Dr. P. NANJUNDASWAMYDepartment of Civil Engineering
S J College of EngineeringMysore – 570 006
Pavement Design Parameters
• Traffic
• Climatic Factors
• Road Geometry
• Subgrade
• Material Properties
• Environment
Design Parameters – Traffic
• Maximum Wheel load
• Contact Pressure
• Multiple Wheel Loads
• Repetition of Loads
• Position
• Impact of wheels
• Iron-tyred vehicles
Traffic Loading - Approaches
Approaches
Fixed traffic• Single load, no load repetitions
Fixed vehicle• No. of repetitions of a std. axle load• Equivalent axle load factor
Variable traffic and vehicle• Stresses, strains and deflections under
each load group separately
Wheel Load and Contact Pressure
Contact Pressure
The influence of contact pressure on stress levels in base, subbaseand subgrade layers are marginal
Contact Pressure
The magnitude of contact pressure determines the quality and thickness of wearing and binder course
Wheel Load
The influence of the magnitude of the wheel load on stress levels in base, sub-base and subgrade layers is significant
Wheel Load
Total thickness of the pavement is mainly determined by the magnitude of the load and not the contact pressure
Axle Configurations and Loads
19t)
Axle Configurations
2 Axle Truck – 16t
3 Axle Truck – 24t
4 Axle Semi Articulated – 34t
4 Axle Articulated – 34t
5 Axle Truck – 40t
LCV
Axle Configurations
Axle Configurations
Axle Configurations
Axle Configurations
Design Vehicle ?
Effect of Wheel Configuration
The effect of axles 1, 2 and 3 on stresses and strains within pavement layers are considered independently
Effect of Wheel Configuration
Within a group of axles, each axle is not considered as independent
Effect of Wheel Configuration
In flexible pavement design by layer theory, only the wheels on one sideare considered
Effect of Wheel Configuration
In rigid pavement design by plate theory, the wheels on both sides are usually considered (even when distance > 1.8 m)
Notice that cars are insignificant and thus usually ignored in pavement design.
1.35
1.85
5.11
0.100.00070
1
2
3
4
5
6
Car Delivery Truck Loaded 18-Wheeler Loaded 40' Bus Loaded 60'
Articulated Bus
ES
AL
s p
er
Ve
hic
le
Shape of Contact area
The true shape of contact area is elliptical
In flexible pavement analysis, it is approximated to circular shape for the ease of calculations
Radius of contact area,
p
Pa
Shape of Contact area
In rigid pavement analysis, circular shape approximation leads to significant error
For the convenience of calculations, the elliptical shape is approximated by a rectangle and two semicircles
Shape of Contact area
0.3 L
0.3 L
0.4 L L
0.6 L
The contact area
In FEM analysis of rigid pavement, equivalent rectangular area of 0.8172 L x 0.6 L is assumed
Multiple Wheel Loads
Equivalent Single Wheel Load (ESWL)
Is a single wheel load having same contact pressure which produces same value of • Maximum stress• Deflection• Tensile stress• Contact pressure
at the desired depth
Graphical Solution
Equivalent Single Wheel Load (ESWL)
Boyd and Foster methodSemi-rational method
Assumptions• Equalancy concept is based on equal
vertical stress• Contact area is circular• Influence angle is 45o
• Soil medium is elastic, homogeneous and isotropic half space
Graphical Solution . . . .
Equivalent Single Wheel Load (ESWL)
Graphical Solution . . . .
Equivalent Single Wheel Load (ESWL)
Graphical Solution . . . .
Equivalent Single Wheel Load (ESWL)
Depth z (Log Scale)
ESW
L (L
og
Scal
e)
P
d/2 2 S
2P
Z1
P1
A
B
Graphical Solution . . . .
