extending flexible pavement life

7/23/2019 Extending Flexible Pavement Life

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xtending

Flexible

Pavement

Life Using Geogrids

By

Jim

Penman

CGeol

FGS

and

Joe Cavanaugh

P E

PROFESSION L

DEVELOPMENT

SERIES

Se

pt

mb

r

2006

U T

H

T

n l

l T O

W

• • • • . e INTERNATIONAL



Professional Development Series

G

ogrids have been

in

common use for more than 25 years.

Whi

le they

have gained widespread acceptance as a so

luti

on to problems associated

with roads constructed on soft or problematic subgrades, their

use

on

competent subgrades has been less common.

Clear

well-established design methodology is

now available that allows the design engineer to quantify the benefits of using geogrids to

extend pavement design lif

e.

This approach can be applied for the design

of

major highways

or light-duty pavements associated with local housing or retail store developments.

Instructions

The Professiona

l

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Learning Objectives

1) Understand the mechanisms by

which

geogrids rein

force pavement structures and

how

the benefits

of

using geogrids can

be

quantified.

2) Develop a general understanding of the design

methodology

currently prescribed

by

AASHTO for

including the benefits

of

geogrid reinforcement in flex

ible pavement structures.

3) Gain insight on

how

these techniques can provide cost

effective solutions, even on relatively low-volume pave

ments.

4) Develop a ba

sic

understanding

of

state-of-the -

art

mechanistic-empirical design techniques and how

these methods will enhance pavement design practices

in the future.

Professional Development Series Sponsors

CONTECH Earth Stabili

zation

So

lutions

Inc.

Tensar

International

Corporation

Geogrid technology has developed steadily since the

products were first introduced in the early 19805. The initial

geogrids rapidly gained popularity

within

the civil engineer

ing industry, principall y due to their ability to provide simple,

cost-effective solutions in various roadway and grade separa

tion applications.

A

geogrid is a regular grid structure

of

polymeric material

used

to

reinfOfCe

soil

or

other geotechnical engineering related

materials. Products generally are classified

as

either uniaxial

geogrids

or

biaxial geogrids, depending upon whether their

Figure 1: Uniaxial UX) and

biaxial BX) geogrid

strength

is

predominantly in one

or

two directions. Uniaxial

geogrids are principally used in

grade separation applications

such as retaining walls and steep

slopes; biaxial geogrids are used

mainly in roadway applications.

Examples of

both

geogrid types

are shown in Figure

1.

This article is principally

concerned

with

the use

of

biaxial

geogrids in base reinfOfCement

applications. In these situations, the existing subgrade is

of

a

firm

nature or has been rendered such through the use

of

a

subgrade

improvement

te<:hnique. One of the principal failure

Figure

2:

The inclusion of

8X

Geogrids provides lateral confine

ment of the

base,

which

results

in enhanced pavement

pe

rform

ance - either

an

increase in the pavement life, a decrease in the

required thickness of the pavement, or a combination of the two.

_ ~

.

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Extending

Flexible

Pavement

Life

Table

l:Keygeogrid propertiesdescribed

by1he

us.

ArrrryCorpsciEngineEfS

Geogrld Prop rty

Rib

Shape

properties Thickness

Aperture

properties

Junction strength

Overall

Stiffness

Size

Shape

Stiffness

Torsional

strength

Stability

Judgment

Rectangular

Is

better

Thicker Is better

High stiffness

Is

better

Should

be

matched totill type used

Round or square

Is

better

High stiffness

Is

better

High compared to rib strength (>90%)

High

Is

better, minimum of

0.65 cm-kgr

recommended

High

Is

better

mechanisms of a pavement under

these

firm

subsoil

conditions

is

rutting resulting from progressive lateral movement of the

aggregate base course during traffic loading (Figure 2).

The amount of lateral movement can be reduced greatly

by including a biaxial geogrid within, or at the bottom of, the

base

course

layer,

Partial penetration of coarse aggregate

particles through the geogrid apertures and subsequent

compaction results

in

mechanic

al

interlock or confine

ment of the aggregate particles.

