Transportation Engineering

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TRANSPORTATION ENGINEERING

UNIT - 1 HIGHWAY PLANNING AND DEVELOPMENT IN INDIA

CONTENTS:

Aims / Objectives:

1.1. Introduction

1.2. Different Modes of Transportation

1.3. Characteristics of Road Transportation

1.4. Highway Planning and Development in India

1.5. Summary

1.6. Self Assessment Questions

1.7. Books for Reference.

Aims / Objectives:

Transportation system is required for carrying passengers and goods from one place to

the other. The different modes of transport are Roads , Railways , Airways and Water ways.

Of all communications , highway or road is the nearest communication to man and is the

only means of transportation that offers to the whole community alike. Merits and demerits of

roads as a means of transport are presented.

Due to lack of rational and scientific planning road development in India has suffered

a major set back. This set back has become a major hurdle in Nation’s march towards

progress and prosperity. The government of India formed a Road Development Committee ,

under the chairman ship of Mr. M.R. Jayakar , in 1927 to examine the desirability of

developing the road system of India and the means by which such development could be

achieved. The committee submitted its report in 1928 which may be considered as a major

land mark in the planned development of roads in our country. The recommendations of the

Jayakar Committee and their impact on the planning and development of roads in modern

India have been explained.

1.1. Introduction:

Transportation contributes to the economic , industrial and cultural development of

any region or country. Transportation is also essential for strategic movement in emergency

for the defence of the country and to maintain better law and order. A study of the economic ,

industrial and cultural development of the advanced nations like the United State , United

Kingdom , Japan , Germany and others indicate that the progress and prosperity of any

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country or nation may be linked up with the efficiency and adequacy of its transportation

systems.

1.2. Different Modes of Transport:

The three media surrounding us , Land , Water and Air , have been used effectively

for the development of transportation modes. Land has given scope for the development of

road and rail transport. Water and air have developed water ways and air ways. Water ways

include seas , rivers , canals and lakes for movement of ships and boats.

The choice of a transportation system depends upon (i) length of haul (ii) weight and

size of consignment (iii) traffic density (iv) nature of route and (v) quality of service.

1.3. Characteristics of Road Transport:

Some of the important characteristics of this mode of transport are as follows:

(i) Of all communications , road communication is the nearest to men.

(ii) Road transport is the only means of transport that offers itself to the whole community

alike.

(iii) Roads are used by various modes of transport , that is , by-cycles , rickshaws , animal

drawn carts and carriages , automobiles , etc. ; but railways , airways and waterways are used

by rolling stock , aeroplanes and by ships and boats respectively.

(iv) Construction and maintenance cost of roads is cheaper than that of railway tracks , docks

and harbours and airports.

(v) Stage Construction is feasible for roads.

(vi) Roads can be constructed to penetrate interior of any region and to connect villages. This

advantage becomes particularly evident when planning the communication system in hilly

regions and scarcely populated areas. Provision of railways in such areas become

uneconomical.

(vii) Road transport offers a complete freedom to the road users to transfer the vehicle from

one lane to another and from one road to another according to the need and convenience.

(viii) Road transport offers a flexible service , free from fixed schedules.

(ix) In particular for short distance travel and short hauls road transport saves time and is

economical.

(x) Road transport offers door to door service.

(xi) Road Transport has a high employment potential.

(xii) Road Transport causes parking problems of serious proportions in city streets.

(xiii) One of the serious disadvantages of road transport in its poor record of safety.

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(xiv) Road transport has been one of the major causes of environmental pollution. Noise ,

fumes , vibrations , loss of aesthetics , ribbon development. Cluster of advertisements along

highways - are some of the ill-effects.

1.4.0. High-way Planning:

Planning is a pre-requisite for any development programme. This is particularly

necessary when long range comprehensive plans have to be drawn.

Before going to the stage of planning , one should fix up the main objectives of a

programme. Following are some of the main objectives of highway planning.

(i) To provide for efficient , safe , economical , comfortable and speedy movement of goods

and people.

(ii) To plan anticipated future developments.

(iii) To plan for a road system having maximum utility within available resources.

(iv) To phase the road development programme from financial considerations.

(v) To evolve a financial system.

1.4.1. Planning and Development in India:

The history of roads dates back to the period before the advent of recorded history.

The various civilisations of the world that are known for their excellence and attainments

have left traces of their art of road building.

In Mauryan age , considerable importance began to be attached to roads as trade ,

agriculture and cultural activities flourished. Rajapaths (main highways) and vanijapathas

(trade routes) were constructed. Kautilya , Prime minister of Emperor Chandragupta Maurya ,

laid down rules for the construction of roads for different types of traffic in his book

‘Arthashasthra’. During the Pathan and Mughal periods , roads were greatly improved. Some

of the highways either built or maintained by Mughals and other rulers received great

appreciation from the foreign visitors who visited India during the reign. Roads were built

running from North-West to Eastern areas through Gangetic plains , linking also the coastal

and central parts. Later the fall of the Mughal empire led to scant attention to

communications ; and the conditions of the roads deteriorated considerably. At the beginning

of the British rule , roads were constructed by British Military Engineers on the remains of

old roads which existed. These roads connected important military and business centres.

Military maintenance of roads was not quite adequate and in 1865 Lord Dalhousie ,

the then Governor General , formed the public works department in more or less the same

form that exists today. Engineering Colleges were established to train civil engineers.

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Important roads were provided with metalled beds and were bridged. Specifications were

formed for construction of roads. By the end of 19th century , these efforts resulted in the

establishment of a good system of trunk roads in the country. With the development of

railways (in the latter part of 19th century) a set back to the rapid development of roads

occurred. Government of India act 1919 put a further damper on road development as the

subject of roads was purely a provincial charge and central government remained concerned

with roads of strategic importance.

After world war I , motor vehicle traffic on roads increased. The existing roads were

not capable to withstand both bullock-cart traffic and motor vehicles. This demanded better

road network which can carry the mixed traffic. In November 1927 , Government of India

appointed a committee called the Road Development Committee under the chairmanship of

Mr. M.R. Jayakar. The committee , known popularly as the Jayakar committee was required

to:

(i) examine the desirability of developing the road system of India and the means by which

such development could be achieved.

(ii) examine the possibility , having regard to the distribution of functions between the centre

and state governments , of co-ordinating the activities of the different governing authorities in

the country by the formation of a central road board or otherwise.

1.4.2. Recommendations of Jayakar Committee: The committee’s report (1928) may be

considered as a major landmark in the planned development of roads in our country. The

most important recommendations made by the committee are:

(i) The road development in the country should be considered as a national interest as it has

become beyond the capacity of state governments and local bodies.

(ii) An extra tax should be levied on petrol from the road users to develop a road

development fund called ‘Central Road Fund’.

(iii) A semi-official technical body should be formed to pool the technical knowledge from

various parts of the country and to act as an advisory body on various aspects of roads.

(iv) A research organisation should be instituted to carry research and development work and

to be available for consultation.

Most of the recommendations of Jayakar committee were accepted by Government of

India and were implemented subsequently.

1.4.3. Central Road Fund: Following the recommendations of Jayakar Committee for

funding the roads , CENTRAL ROAD FUND was created in 1929 with additional duty

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levied on petrol and was intended for road development. Twenty percent of the same will be

retained by the central government for meeting the expenses of road development of the

country and the remaining will be distributed among the states in the ratio of actual

consumption of petrol or revenue collected.

1.4.4. Indian Roads Congress (IRC): A semi-official technical body known as the Indian

Roads Congress (IRC) was formed in 1934 by the government of India. This , it may be

recalled , was one of the recommendations of the Jayakar Committee.

Now IRC is an active body of national importance controlling standardisation ,

specifications and recommendations as regards design and construction of roads and bridges.

The IRC publishes journals , research publications and standard specifications on various

aspects of Highway and Traffic engineering. The IRC works in collaboration with the Roads

wing of the Ministry of Shipping and Transportation , Government of India. It is responsible

for the various highway development plans of our country.

1.4.5. Motor Vehicles Act:

In-order to have control over the driver , vehicle ownership and vehicle operation on

roads , the Government of India broughtout for the first time the Motor Vehicles Act in 1939.

This act has been thoroughly revised in the year 1988.

1.4.6. Nagpur Conference:

At the initiative of IRC , a conference of the chief engineers of all states was held at

Nagpur in 1943 to finalise the road development plan for the country as a whole. This may be

considered as a landmark in the history of road development in India , as it was the first

attempt to prepare a co-ordinated road development programme in a comprehensive and

scientific manner. In this conference a 20 year plan , for the period 1943-63 , popularly

known as Nagpur Plan , was finalised. All roads were classified into five categories and a

target of 16 KM of road per 100 sq. km area of the country was aimed at.

1.4.7. Central Road-Research Institute (C.R.R.I):

One of the recommendations of the Jayakar Committee was to set up a central

organisation of research. Accordingly , an institute for carrying out research in various fields

of highway engineering , called the Central Road Research Institute (CRRI) was started at

New-Delhi in 1950. This institute is mainly engaged in applied research and offers technical

advice to state governments , other organisations and industries on various problems

concerning highways.

1.4.8. National Highway Act:

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The National Highway Act was passed in the year 1956. According to this act , the

development and maintenance of National Highways is the responsibility of the Central

government. The Central government is empowered to declare any other highway as National

Highway or to omit any of the existing National Highway from the list.

1.4.9. Border Roads Development Board:

This board was set up in March 1960 under the chairmanship of the prime minister. A

General Reserve Engineering Force (GREF) consisting of unskilled and skilled labour ,

supervisors and engineers from all parts of India was raised. This organisation is to take up

construction of roads at high altitudes in extremely varied , difficult and hostile terrain under

unfavourable climatic conditions.

1.4.10. Second Road Development Plan (1961 ‘ 81):

The second twenty year road development plan for the period 1961 - 81 was initiated

by the IRC and was finalised in 1959 and is also known as the Bombay Road Plan. The plan

gave due considerations to the developments those are taking place and developments that

have to take place in our country in various fields during the plan period. The target of road

length contemplated during this plan period was 32 per 100 Sq. Km of area covered.

Though the road length envisaged in this plan has been exceeded , the standards to

which these roads have been brought , leave scope for considerable improvement. The

position of rural roads is far from satisfactory , Constraint of resources have been the major

bottleneck for this state of affairs.

1.4.11. Highway Research Board:

Recognising the need for faster research on highway matters , the Indian Roads

Congress has established a Highway Research Board on October 24 , 1973. Its main

functions are to advise the government about the road research programme required for the

conditions prevailing in our country , correlate the research information from various

organisation in India and recommend priorities about various road research problems. It will

also obtain feedback of research findings and evaluate the same , collect and disseminate the

results of research.

1.4.12. National Transport Policy Committee (NTPC):

This committee was formed in the year 1978 to prepare a comprehensive national

transport policy for the country , keeping in view the objectives and priorities set out in the

Five Year Plans. This Committee submitted its report in 1980 and most of the

recommendations of the Committee have been accepted by Government of India. Some of the

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important recommendations include (i) Liberalisation of transport sector , inclusion of

transportation in priority sector and optimal inter-modal mix between railway and road

transport based on resource - cost consideration and energy conservation.

(ii) Development of roads in rural , hilly and tribal areas ; strengthening of national highways

; increasing the funds for maintenance of roads ; to connect all the villages with all weather

low-cost roads within the next twenty years , and

(iii) Separate recommendations for various factors connected with development and growth

of road transport by the year 2001.

1.4.13 Third Twenty Year Road Development Plan (1981 - 2001): This plan is also called

‘Lucknow Road Plan’ and has been prepared keeping in view the growth pattern envisaged in

various fields by the turn of this century. Some of the points which were given due

consideration while formulating the plan are improvement of transportation facilities in

villages , towns and small cities , conservation of energy , preservation of environmental

quality and improvement in road safety.

1.4.14. National Highway Authority: National Highway Authority was constituted by the

Ministry of Surface Transport with effect from June , 15 , 1989 with the following objectives:

(1) National Highways carry 1/3 of the total road traffic and to cope up with the increasing

demands of traffic widening of existing sections to four lanes and construction of express

ways on the high traffic density corridors are required. The funds for this are met with from

external financing institutions like the World Bank and Asian Development Bank.

Construction , maintenance and operation of National Highways , hither to done by State

P.W.D’s will ultimately be taken over by National Highway authority.

(2) Construction of toll based expressways having grade separated , divided carriage ways

will serve as a vailable alternative facilities. These require initial budgetary provision. Within

10 to 15 years the cost of the facilities along with interest is recovered through tolls ; then the

revenues generated through tolls will be very high in comparison with maintenance and

operation costs. As such ultimately the authority will have enough internal resources of its

own for construction of an expressway net work.

(3) With the National Highways Authority of India , taking over execution of National

Highway Projects , it will be possible to ensure uniformity and continuity in the improvement

to National Highway system , introduction of modern management and operation techniques ,

and optimum mechanisation and new technology in road construction besides deployment of

modern equipment for energy saving and pollution control.

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(4) Because of economics better equipment will be deployed and a continuous flow of large

scale projects would be feasible with one executing agency handling National Highway

Projects.

1.5. Summary:

The basic function of a transportation system is to carry passengers and goods from

one place to another. Social , cultural and economic development of a region or country

depends upon its transportation system. An efficient transportation system is necessary for

maintaining law and order with in the country and for boarder security.

The different modes of transport are roads , railways , airways and waterways. Out of

these roads are the most popular and major means of transport and are considered as vital

means of communication.

The Jayakar Committees report may be considered as a major land mark in the

planned development of roads in our country. Various phases of planning and development of

highways in India , during the 20th century , have been dealt with in detail.

1.6. Self Assessment Questions:

(1) Briefly out line the historical development of highways in India.

(2) Discuss the merits and limitations of roads as transportation means.

(3) Write notes on

(a) Jayakar Committee’s Recommendations

(b) Indian Roads Congress

(c) National Transportation Policy Committees

(d) Objectives of highway planning.

1.7. Books of Reference:

(1) Bindra , S.P. (1977) - A course in Highway Engineering , Dhanpat Rai and Sons , New -

Delhi.

(2) Kadiyali , L.R. (1984) - Principles and Practice of Highway Engineering - Khanna Tech

Publications , New - Delhi.

(3) Khanna , Dr. S.K. and Justo , Dr. C.R.G. (1991) - Highway Engineering , Nem chand and

Bro., Roorkee.

***

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UNIT - 2

ROAD DEVELOPMENT PLANS OF INDIA CONTENTS:

Aims / Objectives:

2.1. Introduction

2.2. Classification of Roads

2.3. Road Development plans

2.4. Rural Road Patterns

2.5. Master Plan Preparation

2.6. Summary


2.8. Reference

Aims / Objectives:

Highways have been universally recognised as vital means of transport every where.

Three long term plans have been drawn for the development of roads in India for the period

1943 to 2001. Important features of these three plans - Nagpur Plan , Bombay Plan and

Lucknow Plan have been explained. Classification of roads based on a rational basis is quite

essential for planning of highways in a region or country. Classification of roads based on

their location and importance , adopted in the Nagpur Plan , and the modified system of

classification of roads adopted in the third 20 year development plan have been discussed.

Road patterns generally adopted for rural roads have been included in all long term planning

programmes. ‘Master Plans’ play a very important role. Various stages in the preparation of

master plans are also indicated in this unit.

2.1. Introduction:

Nagpur Plan is the first plan prepared in India on scientific principles , for the

development of highways for the period 1943 - 63. The targets of this plan have been

achieved by the end of the second five year plan i.e., by 1961. As such a perspective plan for

the period 1961 - 81 , known as ‘Second Twenty Year Plan’ was drafted by the Roads wing

of Government of India. The roads in these two plans have been divided into five groups

based on their location and importance. Star and Grid pattern of roads have been adopted.

Transport planners realised that prosperity of the country takes place only when

transportation facilities are extended to rural and undeveloped areas. Further , aspects like

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environmental protection , energy conservation and improvement in road safety have also to

be included because of the expected very high rate of growth of road traffic by the turn of this

century. The Lucknow plan for the period 1981 - 2001 has been drafted including all the

above aspects.

2.2. Classification of Roads:

Roads are classified based on a number of considerations , some of the important

consideration are ;

(a) Structural Behaviour:

Based on their structural behaviour roads are classified as Flexible pavements and

Rigid pavements. These are discussed in units 9 and 10.

(b) Material of Construction:

Based on the materials of construction roads are classified as Earth roads , Gravel

roads , Water Bound Macadam roads , Bituminous roads and Cement Concrete roads. These

are discussed in Units - 11 and

(c) Service Conditions:

Based on their service conditions during different seasons of a year , roads are divided

into two categories - all weather roads and fair weather roads.

(d) Traffic Volume or Load transported or Location and Function:

Classification systems based on the traffic volume or the load transported have been

arbitrarily fixed by different agencies and there is no common agreement regarding the limits

for each classification group. However , in the system based on location and function ,

different categories may be defined clearly.

The Nagpur plan and the Lunknow plan have classified the roads based on their

location and importance. In the Nagpur plan roads have been classified into five major

categories as follows:

(i) National Highways (NH) (ii) State Highways (SH) (iii) Major District Roads (MDR) (iv)

Other District Roads (ODR) and (v) Village Roads (VR).

The above system of classification of roads has been modified in the Lunknow plan

(1981 - 2001) and the roads in the country have been classified into three groups.

(i) Primary System: - This system consists of two categories of roads (a) Express ways and

National Highways (NH).

(ii) Secondary System:- The secondary system consists of two categories of roads , namely ,

State Highways (SH) , Major District Roads (MDR).

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(iii) Tertiary System or rural roads:- This system includes Other District Roads (ODR) and

Village Roads (VR).

Each of the above categories are explained below.

2.2.1. Express Ways:

Express ways are separate class of highways with superior facilities and design

standards and are meant as through routes having very high volume of traffic. The express

ways are to be provided with divided carriage ways , controlled access , grade separations at

cross roads and fencing. These highways should permit only fast moving vehicles. Express

ways may be owned by the Central government or a State government depending on whether

the route is a national highway or state highway.

2.2.2. National Highways:

Main highways running through the length and breadth of India , connecting foreign

highways , capitals of large states , ports and including roads required for strategic

movements for the defence of India are classified as National Highways. They constitute the

frame on which the entire road communication system of the country based. They must give

uninterrupted road communication throughout the year and should be of fairly high grade

construction. All National Highways vest in the union Government of India as per the

National Highway Act 1956 and is the responsibility of the centre to develop and maintain

properly all national highways.

2.2.3. State Highways:

These highways are other main trunk or arterial roads of a state , connecting up with

the National Highways or highways of adjacent states and linking the district head quarters

and important cities within the state. The state highways are the main arteries of traffic within

a state. They are to be of the same standards as National Highways.

2.2.4. District Roads:

District roads are roads traversing each district serving areas of production and

marketing and connecting these with each other or with national and state highways or

railways or important navigational routes. They should be capable of taking road traffic into

the heart of rural areas throughout the year with only minor interruptions. District roads are

divided into two classes on the basis of traffic.

(i) Major District Roads (M.D.R) for higher order of traffic.

(ii) Other District Roads (O.D.R) for lower order of traffic.

2.2.5. Village Roads:

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Village roads are roads connecting villages or groups of villages with each other and

to the nearest district , state or national highway or railway or navigational routes. They are in

essence roads from villages to a market or to a main route.

The road systems within the Urban areas are classified as ‘Urban Roads’ and will

form a separate category of roads to be taken up by the respective urban authorities.

2.3. Long Term Road Plans:

Selient features of the three plans are presented briefly.

2.3.1. Nagpur Plan:

This is the first plan prepared on scientific principles for the development of highways

in India. At the initiative of the IRC , chief engineers of the various states met at Nagpur for

drafting a highway development plant for the period 1943-63. According to this plan the road

net work in the country was divided into five categories:

(i) National Highways (ii) State Highways (iii) Major District roads (iv) Other district roads

and (v) Village roads. Requirements of each of these categories of roads are given in article

2.2.

The target for the total length of the roads was fixed as 16 km per 100 Sq. Km of the

area covered. Based on ‘Star and Grid pattern’ of road network two sets of formulae have

been developed. One for the total length of NH , SH and MDR and another for the length of

ODR and VR. These formulae have been developed taking into consideration the

geographical , agriculture and population conditions.

2.3.2. Second Twenty Year Road Plan:

The length of roads envisaged under the Nagpur plan was found to have been

achieved by the end of the second plan i.e., by 1961 , but the road system was deficient in

may respects. The changed economic , industrial and agricultural conditions in the country

warranted a review of the country’s rapidly growing economy. Accordingly roads wing of

Government of India received the situation and drafted a perspective plan for road

development for the period 1961 - 81 , known as second twenty year road plan.

Five different formulae based on the ‘star and Grid’ pattern of roads have been

formulated for the five categories of roads proposed in Nagpur plan. The target of this plan

has been fixed as 32 km of total length of road per 100 Sq. km of area covered.

Comparison of Nagpur Road Plan and Second 20-Year Road Plan:

(i) The Nagpur plan has a target length of 16 km per 100 Sq. Km area covered where as the

second 20 year plan has double this length.

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(ii) Nagpur road plan gives two formulae. One for total length of fast category of roads ,

namely N.H , S.H and M.D.R and the second formula for finding the total length of O.D.R.

and V.R. Hence it is not possible to get the road length for each category of the roads

separately. In the second 20 - year plan it is possible to find out the length of each category of

roads as five different formulae are available for the five different categories.

(iii) The Nagpur plan formulae for the road lengths divide the area into two categories -

Agricultural area and Non-Agricultural area. In the second road plan , the area is divided into

three categories - developed and agricultural area , semi-developed area and undeveloped and

uncultivated area.

(iv) In developing formulae for the different categories of roads in the Nagpur plan , villages

and towns are divided into six groups based on population. All towns with population greater

than 5000 , are grouped together. In the second 20 year plan , villages and towns have been

divided into nine different population groups. All towns with population greater than

1,00,000 are kept in one group.

(v) In Nagpur plan the length of railway track in the area was deducted from the total length

of road required. Such a deduction was not allowed in the second 20 year formulae , as it was

realised that the highway system should develop independently.

(vi) In the second 20 year plan , a development factor of 5% only is allowed where as in the

Nagpur plan this factor is 15%.

(vii) In the second 20 year road plan , provision was made for express highways. (Highways

provided for the movement of heavy volumes of motor traffic at higher speeds and have

atleast four lanes).

(viii) In general it may be said that the second 20 - year road plan has been developed on a

more rational and scientific basis than the Nagpur road plan.

Deficiencies of the Road Plans:

The two plans are mainly centered around planning a network of roads. These plans

are hardly based on and evolved from principles of transport planning. The plans are not

correlated to transport needs and are not based on systematic transport surveys.

The plan formulae for evolving the road length indicate that greater length was

apportioned for developed areas than semi-developed and underdeveloped area. The back-

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wardness tended to be perpetuated rather than conditions of communications and accessibility

improved.

2.3.3. Third Twenty Year Road Development Plan (1981 - 2001):

Salient features of the Lunknow plan are as follows:

(i) The future road development should be based on the modified classification consisting of

primary , secondary and tertiary road systems as mentioned in Article 2.2.

(ii) The road network should be developed so as to preserve the rural oriented economy and

to develop small towns with all essential facilities. All villages with population over 500

(based on 1981 census) should be connected by all weather roads by the end of this century.

(iii) The N.H. net work should be expanded to form square grids of 100 km sides so that no

part of the country is more than 50 km away from a N.H.

(iv) The over all road density in the country should be increased to 82 km per 100 Sq km area

by the year 2001.

(v) The lengths of SH and MDR required in a state or region should be decided based on both

the area and the number of towns with population above 5,000 in the state or region.

(vi) Express ways should be constructed along major traffic corridors to provide fast travel.

(vii) All the towns and villages with population over 1500 should be connected by MDRs and

villages with population 1000 to 1500 by ODRs. There should be a road within a distance of

3.0 km in plain and 5.0 km in hilly terrain connecting all villages or groups of villages with

population less than 500.

(viii) Roads should be constructed in less industrialised areas to attract growth of industries.

(ix) Long term plans for road development should be prepared at various levels. The road

network should be scientifically decided to provide maximum utility.

(x) The existing roads should be improved by rectifying the defects in road geometrics ,

improving the riding quality of the pavement surface and strengthening of the pavement

structure to save vehicle operation cost and thus to conserve energy.

(xi) There should be improvements in environmental quality and road safety.

Determination of Road Lengths:

I. Primary System:

(i) Express ways of total length 2000 Km to be developed for fast travel based on traffic

requirements.

(ii) Total length of NH in the country or in a state in Km = Total Area of the country or state

in Sq. Km / 50.

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II Secondary System:

(i) Length of State Highways (SH) in Km.

(a) By total area: S.H , Km = Area of the State in Sq . Km / 25

(b) By total number of towns and area in the state ,

SH , Km = 62.5 x no. of towns in the state - area of state , sq . km

50

(ii) Length of MDR in a State in Km.

(a) By total area , MDR Km = Area of State in Sq. Km / 12.5

(b) By no. of towns in the State , MDR , Km = 90 x no. of towns in the state.

(III) Tertiary System or Rural Roads:

(i) Length of Rural Roads (ODR and VR) in each state

= Total length of roads in the State - Length of (NH + SH + MDR) in the state.

Note:

Total length of roads in a state = Area of the State in Sq Km

10082×

2.4. Road Patterns:

The following are the road patterns (fig 2.1) used for rural roads.

(i) Grid or Rectangular or Block pattern.

(ii) Radial pattern

(a) star and Block

(b) star and circular

(c) star and grid.

(iii) Hexagonal pattern

(iv) Minimum travel pattern.

The choice of a road pattern depends upon: layout of town showing industrial ,

agricultural and production centres , terrain and topography and choice of the planner.

2.4.1. Grid pattern:

In this type (fig 2.1A) roads are perpendicular to each other. It is easy to set out this

pattern and is suitable for flat countries without any predominant natural features. This

system has been adopted in the city roads of Chandigarh , This pattern produces

monotonously long sets flanked by dull blocks of buildings. It encourages an even spread of

traffic over the entire grid. It is easy for the through traffic to bypass a definite control area in

the middle of the grid. This pattern is not quite convenient from traffic operation point of

view.

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2.4.2. Radial pattern:

Fig 2.1 B , 2.1 C and 2.1 D show the various systems of radial road patterns.

In this system a number of roads radiate from central core known as focal point. This

system of roads lead to congestion of centre (fig 2.1B)

The ring roads (figs 2.1C and 2.1D) are circumferential highways to permit traffic to

avoid centre of town. The location , number and design of ring roads depend upon the

population of the town , size , layout and usage of central area.

The inner ring road deflects traffic which has no need to traverse the central area and

the outer ring road is used by through traffic of the town as distribution between radials. The

outer ring roads are located within the outer fringe of present and future development.

The Connaught place in New-Delhi has radial and circular pattern of road network.

The Nagpur Road Plan (1943 - 63) and the II Road Plan (1962 - 81) were formulated on the

basis of star and grid pattern.

The advantages of this pattern are not much because (i) Towns are not circular (ii) It

is not possible to join a ring route at any point and (iii) the relative advantages of routes are

different.

2.4.3. Hexagonal Pattern:

In this pattern the roads are arranged to form a hexagonal shape (fig 2.1 E). Each

system has one of its road common with another system of hexagon.

2.5. MASTER PLAN:

Master plan is the final road development plan for the area under study - a city or a

district or a state or a country. It is an ideal plan showing the full development of the area at a

future data. It serves as a guide to the planner to improve some of the existing roads and plan

net work of new roads. Master plan of an area helps in controlling the industrial picture of the

fully developed area in a planned and scientific manner.

The various stages in preparation of a master plan are

(a) Data collection: This include data regarding existing land use , population , industrial and

agricultural growth , traffic flow , topography and future trends.

(b) Preparation of draft plan based on future trends and invite suggestion from public and

experts.

(c) Revision of draft plan in the light of discussions and comment from public and experts ,

and

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(d) Compare the various alternate proposals of road system and determine the sequence in

which the master plan will be implemented.

In India , in the various development plans considered , targets for the lengths of

roads have been fixed based on population , area , number of towns , agricultural and

industrial growth. Similar system may be adopted in preparing master plan also.

2.6. Summary:

For the development of roads in our country , three long term plans have been drawn.

The first plan , popularly known as Nagpur plan , drawn for the period 1943 - 63 may be

considered as a land mark in the development of roads in our country.

Depending on the development of the area and population served , roads have been

divided into five groups - National Highways , State Highways , Major District Roads , Other

District Roads and Village Roads. A target of 16 Km of road for every 100 Sq.Km of area

covered was fixed for this plan period. As the targets of Nagpur plan could be achieved by

1961 , a second road development plan for the period 1961 to 1981 , also known as Bombay

plan , was formulated. The targets of this plan were double that of the Nagpur plan. In both of

these plans star and gird pattern of roads has been used for developing the formulae for the

different categories of roads proposed in the Nagpur Plan.

Both the Nagpur plan and Bombay plan have been hardly based on and evolved from

principles of transportation planning. The plan formulae for evolving the road length indicate

that greater length was apportioned for developed areas than for semi-developed or under

developed areas. The backwardness tended to be perpetuated rather than conditions of

communications and accessibility improved. Taking care of these short coming and keeping

in view the growth pattern envisaged in the various fields by the turn of the century , a third

road development plan for the period 1981-2001 has been prepared. Attention was paid while

drafting the plant to present the quality of environment , to improve road safety and to

conserve energy.

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Fig 2.1 ROAD PATTERNS

Roads have been classified into three groups - primary system consisting of the Express ways

and National Highways , Secondary System comprising of State Highways and Major

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District Roads and Tertiary system or Rural roads which include other district roads and

village roads for the purpose of the Lucknow plan.

Road patterns adopted in rural roads have also been discussed in this unit. Data to be

collected and the various steps in the preparation of Master Plans for highway development

have also been dealt with in detail.


1. Compare the I and II - 20 year Road development plans clearly bring out the deficiencies

in these two plans.

2. Discuss important aspects of Lucknow plan indicating how the total lengths of different

categories of roads can be calculated ?

3. Write notes on

(a) Rural Road Patterns

(b) Master plans for Roads

(c) Classification of Roads.

2.8. BOOKS FOR REFERENCE:

1. Bindra , S.P. (1977) - A course of Highway Engineering.

Dhanpath Rai & Sons , New Delhi.

2. Kadiyali L.R. (1984) - Principles and practice of Highway Engineering -

Khanna Tech. Publications , New Delhi.

3. Khanna , Dr. S.K., and Justo , Dr. CEG - Highway Engineering ,

New Chand and Bros., Roorkee.

***


UNIT - 3

HIGH WAY GEOMETRIC DESIGN - 1 CONTENTS:

AIMS / OBJECTIVES:

3.1. Introduction

3.2. Pavement Surface Characteristics

3.3. Width of Pavement or Carriage way

3.4. Right of way

3.5. Camber

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3.6. Sight Distance - Stopping Sight -Distance

3.7. Sight Distance - Over Taking Sight -Distance

3.8. Sight Distance at Intersections.

3.9. Sight Distances - Miscellaneous

3.10. Criteria for Sight Distance

3.11 Factors Effecting Horizontal Alignment Design

3.12. Summary


3.14. Books of Reference.

AIMS / OBJECTIVES:

Various geometric features of a highway have to be designed on scientific principles

so as to provide maximum efficiency to traffic operations with safety , comfort and economy.

The geometric features discussed in this unit are Cross-sectional elements , sight - distance

requirements , and factors that affect the control and design of horizontal alignment.

3.1. INTRODUCTION:

Geometric design of a highways deals with dimensions of various highway features

such as alignment , slopes , widths , sight distances , gradients etc. In the early phase of road

development , greater emphasis was used to be laid on structural design of road way rather

than on the geometric design. With the increase in number and speeds of motor vehicles

emphasis has been shifted to the geometric design. The geometric layout should be so

designed as to provide maximum efficiency to traffic operations with safety , comfort and

economy.

It is possible to design and construct the pavement of a road in stages ; but it is very

expensive and rather difficult to improve the geometric elements of a road in stages at a later

time. Therefore , it is important to plan and design the geometric features of the road during

the initial alignment itself taking into consideration the full growth of traffic and the

possibility of the road being upgraded to a higher category.

Geometric design of a highway , in general , deals with the following elements.

(i) Cross - sectional elements

(ii) Sight -Distance Considerations

(iii) Horizontal and Vertical alignment details

(iv) Intersection elements.

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In this unit , design aspects of the cross-sectional elements , sight distance and factors

affecting the horizontal alignment are considered.

Highway geometrics depend upon the topography , locality and type and intensity of

traffic for which the road is intended. Comprehensive design standards for roads have been

evolved after considerable thought by the Indian Roads Congress (IRC) and the Roads Wing

of Ministry of Transport (MOT).

3.2. PAVEMENT SURFACE CHARACTERISTICS:

The important surface characteristics of the pavement are friction , roughness , light

reflecting characteristics and drainage of surface water.

3.2.1. Friction:

The friction between the vehicle tyre and the pavement surface is a very important

factor. It affects the operating speed , distance requirements in stopping and accelerating

vehicles , and the force resisting centrifugal force while a vehicle negotiates a curve.

Important factors affecting the frictional resistance of the pavement surface are (i) The type

of road surface , (ii) condition of the pavement (iii) the type and condition of the tyre (iv) the

speed of vehicle (v) the extent of brake application and (vi) the load , and the tyre pressure.

Longitudinal friction comes into play when brakes are applied for stopping a vehicle

and it depends on the speed of the vehicle and the surface conditions of the pavement. I.R.C.

recommends the longitudinal friction coefficient values of 0.35 to 0.40 depending on the

speed. In the case of horizontal curve design , IRC recommended lateral coefficient of

friction of 0.15. This low value has been suggested for the worst possible surface condition

such as mud on pavement surface on horizontal curve with super -elevation (as it is essential

to prevent lateral skid).

3.2.2. Pavement Unevenness:

Pavement unevenness increases fuel consumption and operating cost of the vehicles ,

reduces the speed , safety and comfort of travel. Uneven surfaces increase fatigue and

accidents. As such pavement surfaces should be maintained as even as possible.

The pavement surface condition is measured by “UNEVENNESS INDEX’ , which is

the cumulative vertical undulations of the road. It has been found from tests that it is

desirable to keep the unevenness index low and preferably less than 150 cm/km , for good

road surfaces of high speed highways. Values of more than 350 cm/km is considered very

uncomfortable even at a speed of 50 Kmph.

3.2.3. Light Reflecting Characteristics:

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Night visibility very much depends upon the light reflecting characteristics of the

pavement. Light reflecting characteristics of the pavements are colour and surface condition

(dry or wet) of the pavement. The glare caused by the reflection of head lights is considerably

more on wet pavement surface than on a dry pavement. Though light coloured or white

pavement surface give good visibility at night , they produce glare and eye strain during

bright sunlight. Black top pavement surface on the otherhand provides very poor visibility at

nights , especially when the surface is wet.

3.3. WIDTH OF PAVEMENT OR CARRIAGE - WAY:

The pavement or carriage way width depends on the width of a traffic lane and

number of lanes. The carriage way width intended for one line of traffic movement may be

called ‘traffic lane width’. This lane width is determined on the basis of the width of the

vehicle and the minimum clearance to be provided for safety. When the side clearance is

increased (upto a certain limit) , there is an increase in the operating speed of the vehicle and

hence an increase in the capacity of a traffic lane. Keeping all these in view , a width of

3.75m is considered desirable for a road having single lane for vehicles of 2.50m width

(width of design vehicle). For pavements having two or more lanes , width of 3.5m per lane is

considered.

In the case of a single lane carriage way of width 3.75m a side clearance of 0.625m

would be obtained as shown in Fig 3.1 (A) and in the case of two lane pavement of width 7m,

a minimum clearance between two lanes of traffic would be 1.00m for standard vehicle as

shown in Fig 3.1 (B).

Fig 3.1. WIDTH OF PAVEMENTS

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The number of lanes required in a highway depends on the predicted traffic volume

and the traffic capacity of one lane. In developing countries like India , the traffic is of mixed

nature consisting of both slow and fast moving vehicles between which there are wide

variations in speed. In such cases , it is common practice to express the traffic capacity in

terms of PASSENGER CAR UNITS (PCUS’).

The weightage values for various classes of vehicles as recommended by the MOT

are given in Table 3.1. Factors affecting PCU are (i) average speed of the vehicle class ,

under the prevailing roadway and traffic conditions , (ii) average width and length of the

vehicle class , and (iii) average transverse gap and longitudinal gap between vehicles of the

same class in the speed range under consideration of a compact stream flow.

TABLE 3.1. PASSENGER CAR UNITS (PCU):

Sl.No. Type of vehicle PCU

(1) Bicycle 0.5

(2) Motor cycle , scooters 0.75

(3) Light Commercial Vehicles 1.0

(4) Automobiles 1.0

(5) Cycle Rickshaws 1.5

(6) Trucks and Busses 3.0

(7) Animal drawn Vehicles 4.0 to 8.0

Technical group of the Ministry of Transport recommended the following criteria for

designing carriage way width used on highway capacity expressed in passenger car units.

(Table 3.2).

TABLE 3.2

Sl.No. Road Type Recommended Capacity

PCUS’ / day

1. Single lane road with satisfactory earth

shoulders

Upto 1000

2. Single lane road with 1.0m wide all

weather shoulders on either side

Over 1000 but less than 2500

3. Two lanes under ideal conditions with

eathen shoulders

Over 2500 but less than 10,000

4. Four lanes , divided highway (depending

on traffic , access control etc.)

20,000 to 30,000

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The traffic at a future data (end of design period) in PCU / day for design purposes is

generally computed by using the following formula:

A = P (1 + r)n + m 3.1

The width of carriage way for various classes of roads as standardised by the IRC are

given in Table 3.3.

Table 3.1. WIDTH OF CARRIAGE WAYS FOR VARIOUS CLASSES OF ROADS:

Sl.No

.

Class of Road Width of carriage-way in metres

1. Single lane 3.75 for all roads ; may be decreased

to 3.0m for village roads.

2. Two lane without raised kerb 7.0

3. Two lane with raised kerb 7.5

4. Intermediate carriageway (Except on

important roads)

5.5

5. Multiple lane pavements 3.5 / lane

Where A = Design number of PCU / day

P = Existing number of PCU / day

r = Annual rate of increase in traffic (taken as 7.5% in the absence of any data)

n = Number of years between last census and the year of construction or improvement

and m = design period in years.

The value of P in formula 3.1 , should be seven day average of heavy vehicles found

from 24 hours counts.

3.3.1. Traffic Separators or Medians:

In some highways traffic separators and medians are provided between two sets of

traffic lanes intended for traffic moving in opposite directions. The main function of the

traffic separators is to prevent head - on collision between vehicles moving in opposite

directions in adjacent lanes. In such highways the road width depends on the pavement width

(or lane width and number of lanes) and the width of traffic separators. Apart from preventing

head-on collision of vehicles , separators may also help to

(i) Channelise the traffic into streams at intersections.

(ii) Shadow the crossing and turning traffic , and

(iii) Segregation of slow traffic and to protect pedestrians.

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The traffic separators may be in the form of pavement markings , physical dividers ,

or area separators. Pavement markings are the simplest of these. The mechanical separators

should be designed in such a manner that even if the wheels of a vehicle encroach , no part of

the vehicle body should be damaged.

Area separators may be medians , dividing islands or parkway strips dividing the two

directions of traffic flow , (fig 3.2). It is desirable to have a wide area separators of 8 to 14m

width. A minimum of 6m is required to reduce head light glare. The glare can be reduced in

narrow strips by planting trees or shrubs.

For medians desirable minimum width of 5.0m on a rural highway may be reduced to

3.0m where land is restricted. On long bridges the width of the median may be reduced upto

1.2 to 1.5m. The medians should preferably be of uniform width throughout. On urban

highways with 6 lanes or more , medians should invariably be provided ; absolute minimum

width being 1.2m and desirable minimum being 5.0m.

3.3.2. Kerbs:

Kerbs (curbs) indicate the boundary between the pavement and shoulder or sometimes

islands or footpaths or parking space. These are classified as ‘barrier’ and mountable kerbs.

Barrier kerbs are designed to discourage ,

Fig 3.2. KERBS AND TRAFFIC SEPARATOR

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Fig 3.3. TYPES OF KERBS

vehicles leaving the pavement. The face may be vertical or sloping and the height may range

from 15-25 cm. (Fig 3.3 a). A smaller height may be adopted for pedestrian or refuge islands.

Mountable kerbs are those which can be easily crossed by vehicles if required (fig 3.3 B).

They are used at medians and channelising islands.

In rural areas submerged kerbs are provided at pavement edges between the pavement

edge and shoulders of rural roads. These kerbs provide lateral support for the granular base

course of flexible pavements.

3.3.3. Shoulders:

The shoulder is that portion of the roadway contiguous with carriageway and is

intended for accommodation of stopped vehicles , emergency use and lateral support of base

and surface courses. The width of the shoulder should be adequate to accommodate stationary

vehicles fairly away from the edge of the adjacent lane. The shoulder should have sufficient

load carrying capacity to support a loaded truck even in wet weather.

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Fig 3.4. CROSS SECTION DETAILS

Cross sectional details of a highway are shown in fig 3.4. Width of formation or

roadway is the sum of widths of pavement or carriage-way including separators if any , and

the shoulders. Formation width is the top width of highway embankment or the bottom width

of cutting excluding the side drains as shown in fig 3.4. The widths of road way as

standardised by the IRC are given in Table 3.4.

3.4.1. Right of Way:

The right of way is the area of land acquired for the road along its alignment. The

width of this acquired land , is known as ‘Land -width’ or ‘Right of Way Width’ and depends

on the importance of the road and possible future development. The recommended land width

for different categories of roads as per the IRC are presented in Table 3.5.

NOTES: (i) In multilane highways , roadway width should be adequate for the requisite

number of traffic lanes besides shoulders and central median.

(ii) The minimum roadway width on single lane bridge is 4.25m. While acquiring

land for a highway it is desirable to acquire more width of land as it may be difficult to

acquire land at a future date for widening or other improvements.

In order to prevent ribbon development along the highways , it is sometimes

necessary to establish ‘Building lines’ and ‘control lines’ with the following definitions.

Table 3.4. Width of Roadway of Various Classes of Roads:

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Roadway width , m:

Sl.No. Road classification Plain and Rolling

terrain

Mountainous and

Steep terrain

1. National and State Highways

(a) Single lane

(b) Two lanes

12.0

12.0

6.25

8.80

2. Major district road

(a) Single lane

(b) Two lanes

9.0

9.0

4.75

----

3. Other District Roads

(a) Single

(b) Two lanes

7.5

9.0

4.75

----

4. Village roads - single lane 7.5 4.00

Tables 3.5 RECOMMENDED LAND-WIDTH FOR DIFFERENT CLASSES OF

RURAL ROADS (METRES):

Plain and rolling terrain Mountainous and steep

Terrain

Sl.

No.

Road

Classification

Open areas Built-up ares Open areas Built-up

areas

Normal Range Normal Range Normal Normal

1. National and

State

Highways

45 30-60 30 30-60 24 20

2. Major distric

Roads

25 25-30 20 15-25 18 15

3. Other district

Roads

15 15-25 15 15-20 15 12

4. Village Roads 12 12-18 10 10-15 9 9

Control line is a line which represents the nearest limits of future uncontrolled

building activity in relation to a road. This signifies that though building activity is not totally

banned between building line and control line , the nature of buildings permitted here are

controlled.

Current standards for building and control lines as per IRC are presented in Table 3.6.

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Table 3.6. RECOMMENDED STANDARDS FOR BUILDING LINES AND

CONTROL LINES:

Plain and rolling terrain Mountainous and steep

terrain

Road

classification

Open areas Built-up areas Distance between building

line and road boundary (set

back) , m

Overall

width

between

building

lines , m

Overall

width

between

control

lines , m

Distance between

building line and

round boundary

(set-back) , m

Open areas Built-up

areas

N.H and S.H. 80 150 3 to 6 3 to 5 3 to 5

M.D.R 50 100 3 to 5 3 to 5 3 to 5

O.D.R. 25/30* 35 3 to 5 3 to 5 3 to 5

V.R. 25 30 3 to 5 3 to 5 3 to 5

Note: *If the land width is equal to the width between building lines indicated in this column

, the building lines should be set back 2.5m from the road land boundary.

3.5. CAMBER OR CROSS SLOPE:

Camber or cross - slope is the slope provided to the road surface in the transverse

direction to drain off rain water from the road surface. Usually camber is provided on straight

roads by raising the centre of the carriage way with respect to the edges forming a CROWN ,

or highest point on the centre line. At horizontal curves , the surface drainage is affected by

raising the outer edge of the pavement with respect to the inner edge while providing the

desired superelevation.

The rate of camber is usually designated by 1 in n which measures the transverse

slope in the ratio 1 vertical to n horizontal. Camber is also expressed as a percentage. If a

camber is Y% , the cross slope is Y in 100.

The rate of camber depends on:

(i) The type of pavement surface and (ii) the amount of rainfall.

The minimum camber needed to drain off surface water may be provided keeping in

view the type of pavement surface and the amount of rainfall in the locality. The values of

camber recommended by the I.R.C. are given in Table 3.7. A range of values are given with a

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view that in localities with lesser rainfall , a flatter cross slope may be adopted and in places

of higher rainfall a steeper camber may be adopted.

Table 3.7. RECOMMENDED VALUES OF CAMBER (IRC):

Sl. No. Type of road surface Range of camber in areas of rainfall range

Heavy to light

1. Concrete and high type

bituminous surface

1 in 50 (2%) to 1 in 60 (1.7%)

2. Thin bituminous surface 1 in 40 (2.5%) to 1 in 50 (2%)

3. Water bound macadam and

gravel pavement

1 in 33 (3%) to 1 in 40 (2.5%)

4. Earth 1 in 25 (4%) to 1 in 33 (3%)

The cross slope for shoulders should be 0.5% steeper than the cross slope of adjoining

pavement , subjected to a minimum of 3 percent: maximum value of 5.0% for earth

shoulders.

3.5.1. SHAPES OF CAMBER:

The camber is given a parabolic , elliptic or straight line shape. In parabolic or elliptic

shape the profile is flat at the middle and steeper towards the edges which is preferred by fast

moving vehicles as they have to cross the crown line during overtaking operations frequently.

When very flat slope is to be provided as in cement concrete pavements , straight line shape

of camber may be provided as shown in Fig 3.5. Some times a combined camber with

parabolic central portion and straight line at the edges (Fig 3.5) is preferred when animal

drawn vehicular traffic with steel tyres is heavy.

3.5.2. Camber Boards:

For providing the desired amount and shape of camber , camber boards or templates

are prepared with the specified camber. These are used to check the lateral profile of the

finished pavement during construction.

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Fig 3.5 SHAPES OF CAMBER

WORKED OUT EXAMPLE:

3.1. Give dimensions of the camber board for two lane bituminous surface to be constructed

in an area of heavy rainfall.

Solution: Since it is a two lane highway , width = 7m = 700cm

parabolic camber is proposed.

In an area of heavy rainfall , camber is 1 in 40 (Table 3.7) Height of pavement at the

centre over edges = 7002

140

8 75× = . cm.

Equation for parabolic shape of camber is

Y = 2x2 / nw 3.2

where y is the ordinate at a distance

x from the crown of the pavement.

(Various terms are as shown in Figure).

For various values of x , y is calculated from the above formula.

x in cm y in cm

50 0.1785

100 0.714

150 1.607

200 2.857

250 4.464

300 6.428

350 8.750

These values are plotted as shown in figure of camber board.

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Fig Camber Board

3.6. SIGHT DISTANCE:

Sight distance is the length of the highway ahead visible to the driver. It is the element

, which has the most important influence on highway safety and efficiency in operation. A

knowledge of the sight distance requirement is needed in the design of straight lengths ,

intersections as well as on the horizontal and vertical curves.

Sight distance available from a point is the actual distance along the road surface ,

which a driver from a specified height (1.20 m) above the carriage way has the visibility of a

stationary (0.15 m height) or moving object (1.20 m height). In other words , it is the length

of the road visible ahead to the driver at any instance. Restrictions to sight distance may be

caused at horizontal curves by objects obstructing vision at the inner side of the road or at

vertical summit curves or at intersections.

Three sight distance situations are considered in the design.

(i) Stopping or absolute minimum sight distance.

(ii) Safe overtaking or passing sight distance , and

(iii) Safe sight distance for entering into uncontrolled intersection.

3.6.1. Analysis of stopping sight distance:

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Stopping sight distance is the distance required by the driver of a vehicle travelling at

a given speed to bring the vehicle to stop after an object on the roadway becomes visible.

Because of the importance to safety all highways must be designed to provide minimum

stopping sight distance throughout their length. This is also called sometimes ‘non-passing

sight distance’.

The stopping distance is the sum of (i) the distance travelled by the vehicle during the

total reaction time known as LAG DISTANCE and (ii) the distance travelled by the vehicle

after the application of brakes , before coming to a dead stop position , known as the

‘BRAKING DISTANCE’.

(i) Lag Distance:

The total reaction time of the driver is the time taken from the instance the object is

visible to the instant the brakes are effectively applied. The amount of time gap , total

Reaction time , depends on several factors.

Some traffic engineers have split the total reaction time into four parts based on

‘PIEV’ theory. According to this theory the total reaction time of the driver is split into the

following four parts.

(a) Perception time , is the time required to perceive an object or situation.

(b) Intellection time , is the time required for comparing the different thoughts , regrouping

and registering new situation.

(c) Emotion time is the time required during emotional sensation and disturbance , and

(d) Volition time is the time required for the final action.

It is possible that the driver may apply the brakes or take any avoiding action by the

reflexive action even without thinking. This is shown in Fig 3.6. Which illustrates the PIEV

process.

P - Perception

I - Intellection

E - Emotion

V - Volition.

The total reaction time depends upon the physical characteristics of the driver ,

psychological factors , environmental conditions , purpose of trip and speed of the vehicle.

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Fig 3.6. Reaction Time and PIEV process

If ‘v’ is the design speed in m / sec and ‘t’ the total reaction time of the driver in

seconds , then

Lag distance (metres) = vt 3.3a

If V is the design speed in kmph , then the lag distance , metres

= V × 100060 × 60

× t = 0.278 Vt 3.3b

A total reaction time of 2.5 seconds is recommended. As such under most situations ,

the lag distance , (metres)

= 2.5 v = 0.278 x 2.5 x V

(ii) Braking Distance:

Braking distance is obtained by equating the work done in stopping the vehicle to the

kinetic energy of motion.

Case (a):

Let the vehicle move on an ascending gradient of +n% . The forces acting against

motion and helping to stop the vehicle are

(i) Frictional force F acting down the gradient.

(ii) The component of gravity of the vehicle W , acting parallel to the surface and acting

downwards and equal to W sin α = W tan α = Wn / 100.

Figure

The frictional force F = f.W. Where f is the coefficient of friction (longitudinal).

CE409/16 35

If l is the braking distance then the work done in stopping the vehicle = F. l +W n100

l

= (f + n / 100) Wl.

The kinetic energy at the design speed v , m / sec will be = 12

mv2 =12

(W/g) v2 (g

being the acceleration due to gravity).

Equating the work done in stopping the vehicle to the kinetic energy.

(f + n / 100) Wl =12

(W / g) v2

or braking distance l (metres) = v2 / 2g (f + n / 100) 3.4a

Case (b):

In a descending gradient of -n% , the braking distance ‘l’ increases as the component

of gravity now opposes the braking force. Hence , the braking distance may be obtained from

the equation.

l = v2 / 2g (f - n / 100) 3.4b

Case (c):

On a level surface n = 0 , then the braking distance may be obtained from the equation

l = v2 / 2gf 3.4c

In general , the expression for braking distance may be written as

l =v 2

2g f ± n / 100( )=

v 2

2g f ± 0.01 n( )3.4d

Stopping distance = Lag distance + Braking distance

S.D. (m) = vt + v 2

2g f ± 0.01 n( )3.5a

Since the total reaction time is taken as 2.5 seconds and V is the design speed in Kmph ,

equation , 3.5a , may be written as

S.D (metres) = 0.278 Vt + 0.278 V( )2

2 × 9.81 f ± 0.01 n( )(Since g = 9.81 m / sec).

= 0.278 Vt + V 2

254 f ± 0.01 n(( ) 3.5b

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Throughout the above analysis , it was assumed that the brakes are quite effective ,

i.e., of 100% efficiency. If the efficiency of the brakes is only n% , then

S.D (metres) = 0.278 Vt + V 2

254 f ± .01 n( )× .01 n3.5c

The coefficient of friction ‘f’ depends upon a number of factors like the type and

condition of the pavement surface , tyres and speed of the vehicle The IRC recommended the

following values of f based on the speed of vehicles.

Table 3.8. Recommended values of Coefficient of Friction:

Speed in Kmph 20-30 40 50 60 65 80 100

Long. coefficient of friction f. 0.40 0.38 0.37 0.36 0.36 0.35 0.35

The minimum sight distance should be equal to the stopping distance in one way

traffic lanes and also in two - way traffic roads when there are more than one lane of traffic.

In roads with restricted width , in single lane roads , when two-way movement of traffic is

permitted , the minimum sight distance should be equal to TWICE the stopping distance to

enable both the vehicles coming from opposite directions to stop. The Stopping Distance

should invariably be provided throughout the length of the road and hence this is also known

as Absolute Minimum Sight Distance.

Worked Example:

3.2. Calculate the minimum stopping sight -distance on a highway at a descending gradient of

6%. Design speed may be taken as 80 kmph. If the road is single lane one , with two way

traffic , what is the sight distance to be provided ?

Solution:

Total reaction time may be taken as 2.5 sec.

For the speed of 80 kmph , f = 0.35 (from Table 3.8). Minimum stopping sight

distance on a descending gradient is

= 0.278 Vt + V 2

254 f − 0.01 n( )

= 0.278 × 80 × 2.5 +802

254 0.35 − 0.01 × 6( ) = 142.5m.

As there is two way traffic in a single lane , minimum sight distance = 2 (stopping

sight distance).

= 2 × 142.5 = 285 m.

CE409/16 37

3.7. OVERTAKING SIGHT DISTANCE:

It is the minimum distance open to the view of the driver of a vehicle intending to

overtake a slow vehicle ahead with safety against the traffic in the opposite direction. The

overtaking sight distance is measured along the centre line of the road which a driver with his

eye level 1.2m above the road surface.

Important factors on which the minimum overtaking sight distance or the safe passing

sight distance depends are:

(i) Speeds of the overtaking , overtaken and the vehicle coming from the opposite direction.

(ii) Spacing between the vehicles.

(iii) Skill and reaction time of the driver , and the

(iv) Slope of the road.

3.7.1. Analysis of Overtaking Sight Distance:

Figure 3.7 shows the elements that go to make up the overtaking sight distance. In Fig

3.7 A is the ‘overtaking vehicle’ travelling at the design speed and B is the ‘overtaken

vehicle’ moving slowly on a two lane road. The vehicle C is the ‘on coming vehicle’ coming

in the opposite direction at design speed.

Fig 3.7 Elements of taking manoeuvre

The overtaking distance may be divided into three parts:

(i) d1 - the distance travelled by the overtaking vehicle A. during the reaction time ‘t’ from

position A1 to A2 .

(ii) d2 - the distance travelled by vehicle , A from A2 to A3 during the actual overtaking

operation.

(iii) d3 is the distance travelled by the on-coming vehicle C , from C1 to Cz during the

overtaking operation of A.

The vehicles A and C are travelling at a design speed of V kmph or v m / sec.

CE409/16 38

The overtaken vehicle B is moving slowly at a speed of Vb kmph or vb m / sec.

In a two-way lane road , the opportunity to overtake depends on the frequency of

vehicles coming from the opposite direction and overtaking sight distance available at any

instant.

The overtaking phenomenon may be assumed as follows:

(i) When it is decided to overtake , the overtaking vehicle A , reduces its speed to the speed

of the slow moving vehicle B and moves behind it during the reaction time t , till there is

opportunity for safe overtaking.

This distance d1 , shown in fig 3.7 = vb x t (metres) where t is the reaction time of the

driver in seconds.

This reaction time is taken as 2 seconds as an average value.

Then d1 = vb x t = vb x2 metres 3.6

(ii) From position A2 , the vehicle A starts accelerating , shifts to the adjoining lane ,

overtakes the vehicle B , and shifts back to its original lane ahead of B in position A3 . The

distance between the positions A2 and A3 is taken as d2 .

