When a fast moving vehicle passing from a straight path suddenly negotiates a horizontal curve is termed as Superelevation. The centrifugal force acts on the vehicle from inside towards the outside of the curve which tries to push the vehicle off the track. This centrifugal force always acts at right angle to the direction of motion. It disturbs the stability of the vehicle. The frictional resistance between the wheels and road surface will act in the opposite direction.
When the transverse inclination providing to the pavement surface on curves by raising the outer edge with respect to inner edge to counter act the centrifugal force. Also, To reduce the tendency of vehicle to overturn is termed as Superelevation. It is express as the difference of heights of two edges of the carriage way to the width of carriage way. It is denoting by ‘e’.
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e = V²/127 R
V = Speed of vehicle in km/hr
R = Radius of curve in metres.
Thus a superelevation of 1 in 15 means that for a 15 m wide road. The other edge of the carriage way is raising above the inner edge of the curve by 1 metre. It should be remember that greater the superelevation, more is the inconvenience to the slow traffic. Hence the superelevation providing should be minimum as possible but it can not be less than the camber giving to the remaining part of road for effective drainage. The super elevation generally varies from 1 in 14 to 1 in 16 but in special circumstances, the maximum superelevation can be provide up to 1 in 10.
METHODS OF PROVIDING SUPERELEVATION
Introducing super elevation on horizontal curves is an important feature of highway construction. The horizontal curve with a desire super elevation in road terminology is termed as a ‘Super elevated curve’. A super elevated curve consists of a circular curve, with transition curves on both the ends. A transition curve thus joins the straight portion of the road with one end of the circular curve.
At the end of the straight portion of the road curve, which is also the beginning of the transition curve, there is a provision of usual camber to the road surface. “The principle of introducing super elevation lies in the gradual changing of this camber surface at the start of the transition curve to a one way slope. i.e. the requiring super elevation at the end of the transition curve which is also the beginning of the circular curve”. This full and requiring super elevation is maintain on the entire circular curve up to its end which is also the beginning of the next or forward transition curve.
To change the cambered section of the road into a super elevated one, two steps are taking.
- Firstly, the camber on the outer edge is eliminate gradually till the road pavement has one straight tilt or slope from the inner to the outer edge (Elimination of crown method).
- Secondly, this straight line slope is gradually increase till the desire super elevation is attain. (Rotating pavement method).
It may be note that on the next transition curve, this design super elevation must be gradually remove. The cross-slope should be gradually change to the cambered road surface in the similar manner in which it was introduced but in the reverse order. The two important methods of providing super elevation to the curves are given below:
- Outer slope rotation method. (Elimination of crown method)
- Diagonal crown method.
Outer slope rotation method. (Elimination of crown method)
In the first method, the outer slope of the road is slowly rotate around the crown by progressively decreasing its inclination or gradually raising the outer edge. This inclination of the outer slope tangential to the crown surface firstly reaches horizontal. Then it is slowly increase till it has attained the same inward slope. The main disadvantage of this method is the difficulty of draining water in some portion of the road length where the slope on the outer edge is less (gradually decreased) than the camber provide on the road. This method is commonly use on Indian highways. The above disadvantage will not count much if the road has a good longitudinal slope.
Diagonal crown method.
In the diagonal crown method, the cambered section of the road is eliminate by progressively shifting the crown towards the outer edge along the extension of the inner slope (as shown in figure).
This shifting of the crown is continue, till it reaches the outer edge. This method is usually not recommend as some portion of the road will have an excessive slope more than the required super elevation (negative super elevation).
Once the profile of the road has been brought to a single straight line slope by any of the above methods. The designed super elevation is attained by any of the following methods;
- By progressively revolving the straight line slope about the central line. Thereby decreasing the inner edge and elevating the outer edge simultaneously both by half the total amount of super elevation (e). This method is preferred as grade in centre line remains unchanged. (No bumps).
- By progressively revolving the slope about the inner edge thereby raising the centre line and outer edge simultaneously. It is prefer in flat topographic areas where rainfall is heavy. (Easy drainage).
ADVANTAGES OF SUPERELEVATION
The super elevation provided to the horizontal curves affects the following:
- The stability of fast moving vehicle is increase and their tendency to skid or overturn decreases.
- The distribution of pressure on both the wheels will be equal and thus wear and tear of vehicles decreases.
- Intensity of stresses on the foundation decreases.
- In super elevated curves, the vehicles need not necessarily be slow down.
- No gullies or ditches are require on outer edges of super elevate curve.