Table 2:Typicallayer coefficients for

pa

vement materials

Material

As

phalt surface cour

se

Aspha

lt

base

cour

se

Dense-graded aggregate

Granular sub-base

Geogrid technology

Typical layer

coefficie

nt

,

0 1

0.40 - 0.44

0.30

-

OAO

0.10 - 0.14

0.06 - 0.1 0

The principal benefit of using a geogrid within the

unbound aggregate component of a flexible pavement is

ess

rutting at the surface because of reduced late

ral

spreading of

the unbound aggregate. However,

an

additional feature of the

reinforcement is that the geogrid-confined aggregate r

esults

in a much stiffer

base

course layer and a lower dynamic deflec

tioo of the pavement structure during traffic loading.

Fat

igue

cracking of the asphalt

is

therefore reduced

because

of the

presence of the geogrid reinforcement.

In

ord er for geogrids

to wor

k successfully

in base

reinforce

ment applications, they must

have

the capacity

to

facilitate

efficient load transfer between the aggregate and the

geogrid. Webster (1992) reported on a large-scale

resea

r

ch

program undertaken by the

U.S.

Army Corps of Engineers

(Corps) to investigate and determine the key physical proper

ties

of a geogrid requir

ed to

create optimal interaction and

load transfer. A summary of the

key

material properties deter

mined in the study are presented

in

Table 1.

Ta

bl

e

3:

Typica l

drainag

e coe

ffi

c

ient

s for

unbound pavem

en t materials

Qu.lI.,. of

cl<ai

n.. .

E>c<.Uent

'

r

-

'1 poor

Pfopootlotl

oft "",

p_ ment I. opproachlng .

u, .

...

< I I _S S_2S

1.40-1.15

1.1

5 -

1.10 1

l(l-1.2O 1.20

1.35 - 1.25 L25 - 1.15 1.15 -

1.

00

'00

1.15-1.15 1.15

- I.oS

1.

00-0.00

.00

I.IS-1.05 1.0

5 -

O.ao

0.00-0.60

.00

1.

05 _ 0.95 0.95 _0,75 0,75 _ 0.40

000

Current design practice for

flexible pavements

The American Association of State

Highway and Tra nsportation Officials

(AASHTO) provides guidelines for the design of

flexible pavements in its current design guide

AASHTO,

1993), The design methods described in the guide are

based

on a purely empirical approach following a set of large-scale

tests

undertaken

in

Ottawa, 111.

in

the late 1950s.

The designer is required to know the following input

parameters for a proposed pavement section:

Structural

Number SN)

- This is determined by

adding the structural cOfltributioflS from

each

of the pave

ment

layers, as

shown

in

Figure

3.

Structural Number

SH _ I

IOAl

_ OA2

ACC B I

a

0.01(1

SH _ Z.OAO_O.80

'1

liliIIIIiIII

'

, - ~ : : ~ ~ ~ a_ O

I4.m

_ l.O

~ C ' N C C O C • , C . ~ o ~• o 7 . ~ o . . . ~ : - . . .

SH_ . 0.

. 1 . 0

_ O

&S

• - IIY'I' coet'llclent (lypIc:al .a l

. . . . .

hown In r oblel)

m _ dralnage f .o, Ityplcal

.a

l

. . . . .hown In

Ta b 3)

Figure 3:

Ca

lculati

on

of

the

St

ructural Number for a

pavement

section.

Standard Nonnal Deviate

ZR

- This

parameter det

er-

mines the probab il ity that a road will maintain

an

acceptable

level of serv iceab ility during i

ts

design life. Typical values of reli-

ability recommended by MSHTO

are

presented

in Table 4, and

the relationship between reliability and the required input

parameter, ZIl is shown in

Table 5.

Standard Deviation -

Th is parameter d

escr

i

bes

the

reliability of the input parameters selected for the local condi

tions. Default values of 0.40

to

0.50 are recommended for

flexible pavements.