From the geometry of the figure d2 = b + 2s where s is the spacing to be maintained

between the vehicles and is given by the formula.

s = (0.7 vb + 6) metres 3.7

Let the time taken by vehicle A to overtake vehicle B be T sec. During this time the

vehicle A moves from A2 to A3 over a distance of d2 and the vehicle B , moves from B1 to B2

over a distance b.

Then b = vb . T (metres) since vb is the speed of vehicle B.

Thus d2 = vb T + 2s (from the geometry , fig 3.7) 3.8.

The vehicle A , travelling with an initial velocity of vb , accelerated at ‘a’ metres / sec

, travels a distance of d2 in time T seconds , then

d2 = vb T + 1 / 2 (aT2) 3.9

Equating equations 3.8 and 3.9.

d2 = vb T + 2S = vbT +12

aT2

or Tsec = 4s / a 3.10

where s = (0.7 vb + b) from equation 3.7.

Hence d2 = vbT + 2s 3.10.

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(iii) During the reaction Time T seconds , the on coming vehicle travelling with a speed of v

m / sec covers a distance d3 given by

d3 = v T metres 3.11

The overtaking distance is the sum of d1 , d2 and d3 and hence

O.S.D. (metres) = d1 + d2 + d3

= vbt + (vbT + 2s) + vT

= Vb x2 + (vbT + 2s) + vT 3.12a

= 2 x 0.278 VB + 0.278 VbT + 2S + 0.278 VT 3.12b

where Tsec = 14.4S / AS = (0.2 Vb + 6) metres - spacing between the vehicles

A = Acceleration in kmph / sec.

Table 3.9 may be used for finding max. acceleration of vehicles at different speeds.

Table 3.9 Maximum Overtaking Accelerations:

Speed Max. Overtaking Acceleration

V Kmph v m / sec. A Kmph / sec. a m / sec2

25 6.93 5.00 1.41

30 8.34 4.80 1.30

40 11.10 4.45 1.24

50 13.80 4.00 1.11

60 18.00 3.28 0.92

80 22.20 2.56 0.72

100 27.80 1.92 0.53

In case the speed of the overtaken vehicle is not given , Vb may be assumed as (V -

16) kmph or vb = (v - 4.5) m / sec.

At overtaking sections the minimum overtaking sight distance should be (d1 + d2 + d3)

when two way traffic exists. On divided highways , the overtaking sight distance need be

only (d1 + d2) as in one way movements as no vehicle is expected to come from the opposite

direction.

The IRC suggests that , on divided highways with four or more lanes , it is not

necessary to provide usual overtaking sight distance ; however the sight distance in such

highways should be more than the stopping sight distance.

3.7.2. Effect of Grades on Overtaking Sight Distance:

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Appreciable grades in the road , both ascending as well as descending , increase the

sight distance required for safe overtaking. On up grades , the acceleration of the overtaking

vehicle will be less and hence the passing will be difficult , but the overtaken vehicle may

also decelerate in steep ascending gradients and compensate to some extent the passing sight

distance requirement.

In down grades though it is easier for the overtaking vehicles to accelerate and pass ;

the overtaken vehicle may also accelerate and cover greater distance ‘b’ during the overtaking

time.

Therefore , the overtaking sight distance , at both ascending and descending grades

are taken as equal to that at level stretch. However , at grades , it is desirable to provide

overtaking sight distance more than that required at level.

3.7.3. Overtaking Zones:

It is desirable to construct highways such that the length of the road visible ahead at

every point is sufficient for safe overtaking. This is seldom practicable and there may be

stretches where the safe over taking distance cannot be provided. In such zones where

overtaking is not possible , sign posts should be installed indicating “Overtaking Prohibited”

before such restricted zones start. But the overtaking opportunity for vehicles moving at

design speed should be given at frequent intervals. These zones which are meant for

overtaking are called “Overtaking Zones”.

Fig 3.8 shows an overtaking zone. The minimum length of the zone is to be 3 times

the overtaking sight distance. It is desirable to keep the length of the zone as five times the

overtaking sight distance. At these sections , there should be sufficient pavement width for

safe overtaking operations. The beginning and end of these zones should be indicated well

ahead by installing suitable sign boards.

CE409/16 41

Fig 3.8 Overtaking Zone

WORKED EXAMPLE:

3.3. Calculate the passing sight distance for a two way traffic high way for which the design

speed is 60 kmph. The rate of acceleration of the fast moving vehicle may be assumed as 3.6

kmph/sec and the speed of the overtaken vehicle may be assumed as 40 kmph. What will be

the passing sight distance if only oneway traffic is allowed ? Also find out the length of the

overtaking zone and show the position of sign posts in overtaking zones.

Solution:

Given VA = 60 kmph Vb = 40 kmph ; A = 3.6 kmph / sec.

Then spacing between the vehicles = s = 0.2 Vb + 6. = 0.2 × 40 + 6 = 14m.

Overtaking time , T sec = 14.4S / A = 14.4 ×14 /3.6 = 7.48 sec.

Distance covered during the reaction time , d1 = 0.28 Vbt = 0.28 × 40 × 2 = 22.4 m.

Distance covered during the overtaking period = d2 = b + 2S = 0.28 Vb T + 2S

= 0.28 × 40 × 7.48 + 2 × 14 = 111.76 m.

Distance travelled by the oncoming vehicle during T sec.

= 0.28 VT = 0.28 × 60 × 7.48 = 125.67.

Case 1: Over taking sight distance in case of two way Traffic road

= d1 + d2 + d3 = 22.40 + 111.76 + 126.67 = 259.83 or 260m.

Case 2: When there is only one way traffic , OSD =

= d1 + d2 = 22.4 + 111.76 = 134.16 or say 135m.

Case 3: Length of overtaking zone - minimum = 3 × 260 = 780m

Length of overtaking zone - desirable = 5 × 260 = 1300m.

CE409/16 42

Sign posts SP1 and SP2 should be kept 260m ahead of the beginning and end of the

zone. (Refer to Fig 3.8).

3.8. SIGHT DISTANCE AT INTERSECTIONS:

In planning highway intersections , it is essential to provide sufficient corner sight

distance to allow an approaching vehicle to have an unobstructed view of the entire

intersection. The sight line is obstructed by the presence of a structure or other objects at the

corner of the intersection. Sufficient right of way should be acquired at intersections to allow

unobstructed line of sight. The driver should be able to see the vehicle approaching the

intersection from the cross road in sufficient time before , to provide for total reaction time

and change the speed of the vehicle.

The design of sight distance at intersections may be based on three possible

conditions:

Fig 3.9. SIGHT DISTANCE AT INTERSECTION

(i) Enabling the approaching vehicle to change the speed to avoid conflict at the point C (in

fig 3.9). Hence the two sides AC and BC of the sight triangle should be equal to at least the

distance covered by a vehicle travelling at the design speed in two seconds. But this sight

distance being too less , should be increased in all possible cases.

(ii) Enabling the approaching vehicle to stop. In this case the two sides of the triangle , AC

and BC should be equal to the stopping distance , and

(iii) Enabling the stopped vehicle to cross a main road.

From safety considerations , the sight distance at uncontrolled intersections should

fulfill the above three conditions. The higher of the three values is taken as the sight distance

at uncontrolled intersections. At rotaries , the sight distance should be atleast equal to the safe

CE409/16 43

stopping sight distance. At signalled intersections the above three requirements are not

applicable.

3.9. SIGHT DISTANCE - MISCELLANEOUS:

The following criteria are also taken in fixing up sight distance.

3.9.1. Intermediate Sight Distance:

This is defined as twice the stopping sight distance. When overtaking sight distance

cannot be provided , intermediate sight distance is provided to give limited overtaking

opportunities to the fast moving vehicles. This is also some times called ‘Meeting Sight

Distance’.

3.9.2. Head - Light Sight Distance:

This is the distance visible to a driver during night driving under the illumination of

the vehicle head light. This is used for design of valley curves.

3.10. CRITERIA FOR SIGHT DISTANCE REQUIREMENTS:

Throughout the length of the highway , the absolute minimum sight distance required

which is equal to the stopping sight distance should be provided. On horizontal curves , the

obstruction on the inner side of the curve should be cleared up , to provide the absolute

minimum sight distance. At uncontrolled intersections sufficient clearance to the sight line

may be given to provide stopping sight distance.

Overtaking facilities have to be provided in highway alignment at frequent intervals.

At such sections liberal overtaking sight distance should be provided. At sections of the road

where the requirements of overtaking sight distance cannot be provided , as far as possible

intermediate sight distance equal to twice the stopping sight distance may be provided. On

sharp horizontal curves , where overtaking sight distance requirements cannot be satisfied ,

overtaking should be prohibited by regulatory signs.

3.11. HORIZONTAL ALIGNMENT DESIGN:

Important factors that affect the control and design of horizontal alignment are:

3.11.1. Controlling Factors:

(i) Alignment should be as uni-directional as possible , but it should be consistent with the

topographical features of the area and should follow the natural contours of the country. This

is desirable from the construction , maintenance and aesthetic point of view. At the same time

the number of curves to be provided should be the minimum. Passing sight distance should be

provided all along the road as far as possible.

CE409/16 44

(ii) Use of maximum radii of curvatures should be made for the prescribed design speeds

since this leads to uniform speeds and greater safety.

(iii) Sharp curves should be avoided and consistency in the design should be maintained as

far as possible.

(iv) Reverse curves and compound curves which create difficulties to the driver to control the

vehicle should be avoided as far as possible.

(v) Sites for bridges and rail - road crossings must be carefully selected , and curved

approaches to such crossings must be avoided.

3.11.2. Design Factors:

The following factors affect the design of the horizontal alignment.

(i) Design speed of the highway

(ii) Superelevation

(iii) Radius of the horizontal curve

(iv) Extra widening of pavement on the curve

(v) Length of transition curves

(vi) Gradients , and

(vii) Vertical curves.

3.12. DESIGN SPEED OF THE HIGHWAY:

The main factor which effects the geometric design of various elements of a highway

is the design speed. The design speeds of the roads depends upon (i) the class of the road and

(ii) the terrain. The terrains have been classified as follows , based on the cross-slope of the

country.

TABLE 3.10. TERRAIN CLASSIFICATION:

Terrain classification Cross-slope of country -percent

Plain 0 - 10

Rolling 10 - 25

Mountainous 25 - 60

Steep Greater than 60

The design speed (ruling and minimum standardised by the I.R.C for different classes

of roads on different terrains are presented in Table 3.11. Speed restrictions when warranted

due to safety considerations on highways should be specified by exhibiting proper regulatory

signs.

TABLE 3.11. DESIGN SPEEDS:

CE409/16 45

Design speed in Kmph for various terrains

Road Classifications Plain Rolling Mountainous Steep

Ruling Min Ruling Min Rulin

g

Min Rulin

g

Min

National and State

Highways

100

80

80

65 50 40 40 30

Major District Roads 80 65 65 50 40 30 30 20

Other District Roads 65 50 50 40 30 25 25 20

Village Roads 50 40 40 35 25 20 25 20

3.13. SUMMARY:

Geometric design of a highway deals with the dimensions of highway features such as

alignment , slopes , widths , sight distances etc. These elements should be designed on

scientific basis to provide maximum efficiency to traffic operations with safety , comfort and

economy.

Pavement surface characteristics like friction , unevenness , light reflecting properties

and drainage of surface water influence the design of a highway geometry.

The width of a pavement on carriage way depends on the number of lanes and the

width of one traffic lane. The IRC recommends a width of 3.5m per lane in a multiple lane

highway. number of lanes to be provided depends on the total intensity of traffic and the

capacity of each lane. When mixed traffic exists , the capacity of highway is expressed in

terms of Passenger Car Units (PCU). The weightage values for various classes of vehicles

have been specified by the IRC and MOT.

In order to prevent head on collision between vehicles moving in opposite directions

in adjacent lanes of multiple lane roads , traffic separators are provided. In such highways ,

the road width depends on the pavement width and width of traffic separators, if any. The

traffic separators may be in the form of pavement markings , physical dividers or area

separators.

Width of land to be acquired along the highway alignment , ‘Land width or Right of

way Width’ , depends on the importance of the road and possible future expansion. The IRC

has specified the land widths for various classes of roads.

Camber or Cross - slope is the transverse slope provided to the road surface to drain

off the rain water. This camber to be provided depends on the type of road surface and the

amount of rainfall in the region.

CE409/16 46

Sufficient length of highway should be visible to the driver ahead for safe and

efficient driving. Three types of sight distances are considered in the geometric design of

highways.

Stopping sight distance or absolute minimum sight distance is the distance required to

stop a vehicle moving at design speed before coming into collision with another moving or

stationary object. For safe overtaking operation , sufficient length of the road should be

visible to the driver ahead throughout the length of the road. This is called overtaking sight

distance. Where such overtaking opportunities are not available ‘Overtaking Zones’ are

provided at frequent intervals along the length of the road to facilitate overtaking. For

vehicles entering uncontrolled intersections safe sight distances should be provided.

Criteria for sight distance requirements on highways has been presented based on the

IRC standards.

Important factors that affect the design of horizontal alignment of a highway have

been presented.

3.14. SELF ASSESSMENT QUESTIONS:

(1) Define the terms: (i) Sight Distance (ii) Passing Sight Distance and (iii) Stopping Sight

Distance.

(2) Explain briefly how the sight distance affects the design and construction of a highway.

(3) Why is straight line camber provided for concrete roads.

(4) What do you understand by (i) Right of way and (ii) Road camber.

(5) What are the factors that affect the lane width and the number of lanes to be provided for

a straight highway ?

(6) Design the overtaking sight distance for a national highway of two lane width with two

way traffic. Design speed may be assumed as 100 kmph. All other data may be assumed.

ANS : 760.

(7) Calculate the stopping sight distance for a road for which the design speed is 60 kmph.

Brake efficiency is 40%.

ANS. : 141 M.

(8) Write Notes on

(i) Kerbs (ii) Traffic separators (iii) Overtaking Zones and (iv) Sight distance at uncontrolled

intersections.

3.15. BOOKS OF REFERENCE:

CE409/16 47

(1) Bindra , S.P. (1977) - A course in Highway Engineering , Dhanpath Rai and Sons , New -

Delhi.

(2) Kadiyali , L.R. (1984) - Principles and Practice of Highway Engineering , Khanna Tech.

Publications , New Delhi.

(3) Khanna , Dr. S.K. and Justo , Dr. C.E.G. - (1991) - Highway Engineering , Nem Chand

and Bros., Roorkee.

(4) Ribbon Development along Highways and its prevention , special Report 15 , the IRC ,

New - Delhi , (1974).

(5) Recommended Practice for Sight Distance on Rural Highways , The IRC , New-Delhi ,

1976.

***


UNIT - 4

HIGH-WAY GEOMETRIC DESIGN - II CONTENTS:

Aims / Objectives:

4.1. Introduction

4.2. Superelevation

4.3. Radius of Horizontal Curve

4.4. Extra Widening of Pavement on Curves

4.5. Transition curves

4.6. Set-back distance on horizontal Curves

4.7. Gradients

4.8. Vertical Curves - Summit curves

4.9. Vertical curves - Valley curves


4.11. Summary

4.12. References.

Aims / Objectives:

Design of horizontal curvature of a highway is the most important feature as it

influences the efficiency and safety of the highway. The maximum comfortable speed on a

horizontal curve is primarily dependent upon the radius of the curve and super elevation of

CE409/16 48

the carriage way. Other factors that affect the vehicle safety and speed on the horizontal

curves are extra carriage way widths , presence of transition curves and provision of

sufficient sight distance. Design principles of these features in a horizontal alignment are

presented.

Vertical alignment of a highway consists of two elements: gradients and vertical

curve. In order to have smooth and safe movement of traffic , the changes in the gradients

have to be smoothened out by vertical curves. Design principles of these elements are

discussed.

4.1. INTRODUCTION:

Many a time it may not be possible to have a straight alignment to a highway and

changes in direction have to be incorporated. This may be due to the obligatory points that

decide the alignment of the highway or due to the necessity of breaking the monotony or

driving and keeping the driver alert. In order to have smooth vehicle movements on the roads

, the changes in the straight direction should be smoothened out by the horizontal curves. The

alignment should enable consistant , safe and smooth movement of vehicles operating at the

design speeds. Various factors that affect the design of horizontal alignment are discussed.

The vertical profile of a road will have level stretches as well as slopes or gradients.

The changes in gradients have to be smoothened by means of vertical curves in order to have

smooth and safe flow of traffic. Two types of vertical curves are met with in a vertical

alignment. The design principles of the vertical curves - Summit and Sag or Valley curves are

also presented in the following paragraphs.

4.2. SUPER ELEVATION:

In order to counteract the affects of centrifugal force on the vehicle the highway is

super-elevated on curves. The effects of centrifufal forces on the vehicles are discussed.

4.2.1. Centrifugal Force:

When a vehicle travels around a curve of constant radius R(metres) , at a constant

speed of v metres / sec., it experiences a horizontal outward force through its centre of

gravity , known as ‘Centrifugal Force’ and is given by the equation.

P = Wv2 / R g 4.1

where P = Centrifugal Force in Kg.

W = Weight of vehicle , Kg.

V = Speed of vehicle m / sec

R = Radius of circular curve , m.

CE409/16 49

g = acceleration due to gravity m / sec2 .

or P / W = v2 / Rg 4.2

The ratio of P / W is knows as ‘Centrifugal Ratio’ or ‘Impact Factor ‘.

The centrifugal force acting on a vehicle negotiating a curve tends to (i) skid the

vehicle laterally outwards and (ii) overturn the vehicle about the outer wheel.

These effects are discussed below.

(a) Tendency to skid - Lateral skid may occur due to centrifugal force P (Fig 4.1a) if the

lateral resisting force (FA + FB) is lower. For equilibrium condition.

P = FA + FB = f (RA + RB) = f(W)

where RA and RB are the normal reactions at the wheels A and B ; W = weight of vehicle and

f is the coefficient of lateral friction. since P = W v2 / Rg (from equation 4.1)

we get

Pressure distribution under the wheels

(A) SKIDDING (B) OVERTURNING

Fig 4.1. Effect of centrifugal Force

P = Wv2 / Rg = f.W 4.3

or v2 / Rg = f

But v2 / Rg is the centrifugal ratio.

Thus when the centrifugal ratio (P / W) attains a value equal to the coefficient of

lateral friction (f) there is a danger of lateral skidding.

(b) Tendency to overturn: Overturning takes place about the outer wheel B (Fig 4.1B). The

overturning moment due to the centrifugal force , P = Ph and the balancing moment due to

the weight of the vehicle , W = Wb / 2. In the equilibrium condition.

P. h = W. b / 2

CE409/16 50

or P / W = b / 2h 4.4

where h is the height of C.G. above road surface

b = wheel track of vehicle.

Thus when the centrifugal ratio (P / W) attains a value of b / 2h , there is a danger of

overturning.

Due to the tendency of overturning , the pressure under the outer wheel (RB) will be

more than that under the inner wheel (RA). In the limiting equilibrium condition for

overturning , the pressure at the inner wheel becomes zero.

In order to prevent skidding and overturning on a horizontal curve , the centrifugal

ratio should be less than ‘f’ and b / 2h.

When the pavement surface is horizontal , the vehicle has to fully depend on the

coefficient of lateral friction ‘f’ to resist the lateral skidding. The tendency to skid or overturn

can be reduced by reducing the centirfugal force acting on the vehicle either by reducing the

speed of the vehicle ‘v’ or by increasing the radius ‘R’ of the curve.

4.2.2. Super-elevation:

Super elevation , banking or cant may be defined as the raising of the outer edge of

the road over the inner edge along the curve in order to counter-act the effect of centrifugal

force in combination with friction between the road surface and tyres developed in the lateral

direction.

Fig 4.2. SUPER ELEVATED PAVEMENT SECTION

The superelevation ‘e’ is expressed as a ratio of the height of the outer edge NL = E

with respect to the horizontal width (B). From Fig 4.2

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e = NL / ML = tan θ.

In practice , the value of ‘e’ will not exceed 7% or 0.07.

Hence e = tan θ , sin θ = NL / MN = E / B

where E is the total super elevated height of outer edge , e is the super elevation ratio

and B is width of the pavement on curve.

4.2.3. Analysis of Super-elevation:

The forces acting on the vehicle , moving on a super elevated circular curve of radius

‘R’ meters with a speed of v m / sec. or V kmph are as follows (Fig 4.3)

Fig 4.3. Analysis of Super-Elevation

(i) Centrifugal force Wv2 / Rg acting horizontally through centre of gravity of the vehicle.

(ii) The weight ‘W’ of the vehicle acting vertically downwards through the centre of gravity ,

and

(iii) The frictional force (FA + FB) developed between the wheels and the pavement surface

acting trasversely along the pavement surface towards the centre of curve.

In fig 4.3 , these forces acting on the vehicle have been resolved parallel and

perpendicular to the surface. The centrifual force , in this case , is opposed by frictional force

along the pavement surface and the component of the force of gravity. For equilibrium

conditions

P cos θ = W sin θ + FA + FB

= W sin θ + f (RA + RB)

= W sin θ + f (W cos θ + P sin θ).

or P(cos θ - f sin θ) = W (sin θ + f cos θ)

or by dividing through out by W cos θ , we get

CE409/16 52

P / W = (tan θ + f) / (1 - f tan θ).

The value of f , the coefficient of lateral friction , is taken as 0.15 for design purposes.

The value of transverse slope Tan θ = e , seldom exceeds 0.07. Hence the value of f tan θ is

less than 0.01. Thus the value of (1 - f tan θ) in the above equation is equal to 0.99 and may

be approximated to 1.

Therefore P/W = tan θ + f = e + f

but P / W = v2 / Rg (from equation 4.2)

Hence e + f = v2 / Rg 4.6

If the speed of the vehicle is V in kmph and since g = 9.8 m / sec2 ,

we have

e + f = (0.278 V)2 / 9.8 R = V2 / 127 R 4.7

R being the radius of curve in metres

When the coefficient of friction is taken as zero , (i.e.,) f = 0 , the super elevation

provided is called ‘EQUILIBRIUM SUPER ELEVATION’ and is given by the expression

e = V2 / 127 R.

In places where super elevation is not provided , say at rotaries , due to practical

difficulties , the frictional force has to fully countract centrifugal force that is f = V2 / 127 R.

In this case , the speed of the vehicle negotiating the curve has to be restricted to

V = 127 f . R4.3.4. Limits of Super-elevation:

(i) Maximum Limit - For transportation of commercial goods on our roads , animal drawn

carts are being used. Since the centre of gravity of a loaded animal drawn cart is quite high it

is just possible that a high rate of super-elevation on roads may raise the outer edge so high

that the bullock carts may topple over. As such , a maximum rate of super elevation of 7%

(0.07 or 1 / 15) has been recommended by the IRC , for plain and rolling terrain. On hill

roads not bound by snow a maximum superelevation of 10% and on Urban roads with too

many intersections , a value of 4% is recommended.

(ii) Minimum limit - From the drainage considerations , it is necessary to have a minimum

cross slope to the pavement surface. If the super-elevation to be provided (from equation 4.7)

works out to be less than the camber of the road surface , then the minimum superelevation to

be provided on the horizontal curve may be limited to the recommended camber on the

surface.

4.2.5. Design of Super-elevation:

CE409/16 53

Design of super elevation for mixed traffic is a complex problem , as different

vehicles ply on the road with a wide range of speeds. Various steps in the design of super

elevation in practice are as follows:

(i) The super-elevation is calculated for 75% of the design speed (v m / sec or V kmph) ,

neglecting friction

e = (0.75 v)2 / Rg = (0.75 v)2 / 127 R 4.8

(ii) If the calculated value of ‘e’ is less than 0.07 , the value so obtained is provided. If the

value of e , calculated as per (i) exceeds 1 / 15 or 0.07 , then provide the maximum super

elevation equal to 0.07 , and proceed to the following step.

(iii) Check the coefficient of friction developed for the maximum value of e = 0.07, using the

equation.

f = (v2 / Rg - 0.07) = (V2 / 127 R - 0.07) 4.9

If the value of f thus calculated is less than 0.15 , the super elevation is safe for the

design speed. If not calculate the restricted speed as given in step (iv).

(iv) The allowable speed (va m / sec or Va kmph) on the curve is calculated by considering

the design coefficient of lateral friction and the maximum super elevation from the equation.

e + f = 0.07 + 0.15 = 0.22 = va2 / Rg = Va

2 / 127 R 4.10

If Va is greater than V , the design is adquate and e = 0.07 is provided. If not , that is

Va < V , the design speed is restricted to Va , and appropriate caution and speed limit signs

are provided.

As it is not desirable to restrict the speed on important highways , if the site

conditions permit , it is better to re-align the curve with longer radius so that the design speed

could be maintained.

4.2.6. Methods of Providing Super elevation:

The road cross-section at the straight portion is cambered with crown at the centre of

the pavement and sloping downwards the edge. But the cross-section in the circular portion

of the road is super-elevated with uniform tilt sloping down from the outer edge of the

pavement upto the inner edge. Thus , the curved camber section at the straight before the start

of the transition curve should be changed to a single cross-slope equal to the desired

superelevation at the beginning of the circular curve. This change may be conveniently

attained at a gradual and uniform rate throughout the transition length of the horizontal curve.

(Fig 4.4). This is done in two stages: (i) the camber is neutralised gradually till the road has

one straight line slope from the outer to inner edge (ii) then the straight slope is gradually

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increased till the designed superelevation is attained. After the outer edge is raised to be in

one straight line as shown in Fig 4.4 c , the calculated super elevation is given in one of the

two ways shown in Fig 4.4 d1 or d2 . In d1 the desired superelevation E is attained by raising

the outer edge revolving about the inner edge. In Fig 4.4 d2 , the desired superelevation is

attained by raising the outer edge by E / 2 and depressing the inner edge by E / 2. The later is

preferred since the grade of the centre line remains the same. The first case is preferred in

dead flat country , where the second method cannot be adopted for fear of taking the inner

edge in cutting and consequent trouble of drainage.

Fig 4.4. Attainment of Super - Elevation

A transition curve should be sufficiently long to enable these changes to be made

gradually. About 1/2 of the transition curve will be required to eliminate the crown. The

super elevation is introduced by raising the outer edge of the pavement at a rate of 1 in 150 in

plains , 1 in 100 in built up areas and 1 in 60 in hilly areas as per the recommendations of the

IRC.

In very flat curves , the centrifugal force developed will be very small and in such

cases the normal camber may be retained on the curve. The IRC recommended the radii of

horizontal curves beyond which normal cambered section may be maintained and no

superelevation is required for the curves.

4.3. Radius of Horizontal curve:

The centrifugual force which is counter-acted by superelevation (e) and lateral friction

f is given by equations 4.6. or 4.7 substituting the values of maximum superelevation e= 0.07

and the design coefficient of lateral friction ‘f’ = 0.15 , we have

e + f = 0.07 + 0.15 = 0.22 = v2 / Rg = V 2

127 R

or R = v 2

0.22 g=

V 2

127 × 0.22

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Minimum Radius of the horizontal curve is obtained by substituting minimum value

of the design speed in the above equation.

Rmin = v2min / 0.22 g = V2

min / (127 × 0.22) 4.11

It is likely that the standard design speeds are raised in years to come and it is not

desirable to adopt the radius of the curve given by equation 4.11. The radius of the curve to

be adopted for design should be higher than the minimum radius. The ‘Ruling Minimum

Radius’ for which the curves have to be designed is given by

Rruling = v2ruling / 0.22 g = V2

ruling / (127 × 0.22) 4.12

Values of Vruling and Vmin as recommended by the IRC for various classes of roads are

presented in Table 3.11.

4.4. Extra Widening of Pavement on curves:

The widening of pavement at horizontal curves is required (a) For mechanical reasons

and (b) For psychological reasons.

(a) Mechanical Widening - On curves the vehicles occupy a greater width because the rear

wheels track inside the front wheels as shown in Fig 4.5. This is sometimes called ‘OFF

TRACKING’ . The extra width to be provided on curves to account for the off tracking due

to rigidity of wheel base is called mechanical widening ‘Wm’ , and may be calculated as

follows.

Fig 4.5. Widening on curves

From fig 4.5. the extra widening Wm = R - (r + w)

But (r + w) = R2 2− l

∴ = R - R2 2− l

or R2 - l 2 = (R - Wm)2

R2 - l 2 = (R - Wm)2

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l 2 = [R2 - (R - Wm)2] = (R + R - Wm) (R - R + Wm) = Wm (2R - Wm)

or Wm = l 2 / (2R - Wm) = l 2 / 2R per lane 4.13

(b) Widening due to psychological reasons - Widening of pavement is required on curves for

the following reasons:

(i) On curves drivers have difficulty in steering the vehicles to keep to the centre line of the

lane.

(ii) Drivers have psychological shyness to drive close to the edges of the pavement on curves.

An emperical formula has been recommended by the IRC for finding the additional

widening ‘Wps’ for psychological reasons , which is dependent on the design speed and the

radius of the curve and is given by

Wps = V / 9.5 R 4.14

The extra widening We (metres) required on a horizontal curve is given by

We = Wm + Wps = n l 2 / 2R + V / 9.5 R 4.15

where n = nuimber of leanes of traffic

l = length of wheel base of longest vehicle. This value may be taken as 6m for

commercial vehicles unless otherwise specified.

V = Design speed in kmph.

R = Radius of horizontal curve in metres.

We = total extra widening in metres.

4.4.1. Methods of Introducing Extra Widening:

The widening is introduced gradually starting from the begining of the transition

curve and progressively increased at a uniform rate , till the full value of designed widening

‘We’ is reached at the end of the transition curve where full value of superelevation is also

provided. Usually the widening is equally distributed , that is , We / 2 each on the inner and

outer-sides of the curve. But on sharp curves of hill roads the extra widening may be

provided in full on the inside of the curve (Fig 4.6).

4.5. Transition Curves:

A transition curve has a radius which decreases from infinity at the tangent point to a

designed radius of the circular curve. The rate of change of radius of transition curve will

depend on the equation fo the curve. The functions of a transition curve in a horizontal

alignment are as follows:

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(i) To introduce gradually the centrifugal force between the tangent point and the beginning

of the circular curve , avoiding sudden jerk on the vehicle.

(ii) To enable the driver turn the steering gradually for his own comfort and security.

(iii) To enable gradual introduction of designed superelevation and extra widening of

pavement at the start of the ciruclar curve.

Fig 4.6. Widening of pavement on curves

The IRC recommends the use of the spiral as transition curve in the horizontal

alignment of highways due to the following reasons.

(i) The spriral cruve satisfies the requirements of an ideal transiton curve. (Ls . R = constant).

(ii) The spiral curve can be easily laid in the field.

4.5.1. Length of Transition Curve:

The length of transition curve is designed to fulfil two conditions.

(a) Rate of change of centrifugal acceleration to be developed gradually.

(b) Rate of introduction of designed superelevation to be at a reasonable rate and

(c) The IRC Formula

(a) Rate of change of centrifugal acceleration:

The length of transition curve is given by

LS = v3 / CR = V3 / 46.5 CR 4.16

where LS is the length of transition curve in metres.

C is the rate of change of centrifugal acceleration m / sec2 / sec.

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v , V and R have the same significance as earlier and

C = 80 / (75 + V) m/sec2 , (0.5 < C < 0.8) 4.17

that is the minimum and maximum values of C are limited to 0.5 and 0.8 respectively.

(b) Rate of introduction of super-elevation:

This has already been discussed in article 4.2.6.

If the pavement is rotated about the inner edge , the length of transition curve (Ls) is given by

LS = E N = eN (W + We) 4.18

If the pavement is rotated about the centre line , length of transiton curve (Ls) is given by

Ls = EN / 2 = eN / 2 (W + We) 4.19

where Ls is the length of transition curve in metres.

E is total raising of the pavement with respect to inner edge (metres)

W normal width of pavement in metres

We extra width of pavement at circular curve , metres

e the super elevation rate (designed based on 75% of design speed)

N rate of super elevation allowed - 150 in open country , 100 in built up area and 60

in hilly area.

(c) The IRC Formula:

According to the IRC , the length of the horizontal transition curve ‘Ls’ should not be

less than the value given by the following formulae based on the terrain classifications.

(i) For plain and rolling terrain

LS = 2.7 V2 / R 4.20

(ii) For mountainous and steep terrains

LS = V2 / R 4.21

The length of the transition curve for design should be highest of the three values

considered in a , b and c.

When the radius of the circular curve is very large transition curve need not

necessarily be provided. Such radii for various design speeds have been specified by the IRC.

The circular curve gets shifted in position due to the introduction of the trransition

curves at both the ends. The shift ‘S’ of the circular curve is given by

S = L2S / 24 R 4.22

4.6. Set-back Distance on Horizontal Curves:

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The clear distance or set-back distance m (fig 4.7) required from the centre line of a

horizontal curve to an obstruction on the inner side of the curve to provide adequate sight

distance depend upon the following factors:

(i) required sight distance

(ii) radius of horizontal curve

(iii) Length of curve

Let ‘α‘ be the angle subtended by the arc length equal to the required sight distance

‘S’ at the centre of the horizontal curve. Then

α = 180S / RT degress.

Fig 4.7

The distance from the obstruction to the centre is R cos α / 2.

Then the set back distance CD = m required from the centre line is given by

m = R - R cos α / 2 4.23

In the case of wide roads with two or more lanes if ‘d’ is the distance between the

centre line of the road and the centre line of the inside line in metres , the sight distance is

measured along the middle of the inner side lane and the set back distance ‘m1’ is given by

m’ = R - (R - d) cos α / 2 4.24

where α‘ = 180S / π (R - d) degress

d is usually taken as total width of pavement / 4 = (w + We) / 4 4.25

The IRC has suggested equation 4.24 for finding the set back distance required at

horizontal curves. The clearance of obstruction upto the set back distance is important when

there is a cut slope on the inside of the horizontal cuve.

Worked example 4.1 illustrates complete design of the horizontal alignment.

Worked Example:

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4.1: A national highway is passing through a plain terrain. Design all the geometric features

of the curve assuming suitable data. Also specify the minimum set back distance from the

centre line or the two lane highway on the inner side of the curve upto which the buildings etc

., obstructing the vision should not be constructed so that the overtaking sight distance is

available through out the circular curve. Assume the length of the circular curve greater than

overtaking sight distance.

Solution:

The various geometric elements to be designed are:

(i) Ruling minimum radius (ii) Super elevation (iii) Extra widening (iv) Length of transition

curve (v) Overtaking sight distance and set back distance.

Data assumed:

For National Highway through Plain Terrain

Ruling speed = 100 kmph ; No. of lanes 2

Width of pavement = 7m

Speed of overtaking vehicle = (100 - 16) = 94 kmph

For overtaking reaction time of driver = 2 sec.

(i) Ruling minimum radius = V2 / 127 (a + f) = 1002 / 127 (0.07 + 0.15) = 357.9 m or say

360m.

(ii) Super elevation Design:

Super elevation is designed for 75% of the design speed.

e = (0.75 V)2 / 127 R = (0.75 x 100)2 / 127 x 360 = 0.123

as the value of e is greater than 0.07 , it is restricted to 0.07 and check for friction is made

f = V2 / 127 R-e = 1002 / 127 x 360 - 0.07 = 0.149

As this value of f is less than 0.15 , superelevation of 0.07 may be provided.

(iii) Extra -width of pavement to be provided

We = nl2 / 2R + V / 9.5 R = 2 × 62 / 2 × 360 + 100 / 9.5 360 = 0.65m

Total width of pavement = 7 + 0.05 = 7.05m.

(iv) Length of transition curve is calculated based on the following considerations and the

longest of all the three lengths is adopted. (a) based on the rate of change of centrifugal

acceleration , C , rate of change of centrifugal acceleration.

= 80 / (75 + V) = 80 / (75 + 100) = 0.457

The minimum specified value of C = 0.5 is to be adopted.

Length of transition curve = LS = V2 / 46.5 CR = 1003 / 46.5 × 0.5 × 360 = 123.46 m .

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(b) Based on rate of introduction of superelevation - As the terrain is plain , the

superelevation is introduced at a rate of 1 in 150 by rotating the pavement about the inner

edge.

∴ LS = N . e (W + We) = 150 × 0.07 (7 + 0.65) = 80.32 m

(c) based on the minimum length requirements of IRC.

LS = 2.7 × V2 / R = 2.7 × 1002 / 360 = 75 m.

The highest value of these three lengths is 123.46 m.

Hence adopt a transition curve of length 125m .

(v) Overtaking Sight Distance:

(a) Distance covered during reaction time d1 = 0.278 Vbt = 0. 278 × (100 - 16) × 2 = 46.7 m.

Minimum spacing between the vehicles = S = (0.2 VB + 6) = (0.2 × 84 + 6) = 22.8 m.

Time for overtaking Tsec = 14.4S / AA - rate of acceleration for 100 kmph = 1.92 kmph / sec.

∴ Tsec = 14.4 × 22.8 /1.92 = 13.02

(b) Then d2 = b + 2S = 0.278 VBT + 2S = 0.278 × 84 × 13.82 + 2 × 22.8 = 351.04 = 352 m.

(c) Distance covered by the on coming vehicle

d3 = 0.278 V T = 0.278 × 100 × 13.08 = 363.0 m

Overtaking sight distance = d1 + d2 + d3 = 761.2m

(vi) Set back distance:

d = distance between the centre line of the road and the centre line of the inside

pavement is taken as width of pavement / 4 = 7.65 / 4 = 11.91m.

α‘ / 2 = 180 × 762 / 2 π (360 - 1.91) = 60.6

Set back = 360 - (360 - 1.91) cos 60.6 = 187 m.

Answers:

Ruling Mini Radius = 360 m.

Super elevation e = 0.07

Extra widening = 0.65m

Length of Transition curve = 125m.

overtaking sight distance = 690 m.

Set back distance = 187m.

4.7. Gradients:

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Gradient is the rate of rise or fall along the length of the road with respect to the

horizontal. A rising or ascending gradient is designated by plus (+) sign while a descending

gradient by negative (-) sign. Gradient is expressed as a ratio of 1 in n (one unit of vertical to

n units of horizontal) and some times as a percentage that is n in 100. The angle which

measured the change of direction at the intersection of two grades is called the ‘Deviation

Angle’ and is represented by N , and is equal to the algebraic difference between the two

grades. In Fig 4.8 , the deviation angle N = ∠DBC = ∠BAC + ∠BCA = n1 - (- n2) = n1 + n2.

Where n1 is the ascending gradient of AB and (-n2) is the descending gradient of BC.

Fig 4.8 MEASUREMENT OF GRADIENT

Gradient in roads should not be very steep. Steep grades not only make it difficult for

vehicles to climb over them , but also increase operational cost of the vehicles. Designer

should try to provide as easy gradient as possible provided earth work is not unnecessarily

increased.

Gradients are classified into the following types:

(i) Ruling gradient

(ii) Limiting gradient

(iii) Exceptional gradient , and

(iv) Minimum gradient.

4.7.1. Ruling Gradient:

This is the maximum gradient normally adopted. Its value depends upon the type of

terrain , length of grade , speed of vehicles , power of vehicles , type of traffic and presence

of horizontal curves in the road alignment. Based on experience , for mixed traffic conditions

, values of ruling gradients as recommended by the IRC are presented in Table 4.1.

4.7.2. Limiting Gradient:

Limiting gradient is steeper than ruling gradient and is provided at places , where by

adopting slightly steeper gradient a lot of saving in earth work and other aspects can be

affected. The length of the limiting grades should not be continuous but limited in length.

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Every limiting gradient should be followed by a stretch of road having a very small gradient

or levelled ground. In hilly areas these gradients are very common. Recommended values of

limiting gradients of roads in differenct terrains are as presented in Table 4.1.

4.7.3. Exceptional Gradient:

Exceptional gradients are steeper than limiting gradients and have to be provided only

under unavoidable circumstances. However the excceptional gradient should be strictly

limited only for short stretches not exceeding about 100 metres at a stretches. Recommended

values of this gradient for different terrains are also presented in Table 4.1.

Table 4.1. Gradients For Roads in Different Terrains:

Terrain Gradient in percent

Ruling Limiting Exceptional

1. Plane or Rolling 3.3 (1 in 30) 5.0 (1 in 20) 6.7 (1 in 15)

2. Mountainous terrain and steep

terrains having elevation more than

3000m above M.S.L.

5.0 (1 in 20) 6.0 (1 in 16.7) 7.0 (1 in 14.3)

3. Steep terrains upto 3000m height

above M.S.L.

6.0 (1 in 16.7) 7.0 (1 in 14.3) 8.0 (1 in 12.5)

The maximum length of ascending gradient which a loaded truck can operate without

undue reduction in speed is called ‘Critical Length of Grade’ for design. A reduction of speed

of about 25 kmph , may be considered as reasonable limit. The critical length of ascending

gradient should therefore be limited to lower values at steeper gradient.

4.7.4. Minimum Gradient:

It is desireable to have certain minimum graident on roads from drainage point of

view , provided topogrophy favours this. The minimum gradient would depend on the rainfall

, run off , type of soil , topography and other site conditions.

A gradient of about 1 in 500 may be enough to drain off water in concrete drains ; but

on inferior surfaces of drains a slope of 1 in 200 may be needed where as on soil drains

steeper slopes upto 1 in 100 may be needed.

4.7.5. Compensation in Gradients on Horizontal Curves:

If a horizontal curve is also located on an ascending road , vehicles while negotiating

curve and grade will have to come across the joint resistance offered by the curve and grade

effectively. It is necessary in such cases , the total resitance due to grade and curve should not

CE409/16 64

exceed the resistance due to the maximum value of the gradient specified. The gradient in

such cases is slightly reduced so that the vehicle may deal with resistance offered by the

curve and grade effectively. This reduction in gradient is called ‘Grade Compensation’ and is

calculated from the following formula.

Grade compensation , (percent) = (30 + R/R) 4.26

Where R is the radius of curve in metres.

Maximum valve of the grade compensation is limited to 75 / R. According to the IRC

, the grade compensation is not necessary for gradients better than 4% (1 in 25).

4.8. Vertical Curves:

The changes in grades in the vertical alignment of a highway are smoothened out by

introducing vertical curves in order to have smooth vehicle movements. The vertical curves

used in highways may be classified into two types

(i) Summit or Crest Curves

(ii) Sag or Valley Curves.

In order to design the vertical curves the following assumptions are made:

(i) The curve is so flat that the length of the curve is equal to the length of the chord.

(ii) The portions of the curve along the two tangents on either side of the point of intersection

are equal.

(iii) The angles subtended by the tangents with the horizontal are so small that the tangents of

these angles are equal to the angles in radians themselves.

4.8.1. Summit Curves:

Some of the cases where summit curves with convexity upwards are formed are

illustrated in Fig 4.9.

When a fast moving vehicle travels along a summit curve , the centrifugal force will

act upwards , against gravity and hence a part of the pressure on the tyres and springs of the

vehicles is released and there is no problem of discomfort to the passengers. While other

curve forms can be used with satisfactory results , the tendency , has been to utilize the

parabolic curve in profile alignment design. This is primarily because of the ease with which

it can be laid out as well as enabling the comfortable transition from one grade to another.

Normally vertical curves of this type are not necessary when the deviation angle does not

exceed 0.5%.

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N = Angle of Deviation = Algebraic difference between grades

Fig 4.9 SUMMIT CURVES

The only problem in the design of summit curves is to provide adequate sight

distance. The stopping sight distance or the absolute minimum sight distance should

invariably be provided at all sections of the road system and also on summit curves. As far as

possible , safe overtaking sight distance or at least intermediate sight distance should be

available on the curves.

4.8.2. length of Summit curve:

Parabolic summit curves are generally adopted , the euqation of which is given by

y = x2 / a , with value of a = 2L / N 4.27

where N is the deviation angle and L is the length of the curve x and y are the co-

ordinates of any point P on the curve.

Length of curve ACB = Chord Length AB

EF - Stopping sight Distance

EG - Overtaking Sight Distance

Fig 4.10 LENGTH OF SUMMIT CURVE

x and y are as defined in the Fig 4.10.

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While designing the length of the parabolic summit curve , it is necessary to consider

both the stopping sight distance (SSD) and overtaking sight distance (OSD) separately. These

two cases are discussed.

(a) Length of Summit Curve for Stopping Sight Distance (SSD):

Two cases are to be considered in deciding the length.

(i) When the length of the curve is greater then the stopping sight distance (L > SSD).

(ii) When the length of the cuve is less than the stopping sight distance (L < SSD).

Case (i) When (L > SSD):

The general equation for the length of the parabolic curve is given by

L = NS2 / ( 2H + 2h )2 4.28

Here L = Length of summit curve is metres.

S = Stopping sight distance in metres.

N = Angle of deviation

H = Height of the eye level of the driver above road surface = 1.2m.

h = Height of obstruction above the pavement , surface = 0.15m.

Substituting for H and h , the values recommended by the IRC in equation 4.28 , we

get ,

L = NS2 / 4.4 4.29

Case (ii) , when L < SSD:

The length of the parabolic equation may be obtained from

L = 2S - ( 2H + 2h )2 / N 4.30

Substituting for H and h , the recommended values

L = 2S - 4.4 / N 4.31

Knowing the stopping sight distance S , the length of summit curve may be obtained

using any one of the two appropriate formulae (4.29) or (4.31).

(b) Length of Summit Curve for Safe Overtaking Sight Distance (OSD) or Intermediate

Sight Distance (ISD):

Under this consideration also two cases have to be considered in deciding the length

of the summit curve. They are:

(i) When the length of the curve is greater than the Overtaking Sight Distance (L > OSD) or

Intermediate Sight Distance (L > ISD).

(ii) When the length of the curve is less than the Overtaking or Intermediate Sight Distance

(L < OSD) or (L < ISD).

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Case (i) When L > OSD or > ISD:

For this case also the same general equation (4.28) is applicable except that H = h =

1.2 , heights of the eye level of the drivers above the surface of the road since overtaking or

intermediate sight distance is considered for S. Then substituting H = h = 1.2 in equation

4.28.

L = NS2 / 8H = NS2 / 9.6 4.32

Case (ii) When L < OSD or L < ISD:

The same general equation 4.30 can be used in this case also ;

Substituting H = h = 1.2 and simplifying

L = 2S - 8H / N = 2S - 9.6 / N 4.33

Here in equations 4.32 or 4.33

L = Length of Summit curve in metres

S = Overtaking or Intermediate Sight Distance

and N = Angle of deviation.

The length of the vertical summit curve in calculated knowing the overtaking sight

distance / intermediate sight distance using the appropriate formulae out of the equaltion 4.32

or 4.33.

4.9. Valley or Sag Curves:

A vertical curve having concavity upwards is called a valley curve. Some of the cases

where valley curves are formed are illustrated in Fig 4.11. At valley curves the centrifugal

force acts downwards and add to the weight of the vehicle. For gradual development of

centrifugal force the best shape of the valley curve is a transition curve.

Fig 4.11 VALLEY CURVES

Cubic parabola is the preferred shape of the vertical valley curve.

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Whereas in the case of summit curves the most important factor is the length of the

curve necessary for safety , there are at least four widely accepted criteria for determining the

minimum length of the valley curves.

They are:

(i) the vehicle head light sight distance. (British Practice)

(ii) the motorist’s comfort (Indian Practice).

(iii) drainage control , and

(iv) general aesthetic considerations.

4.9.1. Length of the Valley Curve:

Length of the vally transition curve is designed based on two criteria:

(i) the allowable rate of change of centrifugal acceleration.

and (ii) the head light sight distance.

The higher of the two values is adopted. Usually the second criteria of head light sight

distance is higher and therefore governs the design.

Case (a) Length of valley curve based on the allowable rate of change of centrifugal

acceleration.

Fig 4.12 Length of Valley Curve

The valley curve ACB of length ‘L’ is made fully transitional by providing two

similar transition curves of length AC = CB = Ls - L. (Fig 4.12).

Thus L = 2LS

The length of transition curve is given by

LS = v3 / CR But R = Radius of curve = LS / N.

∴ LS = V3N / C. LS , substituting for R.

or LS2 = Nv3 / C

or LS = (Nv3 / C)1/2

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But L = 2 LS

∴ L = Length of valley curve = 2 (Nv3 / C)1/2 4.34

Here N is the angle of deviation of the gradients.

v is the speed of metres / sec.

and C is the allowable rate of centrifugal acceleration in m / sec2 / sec.

Recommended value of C = 0.6m / sec3 . If V is the speed in kmph , then substituting

for C and V in equation 4.34 , 0.6 m / sec3 and v = 0.278 V respectively , we get

L metres = 0.38 (NV3)1/2 4.35

Case (b) Length of valley curve for Head Light Sight Distance - In this , two cases are

considered. They are:

(i) Length of valley curve (L) greater than the stopping sight distance (S): L > SSD.

(ii) Length of valley curve (L) Less than the stopping sight distance (S): L < SSD.

Case (i) when L > S:

L = NS2 / (1.5 + 0.035S) 4.36

Case (ii) when L < S:

L = 2S - 1.5 + 0.035S( )

N4.37

Worked Examples:

4.2 A valley curve is to be designed for the following conditions. A descending gradient of 1

in 25 meets an ascending gradient of 1 in 30. Calculate length of the valley curve for a design

speed of 100 kmph for both comfort and head light sight distance.

Solution:

Angle of deviation N = - 1 / 25 - 1 / 30 = - 11 / 150

Case (i) Length of valleys curve based on motorist’s comfort.

L = 0.38 (NV3) 1/2 = 0.38 (11 / 150 × 1003)1/2 = 102.9m.

Case (ii) Head light sight distance.

Stopping sight distance = 0.278 Vt + V 2

254 fwhere t = reaction time = 2.5 sec.

f = coefficient fiction = 0.35.

S = 0.278 × 100 × 2.5 + 1002 / 254 × 0.35 = 182 m.

When the length of the curve > stopping sight distance.

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L = NS 2

1.5 + 0.035S( ) =11/150 ×168 2

1.5 + 0.035 × 168( ) = 300.6 m.

Hence length of valley curve = 310 m (Answer).

Example 4.3: An ascending gradient of 1 in 60 meets a descending gradient of 1 in 40. A

vertical curve is to be designed for a speed of 80 kmph so as to have a stopping sight distance

of 150m and an overtaking sight distance of 500m. Due to the site conditions the length of the

vertical curve is to be restricted to 500m.

Solution:

N = 160

140

124

+ =

Case (i) when the length of the curve is > stopping site distance.

L = NS2 / 4.4 = 1 / 24 × 1502 / 4.4 = 213m

As the length of the curve 213m > stopping sight distance of 150 m , the assumption

is O.K. As the length of curve is less than the prescribed value , it may be adopted.

Case (ii) Overtaking sight distance consideration.

In this case , L > OSD , need not be considered as the overtaking sight distance is

given as 500 and the length of the curve is to be restricted to 500m.

When L < OSD

L = 2S - 9.6 / N = 2 × 500 - 9.6 / 1/ 24 = 770m.

As this length is greater than OSD , the assumption is not correct.

Case (iii): To provide for limited opportunities of overtaking , intermediate sight distance = 2

SSD may be considered.

When L > ISD

L = NS2 / 9.6 = 1/24 ×300 2

9.6= 388m.

As this length is less than 500m , the length of summit curve may be fixed as 388m ,

giving limited opportunities of overtaking.


(1) What are the effects of centrifugal force on a vehicle moving along a horizontal curve ?

Why roads are superelevated on curves ?

(2) In the design procedure for super-elevation , for calculating e , only 75% of the design

speed is considered. Why ?

(3) On curves of large radius , roads are not superelevated. Why ?

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(4) Calculate the safe driving speed on a curve with radius 200m. (the super elevation = 0.07

and coefficient of friction = 0.15) is the curve meeting the standards of M.D.R in plain

terrain.

(Ans: Speed = 74.75 kmph., No.)

(5) While aligning a hill road with a ruling gradient of 5% , a horizontal curve of radius 100m

is ecountered. Find the compensated gradient at the curve.

(Ans: 4.25%)

(6) A vertical summit curve has a deviation angle of 0.045. Design a summit curve to have a

passing sight distance of 500m for the following conditions.

(i) Length greater than passing sight distance and

(ii) Length less than passing sight distance.

(Ans: 1172m).

(7) A descending gradient 1 in 25 meets an ascending gradient of 1 in 30. Calculate the length

of valley curve for a design speed of 60 kmph. (Ans: 47.5m).

4.11. Summary:

A well designed horizontal alignment of a highway should permit consistent , safe and

smooth movement of vehicles operating at the design speed. The elements of a horizontal

alignment are superelevation , radius of horizontal curve , widening of pavement on curves

and transition curves. A detailed design procedure of these elements is discussed.

On Horizontal curves roads are superelevated , that is , the outer edge of the pavement

is raised over the inner edge to counteract the effect of the centirfugal force. Considering the

mixed traffic conditions existing on our highways , a maximum rate of superelevation of 7%

has been recommonded by the IRC. Maximum amount of coefficient of lateral friction

suggested is 0.15.

Considering the off tracking effect of the wheels of a vehicle and the psychological

reasons of the driver , the pavement width is increased on horizontal curves. This extra width

to be provided is a function of the design speed , length of wheel base , number of lanes of

traffic and the radius of the horizontal curve.

Transition curves are used in the horizontal alignment between , the straight portion

and circular curve to facilitate the introduction of super elevation and extra widening of the

pavement at a uniform rate to provide a smooth and confrortable movement of the vehicles.

The length of the transition curve is based on (i) the rate of introduction of superelevation (ii)

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permissible rate of change of radial acceleration and (iii) minimum length as per the IRC

formula. Length of transition curve to be adopted should be the highest of these three values.

To provide adequate sight distance on curves , obstructions have to be set back from

the centre line of the road by a certain distance. This distance depends on the sight distance

considered -stopping sight distance or overtaking sight distance.

Vertical alignment of a highway consists of gradients and curves along the profile of

the road. The changes in the grades in the vertical alignment are smoothened by providing

vertical curves. While designing the vertical alingment of the highway , it must be ensured

that minimum stopping sight distance requirement is met with.

Gradients in the roads should not be very steep. Steep gradients not only make

difficult for the vehicles to climb over them but also increase the operational cost of the

vehicles. As easy gradients as possible have to be laid , provided the earth work is not

unnecessarily increased. Gradients are classified into four classes - Ruling Gradient ,

Limiting Gradient , Exceptional Gradient and Minimum Gradient. The IRC has specified

limiting values for these gradients based on the terrain classification.

If a horizontal curve is also located on an ascending gradient steeper than 4% , the

gradient has to be reduced so that the vehicles may deal with resistance offered by the curve

and grade effectively. This reduction in gradient is called ‘Grade Compensation’.

Vertical curves may be either , summit curves or valley curves. Summit curves have

convexity upwards and parabolic. The only problem in the design of summut curves is to

provide adequate sight distance.

Valley curves have concavity upwards and are designed as transition curves. Cubic

parabolic is the preferred shape of vertical valley curves. There are four widely accepted

criteria for designing the minimum length of the valley curve. They are

(i) the vehicle head light distance

(ii) the motorist comfort ,

(iii) drainage control , and

(iv) aesthetic considerations.

4.12. References:

1. Bindra , S.P - )1977). ‘A Course in Highway Engineering’.

Dhanpath Rai and Sons , Delhi.

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2. Kadiyali , L.R. (1984) - ‘Principles and Practice of Highway Engineering’ , Khanna Tech

Publications , New-Delhi.

3. Khanna , Dr. S.K. and Justo , Dr. C.E.G (1991) - ‘Highway Engineering’ . Neem Chand

and Brothers , Roorkee.

***


UNIT - 5

TRAFFIC ENGINEERING - I CONTENTS:

Aims / Objectives

5.1. Introduction

5.2. Scope of Traffic Engineering

5.3. Traffic Studies

5.4. Traffic Volume Studies

5.5. Speed Studies

5.6. Speed and Delay Studies

5.7. Origin and Destination Studies

5.8. Traffic Capacity

5.9. Road Accidents


5.11. Summary

5.12. References

AIMS / OBJECTIVES:

The fundamental objective of traffic engineering is to achieve safe , free , rapid and

efficient flow of traffic. Factual studies of traffic operations provide the foundation for

developing methods for improvement , in general , and for solving specific traffic problems.

Traffic volume , speed , speed and delay , origin and destination , capacity and accident

studies which form the basis for traffic data are presented in this unit.

