1. COFFEY GEOTECHNICS
Outline of This Presentation
• PAVEMENT TYPES (by use)
– Airport
– Container Terminals
– Open Cut Mines
– Public Roads
– Temporary Haul Roads
• DESIGN METHODS & VEHICLE LOADS
– Empirical
– Mechanistic
• PAVEMENT MATERIALS
– Focus is on natural gravels in Western Australia
FUTURE TRENDS

2. COFFEY GEOTECHNICS
WHY USE DIFFERENT DESIGN METHODS
• Wheel loads, tyre pressure and tracking
differ.
• Stress affects modulus.
• Performance limitations differ.

3. COFFEY GEOTECHNICS
Airport Pavements
• Wheel Loads>>3 tonne
• B717 13.5 tonne per tyre
• Very High Tire Pressures
• (Learjet 60 is 1480kPa)
• Low repetitions
• No drive through the
wheels
• Tires rotate
independently
• Jets land at about
350km/hour

4. COFFEY GEOTECHNICS
Airport Pavements
• Track width varies
• Wander is significant
• Load varies by a large
margin.
• Airport Pavement
Structural Design System
(APSDS), uses
cumulative damage
factors to combine the
effect of different aircraft.

5. COFFEY GEOTECHNICS
Military Aerodromes
Foreign Object Debris (FOD)
is a major design factor.
No cracking please!
.

6. COFFEY GEOTECHNICS
Surfacing for Aerodromes
• CASA Circular AC 139-25(0) permits spray seals up to tyre
pressures of 1000kPa.
• For higher tyre pressures asphalt is required (except with a
concession).

7. COFFEY GEOTECHNICS
Container Terminals: Highly Channelized Loading With
Straddle Carriers
12 tonne per
tire
GPS controlled
straddle carrier
(zero wander).

8. COFFEY GEOTECHNICS
Open Cut Mines
• Wheel Loads>> 3 tonne
• 60 tonne + per tire
• Pavement is typically
unsealed.
• Wander large (human
drivers) or negligible
(GPS controlled,
driverless).

9. COFFEY GEOTECHNICS
Public Roads
• Wheel Load < 3 tonne
• Bigger load = more
tires
• Most trucks are the
same width
• Truck tires at about
700kPa
• Equivalent Standard
Axles
• Single axle
• ESA 8.2 tonne on 4
tires
• Many repetitions
• 105 to 108

10. COFFEY GEOTECHNICS
TRAFFICABILITY FOR CONSTRUCTION EQUIPMENT

11. COFFEY GEOTECHNICS
TRAFFICABILITY OVER CLAY
• A simple procedure using Clay moisture
content and Atterberg Limits (concept
proposed by DeFlice)
• Su at Liquid Limit = 1.7 kPa
• Su at Plastic Limit = 170 kPa

12. COFFEY GEOTECHNICS
Trafficability over Clay
• Liquidity Index (IL) = 1 at Liquid Limit
• Liquidity Index = 0 at Plastic Limit
• Su = 170 * 10-2IL (Atkinson)

13. COFFEY GEOTECHNICS
Trafficability over Clay
• Check that vertical
stress, factored for
repetitions
• (1 + Log N)
• at the top of the clay
is less than
• Ncu * 170 * 10-2IL

14. COFFEY GEOTECHNICS
Traffickability over Soft Clay
• Geo-reinforcement .
• Ncu increases
• Load spread angle
increases .
• Sensitivity to load
repetitions decreases

15. COFFEY GEOTECHNICS
Pavement Design Methods
EMPIRICAL METHODS
Typically a chart linking thickness to load and subgrade CBR
– Main Roads WA Engineering Road Note 9 for highways
– ARRB AP –T36/06 for local streets (Australian)
– US Army Corps of Engineers CBR method for airports
MECHANISTIC METHODS
Typically use elastic models
– AUSTROADS for roads
– SAMDM for roads (South African)
– APSDS 5 for airports (Australian)

16. COFFEY GEOTECHNICS
ERN 9 (Empirical, Sprayed seals)

17. COFFEY GEOTECHNICS
Origins of Main Roads WA ERN 9 (Empirical Method)
• Started with US Army Corp of Engineers (circa 1940) chart for
airport pavement design.
• (source: Geoff Jameson)

18. COFFEY GEOTECHNICS

19. COFFEY GEOTECHNICS
Origins of ERN 9
• Porter (1942) assumed that lightly trafficked road is
equivalent to 7000 lb wheel load and heavily
trafficked road is equivalent to 12000 lb wheel load.
• E H Davis (RRL 1949) assessed 7 roads in UK and
compared results to Porter’s curves.
• RRL(1955) adapted Porter’s curves to design of
roads for new estates.