Change n Serviceability apSI)

-

Th is describes the

loss

in serviceability during the design life of the road and is

dictated by acceptable levels of c

ra

cking, rutting, etc.

An

initial serviceability, P of 4.2 is normally assumed, and

AASHTO recommends a terminal serviceability,

P

r

of

2.S or

higher for a major highway and 2.0 fo r highways with

less

traffic. Once P and P

r

are determined, apsi

'

P

-

Pro

Subgrade Resilient Modulus M

R

) -

Th

is

defines the

strength of the subgrade

or

foundat ion

1ayer

on which the

Tabl

e4:

Recommended

reliability

for

roads based on AASHTO (1

993

)

F u n c t i o n ~ c I ~ s s i f i c a t i o n

Interstate and other

f.eeways

Principal Arte.ials

Collectors

Local

Recommended

level of reliability

(' til

Urban

Rural

85-99.9 80-99.9

SO-99

75

-95

SO-95

75

-95

50 - 80 50 -

SO

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main pavement sits.

Once these input parameters

have been

determined, it is possible to calculate the

allowable traffic capacity, W

1

for a particular

pavement section using the following equation:

Jog ,, (w,. ) _ Z. S, +9.36Jog,, (SN + J) - 0.20 +

I

PSI

I

og 4 2 U

. ' 1094 +2.321og M.-8.07

0.40+ SN+ l

/

The allowable traffic capacity determined using this equa

tion

is

quoted in Equivalent Standard Axle

Loads (ESALs). To

put

this into perspective, a typical, fully laden 20-ton truck

would impose a load equivalent to approximately 5

ESALs.

Geogrids

in

flexible pavement designs

Geogrids were invented in the late 19705 and sold

commercially for the first time in the early 1980s. Clearly, they

were

not used

in the original road test

used

to develop the

current

AASHTO

design methodology for flexible pavement

design. However, guidance for incorporating geogrids for

base

reinforcement

in

flexible pavements is given in the

Interim Standard

PP46-01

published by

AASHTO in

2001.

This document recognizes that geogrids used

in

flexible

pavements provide one or both of the following benefits:

• extension of pavement design life; and

• reduction of pavement layer thickness.

t is

further stated

in AASHTO s

PP46-01

that

to

quantify

these performance benefits for a particular geogrid, it is

n e e s ~ r y

to undertake large-scale performance testing under

carefully controlled (OnditiOflS. A good summary of the test

ing undertaken during the first 20 years since geogrids were

introduced s provided by

Perkins

and Ismeik (1999).

Irrespective of the type of test undertaken, the objective

s

the ~ m - quantify the improved performance

of

geogrid

reinforced pavement sections compared with unreinforced

test sections.

133 kN 133 kN

Un re inforced Reinforced

Figure 4: Performance benefits for extended design life

Table 5: Relationship bet

wee

n Reliability and

Standard Normal Deviate,4,

Reliability Z, Reliability Z,

99 99

-3.750

92

-1

.405

99 9

-

3.090

91 -1

.340

99

-2.3

27

90

-1

.282

98

-2 .054

85

-

1.03

7

97

-

1.

881

80

-0.84

1

96

-

1.7

51

75

-0

.674

95

-1.645

70 -0

.524

9

-1.55S

60

-0.253

93

-1

.476

50 0

Quantifying extended design life for geogrid-

reinforced pavements

Consider the two pavement sections shown in Figure

4.

The sections

are

identical apart from the fact that the rein

forced pavement contains a geogrid at the subgrade-base

course interface.

The parameter generally used to quantify the extension

of

pavement design life using geogrids

is

the Traffic Benefit Ratio

TBR). This

is

defined

as

follows:

T R = No. of

cycles

for

given

deformation in

reinforced

section

No.

of

cycles for giveo defOfmation

in unreinfOfCed

sectioo

The results shown in Figure 4 are from the Corps testing

undertaken by Webster (1992). In this simple example, the

TBR would

be

calculated

as

follows:

TBR

= 500/106 =

4.72

In other words, the

use

of a geogrid in this pavement

section extended the pavement design life (for a 1 inch

surface rut) by a factor

of

4.72.