5.1. INTRODUCTION:

Traffic engineering is a comparatively new branch of engineering , and has grown

with the increase in traffic in recent years. As vehicular traffic began to increase , the

congestion on streets began to hamper the safe and efficient movement of traffic. More and

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more accidents were caused and serious problems of parking and environmental pollution

began to be felt. If was therefore necessary to give increasing attention to the operational

characteristics of highway transportation.

Traffic engineering is the science of measuring traffic and travel , the study of the

basic laws relating to traffic flow and generation and application of this knowledge to

professional practice of planning , designing and operating traffic systems to achieve safe and

efficient movement of persons and goods.

Highway engineering and traffic engineering are related subjects and the latter may be

considered to be an off-shot of the former. However , now-a-days traffic engineering has

been recognised as a specialised branch.

5.2. Scope of Traffic Engineering:

Traffic engineering may be considered as a special technique in highway design and

control of traffic. Traffic engineering deals with the direction and control of vehicular traffic

and pedestrians on existing highways. The planning , design and operation of all devices that

aid in the highway safety and free flow of traffic fall within the scope of this subject.

Traffic characteristics are quite complex and mainly dependent upon the

characteristics of the road user and the vehicle and require particular attention. The factors

which affect the characteristics of road user are physical , mental , psychological and

environmental. Vehicle size , power , speed and braking system effect the vehicular

performance on the road and need special attention. Apart from these , the various studies to

be carried out include volume , speed , speed and delay , origin and destination , capacity ,

parking and accident studies.

Various aspects that are covered under traffic operations are regulations , control and

warrants for application controls. Regulations may be in the form of laws and ordinances or

other traffic regulating measures such as speed limit , parking restrictions etc.,. Installation of

traffic control devices like signs , signals and islands are the most common measures to

regulate and control traffic. Actual adoption of traffic management measures such as traffic

regulations and control need adequate attention.

Enforcement of traffic regulations and education of the masses towards a proper traffic

behaviour are akin to Traffic Engineering and these three things are some times designated

as the three E -S of traffic safety.

5.3. TRAFFIC STUDIES:

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Traffic studies are carried out to collect factual data regarding traffic operations.

These studies help in deciding the geometric design features and traffic control for safe and

efficient traffic movements. These studies are also known as ‘Traffic Surveys’ or ‘Traffic

Census’. The following traffic studies are discussed in the following pages.

(i) Traffic Volume studies

(ii) Speed studies

(iii) Speed and Delay studies

(iv) Origin and Destination studies (0 - D studies)

(v) Traffic Capacity studies and

(vi) Accident studies

5.4. TRAFFIC VOLUME STUDIES:

The term ‘Traffic Flow’ and ‘Traffic Volume’ are used interchangeably to define the

number of vehicles that pass a given point on the highway in a given period of time. It is this

information that is of most value to the highway planner. When the traffic is composed of a

number of types of vehicles , mixed traffic , it is the normal practice to convert the flow into’

Equivalent Passenger Car Units’ (PCUS) by using the specified equivalency factors. The

flow is then expressed as PCUS per hour or per day.

A complete traffic volume study includes the classified volume study by recording the

volume of various types and classes of traffic , the distribution by direction and by turning

movement per unit time.

The objects and uses of traffic volume studies are given below:

(i) Relative importance of roads may be fixed in deciding the priority for improvement and

expansion.

(ii) Helps to evaluate the existing facilities and plan new facilities in regard to traffic

operation and control.

(iii) Helps in the analysis of traffic trends and patterns.

(iv) Classified volume study helps in structural design of pavements and geometric design of

roads ; design of side walks , pedestrian crossings etc.,

(v) Helps in planning regulatory measures.

(vi) Turning traffic studies are useful in design of road intersections , light signal timings and

other control measures.

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The volume of traffic flow varies from time to time , hour to hour in a day , day to day

in a week , and week to week in a year. Hence if a true picture is to be obtained , the hourly

traffic volume should be known with daily and seasonal variation pattern.

5.4.1. COUNTING OF TRAFFIC VOLUME:

Traffic volume counts are made either by mechanical counters or manually.

(a) Mechanical counters: These automatically record the total number of vehicles crossing a

section of a road in a desired period. These recorders are capable of recording impulses

caused by the traffic movements on a pneumatic nose placed across a roadway or by

electrically operated counters. In some cases , the impulses caused by vehicles of light weight

, and in particular pedestrians traffic , may not be enough to actuate the counter. Other

methods of working of the mechanical detectors are by photo-electric cells , magnetic

detectors and radar detectors.

The main advantage of these counters is that they can work throughout the day and

night for the desired period , recording the total hourly volume , which may not be practicable

in manual counting. The main drawback in this method of traffic count is that it is not

possible to get the traffic volumes of various classes of traffic in the stream and the details of

turning movements.

(b) Manual Counts: This methods employs a field team to record traffic volume on the

prescribed sheets. By this method it is possible to obtain data which cannot be collected by

mechanical counters , such as vehicle classification , turning movements , and counts where

the loading conditions or number of occupants are required. However , it is not practicable to

have manual count for all the 24 hours of the day and on all the days in a year. Hence resort is

made to statistical sampling techniques to arrive at the peak hourly volumes as well as the

average daily traffic volume.

TABLE - 5.1 MODEL ENUMERATORS FORM:

Location of Junction: Road Classification

Name of Approach Kilometreage:

Data Route No.

Hour of Starting Hour of Ending

District State

Vehicle

Category

Left Turning Straight Right

Turning

Grand Total

PCU

Enume Total PCU Enu Total PCU Enu Total PCU

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ration me

ration

me

ration

A specimen enumerator’s form (Table 5.1) is shown below. The data may be recorded

conveniently by five dash system whereby vertical strokes are encountered for the first four

vehicles , followed by an oblique stroke for the fifth vehicle so as to depict a total of five.

5.4.2. Presentation of Traffic Volume Data:

The date collected during the traffic volume studies are sorted out and are presented in

any of the following forms depending upon the requirements.

(i) Variation charts showing hourly , daily and seasonal variations of the traffic are prepared.

These help in deciding the facilities and regulations needed during peak traffic periods.

(ii) Trend charts show the volume trends over a period of years and are useful for planning

future expansion , design and regulations.

(iii) Annual Average Daily Traffic (AADT or ADT) - Numerically , the ADT is the total

annual volume of traffic divided by the number of days in the year. The ADT is readily

obtained where continuous counts of traffic are available. ADT volumes are useful in the

economic study of highways and also in the design of structural elements of the road.

(iv) Traffic Flow Maps along the routes and volume flow diagram at intersections as shown

in Fig 5.1 are drawn. These help to find the traffic volume distribution at a glance and for the

intersection design.

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Fig 5.1. Volume Flow Diagram

(ii) Thirtieth Highest Hourly Volume is generally adopted as the design volume in most of

the advanced countries. This is obtained from a plot between the hourly volume of traffic

expressed as a percentage of ADT arranged in a descending order and the number of hours in

a year. The slope of the curve becomes steeper for the busier hours of the year while if

flattens for the slacker hours. Road facilities designed according to the peak hourly volume of

the year will be uneconomical and those according to the ADT will be in adequate for most

of the time of the year. Highway facilities designed according to the 30th highest hourly

traffic volume of the year is found to be satisfactory both from the economic and adequacy

point of view and traffic congestion on such roads is expected for only 29 hours of the year.

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Fig 5.2. HOURLY TRAFFIC VOLUME

5.5. SPEED STUDIES:

The term ‘Traffic Speed’ is often used loosely when describing the rate of movement

of traffic. To the highway engineer there are many different types of speed , each of which

describe the rate of traffic movement under specific conditions and for a specific purpose.

The vehicle speeds of most interest are spot speed , running speed and journey speed and they

are discussed here. ‘Spot Speed’ is the instantaneous speed of a vehicle at a specified section

or location.

‘Running Speed’ is the average speed maintained by a vehicle over a given course

while the vehicle is in motion.

Running Speed = Length of Course / Running Time 5.1

It is to be noted that ‘running time’ excludes that part of the journey time when the

vehicle suffers delay. ‘Journey Speed or Overall Travel Speed ‘ is the distance between two

points divided by the total time taken by the vehicle to complete the journey , including all

delays incurred on the route. Thus ,

Journey Speed = Distance

Total Journey Time (including delay)

‘Average Speed ‘ is the average of the spot speeds of all vehicles passing a given

point on the highway.

There are two definitions for the average of a series of spot speed measurements ,

namely , space-mean speed and time - mean speed.

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‘Space mean Speed’ represents the average speed of vehicles in a certain road length

at any time. This is obtained from the observed travel time of the vehicles over a relatively

long stretch of the road. Space mean speed (Vs) is calculated from:

Vs = 3.6 d . n / tik=1

n

∑ 5.3

‘Time -mean speed’ represents the speed distribution of vehicles at a point on the road

way and it is the average of instantaneous speeds of observed vehicles at the spot. Time mean

speed (Vt) is calculated from:

Vt = Vi / ni=1

n

∑ 5.4

Where Vs is the speace-mean speed in kmph.

Vt is the time-mean speed in kmph.

d = length of road considered in metres.

n = number of individual vehicle observations.

ti = observed travel time (secs) for the ith vehicle to travel distance ‘d’.

vi = observed speed of the ith vehicle.

5.5.1. OBJECTS AND USES OF SPOT-SPEED STUDIES:

The spot speed studies may be useful for the following purposes:

(i) To use in planning traffic control and in traffic regulations.

(ii) To use in geometric design

(iii) To use in accident studies

(iv) To study traffic capacity , and

(v) To decide the speed trends.

The following factors effect the spot speed: Physical features of the road , road side

developments , environmental conditions; enforcement ; traffic conditions ; driver ; vehicle

and purpose of travel.

5.5.2. METHODS OF MEASURING SPOT SPEEDS:

There are a number of methods of measuring the spot speeds. Only two methods , viz,

Direct timing procedure and the Enoscope method are described below.

(i) Direct Timing Procedure: This is one of the simplest methods for spot speed

determination. Two reference points are marked on the pavement at a suitable distance apart

and two observers are stationed one at each reference point. The observer standing at the

reference point which the vehicles pass first , signals that a vehicle to be timed is passing the

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point and the second observer then starts a stop watch. The second observer stops the stop

watch when he observes the same vehicle passing the reference point. From these

observations the speed (kmph) is calculated. The dis-advantage with this method is that it

involves the reaction time of the two individual observers.

(ii) Enoscope Method: A simple device called (ENOSCOPE) is used for spot speed studies.

It is nothing but a mirror box supported on a tripod stand. In its simplest principle , the

observer is stationed on one side of the road , and starts a stop watch when a vehicle crosses

that section. An Enoscope is placed at a convenient distance say 50 metres in such a way

Fig 5.3. SPOT SPEED STUDY BY ENOSCOPE

that the image of the vehicle in seen by the observer when the vehicle crosses the section

where the enoscope is fixed (fig 5.3) and at this instant the stop watch is stopped. Thus the

time required for the vehicle to cross the known length is found and is converted into speed in

kmph. The main advantage of this method is that it is simple and cheap equipment easy to

use. The disadvantage is that the progress is so slow that it is difficult to spot out typical

vehicles and the number of samples observed is less. There is also possibility of human error.

5.5.3. PRESENTATION OF SPOT SPEED DATA:

The spot speed data can be analysed in the following two ways - (i) Graphical

Analysis and (ii) Mathematical analysis.

(i) Graphical Analysis: From the spot speed data of the selected samples , frequency

distribution tables are prepared by arranging the data in groups covering various speed ranges

and the number of vehicles in such range. Table 5.2 shows a specimen frequency distribution

table.

TABLE - 5.2

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Speed Class

kmph

Mean speed

kmph

Number of

Vehicles

Frequency Cumulative percent

From the results presented in Table 5.2 , cumulative speed distribution curve (Fig 5.4)

and frequency distribution curve of spot speeds (Fig 5.5) are plotted.

Fig 5.4. CUMULATIVE SPEED FIG 5.5. FREQUENCY DISTRIBUTION

DISTRIBUTION CURVE CURVE FOR SPOT SPEEDS

The cumulative curve (Fig 5.4) is most useful in determining the speed above or

below at which certain percentage of vehicles are travelling.

‘Medium speed’ is the middle or 50% percentile speed. At this speed there are as

many vehicles going faster as there are going slower.

‘The 85 percentile speed’ is the speed below which 85 percentile of vehicles are being

driven. This is the speed at which motorists drive considering the safety condition of the

highway. This speed is adopted for the ‘Safe speed’ limit at the zone . ‘The 98 percentile

speed’ is taken for ‘geometric design’ of the highway ‘The 15 percentile speed’ is the speed

value which should be utilised as the minimum speed on the highway. It has been found that

vehicles travelling below this value on high speed roads are potential accident hazards.

‘Model speed’ is obtained from the frequency distribution curve (Fig 5.5) and is the speed at

which the greatest number of vehicles travel.

(ii) Mathematical Analysis: In this analysis the arithmetic mean or average speed is

computed from frequency distribution table by multiplying the number of vehicles in each

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speed class by the mean speed of that class , summing up this amount and dividing this total

by the total number of vehicles observed. Mathematically it is expressed as.

x−

= fii=1

n

∑ xi / n 5.5

where x−

is the average speed (spot)

x−

I is the average speed of the ith speed interval.

fi is frequency of ith group.

n is the total number of observations.

5.6. SPEED AND DELAY STUDIES:

The object of speed and delay studies is to find the amount , location , duration ,

frequency , and causes of delay in the traffic stream as well as the amount of time spent in

actual motion from point to point along a given route. This study is carried out to analyse the

following.

(i) Congestion

(ii) Sufficiency ratings or congestion indices

(iii) Before and after improvement studies

(iv) Traffic assignment to new facilities

(v) Economic studies

(vi) Trend studies

(vii) Determination of efficiency of a street.

5.6.1. CAUSES OF DELAY:

The causes of delay can be classified into the following two categories:-

(i) Fixed delay: It occurs primarily at intersections. eg - Traffic signals , stop signs and rail-

road crossings.

(ii) Operation delay: It can be either due to internal friction within the stream or due to other

traffic movements as explained below.

Operational delay caused by internal friction within the stream is a result of (a)

congestion due to high volumes , (b) lack of capacity and (c) merging and weaving

manoeuvres.

Operational delay caused due to other traffic movement is a result of (a) parking and

unpacking of vehicles (b) turning vehicles (c) pedestrians (d) stalled vehicles and (e) double

parking.

5.6.2. METHODS OF SPEED AND DEALY STUDIES:

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The methods used are (i) Floating car method (ii) Elevated observer method and (iii)

Licence plate method.

(i) Floating Car Method: A test car is allowed to float with traffic. The test car should

overtake as many vehicles as possible. An observer is seated in the floating car with two stop

watches. One of the stop watches is used to record the times at various control points. The

other stop watch is used to find the individual delays. The times , locations and causes of

these delays are recorded by another observer on a suitable tabular form or by voice

recording equipment. In this method the detailed information is obtained concerning all

phases of speed and delay.

(ii) Elevated Observer Method: Observers are stationed at elevated points such as high

buildings from which a considerable length of road may be observed. Observers select

vehicles at random and record time , locations and causes of delay. It is difficult to secure

suitable points for observation all along the routes to be studied.

(iii) Licence Plate Method: Investigators stationed at control points along the route enter on

a time control basis the licence plate numbers of passing vehicles of selected sample. From

the office computations , travel time of each vehicle can be found. But the method does not

give important information such as causes of delays and the duration and number of delays

within the test section.

5.7. ORIGIN AND DESTINATION (O - D) STUDIES:

These studies are carried out to collect factual information about desired lines of

travel with a view to provide the most effective transportation system for the traffic. The

purpose of this study is to get answers to the following questions.

(i) Why people travel ? (Purpose of trip)

(ii) When people travel ? (time and direction)

(iii) How people travel ? (mode of transport - bus , car , cycle , foot etc.,)

(iv) Where the people want to go (Origin and destination)

(v) Where and why people stop ? ( To determine concentration of vehicles warranting

parking facilities).

These studies are most essential in planning new highway facilities and in improving

some of the existing systems.

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If this study , for example , indicates a high percentage of through traffic , a by-pass

can be considered. This survey also helps in solving zonal as well as regional traffic

problems. The uses of this study may be summarised as follows:

(i) To provide wide and better roads (express ways) along the maximum desired lines of

travel.

(ii) To plan by - passes and establish preferential routes for various types of traffic.

(iii) To plan public transportation systems in cities.

(iv) To evaluate the existing facilities and plan for an improvement or new facilities as the

case may be , and

(v) To fix dimensions and design standards for road bridges etc.,

5.7.1. METHOD OF COLLECTING O - D DATA:

Some of the popular methods of collecting O and D data are as follows:

(i) Road side Interviews with Drivers: Trained personal are stationed at pre-selected

stations. The vehicles are stopped and each driver is asked to answer the following questions

(sample questions are only given).

(a) Origin

(b) Destination

(c) Purpose of Trip

(d) Route selected to reach the destination

(e) Location of stops and purposes.

(f) Also note is made of the type of vehicles and

(g) number of passengers in the vehicles.

This method is quick , but the main drawbacks of this method are wastage of time of

traffic , possibility of traffic congestion and possible resentment from the road users.

(ii) Post-Card Survey - When the traffic is heavy and cannot be stopped long enough for

interviews , prepaid business reply Post Cards with return address and on which a

questionnaire to be filled in , along with a request to answer them and also the purpose of the

study are distributed to the drivers as they pass the station. The stations can be located where

the vehicles have to proceed slowly as at toll gates. A return of 10% or less is considered too

small. Return of well planned studies range from 25-50%. Further if the questionnaire is not

answered properly , the information hence compiled may not reflect the true picture.

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(iii) Home Interviews: In this method about 10% of the population is interviewed by trained

personnel. On the basis of the interview travel data are compiled and used for carrying out

necessary improvements and planning new strategy.

(iv) Work-Place Interviews: In this method the place of interview is shifted to the work

place so that time and manpower required for conducting the survey may be minimised. Trip

particulars could be collected from many employees at one instant.

5.7.2. PRESENTATION OF O - D DATA:

Two methods of presentation of O - D survey data O - D Matrix method and Desire

line chart method are discussed.

(i) O - D Matrix: This is the most convenient form , in which the origin zones and

destinations are presented as a matrix. (Fig 5.6). The horizontal axis of the matrix represents

the destination zones and the vertical axis of the matrix represents the origin zones. The zones

may be further classified into internal and external zones , if the survey covers both internal

and external zones. The number of trips are entered in the cells of the matrix. In this (Fig 5.6)

t2-3 represents the number of trips originating in zone 2 and terminating in zone - 3.

(ii) Desire - Line Chart: This is the most popular pictorial line representation by means of

desire line chart. In this chart the trips between any pair of zones are represented by a straight

line connecting the controids of the two zones and having a width drawn to a suitable scale to

represent the actual volume of traffic. A typical desire - line chart is shown in Fig 5.7.

Fig 5.6 D - Matrix

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Fig 5.7. Desire Line Chart

5.8. TRAFFIC CAPACITY:

The term capacity is used here to define the ability of a road to accommodate traffic

under given circumstances. Some of the terms related to traffic capacity studies are defined

below.

(i) Traffic Volume: This represents the number of vehicles moving in a specified direction

on a given road way that pass a given point during specified unit of time. Traffic volume is

expressed as vehicles per hour or vehicles per day.

(ii) Traffic Density: This is the number of vehicles occupying a unit length of lane of a road

way at a given instant , usually expressed as vehicles per kilometre.

Traffic Volume = Traffic Density × Traffic speed 5.6

The highest traffic density will occur when the vehicles are practically at a stand still

on a given route and in this case the traffic volume will approach zero.

(iii) Traffic Capacity is defined as the number of vehicles passing a point on a highway in a

unit period of time. In other words it is the ability of a road way to accommodate traffic

volume. It is expressed as vehicles per hour per lane or road way.

Capacity and volume are measures of traffic flow and have the same units. Volume,

represents an actual rate of flow and responds to variation in traffic demand where as

capacity indicates the maximum rate of flow with a certain level of service characteristics that

can be carried by the road way.

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(iv) Basic Capacity is the theoretical capacity of a lane or a road way. Two roads having the

same physical features will have the same basic capacity irrespective of traffic conditions , as

they are assumed ideal.

(v) Possible Capacity: This is defined as the maximum number of vehicles that can pass on a

lane or roadway during one hour under prevailing roadway and traffic conditions. This

capacity is generally much lower than the basic capacity of the road way as the prevailing

road way and traffic conditions and seldom ideal. Under the worst road way and traffic

conditions , the possible capacity of the road way may approach zero. When the prevailing

roadway and traffic conditions approach the ideal conditions , the possible capacity would

approach the basic capacity. Thus the possible capacity varies from zero to basic capacity.

(vi) Practical Capacity or Design Capacity: This is the maximum number of vehicles that can

pass a given point on a lane or road way during one hour without traffic density being so

great as to cause unreasonable delay , hazard or restrictions to the drivers freedom to

manoeuvres under the prevailing road way and traffic conditions. It is the practical capacity

that is of primary interest to the designer.

5.8.1. DETERMINATION OF BASIC CAPACITY:

Basic capacity of a single lane may be obtained from the relation

C = 1000V / S 5.7

where V is the speed of the vehicle in kmph.

S is the average centre of the spacing of vehicles in meters

C is the capacity of a single lane in Vehicles / hour.

The average spacing ‘S’ between centre to centre of vehicles is equal to the average

length of vehicles plus the clear spacing between the vehicles in the stream. Clear-spacing

may be assumed as the safe stopping sight distance with the reaction time assumed as 0.70 to

0.75 seconds. If the average length of the vehicles is ‘L’ (metre) ; then

S = Vt + L = 0.278 Vt + L 5.8

Considering a reaction time of 0.7 seconds (i.e.,) t = 0.7 seconds.

Smetres = (0.7 v + L) = (0.2V + L) 5.9

Theoretical capacity may also be obtained if the minimum time head way (Ht)

seconds is known. The time interval between the passage of successive vehicles moving in

the same line and measured from head to head as they pass a point on the road is known as

the time head way. It has been observed that with the increase in speed of the traffic stream ,

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the time head way decreases and after reaching a minimum value (Ht) at an optimum speed ,

starts increasing (Fig 5.8). The maximum theoretical capacity is then given by

C = 3600 / Ht where C is the capacity per hour 5.10

Fig 5.8 VARIATION OF MINIMUM SPACING AND HEADWAY WITH SPEED

The relationship between speed and maximum capacity of a traffic lane is shown in

Fig 5.9. The peak value of the theoretical maximum capacity is reached at an optimum speed.

As the speed is increased further , the maximum capacity of the lane starts decreasing due to

an increase in the time headway at the speed range.

Fig 5.9 Speed and Capacity

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The practical capacity of a traffic lane is affected by factors like lane width , lateral

clearance , width of shoulders , commercial vehicles , alignment , presence of intersections ,

stream speed , number of lanes of traffic movement , driver characteristics and composition

of traffic etc.,. The practical capacity values suggested by the IRC for the purposes of design

of different types of roads in rural areas are given in Table 5.3.

Table 5.3 Capacity of Roads in Rural Areas:

Type of Road Capacity PCU / day Both Directions

1. Single lane with 3.75m wide carriage way

and normal earth shoulders

1000

2. Single lane roads with 3.75m wide carriage

way and 1.0m wide hard shoulders

2500

3. Roads with intermediate lanes of width 5.5m

with earth shoulders.

5000

4. Two lane roads with 7.0m wide carriage

way and earth shoulders

10000

5. Four lane divided highway (depending on

traffic access control etc.)

20000 to 30000

On typical urban roads of pavement width 7.0 to 7.5m , the practical capacity may be

assumed to be 600 to 1100 PCUs per hour depending on the various factors affecting the

capacity.

5.9. ROAD ACCIDIENTS - CAUSES AND PREVENTION:

The spectacular increase in the number of motor vehicles on the road has created a

major special problem - road accidents. Road accidents may involve property damage ,

personal injuries or even causalities. The traffic engineer is concerned because many features

of the highway affect the safety of the vehicle and other road users. The analysis of the

accident statistics provides clues to the many factors that lead to the accidents and to the

improvements that may be desired. Based on the statistics the traffic engineer must devise

ways to reduce the accidents through better planning , design , construction , maintenance

and traffic operation. The traffic engineer is also concerned about the regulation and

management of traffic to ensure safe travel. Accident data supply valuable information to

control , regulate and manage the traffic more efficiently. The cost of traffic accidents help

the traffic engineer in evaluating an improvement scheme aimed at reducing the accidents.

5.9.1. CAUSES OF ACCIDENTS:

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The road accidents may be caused due to a combination of several reasons and seldom

due to a particular reason. Hence it is often not possible to pin-point a particular single cause

of an accident. However , it has been established that there are four basic elements involved

in traffic accidents.

(i) The road users (ii) the vehicles (iii) the road and its condition and (iv) miscellaneous

factors. These factors are discussed.

(i) Road User: The road users responsible for the accident may be the driver of one or more

vehicles involved , pedestrians or the passengers. Accidents may occur due to the mistakes ,

carelessness and deliberate actions of the road users. Excessive speed and rash driving ,

carelessness , violation of rules and regulations , failure to see and understand the traffic

situation , sign or signal , temporary effects due to fatigue , sleep or alcohol on the part of the

driver may lead to accidents. By violating regulations and by careless use of the carriage way

meant from vehicular traffic , pedestrians are responsible for road acceidents. Alighting or

getting into moving vehicles by the passengers may be also lead to accidents.

(ii) Vehicle: Vehicle defects such as failure of brakes , steering system , tyre bursts or

lighting system and any other defects in the vehicle may lead to road accidients.

(iii) Road Conditions: Surface conditions of the road like slippery surface , pot holes and

other defects have been responsible for road accidents. Defective geometric design of the

highway , improper lighting and improper traffic control devices also have been responsible

for road accidents.

(iv) Miscellaneous Factors: Un favourable weather conditions , stray animals on the road ,

badly located advertisement boards or service stations etc ., may also lead to road accidents.

5.9.2. ACCIDENT STUDIES AND RECORDS:

The usefulness of an accurate and comprehensive system of collection and recording

accident data cannot be over emphasized. Such data serves to identify the spots prone to

accidents and to suggest means of overcoming the deficiancies that lead to accidents.

The various phases in accidient studies are collection of accident data , preparation of

reports , location files and diagrams and the application of the above records for suggesting

preventive measures.

(i) Collection of Accident data: This is the first step involved in accident studies. Standard

forms are available for this purpose. Some of the details to be collected are date and time of

accident , classification of accident , location , primary causes , cost of accident , nature of

accident etc.

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(ii) Accident Reports: The accident data collected as given above is furnished in the form of

a report with all facts which might be useful in the subsequent analysis. The accidents should

be reported to the police authorities who would take legal action especially in more serious

accidents involving injuries , casualities , or severe damage to property.

(iii) Accident Records: These include all particulars of the accident , location and other

details. These records are maintained by means of location files , spot maps , condition and

collision diagrams.

(a) Location Files: These are useful to identify locations of high accident incidence and to

keep a check on the places. These files should be maintained by each police station for the

respective jurisdiction.

(b) Spot Maps: These show the accident spots. Pins or symbols are used on these maps. The

common legend used for spot maps are as given below.

(c) Condition Diagrams: A condition diagram is a drawing showing all important physical

condtions of an accident location to be studied.

Fig 5.10 LEGEND FOR SPOT MAPS

The important features generally to be shown are road way limits , kerb lines , bridges ,

culverts , trees and all other details of the road way conditions , obstructions to vision ,

property lines , signs , signals etc.,. There are standard sysmbols used in showing various

details. The condition and collision diagrams may be combined together in a single sketch , if

necessary.

(d) Collision Diagram: This diagram (Fig 5.11) is the most valuable tool since it indicates

graphically the nature of the accident recorded at any particular location. The diagram need

not be drawn to scale , but should show by arrow indications the movements which led to

each accident. The data and houe of ech accident are shown along side of one of the arrows.

If weather and visibility conditions are important these are also indicated. Collision diagrams

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, are most useful to compare the accident pattern before and after the remedial measured have

been effected.

5.9.3. ACCIDENT PREVENTION:

The variious measures to decrease the accident rates may be divided into three

groups:

(i) Engineering (ii) Enforcement and (iii) Education

(i) Engineering - Engineering measures for prevention of accidents include.

(a) Proper design (geometric) of streets and highways.

(b) Before and after studies of preventive measures to study their efficiency

(c) Road lighting , and

(d) Maintenance of vehicular defects.

(ii) Enforcement - The various enforcement measures include:

(a) Speed zoning

(b) Traffic laws and ordinances

(c) Traffic signals

(d) Traffic markings

(e) Channelization

(f) Pedestrian control

(g) Control of parking

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Fig 5.11 Collision Diagram

(iii) Education - This includes in making the future citizen traffic minded by children

education. The IRC has recommended to provide ‘traffic training parks’ in cities and towns

for the traffic education of children. Traffic education can also be imparted with the help of

T.V., films and documetaries and imposing traffic safety week. Posters exhibiting the serious

results due to carelessness of road users may also be useful. Training classes may be

conducted for the drivers. The IRC has been organising ‘Highway Safety Workshops’ in

different regions of the country.

The three measures discussed above to prevent road accidents , namely Engineering ,

Enforcement , and Education , are generally termed as “3 - Es” of preventing road accidents.


1. Discuss the various traffic studies and their importance.

2. Explain spot speed , running speed , space-mean speed , time mean speed and average

speed.

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3. Distinguish among the following - Traffic volume , Traffic capacity , basic capacity ,

possible capacity and practical capacity.

4. How do you arrive at the following speeds ? (a) Safe speed (b) Speed for geometric design

of highway elements.

5.11. SUMMARY:

Traffic engineering deals with the improvement of traffic performance to achieve

efficient , free , safe and rapid flow of traffic. To achieve these objectives a number of studies

have to be carried out to collect data regarding the traffic operations. These traffic studies are

known as “Traffic Surveys’ or ‘Traffic Census’.

Some of the important traffic studies that are carried out are traffic volume studies ,

speed studies , speed and dealy studies , origin and destination studies , capacity studies and

traffic accident studies.

Main objectives , popular methods of carrying out these traffic studies , presentation

and analysis of the data obtained have been explained under each group.

5.12. REFERENCES:

1. Kadiyali , L.R. (1978). ‘Traffic Engineering and Transport Planning , Khanna Publishers ,

Delhi.

2. Kadiyali , L.R. (1984) - Principles and Practice of Highway Engineering ‘Khanna Tech.

Publishers , Delhi.

3. Khanna , Dr. S.K and Justo , Dr. C.E.G - (1991) - Highway Engineering’ , Nimchand and

Bro., Roorkee.

4. O’Flaherty , C.A. - (1974) - “Highways - Vo12 , Highways and Traffic’ - Edward Arnold ,

London.

***


UNIT - 6

TRAFFIC ENGINEERING - II CONTENTS:

AIMS / OBJECTIVES

6.1. Introduction.

6.2 Traffic Regulation Measures

6.3. Traffic Control Devices

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6.4. Road Intersection Design - At Grade Intersection

6.5. Grade Separated Intersections


6.7. Summary

6.8. References

AIMS / OBJECTIVES:

Highway intersections are places of traffic hazards because of the complex

characteristics of traffic at these places. The intersections have to be designed well and traffic

has to be controlled and regulated in order to effect safe , free and efficient movement of

traffic. Principles of traffic regulation and control and the design principles of intersections

are discussed.

6.1. INTRODUCTION:

For the safe traffic operation on highways adequate regulations and controls have to

be imposed. Traffic regulations should be framed in such a way that effective control on

drivers , vehicles and other road users may be exercised. Compliance of traffic regulations

and laws are obligatory for all road users. These regulations should be rational and should

evoke respect by road users and not be disregarded. Highway traffic regulations include

measures like control both on the driver and the vehicle , enforcement of certain basic rules

for use and conduct and one way streets. Traffic signs , traffic signals , road marking and

traffic islands are used for controlling the traffic on the streets and at the intersections.

On straight roads there is no problem in traffic movements. At intersections traffic

movements are quite involved because the traffic has to perform through , turning and cross

movements. Highway intersections are thus points of traffic hazard which effect the safety ,

efficiency , capacity and cost of operation of the whole road. Hence road intersections have to

be designed on scientific principles in order to improve the efficiency of the road system and

to avoid possibilities of road accidents. Intersection problems are generally unavoidable when

all the approaching roads at an intersection are at the same level ; these are called

‘Intersections at Grade’. However , on express highways , these problems are avoided by

providing ‘Grade Separated Intersections’.

6.2. TRAFFIC REGULATION MEASURES:

Traffic regulation and controls aim at reducing congestions and accidents. They , even

though considered as an encroachment on the rights of the individuals , are necessary for

public safety , welfare and convenience. A careful and detailed study of the facts from traffic

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surveys , accident studies , driver records etc., are necessary for developing effective methods

of traffic regulations. Some of the traffic regulatory measures are discussed.

6.2.1. CONTROL OF BOTH DRIVER AND VEHICLE:

This is achieved by control of vehicle ownership , vehicle inspection , vehicles size ,

vehicle weight , vehicle registration , route permit and driving licences.

6.2.2. ENFORCEMENT OF BASIC GENERAL RULES:

Certain basic general rules are laid down for road use and conduct. Non-compliance

of these rules is an offence. In this regard , certain regulations are imposed on the vehicle

speed , overtaking and passing , turning and stopping , parking on roads etc. In India , traffic

flow regulation is ‘Keep to left’. Over taking should always be done from the right side. The

traffic flow in traffic round abouts and other control islands should be regulated in a

clockwise direction.

6.2.3. ONE-WAY STREETS:

To ensure smooth flow of traffic in congested streets one way system of traffic flow

should be enforced. One way street system reduces delay to the vehicle and improves the

facility of traffic movement. It increases the capacity of a street , permits higher operating

speeds and results in a reduction in the number of accidents. However , the severity of an

accident , if any , may be more. It also results in economic savings due to lowering of

motorists journey time and lessening need for police control. Such regulations are possible

only when there is a network of roads connecting two bigger roads so that the additional

distance to be travelled by some vehicles through these one way streets is not much.

6.3. TRAFFIC CONTROL DEVICES:

The various aids and devices used to control , regulate and guide traffic may be called

‘Traffic Control Devices’. The most common among these are (i) signs , (ii) signals (iii) road

markings and (iv) traffic islands. These are explained below.

6.3.1. TRAFFIC SIGNS:

Traffic signs perform a number of functions:

(i) They give timely warning of hazardous situations when they are not self-evident.

(ii) They are of great help in regulating traffic and

(iii) They give information as to highways routes , directions and places of interest.

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These traffic signs should be backed by law in order to make them useful and

effective. These are divided into the following three types:

(a) Danger signs , also know as Warning Signs.

(b) Signs giving definite instructions also known as Regulatory signs. These are further

divided into (i) Prohibitory Signs and (ii) Mandatory Signs and

(c) Information Signs.

(a) Danger Signs or Warning Signs or Cautionary Signs:

Danger signs are used when it is deemed necessary to warn traffic of existing or

potentially hazardous conditions on or adjacent to a highway or street. The use of warning

signs should be kept to a minimum , because their unnecessary use tends to bread disrespect

for all signs. The warning signs recommended by the IRC are illustrated in Fig 6.1. The side

of the triangle is 900mm for a standard size and 600 mm for a reduced size. These signs have

a red border and the symbols indicated there in are in black colour against a white back

ground.

(b) Regulatory signs: These signs are intended to inform the highway users of traffic laws or

regulations. These are of two types:

(i) Prohibitory Signs: These signs given definite negative instructions prohibiting the

motorist from making particular manoeuvres. eg: over taking prohibited , one-way traffic ,

waiting restrictions , restrictions on dimensions , speed of vehicle etc. These are of circular

shape , with a dia. of 600mm for signs of the standard size and 400mm for reduced size. The

signs have a red border. The colour of the background is white for speed control and blue for

other signs. The symbols are black in colour on white background and white in colour on

blue background. These signs are illustrated in Fig 6.2.

(ii) Mandatory Signs are part of regulatory signs and are intended to convey definite positive

instructions when it is desired that motorists take some positive action. The two important

mandatory signs are the STOP sign and YIELD or GIVE WAY sign Fig. 6.3. The stop sign

requires all the vehicles to come to a halt before the stop line. Stop sign is an octagon with a

white border and a red background , the side of the octagon being either 900 mm or 600 mm.

It shall be used in combination with a definition plate carrying the message ‘STOP’.

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Fig 6.1 WARNING SIGNS

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Fig 6.2 REGULATORY SIGNS

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THE YIELD or GIVE WAY sign (Fig 6.3) is a downward pointing equilateral

triangle having a red border and a white background. The side of the equilateral triangle is

either 900 mm or 600 mm. It shall be used in combination with a definition plate carrying the

message “GIVEWAY”. This sign is used to assign right-of way to traffic on certain

approaches to an intersection. Vehicles controlled by a YIELD sign need stop only when

necessary to avoid interference with other traffic.

(c) Informatory Signs: These signs are intended to guide the road user and to give such

information as may be of interest during travel. An informatory sign is made of a rectangular

board of specified size. The common informatory signs used are ‘Road Junction approach’ ,

End of speed limit; and ‘Parking Sign’.

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Fig 6.3 STOP AND GIVE WAY SIGNS

These informatory signs are shown in Fig 6.4. Route marker signs are provided before

intersections , particularly to indicate the National Highway route (Fig 6.5).

Fig 6.4. INFORMATORY SIGNS

Locations of Traffic Signs: In India , the signs are located on the left side of the road. The

signals are mounted at a height of 1.5m for unkerbed roads and 2m for kerbed roads.

Stop Signs: To be located at the point where the vehicle has to stop or as near thereto as

possible - say 3 metres ; in the case of pedestrians crossings 1.2m ahead.

Give way sign: To be located as near to the point where the vehicle is to stop - say at a

distance 3m.

Warning Sign: In the case of urban location 50m ahead of the hazard and non-urban

locations depends on the type of terrain and importance of the road.

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FIG 6.5. ROUTE MARKER SIGN

6.3.2. TRAFFIC SIGNALS:

These are provided at large intersections for safe and efficient movement of vehicular

traffic and pedestrians. At small intersections , traffic is controlled by the traffic police ,

alternately showing ‘stop’ sign to the incoming roads.

Advantages of traffic signals are:

(i) Movements of traffic at the intersections is more orderly and safe.

(ii) Traffic handling capacity at intersections is increased.

(iii) They improve the quality of traffic flow as compared to the police control.

(iv) The signal indications can very easily be understood even in foggy weather or at night.

Disadvantages of traffic signals are:

(i) They generally increase the total vehicle delay at intersections during off-peak hours.

(ii) They cause an increase in rear-end collisions.

(iii) Electric power failure of the signal installations may lead to serious and wide spread

traffic difficulties.

A typical signal head is shown in Fig 6.6. Traffic signals have three coloured light

glows facing each direction of traffic flow. The red light is meant for STOP , the green light

indicates GO and the amber of yellow light shows CLEARANCE TIME for the vehicles

which enter the intersection area by the end of green time , to clear off. These are placed on

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the left hand side of the road and are illuminated in such a way that they are visible from a

distance 0.4 km under normal atmospheric conditions. Based on their operating principles

and functions traffic signals are classified into the following groups.

Fig 6.6. TRAFFIC SIGNALS

(i) Traffic Control Signals: (a) Fixed - time signals , (b) Manually operated Signals and (c)

Traffic Actuated (Automatic) Signals.

(ii) Pedestrian signals , and

(iii) Special Traffic Signals.

6.3.3. Road markings:

Traffic markings are made of lines , patterns words , symbols or reflecters on the

pavement , kerbs , sides of islands or on fixed objects near the road way. Traffic marking may

be considered as special signs intended to control , warn , guide or regulate the traffic. The

markings are made using paints in contrast with the colour and brightness of the pavement or

other back ground. Light reflecting paints are also used for traffic markings. In order to

ensure that the markings are seen by the road users , the longitudinal lines should be atleast

10cm thick and the transverse lines should be made in such a way that they are visible at

sufficient distance in advance to give road users adequate time to respond.

The various types of markings may be classified as:

(a) Pavement markings (b) Kerb markings (c) Object markings and (d) Reflector unit

markings.

(a) Pavement Markings: These are of the following categories:

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(i) Centre line , (ii) Traffic lane lines , (iii) No overtaking zone markings , (iv) Obstruction

approach markings. (v) Stop lines , (vi) Pedestrian crossings (vii) Cyclist crossings , (viii)

Route direction indications (ix) Word messages (x) parking space limits , (xi) Bus stops etc.

In general solid lines are restrictive in nature and it is an offence to cross or straddle

such a line. Broken lines are permissive in character , and vehicles can cross such a line

provided this can be done with safety. All carriage markings except those intended for

parking restrictions are made in white colour. Yellow paint is used for markings intended for

parking restrictions and continuous centre and barrier line markings.

(b) Kerb Markings: These may indicate certain regulations like parking regulations. To

increase the visibility from a long distance kerbs of all islands are normally painted with

alternate black and white stripes.

(c) Object Markings: Obstructions on or near the road way are hazardous and hence should

be marked properly. Typical obstructions are supports for bridges , signs and signals , level

crossings , narrow bridges , culvert head walls etc. Obstructions in the carriage way are

marked by alternate black and white stripes , sloping down at an angle of 450 towards the side

of the obstruction on which traffic passes. When the vertical clearance of an under pass is less

than the prescribed minimum , the clearance is marked by vertical stripes alternatively black

and white.

(d) Reflector Unit Markings: These are used as hazard markers and guide markers for safe

driving during night. Hazard markers reflecting yellow light should be visible from a long

distance of about 150 metres.

6.3.4. TRAFFIC ISLANDS:

Traffic islands are raised areas constructed within the roadway to establish physical

channels through which vehicular traffic may be guided. Traffic islands may be classified

based on their functions as (i) Division Islands , (ii) Channelising Islands and (iii) Rotaries.

Division islands have been dealt with already (Art 3.3) , channelising islands and

rotaries are discussed in article 6.4.

6.4. ROAD INTERSECTION DESIGN:

Road intersections can be either Intersections at grade or Level or Grade separated

intersections. In the case of at grade or level intersections approaching roads meet at one

level , but in the case of grade separated intersections , intersecting roads are separated in

level by passing one road over or below the other.

6.4.1. AT GRADE INTERSECTIONS:

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In the case of at grade intersections , traffic have to perform through , turning and

crossing movements. As such , intersections are points of traffic hazards which effect safety ,

speed , efficiency , capacity and cost of operation of the road. Hence design of road

intersection is vital for improving road system and to avoid possibilities of road accidents.

Along with vehicular traffic , pedestrian traffic has to be tackled at the road intersections.

Fig 6.7. CONFLICTING AREAS

At grade intersection , how best it may have been designed , traffic speed has to be

reduced. More over , several conflicting points are developed where possibilities of accidents

always persist. Depending on the number of conflicting points , the common area at the

intersection may be divided into major conflicting area , and minor conflicting area ;

especially major area should be reduced to a minimum. This is obtained by the construction

of channelising islands as shown in Fig 6.7.

6.4.2. BASIC REQUIREMENTS OF INTERSECTIONS:

(i) At the intersection the area of conflict should be as small as possible.

(ii) The relative speed and particularly the angle of approach of the vehicles should be small.

(iii) Adequate visibility should be available for vehicles approaching the intersection.

(iv) Sudden change of path should be avoided.

(v) Various geometric features should be adequately designed.

(vi) Proper signs should be provided on the roads approaching the intersection.

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(vii) Good lighting at night is desirable.

(viii) At large intersections special provisions should be provided for handling the cyclists

and pedestrians separately.

6.4.3. TYPES OF GRADE INTERSECTIONS:

Intersections are classified into different categories based on the geometry of the

intersecting roads and based on the type of channelisation provided.

(a) Based on Geometry of Intersecting Roads: Various types of intersection are shown in

Fig 6.8.

Fig 6.8. FORMS OF INTERSECTIONS

(b) Unchannelised Intersections: They are the simplest type but most dangerous and

inefficient in traffic operation. There is absolutely no restriction to vehicles to use any part of

the intersection area ; there is increase in the maximum conflicting area resulting in more

number of accidents unless controlled by a police man. These unchannelised intersections

may be either plain or flared (Fig 6.9). In the case of plain intersections no additional

pavement width is provided for turning movements and therefore they are the most

economical form of intersections. On the other hand , in the case of flared intersections the

width of intersecting roads is increased at the junction. The extra widths are usually provided

on either side of the intersection carriage way area. Since the paths of turning vehicles are not

restricted or controlled , there is possibility for reduction in the number of accidents.

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Fig 6.9. UNCHANNALISED INTERSECTIONS

(c) Channelised Intersections: These are achieved by introducing islands into the

intersectional area , thus reducing the total conflict area available in the unchannelised

intersection. These islands are normally triangular in shape and help to channelise the turning

traffic , to control their speed and angle of approach and to decrease the conflict area at the

intersection.

Channelisation islands should be so located with respect to the approach that their

visibility is clear and the intended path is automatically selected. It is better to provide small

number of larger sized islands rather than large number of small sized islands. The minimum

desirable size of island is 5 square metres in area , and it is better to have an area of 8m2 .

These islands also serve as refuge islands for pedestrians and location for other traffic control

devices. Channalisation may be either partial or complete with divisional and directional

islands and medians (Fig 6.10). From traffic operation point of view there is better control on

the traffic entering or leaving the intersection and these are considered superior to all paved

types. However , one of the crossing vehicles will have to stop while the other proceeds.

Fig 6.10 CHANNELISED INTERSECTION

(d) Rotary Intersections: It is a channelised road intersection , where all the traffic from

approach roads is made to move round a large control island in a clockwise direction before

weaving out into their desired direction of road radiating from the rotary. Necessity of

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stopping at road intersection is eliminated. All the traffic streams merge into a common

stream around the centre island and then diverge out to the desired , radiation road. On rotary

, crossing conflicts are completely eliminated (See Fig 6.11).

Design Factors of Rotary:

In designing a rotary the following elements are considered.

(i) Design speed: As the vehicles approach the rotary with reduced speed , rotaries on Indian

roads are designed for a speed of 40 kmph in rural areas and for a speed of 30 kmph in urban

areas.

Fig 6.11. ROTARY INTERSECTION

(ii) Shape of Central Island: The shape of the central island depends on the number and

layout of the intersecting roads around it. The central islands should be without any corners

and if corners are provided they should be provided with large radius. The various shapes

considered to suit different conditions are circular , elliptical , turbine and tangent shape (Fig

6.12). If all the roads are equispaced around the rotary and carry equal importance , circular

central island is the most suitable. When the layout of the intersecting roads is four or more ,

an elliptical shape is preferred. In such a case , the major axis of the ellipse is along the axis

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of the import roads. Turbine shape forces reduction in speed of vehicles and enables speeding

up of vehicles going out. At night the head light glow is a limitation of the design.

Fig 6.12. SHAPES OF ROTARY ISLANDS

(iii) Radius of Rotary: If the centre island is circular it will have a constant radius throughout

its perimeter. But if it is of any other shape , it will have different radii at different locations.

Super elevation is normally not provided at the road around the centre island and it is only the

friction that will be counteracting the centrifugal force.

Hence the radius of the curve ® is given by

R (metres) = V2 / 127 f 6.1

where f is the coefficient of friction taken as 0.43 and 0.47 for speeds (V) of 40 and 30 kmph

respectively after allowing a factor of safety of 1.5.

(iv) Weaving Angle and Weaving Distance: The angle between the paths of vehicles

entering the rotary from a particular radiating road and that of another vehicle leaving the

rotary on an adjacent road is known as weaving angle (Fig 6.11). The value of the weaving

angle should be kept small , but not less than 150 .

Vehicles entering the rotary from a road and leaving towards another radiating road

have to first merge into one way traffic flow in the roatary roadway around the central island

and than weave out to diverge from the flow to the required road outlet. The weaving

operation including merging and diverging can take place between two channelising islands

of adjacent intersecting legs ; and the length of the roadway is known as ‘Weaving length ‘

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(Fig 6.11). The recommended values of minimum weaving lengths are 45 m and 30 m for

speeds of 40 kmph and 30 kmph respectively. The maximum weaving length may be taken as

twice the above minimum recommended weaving length.

(v) Width of Rotary Road Way: The minimum width of the roadway between the edge of

the central island and adjoining kerb is the effective width of the rotary or of the weaving

section and this by and large determines the capacity of the rotary.

The width of non-weaving section e2 of the rotary should be equal to the widest single

entry to the rotary and should generally be less than the width of weaving section. The width

‘W’ of the weaving section of the rotary should be one traffic lane wider than the mean width

of entry (e1) and nonweaving section (e2). That is

W = (e1 + e2) / 2 + 3.5 m 6.2

(vi) Channelising Islands: These should be provided at the entrance and exit of the rotary to

prevent undesirable weaving and turning and to reduce the area of conflict. These islands also

help in forcing the vehicles to reduce their speed to the design speed of the rotary and to serve

as a convenient place foe erecting traffic signs and as a pedestrian refuge. They have kerbs of

15 to 21 cm high.

(viii) Sight Distance and Grade: The sight distance at the rotary should be as large as

possible and in no case less than the safe stopping sight distance for the design speed. It is

preferable to locate a rotary on level ground. It may also be located on the area which is on a

single plane , with the slope not exceeding 1 in 50 with the horizontal.

(viii) Lighting: The minimum lighting required is one each on the edge of the central island

facing each radiating road (points A in Fig 6.11). Additional lights ‘B’ may be provided when

the central island is larger than 60m dia. Lights ‘C’ may also be provided near the entrance

curves if the pedestrians are large in number.

(ix) Traffic signs: The standard traffic (warning) sings indicating the presence of rotary

intersection should be installed at all approaching roads. At night a red reflect or red light is

placed at about one metre above the road level on the nose of each directional island and on

the kerb of the central island and channelising islands are marked by alternating black and

white stripes to improve visibility.

(x) Provision for Cyclists and Pedestrians: Pedestrians cause hindrance to the free flow of

vehicular traffic. As such they should be isolated from the vehicular traffic using rotary by

providing subways or overbridge , though they prove to be costly.

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Recommendation of Traffic Rotaries: Traffic rotaries should be recommended when the

total volume from all the incoming intersecting roads exceed 500 vehicles / hour , but not

more than 5000 vehicles / hour. Under conditions of mixed traffic , the IRC recommends that

traffic rotaries should be installed when the vehicular traffic (motor traffic) is about 50% or

more of the total traffic , on all the intersecting roads. If fast right turning traffic is more than

30% of the total traffic , rotary is again justified.

Advantages of Traffic Rotaries: Some of the important advantages of traffic rotaries are:

(i) The traffic handling capacity of a rotary is higher than that of any other type of at-grade

intersection.

(ii) It makes all the radiating roads to carry traffic almost to their full capacity.

(iii) It is the safest of all types of at-grade intersections.

(iv) Traffic coming from and turning to any direction gets equal preference.

(v) Operational cost of vehicles is smallest because vehicles do not have to stop and re-start.

(vi) Rotary functions by itself and there is no need of traffic pole or signals to control the

traffic.

(vii) Number of accidents and severity of accidents are low because of the relative low speed

of vehicles.

(viii) When the number of intersecting roads is 4 to 7 , rotaries can be constructed with

advantage.

Disadvantages: Limitations of traffic rotaries are as follows:

(i) Area required for the construction of traffic rotaries is very large and hence may not be

available in built-up areas.

(ii) If the number of intersecting roads is more than 7 , traffic rotaries are unsuitable.

(iii) Extra length to be traversed by right turning traffic and crossing traffic is quite large and

they may try to violate the traffic regulations by adopting to short cuts.

6.5. GRADE SEPARATED INTERSECTIONS:

These types of intersections generally have very large initial cost and cause the least

delay and hazard to the crossing traffic. The grade separation is achieved by means of vertical

level separation of the intersecting roads by means of a bridge , thus eliminating all crossing

conflicts at the intersection. The grade separation structure may be an ‘over pass’ or an ‘under

pass’. Transfer of route at the grade separation is provided by interchange facilities consisting

of ramps. Interchange ramps may be classified as direct , semi-direct or indirect as shown in

Fig 6.13. The direct interchange ramp involves diversion and merging from the right side.

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Semidirect ramp allows diverging to left , but merging is from the right side. In the indirect

method of interchange ramp , diverging and merging is from the left. In this case , there are

less hazards than diverging from right and merging from right. But the distance to be

traversed in the indirect interchange is more. Gradient on ramps should be limited to 4 to 6

percent.

Fig 6. 13 INTER CHANGE RAMPS

Advantages and Dis-advantages:

Following are the advantages of grade separation:

(i) Maximum facility is given to crossing traffic. There is increased safety for turning traffic

also.

(ii) There is increased comfort , saving in travel time and vehicle operation cost.

(iii) Capacity of grade separated intersections can practically approach that of the two cross-

roads.

Disadvantages of Grade separation are

(i) This type of interchange is very costly.

(ii) In built up or urban areas or where the topography (flat or plain terrain) is not favourable

; construction of grade separation is costly , difficult and undersirable.

6.5.1. GRADE SEPARATION STRUCTURES:

Various types of bridge structures are used to separate the grades of two intersecting

highways. There should be vertical clearance of atleast 4.3 metres and if double decked

vehicles are anticipated , the clearance should be 5.2 metres. The grade separated intersection

may be an OVERPASS or an UNDER PASS.

When the major highway is taken above the general ground level by raising its profile

by an embankment and an over bridge it is called on ‘Over Pass’. On the otherhand , if the

highway is taken depressing it below the ground level to cross another road by means of an

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under-bridge it is known as an Under Pass’. Choice of over pass or underpass depends on the

topography , vertical alignment , drainage , economy and preferential aspects for one of the

highways. An overpass gives drivers less feeling of restriction and confinement and less

problems of drainage. On the other hand an underpass may be more advantageous where the

major road can be built close to the existing ground surface. An overpass allows for stage

construction where as in the case of under pass there is no possibility of stage construction.

6.5.2. INTERCHANGES:

Grade separated intersection with complete interchange facilities is essential to

develop a highway with full control of access. Some of the types of interchanges are shown

in Fig 6.14. Of all these complete clover leaf fulfils all the requirements of turning traffic

involving simplest traffic manoeuvres , namely , diverging to the left and merging from the

left by providing four indirect ramps.

Fig 6.14 DIAGRAMMATIC EXAMPLES OF TRAFFIC MOVEMENTS AT

INTERCHANGES

6.6. SELF-ASSESSMENT QUESTIONS:

(i) List out the common types of pavement markings.

What is the significance attached to ,

(i) a single broken line in white and

(ii) continuous line in yellow.

(2) What is the difference between a prohibitory type of regulatory sign and a mandatory type

of regulatory sign. Give atleast one example for each type.

(3) What are the merits of channelisation of an intersection ?

(4) Discuss the merits and de-merits of different types of interchange ramps used in grade -

separated intersections.

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6.7. SUMMARY:

For safe operation of traffic on a highway adequate regulations and controls have to

be imposed on the traffic. Regulatory measures include methods like imposing control on

both the vehicle and driver , enforcing certain basic rules for road use and conduct and

introduction of one-way system of traffic flow in congested streets. The various aids used to

control , regulate and guide the traffic are known as Traffic Control Devices. These include

traffic signs and signals , road markings , traffic islands and rotaries. Various aspects of these

traffic control devices such as classification , location and design principles have been dealt

with in detail.

Road intersections can be either intersections at-grade (Level) or grade-separated

intersections. In the case of at-grade intersections approaching roads meet at on a level ; but

in the case of grade separated intersections , intersecting roads are separated in level by

passing one road over or below the other by means of a bridge.

At grade intersections the traffic movements are quite complex and effect safety ,

speed , efficiently and capacity of operation of the road. Hence design of a road intersection

is vital for improving the road system and for avoiding possibilities of accidents. This is

achieved by reducing the conflicting points to a minimum by judicious design and location of

various types of traffic islands. Rotary intersections are the safest type of all the at-grade

intersections where crossing conflicts are completely eliminated. These have highest traffic

handling capacity also.

