پنجشنبه ۲۲ شهریور ۰۳ | ۲۱:۲۵ ۷ بازديد
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Chapter – 2
ROADS
The economic, industrial, social and cultural development of any country is directly
linked with the transportation facilities existing in that country. Following are the four major
modes of transportation.
* Roadways * Railways * Waterways * Airways
Each of these modes have their own merits and limitations. However, the transportation
through roadways is the only mode which gives maximum service to the users, because of the
flexibility and freedom it offers to the users.
Some Definitions
Terrain: It is the landscape. As per IRC, it can be classified as in Table 1.
Table – 1: Classification of Terrain
% cross slope of the ground Classification
0 – 10 Plain / Level
10 – 25 Rolling
25 – 60 Mountainous
> 60 Steep
Formation: It is the ground surface obtained for the construction of road after the earth works
are over.
Road: It is a solid and convenient path over which vehicular traffic takes place lawfully.
Pavement: It is the surface structure over the soil sub-grade constructed in order to provide
smooth and comfortable ride for the vehicles.
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HIGHWAY CROSS SECTIONAL ELEMENTS
1. Carriage way (Pavement) (Fig. 1): It is that part of a road which carries vehicular traffic.
Its width depends upon the width of the vehicle, minimum side clearance to be provided
for safety and number of lanes. (* The carriage way intended for one line of traffic
movement is called a traffic lane). It should be sufficient enough to carry the design
traffic. Hence, there can be a single lane or more lanes of traffic. The widths of carriage
way for roads with different lanes are given in Table 2.
Table 2: Widths of carriage way (as per IRC)
No. of lanes Width of carriage way
Single lane 3.75 m
Two lanes without raised kerbs 7 m
Two lanes with raised kerbs 7.5 m
Multi – lane pavements 3.5 m per lane
Note:
i. On unimportant roads, an intermediate carriage way of width 5.5 m may also be
adopted instead of a regular two lane width.
ii. On village roads, the lower limit of the width of carriage way of a single lane is 3 m.
2. Shoulder (Fig. 1): It is a strip of land provided on either side of the carriage way, which
has sufficient strength to support a loaded truck in wet weather. It serves
as an emergency lane for parking break-down vehicles
as a service lane
sometimes, for over taking
It is desirable to have a minimum shoulder width of 4 – 6 m. As per IRC, the
minimum shoulder width is 2.5 m.
3. Road way width (Width of formation) (Fig. 1): It is the total width of carriage way
together with shoulder on either side. It is nothing but the finished top width of the earth
work in cutting or in embankment. For divided highways, it includes the central medians
also. The standardised widths of roadway are listed in Table 3.
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Table 3: Width of road way for different classes of roads (as per IRC)
Class of road
Road way width: m
On plain and rolling
terrain
On mountainous and deep
terrain
National and State Highways
a. Single lane
b. Two lane
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6.25
8.8
Major district roads
a. Single lane
b. Two lanes
9
9
4.75
–
Other district roads
a. Single lane
b. Two lane
7.5
9
4.75
–
Village Roads
Single lane 7.5 4
4. Right of way (Road land width) (Fig. 1): It is the land acquired for the road along its
alignment. While deciding it, cost of the land and future plans of upgrading the proposed
road should also be kept in mind. Table 4 lists the right of way for different classes of
roads.
Table 4: Recommended right of way for different classes of roads in m
Class of
road
Plain and Rolling Terrain Mountainous and steep
terrain
Open area Built-up area Open area Built-up
area
Normal Range Normal Range Normal Normal
NH & SH 45 30-60 30 30-60 24 20
MDR 25 25-30 20 15-25 18 15
ODR 15 15-25 15 15-20 15 12
VR 12 12-18 10 10-15 9 9
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5. Traffic separator (Fig. 2): It is a strip of land that may be provided on the road, which
helps to prevent head-on collisions between the vehicles moving in opposite directions on
adjacent lanes (in which case it is called as median or dividing island), to channalise the
traffic at intersections, to segregate slow traffic and to protect pedestrians. These are
normally 3 – 5 m wide.