The

AASHTO

design guide methodology described above

can

be used to

calculate the allowable traffic for

an

unrein

forced pavement. To determine the extended pavement life

when using a geogrid

in

the

same

pavement

sectiOfl,

this value

is

simply multiplied by the appropriate

T R

value for the

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Extending Flexible Pavement

Life

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; J:O ... Asphlltl l '

6.0

In

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C·

~ n k > l < e d . , . , a

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80%

reduct loll

III

, ... ' . . . - 11 <IIP'<lty

ofu..,

Pl'Y""'ent.

Figure

5: Premature p.1vement failure.

geogrid concerned.

Keep

in mind, however, that in accordance

with the directions given

in

PP46-01, any T R values used for a

particular geogrid must

have been

determined using testing

methods correlated to observed field perfOfTllance .

Geogrid use in local subdivision developments

The previous sections described the general methods by

which geogrids

can be used

to extend pavement design life.

This section focuses on how this technology can be applied

to

solve

a specific problem associated with relatively light

duty pavements.

As

the population of our towns and cities continues to

expand rapidly, new or recently constructed housing, in the

form of subdivision developments, is becoming increasingly

commonplace. One of the more frequent problems

associ

ated with the roads

in

these developments

is

a direct result of

their method of construction.

Phased

construction (Figure 6) has become an extremely

common practice, particularly

so in

residential developments.

To build a roadway to gain site

access,

contractors initially

place the aggregate component of the pavement and, usually,

a thin asphalt layer on top. This technique

is

surface layers

at considerable expense.

If ruts

have

developed, it will

also

be necessary

to

replace the granular foundatiOfl

layer s).

Consider the three pavement sec tions

shown

in

Figure

5.

The trafficking capacity

in each

case

has been calculated using the AASHTO guidelines

described above.

Strange

as

it may sound,

in

this typical example, leaving off

the 1.S inches of asphalt surfacing during a phased-construc

tioo procedure (Section B) reduces trafficking capacity

of

the

pavement by more than 80 percent.

For

subdivisioo

roads,

however, the majority of the total trafficking

is

experienced

during construction

of

the road itself and the surrounding

housing. Therefore,

it

is

not

surprising that when the surface

layer is installed at the end of construction, the rest

of

the

pavement structure is approaching the end

of

its design life.

Placement of

an

additional thin surface layer results in some

additional trafficking capacity, but a year or

two

later the road

starts to show the

sort of

surface

distress

indicative of problems

associated with the structural integrity of the lower layers.

The simple solution to this problem s a layer of geogrid

installed at the bottom or within the

base

course during initial

construction. The allowable trafficking determined for

SectiOfl

C in Figure 5

was

calculated by applying a T R value of 6 (typi

cal

for a high quality geogrid with the required supporting

performance data) to the trafficking capacity for Section

B.

The

outcome is that the trafficking capacity of the thinner road used

during the construction

phase exceeds

that for which the

completed road (Section A)

was

originally designed.

From the road owner's perspective, for relatively little addi

tional expense at the start of construction, the lifetime

of

their

road is extended enormously, and expensive and disruptive

rehabilitation or reconstruction activities are avoided.

Geogrid use in retail store developments

Another

use of

geogrid technology

can be

found in the

particularly useful when local trenches

are

required for installation

of

utility pipes and

cables.

Once the overall

site

development

is

completed, the remaining asphalt is plaCed,

ensuring that the road is in pristine condition

on Day 1 of its formal

use.

Or

is

it?

Figure

6: A typical subdivision

road

during construction

surface aspha

lt

layer has

not yet

been

installed.

on

which

the

th in

Pavement distress

in

the form of asphalt

cracking at the surface

is

common on roads

within subdivisions (Figure

7). In

many

cases,

these cracks start to appear shortly after

constructiOfl - perhaps

as

soon

as

one or

two

years.

