TYPES OF GRADIENT
[1] Maximum Gradient: -It
is the maximum or steepest gradient which is to be permitted on the road, which
on no account is to be exceeded. It is also called Limiting Gradient. Its value
has been fixed 1in 15 for Hills and 1 in 20 for other roads. These steeper
gradients are sometimes to be provided to avoid deep excavations.
[2] Ruling Gradient: -Ruling
gradient is defined as the suitable gradient within which the engineer must endeavour
(koshishkarna) to design the road. The I.R.C has recommended the value of
ruling gradient as 1 in 20 in Hills and 1 in 30 in plains.
[3] Minimum Gradient: -The
gradient which has to be provided for the purpose of road drainage is called
minimum gradient and its value is usually fixed as 1 in 200. For cement
concrete roads, a minimum gradient of 1 in 330 can be provided.
[4] Average gradient: -It
is defined as the total rise or fall between any two points chosen on the
alignment divided by the horizontal distance between the two points.
[5] Exceptional gradient:-During
the alignment of a road, there may come certain patches of length, where a
gradient may have to be provided which may be either less than the minimum
known as Exceptional gradient. Exceptional gradient should not be provided in a
length more than 100metres, in any case.
ROAD MARGIN: - The portion of the road beyond
the carriageway and on the roadway can be generally called road margin. Various
elements that form the road margins are given below.
SHOULDER:
-Shoulders
are provided along the road edge and is intended for accommodation of stopped
vehicles, serve as an emergency lane for vehicles and provide lateral support
for base and surface courses. The shoulder should be strong enough to bear the
weight of a fully loaded truck even in wet conditions. The shoulder width
should be adequate for giving working space around a stopped vehicle. It is
desirable to have a width of 4.6 m for the shoulders. A minimum width of 2.5 m
is recommended for 2-lane rural highways in India.
WIDTH
OF CARRIAGE WAY: -Width of the
carriage way or the width of the pavement depends on the width of the traffic
lane and number of lanes. Width of a traffic lane depends on the width of the
vehicle and the clearance. Side clearance improves operating speed and safety.
The maximum permissible width of a vehicle is 2.44 and the desirable side
clearance for single lane traffic is 0.68 m. This require minimum of lane width
of 3.75mt for a single lane road.
KERBS:
-Kerbs
indicate the boundary between the carriage way and the shoulder or islands or
footpaths. Different types of kerbs are as:
[1]
Low or mountable kerbs:This
type of kerbs is provided such that they encourage the traffic to remain in the
through traffic lanes and also allow the driver to enter the shoulder area with
little difficulty. The height of this kerb is about 10 cm above the pavement
edge with a slope which allows the vehicle to climb easily.
[2]
Semi-barrier type kerbs: -
When the pedestrian traffic is high, these kerbs are provided. Their height is
15 cm above the pavement edge.
[3]
Barrier type kerbs: - They are
designed to discourage vehicles from leaving the pavement. They are provided
when there is considerable amount of pedestrian traffic. They are placed at a
height of 20 cm above the pavement edge with a steep batter.
FORMATION
WIDTH: -Width of
formation or roadway width is the sum of the widths of pavements or carriage
way including separators and shoulders. This does not include the extra land in
formation/cutting. The values suggested by IRC are as:
Width
of formation for various classed of roads
|
Road
Classification |
Roadway width
in mt. |
|
|
Plain and in
rolling terrain |
Mountainous
and steep terrain |
|
|
NH/SH |
12 |
6.25 – 8.8 |
|
MDR |
9 |
4.75 |
|
ODR |
7.5 – 9 |
4.75 |
|
VR12 |
7.5 |
4.0 |