Example
Find ESWL at depths of 5 cm, 20 cm and 40 cm for adual wheel carrying 2044 kg each. The center to centertyre spacing is 20 cm and distance between walls of thetwo tyres is 10cm
Solution
For desired depth z1 = 5 cm, which is half the distancebetween the walls of tyre, ESWL = P = 2044
For z3 = 40 cm, which is twice the tyre spacing, ESWL =2P = 4088 kg
Graphical Solution . . . .
Equivalent Single Wheel Load (ESWL)
Depth z (Log Scale)
ESW
L (L
og
Scal
e)
P
d/2 2 S
2P
Z2
P2 = 3.5
A
BLog10(d/2) = 0.7 Log10(P) = 3.3
Log10(2S) = 1.6Log10(2P) = 3.6
Log10(Z2) = 1.3
P2 = Antilog (3.5)= 3162 kg
Graphical Solution . . . .
Equivalent Single Wheel Load (ESWL)
5.38log
4log301.03.3log
10
10210 P
kgP 316210 5.32
Equal Vertical Stress Criterion
From Boussinesq’s Theory
or
PS
ZA
Pd Pd
Z
1 32
Sd
Sd/2
σzs Maximum vertical stress at A
σzd Maximum of vertical stresses at 1, 2 and 3
Equal Vertical Deflection Criterion
Foster and Ahlvin (1958) PS
ZA
Pd Pd
Z
1 32
Sd
Sd/2
ws Maximum vertical deflection at A
wd Maximum of vertical deflections at 1, 2 and 3
and
Equal Vertical Deflection Criterion
Huang (1968)ESWL based on interfacedeflection of two layeredsystems
Other Criteria
• Equal Tensile Strain
• Equal Contact Pressure
• Equivalent Contact Radius
Equivalent Single Axle Load (ESAL)
Is the equivalent repetitions of Standard Axle during the design life of pavement
IRC terms this ESAL as Cumulative number of standard axles during the design life
The number of repetitions of different types of axles are converted into equivalent repetitions of standard axle by using Equivalent Axle Load Factors (EALF)
Equivalent Axle Load Factor (EALF)
Defines the damage per pass to a pavement by an axle relative to the damage per pass of a standard axle
Exact EALF can be worked out only by using distress models
Approximate EALF can be worked out using the fourth power rule
Standard Axle Load Single axle : 8160 kgTandam axle : 14968 kg
Vehicle Damage Factor (VDF)
Instead of converting each axle pass into equivalent standard axle passes, it will be convenient to convert one truck pass into equivalent standard axle passes
The factor that converts – VDF
VDF is the number of standard axles per truck
Determining VDF
Sample Axle Load Survey
Sample Axle Load Survey
Computation of VDF
Traffic on Design Lane
Need for Distribution Factors
Traffic on Design Lane
Worked out by finding the
Directional Distribution Factor (0.5 to 0.6)
Proportion of ADT of trucks occurring in the maximum direction
Lane Distribution Factor
Proportion of trucks occurring on the design lane which depends on
Number of lanes and Traffic volume
Factors Suggested by IRC
No. of Traffic lanes in two
directions
Percentage of trucks in
Design Lane
1 100
2 75
4 40
Undivided Roads (Single Carriageway)
Factors Suggested by IRC
No. of Traffic lanes in two
directions
Percentage of trucks in
Design Lane
1 100
2 75
3 60
4 45
Divided Roads (Dual Carriageway)
Design Period
Depends on
traffic volume growth rate capacity of road and possibility of augmentation
Flexible Pavement
15 years – NH, 20 years – Express ways & Urban Roads, 10 to 15 years – Other Roads
Rigid Pavement
30 years. When Accurate prediction not possible – 20 years
Design Traffic
N = Cumulative std. axle repetitions during design period (expressed in msa)
A = Initial traffic intensity (CVPD)D = Lane distribution factorF = Vehicle damage factorn = Design life (years)r = Annual rate of growth for commercial vehicles
Average annual growth rate – 7.5%
Thank you