Engineering Soil Classification

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Engineering Soil Classification

• By D.A.Cameron
• 2006

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The Soil Phases
PHASE DIAGRAM
THE SOIL SYSTEM
The Soil System
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New Terms
Density ? rho Unit weight ?
gamma
e.g. ?water ?w 1 t/m3 or 1 g/cc ?
?w ?w x g 9.81 kN/m3
Unit weight of concrete ? 25 kN/m3 Soil varies
between ? 15 - 21 kN/m3
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Other densities

• Soil dry density, ?d
• Particle density, ?s

Mass of soil / total volume
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Introduction to soil terms, contd

• Particle densities range between 2.6 and 2.7 t/m3
• Moisture content, w
• based on mass of water
• gravimetric rather than volumetric

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Moisture Content
Mw
Ms
Mo Mw Ms
Mw Mo - Mf
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Moisture and Density

• Prove that
• where, w water content (just a ratio, not !)

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More soil terms.

• Void Ratio, e
• Degree of saturation, SR

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VOID RATIO
V Vs Vw Va
Mw, Vw
Ms, Vs
Solids
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Soil Consistency

• DENSITY of granular soils
• loose, medium dense, dense,
• or very dense
• STRENGTH of fine-grained soils
• soft, firm, stiff or hard

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SOIL CLASSIFICATION

• I. Pedological
• - classification based on the development
• of a soil profile due to weathering
• e.g. Great Soil Groups
• Taylor, Thomson and Shepherd
• Bulletin 46, Dept. Mines, S.A., 1974

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Major Forms of Iron Effect on Soil Color
Form Chemical Formula
Color Ferrous oxide FeO
Grey Ferric oxide (Hematite) Fe2O3
Red Hydrated ferric oxide
(Limonite) 2Fe2O3 ?3H2O Yellow
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RB9 - Taylor et al (1974)
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Great Soil Groups

• Adelaide Map Shallow exploration
• map unit soil association
• dominant soil types
• e.g. RZ / TR or BS / TR / BE
• UNCERTAINTY IS EVIDENT!

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II. Unified Soil Classification System (USCS)

• Based on...
• Particle size
• - gravel, sand, silt, clay fractions
• Particle size distribution
• - grading
• Plasticity

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Symbols of the USCS coarse grained
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Defining Particle Sizes
Grain size (mm)
0.002
0.2
2.36
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200
0.075
0.6
6.0
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Basic Soil Type
F M C
F M C
CLAY
SILT
SAND
GRAVEL
COBBLES BOULDERS
Fine-grained soil
Coarse-grained soil
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Sieve Analysis - coarse soils
Gravel (G)
Sand (S)
Silt (M)
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Sieve Analysis - coarse soils
Gap graded (P)
Sand (S)
Silt (M)
Gravel (G)
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Poorly Graded (P) or Gap-Graded
GP-SP
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Particle Size Distribution Terms
P - Poorly graded (uniform sizes)
W - Well graded Good mix of sizes
P - Poorly graded Missing range of sizes
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Defining equations

• The Uniformity Coefficient,
• Cu D60/D10 inverse of gradient of PSD!
• If Cu ? ?5 ? uniform soil, P
• If Cu gt ?5 ? well-graded soil, W
• 4 for gravel and 6 for sand (AS1726)

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Fine-grained Soils

• Too fine for sieving
• Sedimentation and/or laser equipment?
• Even then, sizes say nothing about clay
  mineralogy and potential soil behaviour!

?Fine-grained soils are defined by how plastic
they are
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Symbols for Fine Grained Soils
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Consistency Limits of Fine Soils

• Defining water contents
• 1. LIQUID PHASE
• - fluid, low shear resistance
• 2. PLASTIC PHASE
• - easily moulded
• 3. SOLID PHASE
• - strong, resists deformation

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ATTERBERG LIMITS
solid
liquid
The plastic zone
Max.
Moisture content
0
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CONSISTENCY LIMITS
Change in Volume
PL
LL
Moisture content ()
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Atterberg Limit Tests

• PLASTIC LIMIT, PL
• - by rolling soil into threads
• - m.c. at which soil breaks at a 3 mm dia.
  thread the plastic limit

• LIQUID LIMIT, LL
• - basically the m.c. at which the soil
  can fairly readily be sheared

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Casagrande Liquid Limit
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Casagrande LL Device

• Each blow must be consistent
• fall height 10 mm
• hardened base
• 25 blows for closure over 10 mm length defines
  the LL
• Usually 15-22, 22-30 and 30-38 blows
• need moisture contents for each state, dry to wet
  of LL
• interpolate mc for 25 blows

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The Plasticity Chart
PLASTIC INDEX ()
Example LL 75 PL 32
LIQUID LIMIT ()
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Linear Shrinkage Test

• Change in length of half a cylinder prepared to
  LL and oven-dried
• Basically provides volume change of a remoulded
  soil over
• ?w plastic index

e.g. Lo 250 mm ?L 25 mm ?LS 10
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Field Tests of the USCS for fine-grained
soils

• Dry strength
• relative strength of a dry ball of soil
• prepared at PL
• Toughness
• near PL when remoulded
• Dilatancy
• volume change upon shearing
• prepared at LL

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TOUGHNESS how tough is it to remould?
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Dilatancy - (field test)

• Discard coarse material and wet the remainder
• Form a pat in your cupped hand
• Tap the back of your hand until water rises to
  surface
• (surface sheen)
• Stretch palm of hand
• Shears the pat of soil
• Does water stay or disappear?

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Classification by Field Test
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Interpretation of Field Tests

• Dry strength is low for O and M soils of low
  plasticity
• Dry strength increases with plasticity
• Dry strength is greater for clay soils
• Toughness increases with plasticity
• Silts are dilatant but clays are not!
• dilation increase in volume (with shearing)

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Classification of Mixed Soils

• Wet sieve on 0.075 mm sieve
• gt 50 retained? coarse
• Sieve on 2.36 mm sieve
• lt 50 retained? Sand
• Sieve for fines
• lt 5 SP or SW (fines insignificant)
• gt12 SC or SM (plasticity?)

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SUMMARY

• Soil classification for engineering purposes is
  based on
• 1. Fundamental particle sizes
• AND
• 2. Particle size distributions
• OR
• 3. Soil plasticity
• (LL, PI, LS and/or field tests)