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BASIC CONCEPT OFPAVEMENT DESIGN

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Factors for Pavement Type

Terrain

Materials

Population

Politics

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Factors Affecting Pavement Design

Materials and Properties

Climate

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Factors Affecting Pavement

Design

Subgrade

Traffic

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Rainfall

Temperature

Climate

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Materials and Properties

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Subgrade

Foundation of the Road Pavement

The existing site soil upon which the pavement

structure is constructed.

The soil prepared to support a Pavement

Structure or system.

In highway design, it is the native material

underneath a constructed pavement

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Unsuitable Subgrade Material

- Material from swamps, marshes, peat,logs, stumps, roots and otherperishable or combustible material

- Surface Soil and highly organic clayand silt

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Suitable Subgrade Material

Material having LL < 30% and

< 30% passing the 75 microns sieve to

BS 410

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Variation of Soil Materials

Soil material varies, hence must begrouped in terms of their characteristics

From pavement design and engineeringstandpoint, there are two (2) generalcharacteristics that are very important.

- Soil Classification

- Soil Strength

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Soil Classification

Provides the designer a good idea of thegradation and constituents of the in-situmaterials.

Two (2) systems are normally used:

- Unified System

- AASHTO System

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Soil Strength

Most important soil characteristics

CBR

Unconfined Compressive Strength

Resilient Modulus

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Traffic

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Motor Cycle

http://www.cycletrader.com/find/listing/2005-BIG-DOG-MOTORCYCLES-Mastiff-93551454

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Cars

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Pick-Up/Vans

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Mammy Wagons

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Mammy Wagons

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Small Bus

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Large Bus

http://www.tootoo.com/d-p14702489-Large_Size_City_Bus_C10_12_-4f171d4c45/

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Light Truck

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Medium Truck

http://www.google.com/imgres?imgurl=http://jcwinnie.biz/wordpress/imageSnag/hino-dutro-hybrid.jpg&imgrefurl=http://jcwinnie.biz/wordpress/?p=1993&h=302&w=425&sz=39&tbnid=mUWa6nRzPyoJ::&tbnh=90&tbnw=126&prev=/images?q=medium+truck+pictures&sa=X&oi=image_result&resnum=2&ct=image&cd=1

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Heavy Truck

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Heavy Truck

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Semi Trailer Heavy

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Other Heavy Trailers

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TRAFFIC

Normal

Diverted

Induced

Generated

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Traffic Survey

Total Traffic Volume (AADT)

Manual Counts

Automated Counts

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Manual Traffic Counting

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Automatic Traffic Counter

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Traffic Survey

Axle Load Survey

Magnitude of the Load

Axle Configuration

A l L d S

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Axle Load Survey

Permanent Weighbridges

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Typical Axle Single Axle

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Tandem Axle

A l L d S

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Axle Load Survey

Permanent Weighbridges

A l L d S

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Axle Load Survey

Temporary Weighbridges

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Damaging Effect of Axle Load

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Damaging Effect of Axle Load

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Permanent Weighbridges

Advantages

Sampling

Less Delay and LessObstruction to Traffic

Roadside Interview

Disadvantages

Expensive in terms of

Operational Cost

Drivers will divert fromroute of measurement

Vandalism of Equipment

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Temporary Weighbridges

Advantages

No Vandalism of

Equipment

Default Drivers can beCaptured in the Survey

Disadvantages

High Operational Cost

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Magnitude and Effect of Load

Standard axle Load = 80KN

LEF = (P/Pr) 4.5

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Magnitude and Effect of Load

The relationship can be illustrated by thefollowing example:

Increasing the axle load by 32 % from 80kN to 105 kN will triple the damaging effect

A 32 % reduction in axle load from 80 kNto 54 kN will reduce damaging effect to20 %.

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Magnitude and Effect of Load

Expressed in terms of number of passes

A pavement designed to sustain 1 millionpasses with an 80 kN axle load would show thesame amount of damage after only one third ofa million passes with a 105 kN axle

It would tolerate 5 million passes with a 54 kNaxle.

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Factors Affecting Axle Loads

Axle type and spacing (single - tandem - triple)

Wheel type; dual, wide base single, normalsingle

Uneven load distribution on dual tyres

Tyre pressure

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DesignTraffic

ND = ADTx (FTG)x(DDF)x(LDF)x(Pt)xTf

ND = Design ESALs for design lane

ADT = Initial two-average daily traffic

FTG = Future total growth factor

DDF = Directional distribution factor

LDF = Lane distribution factor

Pt = Percent heavy trucks

TF = Average truck factor or ESALs/truck

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Basic Design Concepts

Empirical Method

Analytical Method

Analytical-Empirical Method

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Empirical Method

Advantages

Easy to use and apply

Adapted to particularregions

Disadvantages

Limited design traffic

Cannot work with newmaterials

Assume that the road

would have failed at endof design life

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Analytical Method

Horizontal tensile strain at the bottom ofthe bituminous layer controls the

development of fatigue cracking

The compressive vertical strain at the topof the subgrade controls permanentdeformation

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Analytical/Empirical Method

Analytical methods used to design andvalidated with field measurement

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GHA Pavement Design Method

Traffic

Subgrade Resilient Modulus

Initial and Terminal Serviceability

Reliability

Standard Deviation

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