Highway Engineering TRAN 3001 Lecture 7

8/9/2019 Highway Engineering TRAN 3001 Lecture 7

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TRAN3001Highway Engineering

Lecture 7Structural Design

Flexible PavementDesign

Pavement Layers


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Structural Design

The purpose of structural esign is to li!it thestresses transferre to the founation "soil# $y tra%cto a safe le&el at which efor!ation of the founationis insigni'cant while at the sa!e ti!e ensuring thatthe roa structure oes not eteriorate to any serious

e(tent within a speci'e perio of ti!e "esign perio)10 to *0 years#+

,a&e!ent wear-istress is the egraation ofpa&e!ent .uality ue to cli!ate "water ante!perature# as well as loaing $y tra%c+ /hen roasare esigne it is assu!e that ae.uate routine ane!ergency !aintenance will $e carrie out uringthe esign perio of the roa an that at the en ofthe esign perio a relati&ely low le&el of

eterioration will ha&e occurre+


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Structural Design "ont#

/earing course Surfacing2asecourse "asphalt#

,a&e!ent Roa$ase

Su$$ase or!ation Su$grae

Ele!ents of a le(i$le ,a&e!ent


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Flexible Pavement

/hether it is a pa&e!ent for a !a4or highspee highway or a local-resiential roa

the $asic structural ele!ents areessentially the sa!e) surfacing an istinctlayers superi!pose on the su$grae+

Surfacing This is the upper!ost layer of thepa&e!ent an will nor!ally consist of$itu!inous surfacing !aterial+


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Flexible Pavement (Cont’d)

Surfacing "ont#

or resiential an other !inor roas the

$itu!inous surfacing !ay $e a single layerof asphalt or e&en a surface ressingco!prising spraye "cut5$ac6# $itu!eno&erlain with stone chips+or !a4or an high spee roas the$itu!inous surfacing usually co!prises twolayers+


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Flexible Pavement (Cont’d)Surfacing "ont#

These are 6nown as the wearing course "top

layer# an the $asecourse or $iner course"$otto! layer#+As the tra%c runs on the wearing course this!ust $e a high .uality) ura$le an

i!per&ious surface+ The $asecourse) whichis not as i!per&ious) etc+ as the wearingcourse) acts as a regulating layer whichi!pro&es the riing .uality of the wearingcourse+


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Surfacing "ont#

The pri!ary functions of the surfacing are to

pro&ie a safe an co!forta$le tra&ellingsurface for tra%c an to protect the unerlyinglayers fro! the a&erse eects of cli!ate an

tra%c "&ehicles $ra6ing) s6iing) etc+#+

or esign purposes the surfacing is generallynot consiere to contri$ute to the strength ofthe pa&e!ent) which errs on the sie of safety+


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Roa$ase

/ith respect to structure) this is the !osti!portant layer of a 8e(i$le pa&e!ent as ithas to istri$ute the applie surface loassafely to the unerlying layers+

As the !ain loa5spreaing layer of thepa&e!ent) roa$ase !aterial !ust $e ofhigh .uality an for !a4or-high speehighways crushe stone will usually $e

speci'e+


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Flexible Pavement

The esign of a 8e(i$le pa&e!ent is $ase onthe lateral istri$ution of the tra%c loa with

epth which trans!its the pressure fro! thetop to the unerlying layers of the pa&e!entan then to the su$grae "founation#+

The loa is istri$ute o&er a s!all area at the

surface) $ut as the epth increases) the sa!eloa is istri$ute o&er a larger area+


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Road-


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Flexible Pavements (Cont’d)or 8e(i$le pa&e!ents) the following are the &isi$le signs

ofwear-istress:

rac6ingrac6ing can originate in the $itu!inous surfacing at the

top of the pa&e!ent $ecause of fatigue cause $y thenu!$er of applie wheel loas+

rac6ing can also result fro! tensile stresses ue tote!perature changes+

Re8ecti&e crac6ing of the "top# $itu!inous surfacingoccurs as a result of crac6s or fractures in the

unerlying layers+


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Rutting

Rutting is the e&elop!ent of epressions in the

pa&e!ent surface along the wheel paths)typically with a with of se&eral centi!etres ana length of tens to thousans of !etres+ The two !ain categories are primary rutting,

which is ue to per!anent efor!ation of the$itu!inous surfacing an secondary rutting ueto per!anent efor!ation in the layers $elow theasphalt+