6. Parking lanes (Fig. 2): Normally, these are provided on urban roads for parking vehicles.
A width of about 3 m is normally required for these lanes.
7. Cycle tracks (Fig. 2): These are provided in urban areas where the volume of cycle traffic
on the road is very high. Minimum width suggested for these tracks is about 2m.
8. Foot paths (Side walks) (Fig. 2): These are the path ways provided on either side of the
road exclusively for the movement of pedestrians. These are provided to protect the
pedestrians from accidents. Minimum width of foot path should be about 1.5 m.
9. Kerb: It is the boundary between the pavement and shoulder / footpath / parking space.
It may be of hard stone or of good quality precast concrete. In urban regions, it is
describe to provide kerbs.
COMPONENT OF A PAVEMENT / ROAD
Based on their structural behaviour, pavements can be of two types namely, flexible
pavements and rigid pavements.
Flexible pavement: It has four components (i) Soil sub-grade (ii) Sub-base course (iii) Base
course (iv) Surface course (Fig. 3)
Sub grade: It is the natural soil on which the roads are constructed. It can also refer to the top
of the embankment over which the pavement is laid. The surface of the sub-grade is
generally known as formation. Entire load of the structure and that of traffic on the road
surface are ultimately transferred to the sub-grade. If natural sub-grade is not of good
quality, it requires strengthening through compaction.
Sub-base course: The layer which is immediately above the sub-grade is the sub-base. It
consists of granular materials (naturally occurring / compacted / stabilized with cement or
lime). This layer shall be stronger than sub-grade. It performs the following functions.
It distributes the traffic loads to sub-grade underneath
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It acts as a good drainage layer
It prevents moisture migration from the sub-grade upwards
Natural sand, murrum, gravel, crushed stone, crushed concrete, laterite etc. can be the
granular material constituents of sub-base course.
Note: The provision of sub-base course is optional and situation dependent. If the traffic
loads are heavy and the sub-grade quality is low, the design thickness is more. In such cases,
it becomes inevitable to provide the sub-base course.
Base course: This is the main load bearing layer, underlying the wearing or surface course.
The material of the base shall be of high quality. It is normally made of crushed / broken
aggregates mechanically interlocked by rolling and bonded together with screenings, soil
binder and water (Water Bound Macadam). The main function of this layer is to withstand
the high traffic loads imposed at the surface and to distribute them to the underlying layers of
the pavement.
Surface course (Wearing course): It generally consists of a binder course (normally
bituminous macadam) and a 2 – 5 cm thick bituminous surface dressing. The functions of
this surface are
to withstand the stresses induced by the traffic, without undergoing unacceptable
deformations.
to prevent the ingress of water (i.e. water proofing the base course)
to provide a smooth, skid resistant riding surface.
Rigid Pavement: It has 2 – 3 components – Natural sub-grade, base course (optional) and
cement concrete slab (Fig. 4). The cement concrete slab itself can serve as a wearing surface
as well as an effective base course.
Some Definitions
Carpet: It is nothing but the top surface of a pavement consisting of coarse aggregates of 12.5
and 10 mm sizes, pre-mixed with bitumen or tar binder compacted to a thickness of 20 mm.
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For good performance, this course has to be covered by a suitable seat coat such as premixed
sand – bitumen seat coat before opening to traffic.
Camber (Cross Slope) (Fig. 5): It is the slope provided to the road surface in the transverse
direction to drain off the rain water from the road surface.
Super elevation (Fig. 6): Whenever a vehicle negotiates a horizontal curve on the road, by
virtue of its speed, it is subjected to a centrifugal force which tends to throw the vehicle away
from the centre of curvature of the curve. To counteract this effect, the outer edge of the road
/ pavement is raised with respect to the inner edge throughout the length of the horizontal
curve. This transverse slope given to the pavement surface is known as super elevation or
cant or banking.