Once the cracking

starts to

develop, the

deterioration accelerates very quickly. Pavement

distress

depicted by alligator cracking s the

most common in

such

developments and

points to

an

overall integrity problem

as

the

pavement approaches the end of its design life.

Under these circumstances, the current owner

of the road will need to replace the road's

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development

of

pavements around retail

stores. Typically, thicker, heavy-duty pave

ments

are

adopted

in

the loading

areas

around such stores, while thinner, lighter-duty

pavements

are

used

for the

car

parking

areas,

One of the main problems associated with this approach s

the potential for a bath tub effect - the subgrade is at a

lower

level

in the

areas

of the heavy-duty pavements,

These

areas are

prone

to

water ingress and build up, resulting

in

a

reduction in the long-term strength of the pavement.

In

colder regions, these

areas

are

also

more susceptible

to

the

effects of freeze-thaw activity. Both of these situations reduce

the design life of the pavement, but there are additional prac

tical problems for the contractor associated with this more

complicated method

of

construction.

Consider the

two

sets of pavement sections shown in

Figure

8, In each case,

the trafficking capacity

of

the geogrid

reinforced sections s at least

as

great

as

the unreinforced pave

ment sections.

Clearly,

the reinforced sections offer cost

benefits because they

are

thinner and require less material.

However, the major advantage of this scenario is gained from

the fact that the light- and heavy-duty sections

are

of the

same

thickness, which creates a uniform subgrade elevation, In

addition to offering protection against the bathtub problems

described above, the reinforced se<:tions offer significant mate

rial cost savings. Additional benefits result from increased

speed of construction - fewer stake-out procedures,

less

undercut/disposal of fill, and simpler construction.

A glimpse

into the future

As

previously stated, the current design approach prescribed

by

AASHTO

is

based

purely on empirical

results

from the large

scale

field tests undertaken in Ottawa,

III.,

in the late 19505.

New pavement design approaches,

based

on

me<:hanistic

empirical (M-E) principles,

are

now being developed and

refined by

AASHTO

and other entities.

DesIgn

Scenario - Proposed

a ~ e n t

Section

Stlnda

rd

Duty

Heavy

Duty

Standa r

Duty

lU,OOO Es-.I:s

121.000 ESAI. .

Deslan Scenlrlo 8 . K I 1 I a i l y ReInforced P a ~ , e n t Sect10n

Standa r Duty

Heavy

Duty Stlndard

Du

ty

8lu1 Geoptd

- - -

65 000 ESAI. . 330,000

Es-. .'s

165,000

ESAI. .

F

gure 8: Biaxial

geogrids

create

uniform

subgrade elevations.

Essentially,

M-E pavement design involves the

use of

numerical modeling

te<:hniques

to

predict accurately the

stresses and strains developed in a particular pavement

section

as

a result of traffic loading. The mechanistic (or theo

retically-based) predicted performance

s

then calibrated with

field tests (empirical data) to validate the methodology.

Figure 7: Condition of a sutxlivision

road

only two to three years after

fina

l paving.

Official publication of the new

AASHTO

design guide may

still

be several years away, but

the availability

of

M-E-based design methods incorporating the

use

of geogrids within the pavement structure

is

imminent.

Researchers

at the University

of

Illinois at Urbana-Champaign are about to

publish the results of a four-year project investi

gating the

use

of geogrids in

base

reinforce

ment applications. Although this type of work

has been undertaken previously by several

authors, the

scale

of the testing undertaken at

the University of Illinois

to

develop accurate

transfer functions s unprecedented. Similarly,

the discrete element modeling approach

used

to define the interaction between the geogrid

and surrounding

soil is

revolutionary.

PDH

This eagerly anticipated advancement

in

pavement design will be published by the

University of Illinois at the 86th Annual Meeting

of the Transportation Research Board (TRB)

in

Washington, D.C., Jan. 22-26, 2007.