20. COFFEY GEOTECHNICS
Origins of Main Roads WA ERN 9 (Empirical Method)
• Maclean (1959) assigned numbers of heavy vehicles
per day to RRL (1955) chart
• Country Roads Board (Victoria) adopted McLean’s
chart with minor modification of the traffic curves

21. COFFEY GEOTECHNICS
Main Roads WA ERN 9 (Empirical Method)
• Circa 1977: Commonwealth intervention.
• NAASRA (1979)- non linear regression analysis on
Country Roads Board chart.
• 1 ESA per heavy vehicle
• 20 year design life to develop chart for thickness
CBR and ESA.
• Main Roads WA adopted the NAASRA chart in ERN 4
(1981) and later in ERN 9 (1988)
• Chart extrapolated by MRWA to 2 x 108 ESA

22. COFFEY GEOTECHNICS
VEHICLE LOADS
McAdam (1824)
• ...Stage coaches, with their present system of
loading , and velocity of travelling upon very narrow
wheels, damage the roads in a much greater
proportion than the compensation derived from the
toll.

23. COFFEY GEOTECHNICS
VEHICLE LOADS
• Wakelan HT (1916) “The damage which is
undoubtedly been caused to road surfaces,
by mechanically propelled vehicles during
the last two or three years, in particular, has
been of an extraordinary nature”.

24. COFFEY GEOTECHNICS
Regulation Axle Loads in WA (single axle dual tyres)
• 1919 4.89 tonne 15
Leagl Load Single Axle
• 1928 5.08 tonne 13
WA (Tonne)
• 1947 7.71 tonne 11
• 1960 8.16 tonne 9
• 1977 8.50 tonne 7
• 1988 9.0 tonne 5
3
1900 1950 2000 2050
MRWA allows 12 tonne
on a single axle under Year
concessional permit.

25. COFFEY GEOTECHNICS
Austroads Method for Assessing Damaging Effects of
Different Loads and Axle Groups
• Basic data from AASHO Road test
• Assumption: Equal surface deflection gives equal
pavement damage?????
• Single axle with dual tyres 8.2 tonne
• Single axle with single tyres 5.4 tonne
• Tandem axle with dual tyres 13.8 tonne
• Triple axle with dual tyres 18.5 tonne

26. COFFEY GEOTECHNICS
Loads other than Reference Load (Austroads Method)
• All loads expressed as equivalent repetitions of 80kn
(8.2 tonne, 18000 lb ) single axle load.
• N= (Group Load/Reference Load)n
• “n” varies from 4 to 12
• ESA uses n = 4
• SAR use n = 5 to 12

27. COFFEY GEOTECHNICS
Weaknesses in the AUSTROADS Approach to Loads
• Tire pressure not included in load equivalency
• Pore pressure not considered
• Tangential loads ignored
• The AASHO Road Test on which the Austroads damage
relationship was based had the following limitations:
– Straight
– Flat
– Constant Speed
– Largest vehicle was a 5 axle semi trailer (we have up to 27
axles on a road train in WA)

28. COFFEY GEOTECHNICS
Flexible Pavement Design
• Flexible pavements on Public Roads are
fairly unique amongst engineering
structures.
• They are unusual in that the design load
(single axle, 8.2 tonne) is less than loads
routinely applied now (eg 12 tonne under
permit)

29. COFFEY GEOTECHNICS
Road train
in WA
hauling
iron ore.
Significant
tangential
force
applied by
drive axle.

30. COFFEY GEOTECHNICS
Mechanistic Design
• Pavement layers typically modelled as elastic or
visco elastic
• Transfer function is an equation that relates the
allowable number of load repetitions to some
parameter (eg strain) calculated using the elastic
model
• N = a (strain) b

31. COFFEY GEOTECHNICS
Mechanistic Design for Public Roads
• Numerous Methods
• Austroads (2010) is an example.

32. COFFEY GEOTECHNICS
AUSTROADS METHOD
• Transfer Function for Vertical Subgrade Strain
(Roads).
• N = (9300/microstrain)7
• This is an empirical transfer function for highways. It
is not a fundamental equation from physics.