Grade separated intersection may be an over pass or an underpass. In the case of an

overpass the major highway is taken above the level of another by means of an overbridge

and an embankment. On the other hand , if the major highway is taken depressing it below

the ground level to cross another road by an underbridge , it is called an under pass. Choice of

overpass or underpass depends on the topography , vertical alignment , drainage , economy ,

and preferential aspects for one of the highways. Transfer of routes at a grade separation is

effected by means of ramps. Interchange ramps may be classified as direct , semi-direct or in-

direct. The indirect ramps are to be preferred as the traffic movements are less hazardous as

the diverging and merging of traffic is from the left. Complete clover leaf fulfils all the

requirements of turning traffic.

6.8. BOOKS OF REFERENCE:

1. Kadiyali , L.R. (1978) - ‘Traffic Engineering and Transport Planning’ - Khanna

Publications , Delhi.

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2. Khanna , Dr. S.K. and Justo , Dr. C.E.G. - (1991) ‘Highway Engineering ‘ , Nem Chand

and Bros , Roorkes.

3. O’ Flaherty , C.A. (1974) - “Highways - Vol - 1 , Highways and Traffic” - Edward Arnold

, London.

4. IRC: 65 - (1976) - “Recommended Practice For Traffic Rotaries”.

***


HIGHWAY MATERIALS - I

UNIT - 7

SUB-GRADE SOIL AND MINERAL AGGREGATES CONTENTS:

Aims / Objectives

7.1. Introduction

7.2. Sub-grade soil

7.3. Mineral Aggregate

7.4. Summary


7.6. References

AIMS / OBJECTIVES:

A variety of materials are used in the construction of highway and air field

pavements. These materials include soil , aggregates , bituminous materials and cement. A

number of tests have to be carried out on these materials to determine their strength

characteristics and also their suitability for road construction. Some of the important tests

specified for the sub-grade soil and mineral aggregates are discussed.

7.1. INTRODUCTION:

Soil is the foundation material for all highways , whether it be in the form of

undisturbed in-situ sub-grade materials or transported and reworked embankment material. In

addition to the highway pavement itself , the flanking shoulders as well are very often

composed of the soil. Thus the soil plays the most important role in the highway construction

and it is axiomatic that the highway engineer should have a thorough understanding of soils

and how they behave.

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Mineral aggregates are the basic materials of highway pavement construction. Not

only do they support the stresses occurring within the pavement , but in addition the

aggregates in the road surface must resist wear due to abrasion by traffic as well as the direct

weathering effects of natural elements. Therefore the properties of the aggregates are also of

considerable significance to the highway engineer.

7.2. SUB-GRADE SOIL:

Sub-grade soil provides support to the pavement from beneath and as such the sub-

grade should possess sufficient stability under adverse climate and loading conditions. Soil

when used in embankment construction , in addition to stability , should be incompressible.

Soil is used as a binding material also. Compacted soil and stabilised soil are often used in

sub-base or base-course of highway pavement. As such , soil is one of the principal highway

materials.

7.2.1. Classification of Soils: The purpose of soil classification is to arrange various types of

soils into groups according to their engineering properties and various other characteristics.

Soils possessing similar characteristics can be placed in the same group. For general

engineering purposes soils may be classified by the following systems.

1. Particle size classification

2. Textural classification

3. Highway Research Board (HRB) classification , and

4. Unified soil classification and I.S. Soil classification.

The unified soil classification system and I.S. soil classification system are based on

both grain size analysis and plasticity properties of the soil and are applicable to any use. The

I.S. (1498 - 1970) presents classification and identification of soils for general engineering

purposes. Characteristics of soil pertinent to roads and air fields also have been discussed.

The student is required to refer to the course material in Soil Mechanics for further details.

Highway Research Board (HRB) system of classification of soils finds direct

application in the design of flexible pavements. The course grained soils are divided into

three groups A - 1 , A - 2 , A - 3 , based on the gravel and sand content. Fine-grained soils are

divided into four groups - A - 4 , A - 5 , A - 6 and A - 7. In order to classify the fine grained

soils within one group and for judging their suitability as subgrade materials , an indexing

system has been developed and is termed as ‘GROUP INDEX’. Group Index is a function of

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percentage material passing 0.075 mm mesh sieve , liquid limit and plasticity index of the

soil and is given by

Group Index (G.I) = 0.2a + 0.005 ac + 0.01 bd 7.1

where

a = that portion of the material passing 0.075 mm mesh sieve , greater than 35 and not

exceeding 75 percent (expressed as a whole number from 0 to 40)

b = that portion of the material passing 0.075 mm mesh sieve , grater than 15 and not

exceeding 55%. (expressed as a whole number for 0 to 40).

c = that value of liquid limit in excess of 40 and less than 60 (expressed as a whole

number 0 - 20).

d = that value of the plasticity index exceeding 10 and not more than 30 (expressed as

a whole number between 0 to 20).

According to this formula , the minimum possible value of group index is zero and the

maximum value is twenty. Higher the value of group index poorer is the soil as a sub-grade

material.

Table 7.1. H.R.B. Classification

Group

classifi

cation

A - 1 A - 3 A - 2 A - 4 A - 5 A - 6 A -

7

Group

Index

0 0 0-4max. 8max 12max 16max 20m

ax

Usual

signific

ant

materia

l

Stone

fragements ,

gravel and

sand

Fine sand Silty &

clayey

gravel &

sand

SILTY SOIL CLAYE

Y SOIL

General

rating

as sub-

grade

Excellent to good Fair to poor

General

Classifi

cation

Generally

less passing

materials

75micron

35% or

sieve

Silt clay.

35%passing

materials

75

(More

micron

than

sieve)

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Worked Example:

7.1. On a sieve analysis it has been found that the soil contains 55% of materials passing

0.075 mm , the liquid and plastic limits are 40% and 25% respectively. Find out the group

index.

Solution: a = 55 - 35 = 20

b = 55 - 15 = 40

c = 40 - 40 = 0

d = (40 - 25) - 10 = 15

G.I. = 0.2 (20) + 0.005 (20) (0) + 0.01 (40) (15) = 10.

7.2.1. Soil Compaction: Compaction is the process by which the dry density of the soil is

increased. The compaction process may be accomplished by rolling , tamping or vibration.

Increase in the dry density of a soil results in the increase of its strength and bearing capacity

, decreases the tendency of the soil to settle under repeated loads and brings about decrease in

the permeability of the soil. The dry density of the soil is a measure of its degree of

compaction. The degree of compaction of the soil depends on (i) moisture content of the soil

(ii) the amount of compactive effort and (iii) the nature of soil.

For a given soil and compactive effort , dry density increases with moisture content

upto a particular value and then with further increase in moisture content , the dry density

decreases. This moisture content at which maximum dry density is attained is called

‘Optimum moisture content’ (OMC) and the corresponding dry density is called the

‘Maximum Dry Density’ (MDD). For a given soil , the O.M.C. decreases an M.D.D.

increases with an increase in the compactive effort.

Fig 7.1. COMPACTION STUDIES

Well graded course -grained soils attain a much higher density and lower optimum moisture

content than fine grained soils.

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To aid field compaction of soils two types of tests are carried out in the laboratory -

I.S. - Light compaction test or I.S. Heavy compaction test. The student may refer to the

course material in ‘Soil Mechanics’ for further details.

The tests used to evaluate the strength properties of soils may be divided into three

groups:

(i) Shear Tests: These tests are carried out to determine the shear parameters cohesion (C)

and angel of internal friction (φ) of a soil. Some of the commonly known laboratory shear

tests are direct shear test , traiaxail shear test and unconfined compression test. Vane shear

tests are carried out on soft soil samples either in the laboratory or on in-situ soil in the field.

For further details reference may be made to the course material in soil mechanics.

(ii) Bearing Tests: These tests are loading tests carried out on the sub-grade soils in -situ with

a load bearing plate to evaluate the sub-grade power of the sub-grade for use in flexible

pavement design or to determine the modulus of sub-grade reaction of the soil for use in rigid

pavement design.

(iii) Penetration Tests are small scale bearing tests in which the size of the loaded area is

relatively much smaller and ratio of the penetration to size of loaded area is much greater

than the ratios in bearing tests. These tests are carried out either in the laboratory or in the

field. The California Bearing Ratio Test (CBR) is the commonly used penetration test.

The bearing and penetration tests are presented here.

7.2.3. Bearing Tests: These are in-situ loading tests used to determine the following:-

(i) Determination of modulus of sub-grade reaction

(ii) Determination of modulus of deformation

(iii) Determination of bearing capacity of the sub-grade and

(iv) Evaluation of existing surface or base of a pavement.

While conducting the test the conditions of the soil in the test area ought to be those

which are likely to exist when the subgrade has reached a state of relative equilibrium

subsequent to construction of the road. In case of natural soils the top 25cm of the soil before

testing is removed.

The apparatus consists of a set of pates of diameter 75 cm , 60 cm , 45 cm and 30 cm

(or square plates of these sizes) , equipment to apply a load to the plate and instruments to

measure the load and settlement of the plate. Generally load is transmitted to the plate by a

hydraulic jack acting against a reaction. A datum frame whose supports rest on the ground at

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points unaffected by the settlement of the plate is used for the measurement of settlement of

the plate (Fig 7.2).

The bearing plate is placed on the test area and before starting the test a seating load

equivalent to a pressure of 0.07 kg / cm2 is applied and released after a few seconds. For

finding modulus of sub-grade reaction a 75 cm size plate and for evaluating the existing

surface or base courses 30 cm size plate is to be used. A load sufficient to cause

approximately a 0.25 cm settlement is applied and when there is no perceptible increase in

settlement or in the case of clayey soils when the rate of increase of settlement is less than

0.025 mm / minute , the average of dial gauge readings is noted. At the same time the load is

also noted.

Fig 7.2. SET UP FOR PLATE LOAD TEST Fig 7.3. LOAD - SETTLEMENT CURVE

The load is again increased so as to have an additional settlement of approximately

0.25 mm. This procedure is repeated till the settlement reaches 1.75. A plot is made between

the bearing pressure of the sub-grade soil and its mean settlement. (Fig 7.3). From this the

modulus of sub-grade reaction (K) is taken as the slope of the line passing though the origin

and the point on the curve corresponding to 0.125 cm. settlement , i.e.,

K = p / 0.125 kg / cm2 / cm or kg / cm3 7.2

In order to determine the modulus of sub-grade reaction quickly , the U.S. Corps of

Engineers have suggested to apply a load to produce a pressure of 0.7 kg/cm2 in 10 seconds

after seating the plate as before and held until there is no increase in settlement or in the case

of clayey soil until the rate of settlement is less than 0.05mm per minute. If ∆ is the average

settlement in cm corresponding to a pressure of 0.7 kg / cm2 , then

K = 0.7 / ∆ kg / cm3 7.3

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In case standard plate of 75 cm dia is not used , the modulus of sub-grade reaction can

be obtained from

K = K1 a1 / a 7.4

Where K1 is the modulus of sub-grade reaction corresponding to the field test carried out with

plate of area a1 and ‘a’ is the area of the standard plate of 75 cm dia.

7.2.4. C.B.R. Test:

This test is developed by California Division of Highway. C.B.R. test is carried out on

a compacted soil in a mould 15 cm in dia and 17.5 cm height. (Fig 7.4). The loading plunger

is 50 cm dia. Briefly the test consists of causing the plunger penetrate a pavement material at

1.25 mm/minute. The material is compacted to its maximum dry density or field density at

the optimum moisture centent or the filed moisture condition. A displacer disk , 5 cm thick

placed in the mould during compaction enables a specimen of 12.5 cm deep to be obtained.

The test specimens should be prepared by static compaction , but if not possible , dynamic

method may be used as an alternative.

The load penetration curve (Fig 7.5) is plotted. When the curve is concave upwards

initially due to surface irregularities , correction is applied by drawing tangent to the curve at

the point of maximum curvature and shifting the origin to the point where the tangent meets

the horizontal axis. The test loads for 2.5mm. and 5mm (correction values) penetration are

recorded. The C.B.R. (percent) =

Load carried by specimen at defined penetration levelLoad carried by standard crushed stone at the above penetration level

100

× 7.5

Standard load values on crushed stones for 2.5 mm and 5mm penetration are 1370 kg and

2055 kg respectively. C.B.R. values are determined at 2.5 mm and 5 mm penetration and

greater to the two values is termed as CBR value for design purposes.

To simulate worst moisture condition of the field , specimens are kept submerged in

water for 96 hours before testing. Sufficient surcharge weight equal to the actual or estimated

weight of the pavement are placed on the top of the specimen before testing. The minimum

surcharge weight kept is 4.5 kg.

The CBR test is essentially an arbitrary strength test and hence the test procedure is to

be strictly followed for getting dependable results.

7.3. STONE AGGREGATES:

Based on the strength property , the coarse aggregates used in pavement construction

may be divided as hard aggregates and soft aggregates. Generally for the wearing course of

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superior pavement types hard aggregates are preferred to resist crushing effect due to heavy

loads of traffic and to resist weathering conditions. In the case of low-cost road construction

or for use in the lower layers of pavement , soft aggregates can also be used. The soft

aggregates include moorum , kankar , laterite , brick-aggregates and slag.

Fig 7.4 C.B.R. Test step up

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Fig 7.5. LOAD PENETRATION CURVE - C.B.R. TEST

7.3.1. Desirable Properties of Road Aggregate:

(i) Strength: Road aggregates should possess sufficient strength and resistance to crushing.

(ii) Hardness: The aggregates are subjected to constant rubbing or ‘abrasion’ due to moving

traffic. Further due to relative movement of aggregates , there will be mutual rubbing of

stones called ‘attrition’. These two , cause wear of the aggregates and as such they should be

sufficiently hard.

(iii) Toughness: Aggregates in the pavement are also subjected to impact due to moving

wheel loads. The resistance to impact or toughness is hence another desirable property of the

aggregates.

(iv) Durability: The stones used in pavement construction should be durable and should resist

the action of weather. This property is also called ‘weathering’.

(v) Shape of Aggregates: Aggregates may have rounded , cubical angular , flaky , or

elongated particles. Flaky and elongated particles will have less strength and durability when

compared with cubical , angular , rounded particles of the same stone. Hence too flaky and

too much elongated aggregates should be avoided as far as possible. Rounded aggregates may

be preferred in cement concrete mixtures due to better workability where as rounded particles

are not preferred in granular base course , WBM construction and bituminous construction as

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the stability due to interlocking of rounded particles is less. In such construction , angular

particles are preferred.

(vi) Adhesion with bitumen: Aggregates used in bituminous pavements should have less

affinity with water when compared with bituminous materials. Otherwise the bituminous

coating of aggregates will be stripped off in the presence of water.

In order to decide the suitability of the road stones for use in construction , the

following tests are carried out.

(a) Crushing test

(b) Abrasion test or Attrition test

(c) Impact test

(d) Soundness test

(e) Shape test

(f) Specific gravity and water absorption test

(g) Bitumen adhesion test.

These tests are discussed in the following pages:

7.3.2. Aggregate Crushing Tests:

Fig 7.6. AGGREGATE CURSING TEST SET UP

The aggregate crushing value provides a relative measure of resistance to crushing

under gradually applied compression load. To achieve a high quality of pavement ,

aggregates possessing low aggregate crushing value are preferred.

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The apparatus for the test is shown in Fig 7.6. Dry aggregate passing 12.5 mm I.S.

sieve and retained on 10mm I.S. sieve is placed in the mould in three layers and each layer is

tamped 25 times by the tamping rod. The weight of the sample taken is determined (W1).

Plunger is placed on the top of the specimen and a load of 40 tonnes is applied at a rate of 4

tonnes / minute by the compression machine. Then the crushed material is sieved through

2.36 mm I.S. sieve. The weight of the material which passes this sieve is equal to W2 . Then

Aggregate crushing value

= ( )

( )Wt . of material passing I.S. sieve 2.36mm W

Wt .of test specimen W1002

1

× 7.6

Strong aggregates given low crushing value. The aggregate crushing value for good

quality aggregates to be used in base course shall not exceed 45% and the value for surface

course shall be within 30%.

7.3.3. Abrasion Tests:

Abrasion tests are carried out to test the hardness property of road aggregates. The

abrasion tests on aggregates may be carried out using any one of the following methods.

(i) Los -Angeles Abrasion Test

(ii) Deval Abrasion Test and

(iii) Dorry Abrasion Test.

The Los - Angeles -abrasion test is more dependable because (a) the rubbing and

pounding action in the test simulate field conditions better and (b) correlation of Los-Angeles

value with field performance and specifications of the test have been established. Hence this

test only is discussed here.

The machine consists of a hollow cylinder of 70 cm dia and length 50 cm and

mounted so as to rotate about its horizontal axis. The machine rotates at a speed of 33 rpm.

Aggregate of the specified grading and specified weight together with cast iron spheres of

about 4.8 cm dia and weighing about 390 to 445 gms is fed into the machine. The number

and weight of spheres to be used as abrasive charges have been specified based on the

grading of the aggregate sample. The

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Fig 7.7. LOS-ANGELES ABRASION TEST APPARATUS

machine is rotated at 33 rpm for specified number of revolutions (500 or 1000 depending on

the grading of the specimen). Wear takes place due to the relative rubbing action between the

aggregate and steel balls used as abrasive charge. The abraded aggregate is then sieved on 1.7

mm I.S. sieve and the weight of aggregate passing is determined. Then

Percentage wear = Wt . of material passing 1.7 mm I.S Sieve

Wt . of original material100× 7.7

As per I.S.S , Los-Angeles Abrasion value acceptable for good aggregate to be used

for cement concrete roads is 16%. For bituminous mixes this value can be upto 30% and for

bituminous macadam construction this wear value can be upto 50%.

7.3.4. Devals Attrition Tests:

The principle of the test is to allow the sample of aggregate specimen to tumble over

in a rattler. The machine (Fig 7.8) consists of two hollow cylinders of diameter 20cm and

length 34 cm mounted in such away that the cylinders rotate about a horizontal. Specified

quantity of dry aggregate specimen (4 to 5.5 kg) , of any of the specified gradings is placed in

a cylinder. Two tests may be carried out simultaneously. The material is sieved on 1.7 mm

I.S. Sieve and the weight of material passing is determined and is expressed as the percentage

of the original weight of the sample and is reported as attrition value.

CE409/16 128

In the case of Devals abrasion test the abrasive charges consisting of 6 cast iron

spheres of about 4.8 cm dia and total weight of 2500 gms is placed along with the specimen ,

test is run for 10,000 revolutions and percentage wear is calculated.

Fig 7.8 DEVAL ABRASION TEST APPARATUS

7.3.5. Impact Test:

The aggregate impact value indicates a relative measure of the resistance of aggregate

to impact.

Aggregate specimen passing 12.5 mm I.S. sieve and retained on 10 mm. I.S. sieve is

filled in the cylindrical cup (Fig 7.9) by tamping 25 times. The weight of aggregate taken is

determined. The hammer weighing 13.5 to 14.0 kg is raised to a height of 38 cm above the

upper surface of the aggregate in the cup and is allowed to fall freely on the specimen for 15

times. The material is than sieved on 2.36 mm I.S. sieve and the weight of the material

passing is determined. Then

Impact value = Wt . of material passing 2.36 mm I.S. sieve

Wt . of aggregate taken100× 7.8

Impact value less than 10% is considered exceptionally strong , 10 - 20% strong , 20-

30% satisfactory for road surfacing. The aggregate impact value should not normally exceed

30% for aggregates to be used in wearing course: for base course this value should not be

more than 45%.

CE409/16 129

Fig 7.9. Impact test equipment

7.3.6. Soundness Test:

These are accelerated laboratory tests to evaluate the resistance of aggregates to

weathering action due to wet-dry and / or freezethaw cycles. Certain weight of dry aggregate

specimen is first measured in the saturated solution of sodium sulphate or magnesium

sulphate for 15 to 18 hours. Later , the saturated specimen is dried in an oven at 1000C to

1050 C to constant weight. This entire process is known as one cycle of wetting and drying.

Generally five such cycles are repeated and average loss of weight is determined. As per I.S.

383 - 1970 the loss of weight after five cycles should not exceed 12% when tested with

sodium sulphate and 18% when tested with magnesium sulphate.

7.3.7. Shape Test:

These are mechanical measures of particle shape in terms of flakiness index ,

elongation index and angularity number.

(a) Flakiness Index: It is the percentage by weight of particles whose least thickness is less

than 3 / 5 of their mean dimension. In this case , the mean dimension is the average of two

adjacent sieve aperture sizes between which the particle being measured is retained by

sieving. This test is not applicable to particles smaller than 6 mm. The test is carried out by

first separating the aggregate into individual percentages retained on specified sieve sizes and

then passing at least 200 particles from the individual percentages through gauges having

elongated slots whose widths are 0.6 times the individual mean dimensions. The flakiness

CE409/16 130

index is calculated as the total weight of material passing the various gauges expressed as a

percentage of the total weight of the sample gauged. It has been reported by various agencies

that the flakiness index of aggregates suitable for road construction should be less than 15%

and in no case greater than 25%.

(b) Elongation Index: It is the percentage by weight of particles whose greatest length is 1.8

times their mean dimension. The sample of aggregate to be tested is sieved through a set of

sieves and then the individual particles from the fractions are passed through opening on

metal length gauge. The elongation index is calculated as the total weight of material retained

on the length gauge expressed as the total weight of the sample gauged. Generally the

aggregate used in road construction should have elongation values less than 15%.

(c) Angularity Number: It is the amount of the percentage of voids that exceeds 33% when

an aggregate is compacted in a specified manner in a standard metal cylinder. Generally the

angularity number for aggregates to be used in road construction should be between 0 to 11.

7.3.8. Specific Gravity and Water Absorption Test:

For determining the specific gravity , the test sample of aggregate is soaked in

distilled water for 24 hours. Later it is weighed in water. After this it is surface dried and

weighed in air. The sample is again weighed in air after oven drying for 24 hours at 1000 -

1100 C. The specific gravity is calculated by dividing the weight of oven dry sample in air by

the difference between the saturated sample weights in air and in water. Generally the

specific gravity of aggregate varies from 2.6. to 2.9.

For determining the water absorption , the aggregate sample is soaked in distilled

water for 24 hours. Later it is surface dried and weighted in air , and then oven dried for 24

hours at 100 - 1100 C , weighed in air again. The water absorption is expressed as a

percentage of water absorbed in terms of the oven dry weight of aggregates. The water

absorption value of aggregates used in road construction range from less than 0.1% to about

2% for material used in road surfacing , while values upto 4% may be accepted in base course

construction.

7.3.9. Bitumen Adhension Test:

Bitumen and tar adhere well to all normal types of road aggregates provided they are

dry and free from dust. Two problems are observed due to the presence of water. First in the

presence of water it is not possible to coat the aggregates with a bituminous binder. This

problem can be dealt with by drying the aggregates , and by increasing the mixing

temperature. Second problem is stripping of binder from coated aggregate due to the presence

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of water. The stripping is due to the fact that some aggregates have greater affinity towards

water than with bitumen and this displacement depends on the physico-chemical forces acting

on them. Those aggregates which are electro - negative (Silica common constituent of

igneous rocks) and water liking are called ‘HYDROPHILIC. Basic aggregates like limestones

have dislike for water and greater attraction to bitumen as they have positive surface charges.

These aggregates are called ‘HYDROPHOBIC’.

It is important to know the type of charge of aggregates used in bituminous

construction. Now bitumen is also available as cationic (positive) and anionic (negative) and

hence a suitable selection may be made depending on the aggregates available. Cationic (+)

bitumen may be selected for electronegative aggregate and anionic (-) bitumen for

electropositive aggregate.

Several laboratory tests have been developed to ascertain the amount of stripping. Out

of these the static immersion test is quite simple and easy to conduct. In this test , aggregate

coated with bitumen is kept immersed in water maintained at 400 C for 24 hours and the

degree of stripping which may take place is evaluated. The result of the test is expressed in

percent of stone surface that has stripped off. The IRC has specified maximum stripping

value of aggregates as 25% for bituminous surface dressing and penetration macadam etc.

7.4. SUMMARY:

The two basic highway materials , namely , soil and mineral aggregates are

considered in this lesson.

Soil is the foundation material for all highways. In addition , the highway pavement

itself is very often composed of compacted soil and stabilised soil. Therefore , an adequate

knowledge of the properties of soil is essential for proper design and construction of roads

and air field runways.

Generally , unified soil classification system or I.S. soil classification system or

Highway Research Board (HRB) system of soil classification are adopted for classifying soils

for highway purposes. The unified soil classification system and I.S. classification system of

soils are based on both grain size analysis and plasticity properties of the soil and are

applicable to any use. HRB system of soil classification finds direct application in the design

of flexible pavements. In this system coarse grained soils are divided into three groups. A - 1

, A - 2 , and A - 3 based on sand and gravel contents. Fine grained soils are divided into four

groups - A - 4 , A - 5 , A - 6 and A - 7. In order to determine the suitability of these soils as

sub-grade materials , an indexing system has been developed and is called as Group Index.

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The Group Index is a function of material finer than 75 micron size , liquid limit and

plasticity index. The range of group index for any soil is between zero and twenty. Higher the

value of the group Index of a soil , poorer is the quality of soil as a sub-grade material.

The engineering behaviour of a soil mass is influenced by the presence of water and

dry density. For a given soil and energy of compaction , there is a particular moisture content

at which the dry density of the soil attains a maximum value. This particular moisture content

is called the Optimum Moisture Content (OMC) and the corresponding dry density is called

the Maximum Dry Density (MDD). Soil to be used in pavement construction has to be

compacted at this moisture content - OMC to get maximum benefits.

A number of tests are conducted on soil for evaluating their strength properties.

Among these tests results of the California Bearing Ratio (CBR) test and the plate bearing

test find direct application in the design of flexible and rigid pavements respectively.

Mineral aggregates are the basic materials of a pavement structure and it is the prime

material used in the pavement construction. Aggregate primarily bearing stresses occurring

on the roads due to wheel loads have to resist wear due to abrasive action of traffic as well as

the effects of weathering agents. Cubical , angular or rounded aggregates are preferred in

road construction as they satisfy almost all the requirements of a good road aggregate. The

percentage of elongated and flakly particles should not exceed 15 , in a road aggregate as the

presence of these particles adversely effect the strength and workability of paving mixtures.

Further aggregates used in bituminous pavements should have greater affinity towards

bitumen than towards water. Basic aggregates line lime stone have dislike for water and

greater attraction to bitumen and are known as Hydrophobic aggregates. These aggregates

have positive surface charge. Silica aggregates have negative surface charges , are water-

liking and are called hydrophilic. It is important to know the type of surface charge of

aggregates used in bituminous construction. Now bitumen is available as cationic or anionic

and hence a suitable selection may be made depending on the aggregate available.


(1) Explain Group Index and its significance in soil classification.

(2) What are the limitations of the CBR Test and Plate Load Test.

(3) List standard physical tests to which road metal will be subjected in order to ascertain its

suitability. State the minimum standard requirements for each test.

(4) What are the most important three tests to be carried out to assess the suitability for road

aggregates used in the surface course of a W.B.M. road construction.

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7.6. REFERENCES:

(1) Kerbs , D.K. and Walker , R.D. (1974) - Highway Materials , McGraw - Hills Book

Company , New York.

(2) Khanna , Dr. S.K. and Justo , Dr. C.E.J. (1991) - Highway Engineering , Namachand and

Bros , Roorkee.

(3) O’ Flaherty , C.A. (1974) - Highways , volume 2 , Highway Engineering - Edward

Arnold.

(4) (- 1969) , Soil Mechanics for Road Engineers , H.M.SO. Publication.

***


HIGHWAY MATERIALS - II

UNIT - 8

BITUMINOUS MATERIALS CONTENTS:

8.1. Introduction

8.2. Bitumen

8.3. Road Tar

8.4. Comparison of Bitumen and Tar

8.5. Requirements of Bitumen

8.6. Tests on Bitumen

8.7. Tests on Road Tar

8.8 Summary

8.9. Self - Assessment Questions

8.10. References

Aims / Objectives:

A variety of bituminous products are used as binders. Standard tests specified for

determining the suitability of a bitumen for road construction are presented along with the

requirements of each test.

8.1. Introduction:

Bituminous materials are very commonly used in highway construction because of

their binding and water proofing properties. Bituminous materials used in ancient times were

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of natural origin obtained from lakes. The greatest proportion of bitumen used in road

construction today is obtained from crude petroleum oil by fractional distillation process.

Bitumen is a hydro-carbon material of either natural or pyrogenous origin found in

gaseous , liquid , semi-solid or solid form and is completely soluble in carbon - di - sulphide.

Bitumen is made up of colloidal hydro-carbon materials consisting of asphaltenes resins and

oils. When the bitumen contains some inert material of minerals , it is sometimes called

ASPHALT. Asphalt is found in lakes of Trinidad (West Indies) and Bermidez (Venezulla)

and in the rocks of Switzerland , France and Spain.

Bituminous materials used in highway construction may be broadly divided as (i)

Bitumen and (ii) Tar.

Tar is a viscous liquid obtained when natural organic materials such as wood and coal

are destructively distilled in the absence of air.

8.2. Bitumen:

Bitumens artificially produced by the industrial refining of crude petroleum are

known under a number of names such as residual bitumens , straight run bitumens , steam-

refined bitumens and refinery bitumens. The components of a petroleum crude are shown in

Fig 8.1.

Fig 8.1. COMPONENTS OF PETROLEUM CRUDE

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These straight run bitumens vary in consistency from semi-solid to semi-liquid at the

room temperature. These bitumens are classified according to their hardness as indicated by

the penetration test.

The straight run bitumens range have to be heated to get the correct consistency for

use in road construction. There may be instances , however , it may be neither desirable nor

necessary to use a hard bitumen , and preference may be given to use of liquid binders such

as the cut - back bitumen or bituminous emulsions.

8.2.1. Cut-back bitumen:

Cut-backs differ from straight run bitumens in that the bitumen is dissolved in a liquid

solvent which makes it suitable for direct application and manipulation in road construction.

After a cut back has been spread on the particles it is intended to bind , the solvent will

evaporate and leave behind the cementitous bitumen to tie the particles together. The

character and behaviour of a cut-back bitumen in any particular situation is largely dependent

on the character and amount of solvent present. The more volatile the solvent , shorter is the

curing period necessary after using the cut-back before the cohesive properties of the binder

are utilised.

Fig 8.2. Components of Bitumen cut-back emulsions

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As indicated in Fig 8.2 , cut-back may be divided into three main types depending on

the type of solvent used to dilute the bitumen. These are commonly designated as Slow

Curing (SC) , Medium Curing (MC) , and Rapid Curing -cut - back (RC).

(a) Slow - Curing Cut-backs: These require a relatively long time for curing and the binding

strength is developed correspondingly slowly. For this reason these cut backs are best used

with densegraded aggregates which provide a strong interlocking frame work and do not

require immediate cementing action from the binder S.C. cut backs are also used in soil

aggregate roads in warm climates in order to keep the dry soil particles from creating a dust

nuisance.

(b) Medium Curing Cut-back: These cut backs have good aggregate coating properties , so

they are very useful when fine-graded and dusty materials are incorporated in the road

surface and for bituminous stabilisation of soils.

(c) Rapid Curing Cut-back: Since the constituents of these cut backs quickly evaporate

these are used when a quick change-back to the residual semi-solid binding agent is desired.

The volatility of the distillate , that is , it has a relatively low flash point , can render their use

hazardous when they are used in road construction.

The grade of the cut-back or its viscosity is designated by a figure which follows the

initials ; as an example RC - 2 means that it is a rapid curing cut-back of grade 2. Suffix

nemericals 0 , 1 , 2 , 3 , 4 and 5 designate progressively thick or more viscous cutbacks as the

numbers increase. This number indicates definite viscosity irrespective of the type of cut-

back ; that is RC - 3 , MC - 3 and SC - 3 all have the same initial viscosity. ISI has specified

the viscosity values of different grades of bitumen in seconds (standard tar viscometer). The

new designations for grades of cut back bitumen , as per the U.S. practice , are based on

Kinematic viscosities of the mixtures at 600 in ‘CENTISTOKES’.

8.2.2. Bituminous Emulsions:

A bitumen emulsion is a liquid product in which a substantial amount of bitumen is

suspended in a finally divided condition in an aqueous medium stabilised by means of one or

more suitable materials. Usually bitumen or refined tar is broken up into fine globules and is

kept in suspension in water. A small proportion of an emulsifier , (soaps , surface - active

agents and colloidal powders) is used to facilitate the formation of dispersion and to keep the

dispersed binder in suspension. Half to one percent emulsifier by weight of finished emulsion

are usually taken for preparing normal road emulsions. The bitumen / tar content of

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emulsions range from 40 to 60% and the remaining portion being water. The penetration

values of these bituminous emulsions are usually between 190 - 320.

When the emulsion is applied on the road , it breaks down and the binder starts

binding the aggregates , though the full binding power develops slowly as and when the water

evaporates. Emulsions are used in bituminous road construction especially in maintenance

and patch repair works. The main advantage of an emulsion is that it can be used in wet

weather even when it is raining. Emulsions are also used in soil-stabilisation works.

8.3 Road Tar:

Tar is the viscous liquid obtained when the natural organic material such as wood and

coal are carbonised or destructively distilled in the absence of air. Based on the material from

which tar is derived , it is referred to as wood tar or coal tar ; the later is more widely used for

road work because of its superiority.

There are five grades of road tars namely , RT - 1 , RT - 2 , RT - 3 , RT - 4 and RT - 5

based on their viscosity and other properties. RT - 1 is used for surface painting under

exceptionally cold weather as it has very low viscosity. RT - 2 is recommended for standard

surface painting ; RT - 3 may be used for surface painting , renewal coats and premixing

chips for top course and light carpets ; RT - 4 is generally used for premixing tar macadam in

base course. For grouting purposes RT - 5 may be adopted.

8.4. Comparison of Bitumen and Tar:

(i) Both binders appear black to dark brown in colour.

(ii) Bitumens respond less readily than tars , to small changes in temperature.

(iii) Tar may be overheated and spoiled more easily than bitumens ; but is much easier , to

get tar out of road tanker.

(iv) Tar tends to penetrate more easily into open road surfaces.

(v) Bitumen is less brittle at low temperatures , this is because it contains higher percentage

of free carbon.

(vi) Tar is not susceptible to dissolving action of petroleum solvents or distillates. In parking

places , where petrol and oil are likely to spill from vehicles , a tar surfacing may have longer

life than a bitumen one.

(vii) The chemical constituents of tar and bitumen are quite different. Tar is produced by the

destructive distillation of coal or wood , but bitumen is a petroleum product.

(viii) Tar coats aggregates more easily and retains it better in presence of water than bitumen.

But the tar is considered to have much inferior weather resisting property.

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(ix) Bitumen is soluble in carbon disulphide and in carbon tetrachloride ; but tar in soluble

only in toluene.

(x) The free carbon content is more in tar as seen from the solubility test.

8.5. Requirements of Bitumen:

The desirable properties of bitumen depend on the mix type and the construction.

General requirements of bitumens to be used in road construction are:

(i) The viscosity of bitumen at the time of mixing and compaction should be adequate. This is

achieved by heating the bitumen and aggregates prior to mixing or by the use of cut-backs or

emulsions of suitable grade.

(ii) The bituminous material should not be highly temperature susceptible. During the hottest

weather of the region the bituminous mix should not become too soft or unstable. During cold

weather the mix should not become too hard and brittle causing cracking of the surface. The

material should be durable.

(iii) In the presence of water the bitumen should not strip off from the aggregates. There

should be adequate affinity and adhesion between bitumen and aggregate used in the mix.

8.6. Tests on Bitumen:

Bitumen as already discussed is available in various types and grades. I.S.I. and other

agencies have suggested a number of tests to determine the behaviour and suitability of

bitumens and cutbacks. These tests include penetration test , ductility test , softening point

test and viscosity test. For classifying the bitumen and studying the performance of

bituminous pavements the ductility and penetration tests are essential. The other tests like

softening point and flash and fire point tests are more important to guide the paving

technologists during field operations.

8.6.1. Penetration Test:

The penetration test determines the hardness or softness of bitumen by measuring the

depth in tenths of a millimetre to which a standard loaded needle will penetrate vertically in

five seconds. The sample is maintained at a temperature of 250 C. The needle assembly

weights 100 gms. The concept of penetration test is shown in fig 8.3. The mean value of three

measurements is reported as penetration value. It may be noted that the penetration value is

largely influenced by any inaccuracy as regards pouring temperature , size of needle , weight

placed on the needle and the test temperature.

The bitumen grade is specified in terms of the penetration value. 80 - 100 or 80 / 100

grade bitumen means the penetration value of the bitumen is in the range of 80-100 at

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standard test conditions. The penetration test is applied almost exclusively to bitumen. As

road tars are soft , the test cannot be carried out on these materials. All other tests are used for

tars , cut-backs and emulsions.

Fig 8.3. Concept of Penetration Test

The penetration values of various types of bitumens used in pavement construction in

this country range between 20 and 225. 30 / 40 and 80 / 100 grade bitumen are more

commonly used , depending on the construction type and climatic conditions. In hot climates

a lower penetration grade bitumen like 30 / 40 bitumen is preferred.

8.6.2. Ductility Test:

The bituminous binder forms a ductile thin film around the aggregates. It improves

the physical inter locking of aggregates. Under traffic loads the bituminous pavement layer is

subjected to repeated deformations and recoveries. The binder material which does not

possess sufficient ductility would crack and thus provide pervious pavement surface.

Ductility is carried out on bitumen to test this property of the binder. The test is believed to

measure the elasticity and adhesiveness of the bitumen.

The ductility is expressed as the distance in centimetres to which a standard briquette

of bitumen can be stretched before the thread breaks. The test is conducted at 270 C and a rate

of pull of 50 mm per minute. The test set up is shown in Fig 8.4. The cross-section at the

minimum width of the specimen is 10mm x 10mm.

The ductility machine functions as a constant temperature water bath and a pulling

device at a pre-calibrated rate. Two clips are thus pulled apart horizontally at a uniform speed

of 50mm per minute.

The ductility value gets seriously affected by factors such as pouring temperature ,

dimensions of the briquette , level of briquette in the water bath , presence of air pockets in

the moulded briquettes , test temperature and rate of pulling.

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The ductility values of bitumen vary from 5 to over 100 for different bitumen grades.

It is desirable that the ductility value of bitumen is not less than 50 , for satisfactory

performance.

8.6.3. Viscosity Test:

Viscosity is inverse of fluidity. It is a measure of resistance to flow. Furol viscosity

test is conducted on liquid bitumens (cut-backs , Tars and Emulsions). In this test , time in

seconds is noted for 50ml. of the liquid bitumen at specified temperature to flow through an

orifice of specified size. Higher the viscosity of the liquid , more will be the time required to

flow out. For tar tests , orifice of 10mm is specified and is called tar visco meter.

Fig 8.4. Ductility Test

Fig 8.5. Viscosity Test

Degree of fluidity of bitumen at the temperature of its use greatly influences the

strength and durability of the road pavements. For each aggregate gradation of the mix and

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bitumen grade , there is an optimum value of viscosity. Bituminous binders of low viscosity

simply lubricate the aggregate particles instead of providing a uniform thin film for binding

action. Similarly high viscosity does not allow full compaction and resulting mix exhibits

heterogeneous character and thus low stability values.

8.6.4. Float Test:

Bitumens which are more viscous cannot be tested either by Fural viscosity test or by

penetration test. Such bituminous materials can be tested for consistency by float test.

Fig 8.6. Float Test

The apparatus used for float-test is a float made of aluminium. At the bottom of the

float there is hole in which a brass collar can be screwed. The specimen of bitumen to be

tested is filled in the collar cooled at 50 C temperature and is then screwed to the float.

Completed float assembly is then floated in a water bath containing water. The temperature

of water bath is maintained at 500 C. The time required is seconds for water to force its way

through the bitumen plug is noted. The higher the float value , the stiffer is the bitumen.

8.6.5. Softening Point Test:

The softening point is the temperature at which the substance attains a particular

degree of softening under specified conditions. It is usually determined by the ‘Ring and

Ball’ apparatus. Generally higher softening point indicates lower temperature susceptibility

and is preferred in warm climates.

A brass ring containing the test sample of bitumen is suspended in liquid like water or

glycerine at a given temperature. A steel ball is placed upon the bitumen and liquid medium

is then heated at a rate of 50 C per minute. The temperature at which the softened bitumen

touches the metal frame placed at a specified distance below the ring is recorded as the

softening point of a bitumen . Hard grade bitumens possess higher softening point than soft

grade bitumens.

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The softening point of various bitumen grades used in pavings jobs vary between

350C to 700C.

8.6.6. Flash point and Fire point Test:

This test give an indication of the critical temperature at and above which suitable

precautions should be taken to eliminate fire hazards during heating of bitumen. The

definition of flash and fire points as given by ISI are as follows:

Fig 8.7. Softening Point Test

Flash Point: The flash point of a material is the lowest temperature at which the vapour of a

substance momentarily takes fire in the form of a flash under specified conditions of the test.

Fire Point: The fire point is the lowest temperature at which the material gets ignited and

burns under specified conditions of the test.

Flash point test is carried out by Pensky - Martens closed cup or open cup apparatus.

The bituminous material , to be tested is filled in the cup upto the filling mark. The cover of

lid should be put to close the cup in close cup system. Thermometer and other attachments

are also suitably fixed. Sample is then heated at a rate of 50C per minute. During heating ,

specimen is continuously kept stirred. When temperature of the specimen reaches about 150C

Less than the expected flash point , test flame is applied. The test flame is applied again and

again at intervals of 30C rise in temperature till the test flame causes a bright flash in the

interior of the cup of a closed system.

The temperature at this instance is noted which is taken as ‘FLASH POINT’ . In the case of

open cup system , flash may appear at any point on the surface of the specimen.

The material is further continued to be heated till the material gets ignited and

continues to burn from five seconds. The temperature recorded is called the ‘FIRE POINT’.

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For bituminous materials used in road pavements , the closed cup flash point should

not be less than 1750 C.

Fig 8.8. Flash and Fire point test

8.6.7. Specific Gravity Test:

The density of a bitumen binder is a fundamental property used as an aid in

classifying the binders for use in paving jobs. In most applications , the bitumen is weighted

out , finally when used with aggregate system. The bitumen content is converted into volume.

The specific gravity value of bitumen is also useful in bituminous mix design. Density of the

binder is greatly influenced by the chemical composition. Increased amount of mineral

impurities in the binder increase its specific gravity.

Specific gravity of a binder is the ratio of its weight at 270 C temperature to the

weight of an equal volume of water at the same temperature. The specific gravity is

determined by standard displacement method for solid bitumen , by hydrometers for liquid

bitumen and by specific gravity bottle for intermediate grade bitumen.

8.6.8. Solubility Test:

Pure bitumen is fully soluble in carbon-di-sulphide and carbon tetra chloride. If some

quantity remains undissolved , it exhibits the quantity of inert materials present in the

bitumen.

Dissolve 2 gms of bitumen in 100 cc of carbon-di-sulphide or tetra chloride. The

solution is filtered and insoluble materials retained are washed and weighed and expressed as

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a percentage of original sample. This percentage should not exceed by one. When solvent

used is carbon tetra chloride and if black carbonaceous residue left is over 0.5% the bitumen

is considered as CRACKED. The minimum proportion of bitumen soluble in Carbon - di-

sulphide is specified as 99 percent.

8.7. Tests on Road Tar:

The following tests may be carried out on road tars.

(i) Specific gravity test

(ii) Equiviscous Temperature (EVT)

(iii) Viscosity by standard tar viscometer

(iv) Softening point test

(v) Float test

(vi) Insoluble matter in percent by weight in Toluene.

(vii) Amount of phenols by volume

(viii) Naphthalene percent by weight

(ix) Water content

Properties of five grades of road tar as required by I.S.I. are presented in Table 8.1.

Table 8.1. Properties and Requirements of Road Tars

Sl. No. Property Road Tar Grade

RT - 1 RT - 2 RT - 3 RT - 4 RT - 5

1. Softening Point --- --- --- --- 45-50

2. Specific Gravity at 270 C 1.16 -

1.26

1.16 -

1.26

1.18 -

1.28

1.18 -

1.28

1.18 -

1.28

3. Equi-viscous Temperature

range 0C (EVT)

32 - 36 37 - 41 43 - 48 53 - 57 63 - 67

4. Viscosity by 10mm size

Tar viscometer

(i) Temperature 0 C

(ii) Viscosity range in

seconds

35

33-35

40

30-55

45

35 - 60

55

40 - 60

---

---

8.8. Summary:

Bituminous materials are very commonly used in highway construction because of

their binding and their water proofing properties. Bituminous materials used in road

construction may be broadly classified as bitumen and tar. The greatest proportion of bitumen

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used in road construction today is obtained from crude petroleum by fractional distillation

process. Where as tar is obtained by destructive distillation of wood and coal. Based on the

material from which tar is derived it is known as wood tar or coal tar , the latter being used

more widely for road construction because of its superiority.

In order to get the best performance from the bituminous mixes used in road

construction , the viscosity of bitumen at the time of mixing and compection should be

adequate. This is achieved by heating the bitumen prior to mixing or by the use of liquid

bitumens like cut-backs or emulsions of proper grade.

Cut backs are obtained by dissolving bitumen in liquid solvents like volatile and non-

volatile oils , kerosene and gasoline. Based on the volatility of the solvent , cut-backs are

classified as slow curing (SC) , Medium Curing (MC) and Rapid Curing (RC). Grade of cut

back or viscosity is designated by numericals - 0 , 1 , 2 , 3, 4 and 5 , which follow the initials

(example RC - 2 , MC - 3 etc.). The more viscous the cut-back is , higher is the number.

A bitumen emulsion is also a liquid product in which a substantial amount of bitumen

is suspended in a finely divided state in an aqueous medium stabilised by means of one or

more suitable stabilisers. Emulsions are of use specially in maintenance and path repair works

as they can be used in wet weather , even when it is raining.

Road tars are available in five grades and are designated as RT - 1 , RT - 2 , RT - 3 ,

RT - 4 and RT - 5 . Just like the cut backs , more viscous tars are represented by higher

numbers.

Though tar coats aggregates more easily and retains it better , even in the presence of

water than bitumen , is preferred in the modern road construction because of its superior

resistance against weathering and lower susceptibility to temperature changes than tar.

A number of tests have been suggested to determine the behaviour and suitability of

these bituminous materials used in road construction. For classifying the bitumens and

studying the performance of bituminous pavements , penetration test , ductility test , viscosity

test and float test are quite useful. Softening point test and flash and fire point tests are

important to guide the paving technologists during field operations.


1. When do you use float test in preference to orifice viscometer for determining the viscosity

of bitumens ?

2. Distinguish between road emulsions and cut-backs

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3. What are the different types of bituminous materials used in road construction under what

circumstances each of these materials is preferred.

8.10. References:

1. Kerbs , D.K and Walker , R.D. (1974) - Highway Materials , McGraw-Hill Book Company

, New - York.

2. Khanna , Dr.S.K and Justo , Dr. C.E.G (1991) - Highway Engineering , New Chand &

Bros. Roorkee.

3. O’ Flaherty , C.A. (1974) - Highways , Volume - 2 , Highway Engineering Edward

Arnold.

4. Bituminous Materials in Road Construction , HMSO , London.

***


UNIT-9

DESIGN OF HIGHWAY PAVEMENTS - FLEXIBLE

CONTENTS

Aims/Objectives

Introduction

9.1 Design Factors

9.2 Pavement Components and their functions

9.3 Choice of the type of pavement

9.4 Design methods of flexible pavements

9.5 Self-assessment questions

9.6 Summary

9.7 References

AIMS/OBJECTIVES:

The main function of any pavement - highway or airport is to carry heavy wheel loads

and to transfer the same over a wide area of the underlying sub-grade soil permitting the

deformations within the elastic or allowable range. The design of pavements involves a study

of soils and paving materials , their behaviour under load and the design of a pavement to

carry that load under all climatic conditions. Factors affecting the design of pavements in

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general are presented. Group Index and California Bearing Ratio methods of design of

flexible pavement design are discussed.

INTRODUCTION:

Pavements may be divided into two categories. The first, the flexible pavement,

consists of a relatively thin wearing surface built over a base course and sub-base course, and

they rest upon the compacted sub-grade. In contrast, rigid pavements are made up of cement

concrete and may or may not have base course between the pavement and the sub-grade (Fig.

9.1). The thickness of the flexible pavement is meant to include all components of the

pavement above the compacted sub-grade. Thus, the sub-base, base and wearing surface are

the structural components of the pavement. In contrast, in the case of rigid pavement, the

concrete, exclusive of the base is, referred to as the pavement.

A COMPONENTS OF FLEXIBLE PAVEMENT

B. COMPONENTS OF RIGID PAVEMENTS

Fig. 9.1

The essential difference between the two types of pavement is the manner in which

they distribute the load over the sub-grade. The rigid pavement, because of its rigidity and

high modulus of elasticity, tends to distribute the load over a relatively large area of soil; thus

a major portion of the structural capacity is supplied by the slab itself . The major factor

considered in the design of rigid pavements is the structural strength of concrete. For this

reason, minor variations in sub-grade strength have little influence upon the structural

capacity of the pavement. The rigid pavements are analysed and streses are evaluated based

on elastic plate resting over elastic or viscous foundation.

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Base courses are used under rigid pavements for various reasons, including (i) control

of pumping, (ii) control of frost action, (iii) drainage, (iv) control of shrink and swell of sub-

grade and (v) expedition of construction.

Flexible pavements are those which have low or negligible flexural strength, and are

rather flexible in their structural action under loads. The flexible pavement layers reflect the

deformation of the lower layers on to the surface layers.

The load-carrying capacity of flexible pavements is brought about by the load-

distributing characteristics of the Layered system. These pavements consist of a series of

layers, with the highest quality material at or near the surface, Hence, the strength of a

flexible pavement is a result of building up thick layers and thereby distributing the load over

the sub-grade, rather than by the bending action of a slab. The thickness design of the

pavement is influenced by the strength of the sub-grade.

Sub-base courses of flexible pavements are generally made up of cheap, locally

available materials, whereas the base courses are higher quality processed materials. In most

cases the base course consists of crushed stone and in some instances, may contain

bituminous concrete/Empirical and Semi-empirical design methods are used.

As shown in fig. 9.1 base courses are constructed to some distance beyond the edge of

the wearing course (atleast 30 cm). This is done to make certain that the loads applied at the

edge of the pavement will be supported by the under lying layers.

9.1 DESIGN FACTORS:

Pavement design consists of two broad categories - (1) design of the paving mixture

and (2) structural design of the pavement components. Factors affecting the structural design

of pavement components are only discussed here.

The various factors are

i) Design wheel load

ii) Strength characteristics of pavement materials.

iii) Climatic variations.

9.1.1 DESIGN WHEEL LOAD:

Following are the elements which are to be considered in deciding the design wheel

load.

a) Maximum wheel load and its distribution.

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b) Wheel loads configuration.

c) Repetition of loads.

a) Maximum wheel load and its distribution: The IRC has specified 8170 kg. (18000 lbs)

as the maximum legal axle load with a maximum equivalent single wheel load of 4085 kg.

The pressures of wheels are controlled by allowable load per centimetre width of the tyre.

Generally, the wheel load is assmed to be distributed over a circular area. But by

measurement of the imprints, of tyres with different load and inflation pressures it is seen that

the contact areas in many cases are elliptical in shape. Contact pressure is defined as

Contact pressure = Wheel loadContact area

(9.1)

Since in the majority of problems, circular the imprints are assumed, the radius of

contact area is as follow:

a = Ρ / pπ (9.2)

where

a = radius of contact area in cm.

P = wheel load in kg.

p = the pressure in kg/cm2.

Yet another term is also used with reference to tyre pressure - The Inflation pressure.

Theoretically, tyre pressure and inflation pressure mean exactly the same. The relation ship

between the tyre pressure and contact pressure are shown in Fig. 9.2. The contact pressure is

found to be more than the tyre pressure when the tyre pressure is less than 7 kg/cm2 and it is

vice-versa when the tyre pressure exceeds this value. Rigidity factor is defined as the ratio of

contact pressure to tyre pressure. The value of rigidity factor is 1.0 for average tyre pressures

of 7 kg/cm2, higher that unity for lower tyre pressures and less than unity for tyre pressures

higher than 7 kg/cm2. This rigidity factor depends upon the degree of tension developed in

the walls of the tyres. However, practically all these terms mean the same.

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Fig. 9.2 Relationship between tyre pressure and contact pressure.

The vertical stress variation with depth for a pavement loaded with wheel load at the

pavement surface may be obtained by using Boussinesq’s equation for uniformly distributed

load on a circular area given by

z = p ( )

1 3 2 - Z

a + z

3

2 2 /

(9.3)

Where z = is the vertical stress at depth z, kg/cm2

p = tyre pressure (equal to contact pressure), kg/cm2

Z = depth at which z is computed.

a = radius of loaded area.

Using the above equation the variation of vertical stress with depth is plotted in fig.

9.3.

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Fig. 9.3. Vertical Stress Distribution

As seen from the fig. 9.3, the influence of the tyre pressure is predominating in the

upper layers. At a greater depth the effect of tyre pressure diminishes and the total load

exhibits a considerable influence on the vertical stress magnitudes. Tyre pressures of high

magnitude therefore demand high quality of materials in upper layers of pavements. As such,

very hard and strong aggregates have to be used for the wearing surface of the pavements

used by steel tyre wheeled bullock carts, because of the very high stresses on the pavement

surfaces.

The total depth of the pavement is influenced by the Total Wheel load and not by the

tyre pressure.

b) Wheel Load Configuration: Since the wheel load should not exceed the permissible

legal wheel load (4085 kg as per IRC), to keep the maximum wheel load within the specified

limits, and to carry greater load, it may be necessary to provide dual wheel assembly to the

rear axles of the loaded vehicles. In doing so, the effect on the pavement through a dual

wheel assembly is obviously not equal to two times the load on any one wheel. The stress

distribution below a dual wheel assembly is shown in fig. 9.4.

Upto the depth of d/2 each wheel load ‘P’ acts independently and after this point the

stresses induced due to each load overlap. At a depth of 2s and more the stresses induced in

the pavement sub-grade may be considered as due to the influence of both the wheels since

overlap area is quite considerable. Hence due to dual wheels total stress at any depth, greater

than 2s may be considered to be equivalent to a single wheel load carrying 2P load. For

calculating the stress at any depth between d/2 and 2s the concept of `Equivalent Single

Wheel Load- (ESWL) is used. Equivalent single wheel loads may be calculated either on the

basis of equivalent deflection or stress. Mostly equivalent deflection criteria is adopted as it is

more reliable.

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Fig. 9.4 Stress Distribution due to Dual Wheels

Suppose a dual wheel load develops a certain value of maximum deflection (∆) at a

particular depth Z. According to equivalent deflection criteria, the ESWL is that single wheel

load having the same contact pressure which develops the same amount of maximum

deflection at the depth Z. Similarly according to the stress criteria, the ESWL is the single

wheel load developing the same intensity of maximum stress at the depth `Z’ as the dual

wheel load does. The methods of determining the ESWL are outside the scope of this course.

For further information the student may refer to the references given at the end of this lesson.

c) Repetition of Loads: The repeated application of wheel loads to highway pavements

result in not only the elastic deformation but also accumulated plastic deformations which in

turn may lead to pavement failure. The repeated load application also causes breaking down

of base course aggregate materials. Similarly the kneading action of traffic may cause

working upwards of the soil materials in sub-base or sub-grade. As per the finding of

AASHO Road Test, for a given axle load, the pavement thickness required to provide a given

terminal level of service is proportional to the Logarithm of the number of receptions of the

axle load.

9.1.2 Strength Characteristics of Pavement Materials:

For design purposes, it is required that various pavement materials are assigned

strength parameters suitable to the redesign methods employed. Various materials used in the

sub-base and base courses are evaluated by different tests. The general strength values

evaluated are (i) CBR value and (ii) Elastic Modulli.

9.1.3 Climatic Variations:

The climatic variations cause the following effects: (i) Variation in moisture

condition, (ii) Frost action and (iii) Variation in temperature. The pavement performance is

very much affected by the variation in moisture and the frost. This is mainly because of the

variations in stability and volume of the sub-grade soil due to these two effects. Variation in

temperature generally affects pavement materials like bituminous mixes and cement concrete

9.2 Pavement Components and their Functions:

The structure of a flexible pavement (fig. 9.1) mainly consists of (a) soil sub-grade,

(b) sub-base course, (c) base course and (d) wearning course.

