Pavement design for low volumeroads

Guy Doré, ing., Ph.D.NSERC i3C Industrial Research Chair

Canadian Highwayso The Canadian road network: 1.04 million kmo World's seventh biggest network.o Paved roads: 415,600kmo Unpaved roads: 626,700kmo At least 800 000 can be considered “Low

Volume”o In Alberta, ~82% of the road network (126

000/155 000) is classified local roads

What is a « Low Volume Road »

Low volume roads

o BC/AB: AADT < 200o Ont/Sask: AADT < 500o US: AADT < 400

LVR can be classified in 3 categorieso Rural System Roads –

• access to farms, residences, businesses or other properties• access to and within isolated communities• Traffic: light and medium vehicles and occasional heavy trucks.

o Recreational Roads –• Primary Roads, which connect to the external road network, perimeter

roads around the recreation area• Recreation area internal roads• Traffic: cars, trailers, camper-truck units and maintenance vehicles.

o Resource Development Roads –• Resource related roads such as forest and mining roads.• Traffic: predominantly large, heavily loaded trucks.

Characteristics of LVRo Low volume of traffic but

vehicles can be very heavyo Major effect of climatic factorso Failure modes and tolerance

levels differ considerably fromhigher volume roads

o Important constraints relatedto construction andmaintenance of LVR (expertise,distance, available budgets, etc)

Pavement design approacheso Experience-based designo Empirical approacho Mechanistic-empirical approach

Pavement design for LVRo Art or science?o Most methods are experience-based or

empiricalo Most empirical methods are extrapolated

from general pavement design empiricalmodels

o Some mechanistic considerations in a fewmodels

Experience based design methodso One or few predefined

pavement structureso Based on local experienceo General guidelines on layer

thickness as a function of siteconditions

o Example from « Low volumeroad engineering» (Keller andSherar, 2003)

o High risk of over/underdesign 

Empirical design methodso Methods based on empirical relationships between site

conditions, traffic, pavement structure and level of serviceo Relationships developed using observations at experimental siteso Generally developed for high volume roads and

extrapolated/adapted to LVR conditionso Extrapolation of empirical relationships can lead to important

design errors

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1109440,0

5,12,4log20,01log36,9log

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Example of empirical method:AASHTO

M-E methodso Methods based on the calculation of stresses and strains in

the pavement structureo Allowable stresses and strains obtained from empirical

damage functionso Pavement structure adjusted to meet allowable strain criteriao Rationale approach to pavement design for any type of roado Damage functions and criteria need to be adapted to low-

volume road conditions

Design objectives

Allowable strains

Layer thicknesses

Site conditionsSoil and material properties

adm.adm.

MLEA model

adm

Log N

Empirical damage functionsM-E

Pavement design

Example of M-E method: NCHRP 1-37a

Comparison of different LVRpavement design methods

0

200

400

600

800

1000

1200

HammitGiroud and

Noiray TRRL AustRoad AASHTO NCHRP 1-37A FCE MTQ MTOBerthier(France)

Ouest(France)

LVR design (AADT = 500; Clay subgrade)

AC/BST Base Subbase

Pavement design for low volumeroadso LVR represent an essential and a major asset

in Canadao Rigorous design methods need to be used to

maximize the performance and the return onthe investment on these roads

o Based on our study, none of the methods usedin Canada allow for optimization of pavementdesign for low volume roads

Pavement design forlow volume roadso M-E pavement design

methods are accessible andallow for optimization ofpavement design for LVR

o Performance models andcriteria need to be adaptedto LVR and Canadianconditions

o Emphasis on permanentdeformation and frost heave

Considerations for selection ofsurface typeo Selection of optimal solution requires a life-cycle cost

analysiso Cost information is highly variable depending on site

consideredo Reliable Information on performance is difficult to

findo Other site specific constraints need to be considered

(availability of expertise, materials and equipment)

Conclusion

o Local roads or « low volume roads » areextremely important for social and economicdevelopment in Canada

o Rigorous pavement design for LVR is requiredto maximize the performance and the benefitof these structures considering limitedresources

o M-E design appear to be the best approach

Conclusiono Research is needed

• To develop and improve performance models anddesign criteria adapted for LVR and Canadianconditions

• To adapt specification and characterizationprocedures for LVR materials

To allow for optimisation of LVR pavementstructures and selection of optimal surfacingstrategy

THANK YOU !