Berm (Fig. 7): It is the horizontal stretch of land left at the ground level between the toe of an
embankment and the top edge of cutting, provided to ensure the stability of a long slope.
CLASSIFICATION OF ROADS
Roads can be classified based on different criteria.
I. Based on administrative jurisdiction and function:
Primary System
Express way
National Highway (NH)
Secondary System
State Highway (SH)
Major District Roads (MDR)
Tertiary System
Other District Roads (ODR)
Village Roads (VR)
Note: Highways are nothing but roadways having certain minimum standards
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Express way
These are special category of highways with superior facilities and design standards,
meant for very high volume of traffic at high speed. They are provided with divided carriage
ways, controlled access, grade separations at intersections and fencing. Parking, loading and
unloading of goods and pedestrian traffic are not permitted on these highways. They come
under the jurisdiction of central or state government depending upon whether the route is a
national highway or a state highway.
National Highway (NH)
These are the arterial roads of the country for inter-state and strategic traffic. They
connect national and state capitals, major ports, railway junctions and border. All national
highways are designated through numbers.
Ex: NH3: National Highway connecting Atari to Manali
NH21: National Highway connecting Jaipur to Agra
NH44: National Highway connecting Srinagar to Kanyakumari
Fig. 8 shows the cross section of a NH in rural area.
State Highway (SH)
These are the arterial roads of a state for inter-district traffic. They link the national
highways and highways of adjacent states, state capital with district headquarters, important
cities within the state. Sometimes, these may carry heavier traffic volume. The national and
state highways have same design speed and geometric design specifications. Fig. 8 shows the
cross section of a SH in rural area.
Major District Roads (MDR)
These are the rods within a district, for intra-district movements. They connect places
of production and market within the district and connect highways in a district. These have
lower speed and geometric design specifications than the national and state highways. Fig. 9
shows the cross section of a Major District Road.
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Other District Roads (ODR)
These serve to connect rural areas of production with the market centres, taluk head
quarters and other main roads. They have lower design specifications than MDR. Fig. 10
shows the cross section of an ODR.
Village Roads (VR)
They connect villages with each other and with the nearest district roads, state and
national highways. Fig. 10 shows the cross section of a VR.
II. Based on their suitability for usage during different seasons of the year:
All weather roads
Fair weather roads
All weather roads
These roads are negotiable during all weather except at major river crossings.
Fair weather roads
The traffic on these roads may be suspended during monsoon season due to flooding
of the roads.
III. Based on the type of road pavement:
Paved roads
Unpaved Roads
Paved roads
These roads are provided with hard pavement course.
Unpaved Roads
These roads are not provided with hard pavement course. The earth roads and gravel
roads belong to this category.
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IV. Based on the type of pavement surfacing
Surface roads
Unsurfaced roads
Surface roads
These roads are provided with bituminous or cement concrete surface.
Unsurfaced roads
These roads are not provided with bituminous or cement concrete surface.
CLASSIFICATION OF URBAN ROADS
Roads in the urban areas form a separate class altogether and are classified into
Arterial roads
Sub-arterial roads or major roads
Collector streets or minor roads
Local streets
Arterial roads (Fig. 11)
These roads pass within the city limits and link the state and national highway system
with a limited access. They include ring road, by-pass road etc. These roads connect major
suburban centres and central business areas with residential areas. Parking, loading and
unloading activities on these roads are regulated. Pedestrian crossings are allowed only at
intersections. No residential buildings are allowed along these roads.
Sub-arterial roads (Fig. 11)
These roads are similar to arterial roads except that they have a lower level of traffic
mobility.
Collector streets (Fig. 11)
These roads link the residential areas, business and industrial areas with the arterial /
sub-arterial roads. Parking restrictions on these streets will not be there except during peak
hours of traffic
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