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2leeing2leeing presents as $lac6 shiny spots on the surfaceof the pa&e!ent an causes low s6i resistance+ 9t isthe result of poor !i( esign with too !uch $itu!en+

Ra&elling This is the loss of stones in the surface of thepa&e!ent as a result of failure of the $on $etweenthe aggregate an the $iner+

,otholes A pothole is a locali;e collapse in an area of thepa&e!ent structure which is wea6ene) for e(a!ple$y efecti&e roa construction or ingress of water+


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The Design ,rocess

The three !ain steps to $e followe inesigning a new roa pa&e!ent are:

•esti!ate the a!ount of tra%c an thecu!ulati&e nu!$er of e.ui&alentstanar a(les that will use the roao&er the selecte esign life

•assess the strength of the su$grae soil

o&er which the roa is to $e $uilt


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The Design ,rocess "ont#

•select the !ost econo!ical co!$inationof pa&e!ent !aterials an layerthic6nesses that will pro&ie satisfactoryser&ice o&er the esign life of the

pa&e!ent The eterioration of pa&e roas cause $ytrafc results fro! $oth the !agnitue ofthe a(le loas an the nu!$er of ti!es

these loas are applie+


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or the purposes of pa&e!ent esignconsieration is gi&en not only to the totalnu!$er of &ehicles that will use the roa$ut also the a(le loas of these &ehicles+

The loas i!pose $y pri&ate cars ansi!ilar si;e &ehicles are insigni'cant inter!s of structural a!age to the roa+


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or the structural esign of the pa&e!ent) cars

an si!ilar si;e &ehicles can $e ignore anonly the total nu!$er an a(le loaing of thehea&y &ehicles) e'ne as those ha&ing anunlaen "e!pty# weight of 3000 6g or !ore) nee

to $e consiere+,articularly for lightly tra%c6e roas)construction tra%c can $e a signi'cantco!ponent of o&erall tra%c loaing an the

esigns shoul ta6e this into account+


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or 8e(i$le pa&e!ents a design lie of$etween 10 to


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The Design ,rocess "ont#9t !eans that the pa&e!ent will nee to

$e strengthene so that it can continue tocarry tra%c satisfactorily for a furtherperio+

9n orer to eter!ine the total tra%c o&erthe esign life of the roa) the 'rst step isto esti!ate $aseline "current# tra%c 8ows+


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Flexible Pavement (Cont’d) The Design ,rocess "ont#

The esti!ate shoul $e the "Annual# Average DailyTrafc (ADT) currently using the route) classi'e

into the &ehicle categories of cars) light goos&ehicles) truc6s "hea&y goos &ehicles# an $uses+ The ADT is e'ne as the su! of the total annualtra%c in $oth irections "!easure separately#

i&ie $y 3=>+2ecause auto!atic counters for classifying &ehiclesare e(pensi&e an own for perios of!aintenance) !anual counting !ethos are !oreco!!on+


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The Design ,rocess "ont#or !ost ays 1=5hour counts will su%ce)

howe&er at least two ays per wee6 shoulha&e a full


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Tra%c forecasting is an uncertain process)especially in e&eloping econo!ies+@enerally growth in three categories oftra%c is in&estigate: normal trafc,

diverted trafc and generated trafcNor!al tra%c is the tra%c which will usethe route whether or not the pa&e!ent isi!pro&e+


