seesion 6 Pavement Design and Maintenance.pptx.pdf

Engineering

1. Design of Flexible Pavements
DrCharles Adams

2. Importance of Pavements
•Pavements are engineered structures and are important for our
everyday life, commerce and trade, and defense.
•Surface transportation is the most widely used mode of
transportation in the world, and a country’s development is often
measured in terms of its total paved road mileage.
•The construction of roads is and will continue to be a major
industry in developing countries, and as the infrastructure
matures, it will be a major industry in developed countries as
well.

3. Importance of Pavements
•Like any other engineered structure, pavements are expected to be
adequately strong and durable for their design life.
•They are expected to function properly by providing a smooth traveling
surface for the traffic under various conditions of the environment.
•In order to ensure this, pavements must be designed, constructed,
maintained, and managed properly.
•Pavements can be broadly classified into asphalt (or flexible) and concrete
(or rigid) pavements.
•Pavements consist of different layers, more so in the case of asphalt
pavements than concrete ones.
•From the bottom up, these layers are known as the subgrade, subbase,
base, and binder and/or surface. There are certain pavements with asphalt
surface layers on top of concrete layers.

4. Pavement Management
•For any pavement network, a proper inventory of the condition of
different pavements must be kept and utilized effectively to
determine the time/order in which the different pavement sections
should be rehabilitated.
•This is important because of two reasons.
•1) there is never enough funding for rehabilitation of all of the roads
in a network at the same time, and
•2) different roads deteriorate at different rates and hence are in
different conditions at a specific time.
•It is important to “catch” the pavement at the most “appropriate”
condition such that the rehabilitation can be done economically—a
totally damaged pavement will need too much money to
rehabilitate/replace it.

5. Pavement Management
•This process of keeping an inventory of condition and selecting
pavements for rehabilitation/reconstruction is called pavement
management.
•This step includes the use of tools for determining the condition
of existing pavements. The fastest growing method of such
detection is nondestructive testing and evaluation (NDT and
NDE).
•One example of a widely used form of NDT is the falling weight
deflectometer(FWD) that works on the principle of evaluating
deflections of pavements under known loads and making an
assessment of the structure and the condition of the pavement.

6. IMPORTANT ISSUES
1)Drainage is needed to drain water away from the pavement.
2)The materials must be evaluated and selected properly so that they can
withstand the effects of the traffic and the environment.
3)The mix must be designed properly such that it can withstand traffic and
environmental factors.
4)The structure should be designed properly such that it has adequate thickness
to resist excessive deformation under traffic and under different environmental
conditions.
5)The pavement must be constructed properly such that it has desirable
qualities.
6)The pavement must be maintained/managed properly through periodic work,
regular testing, and timely rehabilitation.
7)Generation of knowledge through research is critical for ensuring good
pavements in the future.

7. Functional requirements
•A pavement’s primary purpose is to provide a functional surface
for a specific transportation need.
•The basic function is to withstand load, under different seasonal
environmental conditions, without deforming or cracking, since
either of these distress conditions would reduce the functionality
of the pavement.
•The function of the different layers in the pavement is to spread
out the load on the surface and reduce its intensity with depth,
such that the pressure on the subgrade is much less than the
pressure on the surface and can be tolerated by the subgrade
without undergoing excessive deformation.

8. Flexible Pavement Layers
•A pavement consisting of asphalt mixes (and aggregate and soil layers)
only is referred to as a flexible pavement, since the pavement layers
deflect under a traffic load.
•The typical applied concept of a flexible (or asphalt) pavement is that a
layered structure (Figure 1) with better materials near the top would
distribute the load in such a way that the resulting stress in the bottommost
layer will be small enough so as to cause no significant deformation of the
layer.
•The bottommost layer is the existing layer or the existing layer modified
with some materials. The materials and the thicknesses of the different
layers will be such as to be able to withstand the different effects of
temperature and moisture due to changes in season in a specific location.
•The subbase, in addition to providing structural support, may also serve
as a platform for constructing the base and prevent the fine materials from
the subgrade from contaminating the base layer.

