design of flexible pavemant.pdf
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Design of Flexible Pavement
Design Methodology
For flexible pavements, structural design involves
determining appropriate layer thickness and type of
material or construction in each layer.
The main input parameters in design are future traffic load
and strength of soil.
Two methods of flexible pavement design are common
Empirical design, and
Mechanistic Empirical design.
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Empirical Design
In empirical approach the design is based on the results of
experimentation or experience.
An empirical analysis of flexible pavement design can be done
with or without a soil strength test.
A good example of empirical design method where no soil
test is required is Group Index Method.
Semi-empirical concepts where soil strength is used in design
are CBR method, North Dakota Cone method etc. CBR method
of design is widely known design method for flexible
pavement.
In empirical method design charts are developed from
experience and new design is based on these charts.
Mechanistic-Empirical Design
Empirical-Mechanistic method of design is based on the
mechanics of materials that relates input, such as wheel load,
strength of materials, to an output or pavement response.
In pavement design, the responses are the stresses, strains,
and deflections within a pavement structure and the physical
causes are the loads and material properties of the pavement
structure.
The relationship between these phenomena and their physical
causes are typically described using some mathematical
models. Along with mechanistic approach, empirical elements are used.
The relationship between physical phenomena and pavement
failure is described by empirically derived equations that
compute the number of loading cycles to failure.
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Advantages of the Mechanistic-Empirical design method
It can be used for both existing pavement rehabilitation and
new pavement construction It can accommodate changing load types
It can better characterize materials allowing for
better utilization of available materials
accommodation of new materials
improved definition of existing layer proportion
It uses material proportion that relates better with actual
pavement performance
It provides more reliable performance predictions
It defines role of construction in a better way It accommodates environment and aging effect of materials
in the pavement
CBR Method
This method involves determination of the CBR value of
subgrade, and also sub base as well as base materials for most
critical moisture condition.
Traffic is expressed in terms of number of commercial vehicles
per day and estimated for the design year.
A number of charts or curves are developed from the past
experience relating CBR and traffic. The values in the chart for
the estimated design traffic indicate the total thickness of
construction required above any part icular layer having
material with known CBR. The thickness of any particular layer is thus determined by
subtracting the total thicknesses necessary for this layer from
that of the layer above.
It is a simple and easy to apply method but has many limitations.
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Design of Flexible Pavement by IRC
Method
Introduction
The flexible pavement design method in India is provided by
IRC in two separate guidelines.
IRC: 37 2001 (Guideline for the Design of Flexible
Pavements) is basically applicable for Expressways, National
Highways, State Highways and Major District Roads.
IRC: SP: 72- 2007 (Guideline for the Design of Flexible
Pavements for Low Volume Rural Roads) is developed for
the rural roads
IRC: 37 was first published in 1970, which was revised twice in
1984 and 2001. The current version follows the mechanistic-empirical method where CBR tests are conducted to
determine the strength of soil.
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Basics of IRC: 37 2001
In IRC: 37 2001 the flexible pavement has been modeled as a
three layer structure and stresses and strains at critical locationshave been computed using the linear elastic model developed
under the MORT&H (Ministry of Road Transport & Highways).
Layered elastic approach usually works with relatively simple
mathematical models and thus requires following assumptions
Pavement layer extends infinitely in the horizontal direction
The bottom layer extends infinitely downwards
Materials are not stressed beyond their elastic ranges
Design curves correlating pavement thickness, traffic intensity
[upto 150 msa (million standard axle)] and CBR value of soil are
developed.
In this design following three types of pavement distress
resulting from repeated application of traffic loads are
considered.
Vertical compressive strain at the top of the subgrade. If
the strain is excessive, the subgrade will deform
resulting in permanent deformation at the pavement
surface during the design life.
Horizontal tensile strain at the bottom of the
bituminous layer. Large tensile strains cause fracture of
the bituminous layer during the design life.
Pavement deformation within the bituminous layer.
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Pavement deformation within the bituminous layer can be
controlled by appropriate mix design as per MORTH
specification.
The thicknesses of the granular and bituminous layers are
selected using analytical design approach so that strains at the
critical points are within the allowable limits.
For calculating tensile strains at the bottom of the bituminous
layer, the stiffness of Dense Bituminous Macadam (DBM) layer
with 60/70 bitumen has been used in the analysis.
