Gradient
Gradient is
the rate of rise or fall along the length of the road with respect to the horizontal. The positive gradient or
the ascending is denoted as +n and the negative gradient as –n. the deviation
angle N is , when two grades meet, the angle which measures the change of
directions and is given by the algebraic difference between the two grades (n1
– (-n2 )) = n1 + n2 = α1 + α2.
Example : 1 in 30 = 3.33 % ͌ 2ͦ is a steep, while 1 in 50 = 2 % ͌ 1ͦ 10΄
is a flatter gradient. The gradient representation is illustrated in the figure
given below.
Types of gradient :
Ruling Gradient
The ruling
gradient or the maximum gradient with which the designer attempts to design the
vertical profile of the road. This depends on the terrain, length of the grade,
speed pulling power the vehicle and the presence of the horizontal curve. In
flatter terrain, it may be possible to provide flat gradient, but in hilly
terrain it is not economical and sometimes not possible also. The ruling
gradient is adopted by the designer by considering a particular as the design
and for a design vehicle with standard dimensions. But our country has a heterogeneous
traffic and hence it is not possible to lay down precise standards for the
country as a whole. Hence IRC has recommended some values for ruling gradient for
different types of terrain.
Limiting
gradient
This
gradient is adopted when the ruling gradient results in enormous increase in
cost of construction. On rolling terrain and hilly terrain it may be frequently
necessary to adopt limiting gradient stretches should be limited and must be
sandwiched by either straight roads or easier grades.
Exceptional
Gradient
Exceptional
Gradient are very steeper given at unavoidable situations. They should be
limited for short stretches not exceeding about 100 metres at a stretch. In mountainous
and steep terrain, successive exceptional gradients must be separate by a
minimum 100 metres length gentler gradient. At hairpin bends, the gradients is
restricted to 2.5%.
Minimum
gradient
This is
important only at locations where surface drainage is important. Camber will
take care of the lateral drainage .but the longitudinal drainage along the side
drains require some slope for smooth flow of water .therefore minimum gradient
is provided for drainage purpose and it depends on rain fall, type of soil and
other site conditions. A minimum of 1 in 500 may be sufficient for concrete
drain and 1 in 200 for open soil drains are found to give satisfactory
performance.
Terrain
|
Ruling
gradient
|
Limiting
gradient
|
Experimental
gradient
|
Plain or
rolling
|
3.3 per
cent
(1 in
30)
|
5 per
cent
(1 in
20)
|
6.7 per
cent
(1 in
15)
|
Mountainous
terrain, and steep terrain having elevation more than 3,000 m above the mean
sea level
|
5 per
cent
(1 in
20)
|
6 per
cent
(1 in
16.7)
|
7 per
cent
(1 in
14.3)
|
Steep terrain
upto 3,000 m height above mean sea level
|
6 per
cent
(1 in
16.7 )
|
7
Percent
( 1 in
14.3 )
|
8
Percent
( 1 in
2.5 )
|
Suppose the
road is with zero gradient passing through level land and open side drains are
provided with a gradient of say 1 in 300. It may then be necessary to deepen
the downstream end of the drain by about 3.3 m for one kilometre length of
road. This course is not possible from practical considerations. Hence it is
desirable to have a certain minimum gradient on roads from drainage point of
view, provided topography favours this. The minimum gradient would depend on
rain-fall, run-off , type of soil, topography and site conditions.
A minimum
gradient of about 1 in 500 may be sufficient to drain water in concrete drains
and gutter , but on inferior surfaces of drains a slope of 1 in 200 or 0.5
percent may be needed where as on kutcha open drains ( soil drains ) steeper
slopes up to 1 in 100 or 1.0 percent may be needed.