11. Functions of the Pavement Layers
•Surfacing: the primary function of this layer is to provide a safe, smooth, stable riding
surface, i.e. a carriageway, for traffic; its secondary functions are to contribute to the
structural stability of the pavement and protect it from the natural elements.
•When a surfacing is composed of bituminous materials it may comprise a single
homogeneous layer or course; more usually, however, with heavily-trafficked roads, two
distinct sublayers known as a wearing course and a base course are laid in separate
operations.
•The wearing course forms the uniform carriageway surface upon which vehicles run.
Ideally, it should (i) offer good skid resistance, (ii) allow for the rapid drainage of surface
water, (iii) minimize traffic noise, (iv) resist cracking and rutting, (v) withstand traffic turning
and braking forces, (vi) protect the underlying road structure, (vii) require minimal
maintenance, (viii) be capable of being recycled or overlaid, and (ix) be durable and give
value for money. No one material meets all of these requirements so, in practice, the
selection of a wearing course material depends on the design needs at each site.

12. •The basecourse(in mainland Europe this is called a binder course) is a structural platform which
regulates (i.e. makes even) the top of the underlying roadbase, thereby ensuring that the wearing course
has a good riding quality when built; it also helps to distribute the applied traffic loads. If the wearing
course is impervious, the basecoursecan be composed of a more permeable material.
•The roadbase, which provides the platform for the surfacing, is the main structural layer in a flexible
pavement. As the stresses induced in a flexible pavement by the applied wheel loads decrease with depth,
the main function of the roadbaseis to distribute the loads transmitted to it so that the strength capacities
of the weaker subbase and subgrade are not exceeded.
•Roadbasesin flexible pavements are normally designed to be very dense and highly stable, and to resist
fatigue cracking and structural deformation.

13. •A subbaseis very often present in a flexible pavement as a separate layer beneath the
roadbase. Whether it is present, or how it is used, depends upon its intended function(s). As a
structural layer within the pavement the subbase further distributes the applied wheel loads to
the weaker subgrade below.
•Whilst the subbase material is of a lesser quality (and, thus, is normally cheaper) than the
roadbasematerial, it must be able to resist the stresses transmitted to it via the roadbaseand it
must always be stronger than the subgrade soil.
•Another major function of the subbase is to act as a working platform for, and protect the
subgrade from, site and construction vehicles as a pavement is being built. This is especially
important when the subgrade is of poor quality, e.g. clayey or silty, as the critical load-carrying
period is when the heavy wheel loads used in the laying and compaction of the roadbaseare
applied to the subbase during construction.
•Whilst they are few in number the magnitude of these loads may be great. If the subgrade is
strong, e.g. granular, a subbase may not be needed.
•A well-graded dense subbase may be used (with or without a geotextile filter) to prevent the
upward infiltration of fine-grained subgrade soil into a roadbase. This ‘subgrade intrusion’
function is especially important during construction, when site traffic and compaction loadings
are high.

14. •When the subgrade soil is weak a capping layer may be created to
provide a working platform for equipment used to lay the subbase.
This is most commonly done by improving the top of the subgrade,
e.g. by adding a layer of imported material that is stronger than the
subgrade soil or by stabilizing the upper reaches of the subgrade
with, say, lime or cement.
•The interface between the subbase and the subgrade (or between
subbase and the capping) is termed the formation.
•Its cross-section is normally shaped to reflect the cross slope(s) of
the carriageway, to assist in the lateral drainage of water that might
accumulate within the pavement.
•The basement soil surface in contact with the underside of the
capping is termed the sub-formation

15. Functional requirement
Function of the pavement is to decrease the tire contact stress on the subgrade to a tolerable
level; flexible pavement (a), rigid pavement (b).

