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WS 1: Pavement Design in Cold Regions

Cold Region Road Design Dilemma Example from Swedish Road Design

Code.

By Carl Lenngren, Lund University

INTRODUCTION

Thedesignofpavementstendstothickinordertosustainfrostheaveandparticularly

unevensettlementsatspringthaw.Typicallymostroadsincoldregiononlycarrylimited

amountsoftraffic.Thustheinvestmentcostisrelativelyhighforthesetypesofroads,and

the

amount

of

material

moved

is

high

per

vehicle

tons

travelled.

Roughroadsinducehighusercoststhough.Inthepresentpresentationadiscussionis

carriedonhowtobalancethecoldregionpavementdesignwiththefundsavailablefor

investmentandtheassociatedusercosts.

Asinothercountries,Swedishdesigncodestemmedfromempiricalknowledge.By1968the

designcombattedtwoconcerns,trafficandtheenvironment.Trafficwashandledby

calculatingtheequivalentnumberofstandardaxlesoveradesignperiod.Theenvironment

consistedoftheclimateandthenaturalsoilconditions.Laterondrainagewasaddedasan

environmental

factor

of

concern.

Originally,

the

traffic

design

was

controlled

by

adjusting

the

thicknessofthebasecourselayer.Theenvironmentaldesignwascontrolledbythethickness

ofthesubbaselayer.Asthecriteriaforthesubbaselayerwasgraduallyimproved,other

layerswereintroduced,likefrostprotectionlayersetcetera.Thesemayineffectbe

consideredaslowergradesubbaselayers.

In1986thethicknessesweretabulatedforthevariouscategoriesofclimatezones,soil

materialsanddrainage.Thestrainsatcriticalpointswerecalculatedusinglinearelastic

programslikeBISARtoderivethevariousdesignstructures.Upuntilthen,thethaw

weakening

period

during

the

spring

was

considered

to

be

critical

for

the

entire

design.

By1994amoredirectapproachtothestraincriteriawastakenwherethesituationduring

sixseasonswasconsidered.However,thefrostdesignnowwasaddedasaterminthe

equationindependentofthestraincriteria.Thisdesignisbasedonfrostheaveandthe

weightoftheroad.

Thestraincriteriaarebasedonfatigueofboundlayersandpermanentdeformationrateasa

regressionofthetopofthesubgradeverticalstrain.Inadditionthereisacriterionfor

roughnessbasedonthedesignspeedandroadcategory.

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PROBLEM

Fromauserpointofview,theexpectedtravellingcostshouldbeaboutthesameforthe

sametypeofroadcategoryregardlessofthelocation.However,theroadholdercost

particularlyregardingtheinvestmentismuchhigherincoldclimates.Iftheusercostcriteria

are

persisted

it

may

defer

new

investments

in

cold

climate

areas.

Thenewconceptsofdesignshouldallowforalargervariationoflayerthicknessesand

qualitiesandthereisalsospecificsoftwarefurnishedbythetransportadministrationto

assistroaddesignconsultants.Lowvolumeroadsarenowdesignedthicker.Thismeansa

substantialcostincreaseforatypeofroadthatalreadyhasaverylowproductionperunit

invested. Typically,incoldregionsthepopulationdensityissparse.Anassetmanagement

systemmaynotprioritizeroadswithhighinvestmentcosts.Thequestionthenarisesifthe

ambitionistoohighwiththecurrentcriteria?

CONCLUSION

Thecostsforbuildingandmaintainingroadsincoldclimatescanbeenormousifusercosts

onlygovernthedesign.Somehowitisnotjustifiabletofullycompensatetravelcostswith

roadholdercostsbeingmorethan100hundredtimeshigherperkmorton*kmbeing

travelledfortheroadholder. Theextracostsforrougherroadsmusttosomeextentbe

takenbytheusers.Thereisalimitforroughnessthough.AttheIRIinterval34m/kmroads

with20100trucksperdaywouldincursuchhighusercoststhatthehigherinvestments

wouldbemotivated.Fewroadsshouldhavehigherroughnessvaluesthan6m/kmashigher

maintenancecostcouldbemotivated.

ITEMS TO BE DISCUSSED

Obviously,frostheaveandunevenfrostheaveacceleratepavementdeterioration.Itcanbe

measuredasroughness.Theroughnessinturncanbeattributedtousercosts.Isit

reasonabletoinvestinaheavierstructuretofullycompensatetheusercosts?Orshouldthe

designallowforasomewhathigherdeteriorationtooptimizethetotal(roadholderand

user)costs?