Significance of Shear Strain in

1

• Significance of Shear Strain in
• Unbound Granular Pavements
• Ben Hayward Pavements Engineer

2
Pavement Design in NZ

• Current design method in NZ based on Austroads
  Pavement Design Guide
• Permanent deformation is attributed to excessive
  vertical compressive strain at top of subgrade
• No consideration is given to shear failure in
  unbound granular layers

3
Granular Pavement Design

• Structural layer used to distribute surface load
  to the weaker subgrade.

• Current theory implies that pavement will not
  fail so long as
• High quality material used
• Thickness is sufficient
• Layer is protected from moisture

4
Shear Strain Why Its Important

• NZ road case studies and pavement research
  indicate that shear strains are occurring within
  the basecourse.
• Research has also shown these strains may lead to
  permanent deformation (rutting).
• Significant number of field investigations
  showing failure in the basecourse but minimal to
  no subgrade deformation.
• Thus rutting may not be purely a function of
  vertical compressive strain at the top of the
  Subgrade.

5
Investigation Overview

• Install strain measurement devices in test
  pavements (CAPTIF)
• Apply heavy vehicle loads.
• Measure basecourse shear strain.
• Relate strain measurements with rutting over
  pavement life.

6
CAPTIF Test Facility

• CAPTIF Test Track.
• Flexible unbound pavements.
• Single or dual wheels
• 21kN 60kN wheel load.
• Driven wheels.

7
Strain Measurement

• Free floating induction coils CAPTIF emu system
• Use induced voltage to measure deflection
• Laboratory/Field testing for optimum coil matrix
  design
• Mohrs rosette system adopted

8
Rosette Design Principal Strains
9
Strain Translation from Rosettes

• Original Principal Strains (with 200mm spacing)

• Strains Related Back to Common Point

10
Shear Strain Calculation

• 3 principal strains plotted on Mohrs Circle of
  Strain
• Maximum engineering shear strain is the diameter
  of the circle (based on a Tresca Criterion)

11
Pavement 1 and 2 Designs
12
Pavement Life Phases

• These phases have been used by some pavement
  researchers (e.g. Steven 2006)
• The post construction compaction period and
  continuing service life period were identified in
  the research and shear strain development was
  different during the two periods.

13
Post Construction Compaction Period Analysis

• Strong linear relationship between the initial
  magnitude of the maximum shear strain and the rut
  depth at the end of the post construction
  compaction period.
• This rut depth is very hard to model and most rut
  depth prediction models ignore this section (e.g.
  Steven 2006)

14
Continuing Service Life Analysis

• Rate of change of maximum shear strain is
  proportional to the rate of change of rut depth.
• gt85 of rutting was in basecourse (post mortem
  trenches)
• No basecourse consolidation (post test density
  testing)

15
Importance of Longitudinal Strain

• In both pavements the longitudinal tensile strain
  was significantly higher than the vertical
  compressive strain.
• This could be related to the rut depths in both
  pavements.

16
Importance of Longitudinal Strain

• In both pavements the longitudinal tensile strain
  was more sensitive compared to vertical
  compressive strain over pavement life.
• Longitudinal strain also showed a consistent
  increase over time which was comparable to rut
  depth.

17
Shear Strain Summary

• Longitudinal tensile strains are relatively high
  in unbound flexible pavements and these strain
  appear to be more related to rut depth than
  vertical compressive strains.
• The magnitude of the initial shear strain appears
  to be directly related to the increase in rut
  depth during the post construction compaction
  phase.
• The change in shear strain magnitude following
  the post construction compaction phase
  corresponds to the increase in rut depth after
  the post construction compaction deformation has
  occurred.
• Perhaps this is something to consider as well as
  subgrade strains?

18
Acknowledgements

• Technology NZ Technology Industry Fellowship
• NZ Transport Agency CAPTIF
• Fulton Hogan