16. Rigid Pavement Layers
•Rigid(orconcrete)pavements,whichdeflectverylittleundertrafficloads,
behavedifferentlythanflexiblepavementsunderloads.
•The wearing layer, which is in contact with the traffic, is a Portland cement
concrete slab that ranges in thickness between 5 and 12 in. depending on
traffic loading.
•The thicker pavements are typical of heavier and more repetitive loads. The
slab, due to its higher stiffness as characterized by elastic modulus, usually
distributes the loading across a large pavement area. This in turn reduces the
stresses experienced by the underlying base and subgrade layers.
•Rigid pavements may or may not have a base or subbase layer, and could be
placed directly over the subgrade (Figure 1.3).
•However, in high-performance pavements, a base or subbase is typically
included. Besides providing a wearing course and a contact surface with
traffic, the slab provides friction, drainage, smoothness, noise control, and
waterproofing for the underlying layers.

17. Pavement for roads: There are different types of roads ranging from high-traffic-volume interstate highways to low-traffic-
volume local roads. These roads have different types and volumes of traffic. Accordingly, they are designed and constructed in
different ways. Thicker pavements are constructed for both heavier and high-volume traffic, of which trucks are the major load
applicators. Thinner pavements are for low-volume roads, although in many cases low-volume roads may carry heavy trucks
such as log trucks.
Pavement for airports: Pavements are required in airports in aircraft holding/terminal areas, taxiways, and runways. Just as
road pavements are subjected to a wide range of vehicles, airport pavements are subjected to a wide range of aircrafts—a
small, general aviation airport may have only light aircrafts (e.g., <30,000 lb), while a large hub/major airport would have major
aircrafts. The different pavement areas are runways, taxiways, and aprons. Special considerations such as protection from fuel
are required in many cases.
Pavement for parking lots: Parking lots are essential features in cities and towns, and are commonly found adjacent to
business/office buildings, including those near hospitals, schools, and airports.
Loading and unloading areas in ports and other areas: Heavy-duty pavements are constructed to support equipment and
materials unloaded from ships, rail, and trucks. These areas may also require special protections such as those from fuel
droppings/spillage.
TYPES AND USES OF PAVEMENTS

19. DIFFERENT FEATURES OF TYPICAL ASPHALT PAVEMENTS
•The bottom layer on which the pavement is built is called the
subgrade. The other layers, upward in order, are the subbase, base,
binder, and surface layers.
•In many cases, the pavement can be a full depth asphalt pavement, in
which case all of the layers above the subgrade are composed of hot
mix asphalt constructed in several layers (or lifts).
•In other cases, the subbase and base may be combined to form one
single layer, as far as materials are concerned, but constructed in
multiple layers.
•And finally, above the surface layer of HMA, there may be a very thin
wearing course of specialty material such as open graded friction
course (OGFC), to provide better friction and drainage of water.

20. JOINTED PLAIN CONCRETE PAVEMENTS

21. Continuously reinforced concrete pavement

22. Selecting the type of Pavement
The selection of the appropriate type of pavement for a specific
project should be made on the basis of primary factors that
include
•traffic,
•soil type,
•construction considerations,
•climate,
•Recycling options, and
•cost
and secondary factors, consisting of
•energy and material conservation and
•availability,
•performance history, and
•adjacent pavements.

23. Some basic considerations affecting
pavement design
•Pavement design is basically concerned with protecting the
subgrade, and the various courses within the pavement
structure, from excessive stresses and strains imposed by
commercial vehicles and that it is the wheel loads of these
heavy vehicles (and not of cars) that are the primary
contributors to pavement distress.

24. Pavement Design Process TRL Road Note 31 Method
There are three main steps to be followed in designing a new road
pavement These are:
(i) estimating the amount of traffic and the cumulative number of
equivalent standard axles that will use the road over the selected
design life;
(ii) assessing the strength of the subgrade soil over which the road is
to be built;
(iii) selecting the most economical combination of pavement materials
and layer thicknesses that will provide satisfactory service over the
design life of the pavement (It is usually necessary to assume that an
appropriate level of maintenance is also carried out).