33. COFFEY GEOTECHNICS
Austroads
• From 3 aeroplane
curves to
• N = (9300/microstrain)7
• By extrapolation,
assumption and two
stages of non linear
regression.

34. COFFEY GEOTECHNICS
Subgrade Strain (Airport Pavements)
• N = ( k/ε)b
• The exponent b varies with subgrade stiffness and number of
tires on the landing gear( Wardle & Rodway 2010).
• 1 tire b = 7 to 11
• 2 tires b = 12 to 15
• 4 tires Tandem axle b = 17 to 19
• 6 tires Triple axle b = 27

35. COFFEY GEOTECHNICS
ASPHALT IN AUSTROADS
• Austroads method only considers
fatigue.
• Development of ruts is as critical as
fatigue but is not catered for in the
AUSTROADS design procedure.
• We need wheel tracking tests for rut
potential to become routine for asphalt.

36. COFFEY GEOTECHNICS
ASPHALT IN AIRPORTS
• Fatigue of asphalt in airports is relatively
rare.
• Rut development is a major issue particularly
in holding areas at the end of taxiways.
• Consider the use of polymers (EVA) or
multigrade binders to improve rut resistance.

37. COFFEY GEOTECHNICS
Comparison of AUSTROADS & South African Mechanistic Methods
AUSTROADS 2010 SAMDM
Linear Elastic model YES YES
Sub layering YES IMPLICIT
Anisotropy Anisotropic for
unbound granular
materials Isotropic for all layers
Isotropic for asphalt
and cemented (bound)
layers
Fatigue model for
asphalt based on
YES YES
tensile strain

38. COFFEY GEOTECHNICS
SAMDM & Austroads
AUSTROADS 2008 SAMDM
Fatigue model for
cemented layers based YES YES
on tensile strain
Crushing of lightly Based on ratio of
NO
bound materials vertical stress to UCS
Model for unbound NO Based on principal
base course stresses at mid layer of
base course.
Subgrade based on
vertical strain YES YES

39. COFFEY GEOTECHNICS
LIMITATIONS OF AUSTROADS 2010 method
• Unbound base course- no transfer function
for shear.
• Lightly bound materials – no transfer
function for crushing.
• Asphalt – no transfer function for rutting.
• Horizontal loads not adequately addressed.
• Mathematical problems.

40. COFFEY GEOTECHNICS
70mm Thick Asphalt, 800MPa Granular Base
1200
Granular Thickness (mm)
1000
CBR 4
800
CBR 12
600
400
200
0
1.00E+05 1.00E+06 1.00E+07 1.00E+08
Traffic Loading (ESAs)

41. COFFEY GEOTECHNICS
Road Pavements over Expansive Clay
• Granular cover requirements to manage
shrink swell may exceed CBR based design
• Use AS2870 to estimate soil movement
• Rip and moisture condition the subgrade
• Avoid compacting much above 92% MMDD
• Cover and allow to cure for about 3 months
before finalizing pavement construction

42. COFFEY GEOTECHNICS
Pavement Materials for Rural Roads
• Since the
1950s sprayed
seals on
natural gravel
have
dominated the
rural road
scene in WA.

43. COFFEY GEOTECHNICS
Natural
Gravel
Where do WA
Specifications
come from and
what do the various
tests mean?

44. COFFEY GEOTECHNICS
Factors Leading to WA Selection Criteria
• Culture
• Climate
• Axle loads
• Geology
• Cost

45. COFFEY GEOTECHNICS
Culture
• Engineering training came from UK and USA
(English speaking). Papers in Arabic or Chinese
have had little influence.
• WA Gravel Specifications have AASHTO as their
starting point.
• Influence of the visit by Dr Frank Netterberg in the
1980s.

46. COFFEY GEOTECHNICS
Arid Climate

47. COFFEY GEOTECHNICS
Geology
UK & USA typically use
• Crushed Rock
• River Gravels
• Glacial Outwash
WA (outside of Perth) use
• Pedocretes including
• Lateritic gravel
• Ferricrete
• Calcrete
• Silcrete

48. COFFEY GEOTECHNICS
Cost
• WA has a small
population and a
large area.
• Crude oil suitable
for bitumen
production is
imported.

49. COFFEY GEOTECHNICS
What do the Tests Mean?
Most tests are indirect “indicators”.

50. COFFEY GEOTECHNICS
Particle Size Distribution
• Ideal grading based on Wilhelmi or Talbot Curves
p/P = (d/D) 0.42
• Maximum size < 37.5mm (or 19mm)
• Larger size increases material strength and stiffness
but creates workability issues and may increase
permeability.
• Smaller maximum size correlates to reduced
material strength and stiffness.

