibility silts
• 6. Low to medium compressibility clays
• 7. High compressibility clays
• 8. Peat and organic soils

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Unified Soil Classification

• Each soil is given a 2 letter classification
  (e.g. SW). The following procedure is used.

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Unified Soil Classification

• Each soil is given a 2 letter classification
  (e.g. SW). The following procedure is used.
• Coarse grained (gt50 larger than 75 mm)

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Unified Soil Classification

• Each soil is given a 2 letter classification
  (e.g. SW). The following procedure is used.
• Coarse grained (gt50 larger than 75 mm)
• Prefix S if gt 50 of coarse is Sand
• Prefix G if gt 50 of coarse is Gravel

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Unified Soil Classification

• Each soil is given a 2 letter classification
  (e.g. SW). The following procedure is used.
• Coarse grained (gt50 larger than 75 mm)
• Prefix S if gt 50 of coarse is Sand
• Prefix G if gt 50 of coarse is Gravel
• Suffix depends on fines

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Unified Soil Classification

• Each soil is given a 2 letter classification
  (e.g. SW). The following procedure is used.
• Coarse grained (gt50 larger than 75 mm)
• Prefix S if gt 50 of coarse is Sand
• Prefix G if gt 50 of coarse is Gravel
• Suffix depends on fines
• if fines lt 5 suffix is either W or P
• if fines gt 12 suffix is either M or C
• if 5 lt fines lt 12 Dual symbols are used

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Unified Soil Classification

• To determine if W or P, calculate Cu and Cc

x of the soil has particles smaller than Dx
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Unified Soil Classification

• To determine W or P, calculate Cu and Cc

x of the soil has particles smaller than Dx
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Unified Soil Classification

• To determine W or P, calculate Cu and Cc

x of the soil has particles smaller than Dx
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Unified Soil Classification

• To determine W or P, calculate Cu and Cc

x of the soil has particles smaller than Dx
D90 3 mm
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Unified Soil Classification

• To determine W or P, calculate Cu and Cc
• If prefix is G then suffix is W if Cu gt 4 and
  Cc is between 1 and 3
• otherwise use P
• If prefix is S then suffix is W if Cu gt 6 and
  Cc is between 1 and 3
• otherwise use P

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Unified Soil Classification

• Coarse grained soils
• To determine M or C use plasticity chart
• Below A-line use suffix M - Silt
• Above A-line use suffix C - Clay

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Unified Soil Classification

• Fine grained soils (gt 50 finer than 75 mm)
• Both letters determined from plasticity chart

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Example
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Example

• fines ( finer than 75 mm) 11 - Dual
  symbols required

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Example

• fines ( finer than 75 mm) 11 - Dual
  symbols required
• D10 0.06 mm, D30 0.25 mm, D60 0.75 mm

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Example
Particle size fractions Gravel 17 Sand
73 Silt and Clay 10
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Of the coarse fraction about 80 is sand, hence
Prefix is S Cu 12.5, Cc 1.38 Suffix1
W From Atterberg Tests LL 32, PL 26 Ip
32 - 26 6
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Example
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Of the coarse fraction about 80 is sand, hence
Prefix is S Cu 12.5, Cc 1.38 Suffix1
W From Atterberg Tests LL 32, PL 26 Ip
32 - 26 6 From Plasticity Chart point lies
below A-line Suffix2 M
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Of the coarse fraction about 80 is sand, hence
Prefix is S Cu 12.5, Cc 1.38 Suffix1
W From Atterberg Tests LL 32, PL 26 Ip
32 - 26 6 From Plasticity Chart point lies
below A-line Suffix2 M Dual Symbols are
SW-SM
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Of the coarse fraction about 80 is sand, hence
Prefix is S Cu 12.5, Cc 1.38 Suffix1
W From Atterberg Tests LL 32, PL 26 Ip
32 - 26 6 From Plasticity Chart point lies
below A-line Suffix2 M Dual Symbols are
SW-SM To complete the classification the Symbols
should be accompanied by a description
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