Chapter 4-Structural Design of Pavements

1 Highway Engineering-II CEng-4183
Chapter 4:
STRUCTURAL DESIGN OF
PAVEMENTS
Instructor: Fasika Mekonnen

Fasomeku@gmail.com
Introduction
2
Introduction
3
Structural Design
Flexible Pavement : ERA Manual
4
Structural Design
5
6
Structural Design
Traffic Analysis
7
 Traffic Analysis
 The deterioration of paved roads caused by traffic results from
both to magnitude of the individual wheel loads and their
repetitions.
 It is necessary to consider not only the total number of vehicles
that will use the road but also the wheel loads ( or, for
convenience, the axle loads).
 Points to remember:
 Design Period
 Traffic Volumes
 Traffic forecast
 Axle Load
 Cumulative standard Equivalent Axles (ESAs)
Structural Design
Traffic Analysis: Design Period or Life
8
 Design Period or Life

 Design life is the period the pavement will need to be
strengthened so that it can continue to carry traffic satisfactorily
for a further period.
 Many factors may influence this decision, but some of the points
to consider include:
 Functional importance of the road
 Traffic volume
 Location and terrain of the project
 Financial constraints
 Difficulty in forecasting traffic
Structural Design
Traffic Analysis: Traffic Volumes
9
 Traffic Volumes
Vehicle Classification:
 Small axle loads from private cars and other light vehicles do not cause
significant pavement damage.
 Damage caused by heavier vehicles (commercial vehicles)
 Hence, important to distinguish the proportion of vehicles which cause
pavement damage (commercial vehicles) from total traffic
Structural Design
Traffic Analysis: Traffic Forecast
10
 Traffic Forecast
 Traffic forecasting is uncertain process
 The following factors should be considered in the determination
of the annual growth rate, r:
 Attracted or diverted traffic due to the improvement of existing road
 Normal traffic growth due to the increased number and usage of
motor vehicles
 Generated traffic due to upgrading or constructing the new road
facility
 Development traffic due to change in land use as a result of the new
facility
 The traffic growth rates used to project the traffic should be
checked whether they are appropriate for the different sections
of the project.
Structural Design
Traffic Analysis: Axle Load Survey
11
 Axle Load Survey

 To determine the axle loading, it is important to conduct
axle load surveys at the road side by weighing samples of
vehicles
 Axles load equivalency factor are then determined using:
Traffic Loading on Pavements
Traffic Analysis: ESA
12
 Cumulative Equivalent Standard Axle (ESA)

 Calculate the EALF for each type of vehicle or axle load using the
RN 31 of TRL ( ERA manual) or AASHTO method
 Either lane of a two-lane highway can be considered as the
design lane, where as for multilane highways, the outside lane is
the design lane.
 The identification of the design lane is important because in some
cases more trucks will travel in one direction than in the other, or
trucks may travel heavily loaded in one direction and empty in
other direction.
Table 3-2: Lane Distribution Factors (ERA/AASHTO)
Number of Lanes Percent Traffic (ESAL) in
in each direction design Lane Lane Distribution Factors
(ERA/AASHTO)
1 100
2 80 – 100
3 60 – 80
4 50 – 75
Traffic Loading on Pavements
Traffic Analysis
13
 Determination of Cumulative Traffic Volumes for the design period:

1. Determine the initial traffic volume (AADTo or ADTo )
 For each type of car, bus, truck, Truck trailer
2. Estimate the annual growth rate “i/r” expressed as a decimal fraction, and
the anticipated number of years x” between the traffic survey and the opening
of the road.
3. Determine AADT1 the traffic volume in both directions on the year of the
road opening by:
 For paved road determine one-directional traffic volume for each type of
vehicle.
4. The cumulative number of vehicles, T over the chosen design period N ( in
years) is obtained by:
 Where L is lane distribution factor & BD is directional distribution factor:

 For paved roads, conduct a similar calculation to determine the cumulative
volume in each direction for each type of vehicle.
 Note : The higher of the two direction cum.ESA values used for design.
Structural Design
Traffic Analysis
14
 5. Design Traffic (Cumulative Equivalent Standard Axle Load -

CESAL) – is computed by multiplying the total traffic volume
for each vehicle category (Ti) by its corresponding truck factor
(TFi)
 6. The CESAL is used to determine the traffic class to be
employed for pavement design
Structural Design
15
Generally:
 Step-I:
Determination of
traffic class from
the calculated
ESAs.
 Step-II:
Determination of
Subgrade strength
class from the
given subgradde
soil (CBR(%))
Structural Design
16
 Step-IIl: Selecting the appropriate structural layer

17
18
19
20
21
22
23
Structural Design
24
25
Example-1
 Initial traffic volumes have been established for a link road in terms of
AADT and as follows
 Vehicle Classification 2000 AADT
Car 260
Bus 30
Truck 150
Truck-trailer 200
 The anticipated traffic growth is a constant 5.4% and the opening of the
road is scheduled for 2005. An axle load survey has been conducted in
Table-1 and assumed that the loads are equally represented for each
direction of traffic. Use subgrade CBR values: 9,12,15,10,13,12,10,13,14,14.
 Determine the total ESA and traffic class for flexible pavement.
 Determine design CBR value and subgrade class for flexible pavement
 Design the flexible pavement and select economical pavement structure.
26
Table 1: Axle Load Survey:

Vehicle No. Axle-1 Axle-2 Axle-3 Axle-4
1 6350 12480 8490 9940
2 6450 12240 6290 9470
Truck- 3 5550 13930 8550 10150
trailer 4 4570 15300 2720 2410
5 4190 15060 3110 2800
1 6100 4500 7250 5480

Truck 2 5200 6500 8260 8940
1 5550 6300 5460 -
Bus 2 3300 5100 3300 -
Structural Design
Gravel And Low Standard Roads
27
 Design Principles
 Steps To Be Considered In The Design Process
 Traffic (Baseline flow and forecast)
 Material and geotechnical information (Field survey and
material properties)
 Subgrade (Classification, foundation for expansive soils and
material strength)
 Thickness design (Gravel wearing coarse thickness)
 Materials design
 All-weather Access
 Surface Performance
 Maintenance
Structural Design
28
 Design Method
 The required gravel thickness shall be determined as follows:
 Determine the minimum thickness necessary to avoid
excessive compressive strain in the subgrade (D1).
 Determine the extra thickness needed to compensate
for the gravel loss under traffic during the period
between regravelling operations (D2).
 Determine the total gravel thickness required by
adding the above two thicknesses (D1+ D2).
Structural Design
29
 MINIMUM THICKNESS REQUIRED (D1)

 It is necessary to limit the compressive strain in the subgrade to
prevent excessive permanent deformation at the surface of the road
 GRAVEL LOSS
 GL = f T^2 / ( T^2 + 50) ( 4.2 + 0.092 T + 3.50 R^2 + 1.88V)
 Where
 GL = the annual gravel loss measured in mm
 T = the total traffic volume in the first year in both directions, measured in
thousands of vehicles
 R = the average annual rainfall measured in m
 V = the total (rise + fall) as a percentage of the length of the road
 f = 0.94 to 1.29 for lateritic gravels
= 1.1 to 1.51 for quartizitic gravels
= 0.7 to 0.96 for volcanic gravels (weathered lava or tuff)
= 1.5 for coral gravels
= 1.38 for sandstone gravels
Structural Design
30
 TOTAL THICKNESS REQUIRED

 The wearing course of a new gravel road shall have
a thickness D calculated from:
D = D1 + N. GL
 Where
 D1 is the minimum thickness from Figure 11.3
 N is the period between regravelling operations in years
 GL is the annual gravel loss
 Regravelling operations should be programmed to

ensure that the actual gravel thickness never falls
below the minimum thickness D1.
Structural Design
31
Structural Design
32
 Pavement and Materials

 Depending on the CBRdesign of the sub grade, improved
subgrade layers shall be constructed as required, on which
the gravel wearing course is placed.
 Crossfall and Drainage

 The crossfall of carriageway and shoulders for gravel roads
shall be “4%” as indicated in ERA’s Geometric Design
Manual - 2002.
 This is to ensure that potholes do not develop by rapidly
removing surface water and to ensure that excessive
crossfall does not cause erosion of the surface. Provision of
drainage is extremely important for the performance of
gravel roads.
Structural Design
33
 Material Requirements
 Experience With Local materials
 Marginal materials
 towns (dust nuisance) or steep hills (slipperiness).
 Improved Subgrade Layer
 Subgrade CBR
 at least a minimum CBR of 7% for minor gravel roads and
 at least a minimum CBR 25 % for major gravel roads
 Treatment Of Expansive Formations
 Material Characteristics
 non-expansive, non-dispersive and free from any deleterious
matter.
Structural Design
34
Structural Design
35
 Gravel Wearing Course

 Performance Characteristics Of Gravel Wearing Course
 They should have sufficient cohesion to prevent raveling and corrugating
(especially in dry conditions)
 The amount of fines (particularly plastic fines)
 Gravel Wearing Course material Specification
 Gravel Wearing Course material Specification
 hard durable angular particles of stone or gravel, free from vegetable
 Major Gravel Roads (AADT design =20 To 200)
 design category of DS5 to DS8 (See ERA Geometric Design Manual -2002,
 Type 1 in the new construction of roads having an AADT greater than 50
 Type 4 material may be used in the new construction of roads having an AADT
less than 50.
 Minor Gravel Roads (AADT design <20)
 design category of DS9 to DS10 (Refer to ERA Geometric Design Manual-
2002).
 Type 4 material for new construction and Type 1 for maintenance activities
Structural Design
36
 Gravel Wearing Course material Specification

THANK YOU!
37
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1 Highway Engineering-II CEng-4183

Instructor: Fasika Mekonnen

 Design Period or Life

 Axle Load Survey

 Cumulative Equivalent Standard Axle (ESA)

 Determination of Cumulative Traffic Volumes for the design period:

 Where L is lane distribution factor & BD is directional distribution factor:

 5. Design Traffic (Cumulative Equivalent Standard Axle Load -

 Step-IIl: Selecting the appropriate structural layer

Table 1: Axle Load Survey:

1 6100 4500 7250 5480

 MINIMUM THICKNESS REQUIRED (D1)

 TOTAL THICKNESS REQUIRED

 Regravelling operations should be programmed to

 Pavement and Materials

 Crossfall and Drainage

 Gravel Wearing Course

 Gravel Wearing Course material Specification

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