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Di&erte tra%c refers to the tra%c which will

prefer to use the new-i!pro&e route to getfro! the sa!e origin to the sa!e estination+@enerate tra%c will use the new-i!pro&eroa ue to a cost or ti!e reuction o&er an

alternati&e !eans of transport or $ecause ofthe increase e&elop!ent that is $roughta$out $y the in&est!ent in the roa+


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The Design ,rocess "ont#/hen using "relia$le# past tra%c ata uner whatare e(pecte to $e $asically the sa!e econo!ic

conitions the growth in tra%c can $e e(trapolateforwar for as !any years as the ata e(ists fro!the past+Alternati&ely growth in tra%c can $e relate linearly

to anticipate growth in @ross Do!estic ,rouct"@D,#+Apart fro! o&erall increase in econo!ic acti&ity inthe country) speci'c local e&elop!ents can lea tosteay growth in tra%c on a new- i!pro&e route+


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or purposes of the esign the cumulative

num!er o e"uivalent standard axles can $eeter!ine fro! axle load surveys

These sur&eys usually in&ol&e weighingsa!ples of the tra%c using porta$le&ehicle5weighing e.uip!ent+ There can $e signi'cant ierences$etween the two strea!s of tra%c using a

roa+


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ehicles tra&elling one way) for e(a!ple fro! a

.uarry or oc6) are hea&ily loae $ut aree!pty on the return 4ourney+

9n such cases the results fro! the !ore hea&ilytra%c6e lane shoul $e use when con&erting

co!!ercial &ehicle 8ows to the e.ui&alentnu!$er of stanar a(les for pa&e!ent esign+


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The a!age that &ehicles o to a roaepens on the a(le loas of the&ehicles+or pa&e!ent esign purposes the

a!aging power of a(les is relate to aBstanarB a(le of C+1= tonnes "C)1=0 6g#using e.ui&alence factors which ha&e

$een eri&e fro! e!pirical stuies+


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9n orer to eter!ine the cu!ulati&e a(le loa

a!age that a pa&e!ent will sustain uring itsesign life) the total nu!$er of co!!ercial) i+e+hea&y &ehicles "o&er 3)000 6g unlaen weight#

that will use the roa o&er this perio !ust $e

e(presse in ter!s of the cu!ulati&e nu!$erof e.ui&alent stanar a(les "esa-esal#+


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The e"uivalence actors (#$s) for &ehicle

a(les can $e eter!ine fro! the followinge.uation:

#$ % (Axle Load&'*) +

As shown $y the e.uation the e!piricale&ience is that as loas on a pa&e!entincrease there is an e(ponential increase intheir a!aging eect+


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or a &ehicle with !ultiple a(les the E isco!pute for each a(le an then the&ehicles !ean E+ The !ean E for each type or class of&ehicle tra&elling in each irection !ust

then $e eter!ine+ This !etho of co!puting the E shoulalways $e use as calculations $ase on thea&erage a(le loa are incorrect an lea to

large errors+


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The Design ,rocess "ont# Average Axle Loading (incorrect)

-,*** .g

#$ % (-***&'*) +

% **"orrect# Su! each a(le E

(***&'*)+ / (0***&'*) +% * /

,*** .g 0,*** .g

#$ (average) % *'1


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The proceure for eter!ine the

cu!ulati&e e.ui&alent stanar a(les o&erthe esign life of the roa is as follows:

•esta$lish the a&erage aily one5irectional

tra%c 8ow for each class of &ehicle•forecast the total one5irectional tra%c8ow for each class of &ehicle "$ase on

gi&en growth rate#


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•eter!ine the !ean E "e.ui&alence factor#

of each class of &ehicle for each irection•co!pute the prouct of the cu!ulati&e one5irectional tra%c 8ow for each class of&ehicle an the !ean E for that class

•a the a$o&e proucts to get thecu!ulati&e e.ui&alent stanar a(le loaingfor each irection+ "?se the higher &alue for

th i #

Documents

Highway Engineering TRAN 3001 Lecture 7