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Failure Criteria
Fatigue Cracking
Bituminous surfacing of pavements display flexural fatigue
cracking if the tensile strain at the bottom of the bituminous
layer is beyond certain limits.
Pavement is considered failed if 20% of the surface has
cracked
Fatigue Criteria
Nf= 2.21 x 10-4 [1/t]
3.89 x [1/E]0.854
Nf= No. of cumulative standard axles to produce 20%cracked surface area
t = Tensile strain at the bottom of Bituminous Concretelayer
E = Elastic Modulus of Bituminous Surface (MP
Rutting Failure
Pavement may fail in rutting if excessive compressive strain
develops at the top of subgrade layer
Pavement is considered failed if it exhibits a rut depth of
20mm.
Rutting Criteria
NR = 4.1656 x 10-8 [1/z]
4.5337
NR = No. of cumulative standard axles to produce rutting of 20mm
z = Vertical subgrade strain
Relation Between CBR and Elasticity of subgrade, sub-base,base
E3 (MPa) = 10 x CBR if CBR 5%
E2(MPa) = E3x0.2h0.45
E3 = Elastic Modulus of Subgrade; E3 = Composite Elastic Modulusof Sub-base and Base; h = thickness of Sub-base and Base (mm)
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Traffic Estimation
The method considers traffic in terms of the cumulative
number of standard axles (8160 kg) to be carried by thepavement during the design life.
This requires the following information:
i. Initial traffic in terms of CVPD
ii. Traffic growth rate during the design life
iii. Design life in number of years
iv. Vehicle damage factor (VDF)
v. Distribution of commercial traffic over the carriageway.
The commercial vehicles having laden weight 3 tonnes or
more are to be considered.
Initial Traffic
Initial daily average traffic flow for any road should
normally be based on 7-day 24-hour classified traffic
counts (ADT).
In case of new roads, traffic estimates can be made on the
basis of potential land use and traffic on existing routes in
the area.
Traffic growth rate
Traffic growth rates can be estimated by
i. studying the past trends of traffic growth, andii. establishing econometric models.
If adequate data is not available, it is recommended that
an average annual growth rate of 7.5% may be adopted.
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Initial Traffic
Initial daily average traffic flow for any road should
normally be based on 7-day 24-hour classified traffic
counts (ADT).
In case of new roads, traffic estimates can be made on the
basis of potential land use and traffic on existing routes in
the area.
Traffic growth rate
Traffic growth rates can be estimated by
i. studying the past trends of traffic growth, and
ii. establishing econometric models.
If adequate data is not available, it is recommended that
an average annual growth rate of 7.5% may be adopted.
Design Life
For the purpose of the pavement design, the design life is
defined in terms of the cumulative number of standard axles
that can be carried before strengthening of the pavement is
necessary. It is recommended that pavements
Expressway and Urban roads for 20 years and
National Highways, State Highways should be designed for
a life of 15 years
Other categories of roads for 10 to 15 years.
If full pavement cannot be constructed initially stageconstruction is recommended.
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Vehicle Damage Factor
The vehicle damage factor (VDF) is a multiplier forconverting the number of commercial vehicles of differentaxle loads and axle configurations to the number of standardaxle-load repetitions.
It is defined as equivalent number of standard axles percommercial vehicle.
The VDF varies with the axle configuration, axle loading,terrain, type of road, and from region to region.
The axle load equivalency factors are used to convertdifferent axle load repetitions into equivalent standard axleload repetitions.
The exact VDF values can be arrived after extensive fieldsurveys.
Distribution of Traffic over Carriageway
A realistic assessment of distribution of commercial trafficby direction and by lane is necessary as it directly affects thetotal equivalent standard axle load application used in thedesign.
Until reliable data is available, the following distribution maybe assumed. Single lane roads: Traffic tends to be more channelized on
single roads than two lane roads and to allow for thisconcentration of wheel load repetitions, the design shouldbe based on total number of commercial vehicles in bothdirections.
Two-lane single carriageway roads: 75 % of the
commercial vehicles in both directions. Four-lane single carriageway roads: 40 % of the total
number of commercial vehicles in both directions. Dual carriageway roads: For dual two-lane, dual three-
lane, and dual four-lane carriageways, 75%, 60% and 45%of the number of commercial vehicles in each directionrespectively.
