Understanding the micro behaviour of granular pavements

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UNDERSTANDING THE MICRO BEHAVIOUR OF GRANULAR
PAVEMENT

• Presented by Haran Arampamoorthy Senior
  Research Engineer
• OPUS Central Laboratories
• Gracefield, New Zealand

AFD80 Committee meeting, TRB's 90th Annual
meeting at Washington, DC, on January 25, 2011
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Introduction

• Majority of New Zealand roads are granular
  pavement with chip seal surface (gt90 of
  highways)
• Typical traffic up to 22,000 AADT
• Most state highways use a two coat chip seal

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Chip seal highway (extracted from NZ chip sealing
in New Zealand 2005)
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(extracted from NZ chip sealing in New Zealand
2005)
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(extracted from NZ chip sealing in New Zealand
2005)
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Purpose of the research

• Understand micro behaviour of granular layer to
  minimise failure rate
• Identify the mitigation methods
• Project investigates
• Methods to minimise premature failure
• Improve service life.

Rut depth
Minimise early rutting
Age of pavement
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Ex Chip seal section failed by Shear
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Compacting roller Load and stress

P
P
r
150 mm
Granular layer
x
r
Fr
z
y
Stress computed along the line
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Retaining wall earth pressure approach
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Peak stress
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Stress during compaction

• Level of compaction increases
• confining stress increases up to certain level.
• modulus increases
• particle movement restricted
• plastic strain decreases
• Hypothesis
• Uneven stress distributions with depth shown in
  previous slides causes different particle
  movements at different depth.

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One Possible cause of early rutting

• Uneven particle distributions with depth
• This will worsen further at some locations,
  depending on
• - Compaction Energy distribution
• Roller path, Dense
• layer interface
• Segregation of particles
• Therefore aggregate will not be homogenous in
  terms of particle size and density with depth

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Possible of weakening shear stress during
compaction

• The stress direction changes frequently (depend
  on roller position).
• Repeated particle movement in same directions
• Changing the voids distribution
• Reduces at some depth No changes in
• Increases at another depth overall density
• This makes difficult to identify the density
  variation (To prove further investigation needed)

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Testing problems

• The peak stress is within a narrow band
  (approximately 50mm or even less)
• The depth of this band varies
• All makes it difficult to identify the
  differences using density test (Ex.)
• Nuclear densometer
• Sand cone testing
• Difficult to identify the weaken band, which
  could be a possible cause of initial rutting.

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Possible failure mechanism by wheel load in chip
sealed or unsealed roads within granular layer
Weaken the membrane and chip seal by cyclic
wheel load
Figure 2a Punching shear
Figure 2b Local shear
Further weaken the seal and water infiltrated
Figure 2c General shear
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Particle displaced in region b
Region B in tension
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Finite element analysis
Region B
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OPUS Test Track
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Section of the granular layer from test track
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Loose material
Voids
Dense material
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Need an advanced test method

• Spectral Analysis of Surface waves
• (Ex. Portable Seismic Pavement Analyser)
• Pressure gauge
• Induction coil
• 3D X-Ray
• Any other methods?
• Develop a non destructive test, which can detect
• the week zone

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Mitigations methods

• Option Could be Pre stress the soil
• Apply similar approach of pre-stress concrete
  beam
• Keep the region B (i.e. Tensile region) in pre
  compression
• This could be done during compaction process.
• Practical problems need to be rectified

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Suggestion and questions please

• Thank you