Design of flexible pavemant.pdf

7/27/2019 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 %

Design of flexible pavemant.pdf

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