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Outline of This Presentation

• PAVEMENT TYPES (by use)– Airport– Container Terminals– Open Cut Mines– Public Roads– Temporary Haul Roads

• DESIGN METHODS & VEHICLE LOADS• DESIGN METHODS & VEHICLE LOADS– Empirical– Mechanistic

• PAVEMENT MATERIALS– Focus is on natural gravels in Western Australia

FUTURE TRENDS

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WHY USE DIFFERENT DESIGN METHODS

• Wheel loads, tyre pressure and tracking differ.

• Stress affects modulus.• Performance limitations differ.

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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

wheelswheels• Tires rotate

independently• Jets land at about

350km/hour

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Airport Pavements

• Track width varies • Wander is significant• Load varies by a large

margin.• Airport Pavement

Structural Design System Structural Design System (APSDS), uses cumulative damage factors to combine the effect of different aircraft .

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Military Aerodromes

Foreign Object Debris (FOD) is a major design factor.

No cracking please! .

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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 (exce pt with a concession).

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Container Terminals: Highly Channelized Loading With Straddle Carriers

12 tonne per tire

GPS controlled GPS controlled straddle carrier (zero wander).

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Open Cut Mines

• Wheel Loads>> 3 tonne• 60 tonne + per tire• Pavement is typically

unsealed.• Wander large (human

drivers) or negligible (GPS controlled, driverless).

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Public Roads• Wheel Load < 3 tonne• Bigger load = more

tires• Most trucks are the

same width• Truck tires at about

700kPa• Equivalen t Standard

Axles• Single axle• ESA 8.2 tonne on 4

tires• Many repetitions • 105 to 108

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TRAFFICABILITY FOR CONSTRUCTION EQUIPMENT

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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

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Trafficability over Clay

• Liquidity Index (IL) = 1 at Liquid Limit• Liquidity Index = 0 at Plastic Limit

• Su = 170 * 10-2IL (Atkinson)

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Trafficability over Clay

• Check that vertical stress, factored for repetitions

• (1 + Log N) • at the top of the clay • at the top of the clay

is less than • Ncu * 170 * 10-2IL

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Traffickability over Soft Clay

• Geo-reinforcement .• Ncu increases• Load spread angle

increases .increases .• Sensitivity to load

repetitions decreases

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Pavement Design Methods

EMPIRICAL METHODSTypically a chart linking thickness to load and sub grade 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 METHODSMECHANISTIC METHODSTypically use elastic models

– AUSTROADS for roads– SAMDM for roads (South African)– APSDS 5 for airports (Australian)

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ERN 9 (Empirical, Sprayed seals)

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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)

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Origins of ERN 9

• Porter (1942) assumed that lightly trafficked road is equivalent to 7000 lb wheel load and heavilytrafficked road is equivalent to 12000 lb wheel loa d.

• E H Davis (RRL 1949) assessed 7 roads in UK and • 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.

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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

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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 • 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 10 8 ESA

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VEHICLE LOADSMcAdam (1824)

• ...Stage coaches, with their present system of loading , and velocity of travelling upon very narr ow wheels, damage the roads in a much greater proportion than the compensation derived from the proportion than the compensation derived from the toll.

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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 the last two or three years, in particular, has been of an extraordinary nature”.

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Regulation Axle Loads in WA (single axle dual tyres )• 1919 4.89 tonne

• 1928 5.08 tonne

• 1947 7.71 tonne

• 1960 8.16 tonne

• 1977 8.50 tonne 7

9

11

13

15

Leag

l Loa

d S

ingl

e A

xle

WA

(To

nne)

• 1988 9.0 tonne

MRWA allows 12 tonne on a single axle under concessional permit.

3

5

1900 1950 2000 2050Le

agl L

oad

Sin

gle

Axl

e W

A (

Tonn

e)Year

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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

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Loads other than Reference Load (Austroads Method)

• All loads expressed as equivalent repetitions of 80 kn (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

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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 limitation s:– Straight– Flat– Constant Speed– Largest vehicle was a 5 axle semi trailer (we have up to 27

axles on a road train in WA)

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Flexible Pavement Design

• Flexible pavements on Public Roads are fairly unique amongst engineering structures.

• They are unusual in that the design load • 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)

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Road train in WA hauling iron ore.

Significant Significant tangential force applied by drive axle.

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Mechanistic Design

• Pavement layers typically modelled as elastic or visco elastic

• Transfer function is an equation that relates the • 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

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Mechanistic Design for Public Roads

• Numerous Methods• Austroads (2010) is an example.

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AUSTROADS METHOD

• Transfer Function for Vertical Subgrade Strain (Roads).

• N = (9300/microstrain) 7

• This is an empirical transfer function for highways . It • This is an empirical transfer function for highways . It is not a fundamental equation from physics.

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Austroads

• From 3 aeroplane curves to

• N = (9300/microstrain) 7

• By extrapolation, assumption and two stages of non linear regression.

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Subgrade Strain (Airport Pavements)

• N = ( k/ε)b

• The exponent b varies with subgrade stiffness and n umber of tires on the landing gear( Wardle & Rodway 2010).

• 1 tire b = 7 to 11• 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

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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.AUSTROADS design procedure.

• We need wheel tracking tests for rut potential to become routine for asphalt.

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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 • Consider the use of polymers (EVA) or multigrade binders to improve rut resistance.