25. Factors important to consider in Design
1)The influence of tropical climates on moisture conditions in road
subgrades.
2)The severe conditions imposed on exposed bituminous surfacing
materials by tropical climates and the implications of this for the design
of such surfacing.
3)The interrelationship between design and maintenance.
4)If an appropriate level of maintenance cannot be assumed. it is not
possible to produce designs that will carry the anticipated traffic loading
without high costs to vehicle operators through increased road
deterioration.
5)The high axle loads and tyrepressures which are common in most
countries.
6)The influence of tropical climates on the nature of the soils and rocks
used in road building.

27. TRAFFIC
•The deterioration of paved roads caused by traffic results from
both the magnitude of the individual wheel loads and the
number of times these loads are applied.
•For pavement design purposes 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) of these
vehicles.

28. Select Design Period

Figure 2-1 Traffic Evaluation

Estimate Initial Traffic
Volume (Initial AADT) per
Class of Vehicle

Estimate Traffic Growth

Determine Cumulative
Traffic Volumes over the
Design Period

For
Flexible Pavements

Estimate Mean Equivalent
Axle Load (ESA) per
Class of Vehicle

Estimate
Cumulative ESAs
Over the Design Period
(in one direction)

Select Appropriate Traffic
Class (based on ESAs) for
Flexible Pavement Design

Select Appropriate AADT
for Design of Gravel
Wearing Course

For
Gravel Roads

29. ESTIMATING TRAFFIC FLOWS
•Baseline traffic flow
•Determine the total traffic over the design life of the road
1)Estimate baseline traffic flows ie. The (Annual) Average Daily Traffic (ADT) currently using the route,
classified into the vehicle categories of cars, light goods vehicles, trucks (heavy goods vehicles) and
buses. The ADT is defined as the total annual traffic summed for both directions and divided by 365.
In order to reduce variations and errors
a)Undertake traffic counts for seven consecutive days.
b)The counts on some of the days are for a full 24 hours, with preferably at least one 24-
hour count on a weekday and one during a weekend. On the other days 16-hour counts
should be sufficient. These should be grossed up to 24-hour values in the same
proportion as the 16-hour/24-hour split on those days when full 24-hour counts have
been undertaken.
c)Counts are avoided at times when travel activity is abnormal for short periods due to the
payment of wages and salaries, public holidays, etc. If abnormal traffic flows persist for
extended periods, for example during harvest times, additional counts need to be made
to ensure this traffic is properly included.
d)If possible, the seven-day counts should be repeated several times throughout the year

30. Traffic forecasting
•In order to forecast traffic growth it is necessary to separate
traffic into the following three categories:
(a) Normal traffic. Traffic which would pass along the existing
road or track even if no new pavement were provided.
(b) Diverted traffic. Traffic that changes from another route (or
mode of transport) to the project road because of the improved
pavement, but still travels between the same origin and
destination.
(c) Generated traffic. Additional traffic which occurs in response
to the provision or improvement of the road.

31. •The commonest method of forecasting normal traffic is to extrapolate time
series data on traffic levels and assume that growth will either remain
constant in absolute terms i.e. a fixed number of vehicles per year (a linear
extrapolation), or constant in relative terms i.e. a fixed percentage increase.
•Data on fuel sales can often be used as a guide to country-wide growth in
traffic levels, although improvements in fuel economy over time should be
taken into account.
•As a general rule it is only safe to extrapolate forward for as many years as
reliable traffic data exist from the past, and for as many years as the same
general economic conditions are expected to continue.
•As an alternative to time, growth can be related linearly to anticipated Gross
Domestic Product (GDP). This is normally preferable since it explicitly takes
into account changes in overall economic activity, but it has the disadvantage
that a forecast of GDP is needed. The use of additional variables, such as
population or fuel price, brings with it the same problem If GDP forecasts are
not available, then future traffic growth should be based on time series data.
Normaltraffic.