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Computation of Design Traffic
The design traffic is considered in terms of the cumulative
number of standard axle to be carried (in the lane carryingmaximum traffic) during the design life of the road
Traffic count is done a few years prior to the equation year
of completion of construction. The initial traffic in the
above may be computed as
A =P(1+r)x
P = Number of commercial vehicles in the last count
x = Number of years between the last count and the year
of completion of construction.
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Total Thickness of the Pavement
For design of pavement design curves relating pavement
thickness to the cumulative number of standard axles to becarried over the design life for subgrade CBR values are
developed.
One set of curves is for traffic 1 to 10 msa and the other set of
curves for 10 to 150 msa.
Each set has nine curves for nine different subgrade CBR from
2% to 10%. IRC 37 - 2001 _Design curves.pdf
The total thickness of the pavement can be obtained from
these curves for the given traffic and subgrade CBR.
The thickness consists of granular sub-base, granular base and
bituminous surfacing.
Pavement Layer Composition
The thicknesses and composition of pavement layers are given
in the Pavement Design Catalogues. IRC 37- 2001_Design Catalogue_1.pdf
Subbase Course
Granular materials conforming to clause 401 of MORTH
Specification for Road and Bridge Works are recommended
as sub-base
Materials passing 425 micron sieve should have liquid limit
and plasticity index not more than 25 and 6 respectively.
Materials should have minimum CBR of 20% for traffic upto
2msa and 30% for traffic exceeding 2 msa.
The thickness of sub-base should not be less than 150mm
for traffic upto 10msa and 200mm for otherwise.
If subgrade is having CBR less than 2% provide a layer of
150mm in addition to the subbase layer.
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Subbase materials comprise of natural sand, moorum,
gravel, laterite, kankar, brick metal, crushed stone, crushedslag, crushed concrete or combination thereof meeting the
prescribed grading.
Other granular construction like, Water Bound Macadam,
Wet Mix Macadam may be provided.
Base Course
The thickness of base should not be less than 225mm for
traffic upto 2msa and 250mm for otherwise.
Base course is recommended to use Water Bound
Macadam, Wet Mix Macadam, or other equivalent granular
construction conforming to IRC / MORTH specification. Base course may also be consist of Bituminous Macadam
Surfacing
Surface course consists of either a wearing course or a
binder course with wearing course at the top.
Construction of wearing course may consist of Surface
Dressing, Premix Carpet, Semi-dense Bituminous Concrete,
Bituminous Concrete etc.
Construction of binder course may consist of Bituminous
Macadam, Dense Bituminous Macadam etc.
Where wearing course is of surface dressing or premix
carpet type the thickness of the surfacing should not ne
counted within pavement thickness
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Drainage Measure
Performance of any pavement is seriously affected if adequate drainage measure to prevent accumulation of
moisture in the pavement structure is not prevented. Some
important measures for drainage measures are to:
Provide required camber in all layers to aid quick run-off.
Provide sub-surface drain
Keep the pavement as far above the water table as
economically possible. Difference between bottom of
subgrade and the HFL should not be lass than 0.6m.
Install suitable capillary cut-off.
Extend the granular sub-base over the entire formationwidth in low permeable subgrade.
Provide high permeability drainage layer over subgrade.
Design Example
Design the pavement for construction of a new bypass with thefollowing data:
i) Two lane carriage way;
ii) Initial traffic in the year of completion of construction =400 CVPD (sum of both directions);
iii) Traffic growth rate = 7.5% ;iv) Design life = 15 years;
v) Vehicle damage factor based on axle load survey = 2.5standard axle per commercial vehicle;
vi) Design CBR of subgrade soil = 4%.
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Solution Determination of the design traffic
Lane Distribution factor = 0.75N = [365 (1 + 0.075)15 1) x 400 x 0.75 x 2.5]/0.075
= 7200000
= 7.2 msa
Total pavement thickness for CBR 4% and traffic 7.2 msa
from IRC:37 2001 chart1 = 660 mm
Pavement composition can be obtained by interpolation
from Pavement Design Catalogue (IRC:37 2001).
Bituminous surfacing = 25 mm Semi-dense Bituminous
Concrete + 70 mm Dense Bituminous Macadam
Base course = 250 mm Water Bound Macadam Sub-base course = 315 mm granular material of CBR not
less than 30 %
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