Flexible pavements work by distributing wheel loads across layers to reduce stress. The document discusses flexible pavement design according to Indian Road Congress guidelines for design traffic up to 150 million standard axles. It describes evaluating subgrade strength, calculating design traffic loads, and using CBR and thickness design charts to determine the appropriate flexible pavement layers and thicknesses based on subgrade strength and traffic volume.

1. DESIGN OF FLEXIBLE
PAVEMENTS
By
Supriya Pal

2. Typical Sections of Pavements

3.  Load is transferred to the lower layers by grain to grain distribution
The wheel load acting on the pavement will be distributed to a wider area,
and the stress decreases with the depth.
Load transfer Mechanism for Flexible Pavement

4. Flexible Pavement Rigid Pavement

6. Flexible Pavement Design
As per IRC (37-2001)
1.Overview
Indian Road Congress(IRC) has specified the
design procedures for flexible pavements
based on California bearing ratio (CBR)
values. Following this guidelines applicable to
design traffic up to 150 msa.

7. 2.Scope
These guidelines will apply to design of
flexible pavements for
Expressway(EH),National Highways(NH),
State Highways(SH), Major District
roads(MDR) and other categories of roads.
3.Design procedure
The pavement designs are given for subgrade
CBR value ranging from 2% to 10% and
design traffic ranging from 1 (million standard
axle) msa to 150 msa for an average
pavement temperature of 35 C.

8. • Design wheel load (from traffic volume
study)
• Subgrade soil strength, Field soil dry density
& Moisture Content
Using the following simple input
parameters, appropriate designs could be
chosen for the given traffic and soil
strength:
Design parameters

9. 4.Design Traffic
The method considers traffic in terms of the
cumulative number of Equivalent Standard Axle
Load(ESAL)(8160kg) to be carried by the
pavement during the design life. These requires
the following informations:
A. Initial traffic in terms of Cumulative Vehicle per
Day (CVPD)
B. Traffic growth rate during the design life
C. Design life in number of years
D. Vehicle damage factor(VDF)
E. Distribution of commercial traffic over the
carriage way.

10. Traffic – In Terms Of ESAL (8160 Kg)
During Design Life
A. Initial Traffic
 In terms of Cumulative Vehicles/day
 Based on 7 days 24 hours Classified Traffic
B.Traffic Growth Rate
Establishing Models Based on Anticipated Future
Development or based on past trends
 Growth Rate of LCVs, Bus, 2 Axle, 3 Axle, Multi
axle, HCVs are different
 7.5 % may be assumed- (As per Guidelines
for the design of flexible pavements - IRC
37:2001)

11. C. Design life in number of years
It is recommended that pavements for roads
like NH,SH should be designed for a life of 15
years,EH and urban roads for 20 years and
other categories of roads for 10 to 15 years.
D. Vehicle Damage Factor (VDF)
Multiplier to Convert Number of Commercial Vehicles
of Different Axle Loads and Axle Configurations to the
Number of Standard Axle Load (ESAL) Repetitions
indicate VDF Values.

12. Single Lane Roads
 Total No. of Commercial Vehicles in both Directions
Two-lane Single Carriageway Roads
 75% of total No. of Commercial Vehicles in both
Directions
Four-lane Single Carriageway Roads
 40% of the total No. of Commercial Vehicles in both
Directions
Dual Carriageway Roads
 75% of the No. of Commercial Vehicles in each
Direction
E. Design Distribution Of Traffic

13. Evaluation Of Pavement Component Layers
Subgrade
Road foundation- compacted/ borrowed
earth
• Top Layers of Pavement are
constructed over it
• Evaluation Of subgrade strength by CBR
Test
• Te

14. Subbase and Base
Material – Natural Sand, Moorum, Gravel, Kankar, Brick Metal,
Crushed Stone, Crushed Concrete etc
- Stress Transmitting Medium
- To distribute Wheel Loads
- To prevent Shear and Consolidation
Deformation

15. Surface Course
• Sufficient Surface Texture – Good Skid
Resistance in Wet Weather
• Sufficient Stiffness to Reduce Stresses in the
Underlying Layers
• High Resistance to Environmental Degradation;
durable
• Low Permeability - Water Tight Layer against
Ingress of Surface Water
• Good Workability – Allow Adequate Compaction

16. Required Traffic Volume Data
Initial value of commercial vehicles per day
(CVPD).
Traffic growth rate during design life in %
Design life in number of years.
Distribution of commercial vehicles over the
carriage way

17. Computation of Traffic for Use of Pavement
Thickness Design Chart
365 x [(1+r)n – 1]
N = --------------------------- x A x D x F
r
N = Cumulative No. of standard axles to be catered for the design in
terms of msa
D = Lane distribution factor
A = Initial traffic, in the year of completion of construction, in terms of
number of commercial vehicles per day
F = Vehicle Damage Factor
n = Design life in years
r = Annual growth rate of commercial vehicles

18. CBR Testing
California bearing ratio = (PT/PS)x100
Where
PT = test load corresponding to the chosen
penetration curve, and
PS = standard load for the same depth of
penetration as for PS taken from standard
code.
.

19. Flexible pavement design chart (IRC) (for ESAL< 10
msa)

20. Thickness&composition(mm)
Flexible Pavement Layers (IRC) (ESAL< 10
msa)

21. Advantages of Geotextiles for Flexible Pavement