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7/31/2019 Basic Concept of Pavement Design - Ing. L. Lamptey
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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-935514547/31/2019 Basic Concept of Pavement Design - Ing. L. Lamptey
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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_-4f171d4c45/7/31/2019 Basic Concept of Pavement Design - Ing. L. Lamptey
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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=17/31/2019 Basic Concept of Pavement Design - Ing. L. Lamptey
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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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