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Comparison of AUSTROADS & South African Mechanistic Methods

AUSTROADS 2010 SAMDM

Linear Elastic model YES YES

Sub layering YES IMPLICIT

Anisotropy Anisotropic for unbound granular unbound granular

materialsIsotropic for asphalt and cemented (bound) layers

Isotropic for all layers

Fatigue model for asphalt based on

tensile strainYES YES

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SAMDM & AustroadsAUSTROADS 2008 SAMDM

Fatigue model for cemented layers based

on tensile strainYES YES

Crushing of lightly bound materials

NOBased on ratio of

vertical stress to UCSbound materials vertical stress to UCS

Model for unbound base course

NO Based on principal stresses at mid layer of

base course.

Subgrade based on vertical strain YES YES

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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.• Asphalt – no transfer function for rutting.

• Horizontal loads not adequately addressed.

• Mathematical problems.

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600

800

1000

1200G

ranu

lar

Thi

ckne

ss (

mm

)70mm Thick Asphalt, 800MPa Granular Base

CBR 4

CBR 12

0

200

400

1.00E+05 1.00E+06 1.00E+07 1.00E+08

Gra

nula

r T

hick

ness

(m

m)

Traffic Loading (ESAs)

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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

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Pavement Materials for Rural Roads

• Since the 1950s sprayed seals on natural gravel have have dominated the rural road scene in WA.

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Natural Gravel

Where do WASpecificationscome from andwhat do the various tests mean?

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Factors Leading to WA Selection Criteria

• Culture

• Climate

• Axle loads

• Geology

• Cost

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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 th e 1980s.

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Arid Climate

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Geology

UK & USA typically use• Crushed Rock• River Gravels• Glacial Outwash

WA (outside of Perth) use• Pedocretes including• Lateritic gravel• Ferricrete• Calcrete• Silcrete

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Cost

• WA has a small population and a large area.

• Crude oil suitable • Crude oil suitable for bitumen production is imported .

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What do the Tests Mean?

Most tests are indirect “indicators”.

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Particle Size Distribution

• Ideal grading based on Wilhelmi or Talbot Curves

p/P = (d/D) 0.42

• Maximum size < 37.5mm (or 19mm)• Maximum size < 37.5mm (or 19mm)• Larger size increases material strength and stiffnes s

but creates workability issues and may increase permeability.

• Smaller maximum size correlates to reduced material strength and stiffness.

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Atterberg Limits

• High Plasticity Index (PI) associated with increase d moisture sensitivity.

• Specification limits vary with climate and geology.• Specification limits vary with climate and geology.

• Sesquioxides (Al 2O3 and Fe2O3) and carbonate (CaCO3) affect measurement and modify the effects of plasticity.

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Maximum Plasticity Index Limits from Around the World

• AASHTO Gravel 6• Botswana Calcrete 15• Brazil Lateritic Gravel 15• Ghana Lateritic Gravel 10• Ghana Lateritic Gravel 10• Kenya Natural Gravel 15 to 20• Mali Lateritic Gravel 6 to 16• Nigeria Lateritic Gravel 12

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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• Zambia Lateritic Gravel 6 to 10• NAASRA Natural Gravel 6 to 10• WA Lateritic Gravel 6 to 16• WA Calcrete 12

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Effect of Sesquioxides and Carbonates

• Al2O3 and Fe203 are cementing agents

• Allow an increase in PI (coat the clay • Allow an increase in PI (coat the clay during drying) (Moh & Mazhar 1969)

• CaCO3 particles are porous.

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Cracking of Laterite Pavements is Normal.

Lateritic gravel base, asphalt surface with slurry seal.

Cracks have been Cracks have been like this for more than 23 years.

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Does Plasticity Index Matter?

• Chose limits carefully and and enforce them.

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Does MDCS matter?

• Test has poor reproducibility.

• Pavement performance problems due to low MDCS are extremely rare.MDCS are extremely rare.

• Less important with sealed shoulders or kerbed pavement.

• Offset by good mechanical interlock.

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Repeat Load Triaxial Test (RLTT)

• Cylindrical sample with confining pressure is subject to many thousands of compression load pulses.

• Caution: For materials treated with • Caution: For materials treated with cement, tensile stress under wheel loads may rupture cementing bonds .

This is not modelled in the RLTT

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Repeat Load Triaxial Test

• Recent ARRB research has also thrown much doubt on the value of RLLT for assessing rut performance of unbound assessing rut performance of unbound granular pavements.

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Unsealed Roads

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

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FUTURE TRENDS IN HIGHWAY PAVEMENT DESIGN• Finite element method that includes:

• Estimates of rut depth developmentTransfer functions for granular base and rutting

in asphalt.Provision for tandem and triple axles without the Provision for tandem and triple axles without the

need for ESAsUse of higher design axle loadsAllowance for geogrids

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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 • More stringent requirements for crushed

rock.• Concrete pavements on heavily trafficked

freeways.• Reduced use of full depth asphalt.

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Acknowledgements• A great many people and organisations helped with t his presentation by

providing me with photographs and sharing ideas ove r the last 40 years. I particularly want to thank the following:

• John Atkinson, Bob Andrews, Srijib Chakrabarti, Rus sell Clayton, Andrew Cray, Tony DeFlice, Phil Dight, Martin Ellam, Step hen Emery, Geoff Faro, Paul Fisher, Gabor Hamory, David Harris, Sean Hayes, Geo ff Jameson, Ross Keeley, Landcorp, Reg Leach, Colin Leek,Tony Mansour, Fra nk Netterberg, Kerry Sanderson, Jacqueline Scott, Weeks White.

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Limitation

People using information from this presentation mus t apply and rely on their own skill and judgement to the partic ular issue they are considering.