9.2.1 Sub-grade:

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This is the natural layer of soil prepared to receive the layers of pavement materials

placed over it. The loads on the pavement are ultimately received by the soil sub-grade for

dispersion to the soil mass. The pressure transmitted on the top of the sub-grade soil should

be within the limits, that is, should not cause excessive stress condition or should not cause

excessive deformation beyond the elastic limit. The soil subgrade is evaluated by one of the

following tests - C.B.R, Triaxial compression test or plate bearing test.

9.2.2 Sub-base and base courses:

These layers are made up of broken stones bound or unbound. Sub-base course and

base courses are used in flexible pavements primarily to improve the load supporting capacity

by distributing the load through a finite thickness. Sub-base courses are generally made of

inferior materials than base courses. The sub-base and base courses may be evaluated by

suitable strength or stability tests like plate bearing, CBR or stabilometer test.

9.2.3 Wearing Courses:

The purpose of wearing course is to give a smooth riding surface that is dense. It

resists pressure exerted by tyres and takes up wear and tear due to the traffic. Wearing course

also offers a water-tight layer against the surface water infiltration. In flexible pavements,

normally bituminous surfacing is used as a wearing course. In rigid pavements the concrete

slab itself acts as a wearing course. There is no test for evaluating the structural stability of

wearing course. However, the bituminous mixes used in wearing courses are evaluated by

Marshall Stability test. Plaste bearing test and Bankel man Beam test are also used for

evaluating the wearing courses as well the pavement as a whole.

9.3 Choice of the Type of Pavement:

Various factors to be accounted for the selection of pavement type are:

i) Amount and type of traffic.

ii Sub-grade soil conditions.

iii) Cost of materials, construction and subsequent maintenance charges

iv) Anticipated life of pavement, and,

v) Available finance.

9.4 Design Methods of Flexible Pavement:

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Upto 1970, there was no standard for pavement design and a number of empirical and

semi-empirical methods were being used for determination of pavement thickness. The IRC

in 1970 has brought out (IRC: 37 - 1970).These guide lines have been revised in 1984 (IRC:

37 - 1984).

The design of flexible pavements is normally based on Group Index Method and

California Bearing Ratio method are only discussed here.

9.4.1 Group Index Method:

This method is due to D.J.Steele (1945). The ground index of a soil, which depends

on the percentage fines, liquid limit and plasticity index, is believed to rate the sub-grade soil,

and hence indicates the pavement thickness needed to protect the sub-grade. This method

classifies the traffic into the following three categories.

Traffice Intensity No. of commercial vehicles per day

Light less than 50

Medium 50 to 300

Heavy over 300

The sub-grade soil has been rated based on the group index. The higher the group

index value, weaker is the soil sub-grade and for constant value of traffice volume, the

greater would be the thickness requirements.

To design the pavement thickness by this method, first group index of the soil is

found. The anticipated traffic is estimated and designated as light, medium or heavy as per

the above table or as indicated in fig. 9.5a. The appropriate design curves are chosen from fig.

9.5 b. and the total thickness of pavement and thickness of surface and base courses are found

based on the group index value of the sub-grade soil.

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Fig 9.5 (a)

Fig. 9.5 (b)

MERITS AND DEMERITS OF THE GROUP INDEX METHOD:

This is the most simple method of design because it utilizes directly. the data

normally collected for soil classification procedure. It allows for both traffic conditions and

the sub-grade soil characteristics. Since this method does not take into account the load

dispersal abilities of paving materials, the thickness obtained by this method generally tends

to error on the safe side. The method utilizes the curves which have been developed for

particular conditions of sub-grade compaction and moisture. Hence the use of these curves is

only limited to fields when these conditions are met in.

Worked Example:

9.1 Traffic expected on a road is 250 commercial vehicle and it is required to design the road

pavement by Group Index method. Sub-grade soil is taken from the site when analysed gave

the following information.

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1) Soil passing I.S. Sieve No. 8 60%

2) Liquid Limit 40%

3) Plastic limit 22%

Solution:

Plasticity Index 40-22 = 18

Group Index is given by (Equation 7.1)

GI = 0.2a + 0.005 ac + 0.01 bd

a = 60 - 35 = 25

b = 60 - 15 = 45

c = 40 - 40 = 0

d = 18 - 10 = 8

Substituting in the above equation.

G.I = 0.20 × 25 + 0.005 × 25 × 0 + 0.01 × 40 × 8

= 8.2 or say 8.

Traffice volume 250 vehicles comes under medium category, From Fig. 9.5, for a

group Index of 8. Combined thickness of base, sub-base and surface is 40 cm.

Thickness of selected sub-base only = 18 cm.

Thickness of base only = 8 cm

The pavement section is shown above Fig. 9.7.

9.4.2 California Bearing Ratio Method:

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In 1928, California Division of Highways developed this method of design. CBR tests

were conducted by the California state highway department on existing layers including sub-

grade, Sub-base and base course. Based on the extensive data collected on pavements which

behaved satisfactorily and those which failed, an empirical design chart was developed

correlating the CBR value and the pavement thickness. The basis of the CBR design chart is

that a material with a given CBR requires a certain thickness of pavement layer as a cover. A

higher load needs a thicker pavement layer to protect the sub-grade.

The Indian Road congress has developed a design chart (fig. 9.6) for use in our

country. Different curves have been given based on different volumes of traffic.

Fig. 9.6

Studies carried out by the U.S. Corps of Engineers have shown that there exists a

relationship between the pavement thickness, wheel load, tyre pressure and CBR value of the

sub-grade within a range of 10 to 12%. It is possible therefore, to extend the CBR design

curves for various loading condition using the expression.

tCBR

= P 1.75 - 1pπ

1/2

= 175 1 2. /PCBR

- Aπ

(9.4)

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Where t = Pavement thickness; cm

P = Wheel load, kg

CBR = California Bearing Ratio, in %.

p = Tyre pressure, kg/cm2.

A = Area of contact, cm2.

Pavement Thickness Determination: For this, soaked CBR value of the sub-grade soil and

C.B.R. values of other materials to be used in each layer is a pre-requisite. Depending upon

the anticipated traffic intensity appropriate design curve is chosen from fig. 9.6. Sub-grade

layer is the bottom most layer in the flexible pavement. Hence according to the CBR value

(soaked) of the sub-grade and the curve chosen based on intensity of traffic read the required

pavement thickness from the design chart 9.6. This will give the required cover over the sub-

grade to protect it. Sub-base layer is laid over the sub-grade. Now according to the CBR

value of sub-base material and using the same design curve, the thickness of pavement

required above the sub-base course is obtained again from the fig. 9.6. The thickness of sub-

base is obtained by deducting the thickness of cover required over sub-base from the

thickness of cover required over the sub-grade. Similarly thickness of the other subsequent

layers may be determined. The method is illustrated in the worked out example 9.2.

IRC Recommendations: Some of the important recommendations of the IRC for the CBR

method of design (IRC: 37-1970) are given below:

1. CBR test should be performed on remoulded soil and not on insitu- soil. The specimens

should be prepared by static compaction where-ever possible and otherwise by dynamic

compaction.

2. For design of new roads sub-grade sample should be compacted at O.M.C. to I.S. light

compaction density, when suitable equipment is available in the field for compaction to this

density. Otherwise the soil sample may be compacted to the dry density desired to be

achieved in the field. In the case of old existing roads, samples should be compacted to the

field density of the sub-grade soil at the field moisture content (F.M.C.)

3. Soil sample may kept soaked in water for 96 hours in the case of new-constrictions before

CBR test is made on them. But in cases of areas where rain fall is less than 50 cm and water

table is also quite deep, it is not essential to soak the test sample.

4. At least three samples should be tested before assigning CBR value of a particular soil. If the

variation in the CBR values in the three tests is beyond the specified limits as least six test

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specimen should be tested and average value should be assigned as CBR value. Limits of

variation are as follows.

Upto CBR value of 10% 3

10 to 30% CBR values 5

30 to 60% CBR values 10

5. Top 50 cm thickness of sub-grade should be compacted to such an extent that 95 to 100% I.S.

light compaction (Proctor) density is achieved.

6. Thin layers of bituminous wearing course upto a thickness of 2.5 cm should not be

considered towards the total thickness of the pavement since they do not increase the

structural strength of the pavement.

7. Traffic intensity per day on the road pavement has been divided into seven categories ranging

from A to G as shown in fig.9.6. Traffic is considered in terms of vehicles having laiden

weight exceeding 3 tonnes.

8. To estimate the value of traffic at the end of life span of the pavement, IRC recommends the

following expression.

A = P(1+r)n+10 (9.5)

where A = number of heavy vehicles per day for design

(laiden weight > 3 tonnes).

P = number of vehicles per day at count

r = annual rate of increase of heavy vehicles

(taken as 7.5% in the absence of any data).

n = number of years between the last count and the year of

completion of construction.

Worked Example :

9.2. Soil sub-grade sample was obtained from the site of a road project and the CBR

value was determined as 4%. It is further desired to use the following materials for different

pavement layers.

i) Compacted sub-grade 7% CBR

ii) Poorly graded gravel 20% CBR

iii) Well grade gravel 95% CBR

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iv) Minimum thickness of bituminous concrete surfacing

with CBR of above 80% 5 cm.

The traffic survey revealed the ADR of commercial vehicles as 1200. The annual

growth of traffic is found to be 8%. The pavement construction is to be completed in three

years after the last traffic census.

a) Design the pavement section using the CBR method as recommended by the

IRC.

b) Suggest alternate design without using poorly graded gravel.

c) Discuss the limitations of the CBR method of pavement design in the light of the

above results.

Solution :

Design traffic A = P(1+r)n+10

= 1200 (1+0.08)(3+10) = 3260 vehicles/day.

Curve ‘F’ in the chart 9.6 includes this traffic range. Hence is the design curve.

The thickness of material above different layers of different CBR values are obtained

from fig.9.6 as follows.

Sub-grade soil 4% CBR 55 cm

Compacted sub-grade 7% CBR 40 cm

Poorly grades gravel 20% CBR 21 cm

Well graded gravel 95% CBR 8 cm

Bituminous concrete surface 80% CBR 8 cm.

The designed pavement section for cases (a) and (b) are shown in fig.

CASE -A

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CASE -B

Discussion :

The CBR methods has one serious dis-advantage. It may be seen that the total

thickness of construction remains the same, that is 55 cm, though the pavement component

layers in cases (a) and (b) are of different materials with different CBR values. The thickness

of construction over compacted soil of CBR value 7% is the same in both the cases equal to

40 cm, though in one case poorly graded gravel of CBR 20% is used where as in the second

case, it has been replaced by well graded gravel of CBR value 95%.

The CBR method of pavement design gives the total thickness requirement of the

pavement above a sub-grade and this value of thickness would remain the same irrespective

of the quality of materials used in the component layers.

Equivalency Factors:

Bound type of bases have superior load spreading properties and therefore a certain

reduction in the overall pavement thickenss could be permitted. Well constructed bituminous

macadam base will be considered of twice the strength of water bound macadam road. For

other bound type of base courses like lean concrete, lime pozzolona concrete and soil cement,

the equivalency factor may be taken as 1.5. Corresponding reduction in thickness may be

made.

9.4.3. C.B.R. Method of Pavement Design Based on Cumulative Standard Axle Load.

(IRC : 37 - 1984) : In the C.B.R. method of design discussed so far, the traffic is considered

in units of heavy vehicles (laden weight greater than 3 tonnes) per day in both directions

irrespective of whether the design is for a two lane or a dual carriage way. However, it has

been recognised by the field engineers and research workers alike that other factors like

repetition of axle loads, distribution of traffic over the pavement width (depending on the

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number of lanes) type of terrain and’environmental factors affect considerably the thickness

of pavement. Accordingly the I.R.C. has revised the method of design of flexible pavements.

In the revised method, two entirely different apporaches have been given for arriving

at the total thickness of pavement based on traffic intensity. The two approaches are

discussed in the following paragraphs.

a) Design procedure when the Traffic Intensity is less than 1500 Commercial Vehicles /

day - C.B.R. Method.

This method is similar to method already discussed in para 9.4.2. The C.B.R. design

curves for use in this method are developed for a legal axle load of 10.2 Tonnes as against

8.165 Tonnes used in design curves have been provided for arriving at the total thickness of

the pavement for each of the categories of traffic. The traffic is arrived at by using equation

9.5. By choosing proper curve based on the designed traffic volume, knowing the CBR

values of the sub-grade (soaked condition), sub-base and base course materials, the thickness

of component layers of the pavement are obtained by repeated use of the design curves. The

minimum thickness of sub-base and base courses should be 150mm. For the materials to be

used for base course a minimum CBR of 80 has been recommended.

b) Design procedure when the Traffic Intensity is more than 1500 Vehicles / day - The

Cumulative Axle Load Method

In this method the ‘Equivalent Axle Load Concept’ is used. For Design purpose, the

cumulative number of standard axles (8160 kg) to be carried by the road during its life is

considered as the design traffic. The cumulative number of standard axles to be catered for in

the design is arrived at from the initial volume of traffic, by the following formula :

NS =( )[ ]365 1 1× + −A Fxγ

γ9.6

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9.7 CBR Method of Pavement Design by Cumulative Standard Axle Load.

where NS = The cumulative number of standard axles to be catered for in

design.

r = Annual Growth Rate of Commercial Vehicles

x = Design life in years

F = Vehicle Damage Factor (recommended by the IRC) based on

initial traffic volume and terrain)

A = Initial volume of traffic in the year of completion of

construction, in terms of commerical vehicles / day duly

modified to account for lane distribution (Distribution factors

have also been recommended by IRC)

For the value of the designed cumulative number of standard axles (NS) total

thickness of pavement may be arrived at for the CBR value of the sub-grade referring to Fig.

9.7. Thickness of component layers - surface course, Granular base course and sub-base

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course, and their requirements, based on the value of NS has been recommended by IRC (IRC

: 37 - 1984).

9.5. Self Assessment Questions :

1. Explain ‘Flexible and Rigid pavements and bring out the points of difference.

2. What is ‘ESWL’? Explain the concept in determination of the equivalent load.

3. What is the basis on which thickness of pavement is determined based on Group Index

method ? What are the limitations of this method?

4. Explain the merits and demerits of the CBR method of flexible pavement design.

5. The CBR value of sub-grade soil is 5%. Calculate the thickness of pavement required

given that the wheel load is 4082 kg and tyre pressure is 7kg/cm2 using the U.S. Corps of

Engineers Formula. (Ans. 35.4 cm).

12.6. SUMMARY :

Based on their structural behaviours, pavements may be divided into two classes -1)

Flexible pavements and 2) Rigid pavements. The term flexible pavement is associated with

those pavements which reflect the deformation of the sub-grade and subsequent layers on to

the surface. The design of flexible pavements is based on the load distribution characteristics

of the component layers. The rigid pavements, because of its rigidity and high modulus of

elasticity, tend to distribute the load over a relatively large area of soil. thus a major portion

of the structural capacity is supplied by the slab itself.

Various factors to be considered for the design of pavements are (i) Design wheel

load (ii) Strength characteristics of pavement materials and (iii) climatic variations, have been

discussed.

Design of flexible pavements based on (i) the Group Index Method and (ii) the C.B.R.

method are only explained in this unit.

The group index of a soil is believed to rate the sub-grade soil and hence indicates the

pavement thickness needed to protect the sub-grade. This method of design also takes into

account the traffic intensity in arriving at the thickness of pavement.

The IRC (IRC: 37-1970) recommends a design method based on the CBR values of the

pavement material with a given CBR value requires a certain thickness of pavement layer as

a cover. A higher load needs a thicker pavement layer to protect the sub-grade. Design charts

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are prepared by the IRC relating the thickness of construction required and the CBR value of

the pavement material based on the traffic intensity. For the seven groups A to G of traffic,

seven curves are available in the design chart. The traffic intensity to be taken into account

for design purpose in terms of the number of vehicles per day exceeding 3 tonees laden

weight at the end of design period which is normally taken as 10 years. Design of Flexible

pavements based on the cumulative number of standard axle (IRC : 37 - 1984) is also

explained.

12.7. REFERENCES :

1. IRC : 37 - 1970 ; Guide Lines for Design of Flexible Pavements.

2. IRC : 37 - 1984 ; Guide Lines for Design of Flexible Pavements.

3. Khanna, Dr.S.K. and Justo, C.E.J. (1984) - Highway Engineering, Nemchand and Bros.,

Roorkee

4. Ramana Sastry, Dr. M.V.B.R. and Gopal M.S.P., - Computer Oriented Design For

Flexible Pavements - Indian Highways - June, 1987.

Yoder, E.J. and Witczak (1975) - Principles of pavement Design - John Wiley and Sons,

INC., New - York..

***


UNIT - 10

RIGID PAVEMENT DESIGN

CONTENTS :

Aims / Objectives

10.1. Introduction

10.2. Stresses due to Loading

10.3. Stresses due to changes in Temperature

10.4. Combination of Stresses

10.5. IRC Recommendations

10.6. Joints in Concrete Pavements

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10.7. Self-assessment Questions

10.8. Summary

10.9. References

AIMS / OBJECTIVES :

Stresses are set up in concrete road slab by loading, by changes in temperature and

moisture content and by other causes. Methods of calculation of these stresses and the design

principles of concrete road slabs based on the IRC recommendations are presented. Joints are

provided in concrete pavements to allow for movements of the slabs due to changes in

temperature and moisture content. Considerations for the spacing of these joints are also

discussed.

10.1. INTRODUCTION :

Stresses are set up in a concrete road slab by loading, by changes in temperature and

moisture content, and other causes. The worst combination of stress - producing failure

conditions have to be considered for design of these slabs. Since concrete is very much

weaker in tension than in compression, only maximum tensile stresses need be considered.

As such cement concrete pavement should be designed and controlled on the basis of flexural

strength of concrete of 4000 KN / m2 (40 kg / cm2).

The stresses due to loads and due to changes in temperature are of the same order of

magnitude and are considerably grater than those due to changes in moisture content. The

stresses caused by the resistance of the sub-grade to expansion and contraction produced by

change in temperature and moisture content are relatively smaller in short slabs, but may be

more important in long slabs. The stresses caused by dilatency of the sub-grade are small and

need not usually be considered.

Joints are provided in concrete roads to allow for expansion, contraction and warping

of the slabs caused by the changes in the temperature and moisture content of the concrete.

They are also necessary to allow for the break in construction at the end of days work and to

allow the road to be laid in lanes of convenient width. The number of joints should be kept to

a minimum compatible with the above requirements, because the construction of joints

involves a considerable amount of extra work and is liable to interfere with smooth progress

of concreting. The design of various types of joints is also considered.

10.2. STRESSES DUE TO LOADING :

CE409/16 167

Several theories have been developed on the occurrence of stresses in concrete slabs

resting on a uniform bed. The most commonly used method of calculation of the stresses in a

slab is due to Westergaurd. The theory applies to slabs of finite extent, and expressions are

derived for stresses due to loading at the interior, edges and corners of the slabs. Westergaurd

assumed that the concrete slab is homogeneous and has uniform elastic properties, and that

the reaction of the sub-grade (p) is vertical and proportional to the deflection (∆). In other

words he assumed that the support given to the slab is similar to that given by a dense fluid

and hence that the sub-grade has no shear strength. Then according to Westergaurds

assumption p = K∆ where K is defined as the ‘modulus of sub-grade reaction’. The modulus

of sub-grade reaction is also referred to as ‘Spring constant’ and as the ‘dense liquid

constant’. The modulus of sub-grade reaction is determined by means of the plate load test

using a plate of 75 cm dia or square. The modules of sub-grade is the stress per unit

deflection and the units are KN/m3 (Kg/cm3).

10.2.1. Critical Load Positions :

Westergaurd considered stresses produced by three conditions of loading (i) loading

at positions away from the edges (interior loading) (ii) loading at the edges (but away from

the corners) and (iii) loading at the corners. These positions are shown in Fig. 10.1.

Westergaard also assumed that the wheel load is uniformly distributed over a circular area of

contact in the case of interior and corner loading and over a semi-circular area of contact in

the case of edge loading. The position and direction of the tensile stresses for each of critical

positions of the loading are as follows.

a) Loading at the interior - at the bottom of the slab and of the same magnitude in all

directions.

b) Loading at the edge-at the bottom on the slab parallel to the edge. (Another smaller

tensile stress will occur at the top of the slab at right angles to the edge).

c) Loading at the corner - at the top of the slab parallel to the bisector of the corner

angle.

CE409/16 168

Fig. 10.1. Critical Load Position

The stress equations given by Wester-gaurd are as follows.

a) Interior Loading :

Si = 0.316 P / h2 (4 log10 (l / b) + 1.069) 10.1

b) Edge Loading :

Se = 0.572 P / h2 (4 log10 (l / b) + 0.359) 10.2

c) Corner Loading :

Sc = 3 / h2

−

6.0/21lca 10.3

Here,

Si, Se and Sc = maximum stress for interior, edge and corner loading, respectively,

kg/cm2.

h = slab thickness, cm

P = wheel load, kg

a = radius of contact area (wheel load distribution) cm.

l = radius of relative stiffness, cm.

b = radius of resisting section, cm.

The terms radius of relative stiffness and radius of resisting section are explained

below.

Radius of Relative Stiffness - The pressure deformation characteristics of a cement

concrete slab is a function of the relative stiffness of slab to that of the sub-grade. According

to Wester gaard the radius of relative stiffness is given by the following equation :

CE409/16 169

l = ( )Eh

k

3

2412 1− µ

10.4

Equivalent radius of Resisting Section - Considering the positions of interior loading

and edge loading, the maximum bending moment occurs at the loaded area and acts radially

in all directions. With the load concentrated on a small area of the pavement the question

arises as to the sectional area of the pavement, that is effective in resisting the bending

moment. According to Wester gaard, the equivalent radius of resisting section is

approximated, in terms of radius of load distribution and slab thickness as follows.

b = 1.6 a 0.675 h when a / h 1.7242 + − ≤h2 10.5

where E = Modulus of Elasticity of concrete, Kg / cm2

µ = Poissons ratio of concrete,

k = Modulus of sub - grade reaction - Kg / cm3

Other terms have been already defined.

When a/h > 1.724, b = a.

10.2.2. GOLD BECK’S FORMULA :

Gold beck indicated that many concrete slabs failed at the corners. Gold beck’s

formula for stress due to corner load is given by

Sc = 3 P / h2

10.2.3. BRAD BURY’S EQUATION :

Brad bury assumed partial sub-grade support of the pavement slab at the edges and

gave the following empirical equation based on the use of Westergaard’s theory.

σ = (P / h2) Q

here σ = Stress, kg/cm2

P = Wheel load, kg

Q = Brad bury’s stress coefficient.

The value of stress coefficient, Q is determined by the ratio of 1/b in the case of

interior and edge loading and by the ratio of a / 1 in the case of corner loading. These

values may be used for calculating the wheel load stresses.

CE409/16 170

The stress coefficients Qi and Qe for interior and edge loading are given in Table 10.1

for various values of l/b. The values of Qc for corner loading are given in Table 10.2.

10.2.4. FORMULAE RECOMMENDED BY THE I.R.C. :

The I.R.C. recommends the following two formulae for analysis of load stresses at the

edge and corner regions and for the design of rigid pavements.

1. Load stress Se in critical edge region.

Se - 0.529 P/h2 (1+0.54µ) (4 log10(l/b) + log10b - 0.4048) 10.8

(This is Wester gaard’s Equation modified by Teller and Sutherland).

ii. For load stress Sc at the critical corner region.

Sc = 3 P / h2 ( )1 2 11 2

−

a /.

10.9

(This is Wester gaard’s equation modified by Kelley)

Various terms have already been defined.

WORKED EXAMPLE :

10.1. Calculate the stresses by Westergaard and Brad-burys theory in a concrete pavement

slab given the following data.

DATA : Wheel load = 5100 kg.

Modulus of Elasticity of concrete = 3.0 × 105 kg/cm2

Pavement thickness = 18 cm

Poissons ratio of concrete = 0.15

Modulus of Sub-grade reaction = 6.0 kg/cm3

Radius of contact area = 15 cm

SOLUTION : 1) By Wester gaard’s theory :

1. 1 = radius of relative stiffness = ( )Eh

k

3

2412 1− µ

=( )3 10 18

12 1 015 670 6

5 3

2

1 4

× ×

− ×

=.

.

/

cm

2. Since a/h = 15/18 = 0.833 < 1.724

b = Equivalent radius of resisting section is given by

CE409/16 171

= 16 0 675 16 15 18 0 675 152 2 2 2. . . .a h+ − = × + − ×

= 14.4 cm.

3. Stress at the interior is given by

Sc = 0.316 P/h2 [4 log10 (l/b) + 1.069]

= 0.316 ×5100/182[4 log10 (70.6/14.0) + 1.069]

= 19.3 Kg/cm2.

4. Stress at the edge is given by :

Se = [ ]0572 4 0 359 0 57218

51002 2

. ( / ) . ..ph

l b log10 + = ×

= 4 70 614 0

0 359 2854 2log10..

. . /

+

= kg cm

5. Stress at the corner is given by

Sc = 3P/h2 1 2 0 6 3 510018

1 15 270 6

0 62−

= ×−

al

..

.

= 24.27 kg/cm2

2. Bradburys Coefficients Method :

for the slab l/b = 70.6/14.4 = 4.90

from Table 10.1. We get Qi = 1.21 and Qe = 1.80

TABLE - 10.1

RIGID PAVEMENT DESIGN TABLES

Stress Coefficients for Interior and Edge Loadings (due to Bardbury)

l/b Qi Qe l/b Qi Qe

1 2 3 1 2 3

1.0

1.5

2.0

2.5

3.0

0.34

0.56

0.72

0.85

0.94

0.41

0.61

0.89

1.12

1.30

2.0

8.5

9.0

9.5

10.0

1.48

1.52

1.55

1.58

1.6

2.23

2.33

2.39

2.44

2.49

CE409/16 172

3.5

4.0

4.5

5.0

6.0

6.5

7.0

7.5

8.0

1.02

1.10

1.17

1.22

1.33

1.38

1.41

1.45

1.48

1.45

1.58

1.78

1.81

1.99

2.07

2.14

2.21

2.27

10.5

11.0

11.5

12.0

13.0

13.5

14.0

14.5

15.0

1.63

1.66

1.68

1.71

1.75

1.77

1.79

1.81

1.83

2.54

2.59

2.63

2.68

2.75

2.80

2.83

2.86

2.90

Then stress due to interior loading

Si = 1.21 × 5100/182 = 19.04 kg/cm2

Stress due to edge loading

Se = 1.80 × 5100/82 = 28.34 kg/cm2

Stress due to corner loading

for a 2 1 0 300 154/ . , . = 15 270.6

10.2× = =Qc from Table

Then Sc = 1.54 × 5100/182 = 24.24 Kg/cm2

Qi - Bradbury’s stress coefficient for interior loading

Qe -Bradbury’s stress coefficient for edge loading.

Table 10.2

Stress Coefficient for Corner Loading (due to Bardbury)

a 2ι

Qc a 2ι

Qc a 2ι

Qc

1 2 1 2 1 2

0.00

0.05

0.10

3.00

2.50

2.25

0.25

0.25

0.30

1.86

1.69

1.54

0.40

0.45

0.50

1.27

1.14

1.02

CE409/16 173

0.15

0.20

2.04

1.86

0.35

0.40

1.40

1.27

0.55

0.60

0.90

0.79

Qc - Bradburys stress Coefficient for Corner Loading

10.3 STRESSES DUE TO CHANGES IN TEMPERATURE :

Changes in air temperature are followed by (I) changes in temperature gradient

through the slab which may cause the slab to warp; such warping may be wholly or partly

restrained by the weight of the slab and by the reactions of dowel bars and other load transfer

devices in the joint (2) changes in the mean temperature of the slab causing it to expand or

contract; this movement may be wholly or partly restrained by the frictional resistance of the

sub-grade and the resistance of the joints. These two effects are considered separately here.

10.3.1. WARPING STRESSES : Whenever the top and bottom surfaces of a concrete

pavement simultaneously possess different temperatures the slab tends to warp downward or

upward : but the weight of the slab restrains the warping resulting in warping stresses. This

differential temperature is due to the thickness of the slab because of which the bottom

surface reacts to the change more slowly than top.

If Tp and Tb are the temperatures at the top and bottom of the slab, then the mean

temperature would be 1/2 (Tp+Tb) = t, say. If the slab has no restraint, then the unit elongation

of the top fibers and also unit contraction of the bottom fibers due to relative temperature

condition, each would be equal to E α t/2 where α is the thermal coefficient of concrete.

Fig. 10.2. Warping Stress Coefficient

Now introducing the effect of Poisson ratio, the stresses at the interior region in the

longitudinal and transverse directions as given by Bradbury.

CE409/16 174

Sti = ( )E t C Cx yα µ

µ2 1 2

+−

10.10

The warping stresses at the edge and corner regions of the concrete pavement are :

Ste = CxE t or Cy E tα α

2 2= (which ever is higher) 10.11

and Stc = ( )E t aα

µ3 11

−/ 10.12

Where Sti, Ste and Stc are the warping stresses at the interior, edge and corner positions of the

slab in kg/cm2.

t = temperature differential between the top and bottom of the slab in degree C,

C = (Tp+Tb)/2

Cx = Coefficient based on Lx/1 in desired direction

Cy = Coefficient based on Ly/1 at right angle to the above direction

Lx and Ly are the dimensions of the slab considering along x and y directions of the

length and width of the slab. Other terms are as defined earlier.

The values of the warping stress Coefficients Cx and Cy for cement concrete

pavements are taken from the chart developed by Bradbury and presented in Fig. 10.2.

10.3.2. FRICTIONAL STRESSES :

Due to uniform temperature rise and fall in the concrete slab, there is an overall

expansion and contraction of the slab respectively. Since the slab is in contact with soil sub-

grade, the slab movements are restrained due to the friction between the bottom layer of the

pavement and the soil layers. This frictional resistance therefore tends to prevent the

movement thereby induce frictional stresses in the bottom fiber of the road slab. Stresses in

the slab due to this phenomenon vary with slab length. In short slab, stress induced due to this

is negligibly small where as in long slabs, which would under go movements of more than

0.15cm, considerable amount of frictional stresses develop.

Frictional resistance due to sub-grade restraint,

Sf = WLF2 104×

10.13

here Sf = Frictional stress developed in kg/cm2

W = Unit weight of concrete, kg/m3 (2400 kg/m3).

CE409/16 175

f = Coefficient of sub-grade restraint (about 1.5).

L = Length of slab in metres.

10.4. COMBINED STRESSES :

It is necessary to consider the conditions under which the various stresses in cement

concrete pavements would combine to given the most critical combinations (i) During

summer at mid-day when the slab tends to curl down, then the critical combination of stresses

for the interior and edge region becomes.

= Wheel load stress + Temperature warping stress - Sub-grade restraint stress 10.14

(ii) During winter at mid-day when the slab tends to curl down, the critical combination

of stresses for interior and edge region becomes,

= Wheel load stress + Temperature stress + Sub-grade restraint stress 10.15

(iii) In case of corner region the critical combination of stresses exits when the slab curls

upwards, therefore the magnitude of critical stresses become

= Wheel load stress + Temperature warping stress 10.16

For the interior and edge regions, since the temperature differential during winter is of

lower magnitude than in summer, the critical combination of stresses, therefore, are higher in

summer than in winter and become as the design criteria for the conditions in our country.

WORKED EXAMPLE

10.2 Calculate the combined load and temperature stress for the longitudinal end condition

and corner region on the basis of the following data :

1. Design wheel load = 5100 kg.

2. Thickness of pavement = 20 cm

3. Modulus of sub-grade reaction = 15 kg/cm3

4. Modulus of Electricity of concrete = 3.0 × 105 kg/cm2

5. Poissons ratio of concrete = 0.15

6. Equivalent radius of contact area = 15 cm

7. Slab-dimensions = 4.5m × 3.5 m

8. Thermal coefficient of concrete (α) = 10 × 10-6/oc.

9. Temperature difference = 180C.

10. Safe flexural strength of concrete mix = 38.5 kg/cm2

CE409/16 176

SOLUTION :

Since a/h = 15/20 = 0.75 is less than 1.724

Radius of resisting section = b = 16 0 6752 2. . a + −h h

= 16 20 0 675 202. .× + − ×152

= 14.0 cm

Stress developed due to edge loading

Se = ( )[ ]0572 4 0 3592

. / .ph

l b log10 +

Radius of relative stiffness =1= ( )Eh

k

3

2

1 4

12 1−

+µ

/

= ( )30 10 20

12 1 015 15

5 3

2

1 4

.

.

/

× ×

−

= 60.8cm

Then Se = 0.572 × 5100/202 4 60814 0

0 359 log10..

.

+

= 24.0 kg/cm2

Warping stress along the longitudinal edge is given by

Sce = Cx E α t/2

Length of slab = 4.5 m = Lx

∴ Lx / 1 = 4.5 × 100/60.8 = 7.4

Cx from Fig. 10.2 for Lx/l = 7.40 is 1.02

Sie = 102 30 10 10 10 182

27 545 6

2. . . /× × × × × =−

kg cm

Frictional stress =W L F/2 ×104 = (2400 ×4.5 ×1.5) / (2 ×104) = 0.81 kg/cm2

small, since the slab is a short slab and hence neglected. Combined stress = load stress +

warping stress

= 24.0 + 27.54 - 0.81 = 50.73 kg/cm2

Corner region Sc = 3P/h2 ( )1 20 6

−

a l/.

= 3 ×5100/202 ( )1 15 2 608 28 00 6 2−

=/ . . /.

Kg cm

CE409/16 177

Max. warping stress = Eαt/3(1-µ) la /

= ( ) ( )[ ]3 10 10 10 18 3 1 015 60 8 9155 6 2× × × × − =− / . / . . / 15 kg cm

Frictional stress = Zero at corner region

Combined stress = Load stress + Warping stress

= 28.0+9.15 = 37.15 kg/cm2

10.5. I.R.C. RECOMMENDATIONS :

10.5.1. DESIGN PARAMETERS :

The following design parameters may be assumed.

1. Wheel load = 5100 kg; equivalent circular are of 15 cm

2. Tyre pressure = 6.3 to 7.3 kg/cm2

3. Traffic value may be projected to for 20 years. Based on traffic intensity corrections have

to be carried out to the designed thickness. These corrections are given in IRC : 58 - 1974,

Guidelines for Design of Rigid pavements.

4. Temperature differentials for calculating the warping stresses have been recommended

based on thickness of slab end region and are available in IRC: 58-1974.

5. Modulus of sub-grade reaction K is to be determined using standard plate of 75cm

diameter at 0.125 cm deflection. The minimum K - value of 5.5. kg/cm3 is specified for

laying cement concrete pavement. In clayey sub-grades a suitable sub-base course may be

provided to increase the K -value.

6. The flexural strength of concrete used in the pavement should not be less than 40 kg/cm2.

µ = 0.15 α = 10 × 10-5 per 0C.

E = 3 × 10+5 kg/cm2

10.5.2. CALCULATION OF STRESSES :

1. The wheel load stresses at the edge and corner regions are calculated using the equations

(10.8) and (10.9), respectively.

2. Temperature stresses are calculated using the equations (10.11) and (10.13).

3. The critical combination of stresses in summer is obtained and the flexural stress so

obtained should be less than 40 kg/cm2 for the designed thickness of slab.

4. The design thickness is adjusted for traffic intensity as explained in 10.4.1. above.

CE409/16 178

10.5.3. SPACING OF JOINTS :

The following maximum spacing are recommended.

Nature of

Joint

Maximum spacing

metres

Remarks

1)Expansion

Joints

2) Contrac-

tion Joint

140

90

120

50-60

4.5

Foundation is rough for slabs of all thickness for

25mm wide expansion joint

For smooth foundation surfaces of slabs constructed

in summer for slab thickness upto 20 cm.

For slabs up to thickness of 25 cm.

When the construction is carried out in winter

For unreinforced slabs of all thickness

10.6. JOINTS IN CONCRETE PAVEMENTS :

Joints may be broadly divided into transverse and longitudinal joints. Transverse

joints may be conveniently classified into four groups - expansion joints, contraction joints,

warping joints, and construction joints. Longitudinal joints are required in concrete roads

more than 4.5 m wide to allow for transverse warping and to allow for uneven settlement of

the sub-grade.

10.6.1. SPACING OF EXPANSION JOINTS :

The width of the gap in the expansion joint depends upon the length of slab.

Expansion joint spacing is designed based on the maximum temperature variations expected

and width of the joint. Dowel bars are provided at expansion joints for load transfer form one

slab to the other. It is recommended not to have a gap more than 2.5 cm for an expansion

joint. If δ’ is the maximum expansion in a slab of length Le with a temperature rise from t1 to

t2 in degrees centigrade, then the spacing of the expansion joints is given by

CE409/16 179

Le(metres) = ( )δ1

1100 C t2 − t10.17

where C is the coefficient of expansion of concrete δ’ expansion of the slab in cm. It is

assumed that the joint filler may be compressed upto 50% of its thickness and therefore the

expansion joint gap should be twice the allowable expansion in concrete (i.e.,). 2 δ’.

Fig 10.3. TYPICAL EXPANSION JOINT

10.6.2. SPACING OF CONTRACTION JOINTS :

The slab contracts due to the fall of slab temperature below the construction

temperature. This movement is resisted by the sub-grade drag or friction between the bottom

fiber of the slab and the sub-grade. Length of slab to resist the frictional drag, that is spacing

of contraction joints,

LC = 2 104ScWf

× 10.18

Here

LC = spacing between the contraction joints, m.

f = Coefficient of friction (1.5)

W = Unit weight of slab, kg/m3 (2400 kg/m3)

Sc = Allowable stress in tension in cement concrete.

10.6.3. WARPING JOINTS :

If expansion and contraction joints are properly designed and constructed there is no

need of providing warping joints in addition.

Construction aspects of these joints are discussed n Unit No.12.

10.7. SELF - ASSESSMENT QUESTIONS :

1) Find out the spacing of expansion and contraction joints given the following data.

Expansion gap. 2.5 cm.

CE409/16 180

Laying temperature of concrete = 100C

Slab temperature in summer = 540C.

Coefficient of thermal expansion of concrete = 10 × 10-6 / 0C.

Coefficient of friction = 1.5

Ultimate tensile stress in concrete = 1.6 kg/cm2

Factor of safety = 2.

NOTE : δ1 in the formula = 2.5 / 2 = 1.25 cm.

(Ans : Spacing of Expansion joint = 28.5m, spacing of contraction joint = 4.44m).

2) Explain the terms (a) Modulus of sub-grade reaction (b) Radius of relative stiffness

and (c) radius of resisting section.

3) Distinguish between warping stresses and frictional stresses.

10.8. SUMMARY :

Stresses are set up in concrete road slabs by wheel loading and by changes in

temperature. The wheel load stresses are calculated for three critical load positions, namely,

interior, edge and corner positions of the road slab, based on Westergaurds theory. The

equations given by Estergaurd require considerable amount of trails in their solution, if the

slab thickness has to be determined. Bradbury suggested a simplified procedure in terms of

stress coefficients. The IRC considers the edge and corner loading positions only and

recommends the use of Westergaurds edge load formula modified by Teller and Sutherland

and the Westergaurd corner load formula modified by Kelley for calculating these stresses.

Temperature tends to produce two types of stresses in a concrete road slab. They are

warping stresses and frictional stresses. Whenever the top and bottom surfaces of a concrete

pavement simultaneously possess different temperatures, the slabs tends to warp downwards

or upwards inducing warping stresses. These warping stresses are calculated by the equations

developed by Bradbury. The increase or decrease of pavement average temperature causes

expansion or contraction of pavement slab. When these movements are restrained frictional

stresses are developed in the slabs. For designing the pavement slab critical combination of

these stresses have to be obtained. For Indian conditions, critical combination of tresses take

place on a summer mid-day at the edges.

CE409/16 181

Joints are provided in concrete roads to allow for the movements of the slab due to

changes in temperature and moisture content, but the number of joints should be a minimum.

This lesson discusses spacing of expansion and contraction joints.

10.9. REFERENCES :

1. Bindra, S.P. (1977) - A Course in High Way Engineering - Dhanpathy Rai and Sons,

Delhi.

2. Khanna, Dr. S.K. and Justo, Dr. C.E.G. (1991) - Highway Engineering - Nem Chand and

Bros, Roorkee.

3. - (1955) Concrete Roads, H.M.S.O. Publication.

***


UNIT - 11

CONSTRUCTION OF FLEXIBLE PAVEMENTS

CONTENTS :

Aims / Objectives

11.1. Introduction

11.2. Earth Roads

11.3. Gravel Roads

11.4. Maintenance of Earth and Gravel Roads.

11.5. Water Bound Macadam Roads

11.6. Maintenance of Water Bound Macadam Roads

11.7. Types of Bituminous pavements

11.8. Surface Treatments

11.9. Grouted or Penetration mecadam

11.10. Pre-mix Methods

11.11. Maintenance of Bituminous Roads


11.13. Summary

11.14 References

AIMS / OBJECTIVES :

CE409/16 182

Materials required, principles of construction of and equipment and plants needed for

the construction of earth, gravel, water bound Macadam (WBM) and different types of

bituminous roads are presented in this lesson. Proper maintenance of a road is a must for

realising the best serviceability throughout its life. Usual causes of damage and maintenance

methods of each type of the above roads are also discussed in detail.


In our country there is need to construct wide net work of roads to meet the large

demand of transportation expansion. Since funds available for the construction of large

mileage of roads are also quite meager, it is necessary to have roads which cost less not only

from the construction point of view, but also from the maintenance considerations. Such

types of roads are popularly known as ‘Low-cost Roads’. These Low-cost road should be

amenable for stage construction. Making improvement to the existing roads according to the

traffic needs is called ‘Stage Construction’ or ‘Phased development of the roads’. Earth

roads, gravel roads and the water bound Macadam (W.B.M.) roads fall under this category.

Bituminous or tar roads , which are popularly known as ‘Black Top Roads’ in which

bitumen or tar and mineral aggregates are used are also quite popular in our country. Roads

are constructed with varying aggregate sizes and compositions and with different types of

bituminous binders. Hence there are varying techniques of their construction.

Materials used, their requirements and specifications, equipment and plant needed and

construction procedure for each of the above types of flexible pavements are presented in

detail.

A well designed and constructed road must be maintained well in order to reduce the

operation cost and increase the serviceability of the pavement. A detailed note on the

maintenance aspects of each type of construction is also included.

11.2. EARTH ROADS :

An earth road is the lowest form of the surface. It is the first stage in the development

of a road which is to be further developed as increasing traffic requires. These roads are

generally dusty and form ruts quickly thereby destroying the road crown. The camber

provided to the earth roads is very steep and ranges between 1 in 25 (4.0%) to 1 in 33. (3%)

in order to drain off rain water quickly.

11.2.1. MATERIALS : Soils of the following properties are considered satisfactory for

constructing earth roads.

Base course Wearing course

CE409/16 183

Clay content

Silt content

Sand content

Liquid Limit

plasticity Index

5%

9-32%

60-80%

25%

6%

10-18%

5-15%

65-80%

35%

4 to 10%

Highly expansive clays exhibiting marked swell and shrinkage properties should not

be used. The different construction operations are briefly discussed below,

11.2.2. CONSTRUCTION OPERATIONS : The soil survey is carried out and suitable

borrow pits are located. The trees, shrubs, grass, roots and other organic matter including top

soil are removed before excavating the earth for construction.

The centre line and road edges are marked on the ground along the alignment, by

driving wooden pegs. Reference pegs are also driven to help in following the desired vertical

profile of the road during construction.

The sub-grade is then prepared by cleaning the site, excavating and construction of

fills to bring the road to the desired grade and shaping the sub-grade to the desired camber.

The site clearance may be carried out manually using appliances like spade, pick and hand

shovel. Mechanical equipment like dozer, scraper and ripper may also be used for the

purpose. Construction of fills and excavation of cuts to bring the road to the desired profile

may also be done either manually or using excavation, hauling and compaction equipment.

The borrowed soil is balanced, if necessary and dumped on the prepared sub-grade

and water is added, if necessary to bring it to the optimum moisture content. Then the soil is

rolled in layers such that the compacted thickness of each layer does not exceed 10cm. The

type of roller for compaction is decided based on soil type, desired amount of compaction,

and availability of equipment. At least 95% of dry density of I.S. light compaction is

considered desirable. The camber of the finished pavements surface is checked and corrected,

if necessary.

The compacted earth road will be opened to traffic after it is allowed to dry out for a

few days.

11.2.3. TREATED EARTH ROADS : The earth roads can be improved considerably by

treatment with bituminous material or calcium chloride. Clayey and silty soil roads which are

extremely dusty in dry weather and soften readily in wet weather can be treated by

application of bituminous materials. Calcium chloride when used to treat the earth roads,

keeps them slightly damp by absorbing moisture from the air. It has been observed that

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calcium chloride as a dust palliative is only effective when relative humidity of the

atmosphere is above 31%.

11.3. GRAVEL ROADS :

These roads consist of a carriage way constructed with gravels. These roads are fairly

resistant and are suitable to cater an average dial traffic between 350 to 400 vehicles. The

carriage way of this type of road is provided with a camber ranging between 1 in 33 (3.0%) to

1 in 40 (2.5%).

11.3.1. MATERIALS : Hard variety of crushed stone or gravel of specified gradation is

used. Rounded stones and river gravel are not preferred as there is poor interlocking.

Gravel to be used for construction is stacked along the sides of the proposed road.

11.3.2. TYPES OF CONSTRUCTION : Two types of construction methods are generally

available. They are the ‘Feather Type’ and the ‘Trench Type’ (Fig. 11.1.). In the trench type

the sub-grade is prepared by excavating a shallow trench. Since there is better confinement of

the gravel, the trench type is preferred. The feather type is constructed over the sub-grade

with varying thickness, so as to obtain the desired cross slope for the road surface.

Fig. 11.1. Feather type

Fig. 11.1. Trench type

11.2.3. CONSTRUCTION PROCEDURE : Site is cleared and fills and cuts are completed.

Trench is formed to the desired depth of construction, the trench is brought to the desired

grade and compacted.

The gravel aggregates are placed carefully in the trench so as to avoid segregation and

such that the desired camber is obtained. The layer is rolled using a smooth wheel roller

starting from the edges and proceeding towards the centre, with an overlap of atleast half the

width of the roller in the longitudinal direction. Some quantity of water is sprickled and

rolling is done again so that the compaction is effective. The camber is checked and corrected

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from time to time using a template. A few days after the final rolling, the road is opened for

traffic.

11.4. MAINTENANCE OF EARTH AND GRAVEL ROADS :

In these roads, the usual damages needing frequent maintenance are:

(1) Formation of dust in dry weather.

(2) Formation of longitudinal ruts along the wheel path of vehicles, and

(3) Formation of cross-ruts along the surface after the monsoons due to surface water.

The dust nuisance may be reduced by frequent sprinkling of water or by use of dust

palliatives like calcium chloride. periodical maintenance by spreading moist soil along ruts

and reshaping the camber is necessary.

11.5. WATER BOUND MACADAM ROADS :

In India, Water Bound Macadam (W.B.M) has been the most popular base course

material. Macadam construction means the base course is made of crushed or broken

aggregates bound together by the action of rolling. Water bound macadam construction

should consist of clean, crushed broken aggregates mechanically interlocked by rolling and

bound together with screenings and binding material where necessary with water, laid on a

prepared sub-grade, sub-base, of an existing pavement as the case may be. Generally,

W.B.M. is constructed in thickness ranging from 8 c.m. to 30 c.m. The camber provided is 1

in 33 (3.0%) to 1 in 40 (2.5%).

When used as surface course WBM gets deteriorated rapidly under mixed traffic

condition and so this construction is used as base course and is covered with either

bituminous surfacing or cement concrete surfacing.

11.5.1. MATERIALS : The materials required for WBM roads are coarse aggregates,

screenings and filler material. The crushed or broken stones should be hard and durable and

free from excess of flaky, elongated, soft and disintegrated particles and dirt. The aggregates

should meet the following requirements.

Los-Angeles abrasion value (500 revolutions ) 40% max.

Flaking Index 15 % max.

The coarse aggregate is either of 90 to 40mm size or 63 to 40mm size or 50 to 20mm size, fulfilling the specified gradation requirement. The maximum size to be used depends on the type of aggregate available and the total compacted thickness of the layers. The specified gradation requirements of coarse aggregate are given in Table No. 11.1. Table - 11.1

Specified Gradation Requirements of Coarse Aggregates Used in WBM Construction

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Grading No. Size range mm. Sieve size mm. Percentage by weight passing the sieve

1. 90-40 100 80 63 40 20

100 65-85 25-60 0-15 0-5

2. 63 to 40 80 63 50 40 20

100 90-100 30-70 0-15 0-5

3. 50-20 63 50 40 20 10

100 95-100 35-70 0-10 0-5

The screening and other filler materials used to fill up the voids in the coarse

aggregate should meet the grading requirements given in Table No. 11.2.

Table 11.2.

Grading Requirements For Screenings Used in W.B.M. Construction

Type of screening Sieve size mm Percentage by weight passing

the sieve

12.5 mm (for gradings 1 or 2) 12.5

10.0

4.75

0.15

100

90-100

10-30

0-8

10 mm (for gradings 2 or 3) 10.0

4.75

0.15

100

85-100

10-30

Filler material should contain sufficient clay content to prevent revelling of the surface course

of W.B.M layers. Hence it is recommended that the plasticity index of filler material may be

upto 9.

11.5.2. CONSTRUCTION PROCEDURE : Materials of the required grading for

construction are stacked on the road side.

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The site is cleared and either a trench is cut to the required camber or earth bunds are

made in order to provide the required confinement to the loose aggregates for compaction by

rolling. The weak spots are corrected and rolled before the coarse aggregates are placed. On

clay sub-grades a blanket of granular material like sand is spread to a thickness of about 10

cm.

The coarse aggregates are spread upon the prepared surface from the stock piles such

that the required profile is obtained. The thickness of each layer should be such that the

compacted thickness normally doesn’t exceed 7.5cm. The aggregates are spread evenly to the

required profile by using templates placed across the road about 6m apart. The surface is

checked from time to time ensuring proper grade and camber.

After spreading the aggregates, the same are compacted to a full width by rolling. For

this purpose, a three wheel power roller of 6 to 10 tonnes capacity or tandem roller or

equivalent vibratory roller is used. The rolling is done at edges with roller running forward

and backward, until the edges are firmly compacted. Then the roller is moved from the edges

to the centre, parallel to the centre line such that sufficient overlapping is there.

After the coarse aggregates are thoroughly keyed and set by rolling, the screenings are

spread uniformly to fill the interstices. Dry rolling and brooming is carried out. After dry

rolling, the surface is sprinkled with water; swept and rolled. Hand brooms are used to place

the wet screenings into the voids.

The filler material with plasticity index not more than 9 is applied in two successive

thin layers. After the application, the surface is sprinkled with water. the slurry is allowed to

fill up the voids. Rolling is then done.

The road section is allowed to dry overnight and a layer of sand or earth about 6 mm

thick is speed on the surface, lightly sprinkled with water and rolled.

The shoulders are formed to the same cross slope of the pavement and compacted by

rolling. The traffic is allowed on the WBM when the road is properly dried and set.

11.6. MAINTENANCE OF W.B.M. ROAD :

These roads are damaged rapidly due to heavy mixed traffic and adverse climatic

conditions. In dry weather dust is formed and during rains mud is formed. The steel tired

bullock carts cause severe wear and tear to the WBM surface. The fast moving vehicles raise

dust in dry weather and churn up the mud in wet conditions. Due to the combined effects of

the traffic and the rain water, washing away of the soil binder from the surface takes place,

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resulting in the stone aggregates protruding out or getting loose from the surface layer. Pot

holes and ruts are formed in weak spots.

To prevent the aggregates from getting loosened from the surface course, thin layer of

moist soil binder is spread on the surface, particularly after the monsoon. Dust nuisance can

be effectively prevented by providing a bituminous surface dressing course over the WBM

pavement. Temporary measures include spraying of dust palliatives are taken.

Pot holes and ruts formed should be patched up,. The patch repair work is carried out

by first cutting out to a rectangular shape the defective area to remove the stones unto the

effected depth. Then with the coarse aggregates of the same size, the patch is filled up and

compacted well by ramming, such that the patches area is abut 1cm above the general

pavement surface. This allows for further compaction of this patched portion, under traffic.

Wet soil binder is then applied on the surface of the patched area to fill up the interstices and

the surface is rammed again.

11.7. TYPES OF BITUMINOUS PAVEMENTS :

Based on the methods of construction bituminous pavements may be classified under

the following categories :

I) Surface Treatments. They include

a) Prime Coat

b)Tack Coat

c) Surface Dressing and

d) Seal Coat

II) Grouted or Penetration Macadam.

III) Premix which may be any of the following :

a) Bituminous Bound Macadam

b) Carpet

c) Bituminous Concrete

d) Sheet Asphalt or Rolled Asphalt and

e) Mastic Asphalt.

IV) Bituminous pavements are also classified on the basis of mixing and construction

techniques. They are a) Road mix and b) Central Plant mix.

V) Bitumen and tar require heating to bring them to a proper viscosity for their use. Then

the construction technique is termed as ‘HOT MIX METHOD’. Cut backs and emulsions

when used are applied cold and the technique is known as ‘COLD MIX’.

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11.8. SURFACE TREATMENTS :

These are the works carried out to alter the qualities of a wearing surfaces. The

different types of surface treatments are briefly described below.

11.8.1. PRIME COAT :

This consists of application of a less viscosity cut back eg. RC-O, MC-1 or SC-1 on

an existing base of pervious texture like WBM base. Various functions of a prime coat are :

(I) It plugs capillary voids and water proofs the existing base. Otherwise the binder used for

wearing course is likely to go into the void spaces of the existing base and little binder

quantity will be left for binding the aggregate pieces in the surfacing.

11.8.2. TACK COAT :

When the interface treatment is done for an existing bituminous or concrete pavement

it is called a tack coat. Since in this case the base is comparatively impervious, the quantity of

binder required may be less than the primer. However, the tack coat serves the same purpose

as the a primer.

The surface on which the tack coat is to be applied should be thoroughly swept and

cleaned of dust and other foreign matter. The rate of spread of straight run bitumen should be

5kg/10m2 area for an existing bitumen treated surface and 10 kg/10m2 area for an untreated

WBM surface.

11.8.3. SURFACE DRESSING :

Mixed traffic is likely to destroy the WBM road in a very short time. Bituminous

surface dressing prevents the removal of binding material between the stones in WBM road.

The main functions of surface dressing may be (i) to provide dust free surface over a base

course (ii) to provide water proof layer to prevent infiltration of surface water, and (iii) to

protect the base course.

The surface dressing is done in one or two layers and accordingly called single-coat or

two-coat surface dressing. It includes the application of a thin layer of bitumen followed by

cover material of stone chips or coarse sand, which is then rolled. The grades of bitumen used

are 80-100 and 180-200. The surface dressing work is done only in dry and cleaned weather.

Coarse aggregates should be clean, strong and durable, fairly cubical in shape. The

first coat will be 12 mm thick with aggregate of 12 mm nominal maximum size. When

second coat is used it will be 9mm nominal maximum size aggregates and will be of 9mm

thick.

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For good surface dressing, the base course is well prepared to its profile and is made

free from pot holes and ruts. The base course should be made free from all dust, loose soil

etc. The aggregates should be exposed to a depth of 1.25cm, but at the same time should not

be loosened. This is done by the use of wire brushes and final dusting is done with jute bags.

On the prepared surface using a mechanical sprayer or pouring can, uniform spraying

of prime coat is done. Then the bituminous binder is sprayed uniformly at a specified rate,

care is taken that excessive binder is not applied to localized areas as this would cause

bleeding. After the application of the binder, the cover material of sonte chips is spread to

cover the surface uniformly and rolled with a 6 to 8 tonnes tandem roller. Rolling is done

from the edges proceeding towards the centre longitudinally with over lapping not less than

1/3 of the roller tread. Rolling is continued until the particles are firmly interlocked. This is

the final rolling if the surface dressing is in single coat. If the second coat is applied, then the

rolling is done again after the treatment is done for the second coat.

The surface is checked for its cross - slope and the road section is opened to traffic

after 24 hours.

