low volume road design empirical approach. what are the differences ??

LOW VOLUME ROAD

DESIGN

EMPIRICAL

APPROACH

WHAT ARE WHAT ARE THE DIFFERENCES ??THE DIFFERENCES ??

30060

Roman Road

TYPES OF FAILURETYPES OF FAILURE

FUNCTIONAL FAILUREFUNCTIONAL FAILURE

STRUCTURAL FAILURESTRUCTURAL FAILURE

Do they occur at the same time?Do they occur at the same time?

How do they differ for low volume How do they differ for low volume roads ?roads ?

Do all users agree ?Do all users agree ?

How do we combine the different How do we combine the different views ?views ?

Total or Whole Life CostsTotal or Whole Life Costs Minimise total costsMinimise total costs

Need to…..Need to….. predict road deteriorationpredict road deterioration predict the effects of maintenancepredict the effects of maintenance calculate road agency costscalculate road agency costs predict road user costs predict road user costs

Models such as HDM 4 ??

PLUSPLUS calculate social ‘benefits’calculate social ‘benefits’

What structural design factors What structural design factors does road performance, and does road performance, and therefore design, depend ? therefore design, depend ?

strength of subgradestrength of subgrade traffic loadingtraffic loading

wheel loadswheel loads number of repetitions of wheelsnumber of repetitions of wheels

strength of pavement layersstrength of pavement layers thickness of pavement layersthickness of pavement layers ? ? but also time dependantbut also time dependant but how?but how?

try totry to

For high traffic roads we control as much For high traffic roads we control as much as we can to reduce risks of failure to a as we can to reduce risks of failure to a

very low valuevery low value

But for low volume roads we But for low volume roads we cannot afford to do socannot afford to do so

Pass/fail criteria - no riskPass/fail criteria - no risk

Strength of material

Specification for trunk road

Number ofsamples

1 Estimate traffic loading in equivalent standard axles

2 Multiply traffic by regional factor

3 Estimate subgrade strength (now as an elastic modulus)

4 Select serviceability loss (maximum level of acceptable

deterioration)

METHOD THEN RECOMMENDS A STRUCTURAL NUMBER, SN

BASIC AASHTO METHOD

STRUCTURAL NUMBER (SN)

SN = a1 h1 + a2 h2 + a3 h3 + ....

Where a1 , a2 , a3 etc. are strength coefficients for layers 1, 2, 3, etc. and

h1 , h2 , h3 , etc. are the thicknesses of layers 1, 2, 3

The strength coefficients are related to normal strength measures such as CBR, unconfined compressive strength, Marshall stability, etc.

STRENGTH COEFFICIENT, a2 FOR GRANULAR BASE MATERIALS

Strengthcoefficient

(a2)

CBR value40

0.08

0.050

0.15

0.10

0.14

50 100110 150

a2 = {29.14 (CBR) - 0.1977 (CBR)2

+ 0.00045 (CBR)3} x 10-4

•

•

•

STRENGTH COEFFICIENT, a3 FOR SUB-BASE MATERIALS

5

0.050

0.0251

Strengthcoefficient

(a3)

0.150

0.075

0.100

0.125

10 50 100 200

a3 = 0.01 + 0.065 (log10CBR)

•

•

•

CBR of sub-base

WEIGHTED EQUIVALENT ESA APPLICATIONS

103 104 105 106 107 108

EQUIVALENT THICKNESS De, INCHES45

40

35

30

25

20

10

5

0

15

xx

x

xxx x x

xxxxxx

xx

x x xx

xxx

x

x

xx

x

xx x x

xx

xxx xx

x

xx

x

x

AASHO "DESIGN" EQUATION COMPARED WITH DATA

AASHTO EQUATION

87.7.32.2

1

10944.0

5.12.41.36.9. 10

19.5

010

1002.8

R

f

MLog

SN

PSIPSILog

SNLogSZWLog

Traffic

Structuralnumber

Subgrade CBRAllowable

deterioration

Reliability

Effect of ‘Reliability’Effect of ‘Reliability’

For For

250,000 esa and 250,000 esa and

subgrade CBR = 7%subgrade CBR = 7%

95% Reliability95% Reliability SN = 2.54SN = 2.54

85% Reliability85% Reliability SN = 2.29SN = 2.29

a difference of 62 mm of sub-basea difference of 62 mm of sub-base

STATE OF ROADPSI

TIME or TRAFFIC

ROAD DETERIORATION

FROZEN

FROZEN

SPRING

THAW

SPRING

THAW

Effect of climateEffect of climateThe ‘regional’ factorThe ‘regional’ factor

Illinois in summerIllinois in summer

SN = 2.54SN = 2.54

DryDry

SN = 2.14 (-100mm of sub base)SN = 2.14 (-100mm of sub base)

WetWet

SN = 2.97SN = 2.97 (+110mm of sub (+110mm of sub base)base)

But no guidance available from the Road Test

ROAD NOTE 31ROAD NOTE 31A PAVEMENT DESIGN GUIDE A PAVEMENT DESIGN GUIDE

FOR PAVED ROADS FOR PAVED ROADS

IN IN TROPICAL CLIMATESTROPICAL CLIMATES

TAKES ACCOUNT OF…..TAKES ACCOUNT OF…..