32. •Where parallel routes exist, traffic will usually travel on the quickest or cheapest route although this may
not necessarily be the shortest. Thus, surfacing an existing road may divert traffic from a parallel and
shorter route because higher speeds are possible on the surfaced road.
•Origin and destination surveys should be carried out to provide data on the traffic diversions likely to
arise. Assignment of diverted traffic is normally done by an all-or-nothing method in which it is assumed
that all vehicles that would save time or money by diverting would do so, and that vehicles that would lose
time or increase costs would not transfer.
•With such a method it is important that all perceived costs are included. In some of the more developed
countries there may be scope for modelling different scenarios using standard assignment computer
programs.
Diverted traffic.

33. Diverted traffic.
•Diversion from other transport modes, such as rail or water, is not easy to forecast. Transport of
bulk commodities will normally be by the cheapest mode, though this may not be the quickest.
•However, quality of service, speed and convenience are valued by intending consignors and, for
general goods, diversion from other modes should not be estimated solely on the basis of door-to
door transport charges. Similarly, the choice of mode for passenger transport should not be
judged purely on the basis of travel charges. The importance attached to quality of service by
users has been a major contributory factor to the worldwide decline in rail transport over recent
years.
•Diverted traffic is normally forecast to grow at the same rate as traffic on the road from which it
diverted.

34. •Generated traffic arises either because a journey becomes more attractive by virtue of a cost or time reduction or
because of the increased development that is brought about by the road investment. Generated traffic is difficult to
forecast accurately and can be easily overestimated It is only likely to be significant in those cases where the road
investment brings about large reductions in transport costs.
•For example, in the case of a small improvement within an already developed highway system, generated traffic will be
small and can normally be ignored.
•However, in the case of a new road allowing access to a hitherto undeveloped area, there could be large reductions in
transport costs as a result of changing mode from, for example, animal-based transport to motor vehicle transport. In
such a case, generated traffic could be the main component of future traffic flow.
•The recommended approach to forecasting generated traffic is to use demand relationships. The price elasticity of
demand for transport is the responsiveness of traffic to a change in transport costs following a road investment.
•On inter-urban roads a distinction is normally drawn between passenger and freight traffic. On roads providing access
to rural areas, a further distinction is usually made between agricultural and nonagricultural freight traffic.
Generated traffic.

35. Determination of cumulative
equivalent standard axles
•In order to determine the cumulative equivalent standard axles over the design life of the road, the following procedure shouldbe
followed:
•(i) Determine the daily traffic flow for each class of vehicle weighed using the results of the traffic survey and any other recent traffic
count information that is available.
•(ii) Determine the average daily one-directional traffic flow for each class of vehicle.
•(iii) Make a forecast of the one-directional traffic flow for each class of vehicle to determine the total traffic in each classthat will travel
over each lane during the design life
•(iv) Determine the mean equivalence factor of each class of vehicle and for each direction from the results of this axle loadsurvey and
any other surveys that have recently been carried out.
•(v) The products of the cumulative one-directional traffic flows for each class of vehicle over the design life of the road and the mean
equivalence factor for that class should then be calculated and added together to give the cumulative equivalent standard axle loading
for each direction. The higher of the two directional values should be used for design.

36. The following formula, using the average daily traffic flow for the first year (not the
value at opening to traffic, but the projected average for the year), gives the cumulative
totals:


DT i = 365* AADT1 [ (1+r)
N
– 1] / ( r ) ......Equation 1

where:

DT i is the cumulative design traffic in a vehicle category, for one
direction, and

AADT i = average daily traffic in a vehicle category i in the first year
(one direction)
r = average assumed growth rate, per cent per annum
N = design period in years

37. AXLE EQUIVALENCY
Thedamagethatvehiclesdotoapavedroadishighlydependentontheaxle
loadsofthevehicles.Forpavementdesignpurposesthedamagingpowerof
axlesisrelatedtoa“standard”axleof8.16metrictonsusingempirical
equivalencyfactors.
Inordertodeterminethecumulativeaxleloaddamagethatapavementwill
sustainduringitsdesignlife,itisnecessarytoexpressthetotalnumberofheavy
vehiclesthatwillusetheroadoverthisperiodintermsofthecumulative
numberofequivalentstandardaxles(ESAs).