11.8.4. SEAL COAT :

Seal coat is usually recommended as a final cover over certain bituminous surfaces

which are not impervious. They are open graded bituminous constructions which include

grouted macadam, bitumen bound macadam and premixed carpet. Seal coat is also provided

over an existing bituminous pavement which is worn-out. Seal coat is thus a single coat

surface dressing which is usually applied over the exiting black top surfaces to seal the

surfacing against ingress of water, to develop skid resistance texture and to enliven an

existing weathered surface.

Surface treatments are shown in Fig. 11.2.

Fig. 11.2. Surface Treatments

11.9. GROUTED OR PENETRATION MACADAM :

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In penetration macadam construction, the bitumen is sprayed after the aggregates are

spread and compacted in dry state. The bitumen penetrates into the voids from the surface of

the compacted aggregates, thus filling up a part of the voids and binding some stone

aggregates together. Depending upon the quantity of bitumen spread, and the extent of

penetration it is called ‘Full-Grout’ when the bitumen penetrates to the full depth of

compacted aggregates and ‘Semi-Grout’ when it penetrates upto half the depth. Full grout is

adopted in regions of heavy rainfall and semi-grout is adopted in regions of moderate rain fall

and traffic, results in an open graded structure, its use is some times recommended for base

course construction only.

11.9.1. CONSTRUCTION PROCEDURE :

The construction of a penetration macadam is recommended for thickness of 5cm and

7.5cm. The materials used are as a specified below:

a) BITUMEN/TAR : IRC recommends to use any grade of bitumen from 80-100, 60-70

and 30-40; Road tars, RT-4 and RT-5 could also be used. The quantity of bitumen required

depends on the depth of penetration of bitumen into the compacted aggregate layer desired.

b) AGGREGATES : The physical requirements of stone aggregates are specified by the

following values.

Los - Angeles abrasion value 40% max.

Aggregate impact value 30% max.

Flakiness index 25% max.

Stripping at 40°C after 24 hours immersion 25% max.

The gradings of aggregates as recommended by the IRC are given below in table 11.3

and Table 11.4.

Plant and Equipment : Various equipment needed are bitumen heating device,

bitumen distributors, aggregate spreader, and roller for compaction.

Construction Steps :

The underlying course is prepared and conditioned to uniform grade. The surface is

lightly scarified, brushed and prime or tack coat is applied.

Table 11.3 Grading of Coarse Aggregates for penetration Macadam

Percentage passing sieve size mm. Compacted Thickness mm

50 75

63 -- 100

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50 100 --

38 -- 35 - 70

25 35 - 70 --

19 -- 0 - 15

12 0 - 15 --

9 -- --

4.75 -- --

2.36 0 - 5 0 - 5

Table 11.4 Key Aggregates for Penetration Macadam.

Percentage passing sieve size mm. Compacted Thickness mm

50 75

25 -- 100

19 100 35 - 70

12 35 - 70 --

9 -- 0 - 15

4.75 0 - 15 --

2.36 0 - 15 0 - 5

The coarse aggregates of the desired grading depending on the thickness of

construction, are spread with proper edge protection. The profile is checked for the desired

camber. The rolling is done on the dry aggregates with 10 tonnes roller until the aggregates

are compacted and interlocked. Rolling is commenced from the edges and proceeded to the

centre, the overlap recommended being 30 cm. The dry compacted aggregates are checked

for the desired profile and corrected when necessary.

Over the dry and compacted coarse aggregate the binder is applied uniformly either

with pressure distributor or a hand sprayer. The quantity of bitumen required for this purpose

is 50 and 68 kg per 10m2 for 5cm and 7.5cm compacted thickness respectively.

After the application of bitumen, the key aggregates (Table 11.4) are spread and

rolled. The cross profile is again checked.

The seal coat is applied either immediately or after a few days. The pavement section

is again rolled. Excessive rolling, however, should be avoided. The finished surface is then

opened to traffic after at least 24 hours.

11.10. PRE-MIX METHODS :

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In this group of methods, the aggregate and the binder are pre-mixed prior to placing

and spreading the mix. Each aggregate particle gets coated with uniform thickness of binder

film. Another advantage associated with the premix construction is the increased stability of

the mix. In the premix, the aggregate gradation is carefully selected to give a dense mass

possessing minimum voids.

This method includes bituminous macadam. bituminous carpet, bituminous concrete,

sheet asphalt and mastic asphalt. Construction techniques of these pavements are explained.

11.10.1. BITUMEN BOUND MACADAM :

This bitumen bound macadam, also some times called bituminous macadam, is a

premix immediately laid after mixing and then compacted. It is an open graded construction

suitable only as a base course. When this layer is exposed as a surface course, at least a seal

coat is necessary. This is laid in finished thickness of 5cm or 7.5cm.

MATERIALS :

a) BITUMEN OR ROAD TAR :

The grades of bitumen are 30/40, 60/70 and 80/100 and Road Tar RT-4. Cut back and

emulsions can also be used in cold mix construction technique. The binder content used

varies from 3.0 to 4.5% by weight of the mix.

b) AGGREGATES :

Aggregates used are of low porosity fulfilling the following requirements for base

course.

Los-Angeles abrasion value 50% max.



Stripping at 400C after 24 hrs immersion 25% max.

The IRC specifies the following grading of aggregates for base course construction.

Percentage passing

sieve size, mm

Base Course Compacted Thickness, mm

50 75

I II I II

63

50

40

--

100

--

--

100

90-100

100

--

35-70

100

90-100

35-65

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25

20

12

10

4.75

2.36

0.075

25-70

--

0-15

--

--

0-5

0-3

50-80

--

10-30

--

--

--

0-5

--

0-15

--

---

--

0-5

0-3

20-40

--

5-20

--

--

--

0-5

c) STABILITY REQUIREMENT :

Satisfactory requirements of bituminous mix are specified in terms of Marshall

stability and flow values. A Marshall stability value of 200 kg is recommended for light and

medium traffic, where as a value of 300 kg is recommended for heavy and very heavy

traffic. Flow value should be between 10-40 in terms of 0.1 mm units.

PLANTS AND EQUIPEMENTS :

Various plants and equipment required for the job include : sprayer, mechanical or

improvised hand mixer, spreader and roller.

CONSTRUCTION STEPS :


lightly scarified, brushed and tack coat or prime coat of thin layer of bitumen binder is

applied on the existing layer either using the sprayer or a pouring can.

The bitumen binder, and aggregates as per recommended grading are separately

heated to the specified temperature and are then placed in the mixer chosen for the job and

mixed thoroughly to get a homogenous mixture. The mixture is then conveyed to the site,

placed on the desired location and is spread with rakes to a predetermined thickness. The

camber of the profile is checked with a template. It may also be noted that the compacting

temperature also influences the strength characteristics of the resultant pavement structure. It

is therefore required that a minimum time is spent between the placement of the mix and the

rolling operations.

Soon after the spreading of the mix rolling is done with 8 to 10 tonnes tandem roller.

The rolling is commenced from the edges of the pavement and proceeded towards the centre

and uniform overlapping is provided. The roller wheels should be kept damp to avoid the

bituminous material form sticking to the wheels. The pavement surface should be checked for

longitudinal and cross profile.

11.10.2. BITUMINOUS CARPET :

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Bituminous carpet is a premix prepared from stone chips of 10mm to 12mm size, sand

and bitumen binder. The thickness of such a carpet may vary form 2cm to 2.5cm . The

construction is usually recommended for a surface course layer. The carpet is finally covered

with a seal coat.

MATERIALS REQUIRED :

a) BITUMEN / TAR :

The bitumen binder of 80-100 grade or road tar of grade RT-3 is used.

b) STONE CHIPS :

For the carpet of 2cm thick the stone chips of 12mm and 10mm nominal size are

required. The stone chips should be angular, clean, hard, tough and durable. Medium coarse

sand passing 1.7mm sieve and retained on 1.18mm sieve, which is clean, hard and durable is

also required.

The aggregate fulfilling the following requirements are selected.





EQUIPMENT :

Various equipment and plants required for the job include Sprayer, mixer, spreader

and roller.

CONSTRUCTION STEPS:


lightly scarified, brushed cleaned and tack coat or prime coat is applied on the existing layer

either using the sprayer or a pouring can. The tack coat or prime coat is applied just before

spreading the pre-mix.

The aggregates and bitumen are heated separately upto the required temperature and

then mixed into a thorough and homogeneous mixture in a mixer - mechanical or improvised

hand mixer. The pre-mix is taken out and carried to the site for spreading and rolling. The

spreading is done with suitable rakers. The cross profile of the laid material is checked with

suitable templates.

Soon after the spreading of the mix, rolling is done with a 6 to 9 tonnes tandem or

pneumatic roller. At one operation 15 metre of the premix is laid and rolled. the roller wheels

CE409/16 196

should be kept damp to avoid the bituminous material from sticking to wheels. The rolling is

done until there is no further movement of aggregates in the mix layer.

In areas of low rainfall, a premixed - sand seal coat is applied over the carpet. In areas

of high rainfall (over 125cm per year) a liquid seal is sprayed and covered with a layer of

chippings applied over the carpet. The stone chips are of 6mm size. This layer is rolled by a

light tandem roller to give a smooth finished surface. After the application of seal coat, the

pavement so constructed is opened to traffic after a period of 24 hours.

11.10.3. BITUMINOUS CONCRETE :

The bituminous concrete is the highest quality of construction in the group of black

top surfaces This is a carefully proportioned mixture of coarse aggregate, fine aggregate and

mineral filler coated with bitumen binder. In this both the aggregates and the bituminous

binder are heated to the required temperature prior to mixing. The mixes are properly

designed to satisfy the design requirement of stability and durability. Mineral filler is used to

fill up the voids in the fine aggregate and consists of inert materials (Lower than 600 micron

sieve) stone-dust, cement, hydrated lime, fly ash or other non-plastic materials.

MATERIALS REQUIRED :

a) AGGREGATES :

Aggregates used for bituminous concrete should satisfy the following gradation

requirements.

Table : 11.6. Gradation of Aggregates for Bituminous concrete.

Sieve Size mm Percent by Weight

Limit I Limit II

20.0

12.50

10.00

4.75

2.36

--

100

80 - 100

55 - 75

35 - 50

100

80 - 100

70 - 90

50 - 70

35 - 50

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0.60

0.30

0.15

0.075

18 - 29

13 - 23

8 - 16

4 - 0

18 - 29

13 - 23

8 - 16

4 - 10

Physical requirements of course and fine aggregates are specified as under





b) BITUMEN :

The grade of bitumen depends upon the climatic conditions of a given region.

However grades 30-40, 60-70 or 80-100 are usually recommended for use. The binder

content used ranges form 5.0 to 7.5% by weight of the mix. The exact binder content required

depends upon the results of Marshall test carried out in a laboratory.

c) STABILITY REQUIREMENTS :

The requirements of the mix based on the traffic intensity are presented in Table 11.7.

Table 11.7. Requirements of Bituminous Concrete

Requirements Traffic

Heavy Medium Light

Stability, Kg(min). 340 227 227

Flow Value, 0.1mm units 20-40 20-45 20-50

Voids in mix % 3-5 3-5 3-5

Number of compaction

blows of each end of

Marshall test specimen

75 50 35

PLANTS AND EQUIPMENT :

In order to achieve high quality in construction mechanized construction should be

used.

CONSTRUCTION STEPS :

The underlying course is prepared and conditioned to uniform grade. It is desirable to

lay bituminous concrete surface course on a binder course instead of directly laying it on a

WBM.

CE409/16 198

The premix is prepared in a hot mix plant. The hot mixed material is collected form

the mixer by the transporters, carried to the location and is spread by mechanical paver. The

camber and the thickness of the layer are accurately verified. The control of temperature

during mixing and compaction are of great significance in the strength of the resulting

pavement structure.

Soon after the spreading of mix by paver, the surface should be thoroughly compacted

by rolling with a set or rollers moving at a speed not more than 5 kmph. The initial rolling is

done by 8 to 12 tonnes roller and intermediate rolling is done with a fixed wheel pneumatic

roller of 15-30 tonnes having a tyre pressure of 7 kg/cm2. The wheels of the roller are kept

damp wit water. The number of passes required depends on the thickness of the layer. In

warm weather rolling on the next day helps to increase the density if the initial rolling was

not adequate.

QUALITY CONTROL :

The various field controls include (i) aggregate grading control (ii) binder grade

control (iii) temperature control for aggregates and (iv) temperature control for mix during

mixing and compaction. It is recommended that at least one test for the above quality controls

must be carried out for every 100 tones of mix discharged by plant. The field density should

be checked for every 1000m2 of compacted surface.

11.10.4. SHEET ASPHALT OR ROLLED ASPHALT :

Sheet Asphalt contains high percentage of mortar consisting of sand, filler and

bitumen and is quite dense and impervious. The characteristics of durability, imperviousness

and load transmission are excellent in the sheet asphalt. It has been observed that the elastic

modulus of this layer is equivalent to that of cement concrete under transient stress. This

sheet asphalt is used only in wearing courses.

11.10.5. MASTIC ASPHALT :

This is superior to the other types of surfacing because of its ability to take up very

heavy shear stress without deformation. This is hence recommended for very heavy traffic

cities and placed where the braking and accelerating stresses are very heavy - bus stops and

roundabouts. This is a mixture of bitumen, fine aggregate and filler in suitable proportions

which yield avoidless and impermeable mass. The filler, bitumen binder and fine aggregate

are taken in suitable proportions and they are heated in sequence, and cooked at a temperature

of 2000C; then mastic asphalt has a consistency that it can flow. At this temperature the

mastic asphalt is spread to thickness between 2.5cm to 5cm. On cooling it hardens to a

CE409/16 199

semisolid or solid state. No rolling is required. Mastic asphalt has not found any importance

in our country because of its high cost and non-availability of the grade of asphalt required.

11.11. MAINTENANCE OF BITUMINOUS SURFACES :

Maintenance works of bituminous surfacing mainly consists of (i) Patch repairs, (ii)

Pot hole repairs (iii) Surface treatments and (iv) Resurfacing. These are briefly discussed

here.

11.11.1. PATCH REPAIRS :

Inadequate or defective binding material causes removal of aggregates during

monsoons. Patching may be done on affected isolated areas or sections using a cold premix.

11.11.2. POT HOLES :

Pot holes are cut to rectangular shape and the affected materials in the section is

removed until sound material is encountered. The excavated holes are cleaned and applied

with primer. A premixed material is placed in the sections. The material so placed in the Pot

holes is well compacted by ramming to avoid any raveling. The finished level of the patches

is kept slightly above the original level to allow for subsequent compaction under traffic.

Generally cut-back or emulsion is used as binder.

11.11.3. SURFACE TREATMENT :

Excess of bitumen in the surface material bleeds and the pavement becomes slippery.

Corrugations or rutting or shoving develop in such pavement surfaces. Blotting material such

as stone chips of maximum size 1.25mm or coarse sand is spread and rolled to develop

permanent bond between the existing surface and the new materials.

The binders in the black top surface also get oxidized due to aging. This develops

minute cracks in the pavement surface. Such pavements surfaces are applied with a renewal

coat or seal coat. It may be necessary to apply more than one layer of surface treatments, if

the surface has been seriously damaged.

11.11.4. RE SURFACING :

When the pavement surface is totally worn out and develops a poor riding surface, it

may be economical to provide an additional surface course on the existing surface.

11.12. SELF-ASSESSMENT QUESTIONS :

1. Enumerate the steps for the preparation of sub-grade.

2. Specify the materials required for the construction of WBM roads. Give also a

detailed account of the construction and maintenance procedure.

3. Distinguish between the following :

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a) Bituminous Macadam and Penetration Macadam.

b) Seal Coat and Bituminous Carpets.

c) Fine aggregate and Mineral filler.

4. Give a detailed specification for a two coat surface dressing over an existing WBM

road.

11.12. SUMMARY :

Earth and gravel roads are the lowest type of roads used in our country and may be

used successfully when the traffic intensity is between 30 to 200 tonnes per day. The usual

damages needing frequent maintenance of these roads are formation of dust and formation of

longitudinal and cross ruts. Materials and their requirements, construction and maintenance

aspects of these roads are presented in detail.

Another type of construction that is popular in our country is the Water Bound

Macadam (WBM) road. Water bound macadam construction consist of clean crushed coarse

aggregate mechanically inter locked by rolling and the voids in which are filled with

screenings and binding material with the aid of water on a prepared sub-grade, sub-base or

base of an existing pavement as the case may be. In the case of WBM roads the damage is

caused by the removal of the binding material from the surface resulting in pot holes and ruts.

Construction and maintenance aspects of the WBM roads in discussed in detail.

Bituminous roads when properly constructed maintained and used, have long and

economic lives. Based on the methods of construction these pavements are classified as

surface treatments, grouted or penetration macadam and premix types.

The following aspects of construction are discussed under each category of the black

top roads.

MATERIALS USED :

a) Bitumen / Tar : Grades

b) Aggregates : Physical requirements and grading

c) Plants and Equipment, and :

d) Construction procedure :

Maintenance works of bituminous surfacing mainly consists of patch repairs, pot hole

repairs, surface treatments and resurfacing. Important aspects of these have been discussed.

11.14. REFERENCES :

CE409/16 201


Delhi.


Bros, Roorkee.

3. - Bituminous Materials in Road Construction, HMSO. Publication.

***


UNIT - 12

CONSTRUCTION OF CONCRETE PAVEMENTS

CONTENTS :

Aims / Objectives :

12.1. Introduction

12.2. Construction of Pavement Slab

12.3. Construction of Joints

12.4. Joint Filler and Sealer Materials

12.5. Maintenance of Concrete Pavements


12.7. Summary

12.8. Reference

AIMS / OBJECTIVES :

Cement concrete pavements are very much preferred because they provide an

excellent riding surface and have much longer life than any other type of construction. These

merits of cement concrete pavements can be realised only when they are well designed,

constructed and maintained. Design of rigid pavements is already dealt with. In this lesson,

methods of construction and maintenance of concrete roads are explained.


Cement concrete roads are very high standard roads which are the costliest of all other

types of roads. They provide excellent riding surface and pleasing appearance. Though

initially the cement concrete roads are very costly, because of their long span of life,

excellent riding surface and negligible maintenance cost, these roads prove to be cheaper than

the bituminous roads. The cement concrete material exhibits its characteristics which can be

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predicted by theory and as such a structure made using this material can well be designed on

a rational basis. This indirectly saves cost as the resultant structure gives an excellent

performance.

A few of the draw backs of the cement concrete pavements are :

1) The transversal and longitudinal joints which are unavoidable in construction are

planes of weakness; they also delay the construction and increase the cost of maintenance of

these roads. As such, the number of joints should be a minimum.

2) A minimum period of 28 days curing is required before the cement concrete pavement

could be opened to traffic.

3) Cement concrete pavements require very high initial investment and are not suitable

for stage construction;

Three types of pavement construction are available:

(i) Cement Grouted Layer

(ii) Rolled Concrete Layer, and

(iii) Cement Concrete Slab.

In ‘Cement Grouted Layer’, open graded aggregate mix with minimum size of

aggregates as 18 to 25mm is laid on the prepared sub - grade and the aggregates are dry

rolled. A grout made of coarse sand cement and water is applied on the surface and is allowed

to seep through the aggregate matrix. The technique can be compared with bituminous

grouted or penetration type construction.

In ‘Rolled Concrete Layer’, a lean mix of aggregate, sand, cement and water is

prepared and laid on the prepared sub-grade or sub-base course. Then the surface is rolled by

a ‘Tandem Roller’. The rolling should be completed before the final setting time of cement.

Curing is done as per conventional methods.

The cement grout layer and rolled concrete layer are suitable for base course only.

However the cement concrete slabs serve as both base and surface courses. Construction of

cement concrete slab pavements includes (i) construction of pavement slab and (ii) design

and placement of joints.

12.2. CONSTRUCTION OF CEMENT CONCRETE PAVEMENT SLAB :

There are two methods f construction of cement concrete slab: They are (a) Alternate

Bay Method and (b) Continuous Bay Method.

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(a) ALTERNATE BAY METHOD :

In this method, the cement concrete slabs (bays) are constructed in alternate

succession having the next bay to follow up after a time lapse of one week when normal

cement is used and atleast two days later, in the case of rapid hardening cement. In this

method of construction, the bays are constructed in x.y,z etc leaving gaps of bay x’, y’ and z’

(Fig. 12.1).

Fig. 12.1. Construction Method of Cement Concrete Road

This method has the following advantages and disadvantages :

ADVANTAGES :

(i) Provides additional working space for laying slabs.

(ii) Provides ease in joint construction.

DISADVANTAGES :

(i) Larger number of transverse joints are to be provided, and this increases the cost of

construction and maintenance. It also results in bad riding surface.

(ii) During rainy season, Iran water may get collected in the incomplete bays.

(iii) Needs traffic diversion during construction because the construction is spread over the

complete road way width.

b) CONTINUOUS BAY CONSTRUCTION :

In this method all the bays of one traffic lane are laid continuously (i.e.,) x, y’ z etc. or

x’, y, z’ etc. (Fig. 12.1) without any break. The construction joints are however provided at

the end of the days work. The main dis-advantage of this method is the difficulty of providing

joints. This method is still preferred because during construction half the traffic lane width

could be utilised by the diverted traffic.

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12.2. MATERIALS FOR CEMENT CONCRETE PAVEMENTS :

The various materials used in the construction of cement concrete pavements are

cement, coarse - aggregate, fine aggregate and water. For joints materials required are dowel

bars, joint filler and sealer.

a) CEMENT :

Ordinary portland cement is generally used. When the road has to be opened for

traffic early, rapid hardening cement may be used.

b) COARSE AGGREGATE :

Good quality aggregates should be used and should be free from harmful materials

such as iron, pyrites, coal, mica, clay, alkali, organic impurities etc., It is always desireable to

use round gravel aggregates. The largest size of aggregate in a mix should not exceed one

quarter of the thickness of the slab. The best combination of strength and workability is to be

obtained. The physical requirements of aggregates suitable for cement concrete construction

are as follows :

Aggregate crushing value 30% (max.)

Aggregate impact value 30% (max.)

Los - Angeles value 16% (max.)

Soundness, (average loss in weight after 10 cycles ) 12% (max) in Sodium Sulphate

18% (max) in Magnesium Sulphate

(c) FINE AGGREGATE :

Natural sand should be preferred as fine aggregate, though crushed stone may be

used.

(d) MIX DESIGN OF CONCRETE :

The concrete may be proportioned so as to obtain a minimum modulus of ruptuore of

35 kg/cm2 on field specimens after 28 days curing or to develop a minimum compressive

strength of 280 kg/cm2 at 28 days, or higher value as desired in the design.

12.2.2. EQUIPMENT :

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The equipment needed for the construction of concrete slabs are for batching, mixing,

placing, finishing and curing the concrete pavement. These are explained under construction

steps for cement concrete pavement slab.

12.2.3. CONSTRUCTION STEPS FOR CEMENT CONCRETE PAVEMENT SLAB :

The concrete pavement construction should be done only during dry weather where

the temperature is between 4 to 400C. The various construction stages involved are described

below :

a) PREPARATION OF SUB-GRADE OR SUB-BASE : The foundation layer should

be graded and compacted to obtain a smooth hard surface. In case the cement concrete

pavement is to be laid on the sub-grade, then it should be dressed to the required profile and

cross-section. Generally the sub-grade or sub-base is prepared to a width of atleast 30cm

beyond the edge of the pavement to be constructed. The minimum value of the modules of

sub-grade reaction obtained with a plate bearing test should be 5.54kg/cm3 for the foundation

material.

b) FIXING OF FORM WORK :

In past timber forms were used exclusively in concrete road works. These forms get

warped after having been in contact with wet concrete on very few occasions. Present trend

is, therefore, to use steel forms at least of 3m, length, except on curves of less than 45m

radius where shorter sections are used. These steel forms are of M.S. channel and of depth

equal to the thickness of the pavements. When set to grade, the maximum deviation of the top

surface of any section from a straight line is not exceeded by 3 mm.

c) BATCHING AND MIXING OF MATERIALS :

The coarse aggregates, fine aggregate and cement are proportioned by weight in a

weight - batching plant and placed into the hopper. The cement may be measured by bag, the

weight of which is taken 50 kg.

The mixing of concrete should be done in a power driven batch mixer to ensure

uniform distribution of materials throughout the mass. The minimum mixing time should be

fixed in relation to the mixer type and capacity. The workability of concrete should be

checked by performing slump test.

d) TRANSPORTING AND SPREADING OF CONCRETE :

Soon after mixing, the concrete should be transported and placed on the prepared base

between the form work in such a manner as to avoid seggregation and uneven compaction.

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The concrete should not be dropped from a height greater than 90 cm and should be deposited

within 20 minutes from the time of discharge from the mixer.

e) COMPACTION :

The concrete should be compacted fully using vibrating screed and / or internal

vibrators or hand tampers, known as “Hand Tamping Beam” (Fig. 12.2). Compaction should

be so controlled as to prevent excess mortar working on to the top due to over-compaction.

Any low or high spots should be made good by adding or removing concrete. The slab should

be tested for trueness.

Fig. 12.2. Tamping beam

f) FLOATING AND STRAIGHT EDGING :

The concrete is further compacted by means of the longitudinal float (Fig. 12.3). The

longitudinal float is held in a position parallel to the carriage way centre line and passed

gradually from one side to the other. After this, the excess water gets disappeared, the slab

surface is tested for its grade and level with straight edge (Fig. 12.4).

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Fig. 12.3. Float

Fig. 12.4. Straight Edge

g) FINISHING :

Soon after correcting the surface for profile and just before the concrete becomes

hard, the surface should be finished by belting , brooming and edging. For belting, short

strokes are applied with a two-ply canvas belt (Fig. 12.5) transversely to the pavement

surface. Brooming is always done perpendicular to the centre line of the pavement. The

edging tools are used to carefully finish the edges.

Fig. 12.5

h) CURING :

The initial curing is started soon after the finished pavement surface is able to take the

weight of wet jute mats without leaving any marks thereon. The mats are thoroughly

saturated with water and should extend beyond the pavement edges at least by 0.5m. The

initial curing should be continued for a period of 24 hours.

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During final curing after the removal of the mats, wet earth is banked in the form of a

soil berms. A layer of sandy soil is spread and is kept thoroughly saturated with water for 14

days. In places where water is scarce or pavement is on a steep gradient, impervious

membrane curing method should be used.

The concrete pavement is generally opened to the traffic after 28 days of curing.

12.3. JOINTS IN PAVEMENTS:

Joints are provided in concrete roads to allow for expansion, contraction and warping

of the slab caused by the changes of temperature and moisture content. They are also

necessary to allow for the break in construction at the end of the day and to allow the road to

be laid in lanes of convenient width. Design of spacing of various types of joins is dealt with

n detail in article 10.5. Construction aspects of these joints is discussed here.

12.3.1. REQUIREMENTS OF A GOOD JOINT :

1. A joint must be water - proof at all times

2. A joint should not permit ingress of stone grits.

3. Joint must be permitted to move freely at all times.

4. A joint should not detract from the riding quality of a carriage way.

5. A joint should interfere as little as possible in concreting.

6. A joint should not be cause of an expected structural weakness in a pavement.

12.3.2. EXPANSION JOINTS :

These joints are provided to allow for the expansion of slabs due to rise in slab

temperature above the construction temperature of the cement concrete. Expansion joints also

permit the construction of slabs. These joints are provided in our country at an interval of 18

to 21 metres. A typical expansion joint is shown in Fig. 12.6. The approximate gap width for

this joint is from 2.0 to 2.5 cm.

It may be stated that the break in the continuity of a slab forming a joint adds a

weaker plane in the cement concrete pavement. The stresses induced due to the wheel loads

at such joints are of very high order at the edges and corner regions. In order to strengthen

these locations load transfer devices are used.

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Fig. 12.6. Expansion Joint with Dowel bar

This is done through a system of reinforcement provided at suitable intervals

projecting in the concrete in longitudinal direction upto 60 cm length. Such a device is called

‘DOWEL BAR’. Dowel bars are embedded and fixed in concrete at one end and the other

end is kept free to expand or contract by providing a thin coating of bitumen over it. Metal

cap is provided at this end to offer a space of about 2.5. cm for movements during expansion.

Spacing between the dowel bars generally adopted is 30 cm. The size of the dowel bars will

be about 2 cm to 3 cm.

12.3.3. CONTRACTION JOINTS :

These joints are provided to permit the contraction of the slabs. These joints are

spaced closer than expansion joins. Load transfer at these joints is provided through the

physical interlocking by the type of aggregates employed in the construction and with the

average soil and temperature conditions, the recommended spacing for construction joints are

given in Table 12.1. Since it is recommended to provide contraction joints at closer spacing,

there seems to be no need of providing any load transference, as mainly this will be done by

aggregate interlocking. For added safety some agencies recommend use of dowel bars which

are fully bound in the concrete. (Fig. 12.7).

Table 12.1 : Spacing of Contraction Joints

Type of coarse aggregate Joint spacing in metres

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Granite

Limestone

Gravel

a) Calcareous

b) Siliceous

Slag

7.5

7.5

7.5

4.5

4.5

A) Dummy Joint

B) Construction Joint with Dowel Bow

Fig. 12.7. Construction Joints

12.3.4. WARPING JOINTS OR HINGED JOINTS :

These joints are provided to relieve the stresses induced due to warping. These joints

are rarely needed if the suitably designed expansion and contraction joints are provided to

prevent cracking. Longitudinal joints with tie bars are in this class of joint.

12.3.5. LONGITUDINAL JOINTS :

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These joints are provided in wide cement concrete pavements to prevent longitudinal

cracking. They are provided with the bars and these bars help to maintain the two slabs at the

same level. They are not designed to transfer load from one slab to the other. These “tie bars”

are generally 75 cm long and spaced at 60cm apart. Their sizes depend upon the traffic

intensity, that is, 25mm, 20mm, and 12.5 mm diameter bars are used for very heavy, heavy

and medium traffic intensity respectively. IRC recommends to use plain butt or butt with tie

bar (Fig. 12.8) type of joints.

a) Plain Butt Joint

b) Butt Joint with Tie - bar.

Fig. 12.8. Longitudinal joints

12.3.6. ARRANGEMENT OF JOINTS :

Arrangement of transverse joints are as follows :

a) Staggered arrangement

b) Uniform arrangement, and

c) Skew arrangement

Fig. 12.9. shows these arrangements. It is observed that where transverse joints are

arranged staggered on either side of the longitudinal joint as shown in fig. 12.9A, sympathetic

cracks are often formed in line with transverse joints. It is recommended to provide joints

across the longitudinal joint in the same transverse alignment as shown in Fig. 12.9.B.

It is always attempted to avoid the skew alignment of the joints, but in some typical

layouts at intersections it may be required to provide skew arrangements. At places where

these arrangements cannot be avoided, the acute corners so formed are strengthened with

reinforcement as shown in Fig. 12.10.

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a) Staggered Transverse Joint

b) Uniform Joint

c) Skew Arrangement

Fig. 12.9. Arrangement of Joints

Fig. 12.10. Strengthening of Corner Region

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12.4. JOINT FILLER AND SEALER MATERIALS :

Joint spaces are first filled with compressible filler materials to fill up the gap between

the adjacent slab and the top of joints are sealed using a sealer to prevent infiltration of water.

12.4.1. JOINT FILLER :

Joint fillers should be compressible, elastic and durable. Various types of joint filler

materials usually used are soft wood, impregnated fibre board and cork or cork bound with

bitumen. These pre-formed fillers are made from fibres of soft board, fibre board, coir fibre,

or cork. It is required that the performed strips of these materials are properly bonded

together with bitumen. The bitumen content specified by the IRC is 35% by weight. Various

properties required for the satisfactory filler materials are as follows : (As per IRC)

a) COMPRESSION :

The pressure required to compress the specimen to 50% of its original thickness

should be between 7 to 53 kg/cm2. Loss of weight should not be more than 3%.

b) RECOVERY :

At the end of three cycles of load application, specimen should recover by at least 70

percent.

EXTENSION : When compressed to 50% the extension of one edge, should not be

more than 6.5mm where other edges are rest ran.

12.4.2. JOINT SEALER :

For effective sealing of the joint for a long period, it is essential that the sealing

compound possess the following properties:

a) Adhesion to cement concrete edges,

b) Extensibility without fracture,

c) Resistance to the ingress of grit, and

d) Durability.

Different types of sealing compounds are in use. Bitumen is used either along or with

mineral filler as a sealing compound. Rubber - bitumen compounds are also used for this

purpose. Air blown bitumens are also used with advantage, as they are less susceptible to the

temperature changes.

12.5. MAINTENANCE OF CONCRETE PAVEMENTS :

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A well designed and constructed cement concrete road requires very little

maintenance. Main defects in this type of road is the formation of cracks. It is therefore,

necessary to examine the cracks and causes ascertained before any remedial measure is

adopted.

12.5.1 TREATMENT OF CRACKS :

As fine cracks do not harm the pavements, they may be left without any treatment. If

the cracks are wide enough to allow infiltration of water, grit etc., they should be attended

immediately to avoid further damage to the pavement.

The cracks are thoroughly cleaned of all dirt and any other loose particles. This is

done by using a sharp tool and blowing with air blower. The surface is then applied with a

coat of kerosene to facilitate the bond of the sealing material. The cracks are then filled with

suitable grade of bitumen in liquid form. The sealing material is provided 3 mm extra over

the height required. Sand is spread over it.

12.5.2. MAINTENANCE OF JOINTS :

The joints are the weakest spots and the efficiency of the pavement would be

determined by the proper functioning’s of joints. Large number of failure are observed at or

near the joints. Care should be taken to see that filler and sealer materials are in-tact in the

joints. In the event the joints have lost or damaged, either filler or sealer material or both, the

replacement is immediately done. Vertical edges of the joint are also very essential as

inclined faces get very seriously damaged.


1. In narrow lanes cement concrete pavements are used. Can you justify this practice ?

2. Contraction joints are provided at closer spacing than Expansion joints in concrete

pavements, Why?

3. a) Compare alternate bay and continuous bay methods of construction of cement

concrete roads.

b) With the aid of neat sketches show the different types of joints and their positions .

12.7. SUMMARY :

Cement concrete pavements are preferred because of their excellent riding surface,

very high life and little maintenance. Construction of concrete slab pavements include

CE409/16 215

construction of pavement itself and design and placement of the joints. There are two

methods of construction of cement concrete slab pavements: alternate bay method and

continuous bay method. Each method of construction has its applications and limitations.

Steps involved in the construction of pavement slabs are dealt with in detail. The efficiency

of a cement concrete pavement is largely determined by the spacing and efficient functioning

of the various joints. Usual spacing of the joints and constructional details of different types

of joints are presented in detail.

A well designed and constructed cement concrete road requires very little

maintenance. Maintenance of cement concrete roads include treatment of cracks and

maintenance of joints.

12.8. REFERENCES :


Delhi.


Bros, Roorkee.

3. - (1955) Concrete Roads, H.M.S.O. Publication.

***


UNIT-13

SOIL STABILISED ROADS

Aims / Objectives :

13.1. Introduction

13.2. Mechanical Stabilisation

13.3. Soil-Cement Stabilisation

13.4. Soil-Lime stabilisation

13.5. Bituminous Stabilisation

13.6. Summary

13.7. Self-Assessment Questions

13.8. References

AIMS / OBJECTIVES :

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Soil stabilization means the improvement of the stability or bearing power of the soil

by the use of compaction, proportioning and/ or the addition of suitable admixtures or

stabilisers. Roads which are constructed using these techniques are called ‘Stabilized Roads’

and are gaining popularity in developing countries because of their susceptibility for stage

construction. In this chapter, common methods of stabilisation of road beds like mechanical

stabilisation in which soil and aggregate are proportioned to get a dense mixture and methods

in which admixtures like cement, lime and bituminous materials are used to improve the

stability of locally available soil are discussed.


In the construction of roads, cost of construction can be brought down by selecting

locally available aggregates and soil, and correctly proportioning them for maximum density.

If such a soil - aggregate mixture is adequately compacted to get a mechanically stable layer,

the process is ‘Mechanical Stabilisation’.

If the locally available soil is very poor and can not withstand the expected traffic

loads, it can be improved through compaction after the addition of an inexpensive admixture,

the ‘stabilising Agent’. A number of stabilising agents have been used in highway

construction. The more important of these may be classified as cementing agents, modifiers

or water proofing agents. The more common cementing agents are Portland cement, lime and

lime fly ash mixtures. They act through the formation of cementitious compounds that more

or less permanently bind together individual particles or aggregates of particles. Lime is also

a powerful soil modifier that serves to decrease the plasticity, reduce water content and

increase workability of wet clay prior to compaction. Bitumen is used as a water proofing

agent as well as a cementing compound.

Stabilising agents are selected according to the type of soil, stability problem at hand

and the economics of their use.

13.2. MECHANICAL STABILISATION :

When a granular structure, such as a road base or surfacing has the property of

resistance to lateral displacement under load, it is said to be ‘Mechanically Stable’. In

mechanically stabilized soils, the resistance is provided by the natural forces of cohesion and

internal friction in the soil. Cohesion is mainly associated with the silt and clay content of the

material while internal friction is a characteristic of coarser particles. Mechanical stabilization

is still the stabilization method that is most widely used in road construction throughout the

CE409/16 217

world. Its popularity is based on the fact that it makes possible maximum usage of locally

available materials in highway construction. In this process, aggregates and soils are properly

proportioned and the resulting mixture is adequately compacted to get a stable layer.

Factors affecting mechanical stabilisation of soil-aggregate stabilisation include the

following .

a) Quality and quantity of the soil binder.

b) Strength and gradation of aggregates, and

c) Compaction.

13.2.1. Quality and Quantity of the Soil Binder :

A Stabilised soil - aggregate mass is analogous to cement concrete in that the coarse

aggregate content plays the role similar to that of coarse aggregate in concrete and the soil,

sometimes called the binder soil, with water acts as mortar. The main use of this form of

stabilisation is for sub-base construction of high type of roads and road bases and surface

courses of secondary type of roads. The quality and quantity of the binder soil plays a very

important role on the characteristics of soil-aggregate mixture.

Plasticity characteristics of binder soil effect the performance of the soil-aggregate

mix considerably. Soil with high plasticity index, results in poor stability under soaked

conditions. hence, it is desirable to limit the plasticity index of the soil constituents. salts like

sulphates and presence of mica are also undesirable. But presence of salts like calcium

chloride are considered to be beneficial. Liquid limit for binder soil for base course may be

upto 25% and for surface course upto 35%. Similarly plasticity index value for base course

should not exceed 6% and for surface courses should be between 5 and 10%.

The influence of the quantity of soil in soil-aggregate mixture is illustrated in Fig.

13.1. When the aggregate is without fines (Fig.13.1a) the mass will be stable only when

confined, highly permeable, practically no variation in volume or stability with moisture

variation.

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Fig. 13.1. Typical States of Soil - Aggregates

In fig. 13.1.b, the voids in the compacted aggregate are just filled with compacted

binder, without disturbing the grain to grain contact of the aggregates. In this state, the

density of the mix will be maximum with increased stability even when unconfined due to

higher cohesion, but the mix is less permeable and variation in volume and stability due to

moisture variations much depend on the property of the binder soil. The state when the

aggregates are mixed with excess of fines and compacted is shown in fig.13.1.c. In this state,

there is no contact between the aggregates and they float in the binder soil, the stability is

decreased and mix is considered less desirable with poor drainage, more variation in stability

and volume with moisture variation. Thus proportioning of the mix effects the properties of

soil-aggregate mixtures considerably.

13.2.2. Strength and Gradation of Aggregates :

From the above discussion, it will be appreciated that maximum stability is obtained

for a soil-aggregate mixture when the voids in the compacted aggregate are just filled up,

with the binder - soil (Fig.13.1.b). In this case, the load transfer takes place through the

contact points of the aggregates and in such circumstances aggregates with high crushing

strength are to be used. However, weak aggregates have also been successfully used in

mechanical stabilization work (see article 13.2.5). Grain size distribution of the combined

mix would determine the maximum stability of the mix. A well graded aggregate-soil mix

results in a mix with high dry-density and stability values.

Dense mixtures may be obtained when their particle size distributions follow Fuller’s

Law which is expressed as

p = 100 (d/D)0.5 ---- (13.1)

Where p = percent by weight of the total mixture passing any given sieve size.

d = aperture size of that sieve.

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and D = size of the largest particle in the mixture.

However, this grading does not define the practical limits for useful grading of

aggregates. Considering the various practical requirements to be satisfied, number of standard

specifications for aggregate gratings have been brought out by different agencies like,

AASHO, I.R.C. etc. Table 13.1. presents British recommendations for the gradation of

granular stabilised sub-bases.

Table 13.1

BS, Sieve Size, mm percentage by weight passing

76.0

38.1

9.52

4.76

0.6000

0.075

100

85-100

45-100

25-85

8-45

0-10

The clay content of soil-aggregate stabilised road - base must be very carefully

controlled. Where as in surface course, a non-porous layer is required inorder to prevent

water infiltration, the very opposite is normally true in a road base. Plasticity characteristics

specified for binder soil have already been discussed.

13.2.3. Blending Soils and Aggregate :

In some localities deposits of naturally occurring soils will be found which meet the

soil - aggregate specifications. More often than not, however, two or more soils will have to

be blended in proper proportions to produce mixtures meeting these requirements. Methods

for determining the proportions to blend are still essentially by trail and error. They involve

either estimating trail gradations, preparing test mixtures and testing for gradation and

plasticity, or else estimating the proportions which gives the desired plasticity index,

preparing trail mixtures, and testing them for gradation and plasticity. Some of the more

commonly used guide-procedures for blending soils are discussed below.

a) Proportioning of Materials by Rothfutch’s Method :

This method may be used when a number of materials have to be mixed together for

obtaining a desired gradation. The actual procedure will be apparent from the following

example. First the design gradation is decided based on the recommended grain size

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distribution charts or tables. Such a gradation is presented in columns 1, 2 and 3 of Table

13.2. The grain size distribution of a crusher run stone (A) sand (B) and silty-clay (C) that is

required to be mixed to produce a mechanically stable surface within the limits specified in

column 3, are presented in columns 4,5 and 6 in Table 13.2.

(i) The required grain-size distribution is represented by the diagonal oo’ of a rectangle

(Fig.13.2). The vertical ordinates of the rectangle are graduated for percentages from 0 to 100

on a linear scale. The horizontal scale for sieve aperture size is graduated by drawing for each

sieve size a vertical line that intersects the diagonal at a point where the ordinate equals the

percentage passing that sieve, (i.e.,) 100 percent for 25 mm sieve, 92% for 20mm sieve and

so on.

(ii) The size distribution of the aggregates to be mixed (Table 13.2 columns 4, 5, and 6

are plotted on this scale of sieve size (Fig.13.2) giving the lines BAO’ (Crusher run BFE

(Sand) and OG (silty clay).

(iii) The nearest straight line to these size distributions are drawn with the aid of a

transparent straight edge, by the ‘minimum balanced areas’. They are the dotted lines CO,

BO and OG (the last being coincident with the actual size distribution).

(iv) The opposite ends of these lines are joined, giving the chain lines CD and BG (In this

case, the later coincides with the 0.075 mm sieve size ordinates). The points where these lines

cross the required size- distribution line are marked by the circles L and M. The proportions

in which these three aggregates should be mixed are obtained from each difference between

the ordinates of these points, and are shown on the right hand side of Fig. 13.2.

Sieve Sizes (mm)

Fig. 13.2 : Determination of mixture for stabilised surfacings

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The particle-size distribution that will result form mixing the aggregates in these

proportions is given in column 7 of Table 13.2. The size distribution obtained is within the

limits (column-3).

b) Blending two materials by Plasticity Index :

The proportion is to be blended to give the required plasticity index can be estimated

from the following equations :

a = 100 SB ( )

( ) ( )p p

S p p S p pB

B B A A

−− − −

---- (13.2)

b = 100-a

Where a = percentage of material A in final mixture.

b = percentage of material B in final mixture.

p = Required plasticity index of final mixture.

pA = Plasticity index of material A

pB = Plasticity index of material B

SA = Percentage passing 425 µ sieve in material A.

SB = Percentage passing 425 µ sieve in material B.

Table 13.2

Example of Rothfutch’s Method for Proportioning Mixture of Aggregate (See Fig. 13.2).

I S Sieve

size mm

Percentage passing

Required size

distribution.

Aggregates available Mixture

37% A

45% B

18% C

Limits Average (A)

Crushed

stone

(B)

Sand

(C)

Silty Clay

25

20

10

100

85-100

65-100

100

92

82

95

70

21

--

--

--

--

--

--

98

89

71

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4.75

2.00

0.6

0.075

55-85

40-70

25-45

10-25

70

55

35

18

11

7

2

Trace

100

85

55

Nil

--

--

--

100

67

58

43

18

13.2.4. Construction Method :

The construction materials are collected from the selected borrow pits and stacked

along the sides of the road in the desired proportions.

The equipment needed are for excavation upto shallow depth, haulage for short

distance and for compaction. Machinery or Manual labour may be used for excavation and

haulage. For compaction roller of suitable type and weight is necessary depending on the

materials to be compacted. Construction steps are as follows :

(i) The sub-grade is prepared.

(ii) The materials are mixed in the desired proportions as per the design. Generally the

proportions are converted on volume basis.

(iii) Moisture is added if necessary and the materials are thoroughly remixed.

(iv) The wet mix is spread to the desired grade and compacted by rollers. Rolling is started

form the edges, and with adequate longitudinal overlap, it is continued upto the center.

Rolling is continued till adequate compaction is achieved.

(v) Field control tests are to check moisture content and density after compaction.

(vi) The Stabilized road is opened to traffic often the compacted layer hardens by

drying.

13.2.5. Stabilization using Soft-Aggregates :

When hard variety of aggregates are not available locally, the local soft aggregates

may have to be used for construction in order to keep the construction cost as low as possible.

These aggregates have low crushing strength and low aggregate impact value. If the load

transfer takes place through points of contact between aggregates Fig.13.1.b, they get crushed

due to high stresses at points of contract. This problem can be avoided by using the soft

aggregates with excess of binder soil so that the aggregate to aggregate contact is lost and the

aggregate float in a matrix of binder soil (Fig.13.1.c). This principle has been utilised in the

method of stabilization suggested by ‘MEHRA’ for construction of low-cost roads using soft-

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aggregates like kankar, brick bats laterite etc. The soil and aggregate are mixed in 2:1 ratio

and I.R.C. standardized the procedure of construction.

Mehra’s Method of Stabilization :

Base course material consists of compacted soil with sand content (of size less than

0.425 mm and greater than 0.075 mm) being not less than 50% and plasticity index 5 to 7.

Wearing course material consists of brick aggregates and soil mixed in the ration 1:2.

The sand content (of size less than o.425 mm and greater than 0.075 mm) in the soil should

not be less 33% and plasticity index 9.5 to 12.5. However, when bituminous surface

treatment is desired, the plasticity index is limited to 8-10.

The Mehra’s Method of conduction is briefly as follows :

(i) Soil is collected from approved borrow pits and stacked on road side.

(ii) Water is added upto OMC, and soil is mixed and spread to desired camber and grade.

(iii) 11 1/2 cm thick loose base course material is spread and rolled by 8 tonnes

power roller to a compacted thickness of 7 1/2 cm.

(iv) Surface course material (brick aggregate + soil in the ratio 1:2) mixed with

adequate water is spread to 11 1/2 cm loose thickness and the layer is rolled by 8

tonnes power roller to a compacted thickness of about 7 1/2 cm.

(v) After rolling, the surface is watered and left over night. The surface is again

rolled and finished.

(vi) The road is closed to traffic for four to five days and kept sprinkled with water. For

next few days only rubber tired traffic is allowed and after about 2 weeks the road is

opened to all traffic.

This method of constriction can carry 50 tonnes per day of mixed traffic in places

with light rainfall. With bituminous surface treatment the road can give satisfactory service

upto about 200 tonnes per day even in places with heavy rainfall.

13.3. SOIL-CEMENT STABILISATION :

Of all the methods of soil stabilization now in use, that which utilizes cement as the

stabilizing agent is second only to mechanical stabilization in importance and usage,. Factors

which have helped to make use of portland cement so popular as a soil stabilizer in nearly

every other country in the world, are as follows :

(i) Cement is available readily in most countries as a home product.

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(ii) Cement is manufactured on such a large scale that its price is comparatively

less.

(iii) Use of cement generally involves less care and control than may other method of

stabilisation.

(iv) More information is generally available on cement treated soil mixtures than on other

types of soil stabilisation.

(v) Almost any soil can be stabilised with portland-cement if enough cement is

used in combination with the right amount of water and proper compaction and

curing.

There are three different types of cement treated soil mixtures. The engineer should

distinguish among these so that he will know how and when each may be used most

advantageously.

13.3.1. Types of Cement - Treated Soil Mixtures :

a) Soil-Cement :

In this, sufficient cement is added to the soil to harden it and the moisture content of

the mixture is adequate for compaction purposes and hydrating the cement. This soil -cement

mixture should be capable of meeting the particular criteria resulting to strength or durability.

Most-important applications of soil-cement are as sub-base or/and base course in road ways

and parking areas, airport runways, taxiways and aprons. Soil-cement is not used in road

surfacings as it has poor resistance to abrasion. Another super-imposed material, eg.: a

concrete pavement or bituminous surfacing must be used to protect it from wheels of traffic.

b) Cement - Modified Soil :

In certain situations cement may be used to decrease soil plasticity. Cement generally

brings about a decrease in liquid limit and an increase in plastic limit with a corresponding

decrease in plasticity index. The increase in plastic limit is accompanied by a corresponding

increase in optimum moisture content. Situations which indicate the use of modified soil

include construction over wet plastic sub-grades.

c) Plastic - Soil Cement :

This is an intimate mixture of pulverised soil, cement and enough quantity of water to

produce a mortar like consistency at the time of mixing and placing. This is used primarily as

an erosion control material for lining on the sides of ditches and canals. In general, these

CE409/16 225

mixtures require about 4% or more cement than soil-cement mixture in order to meet the

same criteria.

13.3.2 Mechanism of Stabilisation :

In granular soils, the stabilisation is due to bond between the cementitious

components of hydrated cement and compacted soil particles at the points of contact. In

plastic soils, the free lime given out during hydration plays a prominent part in the reactions.

It brings about cation exchange on the surface of soil particles resulting in changes in the

plasticity characteristics. The soil then becomes friable and the cementitious products then

bind the lump of clay particles.

13.3.3. Factors Influencing Properties of Soil Cement :

The factors which primarily affect soil-cement properties are as follows :

(a) Type of soil

(b) Type and amount of Cement.

(c) Water content

(d) Degree of mixing

(e) Compaction

(f) Curing and

(g) Chemical additives.

(a) Type of soil :

The properties and durability of soil-cement depend upon gradation clay content,

Atterberg limits, specific surface, organic matter and sulphate content of the soil. The cement

requirement of fine grained soils is higher than that of well graded coarse grained soils. It is

because of the fact, the fine grained soils have higher specific surface area. The presence of

organic matter and sulphate retards the chemical action of cement. Generally a soil having

more than 50% passing, 4.75 mm sieve, less than 50% passing 0.075 mm sieve and having

liquid limit less than 40% and plasitc limit less than 18% is considered suitable for cement

stabilization.

(b) Type and Amount of Cement :

The greater the cement content the stronger is the resulting soil-cement properties. It

is now practice (in India and Britain) to specify the desired stabilities of soil-cement mixtures

CE409/16 226

in terms of minimum unconfined strength and not in terms of cement content. However in

U.S. the desired cement content is normally selected to meet durability requirements.

(c) Water Content :

Water is necessary in soil-cement mixture in order to hydrate the cement, to improve

the workability and to facilitate compaction. This water should be relatively clean and free

from harmful amounts of alkalies, acids or organic matter. In the case of soil-cement

mixtures it is important to realise that the optinum moisture for maximum density is not the

same as that for strength. In general, the optimum moisture content for maximum strength

tends to be on the dry side of optimum for maximum density for sandy soils and on the wet

side for clayey soils. The location of optimum value for strength is dependent not only on the

amount of clay present in the soil, but also on the type of clay mineral.

(d) Degree of Mixing :

High mixing efficiencies will result in lower cement content in order to achieve a

given soil-cement field strength. Increasing the moist mixing time and/or delaying

compaction after ending the moist mixing, generally results in an increase in the optimum

moisture content for maximum dry unit weight of soil-cement mixture, while its durability

and unconfined compressive strength are decreased. For these reasons, many specifications

place an upper limit on the length of the time between when moisture is added to the mixture

and when compaction is completed. This upper limit of time is about 2 hours as per many

specifications.

(e) Compaction :

Higher amount of compaction results in increased strength and durability.

(f) Curing :

The soil cement must be moist - cured during the initial stages of its life so that

moisture sufficient to meet the hydration needs of cement can be maintained in the mix.

Higher strength is obtained at higher curing temperatures.

(g) Chemical Additives :

Laboratory studies have shown that when trace chemicals like sodium carbonate,

sodium silicate, calcium chloride etc., have been used as additives to soil-cement moistures,

dramatic improvements in strengths can be obtained. It is a common practice to premix a

small quantity of lime with highly plastic soils to facilitate pulverization. It has been reported

CE409/16 227

that a significant reduction in cement content, without loss of long-term strength, could be

obtained by adding fly-ash to the cement-soil mixture.

13.3.4. Design of Soil-Cement Mix :

There are various mix design methods: The most commonly known being British and

P.C.A. Methods.

(a) British Method :

In this method, design is based on the unconfined compressive strength of 5 cm dia

and 10 cm long specimens cured for seven days. The cement content corresponding to a

strength of 17.5 Kg/cm2 (Fig.13.3.) is taken as design cement content for base courses of

highway pavements with light to medium traffic. However for heavy traffic, a higher strength

of 28 to 35 kg/cm2 may be adopted.

Fig. 13.3. Compressive strength (7-days) and max. Dry Density at different Cement

Contents.

(b) P.C.A. Method :

In this, the design criterion is based on durability of the soil-cement specimens to

withstand the specified wet-dry and freeze-thaw cycles, Durability is decided based on

resistance to loss in weight due to brushing the surface, volume change and moisture content

during the specified durability cycles.


The following three methods are used for construction of stabilized soil roads.

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(a) Mix-in-place method.

(b) Travelling plant method.

(c) Stationary plant method.

The major construction operations for soil-cement road are :

(i) Preparation of sub-grade.

(ii) Thorough pulverisation of the soil.

(iii) Thorough mixing of soil with the required quantity of cement.

(iv) Addition of water and its thorough incorporation in the mix.

(v) Spreading and Compacting

(vi) Curing by placing a suitable protective cover to prevent surface evaporation

losses of water.

The compaction of soil-cement mix is done soon after placing the wet mix. The joint

between old and new work should be laid carefully. Following field control tests have to be

carried out to control the quality of the work.

(i) Cement-content test

(ii) Moisture content test prior to compaction

(iii) Density of compacted layer.

It is also desirable to check depth of the processed layer and surface irregularities of

finished surface.

13.4. SOIL - LIME STABILISATION :

Lime as a soil additive, brings several beneficial changes to soil containing silt and

clay particles. The use of lime in construction of high ways is not new. Romans used

pozzonlana with lime for improving its cementing action and used in their roads about 2000

years ago. As in the case of soil-cement, soil-lime is not suitable for the surface courses of

pavements because of its very poor resistance to abrasion and impact. This process has

gained popularity now-a-days in view of its simplicity, efficiency and economy.

13.4.1. Mechanism of Stabilisation :

When lime is mixed with plastic soil, three stages of reaction can be recongnised.

CE409/16 229

(i) An early stage that occurs within a few minutes to an hour and is marked by a

flocculation, agglomeration and granulation of soil particles, a reduction in plasticity and

swelling potential and considerable improvement in the workability.

(ii) A subsequent stage in which a compacted mixture slowly develops strength as a result

of pazzolanic reactions and the formation of new compounds over a period as long as several

years, and

(iii) A third stage in which lime reacts with carbon-dioxide from the atmosphere and form

carbonates of calcium and magnesium. These carbonates are weak cements and the strength

gain due to carbonation is minimal in comparison with detrimental effects resulting from it.

The first stage may be identified as soil modification or amelioration effects, the

second as soil cementation or pozzolanic reactions and the third stage as carbonation. Both

the first two stages are important to stabilization process, since for wet, plastic sub-grade soils

it is the modification that yields proper soil conditions for the mixing and compaction

necessary for good cementation.

