1.
3 Pavement
1.3.1 Role of transportation:
Transportation contributes to the economic, industrial, social ancultural
development of any country. Transportation is vital for economic development of
any region since every commodity produced needs transport at production and
distribution stages. In the production stage transportation is required for carrying
raw materials like seeds, coal etc. In the distribution stage, transportation is
required from the production centers viz. farms and factories to the marketing
centers for distribution to the retailers and the consumers. The inadequate
transportation facilities retard the process of socio economic development of the
country. The adequacy of the transportation system of a country indicates its
economic and social development.
1.3.2 Pavement types, definition:
A pavement is hard crust constructed over the natural soil for the
purpose of providing a stable and even surface for the vehicles. The pavement
supports and distributes the wheel loads and provides an adequate wearing
surface. Pavements are basically of 2 types:
1. Flexible pavements
2. Rigid pavements
1.Flexible pavement:
The flexible pavement is built up in several layers. The natural soil beneath
the pavement is known as sub-grade. Sub-base is built over the subgrade and
the base is constructed over the sub-base. The top layer is known as surfacing,
which is usually bitumen. The flexible pavement can resist only very small tensile
stresses because of its limited rigidity. Any deformation in the subgrade results
in a corresponding change in the surface of the pavement.
2.Rigid pavement:
The rigid pavement is made of cement concrete. As the concrete layer is
quite strong, the sub-base may not be required. The rigid pavements have high
flexural strength and can resist high tensile stresses. The pavements are capable
of bridging small depressions in the sub-grade.
1.3.3 Components of the pavement:
1.Sub-grade:
The sub-grade is layer of natural soil prepared to receive the layers
payment. The sub-grade should be strong enough to take up the stresses
imposed due to loads without shear failure or excessive deformation. It is the
general practice to compact at least top 80cm layer of sub-grade under
controlled conditions of optimum moisture content at maximum dry density. It
is essential to evaluate strength properties of sub-grade required. As the loads
are ultimately received by the sub-grade, if it is weak, it will fail. The soil is
generally treated to increase its strength and to improve its properties.
2. Sub-base and base courses:
These courses provide a medium to spread the wheel loads to the
subgrade. The courses usually consist of broken stones, brick or aggregates.
Boulder stones, bricks on edges and stabilized are also used for sub-bases.
However, it is preferable to use small size graded aggregates because large
stones and bricks have a tendency to penetrate the wet soil and to cause
undulation and unevenness in pavement. As the stresses in sub-base are much
lower than those in the base, the material used is inferior to that in the base.
The sub-base is also known as soling.
The base and sub-base in a flexible pavement improve the load suspending
capacity of the sub-grade by distributing the load over a large area. In a rigid
pavement, the base course helps in preventing pumping out of the soil from
below and also from frost action.
3. Surface course:
The purpose of the surface course, also known as wearing course, is to
give a smooth riding surface and resist pressure from wheels. The surface course
also provides a watertight barrier against infiltration of surface water.
In flexible pavements, a surface course usually consists of a bituminous
surfacing. In rigid pavements, the cement may act as a base course as well as
surface course. There are many types of surface treatments, depending upon
the availability of materials and plants and the magnitude of the load and the
factors affecting pavement design.
1.4 Objectives of the project
The project is entitled “ A design and analysis as flexible pavement on different
Subgrade pavement “ aims at discussing and evaluating design and cost aspects
of flexible and pavement. The main objective of the projects are:
1. To carryout design of flexible rigid pavements.
2. To determine engineering properties of Sandy and Expansive soils.
3. To collect input parameters of traffic for design of flexible pavement.
4. To carryout rate analysis of various items of work as per design and estimate
construction cost per km in respect of both pavements.
5. To collect input parameters like rates of labour, material and machinery for
evaluating cost aspects of pavement.
Layers of Flexible pavement
Layers of Rigid pavement
2. Literature
2.1 General
Based on the performance of existing designs and using analytical
approach, simple design charts and a catalogue of pavement designs are added
in the code. The pavement designs are given for sub-grade CBR values ranging
from 2% to 10% and des ign traffic ranging from 1 msa to 150 msa for an average
annual pavement temperature of 35 °C. The layer thicknesses obtained from the
analysis have been slightly modified to adapt the designs to stage construction.
Using the following simple input parameters, appropriate designs could be
chosen for the given traffic and soil strength:
Design traffic in terms of cumulative number of standard axles; and
CBR value of sub-grade.
The factors considered for the design of flexible pavement thickness are
CBR value of the subgrade and the cumulative standard loads. The CSA value
depends on the initial traffic, design period, growth rate, VDF values and lane
distribution factor.
2.2 CBR value of subgrade | Section 5 of IRC 37;2012 |
It is recommended that whether in embankment or in cutting, the
subgrade should be compacted well to the highest possible dry density so as to
utilize the full strength of the soil and thus economize on the overall thickness
of flexible pavement. On all important roads such as expressways, national and
state highways, major district roads and urban arterials, the top 500 mm of
subgrade is to be compacted to a minimum of 97 % of density achieved by heavy
compaction (or modified Proctor compaction). Inferior soils with dry density
value less than 1.75 g/cc should not be accepted for use as subgrade material.
