What Is Superelevation?
Superelevation on the horizontal curve is one of the most important features in the construction of the roads. It is very essential to provide superelevation in roads for the safe movement of vehicles on the curved portion of the roads.
It helps the fast-moving vehicles to safely pass through the curved portion of the roads with stability.
The inward transverse inclination which is provided to the cross-section of the pavement of road at the horizontally curved portion of the roads is known as superelevation.
Superelevation is also known as Cant or Banking.
Superelevation in roads is basically provided on the horizontally curved portion of the roads in which the outer edge of the road pavement is raised with respect to the inner edge, thus providing a transverse slope throughout the length of the horizontal curve of the road.
This transverse inclination to the surface of the road pavement is known as Superelevation.
Also Read: Road Development Plans in India
Purpose of Providing Superelevation in Roads
• The main aim of providing superelevation is to counteract the effect of centrifugal force acting on the moving vehicle.
• To prevent the damaging effect on the surface of the roads due to improper distribution of load on the roads.
• To help the fast-moving vehicles to pass through a curved path without overturning or skidding.
• To reduce the maintenance cost of the road on the curved portion.
• To ensure the smooth and safe movement of vehicles and passengers on the curved portion of the roads.
Superelevation is expressed as the height of the outer edge of the pavement with respect to the width of the pavement.
Calculation of Superelevation in Roads
Consider a vehicle moving on the circular horizontal curve of the road.
The speed of the moving vehicle on the circular path is V m/s
The curve of the radius is R metres
The forces which are acting on a vehicle while it is moving on the horizontal circular curve are as follows.
The centrifugal force which is acting horizontally out work through the centre of gravity, P=( W V2/gR)
The weight(W) of the vehicle which is acting vertically downwards through the center of gravity.
The friction which is developed between the wheel of the vehicle and the pavement acting transversely along the pavement surface towards the centre of the curve.
The centrifugal force is opposed by the corresponding value of the friction developed and by a component of the force of gravity due to the Superelevation is provided.
The superelevation is provided on the road by the following formula
e + f = (V2 / 127 R)
R = 0.036 V2
e = rate of super elevation
f = design coefficient of lateral friction= 0.15
g = acceleration due to gravity= 9.81 m/s2
R = radius of the curve
By using this formula the angle of superelevation is also determined.
Minimum and Maximum Superelevation in Roads
Superelevation which is provided on the road is less than the camber of a road then it should be get equal to the camber of the road for proper drainage purpose.
In the case of heavily loaded vehicles having denser material, it is not possible to move this vehicle on the road with a high rate of superelevation.
The Centre of gravity of the loaded vehicles is relatively high so it is not safe to move these vehicles from the curved portion of the road where high superelevation is provided.
In the case of flat curves with a larger radius, the superelevation will be negligible as the centrifugal force which will be developed is very less. In this condition, the normal camber may be retained also on the curve.
Hence, in order to avoid the danger of toppling of the loaded vehicles and ensure the safe movement of the vehicles on the curved portion of the road.
It is essential that there should be a limit of maximum allowable superelevation in the roads.
Indian Road Congress(IRC) has recommended the limit of maximum allowable superelevation in the roads for a plane and rolling terrains and snow bound areas is 7% taking mixed traffic into the consideration.
For, Areas which are not bound by snow the maximum limit of the superelevation which is recommended by the Indian Road Congress is up to 10%.
IRC recommendation for radii beyond which Superelevation is not required.
|Sr No||Design Speed in Kmph||
Radius in Meters of Camber
Method of Providing Superelevation to the Roads
Superelevation plays an important role to counteract the centrifugal force which is acting on the vehicle moving from the horizontal curve of the road.
The attainment of superelevation in roads is done into two parts
• Elimination of the Crown and the cambered section
• Rotation of the pavement by attaining full superelevation
Elimination of the Crown and the Cambered Section
Method – 1.
In the first method, the outer half of the cross slope is rotated about the crown at the rate such that the surface of road falls on the same plane and the elevation of the centre line is not varied.
The outer half of the cross slope is brought to the level rotating about the crown line at the start of the transition curve.
There is no point on the curve that will have a negative superelevation at the outer half of the pavement at the start of the transition curve.
Method – 2.
In the second method, the crown is gradually shifted by increasing the width of the inner half of the cross-section.
This method is not usually adopted as a portion of the outer half of the pavement which has increasing values of negative superelevation on to a portion of the outer half.
Rotation of the pavement by attaining full Superelevation
In this method, the inner edge of the road is made the pivot point. The Crown, as well as the outer edge, are raised in such a way that the full amount of superelevation is achieved.
The disadvantage of this method is that the centre of the pavement is raised and the entire pavement width and the outer shoulder are to be raised with respect to the inner edge by additional filling of the earth.