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Summary

Extending

Flexible

Pavement

Life

Evalu

ation of Geosynthetic Reinforced Base

Course Layers

in

Flexible Pavements: Part I

Experimental Work, Geosynthetics Inter

national, Vol. 4 No.6, pages 549-604.

•

Webster

S.L. 1992, Geogrid Reinforced Base

Geogrids can be used successfully to extend the design life

of flexible pavements in a variety of

base

reinforcement appli

catioos.

The techniques

are

equally applicable

to

major high

ways and small subdivisioo

roads.

Significant life cycle cost

savings can

be achieved with relatively little additional up

front expenditure.

Current

AASHTO

design methods exist to determine

appropriate pavement sectioos incorporating geogrids into

pavement structures. However, new and extremely innova

tive, state-of-the-art techniques using M-E principles are just

around the comer •

Courses

for Light Aircraft: Test Section Construction, Behavior

Under Traffic, Laboratory Tests and Design Criteria

Geotechnical Laboratory, Department

of

the Army,

Waterways Experiment Station, Corps of Engineers,

Mississippi.

References

• AASHTO 1993, AASHTO Guide For Design

of

Pavement

Structures, American Association of State Highway and

Transportation Officials.

• AASHTO, 2001, Provisional Standard PP46-01:

Recommended Practice For Geosynthetic Reinforcement of

the Aggregate Base

Course

of Flexible

Pavements

April 2001

Interim Edition.

Jim Penm an . CGeol . FG

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• Perkins S.W. and Ismeik M., 1999, A

Synthesis

and

Professional Development Series

Sponsors:

' ~ A U l=f:t:l 9025 Centre Pointe Of.,

Suile

400.

West

Chester, OH 45069

~ ;; 'V ' t±i j (513) 645

-

7877

•

Fax: (513) 645-7993

•

Email:

info@lcontech<pi.com

EART H SUI l l lZATt ON

I 0 L U , I 0 • I I C

ensa[ Web: lWM'.conlech<pi.com

INlERNATIONAL

E

News's Professional Invelopment

Series

Reporting Fonn

Article Title: Exlending Flexible Pavement

life

Using Geogrids

SponSOr5: CONTECH Earth Sl3biliZ<ltion Solutions

I

nc., and

Tensar

Intemational Corporation

Publication Date:

Septembef

2006

V ~ l i d fOf credit

until:

Septembef

2007

Instructions

:

Select one a n s ~ r

for

each quiz question and

clearly

circle

the

appropriate letter.

Provide

all

of

the

requested

contact

information

,

Fax

this Reporting Form

to

(513) 64$.7993.

(You do not

need

to send the

Quiz; o n ~ this Reporting Form

s

necessary

to

be

submitted.)

I abc

d 6)

abc

d

2)

abc

d

7)

abc

d

3) a

b d 8)

a

b ( d

4)

a

b d 9)

a

b ( d

5) abc

d

10) abc

d

Required contact

information

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Name: First Name: Middle Initial:

Title:

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t

read

the

article,

under5tood the

learning objectives,

and completed the quiz questions to

the

best

of

my ability.

A d d i t i o n a l ~

the contact information provided above

is

true

and accurate.

Signature:

Date:

Special Advertl$lng Section - CONnCH Earth Stabllllllion Solutlon$lnc./Ten$

ar

International Corp.

PO" 7



Professional Development Series Quiz

Professional Development Series Sponsors:

~ ~ E C = = =

9025 Centre Pointe Dr., Suite 400, West Chester, OH 45069

t:t±:i (513)

645

-7877 • Fax: (513) 645-7993 • Email [email protected]

Web: W N W . c o n l e c h - q ) ~ . c o m

l i TH SU IH l l A r I O

Tensar.

•

I c

INTERNATIONAL

Quiz Instructions

On the

Professional

Development Series Reporting Form p<lge PDH 7 ,

circle

the correct answer for each

of

the fo llowing questions.