38. Equivalence factor-The Fourth Power LawThe relationship between a vehicle’s EF and its axle loading is normally considered in
terms of the axle mass measured in kilograms. The relationship takes the form:

EF = [Axle Load /8160]
n
(for axle loads in kg) or

F = [Axle load /80]
n
(for axle loads in kN) ......Equation 2

Where:

EF = the load equivalency factor in ESAs, and
n = relative damage exponent

42. Cumulative number of standard axles in the design year

43. Table 2-5: Traffic Classes for Flexible Pavement Design


Traffic classes Range (10
6
ESAs)


T1 < 0.3
T2 0.3 - 0.7
T3 0.7 - 1.5
T4 1.5 - 3.0
T5 3.0 - 6.0
T6 6.0 - 10
T7 10 - 17
T8 17 – 30

44. ExampleInitial traffic volumes in terms of AADTs have been established for 2002 for a section of
a trunk road under study, as follows:


Vehicle classification 2002 AADT

Car 250
Bus 40
Truck 130
Truck-trailer 180


The anticipated traffic growth is a constant 5%, and the opening of the road is scheduled
for 2005. In addition, an axle load survey has been conducted, giving representative axle
loads for the various classes of heavy vehicles, such as given below for truck-trailers (it is
assumed that the loads are equally representative for each direction of traffic):

45. DESIGN EXAMPLE

Initial traffic volumes in terms of AADTs have been established for 2002 for a section
of a trunk road under study, as follows:


Vehicle classification 2012 AADT

Car 250
Bus 40
Truck 130
Truck-trailer 180


The anticipated traffic growth is a constant 5%, and the opening of the road is scheduled
for 2005. In addition, an axle load survey has been conducted, giving representative
axle loads for the various classes of heavy vehicles, such as given below for truck-
trailers (it is assumed that the loads are equally representative for each direction of
traffic):


Axle loads (Kg)
Vehicle No Axle 1 Axle 2 Axle 3 Axle 4

1 6780 14150 8290 8370
2 6260 12920 8090 9940
3 6350 13000 8490 9340
4 5480 12480 7940 9470
5 6450 8880 6290 10160
6 5550 12240 8550 10150
7 5500 11820 7640 9420
8 4570 13930 2720 2410
9 4190 15300 3110 2450
10 4940 15060 2880 2800


The projected AADTs in 2005 can be calculated as (AADTs in 2002) x (1.05)
3
, and
the corresponding one-directional volumes for each class of vehicle in 2005 are:

Vehicle classification One-directional traffic volume in 2005

Car 145
Bus 23
Truck 75
Truck-trailer 104

Selecting, for this trunk road, a design period of 20 years, the cumulative number of
vehicles in one direction over the design period is calculated as:

46. DESIGN EXAMPLE

Initial traffic volumes in terms of AADTs have been established for 2002 for a section
of a trunk road under study, as follows:


Vehicle classification 2012 AADT

Car 250
Bus 40
Truck 130
Truck-trailer 180


The anticipated traffic growth is a constant 5%, and the opening of the road is scheduled
for 2005. In addition, an axle load survey has been conducted, giving representative
axle loads for the various classes of heavy vehicles, such as given below for truck-
trailers (it is assumed that the loads are equally representative for each direction of
traffic):


Axle loads (Kg)
Vehicle No Axle 1 Axle 2 Axle 3 Axle 4

1 6780 14150 8290 8370
2 6260 12920 8090 9940
3 6350 13000 8490 9340
4 5480 12480 7940 9470
5 6450 8880 6290 10160
6 5550 12240 8550 10150
7 5500 11820 7640 9420
8 4570 13930 2720 2410
9 4190 15300 3110 2450
10 4940 15060 2880 2800


The projected AADTs in 2005 can be calculated as (AADTs in 2002) x (1.05)
3
, and
the corresponding one-directional volumes for each class of vehicle in 2005 are:

Vehicle classification One-directional traffic volume in 2005

Car 145
Bus 23
Truck 75
Truck-trailer 104

Selecting, for this trunk road, a design period of 20 years, the cumulative number of
vehicles in one direction over the design period is calculated as:

47. Vehicle classification Cumulative no. of vehicles in one direction over 20 years

Car 365x145[(1.05)
20
-1]/0.05=1750016
Bus 365x23[(1.05)
20
-1]/0.05=277589
Truck 365x75[(1.05)
20
-1]/0.05=905180
Truck-trailer 365x104[(1.05)
20
-1]/0.05=1255184

48. Equivalency factors for the sample of truck-trailers, and a mean equivalency factor for that
class of heavy vehicles, can be calculated as outlined below:


Vehicle No Axle 1 Axle 2 Axle 3 Axle 4 Total
Load Factor Load Factor Load Factor Load Factor Factor
1 6780 0.43 14150 11.91 8290 1.07 8370 1.12 14.54
2 6260 0.30 12920 7.91 8090 0.96 9940 2.43 11.60
3 6350 0.32 13000 8.13 8490 1.20 9340 1.84 11.49
4 5480 0.17 12480 6.77 7940 0.88 9470 1.95 9.77
5 6450 0.35 8880 1.46 6290 0.31 10160 2.68 4.80
6 5550 0.18 12240 6.20 8550 1.23 10150 2.67 10.28
7 5500 0.17 11820 5.30 7640 0.74 9420 1.91 8.12
8 4570 0.07 13930 11.10 2720 0.01 2410 0.00 11.18
9 4190 0.05 15300 16.92 3110 0.01 2450 0.00 16.99
10 4940 0.10 15060 15.76 2880 0.01 2800 0.01 15.88
Mean equivalency factor for truck-trailers = 11.47

49. For the sake of this example, it will be assumed that similar calculations have been
performed, giving mean equivalency factors for buses and trucks of 0.14 and 6.67
respectively.

Finally, the cumulative numbers of ESAs over the design period are calculated as follows,
using the cumulative numbers of vehicles previously calculated and the equivalency
factors:

Vehicle classification Cum. no. of vehicles Equivalency factor 10
6
ESAs

Car 1750016 0.00 0.0
Bus 277589 0.14 0.0
Truck 905180 6.67 6.0
Truck-trailer 1255184 11.47 14.4
Total ESAs = 20.4

Based on the above analysis, the trunk road under study would belong to the traffic
class T8 for flexible pavement design.

50. SUBGRADE STRENGTH
•Thesubgradestrengthistheothermostimportantfactor,apartfromtraffic
loading,whichgovernsthepavementstructuralconfiguration.Thefirststages
ofdeterminingsubgradestrengthinvolvesofdeterminingnominallyuniform
sectionsintermsofsubgradecondition.Thiscanbebasedongeologicaland
soilpropertyassessments,inconjunctionwithotherphysicalassessments
suchastheDynamicConePenetrometer(DCP)testorinsitubearingtests,or
anyothermeansthatallowsrealisticdelineation.

51. Table 3.1: Subgrade classification
1








Subgrade Class Designation
Subgrade CBR
ranges (%)
S1 S2 S3 S4 S5 S6
2 3-4 5-7 8-1415-2930+

52. Volume III Pavement Design Manual
Part 1: Flexible Pavement Design Guide

EXAMPLE OF PAVEMENT DESIGN


Consider a single carriageway pavement, having a road width of 7.0 meters, to be
designed for the following conditions.

Climate. The mean annual rainfall is 1100mm.

Subgrade. The subgrade soil is silty clay. Its design CBR at 100% MDD (standard
compaction) and after 4 days soak is 6% (refer to section 3). Thus according to Table
3.1 the soil is classified in S3 subgrade class.

Traffic. Traffic counts and axle load surveys have shown that the initial daily number
of commercial vehicles and equivalence factors, EF, will be as follows: [refer to section
2.2 for converting real axle loads to ESAs. The average vehicle damaging factor
(equivalent factor) is then obtained for each vehicle class).

2 axle and tandem trucks: 70, EF= 2.0
Trucks with drawbar trailer: 15, EF= 6.0
Articulated units : 8, EF= 6.0
Buses : 20, EF= 1.0

The economic study of the project has recommended a design period of 15 years, and
has forecasted a constant annual growth rate of 2.5%.

