When a vehicle speeds along a straight path and suddenly encounters a curve, we witness the concept of Superelevation in action. Imagine a car taking a turn—the force trying to push it off the road comes into play. This force, known as centrifugal force, acts from the inside towards the outside of the curve, challenging the stability of the vehicle. Meanwhile, the friction between the wheels and the road fights against this force, adding another dynamic to the situation.
Superelevation is the adjustment made to the road surface on curves by raising the outer edge compared to the inner edge. This counteracts the centrifugal force, reducing the risk of the vehicle overturning. Expressed as ‘e,’ it represents the height difference between the two edges of the road divided by the road’s width.
e=V2127R
Where:
V = Speed of the vehicle in km/hr
R = Radius of the curve in meters
For example, if a road is 15 meters wide and has a superelevation of 1 in 15, the outer edge is raised by 1 meter above the inner edge. It’s crucial to balance superelevation because while it aids fast-moving traffic, too much can inconvenience slower vehicles. Typically, superelevation varies from 1 in 14 to 1 in 16, with exceptions allowing up to 1 in 10 in special circumstances.
Introducing superelevation involves careful planning. A ‘Super Elevated Curve’ includes a circular curve and transition curves on both ends. Transition curves smoothly connect the straight road with the circular curve.
In this method, the outer slope of the road gradually rotates around the crown, reducing its inclination until it becomes horizontal. Then, the slope increases until it matches the inward slope. This method is common on Indian highways but faces challenges in draining water in certain road sections.
This method eliminates the cambered section by shifting the crown towards the outer edge along the extension of the inner slope. However, it may result in excessive slopes in some areas.
Once the road profile is brought to a straight line slope, achieving the desired superelevation involves rotating the slope about the central line or the inner edge, depending on topography and drainage considerations.
Implementing superelevation on horizontal curves offers several advantages:
Increased Vehicle Stability : Fast-moving vehicles become more stable, reducing the risk of skidding or overturning.
Equal Wheel Pressure : The distribution of pressure on both wheels becomes more balanced, minimizing wear and tear.
Reduced Stresses on Foundations : Intensity of stresses on the road foundation decreases.
Maintained Speed in Curves : Vehicles can maintain speed through super elevated curves.
No Need for Outer Edge Features : Gullies or ditches on the outer edges of super elevated curves are not necessary.