13.4.2. Factors Affecting Soil-Lime Stabilisation :

(a) Soil :

Clayey and silty soils benefit from the addition of lime. As the gain in strength of soil-

lime system is mainly due to pozzolanic reactions that take place between lime and certain

clay minerals a certain minimum clay fraction in the soil is always looked for. It is suggested

that soils for soil-lime stabilisation should contain atleast 15% material passings 0.425 mm

sieve size and should have plasticity index of atleast 10.

(b) Lime :

Limes can be divided into two categories: Quick limes and Hydrated Limes. After

long curing periods both types of lime produce almost the same effect. However, quick limes

are found to be more effective than hydrates limes. But quick lime has a tendency to cause

skin burns of unprotected workmen and hence need careful handling and protection.

Hydrated lime is, therefore, commonly used in stabilisation jobs.

(c) Moisture Content and Compactive Effort :

The addition of lime results in a decrease of the maximum dry density with an

increase in the optimum moisture content (fig. 13.4). Further, it is observed that an increase

CE409/16 230

in the compactive effort results in an increase of the compressive strength of soil-lime

mixtures considerably.

As in the case of soil-cement, increasing the moist mixing time and delaying

compaction after ending moist mixing result in lowering of the quality of soil-lime mixtures.

I.R.C. stipulates that the maximum timelag between mixing and compacting should not

exceed three hours.

(d) Curing Time and Temperature :

In general, the strength of lime-treated soils increase with curing periods, depending

on the type of soil, brand and amount of lime moulding moisture content and compactive

effort. It is also reported that specimens cured at higher temperature resulted in higher

strengths. Because of the great influence of temperature on the strength obtained, it is highly

recommended that any soil-lime-stabilisation work should be done’in early part of summer.

13.4.3. Effect of Lime on Strength Characteristics :

The addition of lime to soils up-grades them for use as an engineering material to

support loads. The strength of soil lime mixture in terms of unconfined compressive strength

or soaked C.B.R. values, for any given curing period and temperature, increases with lime

content upto an optimum value of lime, and then with further increase with lime content the

strength decreases (Fig.13.4).

Fig. 13.4. Effect of Lime on Properties of Clay.

13.4.4. Additives :

Addition of lime alone with soil often does not increase the strength of the mix

desired. Hence materials which increase the strength of soil-lime are tried as additives.

Portland cement and pozzolanic materials like fly-ash and surke are the most promising

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materials in this respect. Chemical additives like sodium metasilicate, sodium hydroxide and

sodium sulphate are also found to be useful additives.

13.4.5. Design of Soil-Lime Mix :

C.B.R. of lime-soil mixtures has been adopted as strength Criteria in our country.

Samples for C.B.R. test are prepared with varying percentages of lime at maximum dry

density and optimum moisture content corresponding to I.S. light compaction. The specimens

are then cured for 3 days and then soaked for four days in water before testing them for

C.B.R. value. No minimum CBR values are laid down. However, for base courses and

subbase course minimum values of C.B.R. of 80 and 20 are suggested.


The soil to be stabilised scarified. Fresh stocks of lime (Hydrated or quick) are stored

near the site. It is desirable that lime is made available in the form of fine powder.

The equipment needed are for scarifying, pulverising, mixing (in-situ) and

compaction.


(i) Preparation of sub-grade.

(ii) Pulverisation of soil to be stabilised.

(iii) Addition of lime with water and mixing.

(iv) Allowing the mixture to age for about a day for preconditioning the soil, and re-

mixing when pulverisation becomes easy.

(v) Adding rest of lime and water and remixing.

(vi) Spreading to the desired grade and compacting.

(vii) Curing- The soil-lime is protected from drying out and allowed to moist curing.

(viii) Carrying of field control tests - checking moisture content at the time of

compacting and checking the density soon after compaction.

13.5. SOIL-BITUMEN STABILISATION :

Bituminous materials have been first used for modern stabilisation work as dust

palliative on natural soil roads in Southern California in 1898. Since then, bituminous

materials have been used extensively for stabilisation of soils; however, at present time, use

of bituminous stabilised soils is limited to light trafficked roadways in dry areas, where

mineral aggregates are very scarce.

CE409/16 232

13.5.1. Types of Bituminous Materials :

Most commonly used materials are cut-backs and emulsions. Emulsions are used

especially in places where there is scarcity of water.

13.5.2. Mechanism of Stabilisation :

When a bituminous material is dispersed through out a soil, there are two beneficial

features which may occur. First and perhaps the most important one is, it will water proof the

soil, thereby maintaining already existing strength. Two theories “PLUG THEORY” and

“MEMBRANE THEORY” are proposed to explain this phenomenon. Secondly it may act as

a cemeting agent by binding the soil particles together. The “TNTIMATE MIX THEORY”

explains the cementing action. Obviously, in many circumstances, a combination of these two

mechanisms will occur.

13.5.3. Types of Soil-Bituminous Mixtures :

The commonly employed bituminous stabilisation process may be divided into the

following four main types.

(a) Sand - Bitumen Mixtures:

In these mixtures, the bituminous material is expected to provide cementitious

strength to such cohesionless materials as loose beach, river dune or other types of clean

sand. Penetration grade bituminous or road tars, cut backs (rapid or medium curing) with low

viscosity or emulsions are the commonly used binders.

(b) Sand - Gravel Bituminous Mixtures :

Sand-gravel materials falling into this category has a fairly good gradation, possess

good frictional characteristics, but has little cohesion. The function of the added bituminous

material is to act both as binding and water proofing agent.

(c) Soil - Bituminous Mixtures :

In this group bituminous material is used to stabilise the moisture contents of the

cohesive fine grained soils. Soils that can be satisfactorily stabilised in this way should not

have more than 30% passing 0.075 mm sieve, liquid limits less than 30 and plasticity index

CE409/16 233

less than 18. Soils of greater plasticity can not be used because of the difficulties experienced

in dispersing bitumen throughout the system.

(d) Oiled - Earth Treatments :

In this treatment, a slow curing cut back is applied to clean surface of soil which is

stable, well compacted and well drained. The bituminous material, thus applied, penetrates

downwards under the force of gravity and protects the under lying material from deleterious

effects brought about by changes in moisture content and minimizes dust nuisance.

13.5.4. Factors Affecting Properties Of Soil - Bituminous Mixtures :

Important factors that affect properties of soil-bituminous mixtures are discussed

below :

(a) Soil :

The particle size, shape and gradation of the soil influence the properties of soil-

bitumen mixes. A small proportion of fines in the soil is preferred, though high clay content

is not desirable.

(b) Types of Bituminous Materials :

Cut backs and emulsions are normally used in stabilisation work. Cut backs of

different grades give different stability values for a soil. The highest grade that can be mixed

with the soil at the time of construction should be preferred. The type of cut-back is also

chosen depending on the climatic conditions. Emulsions give slightly inferior results than

cut-backs.

(c) Amount of Bitumen :

Increasing proportion of bitumen caused a decrease in the maximum dry density of

soil-bitumen. The stability increases with an increase in bitumen constant upto an optimum

value and further increase in bitumen content resulting in a decrease in the strength. Water

absorption decreases with increase in bitumen content, though a slight increase may be noted

for very low bitumen contents if the specimens are soaked for long period, such as 28 days.

The optimum bitumen content for maximum stability generally ranges from 4 to 6% by

CE409/16 234

weight of dry soil, depending on the soil properties (Fig. 13.5).

Fig. 13.5. Relationship between Max.Density, Comp.strength and Water Absorption

and Bitumen Content for a Sandy Clay.

(d) Mixing :

Improved type of mixing with low mixing period may be preferred. In order to make

mixing possible and to disperse bitumen in fine particles, it is necessary to first mix soil with

water before adding the cut-back.

(e) Compaction :

Better the compaction, higher will be the stability and resistance to water absorption.

The compaction characteristics and the properties of the resulting mix depend on the

compacting moisture content and temperature, aeration of the mix before compacting and the

amount and type of compaction.

(f) Curing :

By curing soil-bitumen, the water and the volatile (of the solvent used in cut-backs

and emulsions) are allowed to evaporate there by allowing the bitumen to be effective to

impart the binding and water proofing actions. The curing period required to achieve desired

stability and loss-in weight would depend on curing temperature and relative humidity.

(g) Additives :

Sometimes plastic soils have been pretreated with lime or cement to overcome mixing

and constructional problems. Certain anti-stripping agents like phosphorous pentaoxide

(P2O5) have been added to bituminous binders while stabilising cohesive soils to improve

their effectiveness.

13.5.5. Evaluation and Criteria for Stability :

There is no standard method of mix design and the method of evaluation, depends on

the type of soil. In the method of evaluating soil-bitumen mixes as established by ASTM, the

specimens are tested in the Hubbard Field Testing Apparatus. Minimum values of extrusion

before and after absorption test are 480 kg and 190 kg respectively. Maximum values of

expansion and water absorption during test should not exceed 5% and 7% respectively. Sand

bitumens are also evaluated by using this test. For gravel bituminous and gravel-sand-

bituminous mixtures C.B.R. and Triaxial tests may be used.

13.5.6. Applications :

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Bituminous stabilized layer may be used as a sub-base or base course of ordinary

roads and even as surface course for roads with low traffic in low rainfall regions.

13.5.7. Construction Procedure :

Materials - The selected soil is pulverised and stacked. From practical considerations,

the following properties for the soil are suggested for proper stabilization:

Passing 4.75 mm Sieve <50%

Passing 0.425 mm Sieve 35-100%

Passing 0.075 mm Sieve 10-50%

Liquid Limit <40%

Plasticity Index <18%

Plants and Equipment - The plants and equipment needed are for scarifying, pulverising,

mixing and compaction.


(i) The soil to be stabilised is pulverised.

(ii) Water is added to the soil and is mixed.

(iii) Cut-back or emulsion is now added and the moist soil, is remixed for proper

distribution of bitumen.

(iv) The mix is spread, graded and compacted.

(v) The compacted layer is allowed to cure.

(vi) Field control tests include - checking of

(a) Pulverisation of wet mixed soil.

(b) moisture content and bitumen content before compaction, and

(c) dry density after compaction.

13.6. SUMMARY:

A stabilised surface may be defined as the surface which has been prepared with

locally available material with or without the use of admixture and compacted so as not to

allow it to loose its stability or bearing power under traffic.

Soil stabilisation methods for high way purposes may be divided into the following

groups.

(i) Mechanical Stabilisation.

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(ii) Cement Stabilisation.

(iii) Lime Stabilisation and

(iv) Bituminous Stabilisation.

When a granular structure, such as a road base or surfacing has the property of

resistance to lateral deformation under load, it is said to be ‘mechanically stable’. A good

mechanically stable base or surfacing usually consists of a mixture of coarse aggregate

(gravel, crushed stone, slag), fine aggregate (natural or crushed stone, sand etc.), silt and clay,

correctly proportioned and fully compacted. The use of correctly proportioned materials is of

particular importance in the construction of low-cost roads and in fact is the principle of

‘Mechanical Stabilisation’. Methods of proportioning locally available materials to satisfy

requirements of the grading are explained.

A method of low-cost road construction in which soft aggregates like bricks-bats,

kankar, laterite etc., are used has been developed by Professor Mehra. In this method,

popularly known as Mehra’s Method of Stabilisation, soil and aggregates are mixed in the

ratio 2:1 and IRC standardised the procedure of construction.

The importance and usage of cement stabilization is next to the mechanical

stabilisation. Most important applications of soil-cement are as sub-base or/and base course

in roads. Soil-cement is not used in road surfacings as it has a poor resistance to abrasion.

The properties and durability of soil-cement depends upon gradation, plasticity, specific

surface, organic matter and sulphate content of the soil. Normally an increase in cement

content produces material of better quality. High mixing efficiency and good compaction are

essential to obtain a material of high quality. When certain chemicals like sodium carbonate,

sodium silicate, calcium chloride are added in small amount dramatic improvement in

strength have been obtained. The suitability of a soil for processing with cement is

determined either by the unconfined compressive strength tests or by durability tests.

Lime stabilized soils are generally used as sub base for high type pavements and bases

for low and intermediate types of pavements. Hydrated or quick lime is used in stabilisation

work. Lime is found to be quite effective in stabilizing silty clayey soils. Important factors

affecting soil-lime properties are type and nature of soils, amount and type of lime, degree of

compaction, curing and chemical additives. Beyond an optimum lime content, the strength of

soil-lime mixes decrease with an increase in the lime-content. It has been found that strength

of soil-lime mix with addition of materials like cement, fly ash and surkhe and trace

CE409/16 237

chemicals like sodium hydroxide, sodium meta silicate and sodium sulphate. So far there is

not standard method for design of soil-lime mix. Generally the amount of lime required is

based on the unconfined compressive strength or the C.B.R. test criteria.

Bituminous stabilization may be used for the construction of sub-base, base or surface

courses of roads. Bituminous materials used for stabilization work are cut-backs and

emulsions. These materials when added to soil impart cohesion or binding action and reduce

water absorption. In the case of soil-bitumen mixes, as in the case of soil-lime mixes, the

stability of the mix-increases with bitumen content upto an optimum limit, and any, further

increase in bitumen content results in a decrease in the strength of the mix. It has been found

that optimum binder content generally varies from 4 to 6% by weight of soil. Higher amount

of compaction renders higher stability and resistance of water absorption. The curing period

depends upon the curing temperature and relative humidity. Portland cement, lime and anti-

stripping agents like phospherous pentoxide (P2O5) are used as additives in soil-bitumen

construction to improve the effectiveness of the mixture. Design of bituminous stabilization

mix is based on the C.B.R. test and modified Hubbard Field test.

After presenting each method of stabilization, the various operations involved in the

costruction of stabilised roads utilising the particular technique, are dealt with.


(1) Explain the basis principles in the following methods of soil-stabilisation:

(a) Mechanical Stabilisation.

(b) Soil - Lime Stabilisation.

(c) Soil - Bitumen Stabilisation

(d) Soil - Cement Stabilisation.

(2) Black cotton soils are highly clayey soils susceptible for large volume changes in

moisture content. What is the method of stabilisation you recommend in areas covered with

such soils ? Why ?

13.8. REFERENCES :

1. Bindra, S.P. (1977) - A course in Highway Engineering, Chanpat Rai and Sons, Delhi.

2. Kerbs, R.D.S.Walker, R.D. (1971) - Highway Materials, McGraw Hill Book Company,

New-Yark.

CE409/16 238

3. Khanna, Dr. S.K. and Justo, Dr.C.E.G. (1941) - Highway Engineering Nemchand and

Brothers, Roorkela.

4. O’ Flaharty, C.A. (1974) . Highway. Vol.2, Highway Engineering, Edward Arnold,

London.

5. Ramana Sastry, Dr. M.V.B.R. (1985) - Soil Stabilisation and Ground Improvement

Techniques - Lecture notes ISTF Summer School on ‘Foundations in Difficult Sub-Soil

Conditions’ - JNTU Engineering College, Kakinada.

6. (1959), Soil Mechanics for Road Engineers, H.M.S.O. Publication, London.

7. (1976), State of the Art: Lime Stabilisation, IRC Special Report - I, Indian

Roads Congress, New Delhi.

8. Soil - Cement Roads, Construction and Book, Published by the Concrete

Association of India, Bombay.

***


UNIT-14

HIGHWAY ECONOMICS AND FINANCE

CONTENTS :

Aims/Objectives

14.1. Introduction

14.2. Highway Users Benefits

14.3. Cost of Highway Transport

14.4. Economic Analysis Methods

14.5. Worked out Problems

14.6. Highway Finance

14.7. Summary


14.9. References

AIMS/OBJECTIVES :

CE409/16 239

For financing a highway development programme the various alternatives for the

entire highway scheme are to be compared and the best alternative, consistent with maximum

possible economy, is to be selected. The philosophy of highway economics is that the highest

possible ratio of the utility to the cost should be secured. Different procedures available for

carrying out economic studies are discussed. Methods of raising or providing funds for

highway projects are also presented under Highway Financing.


Improved highway facilities provide various benefits to the community. With

improved highways, transportation network improves and the travel time and accidents are

minimized. Cost of land adjoining new or improved highway improvements, should be

analysed and summed up to determine its economic viability. The engineering economy is

defined as ‘getting the most in the long run for the amount spent on the engineering project’.

Any proposal for a new or an improvement of road should be weighed by comparing benefits

expected or to be derived and cost of the proposed road work.

For economic analysis of the proposed highway project, following factors will have to

be carefully examined.

i) The annual cost of owning and operating highway facility.

ii) The annual cost of motor vehicle operations on that facility and

iii) The value of other economic benefits.

The sum of annual cost of maintenance of highway and the cost of operating motor

vehicles is called the transportation cost’. To determine this, several factors which effect

highway cost and vehicle operation cost have to be analysed. The ultimate aim is to achieve

the minimum cost of transportation of the road when all costs are taken into consideration

and at the same time providing adequate and safe conditions to the road user.

14.2. HIGHWAY USERS BENEFITS :

Highway provides varied benefits to the commuters. These benefits can be

categorised as

i) Tangible benefits in terms of market values and

ii) Non-tangible benefits.

14.2.1. Tangible Benefits :

CE409/16 240

Tangible benefits are also called quantifiable, primary or direct benefits. These

include the factors and their consequences which can be readily converted to monetary

values, such as :

i) the saving in travel time

ii) door to door delivery

iii) reduction in operation and maintenance cost of the vehicles.

iv) increase in the revenues from motor vehicles in the form of fuel,

vehicle tax, passenger and goods tax, excise and customs duties on

vehicles and spare parts etc.

14.2. Non-tangible Benefits :

Non-tangible benefits are also called non-quantifiable, secondary or indirect benefits.

These include factors which cannot be reduced to supportable and realistic monetary values,

such as :

i) increased land value

ii) increased business, education, social and general community values.

iii) decreased commodity prices

iv) increased values of natural resources.

v) development of recreational facilities such as sight-seeing and sports.

vi) improved mobility of defense forces and

vii) economy in investment on stocks.

14.3. COST OF HIGHWAY TRANSPORT :

Cost of highway transport comprises of the following two elements :

i) Annual cost of a highway

ii) Vehicle operation cost

14.3.1. Annual Cost of the Highway :

The annual cost of the highway consists of the first annual cost and annual

maintenance charges.

The total cost of a highway is comprised of the following :

a) Engineering and design expenditure.

CE409/16 241

b) Cost of right of way and costs of land acquisition.

c) Cost of construction of the formation.

d) Cost of pavement and drainage structures.

e) Cost of maintenance of highway

f) Legal and administrative cost.

The annual first cost is computed by dividing the total first cost by the useful life of

the highway anticipated under the existing and future traffic conditions.

14.3.2. Vehicle Operation Cost :

This is comprised of running cost and ownership cost.

a) Running cost of a motor includes

i) Fuel cost

ii) Oil cost

iii) Lubrication cost

iv) Tyre and tube cost

v) Repair and maintenance charges

vi) Depreciation cost

b) Ownership cost includes :

i) License fee

ii) Registration fee

iii) Insurance fee

iv) Rent of garage

Further the vehicle operation cost is also affected by factors like the average vehicle

operation cost, fixed cost of the vehicles, accident costs and travel time costs. The average

vehicle operation cost depends on factors like (i) Location of the road (ii) gradient and (iii)

type of surface.

The average annual highway cost for a road system may be summed by the formula.

Ca = H + T + M + Cr ---- (14.1)

Where Ca = Average annual cost of ownership and operation

H = Average cost for administration and management at head quarters.

T = Average annual highway operation cost

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M = Average annual highway maintenance cost

Cr = Average annual capital cost of depreciation of investment plus interest on

capital or the capital recovery with return on capital.

Instead of considering the ever all cost of a project, the annual repayment of a capital

loan plus the interest over a specified period of time of the ‘annual capital cost’ is considered

in the analysis. The first cost of a capital improvement is converted into equivalent uniform

annual cost by the formula :

Cr = P. ( )

( )i i

i

n

n

1

1 1

+

+ −

= P (CRP) ---- (14.2)

Where Cr = receipt in a uniform series for n periods to cover P at a rate of

interest i.

P = first cost of improvement of an element of a highway

i = rate of interest per unit period.

n = period of time in number of interest periods.

CRF = Capital recovery factor =

=( )

( )i i

i

n

n

1

1 1

+

+ −

At the end of the service life of a road pavement, some of the items could be assigned

some salvage value. However the salvage value of some other item may be negligible.

The average annual capital cost Cr for a project considering salvage value may be

estimated by the use of the formula

Cr = (C-Vs)( )

( )i i

i

n

n

1

1 1

+

+ −

+ i Vs ---- (14.3)

= (C-Vs) CRF + iVs

Where C = total investment on the construction

Vs = Salvage value at the end of ‘n’ years.

The basis of economic analysis of a highway improvement programme is to determine

the monetary benefits due to the additional expenditure. The analysis also help to decide the

most economical proposal among various alternatives. The economic analysis can be carried

out by any one of the following methods.

CE409/16 243

(i) Annual cost method.

(ii) Rate of return method.

(iii) Benefit-cost method.

14.4.1. Annual Cost Method :

The annual cost of each element of capital improvement is found by multiplying the

appropriate CRF value calculated for the assumed life span. The annual cost may be found

from equation 14.2. The total annual cost of an improvement is the sum of all annual costs of

capital recovery (Cr) plus annual maintenance and road users costs. Annual costs are

calculated for each of the proposals. The alternative which has the smallest total annual cost

is the best choice.

14.4.2. Rate-of Return Method :

The percentage rate of return R is given by

R = ( )O A M

P+ − × 100 ---- (14.4)

Where O = Savings in annual road users cost

A = Annual savings in accident costs

M = Additional maintenance cost per annum

P = Capital cost of improvement

The alternative which promises the highest rate of return of investment is adopted.

14.4.3. Benefit-Cost Ratio Method :

In this analysis the annual benefits are compared with the increase in annual cost.

Benefit - Cost Ratio

= Annual Benefits from the improvementAnnual cost of the improvement

= (R-R1)/(H1-H) ---- (14.5)

where R = Total annual road user cost of existing highway

R1 = Total annual road user cost for proposed highway improvement.

H = Total annual cost of existing road

H1 = Total annual cost of proposed Highway improvements.

CE409/16 244

In order to justify the proposed improvement, the ratio should be greater than 1.0.

High priority schemes usually have ratio of 3 or 4 or even more.

The following worked out examples illustrate the principles of these methods.

14.5. WORKED EXAMPLES :

14.5.1. The following data relates to a WBM Road surface and cement concrete road

surface. Calculate the total annual cost and find out which is the best solution.

Life of pavement may be taken as 30 years.

Data WBM surface Cement Concrete Surface

Length of Road

Surfacing first cost

Rate of Interest

Maintenance cost

Operating cost

A A D T

15 Km

Rs. 20,000/Year

5%

Rs. 300/Km/Year

Rs. 0.40/Vehicle km

100 Vehicles

15 Km

Rs. 50,000/Year

5%

Rs. 80/Km/Year

Rs. 0.25/Vehicle Km

100 Vehicles

SOLUTION :

I) Annual cost of capital recovery

Cc = (C-Vs)( )

( )i i

i

n

n

1

1 1

+

+ −

+ Vs i ----- (14.2)

For highway salvage cost is zero

Hence Cc = C( )

( )i i

i

n

n

1

1 1

+

+ −

+

i) Annual cost of capital recovery for WBM surface

= 20,000 ( )0 05 1 0 05

105 1

30

30

. ..

+−

= 20,000 × 0.06505 = Rs.1,301.00

ii) For the cement concrete surface

= 50,000 ( )0 05 1 0 05

105 1

30

30

. ..

+−

= 50,000 × 0.06505 = Rs. 3,252.50.

CE409/16 245

II) Maintenance Cost per Year :

Water bound macadam surface = 300 × 15 = Rs. 4,500.00

Cement concrete surface = 80 × 15 = Rs. 1,200.00

III) Vehicle Operating Cost :

On water bound macadam surface = 0.40 × 100 × 365 = 14,600.00

On Cement concrete road = 0.25 × 100 × 365 = 9,125.00

Total annual cost of WBM surface =

1,301 + 4,500 + 14,600 = Rs. 20,401.00

Total annual cost of cement concrete road =

3,252.50 + 1,200.00 + 9,125.00 = Rs. 13,577.50

This problem illustrates that the surface that costs the cheapest initially may not be the

best solution.

14.5.2. Calculate the rate of return using the following data

Item New Road Old Road

Annual vehicle cost per km

Average cost per vehicle per km in Rs.

Number of accidents

Cost per accident

Rs. 83,000/-

2.00

2

Rs. 8,000/-

Rs. 96.000/-

2.10

5

Rs. 8,000/-

Assume the cost of new proposed road is Rs. 3,00,000/- and the maintenance cost is expected

to be Rs. 4,000/-.

SOLUTION :

i) Total annual cost of the new road = 83,000 × 2 = 1,66,000/-

Total annual cost of the old road = 96,000 × 2.10 = 2,01,600/-

Saving annual cost due to improvement

= 2,01,600 - 1,66,000 = Rs. 35,6000.00

ii) Cost of accidents on old road = 5 × 8,000 = Rs. 40,000/-

Cost of accidents on new road = 2 × 8,000 = Rs. 16,000/-

Saving in cost of accident = Rs. 24,000/-

CE409/16 246

Then net saving = Total savings - Cost of maintenance

= 24,000 + 35,600 - 4,000 = Rs. 55,600/-

Then rate of return R = 55,600/3,00,000 × 100 = 18.53% Ans.

14.5.3. Annual benefits worth 20 crore rupees with results if improvements are taken up for

roads in a city. The annual repayment of capital and interest on loan taken to finance

the scheme will be Rs. 5 crores. The annual cost of maintenance and operation of the

road system will be Rs. 5 crores. Determine the benefit cost ratio of the investment.

Cost of Total benefits 20 crores

Repayment 5 crores

Net benefit 15 crores

Cost of operation of road system = 5 crores

Then benefit-cost ratio for the project = 15/5 =3.

14.6. HIGHWAY FINANCE :

The government or any other agency finances highway development. The funds for

financing highway projects are generally recovered from road beneficiaries in the form of

direct or indirect taxes. The beneficiaries of a highway project are the motor vehicle users,

the owners of adjacent property and the general public. The proportions of cost of highway

programme to be degraded depends on the extent of benefits derived by that category.

Two methods are followed for raising highway finances. They are (i) pay as you go

method and (ii) Credit financing method.

In pay as you go method, the highway improvement is carried out by the funds

collected in the form of taxes from the different beneficiaries. In India, the responsibility of

financing different roads lies with the Central government, the concerned state governments

or local bodies like corporations, municipalities etc.

The Central government derives funds for highway financing through duties and taxes

levied on motor fuel (20 percent of Central Road Fund), excise duty on vehicles, spares, tyres

etc., and excise duty on lubricants, grease etc. The main source of income for state

government has been Central Road Fund (80% of the fund) since its introduction in 1929.

Other taxes include registration fees for vehicles, fees on driving licenses, road tax, permits

for transport vehicles, passenger tax on buses, sales tax on vehicle parts etc., Toll taxes are

the major sources of income of local bodies.

CE409/16 247

In credit financing system, the payment for highway improvement is made from

borrowed money, say from the World Bank, the Asian Development Bank, Credit bonds etc.,

and the borrowed amount along with interest is repaid from future income.

14.7. SUMMARY :

Engineering economy may be defined as getting the most, in long term, for the

amount spent on an engineering project. Thus any new proposal for highway improvement or

development should be justified in terms of the cost incurred and the benefits derived. The

Highway Tranportation Cost, that is, the total amount spent on the highway, consist of annual

cost of highway and vehicle operation cost.

Benefits derived from a highway project may be included under two groups -Tangible

benefits and Non-tangible benefits. The philosophy of highway economic studies- Annual

Cost method, Rate of Return method and Benefit-Cost ratio method have been explained. The

lowest annual cost, the highest rate of return and the highest benefit-cost ratio are the

principles respectively for the economic justification of a project based on these methods.

Two methods of raising highway finances, namely, pay as you go and credit financing

system have been explained.

14.8. SELF ASSESSMENT QUESTIONS :

1. Distinguish between quantifiable and non-quantifiable benefits derived from highway

projects.

2. Explain the basic principles on which various economic analysis methods of highway

projects are developed.

3. Write notes on ‘Highway Finance’.

14.9. REFERENCES :

1. Bindra, S.P. (1977) - A course in Highway Engineering - Dhanpat Rai and Sons, New-

Delhi.

2. Khanna, Dr.S.K. and Justo, C.E.G. (1991) - Highway Engineering - Nemchand Brothers,

Roorkee.

***


UNIT-15

AIR PORTS

CE409/16 248

CONTENTS :

Aims & Objectives

15.1. Introduction

15.2. Historical Development

15.3. Classification System of Air Ports

15.4. General Layout of an Air Port


15.6. Summary

15.7. References

Aims & Objectives :

Air Ports are vital to the air transportation system. Air transport has gone up by leaps

and bounds during this century. A brief historical development of air transport system in the

world and in particular in India is presented. After presentation of the Common classification

systems of air-ports, a layout of an airport is given to illustrate the various components in an

airport.


The planning and design of an airport requires more intensive study and forethought

as compared to the planning of other modes of transport. This is because aviation is the most

dynamic industry and its forecast is quite complex. Land, Water and Air have been used by

mankind for developing the transport modes like the road way, railway, water way and

airway. Railways and roadways are in competition for providing better transport facilities.

Waterways and air transportation are used for long distance travels.

In a large country such as India railways and road ways do not satisfy the needs of the

tourists, particularly the international tourists, as the travel by these modes is too slow, and

the country is very vast, and the time available for a tourist is in general very short.

Therefore, it is essential to develop Civil aviation in our country.

Air transportation possesses a few distinct advantages as compared to the other modes

of transport. They are :

(1) Rapidity : Aviation maintains the highest speed. Designs have been finalised for

supersonic jets.

CE409/16 249

(2) Continuous journey : This mode of transport is continuous over land and water

without loss of time.

(3) Accessibility : Air transport has the unique ability to open up any region that is

inaccessible by other means of transport.

The air transport has a few limitations as indicted below :

(1) Operating expenses : The operating expenses are generally very high including the

cost of aircraft, traffic control system etc. This mode of transport accommodates very few

travellers and small cargo which have to bear high expenses for unit weight.

(2) Weather conditions : The operation of air transport is greatly affected by weather

conditions - heavy wind, foggy weather too.

15.2. HISTORY OF AIR TRANSPORT :

The first flight by a power driven aircraft which was heavier than air was made by

Orville Wright, a bicycle repairer on Dec.17, 1903, over a distance of 35m near Kitty Hawk,

North Carolina, U.S.A. During the World War I, Germans used aeroplanes for passenger

transport. Long distance air mail service was introduced in May, 1918 between Washington

and New York (U.S.A). International air services were commenced during 1918 and 1919.

First jet aircraft manufactured in Germany made its first flight in August, 1939.

The year 1911 is a Land mark in the history of air transport in our country. A

Frenchman, Hemri Piquet, flew form Allahabad to Naini junction which is at a distance of 7

km. In 1927, the Civil Aviation Department was formed by the British government and this

organisation helped in building up a few aerodromes and bring up of some flying clubs.

Imperial Air Ways Service, a regular weekly service was commenced between Karachi and

Delhi in 1929. In 1932, Tata Airways limited started internal air services and in 1933, Indian

Trans-Continental Air-Ways Limited was formed.

The second world - war helped for the development of aviation in our country. Tata

Airlines changed its name as Air India Limited in July, 1946. A new organisation named as

‘Air India International Limited’ was formed by the Government of India in 1947, for

external air services.

The Government of India have nationalised air-travel in India, in March, 1953 by

passing the Air Corporation Act, under which Air India International and Indian Airlines

were formed as public sector corporations, Air India to operate long range international

CE409/16 250

services and Indian Airlines to operate all domestic and short range international services and

regional routes to adjacent countries, viz., Pakistan, Bangladesh, Srilanka and Nepal.

Air India celebrated its entry into the jet age by starting Boeing 707 services to

London and to New York in the year 1960. Daily Boeing services were inaugurated by the

Indian Airlines in the year 1971. The country for domestic flights is divided into four flight

information regions with centers at Delhi, Bombay, Madras and Calcutta. Realizing the

potential and importance of air transport and tourism, the Government of India created in the

year 1963, the Ministry of Tourism and Civil Aviation.

The National Air Port Authority (NAA) manages the civilian airports in the country

and provides the air traffic services throughout the country. Domestic services in India are

managed by the Indian Air Lines (IA), with Vayudoot providing some short haul services.

The main function of ‘Pavan Hans’ formed in 1985, is to provide helicopter services to Oil

Industry.

The Government of India has formed an autonomous corporation, “The “Helicopter

Corporation of India (HCI)”, in 1986 in order to meet the requirements of helicopters purely

for civil transport needs, where the nature of the terrain or other special reasons do not permit

the use of other modes of transport. Because of the prohibitive cost of transport by

helicopters, its use is extremely limited even in highly developed countries. A number of

private airlines are also operating on the domestic routes. The Government of India

announced air tax policy in 1985 to allow resident and non-resident Indians to start air taxis

services within the country.

The International Air port Authority of India (IAAI) operates the five major

metropolitan airports, namely, Bombay Delhi, Calcutta, Madras and Trivandrum. IAAI also

takes up construction and consultancy work overseas.

The regulatory and licensing functions, bilateral issues, approval of tariffs / schedules

etc., are entrusted to the Director General of Civil Aviation (DGC A).

15.3. MAJOR INTERNATIONAL ORGANISATIONS :

15.3.1. International Civil Aviation Organisation (ICAO) :

The nations of the world have established the ICAO to serve as the medium through

which the necessary international understanding and agreement between nations in all the

technical, economic and legal field is achieved to carry the international services quite

efficiently. This was created in 1944 and is now a specialized agency in relationship with the

CE409/16 251

united Nations. Its head quarters is in ‘Montreal’ and regional offices at Bangkok, Dakar,

Lima, Mexico City, Paris and Nairobi. ICAO provides the machinery for the achievement of

international cooperation in air.

15.3.2 Federal Aviation Administration (FAA) :

The Federal Aviation Administration (established in 1958 and known till 1968 as the

Federal Aviation Agency) functioning under the Department of Commerce of USA,

develops, directs and fosters the coordination of a national system of air ports and directs

their federal airport programme. The various standards and standards of FAA are widely

adopted in airport planning and design in our country and else where.

15.4. CLASSIFICATION OF AIR PORTS :

15.4.1. Functional Classification :

In India, airports can be classified broadly as military and civil airports. There are also

some privately owned air strips. These airports are divided into categories based on their

importance. They are :

(1) International Air Ports : These are five in number and are located at Bombay (Sahara)

Delhi (Indira Gandhi) Calcutta (Subhas Chandra Bose), Madras (Anna) and Trivandrum.

(2) Major Aerodromes Eg. Hyderabad (Begumpet)

(3) Intermediate Aerodrome Eg. Vijayawada and Visakhapatnam

(4) Minor Aerodromes Eg. Rajahmundry.

15.4.2. Classification According to Geometric Design :

In order to specify the geometric of various types of air ports, classifications have

been developed by the International Civil Aviation Organisation (ICAO) and the Federal

Aviation Administration (FAA). These two systems of classifications are discussed in detail

in the unit on Runway Geometric design.

15.5. COMPONENTS OF AN AIR PORT & GNARL LAYOUT :

An airport has the following components.

1. the ‘runways’ for take off and landing of aircrafts.

2. the ‘taxiways’ which connect runways with other ports of the airport, over which the

aircraft can move. eg. aprons, hangars.

CE409/16 252

3. the ‘apron’ where planes park for embarking and disembarking of passengers and loading

and unloading of cargo.

4. the ‘terminal building’ which forms an interface between the air and ground side, and

where passengers and their baggage are processed.

5. the ‘hangars’ where aircraft can be given shatter and maintenance, and

6. ‘air traffic control’ facilities, eg. Control tower, guidance systems, lighting systems etc.

A general layout of an air port is given below.

Fig. 15.1. General Layout of an Air Port


1. Draw a plan of an airport and identify the various components.

2. Write a detailed note on the systems of classification of air ports.

3. Sketch the development of ‘Air transport’ in India.

15.7. SUMMARY :

As a mode of transport, air travel has certain advantages like rapidity, continuity of

journey and accessibility compared to other modes of transport and as such is developing

very fast all over the world and particularly in India. The Civil aviation transport system in

India is based on domestic services being provided primarily by the Indian Air Lines (IA)

with vayudoot providing some short haul services. Air India (AI) operates international

services. Helicopter services to Oil Industry are provided by ‘Pawan Hans’ and helicopter

Corporation of India provides helicopters for Civil transport needs. A number of private

agencies are also operating air transport facilities. International airports are planned,

designed, constructed and maintained as per recommendations and standards of International

CE409/16 253

Civil Aviation Organisation (ICAO), Federal Aviation Agency (FAA) and International

Airport Authority of India (IAAI). Domestic flights are under the control of National Airport

Authority (NAA). A general layout of an airport given provides a clear picture and

understanding of the functions of the major components of an airport.

15.8. REFERENCES :

1. Khanna, S.K., Arora, M.G. and Jain S.S., (1990) - Airport Planning & Design

“Nemchand Bros, Roorkee.

2. Rangwala S.C., and Rangwala P.S., (1992) - ‘Air Port Planning’ - Charotar Publishing

House, Anand.

3. Rao, G.V., (1992) - ‘Air Port Engineering’ Tata Mc Graw-Hill Publishing Co., Ltd., New

Delhi.

4. Shegal S.G., and Bhanot, K.L., (1980) - A Text Book of Highway Engineering & Air

Ports; S.Chand & Co., New-Delhi.

***


UNIT-16

AIR PORTS

Aims / Objectives

16.1. Introduction

16.2. Aircraft Characteristics and Their Influence on the Air Port Design


16.4. Summary

16.5. References

AIMS & OBJECTIVES :

Information concerning aircraft is vital to an airport designer in order that he may

design the various systems and components rationally. This unit deals with aircraft

characteristics related to airport design.


The planning and design of airports requires proper understanding of the present day

aircrafts, their types, size and probable future improvements in their design. Most of these

CE409/16 254

characteristics of an aircraft are affected by the type of propulsion and thrust generating

medium. Accordingly aircrafts may be categorised as piston engine, turbo-props, turbo-jets

and turbo-fans. Some of the important characteristics of an aircraft which will decide the

landing and ground facilities required for their handling and service are discussed in the

following paragraphs.

16.2.1. Type of Propulsion :

Dimensions, capacity, noise and performance characteristics of an aircraft depends

mainly upon the type of propulsion of an air craft.

16.2.2. Dimensions of an Aircraft:

Fig. 16.1. illustrates the various parameters that define the aircraft size. the length (of

the fuselage), the wing span, and the height give the over all dimensions of an aircraft which

CE409/16 255

influences the size of the hangars, the size of the parking aprons as well as taxiway side

clearances. Typical dimension for some modern air crafts are shown in Table 16.1.

CE409/16 256

Fig. 16.1. Aircraft dimensions-definition figure

Table 16.1. Characteristics of Principal Transport Aircrafts

Manu-

facture

r

Model

destina

tion

Wing

span

(m)

Length

(m)

Maxi-

mum

height

(m)

Distan

ce

betwee

n main

gears

(m)

Wheel

base

(m)

Minim

um

turning

radius

(m)

Maxim

um

gross

take

off Wt.

(kg)

Maxim

um

landing

weight

(kg)

Maxim

um

passen

ger

capacit

y (No)

Type

of

engine

No.of

engine

s

No. of

wheels

in main

gear

Main

gear

dimens

ions

(cm)

Dougla

s

DC-6 35.25 30.50 8.72 7.40 9.35 21.60 44,000 6,400 72 Piston 4 2 75

Convai

r

880 36.00 37.85 10.87 5.60 15.90 20.67 84,000 60,500 110 Turbo

jet

4 4 53.7 ×

118

Vicker

s

Viscou

nt 802

28.10 24.55 8.02 7.15 7.45 21.00 29,300 26,600 48 Turbo

prop

4 2 47.5

Bristol Britani

a 300

42.67 37.27 11.20 9.30 10.57 -- 75,000 58,600 138 Turbo

prop

4 4 75 ×

120

Lock

head

1 lectra 29.70 34.37 9.2 9.35 11.10 19.52 52,700 48,500 74 Turbo

prop

4 2 39.8

CE409/16 257

Boeing 707-

120

39.25 43.35 11.47 6.62 15.70 30.30 112800 79,500 179 Turbo

Jet

4 4 85 ×

140

Boeing 707-

120 B

39.25 43.35 11.47 6.62 15.70 30.30 116200 81,000 179 Turbo

prop

4 4 85 ×

140

Boeing 707-

320

42.72 45.87 11.60 6.62 17.70 32.70 187000 90,000 189 Turbo

jet

4 4 85 ×

140

Boeing 707-

320 B

42.72 45.87 11.60 6.62 17.70 32.70 143000 94,800 189 Turbo

Prop

4 4 85 ×

140

Boeing 720 39.25 40.85 11.42 6.57 15.20 29.70 92,800 75,000 140 Turbo

prop

4 4 78 ×

125

Boeing 720B 39.25 40.85 11.42 6.57 15.20 29.70 100000 75,000 140 Turbo

prop

4 4 78 ×

125

Boeing 727 32.52 30.22 10.12 6.05 15.85 21.05 64,500 59,500 114 Turbo

jet

4 2 87.5

CE406/1 258

The purpose of wing of an aircraft is to support the machine in the air when the

engine has given the necessary forward speed. The cross-section of the wing of an aeroplane

in of cambered aerofoil shape as shown in fig.16.2.

Fig.16.2. Various Parts of Wing

The real advantage of this shape is that it receives a current of air in an upward

direction and directs downwards, thus obtaining the lift reaction. The best design of the

aerofoil is the one which gives greater area of negative pressure on the top and the greatest

area of positive pressure on the bottom.

16.2.3. Minimum Turning Radius :

For establishing the path of movement of aircraft on the airport and determining

aircraft position near a terminal area, it is necessary to know the “movement’ capability of

aricraft. The “Turning radius” is a function of the nose-gear angle (Fig.16.1). When the radius

is minimum, it produces excessive tire wear and when large, it is critical as regards the

clearance to adjacent buildings/aircraft.

To determine the minimum turning radius, a line is drawn through the axis of the

‘nose-gear’ when it is at its maximum angle of rotation. The point, where this line intersects

another line drawn through the axis of the two main gears, is called the ‘Center of rotation’.

The distance of the further wing tip form the center of rotation represents the minimum

turning radius.

16.2.4. Minimum Circling Radius :

CE406/1 259

There is a certain minimum radius with which the air-craft can take a turn in space.

This radius depends upon the type of aircraft, airtraffic volume, and weather conditions. The

minimum turning radius for small general aviation aircrafts under VFR (Visual flight

Regulation) conditions is as large as 80 km.

Two nearby airports should be separated from each other by an adequate distance so

that the aircrafts simultaneously landing on them do not interfere with each other. If the

desirable spacing between the airports cannot be provided, the landing and take-off of

aircrafts in each airport have to be timed so as to avoid collision. This will obviously reduce

the capacity of each airport.

16.2.5. Speed of Aircraft :

Presently, the reference datum for speed of aircraft is the speed of sound (1194 kmph

at 0C and 1263 kmph at 300C ) defined as Mach 1. Most of the modern military air crafts are

supersonic (having a speed greater than that of the sound) having a Mach Number greater

than one, where as a majority of transport aircrafts are subsonic, having, Mach number less

than one. The speed referred to in these cases is the true air speed and not ground speed.

Airports serving high speed aircrafts will have a greater frequency of operations and more

facilities for peak hour passenger flow. The larger dimensions of supersonic aircrafts will also

affect the planning and design of airports.

16.2.6. Capacity :

The passenger and cargo capacity, as well as the fuel capacity, have a direct influence

on the facilities required in the terminal building , cargo handling system and fuel-storage

system. The passenger capacity of some modern aircrafts is given in Table 16.1.

16.2.7. Aircraft Weight and Wheel Configuration :

Aircraft weight influences the length as well as the thickness of the runway. The

different components of weight of an aircraft are as follows.

1. ‘Operating Empty Weight’ comprises of its basic weight including the crew, their

baggage and supplies, the necessary service equipment for flight and other items of

equipment considered an integral part of a particular air plane configuration. It does not

include pay load and fuel.

2. ‘Pay Load’ is used to describe the load that earns the revenue meaning there by, the

weight of passengers, mail and cargo.

CE406/1 260

3. ‘Maximum-Zero Fuel Weight’ is defined as the weight above which all additional weight

is in the form of fuel only. It is specified to limit the moments (while in flight) at the junction

of the wing and fuselage.

4. ‘Trip Fuel weight’ is the weight of the fuel carried for a particular trip and depends on

factors like pay load, attitude of flight, speed of aircrafts distance to be traveled, atmospheric

conditions etc.,

5. ‘Reserve Fuel Weight’ is the weight of fuel required to meet any emergency and depends

on trip length, traffic conditions, location of alternate airport in the event of emergency

landing etc.

6. ‘Maximum Gross - Take Off Weight’ is the maximum load which the aircraft is certified

to carry during take off and the air port pavements are designed for this load.

7. ‘Maximum Structural Landing Weight’ is the maximum gross take off weight less the

weight of trip fuel.

8. ‘Maximum Ramp Weight’ is the maximum weight authorized for ground manoeuver

(taxing) certified by government regulations.

Typical weights for some modern aircrafts are presented in table 16.1. Table 16.2.

presents the distribution of aircraft weights based on the range of travel.

Table 16.2 Distribution of aircraft Weight

(as a percent of total weight)

Item Aircraft Type

Short Range Medium Range Long Range

Operating empty Weight Payload Trip fuel Fuel reverse

66 24 64

59 16 21 4

44 10 40 6

Total : 100 100 100

The aircraft while it is on the ground is supported by the tricycle under carriage

system. The principal functions of the under carriage system is to absorb landing shocks and

to enable the aircraft to manoeuver in the ground. Two sets of wheels are provided for the

aircraft to move on the ground and to carry the entire weight of the aircraft. The major

portion of the total load (about 90%) is carried by ‘two main gears’ which are provided in the

fuselage or in the wings near the junction of fuselage and wings. The remaining part of very

CE406/1 261

small portion of the load is carried by the wheels provided at the nose or tail of the aircraft

known as nose or tail gear respectively. The tail wheel arrangement is not preferred in the

modern aircrafts because in the event of powerful wind, the aircraft may lift off the ground or

be pushed backwards even when the aircraft is parked as this position keeps the nose up and

the wings are at greater angle of incidence.

The main reason of using multiple wheel arrangement is to distribute the aircraft load

over a large area of the pavement and thus reduce the pavement thickness. Basic wheel

configurations are shown in Fig. 16.3.

Fig. 16.3. Basic Wheel Configurations.

16.2.8. Fuel Spilling :

Spillage of fuel and lubricants usually occur on loading aprons and hangars. This is

found to have caused some distress to the pavement if it is surfaced with bituminous material.

Research is still underway to find completely satisfactory fuel-resistant seal coat for

bituminous pavements.

16.2.9. Noises :

The aircrafts make lot of noise during landing and take off. More disturbance is

caused during take off than during landing. The problem is more severe in the case of jet

CE406/1 262

aircrafts than with conventional engine. As far as possible, the runway should be so oriented

that there is no urban development in the area under the approach flight path of the aircraft.

16.2.10. Jet Blast :

The turbo-jet and turbo-prop aircrafts eject hot exhaust gases at 2 relatively high

velocities. The velocity of jet blast may be as high as 300 kmph and it may even cause

inconvenience to the passenger boarding the aircraft. Bituminous pavements are adversely

affected by the heat and blast from the jet aircrafts. In cement concrete pavements sealing

material at the joints will be affected, Blast Fences of different types are available to divert

the smoke ejected by the engine.


1. What are the aircraft characteristics that influence the design of airports.

2. Write notes

a) Wheel configuration of aicrafts

b) Aircraft weight.

16.4. SUMMARY :

Influence of aircraft characteristics on the design and planning of air ports is

discussed in detail. Aircraft dimensions, length of the fuselage, the wing-span and the height

influence the size of the hangars, the parking aprons as well a taxiway side clearance.

Minimum Turing radius of an aircraft effects the geometric design of taxiways and positions

of the aircraft on the aprons. Minimum distance between two neighbouring airports should be

not less than two times the minimum circling radius of the largest aircraft utilizing facilities

of these airports. Aircraft speed and capacity influence the design of terminal area. Aircraft

weight influences the length as well as the thickness of runway. Structural design of taxiways

and aprons is very much dependent of the weight of aircraft. Considerations have to be given

to the nuisance of noise while orienting runways. Approach flight paths should not be above

urban development areas. Jet blasts effect the surface of bituminous pavements and as such

concrete pavements have to be preferred.

16.5. REFERENCES :

1. Khanna, S.K., Arora, M.G., and Jain S.S., (1990) - Airport Planning & Design

‘Nemchand Bros, Roorkee.

CE406/1 263

2. Rangwala S.C., and Rangwala P.S., (1992) - ‘Air Port Planning’- Charotar Publishing

House, Anand.

3. Rao, G.V., (1992) - ‘Air Port Engineering’ Tata McGraw-Hill Publishing Co., Ltd., New-

Delhi.


Ports; S.Chand & Co., New-Delhi.

***


UNIT - 17

AIRPORT PLANNING

Aims / Objectives

17.1. Introduction

17.2. Regional Planning

17.3. Data to be collected for the Location of an Airport

17.4. Factors Affecting Selection of Site for an Airport

17.5. Surveys to be conducted and Drawings to be prepared

17.6. Runway Orientation

17.7. Airport Obstructions

17.8. Zoning Laws


17.10. Summary

17.11. References

AIMS / OBJECTIVES :

The planning of an airport is a complex process. In this chapter the general aspects of

airport planning are considered.


The planning of an airport is not a simple process. As a matter of fact, it is such a

complex process that the analysis of one activity without regard to the effect on other

activities may not provide acceptable solutions. This is due to the fact that an airport

encompasses wide range of activities which have different and conflicting requirements. yet

CE406/1 264

these activities are interdependent so that a single activity may limit the capacity of the entire

complex. In this chapter various factors to be considered in the selection of site and planning

of an airport are presented.

There has been a radical development in the nature of air transport and considerable

improvements in the aircrafts themselves. The airport planned has to satisfy not only the need

of the present aircraft, but also the needs of the aircraft which may emerge in, say, 10 years.

This needs more intensive study and fore-thought on the part of the planner planning an

airport as compared to other modes of transport.

17.2. REGIONAL PLANNING :

In the past, the airport planning was made by keeping the local aviation requirement

only in view. But this concept has changed in the recent times. The airport system plan has

not only to satisfy the local needs of aviation, but at the same time, it should be seen that it

fits into the overall development of the entire region or country. If future air planning has to

achieve useful results, it has to be founded on guidelines established on the basis of

comprehensive airport system and master plans. Planning on regional basis would also enable

to implement ‘Zoning Laws’ in the areas where new airports are needed in the near future.

Regional planning is being done by the Civil Aviation Organisation in Collaboration

with various state governments. The regional plan usually provides the following

information:

1. Approximate locations of the airport in the national plan.

2. Classification of airports

3. Location of strips, and

4. Routes of air travel.

The following data is to be collected for a scientific and sound planning on a regional

basis.

17.3. DATA TO BE COLLECTED FOR LOCATION OF AN AIRPORT:

1. Population growth of the area to be served and the character of the population based on

the income groups and activities and trends and needs of the people towards saving of

travel time.

2. Topographical and geographical features of the area.

CE406/1 265

3. Distance, population and economic character of the adjoining areas having air service

should be studied.

4. Characteristics of future air traffic in terms of passengers, cargo, mail etc. and the

airtraffic volume.

Before deciding to go in for a new airport, full consideration should be given to the

possibility of improving the existing airport capacity so as to meet the future traffic demand.

If this can not be done, a new air-port has to be planned. Before thinking of a site for a new

airport, the following information Concerning the new airport is collected.

(i) Peak hourly volume of air traffic to be handled.

(ii) The present and future types of aircraft which may use the airport.

(iii) Facilities to be provided.

Based on this information classification of the future airport, geometric standards of

approaches, runways and taxiways are determined. Thus the planner gets an idea of the

approximate land size required for developing a new airport.

17.4. AIRPORT SITE - SELECTION :

The following factors have to be considered in the selection of a site for a major

airport.

1. Regional plan - The site selected should fit well into the regional plan.

2. Airport use - The selection of site depends upon the use of an airport - Civilian or

military operations. Civilian airports may be used for military operation during emergencies.

Therefore, the site selected should be such that it provides natural protection to the area from

air raids.

3. proximity to other Airports - The site selected should be such that it is away from the

existing nearby neighboring air port by a distance more than twice the circling radius of the

largest type aircraft expected at the airport.

4. Ground Accessibility - The site should be so selected that it is readily accessible to the

users. The door to door time should normally not exceed 30 minutes. Availability of public

transportation facilities further qualifies the suitability of the site.

5. Topography - A raised ground, for example, a hill top is usually considered to be an

ideal site for an airport because of (a) less obstruction in the approach end turning zones (b)

natural drainage (c) more uniform wind, and better visibility due to less fog.

CE406/1 266

6. Obstructions - The approach areas should be free from man-made or natural

obstructions. Any existing obstructions have to be removed at any cost. The future growth of

undesirable structures is controlled by the zoning laws.

7. Visibility - The site selected should be free from visibility reducing conditions, such

as fog, smoke and haze, Fog settles down in valleys and smoke and haze nuisance exists at

sites nearer to industrial area. Therefore trend of future development of industrial area also

should be studied.

8. Wind - Runway is so oriented that landing and take off is done by heading into the

wind. The site selected should be located in the wind ward direction of the city so that

minimum smoke from the industry is blown over the site. As such wind data, over a

minimum period of five years, should be collected.

9. Noise Nuisance - Noise nuisance is more with jet-engine aircrafts than with the

propulsion type of aircrafts. The site selected should be such that the landing and take off

paths of the aircrafts passover the land which is free from residential or industrial

development. Some times a ‘buffer zone’ may have to be provided between the take off end

of the runway and a nearby residential area. If the buffer zone cannot be provided some

acoustical barrier may have to be provided.

If the runway along the prevailing wind direction is oriented such that the flight path

is over a highly developed residential and industrial area, then the noise nuisance may be

reduced by constructing another runway at right angles to the previous direction, if the area is

found scarcely populated. This runway, called ‘preferential Runway’ (Fig.17.1) may be used

as long as the cross-wind components is within the permissible limits.

CE406/1 267

Fig. 17.1. Preferential Runways for Noise Abatement

10. Grading, drainage and soil characteristics - Grading and drainage play an important

role in the construction and maintenance of airports. The cost of drainage and grading can

often be reduced by selecting a site with favourable soil conditions. The most desirable type

of soil for airport construction is one with reasonable amount of coarse grained soil combined

with a suitable natural binder.

11. Future development - Taking into consideration the anticipated future developments

of the airport, larger area should be acquired initially and a master plan has to be prepared.

Zoning ordinances should be implemented to prevent growth of undesirable structures in the

area.

12. Availability of Utilities - In the selection of site, availability of utilities like water -

supply, sewer, electricity, telephone from the town etc., should be considered.

13. Economic considerations - Among the various alternative sites, one which is

economical, when both initial and ultimate stages are considered, should be preferred.

17.5. SURVEYS TO BE CONDUCTED AND DRAWINGS TO BE PREPARED :

The following surveys have to be carried out at the site of proposed airport

a) Traffic Survey

b) Meteorological Survey

c) Topographical Survey

d) Soil Survey

e) Drainage Survey, and

f) Material Survey

The surveys mentioned above help in the preparation of the following drawings for

the finally selected area.

a) Topographical plan with original and finally proposed features.

b) Obstruction map showing safe approach zones and turning zones for the aircraft.

c) Drainage plan with all details of the drainage net-work.

d) Airport Master Plan with complete details of developments at various phases of

construction and development.

CE406/1 268

17.6. RUNWAY ORIENTATION :

Runway is oriented usually in the direction of the prevailing winds. The aircraft takes

off and lands in a direction opposite to the direction of winds (i.e.,) the head-wind-direction.

During take-off, this provides great lift on the wings and the aircraft rises above the ground

much easier and in a much shorter length of the runway. During landing, the head wind

provides a further braking effect and the aircraft comes to a stop in a smaller length of the

runway.