Variability in material Variability in material propertiesproperties

Uncertainty in traffic estimatesUncertainty in traffic estimates Variability in road performanceVariability in road performance

KEY FACTORSKEY FACTORS

Influence of tropical climates on the Influence of tropical climates on the moisture conditions in the subgrademoisture conditions in the subgrade

Influence of tropical climates on the Influence of tropical climates on the nature of soils and rocksnature of soils and rocks

High axle loads and tyre pressuresHigh axle loads and tyre pressures Severe conditions imposed on the Severe conditions imposed on the

bituminous surface by tropical climatesbituminous surface by tropical climates Inter relationship between design and Inter relationship between design and

maintenance maintenance

ESTIMATING EQUILIBRIUM MOISTURE ESTIMATING EQUILIBRIUM MOISTURE CONTENTCONTENT

CATEGORY 3CATEGORY 3

No permanent water tableNo permanent water table

Arid climateArid climate

Rainfall < 250mm paRainfall < 250mm pa



Deep water table but rainfall sufficient to Deep water table but rainfall sufficient to produce seasonal changes under the produce seasonal changes under the

roadroad

Rainfall >250mm pa. per year and seasonalRainfall >250mm pa. per year and seasonal



Water table sufficiently close to the Water table sufficiently close to the surfacesurface

to control the subgrade moisture contentto control the subgrade moisture content

(This depends on the type of soil)(This depends on the type of soil)

0

20

40

60

80

100

4 8 12CBR (DCP) per cent

Cu

mu

lati

ve p

erce

nta

ge

An example of coping with risk

Soaked

CBR

Soil A

Soil B

Common to area

where designs

developed

Rare

5%

5%

15%

8% 300mm ?

300mm

CBR at

equilibrium

moisture

content

Required

pavement

thickness

Consideration of the Road Design Environment for LVSR’s

OPTIMUM OR APPROPRIATE

PAVEMENT DESIGN

AVAILABLE MATERIALSAVAILABLE MATERIALS

THE ROAD DESIGN OR “RISK” ENVIRONMENTTHE ROAD DESIGN OR “RISK” ENVIRONMENT

AVAILABLE MATERIALSAlternative & thin bituminous surfacingsPavement materials

•Marginal materials•Standards

•Subgrade & road formation•Problem soils•Moisture sensitivity•Stabilisation options and treatments


PAVEMENT DESIGN

METHODOLOGY

Pass/fail criteriaPass/fail criteria

Strength of material

Specification for trunk road

Number ofsamples



PAVEMENT DESIGN

PREVAILINGCLIMATE

PREVAILINGCLIMATE


DRAINAGE AND HYDROLOGYDRAINAGE AND HYDROLOGY


PREVAILING CLIMATERainfall (intensity, distribution)Temperature (evaporation & diurnal change)Future change or unpredictability

DRAINAGE AND HYDROLOGYGround & surface water flowHydro-genesisDemand of terrainModifying influences





PAVEMENT DESIGN

METHODOLOGY



PAVEMENT DESIGN

PREVAILINGCLIMATE

PREVAILINGCLIMATE


CONSTRUCTIONCONSTRUCTION








CONSTRUCTIONQuality controlCapacity, training & experienceSelection and use of plantInfluence of construction traffic


PAVEMENT DESIGN

METHODOLOGY



PAVEMENT DESIGN

PREVAILINGCLIMATE

PREVAILINGCLIMATE


TRAFFIC CHARACTERISTICSTRAFFIC CHARACTERISTICS

CONSTRUCTIONCONSTRUCTION

CONSTRAINTS OF THE “GREEN”

ENVIRONMENT

CONSTRAINTS OF THE “GREEN”

ENVIRONMENT

MAINTENANCEMAINTENANCE

OTHERSOTHERS

GEOMETRICS AND

CROSS-SECTION PROFILES

GEOMETRICS AND

CROSS-SECTION PROFILES








GEOMETRICS AND CROSS-SECTION PROFILESRoad widthCrown heightDemand of terrainSealed shoulders

TRAFFIC CHARACTERISTICSAxle loadingTyre pressuresSeasonalityPositionGrowth projections

OTHERTechnology solution

•labour based•Intermediate equip

•Safety•Institutional environment capacity•Financing•Political pressure•Design period•Road side activity

MAINTENANCECapacity & skillsFundingProgramming

CONSTRUCTIONQuality controlCapacity, training & experienceSelection and use of plantInfluence of construction traffic

CONSTRAINTS OF THE “GREEN” ENVIRONMENTConstrained alignmentsAccess to materialsDepletion of resourcesTerrain stability


PAVEMENT DESIGN

METHODOLOGY

low volume road design empirical approach. what are the differences ??

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