 Obtain the cumulative design traffic (DT) for one direction for each vehicle
category according to equation 1 [section 2.2 (iii)]
 Obtain the cumulative standard axle loads for each vehicle category by multiplying
DT with the equivalent factor for each vehicle category.
 Sum up the cumulative standard axles obtained for each vehicle category. The
sum of the cumulative number of standard axles is 1.95 x 10
6
ESA.

The traffic class is thus T4.

Road making materials. Filed investigations and laboratory tests have shown that
latteritic gravel occurs in sufficient quantity near the alignment.

The gravel has soaked CBR in the range 20-25%, and PI in the range of 17 – 20. In
order to fulfill quality requirements and serve as subbase it should be treated with 5%
lime.

A stone source suitable for graded crushed stone, bituminous bas e and asphalt
concrete exists close to the alignment.

The climate shows predominantly wet region. Based on the available materials, traffic
class and subgrade strength, Design Chart W2 is the appropriate chart to be
considered.

Using Traffic Class T4 and Subgrade Class S3 we obtain the following layer thickness:

Asphlat concrete 50 mm
Granular Base (soaked CBR > 80%) 150 mm
Cemented subbase (7 day UCS 1.5 – 3 Mpa) 150 mm
Selected Layer (soaked CBR > 15%) 150 mm

54. Volume III Pavement Design Manual
Part 1: Flexible Pavement Design Guide

EXAMPLE OF PAVEMENT DESIGN


Consider a single carriageway pavement, having a road width of 7.0 meters, to be
designed for the following conditions.

Climate. The mean annual rainfall is 1100mm.

Subgrade. The subgrade soil is silty clay. Its design CBR at 100% MDD (standard
compaction) and after 4 days soak is 6% (refer to section 3). Thus according to Table
3.1 the soil is classified in S3 subgrade class.

Traffic. Traffic counts and axle load surveys have shown that the initial daily number
of commercial vehicles and equivalence factors, EF, will be as follows: [refer to section
2.2 for converting real axle loads to ESAs. The average vehicle damaging factor
(equivalent factor) is then obtained for each vehicle class).

2 axle and tandem trucks: 70, EF= 2.0
Trucks with drawbar trailer: 15, EF= 6.0
Articulated units : 8, EF= 6.0
Buses : 20, EF= 1.0

The economic study of the project has recommended a design period of 15 years, and
has forecasted a constant annual growth rate of 2.5%.

 Obtain the cumulative design traffic (DT) for one direction for each vehicle
category according to equation 1 [section 2.2 (iii)]
 Obtain the cumulative standard axle loads for each vehicle category by multiplying
DT with the equivalent factor for each vehicle category.
 Sum up the cumulative standard axles obtained for each vehicle category. The
sum of the cumulative number of standard axles is 1.95 x 10
6
ESA.

The traffic class is thus T4.

Road making materials. Filed investigations and laboratory tests have shown that
latteritic gravel occurs in sufficient quantity near the alignment.

The gravel has soaked CBR in the range 20-25%, and PI in the range of 17 – 20. In
order to fulfill quality requirements and serve as subbase it should be treated with 5%
lime.

A stone source suitable for graded crushed stone, bituminous bas e and asphalt
concrete exists close to the alignment.

The climate shows predominantly wet region. Based on the available materials, traffic
class and subgrade strength, Design Chart W2 is the appropriate chart to be
considered.

Using Traffic Class T4 and Subgrade Class S3 we obtain the following layer thickness:

Asphlat concrete 50 mm
Granular Base (soaked CBR > 80%) 150 mm
Cemented subbase (7 day UCS 1.5 – 3 Mpa) 150 mm
Selected Layer (soaked CBR > 15%) 150 mm

57. SPECIFICATIONS
•Pavement thicknesses for the various layers
•Material characteristics including
a)Type
b)Strength CBR
c)Compaction characteristics
d)Gradation
e)Consistency limits