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1. Road Note 31/ Catalogue Method
• A Guide to the Structural Design of Bitumen Surfaced Roads in Tropical and Sub-tropical
Countries.
• ORN – 31 is based principally on research conducted in countries throughout the world
by the Overseas Centre, Transport Research Laboratory (TRL), UK, on behalf of the
Overseas Development Administration (ODA).
• Road Note 31 was first published in 1962 and revised in 1966, 1977 and 1993.
• Now Overseas Road Note (ORN) 31, 1993, 4th Edition.
• 4th edition of the Road Note 31 has drawn on the experience of TRL and collaborating
organizations in over 30 Tropical and Sub-tropical Countries.
• 4th edition extends the designs of previous editions to cater for traffic up to 30 million
equivalent standard axles and takes account of the variability in material properties and
construction control, the uncertainty in traffic forecasts, the effects of climate and high
axle loads and the overall statistical variability in road performance.
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2. Design Process
• 3 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 using structural catalogues. (It is usually necessary to assume
that an appropriate level of maintenance is also carried out.)
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3. Pavement Layers
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4. Surfacing:
• This is the uppermost layer of the pavement and will normally consist of a
bituminous surface dressing or a layer of premixed bituminous material.
Where premixed materials are laid in two layers, these are known as the
wearing course and the base course (or binder course).
Roadbase:
• This is the main load-spreading layer of the pavement. It will normally consist
of crushed stone or gravel, or of gravelly soils, decomposed rock, sands and
sand-clays stabilised with cement, lime or bitumen.
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5. Sub-base:
• This is the secondary load-spreading layer underlying the roadbase.
• It will normally consist of a material of lower quality than that used in the
road base such as unprocessed natural gravel, gravel-sand, or gravel-sand-
clay.
• This layer also serves as a separating layer preventing contamination of the
roadbase by the subgrade material and, under wet conditions, it has an
important role to play in protecting the subgrade from damage by
construction traffic.
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6. Capping layer (selected or improved subgrade):
• Where very weak soils are encountered, a capping layer is sometimes necessary.
• This may consist of better quality subgrade material imported from elsewhere or
existing subgrade material improved by mechanical or chemical stabilisation.
Subgrade:
• This is the upper layer of the natural soil which may be undisturbed local material
or may be soil excavated elsewhere and placed as fill.
• In either case it is compacted during construction to give added strength.
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7. 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.
• The loads imposed by private cars do not contribute significantly to the
structural damage.
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8. • For the purposes of structural design, cars and similar sized vehicles can be
ignored and only the total number and the axle loading of the heavy vehicles
that will use the road during its design life need to be considered.
• In this context, heavy vehicles are defined as those having an unladen weight
of 3000 kg or more.
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9. Design Life
• For most road projects an economic analysis period of between 10 and 20
years from the date of opening is appropriate.
• Design life does not mean that at the end of the period the pavement will be
completely worn out and in need of reconstruction; it means that towards
the end of the period the pavement will need to be strengthened so that it
can continue to carry traffic satisfactorily for a further period.
• Condition surveys of bituminous pavements should be carried out about
once a year as part of the inspection procedures for maintenance.
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10. Estimating Traffic Flows
• In order to determine the total traffic over the design life of the road, the first step is to
estimate baseline traffic flows.
• The estimate should be the (Annual) Average Daily Traffic (ADT) currently using the route.
• The ADT is defined as the total annual traffic summed for both directions and divided by
365.
• It is usually obtained by recording actual traffic flows over a shorter period from which the
ADT is then estimated.
• For long projects, large differences in traffic along the road may make it necessary to
estimate the flow at several locations.
• It should be noted that for structural design purposes the traffic loading in one direction is
required and for this reason care is always required when interpreting ADT figures.
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11. Axle Loading : Axle Equivalency
• The damage that vehicles do to a road depends very strongly on the axle
loads of the vehicles.
• For pavement design purposes the damaging power of axles is related to a
'standard' axle of 8.16 tonnes using equivalence factors which have been
derived from empirical studies.
• In order to determine the cumulative axle load damage that a pavement will
sustain during its design life, it is necessary to express the total number of
heavy vehicles that will use the road over this period in terms of the
cumulative number of equivalent standard axles (esa).
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12. • Axle load surveys must be carried out to determine the axle load distribution
of a sample of the heavy vehicles using the road.
• Data collected from these surveys are used to calculate the mean number of
equivalent standard axles for a typical vehicle in each class.
• These values are then used in conjunction with traffic forecasts to determine
the predicted cumulative equivalent standard axles that the road will carry
over its design life.
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13. Determination of Cumulative Equivalent Standard Axles
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25. Numerical Example
In a certain section of single lane road, it is intended to construct the
road pavement. The traffic survey shown that the road caters
present ADT of 132 cv/day. The annual growth rate of traffic is 7%.
The pavement is to be designed for 10 years period and expected
completion period is 2 years from the last date of traffic count. The
87.5 percentile CBR value of the subgrade soil is found to be 5%.
Design the pavement section as per Road Note 31 Guideline.
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26. Solution:
Cumulative no. of std. axles at the end of design period,
N =
𝟑𝟔𝟓 𝒙 𝑨 [ 𝟏+𝒓 𝒏 −𝟏]
𝒓
=
𝟑𝟔𝟓 𝒙 𝑷 𝟏+𝒓 𝒙
[ 𝟏+𝒓 𝒏 −𝟏]
𝒓
Here,
P = 132 cvpd
r = 7% = 0.07
x = 2 years
n = 10 years
Now,
N =
𝟑𝟔𝟓 𝒙 𝑷 𝟏+𝒓 𝒙
[ 𝟏+𝒓 𝒏 −𝟏]
𝒓
=
𝟑𝟔𝟓 𝒙 𝟏𝟑𝟐 𝟏+𝟎.𝟎𝟕 𝟐
[ 𝟏+𝟎.𝟎𝟕 𝟏𝟎−𝟏]
𝟎.𝟎𝟕
= 0.76 x 106 esa
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27. • According to Road Note 31 Guideline,
For, N = 0.76 x 106 esa, Traffic Class = T3
and
For, CBR = 5%, Subgrade Strength Class = S3
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28. Now, from the Design Chart 1
Granular Sub base = 225 mm
Granular Road Base = 200 mm
Surface Course = Surface
Dressing (SD)
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