The CBR test should be carried out at the appropriate moisture content.
The maximum or worst moisture condition of the subgrade under actual site
conditions should be taken into account. The rain fall, depth of water table from
the formation level, cross slope, type of shoulder, depth of road side drains and
type of pavement surfacing should be considered before deciding the testing
moisture content or the requirement of soaking the CBR test specimens.
CBR tests should be conducted on a number of test specimens on each
soil type selected and the average of at least three consistent values taken as the
design CBR value.
The values are considered consistent if the maximum variation in CBR
values (%) obtained for a soil type do not vary more than 2 for CBR values of 5
to 10 and not more than 3 for CBR values 11 to 30. If the variation exceeds these
limits, the average of best six test specimens are taken as the design CBR value.
2.3 Initial traffic, N | Clause 4.1 of IRC 37:2012 |
The field investigations including traffic and axle load studies are often
carried out during the feasibility studies and preparation of detailed project
report. All the heavy and commercial vehicles of gross loads greater than 3.0 t
are considered. Axle load studies are carried out on a minimum sample size of
20% of each classified vehicle class. The data are grouped into different ranges
of axle loads and also number of axles per vehicle. There is often a delay of 2 to
4 years after the studies before obtaining the approvals, award of the work
contract and completing the road construction. The traffic growth during this
intermediate period, m years has to be considered in the design.
The initial traffic N is taken as the estimated traffic after completion of
the road. construction and opening to traffic.
2.4 Design life, n years | Clause 4.3 of IRC 37:2012 |
Flexible pavements are generally designed for a design life of 15 years for
all important highways such as national and state highways. In the case of
expressways, urban arterial and sub-arterial roads longer design period of 20
years is preferable. In all other roads, the design life of 10 to 15 years is generally
adopted in India. In case due to financial constraints, it is not possible to provide
the thickness requirement, it is possible to resort to stage construction
technique; in this case the full thickness requirement is first designed for 15
years design life and the required thicknesses of granular sub-base and base
courses are constructed in the first stage. However instead of thick layers of
bituminous binder and surface courses, in the first stage the bituminous layers
are designed and constructed for a design life of about five years only and before
the expiry of this period, pavement evaluation studies are carried out and the
overlay thickness requirement is designed and constructed for a further design
life of 5 to 10 years, as required in the second stage.
2.5 Growth rate of vehicles, r % per year
|Clause 4.2 of IRC 37:2012|
The average growth rate of each vehicle class is to be determined making
use of the past data and analysis and the growth rate is to be taken into account,
both during the initial period during construction and during the design life. If
the actual growth rate could not be determined, an average growth rate of 7.5%
may be assumed as a very rough approximation.
2.6 VDF values |Clause 4.4 of IRC 37:2012|
The axle loads and the number axles per heavy vehicle are converted in
terms of number of standard axle loads per vehicle, known as vehicle damage
factor or VDF. The standard axle load is taken as 8.16 or 8.17 t and fourth power
law is generally made use of as explained in under EWLF studies. However, for
the design of pavements of less important roads, when the average number of
commercial vehicles are 150 to 1500 per day the VDF values are assumed as 3.5
on rolling and plain terrains and 1.5 on hilly terrain. Similarly for number of CV
more than 1500 per day, the VDF values are assumed as 4.5 on rolling and plain
terrains and 2.5 on hilly terrain.
2.7 Lane distribution factor, D |Clause 4.5 of IRC 37:2012|
The transverse distribution of heavy vehicles across the width of the
carriageway along both the directions (in the case of two-way traffic movement)
is to be taken into account in case this cannot be assessed from actual field
studies, the following recommended guidelines on lane distribution factors, D
may be followed:
a) On undivided roads with single lane carriageway the total number of heavy
vehicles along both the directions are taken or the lane distribution factor D = 1
b) On undivided roads with two-lane carriageway, D = 0.75 and the total number
of heavy vehicles along both the directions is to be considered
c) On undivided roads with four-lane carriageway, D = 0.4 and the total number,
of heavy vehicles along both the directions is to be considered
d) On roads with divided carriageway with two lanes each, D = 0.75 and the
number of heavy vehicles along each direction is considered
e) On roads with divided carriageway with three lanes each, D = 0.6 and the
number of heavy vehicles along each direction is considered -
f) On roads with divided carriageway with four lane each, D-0.45 and the number
of heavy vehicles along each direction is considered.
2.8 Design Traffic |Section 4 of IRC 37:2012|
The design traffic in terms of the cumulative number of standard axles to
be carried during the design life of the road should be computed using the
following equation:
N= {365[(1 + r) ^ n - 1] .A.D.F }/r
Where,
N = Cumulative number of standard axles to be catered for in the design in terms
of msa.
A = Initial traffic in the year of completion of construction in terms of the number
of Commercial Vehicles Per Day (CVPD).
D = distribution factor.
F = Vehicle Damage Factor (VDF).
n = Design life in years.
r = Annual growth rate of commercial vehicles in decimal (e.g., for 5 per cent
annual growth rate, r = 0.05 )
The traffic in the year of completion is estimated using the following formula:
A = P * (1 + r) ^ x Where,
P = Number of commercial vehicles as per last count.
x = Number of years between the last count and the year of completion of
construction