51. COFFEY GEOTECHNICS
Atterberg Limits
• High Plasticity Index (PI) associated with increased
moisture sensitivity.
• Specification limits vary with climate and geology.
• Sesquioxides (Al2O3 and Fe2O3) and carbonate
(CaCO3) affect measurement and modify the effects
of plasticity.

52. COFFEY GEOTECHNICS
Maximum Plasticity Index Limits from Around the World
• AASHTO Gravel 6
• Botswana Calcrete 15
• Brazil Lateritic Gravel 15
• Ghana Lateritic Gravel 10
• Kenya Natural Gravel 15 to 20
• Mali Lateritic Gravel 6 to 16
• Nigeria Lateritic Gravel 12

53. COFFEY GEOTECHNICS
Plasticity Index Limits from Around the World
• Portugal Lateritic Gravel 15
• Uganda Lateritic Gravel 16 to 25
• USACE (ToW) Lateritic Gravel 10
• Zambia Lateritic Gravel 6 to 10
• NAASRA Natural Gravel 6 to 10
• WA Lateritic Gravel 6 to 16
• WA Calcrete 12

54. COFFEY GEOTECHNICS
Effect of Sesquioxides and Carbonates
• Al2O3 and Fe203 are cementing agents
• Allow an increase in PI (coat the clay
during drying) (Moh & Mazhar 1969)
• CaCO3 particles are porous.

55. COFFEY GEOTECHNICS
Cracking of Laterite Pavements is Normal.
Lateritic gravel base,
asphalt surface with
slurry seal.
Cracks have been
like this for more
than 23 years.

56. COFFEY GEOTECHNICS
Does Plasticity Index Matter?
• Chose
limits
carefully
and
enforce
them.

57. COFFEY GEOTECHNICS
Does MDCS matter?
• Test has poor reproducibility.
• Pavement performance problems due to low
MDCS are extremely rare.
• Less important with sealed shoulders or
kerbed pavement.
• Offset by good mechanical interlock.

58. COFFEY GEOTECHNICS
Repeat Load Triaxial Test (RLTT)
• Cylindrical sample with confining
pressure is subject to many thousands
of compression load pulses.
• Caution: For materials treated with
cement, tensile stress under wheel
loads may rupture cementing bonds.
This is not modelled in the RLTT

59. COFFEY GEOTECHNICS
Repeat Load Triaxial Test
• Recent ARRB research has also thrown
much doubt on the value of RLLT for
assessing rut performance of unbound
granular pavements.

60. COFFEY GEOTECHNICS
Unsealed Roads
• South African
• Max 37.5mm
• Oversize Index 5%
• P0.425 x LS 100 to 365
• Grading Coef. 16 to 34

61. COFFEY GEOTECHNICS
FUTURE TRENDS IN HIGHWAY PAVEMENT DESIGN
• Finite element method that includes:
• Estimates of rut depth development
Transfer functions for granular base and rutting
in asphalt.
Provision for tandem and triple axles without the
need for ESAs
Use of higher design axle loads
Allowance for geogrids

62. COFFEY GEOTECHNICS
Future Trends in Pavement Materials on Public Roads in
WA
• Less use of natural gravel.
• Return to bitumen stabilised limestone.
• More stringent requirements for crushed
rock.
• Concrete pavements on heavily trafficked
freeways.
• Reduced use of full depth asphalt.

63. COFFEY GEOTECHNICS
Acknowledgements
• A great many people and organisations helped with this presentation by
providing me with photographs and sharing ideas over the last 40 years. I
particularly want to thank the following:
• John Atkinson, Bob Andrews, Srijib Chakrabarti, Russell Clayton, Andrew
Cray, Tony DeFlice, Phil Dight, Martin Ellam, Stephen Emery, Geoff Faro, Paul
Fisher, Gabor Hamory, David Harris, Sean Hayes, Geoff Jameson, Ross Keeley,
Landcorp, Reg Leach, Colin Leek,Tony Mansour, Frank Netterberg, Kerry
Sanderson, Jacqueline Scott, Weeks White.

64. COFFEY GEOTECHNICS
Limitation
People using information from this presentation must apply and
rely on their own skill and judgement to the particular issue
they are considering.