, . In base reinforcement applicatio ns (firm subgrade conditions),

what

is

the

main function of a geogrid in improving pave

ment

performance?

a) Enhance loild distribution

resu

lt ing in less vertical deflection o

the subgrade.

b) Reduce

la

tera l movemeot of aggregate particles within

the

base

co

urse

layer.

c) Prevent pumping

of

the subgrade.

d Reduce dynamic deflection of the asphalt.

2. Which of

the

following sets of index properties for a geogrid

would likely give

the

best indication of how

the

product

will

perform

in

roadway applications?

a) Ultimilte temile strength; aperture

size.

b) Tensile strength at 2% strain; percentage open area.

c) Aperture stability modulus (torsional rigidity); junction strength.

d) True

ini

t

ia

l modulus

in

use; creep limited strength.

3. Which of th e following are

~

s

ta t

e ments?

a) Geogrids can never provide effective separation between two

layers because

of

the passage of

fin

e mil through their apertures.

b) Geogrids and geotextiles essentia l

ly

use the 5ilme mechanisms

to provide reinforcement

c) The strength of the geogrid used

will

be the predominant factor

that determines the extension of design life for a reinforced pave

ment

d) None, all these statements are

fa

lse.

4.

Vl/hich

of the following is a ~ statement regarding the curre

nt

AASHTO (1993) pavement design approach?

a) l arge-scale trafficking studies conducted in the 1950s form the

basis for the design method.

b) The design approach is based primarily on theoretical predictions

of pavement performance.

c) Benefits of geog rids for base reinforcement are included in the

method.

d) NOlle, all of the above are false.

S. Based on

the

current AASHTO (1993) design approach, with all

other conditions remaining unchanged , what

wo

uld be the

effect of increasing

the subgrade

strength?

a)

An

increase in the overall Structural Number (SN) of the pave-

ment.

b) A li

kely

increase

in

the drainage properties of the pavement

c)

An

increase in the allowable traffic for the pavement.

d)

Al

l of the above.

6. Which of

the

following soil properties would

be

of

great

es t

value

to

an engineer when

de

signing a reinforced pavement on

a relative ly competent formation?

a) Shear strength of the subgrilde.

b

Shear strength

of

the base cou

rse.

c

Consolidation properties of the subgrade.

d) AJI equally valuable.

7. What difference(s) would you observe in

the

visual dist ress

between

s

ubgrade

rutting and base course rutting?

a) The

al

ligator cracks

will

be larger on the section

wi

th base

COUf5

e

rutting.

b The rut profile wil l be wider on the section wi th subgrade

rutting.

c

There is no difference, the rut profiles

will

be the same.

d) None of the above, as rutting always develops

in

both layef5

equally, and therefore you cannot determine the origin of the

rutting.

8. A subdivi sion road is characterized

by

alligator cracking

but

no

surface rutting. What is th e

most lik

ely

mode

of failure for

the

pavement?

a) Reflection of previous cracks caused by thermal expansion

contraction.

b Fatigue fai lure of the road as it reaches the elld of its design life.

c

Presence of a weak binder with in the asphalt.

d) All of the above are equally likely.

9. If

the

silty gravel subbase (overlying

the

subgrade,

and

supporting a dense-graded gravel base layer) is

the

critical

layer in a five-layer pavement section, where would you best

position

the

geogrid?

a) At the subbase-subgrade interface.

b In the middle of the subbase layer.

c At

the subbase-base interface.

d)

Any of the above, because the geog rid

wi

ll provide the same

contributio n at al l three locations.

10. Which of

the

following advantages apply

to

Mechanistic

Empirical design methods compared with the purely empirical

approach currently

adopted

by AASHTO?

a) Designs can account for

va

riations in material properties with

time.

b

The approach can more easily be adapted

to

take account

of

local mil and climatic conditions.

c

Design methods adopted now

will

more easily adapt to changes

in

ve hicl

e ioads

in

the future

as

trucks develop.

d) All of the above.

OPOH

Spedal

Advertising Section - CONTECH Earth Slablizallon Solutlon$ Inc./Ten5M Inlemallonal Corp.

extending flexible pavement life

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