Engineering Soil Classification

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Engineering Soil Classification

• By D.A.Cameron
• 2003

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The Soil Phases
PHASE DIAGRAM
THE SOIL SYSTEM
AIR
WATER
SOIL SOLIDS
Air pockets
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New Terms
Density, ?, rho and 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 about 25 kN/m3 Soil varies
between about 15 - 21 kN/m3
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Other densities

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

Mass of soil/total volume
No air or water!
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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,
M Mw Ms
Ms
Mw
Ms
M Ms
105 oC
gravimetric
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Moisture and Density

• Prove that
• Where w water content (not in !)

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More soil terms.

• Void Ratio, e
• Degree of saturation, SR

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VOID RATIO
V Vs Vw Va
Ma, Va
Void volume, Vv Vw Va
Mw, Vw
Ms, Vs
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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

• 1. 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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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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2. Unified Soil Classification System (USCS)

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

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Defining Particle Sizes
Grain size (mm)
0.002
0.02
2.36
20
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
Uniform soils (P)
Silt (M)
Gap graded (P)
Gravel (G)
mm
Well graded (W)
n
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Sieve Analysis
Uniform size range
deficient here
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Particle Size Distribution Terms
UNIFORM SIZES P
GOOD MIX OF SIZES W
MISSING SIZE RANGES P
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Defining equations

• The Uniformity Coefficient,
• Cu D60/D10 a gradient!
• 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
Plastic Index or PI
solid
liquid
The plastic zone
PL
LL
Max.
Moisture content
0
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Soil drying
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Atterberg Limit Tests

• PLASTIC LIMIT
• - by rolling soil into threads
• - m.c. at which soil breaks at a 3 mm thread
  the plastic limit
• LIQUID LIMIT
• - basically the m.c. at which the soil can
  fairly readily be sheared

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Fall cone apparatus
Casagrande equipment
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Casagrande Liquid Limit
1.
2.
3.
4.
Gap closed
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The Plasticity Chart
PLASTIC INDEX
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 USCSfor 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?
Plasticity?
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NOT SO TOUGH
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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

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DILATANCY - Step 1
Water to the surface
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DILATANCY - Step 2
Opening the hand shears the soil coarser soil
will dilate
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Dilation increase in volume
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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 or field tests)