Compaction of Soil

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Compaction of Soil

• D. A. Cameron
• Intro to Civil and Mining Engineering, 2003
• School of GMC

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1. The Process

• Expulsion of AIR
• - air void volume, Va, reduced
• - moisture content is unchanged or constant

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2. The Purpose of Compaction

• increase
• STRENGTH
• STIFFNESS
• DURABILITY

• decrease
• PERMEABILITY

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3. Earthwork Applications

• Earth dams, Levee banks, Road subgrades,
  Pavement layers, Subdivisions, etc
• Water retaining structures stability with low
  permeability
• Roads - reduce pavement thickness by increasing
  strength
• Subdivisions - reduce footing stiffness by
  increasing foundation strength and stiffness

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4. Laboratory Soil Compaction

• Compaction of all soil materials, except clean
  gravels and sands
• (no fines or fine soil content)
• - achieved by falling weight hammers of known
  mass and drop height
• ? constant energy

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AS1289 - Standard or Modified?

• Standard Compaction
• (Proctor 1930s)
• light compaction (low energy),
• 25 blows / 3 layers
•  
• (b) Modified Compaction
• (AASHO 1940s)
• heavy compaction (high energy),
• 25 blows / 5 layers (thinner lifts)

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Laboratory compaction testing-relevance?

• How does the soil respond when compacted on site?
• So, the laboratory method, which best replicates
  the field compaction equipment on an earthworks
  job, must be chosen.

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Compaction control parameter
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5. The Compaction Curve

• For a particular soil and compactive effort
  ........
• There is a unique relationship between the dry
  density that can be achieved and the moisture
  content of the soil
• Warning NA to clean sands and gravels

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• Removal of all air voids is impractical
• - ?d max at an air voids ratio ? 5
• (A Va / V )
• w at ?d max is termed the
  OPTIMUM MOISTURE CONTENT (OMC)
• lt OMC, the soil is stiff and dry
• Its difficult to re-orientate particles 
• gt OMC, the soil is too deformable
• flows when compacted

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The Shape of the Compaction Curve
5?
Dry Density
Zero air voids line
?d max
Soil too dry and brittle
Soil too wet and deformable
OMC
Moisture content
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INFLUENCE OF SOIL TYPE ON COMPACTION CURVE -
Constant compaction energy
Sand with some fines
Dry Density
Zero air voids line
?d max
Clay
Moisture content
OMC
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Influence of Compaction Energy
Modified Compaction
Dry Density
Standard Compaction
Moisture content
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Influence of Compaction Energy

• The same effect is realised on earthworks
  projects by
• Increasing the mass of compactors
• Compacting in thinner lifts
• Passing over each layer more
• number of passes

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Air Voids line - a good check on validity of
compaction curves - need particle density
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6. Influence of Compaction on Soil Properties

• Soil strength (stability)
• Stiffness (settlements under load)
• Durability (repeated loading)
• Permeability
• (how easily water passes through)

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Compaction and permeability
Dry Density or permeability
B
C
A
kminm
Moisture content
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7. Compaction Practice

• Compacted in thin layers or LIFTS
• (100 to 200 mm for fine grained soil)
• Silts and Clays - need relatively long duration
  loading
• Sands and Gravels - vibration has greatest effect

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Typical Compaction Equipment
one person operation
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Heavy Compaction Equipment
Smooth steel rollers, with or without vibration -
double or single drum
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Sheepsfoot roller
Pneumatic roller
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Broons Hire
Eccentric rollers?
Square rollers?
Deeper compaction?
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8. Field Checks of Density

• DIRECT a) Sand replacement method
• INDIRECT b) radiation nuclear moisture
  density meter
• c) soil penetration testing

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Sand Replacement

• Cylindrical hole cut in soil
• Soil kept and weighed (M), then moisture content
  (w) obtained by drying the soil
• Obtain dry sand (SP) of known density
• (?sand) when poured from a funnel
• Pour dry sand from container into the hole
• Loss in mass of container and sand
• ?M ? volume of hole (V ?M/?sand)

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Hole prepared - plan
Testing with clean dry sand - elevation
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Nuclear Density Meter

• The rate at which gamma rays pass through the
  soil to reach a detector tube at the ground
  surface can be converted to wet density of the
  soil.
• Take a sample for moisture content and so obtain
  Dry Density

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Nuclear density meter
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1. SET - UP
2. MEASURING - ? photon count
source
? detector
direct path
sand pad
indirect path
Hole prepared
count ratio ? wet density
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Count Ratio, DCR

• Count in soil
• (over time, t)
• divided by
• Background ? radiation count
• (over time, t)

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Falling Weight Penetrometer
Falling weight
Soil penetration
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Blowcount/ 300 mm
2
4
6
8
10
12
0
0.3
Depth in soil (m)
0.6
0.9
1.2
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9. Specification of Compaction of Clean Sands
Gravels

• Maximum compaction when either
• bone dry or saturated
• Capillarity resists compaction
• Compaction defined in terms of maximum and
  minimum dry densities
• ?d max and ?d min

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Method clean granular soils

• ?d min - dry sand, poured through funnel,
• - low drop height
• ?d max - saturated sand in cylinder with
  dead weights
• - vibrating table

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alternatively
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Description of coarse-grained soil
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10. Specification of Compaction

• AS3798 Guidelines on Earthworks for Commercial
  and Residential Developments
• Dry Density Ratio, RD
• Ratio of desired dry density to the maximum
  achievable by the chosen laboratory method,
• e.g. 95 (Standard Compaction)
• or 98 (Modified Compaction)

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Laboratory v. Field Compaction

• Try to match the two
• - may need field trials to achieve this
• May have to vary
• Lift thickness
• number of passes
• Compaction equipment

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Field v Laboratory Compaction
Lab Modified Compactive effort
Dry Density
?d max
40 ton roller, 12 passes
100 ton roller, 12 passes
OMC
Moisture content
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Notes on specification

•  

Sometimes moisture contents for compaction need
to be tightly specified. Why? What if a soil
on site is too wet for compaction?
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AS3798 -1990