17.6.1. CROSS -WIND COMPONENT AND WIND COVERAGE : Wind, throughout the

year may not be blowing ‘along the direction of the center line of the runway. During some

period the wind may be making a certain angle (α) with the central line of the runway. If V is

the velocity of the wind, the component of the velocity perpendicular to the center line of the

runway - V sin α is called the ‘CROSS WIND COMPONENT’. For safe landing and take-off

of aircraft, the cross-wind component should be within certain limits as specified below.

Small air-craft 15 Kmph

Mixed traffic 25 Kmph

Big Aircraft 35 Kmph

The percentage of time in a year during which the cross-wind component remain

within the limits specified above is called ‘WIND COVERAGE’. According to FAA, the

runway handling mixed air-traffic should be so planned that for 95 percent of the time in a

year, the permissible cross-wind component doesn’t exceed 25 kmph. For busy airports the

wind converge may be increased to as much as 98 percent to 100 percent.

WIND - ROSE DIAGRAM : For fixing up the orientation of the runway, a wind-rose

diagram is drawn from the wind data collected at the site of the airport. The wind data should

usually be collected at least for a period of 5 years and preferably for a period of 10 years, so

as to obtain an average data with sufficient accuracy. Wind-rose diagrams can be plotted in

two ways.

Type 1: Showing direction and duration of wind

Type 2: Showing direction, duration and intensity of wind

Fixing up the orientation of a runway by drawing the Type 1 of wind-rose diagram is

only explained below.

CE406/1 269

Table 17.1. Wind Data*

Wind

direction

Duration of wind, per cent** Total in each

direction

percent

6.4 - 25 kmph 25 - 40 kmph 40 - 60 kmph

N

NNE

NE

ENE

E

ESE

SE

SSE

S

SSW

SW

WSW

W

WNW

NW

NNW

7.4

5.7

2.4

1.2

0.8

0.3

4.3

5.5

9.7

6.3

3.6

1.0

0.4

0.2

5.3

4.0

2.7

2.1

0.9

0.4

0.2

0.1

2.8

3.2

4.6

3.2

1.8

0.5

0.1

0.1

1.9

1.3

0.2

0.3

0.6

0.2

0.0

0.0

0.0

0.0

0.0

0.5

0.3

0.1

0.0

0.0

0.0

0.3

10.3

8.1

3.9

1.8

1.0

0.4

7.1

8.7

14.3

10.0

5.7

1.6

0.5

0.3

7.2

5.6

Total per cent = 86.5

* Average of 8 years periods

** Percentage of time during which wind intensity, is less than 6.4 kmph is 100 - 86.5 = 13.5

per cent. This period is called calm period and does not infulence the operation of landing or

take-off because of low wind intensity.

A typical wind data is presented in Table 17.1. In this table, the duration of wind for

any one direction covers an angle of 22.5 degrees. The wind-rose diagram is illustrate in Fig.

17.2. The radial lines indicate the wind direction and each circle represents the duration of

CE406/1 270

wind. From table 17.1, it is observed that the total percentage of time in an year during which

wind from the north direction is 10.3 percent. This value is plotted along the North direction.

Similarly other values plotted along with the respective directions. All plotted points are then

joined by straight lines as shown in Fig. 17.2. The resulting diagram (shown in the figure) is

known as the wind-rose diagram. The best direction of runway is usually along the direction

of the longest line on the wind rose diagram. In Fig. 17.2. the best orientation of the runway

is, thus, along the N-S direction.

Fig. 17.2. Wind Rose Diagram Type 1

The direction of wind, in percent (wind converge available) along this direction is

only 10.3+14.3 = 24.6 percent of the year. As this gives a very low wind coverage, and if the

deviation of wind direction upto 22.50, (considering the cross wind component effect) from

the direction of landing and take-off is permissible, the percentage of time in one year during

which the runway can safely be used for landing and take-off will be obtained by summing

the percentages of time, along NNW, N, NNE, SSE & S and SSW directions, (i.e.,)

5.6+10.3+8.1+8.7+14.3+10.0 = 57.0 percent.

‘Calm period’ (i.e.,) the percentage of period during which the wind intensity is low,

less than 6.4 kmph in this case, may also be added to the above period.

Calm period = 100 - Total percent of time wind coverage, that is equal to

(100 - 86.5*) = 13.4 in this problem.

The total percentage of the time therefore comes to 57.0 + 13.5 = 70.5 percent

CE406/1 271

It may be noted that this wind rose diagram does not account for the effect of cross -

wind component.

17.7. AIRPORT OBSTRUCTIONS :

An aircraft while landing or taking-off loses or gains height gradually along an

inclined path called the ‘glide - path’. It is, therefore, necessary that wide areas on either side

of a runway must be kept clear of any obstruction. Obstructions for safe air navigations are

broadly divided into two categories.

(1) Objects exceeding their limits on the ground in the approach and turning zones.

(2) Objects protruding above certain surfaces.

17.7.1. APPROACH AND TURNING ZONES : Wide areas on either side of a particular

runway which must be kept clear of obstructions are called approach areas or approach zones

(Fig. 17.3). The centre line of the approach area coincides with the centre line of the runway.

This area on the ground is trepe zoidal in shape with its width increasing from the runway

end outwards.

Fig. 17.3. Approach Areas and Obstruction Clearance Line

Refer to Table 17.1

A line rising at a particular slope, Say 1 in 50 or 2% in the case of class A airport

from a point 60m form the end of the runway end represents the ‘Obstruction Clearance Line’

in elevation (Fig. 17.3). The imaginary inclined plane containing the obstruction clearance

line and directly above the approach area or zone is termed as ‘Approach Surface’. All

manoeuvers while taking off and landing of aircraft are expected to take place above this

CE406/1 272

approach surface. The tips of obstructions, natural or man made, shall be well below this

approach surface.

The most critical portion of the approach area is its inner most portion known as

“Clear Zone’. It is also shown in fig. 17.3. It should be preferably a level area and except for

fences, ditches and other minor obstructions, all other major obstructions should be removed

form the clear zone.

The ‘Turning Zone’ is the area of airport other than the approach area and it is

intended for turning operations of the aircraft in case of emergencies like failure of engine or

trouble in smooth working of aircraft experienced at the start of take off. In such cases, the

pilot takes a turn and comes in line with runway before landing. The aircraft operates at a

considerably low height in the turning zone and it therefore becomes absolutely necessary to

make this zone free from any obstructions. Some of the standards specified by the ICAO for

the various surfaces indicated above are presented in Table 17.2.

17.7.2. IMAGINARY SURFACES : The imaginary surfaces are the established surfaces

(Fig. 17.4) in relation to the airport and to each runway above which no obstruction should

project. The size of the imaginary surfaces depends on the category of each runway and on

the type of approach planned for that runway. The location of these imaginary surfaces is

referred with respect to the ‘Air Port Reference Point (ARP)’. The ARP represents the

geographical location of an airport, that is, the geometric centre of the landing area. The types

of imaginary surfaces are :

Table 17.2. Standards By ICAO for Various Zones : (Refer to Fig. 17.3)

Sl.No. Surface Runway Code

A B C D E

1.

Approach Area / zone length (m) when the lnading

CE406/1 273

is a) Instrumental b) Non -Instrumental

15000 3000

15000 3000

15000 3000

---- 2500

----1600

2.

Clear Zone when the landing is a) Instrumental Width W1 (m) Width W2 (m) Length L (m) b) Non - Instrumental Width W1 (m) Width W2 (m) Length L (m)

300 525 750

150 270 600

300 525 750

150 270 600

300 525 750

150 270 600

--- ---

75 135 300

------

75135300

3. Approach surface Longitudinal gradient in (%) when the landing is a) Instrumental b) Non - Instrumental

22.5

22.5

23.3

--- 4.0

---5.0

Fig. 17.4. Schematic View of Imaginary Surfaces

a) Approach Surface - Already discussed under 17.7.1.

CE406/1 274

b) Transitional Surfaces - These have a slope of 14.3% (IV:7H) for A, B and C types of

airports and 2% for D and E types of airports and extend upwards from the edges of runway

safety area and intersects the approach surface and inner horizontal surface.

c) Inner Horizontal Surface (IHS) - This is an imaginary surface at a height of 45m

above the established elevation of the runway. The radius of this surface with respect to ARP

is 4000m for AB and C types and 2500 for D and E types of airports.

d) Conical Surface - This surface extends upwards and outwards from the periphery of

inner horizontal surface. This is an inclined surface. As per the ICAO, the specifications for

the conical surfaces are as follows.

Slope (for all types of airports) 5%

Height

(I) Instrument Approach

For A, B Class 100 m

C Class 75 m

(II) Non - Instrument Approach

For A, B Class 100 m

C Class 75 m

D Class 55 m

E Class 35 m

e) Outer Horizontal Surface (OHS) - This surface is not proposed for aerodromes with

runways of length less than 900 m. It is circular in plan with centre located at ARP. The

height of OHS is 150m above the established elevation of the airport. The OHS shall extend

upto 9900 m from the ARP, when the length of the longest runway is less than 1500m and

upto 15,000 m from ARP when the length of the longest runway extend beyond 1500m. The

constructions protruding above this surface shall not be permitted.

17.8. ZONING LAWS :

These refer to the enactment of legislation for a restricted development of the area

surrounding the airport so that no structure protrudes above the approach surface, and thus

causes a hazard to the safe air navigation, especially in the approach areas. Different

maximum construction heights may be permitted in different zones depending on the distance

of a particular locality form the airport and depending also upon whether the locality is within

CE406/1 275

the approach zone or away form it. Zoning is considered to be the most effective method of

protecting the interests of both the airport and those of the owners of the land surrounding it.

The use of land for manufacture of certain items which may result is smoke, nuisance,

obnoxious odour may also be controlled by zoning laws. A zoning ordinance shall not be

oppressive, unreasonable, but shall promote general welfare of public, their health, safety,

morals and comfort.


1. Bring out the factors that need consideration for selection of site of an airport. Discuss the

critical issues involved.

2. Why is it necessary to plan airports on regional basis ?

3. Explain the step by step procedure you will follow for developing an existing airport for

the increased air traffic anticipated.

4. Explain the procedure of Orienting the runway.

5. Explain the significance of the following in an airport planning.

a) Approach zone & approach surface.

b) Clear zone

c) Imaginary Surfaces

d) Zoning Laws.

17.10. SUMMARY :

Aviation is the most dynamic industry and its forecast is quite complex. As such,

airport planning requires more intensive study and forethought as compared to planning of

other modes of transport. Airport planning including airport administration, has to be done on

a regional basis; otherwise it would seriously impair the effective air traffic services of the

country. Data to be collected for upgrading an existing airport to meet the demands of

increased traffic or for location of a new airport has been dealt with in detial.

A number of factors have to be studied critically while selecting a suitable site for an

airport, depending upon the class of airport. Factors that have to be considered in the

selection of site for a major airport have been discussed.

An aircraft lands and takes-off in the head wind direction. The wind data collected at

the site of the airport helps in drawing the Wind Rose diagram. Procedure for drawing the

CE406/1 276

wind rose diagram and orientation of runway for obtaining maximum wind coverage has also

been presented in this chapter.

The site selected for an airport should be such that, it doesn’t obstruct safe landing

and take-off of aircrafts. Air-crafts while landing and taking off loses or gains height

gradually along an inclined path called ‘the glide path’. Further, to permit various

manoeuvers of aircraft for safe navigation. Large areas around the airport have to be free

from obstructions or the heights of obstructions have to be limited depending on their

distance form the airport. The various standards specified by the ICAO or the FAA are

discussed.

The forming of suitable laws is an essential part of an airport master plan and they are

to be implemented as soon as the final selection of the airport site is made. The zoning is

considered to be the most effective method for achieving excellent results from the

functioning of the airport. The airports are involved in two types of zoning, namely, height

zoning and land use zoning.

17.11. REFERENCES :

1. Khanna, S.K., Arora, M.G., and Jain S.S., (1990) - Airpot Planning & Design “Nemchand

Bros, Roorkee.

2. Rangwala S.C., and Rangwala P.S., (1992) - ‘Air port planning’ - Charotar Publishing

House, Anand.

3. Rao, G.V., (1992) - ‘Air Port Engineering’ Tata McGraw - Hill Publishing Co., Ltd.,

New-Delhi.


Ports; S. Chand & Co., New - Delhi.

***


UNIT - 18

RUNWAY AND TAXIWAY DESIGN AND PRINCIPLES OF AIRPORT DRAINAGE

Aims / Objectives

18.1. Introduction

18.2. Basic Runway Length

18.2.1. Corrections for Runway Length

CE406/1 277

18.2.2. Runway Geometric Standards

18.2.3. Configuration of Runways and Capacity

18.3. Taxiway

18.3.1. Geometric Design Standards

18.3.2. Turn around or Bypass Taxi Ways

18.4. Airport Drainage

18.4.1. Surface Drainage

18.4.2. Subsurface Drainage


18.6. Summary

18.7. References

AIMS / OBJECTIVES :

Runway and Taxiway geometrics as per ICAO and FAA standards have been

presented and discussed. Corrections to be applied for arriving at the length of runway at the

actual site of construction, for the elevation, temperature and gradient over the basic runway

length have been explained by working out problems. Special features of airport drainage and

methods of providing surface and sub-surface drainage are also included.


In this chapter, runway and taxiway geometrics as per the standards of ICAO and

FAA have been presented and discussed. In these standards the length of the runway given is

the basic runway length. It is the length of the runway under certain standard assumed

conditions at the airport. As the conditions at the actual site of construction will be far

different from the assumed conditions, corrections have to be applied, for any change in

elevation, temperature and gradient to the basic runway length to arrive at the actual runway

length. The capacity of an airport depends on the number of runways and their configuration,

apart from many other factors. Airport drainage is one of the most important aspects of

airport construction and maintenance. If the runoff is not quickly drained off from the

runways/taxiways/aprons it is hazardous for both the take off as well as the landing of the

aircraft, but adversely influences the pavement performance and life.

18.2. BASIC RUNWAY LENGTH :

It is the length of runway under the following assumed conditions at the airport.

CE406/1 278

1. Airport altitude is at the sea level.

2. Temperature at the airport is standard at 150C.

3. Runway is levelled in the longitudinal direction

4. No wind is blowing on the runway

5. Aircraft is loaded to its full capacity

6. There is no wind blowing enroute to the destination

7. Enroute temperature is standard.

The following performance conditions of the aircraft have to be considered in arriving

at the basic runway length.

Normal landing case

Normal take-off case

Engine failure case

Based on the length of the basic runway ICAO classifies airports into five groups A

through E. (Table 18.1).

18.2.1. CORRECTIONS FOR RUNWAY LENGTH : The basic runway length, as

discussed above is for mean sea level elevation having standard atmospheric conditions.

Necessary corrections have, therefore, to be applied for any change in elevation, temperature

and gradient for the actual site of construction.

(i) CORRECTION FOR ELEVATION : As the elevation increases, the air density

reduces. This in turn reduces the lift on the wings, and the aircraft requires a longer length of

runway to achieve the necessary lift. ICAO recommends that the basic runway length should

be increased at the rate of 7 percent per 300m rise in elevation above the mean sea level.

(ii) CORRECTION FOR TEMPERATURE : The rise in airport reference temperature

has the same effect as that of the increase in elevation. Airport reference temperature (ART)

is defined as the monthly mean of average daily temperature (Ta) for the hottest month of the

year plus one third the difference of this mean temperature and the monthly mean of the

maximum daily temperature (Tm) of the same month of the year.

Thus ART = Ta + Tm Ta−3

CE406/1 279

ICAO recommends that the basic runway length after having been corrected for elevation

should be further increased at the rate of 1 percent for every 10C rise of airport reference

temperature above the standard atmospheric temperature at that elevation.

Std. Atmospheric temperature at the elevation of airport. = 150C - 0.0065 × elevation

of airport above M.S.L. in metres.

(iii) CHECK FOR THE TOTAL CORRECTION FOR ELEVATION PLUS

TEMPERATURE : As per the ICAO, if the total correction for elevation plus temperature

exceeds 35 percent of basic runway length, these corrections should be further checked up by

conducting specific studies at the site by model tests.

(iv) CORRECTION FOR GRADIENT : Steeper gradient results in greater consumption

of energy and much longer length of runway is required to attain the desired ground speed.

The F.A.A. recommends that runway length after having been corrected for elevation and

temperature should further be increased at the rate of 20% for every one percent of ‘effective

gradient’. Effective gradient or average grade is defined as the maximum difference in

elevation between the highest and the lowest points of runway divided by the total length of

the runway.

18.1. WORKED EXAMPLE :

Obtain the length of runway given the following data.

Basic runway length 1620 m

Elevation of airport above MSL 270 m

Mean of Max. daily temp Tm = 43.720c

Mean of Average daily temp Ta = 26.320C

Particulars of longitudinal section of the runway

Chainage (m) 0 100 300 600 800

Elevation (m) 100 101.0 99.0 101.40 101.80

SOLUTION :

Airport reference temperature = 26.32 + (43.72 - 26.32) × 1/3 = 32.120C.

Std. temperature at the elevation of airport = 150 - 0.0065 × 270 = 13.150C.

Rise of Airport Temp above the standard temp = (32.12 - 13.15) = 18.970C.

Effective gradient = 101.80 - 99.00 / 800 × 100 = 0.35%

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CORRECTIONS :

1) Elevation correction = 7100

1620 270300

102× × = m

Total length of runway after elevation correction

= 1620 + 102 = 1722m

2) Temperature correction = 1722 1100

18 97 326 7× × =. . m

Length of the runway after temperature correction = 1722 +327 = 2049m.

3) Gradient correction = 2049 × 20/100 × 0.35 = 143.4m.

Length of the runway after gradient correction = 2049+144 = 2093 m

Check for increase in length after elevation and temperature correction

= 2049 - 1620 / 1620 × 100 = 429 / 1620 × 100 = 26.48%

This is with in the limits specified (i.e.,) less than 35%.

18.2.2. RUNWAY GEOMETRIC STANDARDS : The geometric standards of an airport

depend upon the performance characteristics of the aircraft that will use the airport, the

weather conditions and the services rendered by the airport. The ICAO has classified airports

into two categories. (i) based on the basic runway length and other geometrics using the code

letters A to E. and (ii) based on the ESWL and the tyre pressure of the aircraft which will use

the airport using Code No.1 to 7. These are presented in Table 18.1. and 18.2.

Table 18.1 Runways Geometrics (ICAO)

Air port

types

Basic Runway length m Runway pavement

width m

Maximum

Longitudinal grade %

Max. Min.

A --- 2100 45 1.5

B 2099 1500 45 1.5

C 1490 900 30 1.5

D 899 750 22.5 2.0

E 749 600 18 2.0

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Table 18.2. Classification of Airports Based on Aircraft Wheel Load Characteristics

(ICAO)

Code Single isolated wheel load (kg) Tyre pressure Kg/cm2

1. 45,000 8.5

2. 34,000 7.0

3. 27,000 7.0

4. 20,000 7.0

5. 13,000 6.0

6. 7,000 5.0

7. 2,000 2.5

The following items are considered in the geometric design of runways.

i. Runway Length : To obtain the actual length of runway, corrections for elevation,

temperature and gradient are applied to the basic runway length (Table 18.1) as explained in

the example 18.1.

ii. Runway Width : The width of runway for different types of airports vary from 45m to

18m (Table 18.1). The airtraffic is more concentrated in the central 24m width of the runway.

Further, the outer most machine of a large jet aircraft using the airport should not extend off

the pavement to shoulders. This is because the shoulder is usually of loose or stabilised soil

etc., which is likely to get into the engine and damage it. The outer board engines of large jet

transport aircraft are at about 13.5m from the longitudinal axis of the aircraft. As such a

pavement width of 45m will provide adequate protection to the engine from shoulder material

during normal operations.

iii. Width and Length of the Safety Area :

CE406/1 282

Fig. 18.1. Runway Elements

Safety area consists of the runway, which is paved area plus the shoulders on either

side of runway plus the area that is cleared, graded and drained as shown in Fig. 18.1. The

shoulders are usually unpaved (stabilised or turfed) as they are used by the aircraft only

during emergency. The ICAO recommend, the following widths for safety areas :

Non - Instrumental landing

A, B, C types of Airports 150 m

D, E types of Airports 78 m

Instrumental landing

A, B types of Airports 300 m

In the length direction, the landing strip extends by 60 m beyond the runway at both the ends.

Typical cross - section of runway for the Instrument Landing System (ILS) of approach is

shown in Fig. 18.2.

Fig. 18.2. ILS Runway Cross Section

iv. Transverse gradient : The ICAO recommends the transverse gradient of runways

should not exceed the following limits :

A, B and C types of airports 1.5%

D and E types of airports 2.0%

Minimum limit 0.50%

CE406/1 283

Gradients for shoulders

(a) Upto 75m on either from the centre of runway 2.5%

(b) Beyond 75 m 5.0%

v. Longitudinal and effective gradient : The ICAO recommends the following maximum

values

For longitudinal gradient :


D and E 2.0%

for effective gradient :


D and E types of airports 2.0%

vi. Rate of change of longitudinal gradient : Abrupt changes of the longitudinal gradient

restricts the sight distance and may also cause premature lift off of the aircraft during take off

operations. Too many changes in the gradients over a small length of the runway can also

restrict the sight distance and increase the length of the runway. Thus not only the changes in

grade, but also the distance between such changes, are to be kept within the tolerable limits.

The changes in gradients should be smoothened by vertical curves.

The ICAOs recommendations are as follows :

a) Maximum change in gradient

Class of Airport Max. Rate of change of gradient

in percent for 30m of the vertical

curve

A & B 0.10

C 0.20

D & E 0.40

b) Minimum distance between two successive points of grade intersections

Class of airport Min. distance m.

A & B 300 × absolute numerical value

of change in slope

C 150 × ”

CE406/1 284

D & E 49.5 × ”

vii. Sight distance : The longitudinal gradient of runways is quite gentle and hence there

will be no restriction to sight distance. The problem arises when two runways or a runway

and taxiway intersect with each other. The ICAO recommends as follows L

a) In the case of A, B and C types of airports any two points 3m above the surface of

runway should be mutually visible from a distance equal to 1/2 runway length.

b) For D and E types there should be unobstructed line of sight from any point 3m above

the runway to another point 2.1m above the runway within a distance of at least one half the

length of the runway.

CE406/1 285

18.2.3. CONFIGURATION OF RUNWAYS AND CAPACITY : The number of aircraft

movements which an airport can process within a specified period of time, with an average

delay to the departing aircraft within the acceptable time limit is defined as airport capacity.

The airport capacity depends upon the runway configuration. The pattern of runways to be

adopted at the site of an airport depends on the volume of the air traffic to be handled,

environmental conditions and availability of suitable approaches. The basic patterns of

runways are shown in Fig. 18.3. The actual runway pattern may consist of combination of

two or more number of basic patterns.

Fig. 18.3. Runway Patterns

The capacities of the different patterns are as follows :

1. Single runway : The capacity of this simplest pattern under VIR conditions may be a

maximum of 45 to 60 operations per hour. Under IFR conditions, its capacity reduces to 20 to

40 operations.

CE406/1 286

2. Parallel Runways : The capacity of this pattern depends on the lateral spacing

between the two runways, the weather conditions and the navigational aids available at the

airport.

When the lateral spacing between the runways is 900 m landing and take off (but not

simultaneous landing) can be done on the two runways independently. This pattern permits

100 operations and about 80 operations per hour under VFR and IFR conditions respectively.

If the runways are located as close as 210m, the capacities under VFR and IFR conditions

reduce to 75 to 25 operations per hour and 40 to 45 operations per hour respectively.

If parallel runways are staggered and the terminal area is located in between the two

runways, the taxiway distances to and from the terminal area are reduced. The present trend

for large airports is towards parallel staggered runways which are located about 1500 m apart

so that they are capable of being used simultaneously.

3. Intersecting runways : This pattern of runways is usually adopted when wind in a

particular direction does not provide the required coverage. When the cross wind components

on each runway are favourable for use of both the runways, the capacity depends upon the

direction of landing and take off, the lateral separation of the glide paths of the aircrafts and

the intersection point of runways. The capacity will be maximum when the intersection point

is near the runway ends.

4. Non - Intersecting runways : The capacity depends upon the wind conditions and

visibility. When the flight paths are divergent the capacity may be of the order of 80 to 110

operations per hour. If the flight paths are convergent the capacity reduces to 66 to 80

operations per hour.

18.3. TAXIWAYS :

Taxiways link the various elements of an airport, e.g., runways with aprons and

hangars to provide for a safe and expeditious surface movement of aircraft. The taxiway

system should enable smooth flow, minimizing crossing of runways or other taxiways.

Provision of well-designed exit taxiways increases the runway capacity. Taxiways include

exit and entrance taxiways, parallel and dual parallel taxiways, by pass taxiways, apron

taxiways and others. It is preferable that the entire taxiway system be visible from the

airtraffic control tower. The following conditions decide the layout of taxiways.

1. Taxiways should be so adjusted that the aircraft that have just landed and are taxing

towards the apron, do not interfere with aricrafts taxing for take off.

CE406/1 287

2. At busy airports, taxiways should be located at various points along the runway so

that the landing aircraft leaves the runway as early as possible and keeps it clear for use by

other aircraft. Such taxiways are called ‘Exit taxi ways’.

3. The route for taxiway should be so selected that it provides the shortest practicable

distance from the apron to the runway end.

4. As far as possible, the intersection of taxiway and runway should be avoided.

5. Exit taxiways should be designed for high turn off speeds. This will reduce the

runway occupancy time of aricraft and thus increases the airport capacity.

18.3.1. GEOMETRIC DESIGN STANDARDS : The speed of aircraft on taxiway is much

lower than its speed on a runway during the landing and take off. Thus the design standards

of taxiways are not as rigid as they are for runway. Geometric design standards of taxiways

as per the ICAO are presented in Table 18.3. A cross - section of a taxiway is shown in Fig.

18.4.

Fig. 18.4. Taxiway Cross Section

Whenever there is a change in the direction of a taxiway, a horizontal curve is

provided. For this purpose a circular curve with a large radius is designed so that the aircraft

can negotiate it without significantly reducing the speed. The radius of the curve is given by

R = V2 / 125 F

Where R is the radius in metres

V is the speed in kmph

F is coefficient of friction which may be assumed as 0.13

For airports serving large subsonic aircrafts, a minimum value of radius of 120 m is

recommended whatever be the speed.

CE406/1 288

Fillets are provided at the junction or intersection of two or more number of traffic

ways eg., runways, taxiways or apron. If adequate fillets are not provided, one of the main

gears of an aircraft, is likely to go off the pavement on to the shoulder. As a guide the ICAO

recommends that the radius of the fillet should not be less than the width of taxiway.

Table 18.3 Taxiway Geometric Standards (As per ICAO)

Airport

classifica-

tion

Taxiway

width m

Max. long.

gradeint %

Min.

Trans.

Gradient %

Max. rate

of change

of long

grade per

30 m%

Sight

distance

Safety area

A

B

C

D

E

22.50

22.50

15.00

9.90

7.50

1.50

1.50

3.00

3.00

3.00

1.50

1.50

1.50

2.00

2.00

1.0

1.0

1.0

1.2

1.2

Surface of

taxiway

must be

visible

from 3 m

height for

distance of

300 m for

ABC types

and a

distance of

249 m be

visible

from 2.1 m

for D and

E types

Turfed or

paved

shoulders

are no

mandatory

but are

suggested

if need

exists

From the view point of safety, the two parallel taxiways should be separated by an

adequate distance from each other. The separation distance known as ‘Separation Clearance”

depends on the type of airport, the wing span of the aircraft and the navigational aids

available at the airport. The traffic ways should also be spirited sufficiently from the adjacent

obstructions, e.g., buildings. The ICAO has given recommendations for separation clearances

also.

CE406/1 289

18.3.2. Turn Around or Bypass Taxiways :

Many airports in the initial stages have low airport traffic and may not be provided

with parallel taxiways. As the traffic increases the need of a parallel taxiway may be felt

although it may not always be feasible to provide it from economic considerations. As a

substitute for parallel taxiway, some times a turn around or bypass taxiway is constructed.

This arrangement is shown in Fig. 18.5.

Fig. 18.5. Typical Taxiway Turn Around

18.4.0. AIRPORT DRAINAGE :

Airport drainage is one of the most important aspects of airport construction and

maintenance. If the runoff is not quickly drained from the runways / taxiways / aprons

resulting in the formation of puddles, it is hazardous for both the aircraft take-off as well as

the pavement performance and life. Thus primary functions of airport drainage system are

1. Quick removal of surface run-off

2. Sub - Surface drainage, and

3. Diversion of water inflow from the neighbor hood away from the airport.

The drainage of airport complex has certain special features on account of which the

designing of drainage system becomes quite complicated. The main features are :

1. Excessive area under consideration

2. Varying soil conditions

3. Heavy concentration of wheel loads of aircrafts

CE406/1 290

4. Wide runways, taxiways and aprons.

5. Flat longitudinal and transverse grades, and

6. absence of side ditches.

Basic information to be collected for designing drainage system for an airport

includes :

1. A contour map of the airport site and lands adjacent to it including the natural courses,

their run off.

2. A layout of the entire airport with phases of development including all particulars of

drainage system.

3. Rainfall data

4. Longitudinal and Cross-sectional details of runways, taxiways and aprons.

5. Soil profile with ground water details

6. Infiltration characteristics of soils, and

7. Hydraulic data for the design of drainage system.

18.4.1. SURFACE DRAINAGE : It is necessary to select a design storm for which the

drainage system should be designed. FAA recommends a design storm which is probable

once in five years. However, it is also necessary to check with 10 or 15 year storm in order to

assess the interruption to operations that will arise. The amount of run off may then be

obtained by the following equation (as per PAA).

Q = 0.01 CIA.

Q = run-off from a given drainage basin, m3 / hour

C = Coefficient of run-off. For the entire drainage area consisting of surfaces with

different infiltrate characteristic the run off coefficient is the weighted average one.

I = Intensity of rain fall in cm / hr, and

A = drainage area, m2

The time of concentration is the time taken by water in reaching the inlet of the drain

from the farthest point. The pipe lines then are designed to carry (i) discharge computed for

the time taken for the overland travel of water coming into the inlet, and (ii) the contribution

of dischrage from the other inlets carrying water to join the stream.

CE406/1 291

In certain cases, a technique known as ‘PONDING’ may be more economical.

‘Ponding’ means the provision of a temporary storage of runoff before its entry into the

inlets. Such areas should be kept atleast 20 m away from the edge of the pavements. The inlet

structure for collection of water is a concrete box like structure, with a cast iron or reinforced

concrete grating (Fig. 18.6). Wherever possible they may be located between a runway and a

parallel taxiway. Where there is no parallel taxiway they may be located at the toe of the

slope of the graded area. On large aprons, inlets are necessarily placed in the pavement

structure to ensure better drainage.

Fig. 18.6. Typical runway Drainage system - (d) Detailed cross - section of an inlet

structure

Fig. 18.7. Typical runway drainage system - (c) Detailed Cross - section of subgrade

drain

18.4.2. SUB - SURFACE DRAINAGE :

The primary purpose of sub-surface drainage is to prevent the entry of water into the

sub-grade and the base course of the pavement. A detailed cross-section of a sub-grade drain

CE406/1 292

is shown in Fig. 18.7. Now-a-days geotextiles are being increasingly used for drainage

purposes.


1. What do you understand by the term ‘basic runway length’? Explain the procedure of

determining the actual runway length required at a particular site.

2. Summarize the various runway geometric as recommended by the ICAO.

3. The length of a runway at sea level, standard atmosphere conditions and zero gradient is

1500m. The airport site is at an elevation of 900m, and the ariport reference temperature is

200C. If the proposed runway has an effective gradient of 0.20 percent, determine the actual

runway length required at the site. Draw a neat sketch of the safety area and cross - section of

the runway for this runway as per standard specifications.

4. Present a detailed discussion on ‘Runway Capacity’.

5. Discuss in detail the taxiway geometrics.

6. Explain the necessity and special characteristics of airport drainage.

18.6. SUMMARY :

In this chapter the design details of three important elements in an airport have been

presented in detail. They are the geometric design aspects of runways and taxiways, and the

design of the system for drainage of an airport. The relevant design standards for the

geometrics of different elements of runway and taxiway, as per the ICAO and the FAA have

been discussed in detail. Basic runway length arrived based on the performance

characteristics of aircraft has to be corrected for elevation, temperature and effective gradient

of runway at the location of the airport. These aspects have been explained by a worked

example.

The capacity of an airport depends on the configuration of runways. Different

configurations of runways normally adopted and the factors affecting their capacity have

been discussed.

The airport drainage has unusual requirements and is considered to be a complex

design problem. Its special characteristics, data to be collected for design of drainage system

and methods of providing surface and sub-surface drainage have been presented.

18.7. REFERENCES :

CE406/1 293


Bros, Roorkee.


House, Anand.


New-Delhi.



***

TRANSPORTATION ENGINEERING - AIR PORTS

UNIT - 19

AIRPORT TERMINAL AREA

Aims and Objectives

19.1. Introduction

19.2. Terminal Building

19.3. Apron

19.4. Basic Parking Configurations and Parking System of Aircrafts

19.5. Hangar

19.6. Blast Fences


19.8. Summary

19.9. References

Aims/Objectives :

Terminal area of an airport, is the portion other than landing area which includes

terminal and operational buildings, vehicle parking area, aircraft service hangars etc. The

functions of each of the elements, their location and requirements and interrelationship

between the various elements is the topic of discussion in this chapter.


The key elements of an airport are the runway system, the taxiway system and the

terminal area. The terminal area includes the terminal and operational buildings, vehicle

CE406/1 294

parking area, aircraft service hangars, aprons etc. The terminal and operational buildings

usually house all managerial and operational activities for the air craft. Congestion and

inconvenience to the airport users may be avoided by carefully planning vehicular circulation

and parking system. The terminal area should be connected to taxiways for a quick get away

of the landing aircraft and for the aircraft to proceed for runway ends by the shortest route.

Holding aprons or warm-up pads are usually provided at busy airports to allow for aircraft to

queue in these, for take - off or to get to a gate. High energy jet exhaust causes

inconvenience, discomfort and even injury to the passengers boarding the aircraft, if they are

exposed to jet exhaust. They may also cause damage to the airport equipment, structures and

to the pavements. These damaging effects of jet blasts may be reduced by properly selecting

and designing blast fences. Functions of each of the main elements, considerations in their

location and basic design principles are presented in this chapter.

19.2. TERMINAL BUILDING :

The airport terminal building or buildings should provide adequate facilities for all

passengers arriving and departing from the aircraft and accommodation for airline and

administrative personnel. The building or buildings should include provisions for services

such as booking offices, offices of the management, passport and customs, restaurant,

watcher bureau, book and magazine stalls, public communication system, waiting and bath

rooms. At the exit, land transportation facilities such as transit buses, taxies etc., must be

available.

If the aircraft control tower is needed at the airport, it is usually located on the roof of

the administrative building. Typical layout of an airport terminal building is shown in Fig.

19.1. All the buildings (including hangars and shops) must be positioned with respect to

runways and taxiways so that the necessary manoeuvaring of planes, after landing and before

takeoff is kept at a minimum.

The apron provides a connection between the terminal buildings and the airfield. The

number of gates required for an airport can be obtained from the number of flights expected

during a peak hour. The average times necessary for the aircraft to remain at the gate position

range from 30 minutes for a dogmatic flight to 60 minutes or international flight. Hence, the

number of gate positions is equal to the design aircraft volume (vehicles/hour) divided by the

mean time of aircraft at the gate (hour/vehicles).

19.3. APRON :

CE406/1 295

It is a paved area for parking of aircrafts and loading of passengers and cargo. It is

usually located close to the terminal building or hangars. The size of the apron depends upon

the (i) size of loading area required for each type of aircraft as ‘gate position, (ii) number of

get positions and aircraft parking system. Depending on the volume of traffic and the size of

the airport and the cargo handled by it, a single apron for all these facilities or separate one

for each one of these purposes may be provided.

Fig. 19.1Typical Layout of Air Port Terminal Building

Apart form these ‘Holding aprons’ (also called runup pads or warm up pads) may also

be provided at busy airports at or near the end of the runway (Fig. 15.1) . These serve to hold

planes immediately before taking off so as to wait till the runway is cleared of the other

CE406/1 296

planes using it. Final check up of the aircraft may be done on these aprons. The size of the

pad is large enough so that when an aircraft is unable to take off because of some defect,

another one ready to take off is able to pass it with adequate clearance. These aprons, are

designed to accommodate three or four aircrafts of the biggest size to be handled by the

airport.

With a view of relieving congestion on major airports, some times aprons are

provided for fueling. Such an apron is timed as ‘Fueling Apron’.

According to the ICAO, slopes on aprons should be sufficient to prevent

accumulation of water on the surface of the apron, but should be kept as low as possible.

The apron and terminal area should be so situated that they are equidistant from the

ends and as far as possible to the centre of the runways so that taxing distance is cutdown to a

minimum and is the same whichever of the two ends of the runway is used. This ideal may

seldom be achieved in practice because it may usually involve expensive road or rail access is

not affected by the present or future expansion of the airport. In general terminal area and

apron are situated where access is not affected by the present or future runway clearances and

some adjustments by way of increase in taxing distance is therefore accepted.

19.4. BASIC CONFIGURATIONS AND PARKING SYSTEMS OF AIRCRAFT :

The parking of the aircraft with respect to the terminal can be either the ‘nose in or

angle nose in’ (Fig. 19.2.a) the ‘nose out or angle nose out’ (Fig. 19.2.b) or the parallel (Fig.

19.2.c).

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Fig. 19.2 Basic Parking Configurations of Aircrafts

In the nose in position less of noise during taxing and front door is nearer to the

terminal building. However, great noise will result during the pullout of the aircraft. Hot

blasts are away from the terminal building.

In the nose out type less noise will result during the take-out of the aircraft out of its

gate position. Rear loading door is nearer to the terminal building. Hot blasts are towards the

terminal building.

In parallel type, more space is occupied by the aircraft in the gate position and hot

gases are towards the terminal building.

Aircrafts can be grouped adjacent to the terminal building in various types.

(i) Frontal system

(ii) Open apron system

(iii) Finger system and

(iv) Satellite system

These parking systems are illustrated in Fig. 19.3 and are discussed below.

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Fig. 19.3 Aircraft Parking Systems

(i) Frontal system : It is very simple and economical system. But its use is limited only to

small airports requiring few gate positions.

(ii) Open apron system: In this system, the aircrafts are parked in rows and adopted when

the aircrafts are large in number. Passengers have to walk long distance to reach the aircraft

parked in the outer most rows, and exposed to weather, hot blasts and noise.

(iii) Finger system: In this system the aircrafts are parked on either side of a finger

(straight, Y-shaped, T-shaped) projecting from the terminal building. The finger itself may be

a fenced walk or an enclosed structure. If enclosed, it provides adequate protection to the

passengers from weather, noise, fumes etc.

(iv) Satellite system : Satellites are small buildings located on the apron. Aircrafts are

parked around the satellite buildings which are connected to the main terminal building by

underground tunnels. This system is used at the International airport of Los-Angeles. In this

system less turning is required to manoeuver the aircraft in and out of the gate position. The

major draw back of this system is the large construction cost.

19.5. HANGAR :

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The primary function of a hangar is to provide an enclosure for servicing, overhauling

and for repairing and for repairing of the aircraft. They are usually constructed with steel

frames and covered with galvanised iron sheets. These are provided with machine shops and

stores for spare parts. The size of the hangar depends on the size of the aircraft and its turning

radius. Adequate lighting. in-side the hangar is of prime importance. Construction of single

hangar to accommodate large number of aircrafts may be undesirable both from economy and

other considerations, Viz., difficulty in the maneouvering of aircraft, noise nuisance, fumes,

fire hazards etc. The number of hangars depends on the peak hour traffic and the demand of

hangars on rental basis by different airline agencies.

If the hangar can be located close to the terminal building and loading aprons, such an

arrangement offers many advantages. This arrangement should not come in the way of future

expansion of the terminal area as well as the hangar facilities.

19.6 BLAST FENCES :

Blast Fences are used to deflect and dissipate the energy of high velocity jet exhaust.

Various types of blast fences are available. Metal and Concrete blast fences have been used.

U.S. Crops of Engineers have observed that curved fences perform better than a flat plate

fence or a flat louvered concrete fence. At runway ends and other places where full thrust is

applied, fences of 2.5m to 3.0m heights give satisfactory performances.

Blast protection may also be obtained by other methods. Any obstruction, natural or

man made, will afford some measures of protection, Hedges, bushes, and trees help to reduce

noise to a great extent. Tall and thick hedges may deflect and dissipate high velocity exhaust

energy. Natural terrain may also be used with advantage Rolling terrain may not be able to

dissipate the blast rapidly, but the steep hills may be able to do so.

Heat is also associated with jet wake exhaust. But the temperature dissipates more

rapidly with distance than the velocity. Further, the working personnel, equipment and

structures normally do not occupy the areas where the upper limit of the heat are generated

during the jet operations.


1. What do you understand by terminal area? What facilities are provided in this area?

2. Discuss the requirements of a site for terminal building. Mention also the facilities to

be provided in the terminal building.

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3. What are the different systems of aircraft parking ? Explain the suitability of each

system.

4. Write notes on (i) Hangar (ii) Blast Furnace.

19.8. SUMMARY :

This chapter presented the various elements of the terminal area of an airport. The

terminal usually refers to a building mainly, used for passengers airline and administration

facilities. Its layout is such as to offer the enplaning passengers, the convenient and direct

access from the street side of the building, through the booking and waiting rooms, to the

aircraft loading positions on the apron. Deploying passengers are also provided with direct

route from the aircraft to the baggage claim counter and then to the vehicle platform (street

side). Control tower, weather bureau and other operational services may also be provided in

this building. The correct placement of the terminal building with respect to the runways and

loading aprons result in a more rational approach for the airport development.

Apron is a paved area for parking aircraft for embarking and disembarking of

passengers and loading and the unloading of cargo. It is usually located close to the terminal

building or hangars. In busy airports, aprons may be provided at the end of runways for the

aircraft to wait, if necessary, before take off. These are known as warming up pads or holding

aprons.

The basic parking configurations of aircrafts are (a) nose - in or angled nose in (b)

Nose out or angle nose out and (c) Parallel system. The merits and demerits of each of these

systems have been discussed. Aircrafts can be grouped adjacent to terminal building in

various ways - Frontal system, Open apron system, Finger system and Satellite system. these

parking systems have been illustrated and discussed.

Servicing, Overhauling, and repairing facilities are provided to aircraft in hangars.

These are located close to the terminal building and holding aprons. Adequate lighting and

ventilation inside the hangar are of prime importance.

Blast fences are used, wherever necessary to dissipate the energy of high velocity jet

exhaust. It was found that curved fences are quite effective in the dissipation of jet exhaust

energy.

19.9. REFERENCES :

1. Khanna, S.K. Arrora, M.G., and Jain S.S., (1990) - Airport Planning and Design

‘Nemchand Bros., Roorkee.

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2. Rangwala S.C., and Rangwala P.S., (1992) - ‘Air Port Planning’ - Charotar Publishing

House, Anand.

3. Rao, G.V., (1992) - ‘ Air Port Engineering’ Tata McGraw - Hill Publishing Co., Ltd.,

New-Delhi

4. Shegal S.G., and Bhanot, K.L, (1980) - A Text Book of Highway Engineering & Air

Ports; S.Chand & Co., New - Delhi.

***

TRANSPORTATION ENGINEERING - AIR PORTS

UNIT - 20

VISUAL AIDS AND INSTRUMENT LANDING SYSTEM

Aims / Objectives

20.1. Introduction

20.2. Air Port Marking

20.2.1. Runway Marking

20.2.2. Taxiway Marking

20.2.3. Runway and Taxiway Shoulder Markings

20.2.4. Wind and Landing Direction Indicators

20.3. Airport Lighting

20.3.1. Rotating Beacon

20.3.2. Code Beacon

20.3.3. Boundary Lights

20.3.4. Approach Lighting

20.3.5. Threshold Lighting

20.3.6. Runway Lighting

20.3.7. Taxiway Lighting

20.3.8. Miscellaneous Lighting Systems

20.4. Instrument Landing System


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20.6. Summary

20.7. References

AIMS / OBJECTIVES :

To a pilot who navigates a plane at a considerable height and at an appreciable

distance, it is essential that the pilot is able to identify the runway and taxiway, and other

allied structures in an appropriate manner so that safe landing can be made both during day

and night. The various aids available for this purpose are discussed in this chapter. One of the

most widely used systems of landing namely, Instrument Landing System is presented here.

20.1 INTRODUCTION :

For safe landing and take-off of aircraft, runway threshold, edges and centre line

should clearly be visible to the pilot both during day and night and under bad and good

weather conditions. The pilot must be able to identify taxiways, exit taxiways, aprons and

other allied structures also. For this purpose runways, taxiways and other allied structures

have essentially to be marked in an appropriate manner to make them easily understandable

and detectable by the pilot. Any marking in bright paint identifiable during the day or

provision of night lights (white or coloured) which helps the pilot in locating the airport or in

marking a safe landing or take off are termed ‘Visual Aids’.

In most of the modern airports, the landing of aircraft is aided by using a number of

instruments, popularly known as “instrument Landing System” (ILS).

20.2. AIR PORT MARKINGS :

The airport markings are classified into the following groups :

a) Runway markings

b) Taxiway markings

c) Runway and Taxiway shoulder markings

d) Apron markings

e) Wind direction indicator, and

f) Landing - direction indicator

20.2.1. RUNWAY MARKINGS : All runway markings are white in colour and are of the

following types :

1. Runway threshold marking

2. Runway touch down zone or landing zone marking

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3. Runway edge strip marking.

4. Runway Centre line marking and

5. Runway numbering

Various runway markings are shown in Fig. 20.1. and explained.

Runway number indicates the magnetic azimuth of the runway measured clockwise

from North direction. The marking is given to the nearest of 100. Thus the east end of the

east-west runway is marked with number 9, similarly at the west end, the marking is as 27. If

these are two or more number of panel runways, they are marked as follows :

a) Two parallel runways : L, R (i.e., Left and Right)

b) Three parallel runways : L, C, R (Left, Central and Right)

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c) Four parallel runways : L, LC, RC, R

20.2.2. TAXIWAY MARKING :

Taxiway markings are illustrated in Fig. 20.2. All these markings are painted Yellow.

The centre line of the taxiway is marked with a single strip, 15cm wide. This centre line

normally terminates at the runway edge, except in the case of exit-taxiway where the taxiway

centre line is curved into the centre line of the runway. See Fig. 20.2.b. At intersection of

taxiways the centre lines continue through the intersetion areas as shown in Fig. 20.2.c. A

holding point marker should be painted at all the intersections of the paved taxiways and

runways. The holding position should be at least 30m form the edge of the runway as shown

in Fig. 20.2.

Taxiway Centre line at Taxiway centre line Taxiway intersections the end of runway inclined to runway

Fig. 20.2. Various Taxiway Markings

20.2.3. RUNWAY AND TAXIWAY SHOULDER MARKINGS :

Typical markings are indicated in fig. 20.3. The markings are in Yellow paint.

Runway shoulders are marked with diagonal lines each having a width of 0.9m, Taxiway and

holding apron shoulders are marked with lines perpendicular to the direction of travel of

aircraft. Blast pad at the end of the runway is marked with a chevron pattern.

CE406/1 305

Certain guide lines are marked in yellow, on the apron to help the pilot in

manoeuvering the aircraft on the apron. Usually the guide line is painted to indicate the path

of the nose gear of most critical aircraft. The smaller aircraft can also use the same path and

manoeuver without difficulty.

Fig. 20.4. Segmented Circle Marker

20.2.4. WIND AND LANDING DIRECTION INDICATORS : The wind direction

indicator, which may be a wind cone, is usually placed at the centre of the segmented circle

marker as illustrated in Fig. 20.3. This helps the pilot in locating the airport and observing the

wind direction. As per the ICAO the segmented circle should have an inside diameter of 30m

and the panel width varying from 0.90m to 2.4m. The panels forming the segmented circle

CE406/1 306

are gable roof shaped with a pitch of atleast 1:1. This enhances the visibility of the segmented

circle. In most cases the panels are painted in white. The wind direction indicator is in the

form of a cone as shown in fig. 20.5. For its visibility the wind direction indicator is

illuminated as shown.

Landing direction indicator is in the form of a Tee or Tetrahedron and is placed at the

centre of the segmented. Circle as shown in Fig. 20.6. This indicates the direction of the

active runway of the airport.

Fig. 20.5. WIND DIRECTION INDICATIOR WITH LIGHTING

Fig. 20.6. Landing Direction Indicator

20.3. AIR PORT LIGHTING :

The airport lighting system for safe landing and takeoff of aircraft during night are

presented in the following paragraphs.

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20.3.1. ROTATING BEACON : This beacon rotates at a speed of six RPM projecting

horizontal beams of green and clear light, 1800 apart. It indicates the approximate location of

airport. The beacon is mounted higher than any surrounding obstruction.

20.3.2. CODE BEACON : The beacon is mounted high enough so that its beam clears all

obstructions. It is provided with two 500 Watt bulbs with a green colour screen. It

continuously flashes a Morse code signal designating the airport.

20.3.3. BOUNDARY LIGHTS : The entire landing area of the airport is provided by the

boundary lights spaced approximately 90m apart. They are mounted approximately 0.75m

above the ground. Red marker lights are used to show harardous zones.

20.3.4. APPROACH LIGHTING : While approaching the runway for landing, the approach

lights are the only elements of guidance to the pilot for safe landing. These are extended upto

900m ahead of runway threshold. The approach lighting system accepted by the ICAO is

shown in fig. 20.7. The transverse bars each 4.2m wide and located at 30m intervals along the

extended centre line of runway, give suitable information to the pilot of the angle of roll. At

300m from the threshold, a cross bar 30m wide is provided to serve as a distance marker and

to provide further horizontal references. Approach lights are normally mounted on pedestals

of varying height. Some military airports require that approach lights, within 300m of the

runway threshold, must be nearly flush with the graded approach zone.

20.3.5. THRESHOLD LIGHTING : This consists of terminating bar red lights placed at

60m from threshold indicating the end of the approach lighting. The threshold wing bars,

placed at 30m ahead of the runway serve as distant marker. The threshold, itself is lighted

with continuous line of green lights extending across the entire width of the runway. The

lights may be elevated or of the semi-flush type.

Fig. 20.7. Approach Lighting system

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20.3.6. RUNWAY LIGHTING : After crossing the threshold, the pilot completes a touch

down and then rolls the aircraft on the runway. The lighting pattern is illustrated in Fig. 20.8.

As the pilot crosses the threshold and is about to touch the runway, he finds the central area

of the runway excessively dark. To eliminate this ‘black hole’ effect, a narrow gauge pattern

lighting is provided to light the central position of the runway. In this system, shown in Fig.

20.8, groups of high intensity lights are placed 18m apart and on either side of the centre line

of runway upto a distance of 140m from the threshold. Beyond this distance closely spaced

lights are placed along the centre line which extend upto the other end of the runway. All the

lights on the runway are white in colour. The lights inside the runway are of the flush type

and those on the runway edges are elevated type.

Fig. 20.8. Narron – Gauge Pallirn for Runway Lighting

20.3. TAXIWAY LIGHTING :

The following considerations apply to taxiway lighting system:

1. Taxiway should be clearly identified so that they can not be confused with the runways.

2. Exit taxiways should be so lighted that the pilot is able to locate the exits 360 to 450m

ahead of the point of turn off.

3. There should be adequate guidance along the taxiway.

4. Crossing of taxiway and the runway should be clearly identified

5. Effective and simple presentation of guidance elements to permit rapid aircraft movement

between the runway and apron should be preferred.

The taxiway lighting system is shown in Fig. 20.9.

On the tangent portion, the lights are placed at not more than 60m apart, at a distance

of 3m from either edge along the taxiway. The spacing is reduced on curves and intersections

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to facilitate their clear identification. All taxiway side lights are coloured blue and usually

project not more than 0.32m above the pavement surface.

Fig. 20.9. Taxiway Lighting System

On exit taxiway, in order to clearly identify the point of turn off, lights are placed

along the centre line of the exit taxiway. With this arrangement, the taxiway centre line, at its

junction with the runway centre line, forms a distinct V-shaped pattern. The centre-line lights

are green in colour and placed at 6 to 7.5m distance along the straight length and at 3 to 3.6m

distance along the curves.

20.3.8. MISCELLANEOUS LIGHTING : Hangar and apron areas are usually flood lit. The

flood lights are mounted at least 12m above the pavement to avoid glare in the eyes of the

pilots.

Landing direction indicator and wind direction are also illuminated as illustrated in

fig. 20.6. for their visibility during night and during bad weather conditions.

20.4.0. INSTRUMENT LANDING SYSTEM (ILS) :

The various aids used by an aircraft during its flight are :

a) Enroute aids and b) Landing aids.

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Landing aids consists of the following three components.

a) Localiser antenna

b) Glide slope antenna, and the

c) Outer and middle markers (Fan Markers)

Fig. 20.10. Instrument Landing System (ILS)

The landing procedures is as follows :

Each aircraft is brought to the holding fix by means of enroute aids. It is then taken

over by the airport control. If two or more aircrafts have reached the holding point

simultaneously, they are detained by the control tower and are directed to keep moving round

in space with a vertical separation of atleast 300m. After every two minutes, the airport

control directs the aircraft at the bottom of stack to move out for final approach to the airport.

Simultaneously the aircrafts in the stack are instructed to descend to the next lower level and

keep moving round.

The air craft is then picked up by the ‘Localiser’. The localiser antenna is along the

centre line of the runway and is a radio transmitter of very high frequency omni directional

range (VOR). It emits radio beam signals in a vertical plane and indicates to the pilot whether

he is to the left or right of the correct alignment for approach to the runway. This localiser

provides the alignment guidance along the centre line of the runway.

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The guide slope antenna indicates to the pilot the correct angle of descent to the

runway. This is also a VOR transmitter which transmits signals in the horizontal plane. This

glide slope antenna is provided on one side of the runway as shown in Fig. 20.10.

As the aircraft passes over the outer and middle markers, the pilot receives a high

pitched steady tone and also finds a visual indication in the cock pit. These are small radio

transmitters and these markers define the specific distance of the aircraft along the localiser

course. As the aircraft crosses over the middle marker, the pilot changes over from the

instrumental to the visual flight system. Now the approaching and runway lighting assist the

pilot in visual landing.

The ‘Microwave Landing System’ (MLS), is expected to take over the landing

operations of an aircraft by about the end of century.


1. By means of a neat sketch explain the runway - marking system as per ICAO.

2. Show the approach lighting system for a class - A Airport.

3. Explain how the landing of an aircraft is effected by ILS.

20.6. SUMMARY :

For the safe landing and take off of aircrafts in an airport, a number of aids are

provided in the airport so that the pilot will be able to identify the runways, taxiways and

other pertinent structures not only during the day, but also in the night under all conditions of

weather. These are known as ‘Visual Aids’.

To aid landing and take off during the day, various elements of the airport are marked

in different colour paints. Runway threshold, landing zone, edge strip, centre line and number

are marked in white paint. All the taxiway markings, runway and taxiway shoulder markings

and apron markings are made in Yellow paint.

For flight operations during night lighting system is provided. Approach lights

provide necessary guidance to the pilot is illustrated. The approaching lighting system

accepted by the ICAO is illustrated. The identification of runway threshold has an important

bearing on the landing of aircraft. For this reason, the area near the runway threshold is given

special lighting consideration. The narrow-gauge pattern of lights help the pilot to precisely

touch the runway. The centre line of the runway may be identified easily by the runway

lighting system. Runway lighting system consists of white coloured lights.

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Taxiway lighting system is so designed that they cannot be confused with the runway.

All taxiways side lights are coloured blue and usually project more than 0.32m above the

pavement surface. The centre line lights on exit taxiways are of green colour. Other elements

of the airport like aprons, hangars, wind and landing directions are properly illuminated.

Rotating Beacon and Code Beacon help in identifying the airport location and

designation.

Instrument Landing System (ILS) aids flight operations when the visibility is

inadequate during night time or due to cloudy or foggy weather. Instrument approach and

landing procedure has been illustrated and briefly explained.

20.7. REFERENCES :


Bros, Roorkee.


House, Anand.


New-Delhi.



***

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