Super elevation of the road

 When a vehicle negotiates a curved path, it is subjected to an outward force known as centrifugal force. To resist this force, the outer edge of the road is generally raised above the inner edge. This is known as superelevation or cant or banking. It is also defined as the inward tilt given to the cross-section of the road surface, throughout the length of the horizontal curve. The superelevation is expressed as the difference of heights of two edges of the carriageway to the width of the carriageway. The main advantages of providing superelevation are

(a) To achieve the higher speed of vehicles.

(b) (b) to increase the stability of the fast-moving vehicles, when they negotiate a horizontal curve, and

(c) To decrease the intensity of stresses on the foundation.

It may be noted that the greater the superelevation, the more will be convenient to the slow-moving traffic. The superelevation should not be less than the comber. In the absence of superelevation on-road along curves, potholes are likely to occur at the outer edge of the road. To prevent the overturning of the bullock cart on the curves, the Indian Road Congress(IRC) has prescribed the maximum value of superelevation as 1 in 15.

         If

                          e = Rate of superelevation.

                          f = Lateral friction coefficient = 0.15,

                          V = Speed of the vehicle in kmph, and

                           R = Radius of curvature in the meter.

Then               
 e+f = V²/127R

When the coefficient of friction is neglected, then the superelevation

E = V²/127R

From practical considerations, it is suggested the superelevation should be provided to fully counteract the centrifugal force due to 75% of the design speed(V) by neglecting lateral friction developed. Thus, the maximum rate of superelevation is given by

        

 E = (0.75V)²/127R = V²/225R

Attainment of superelevation in the field condition:

The introduction of superelevation on a horizontal curve in the field is a very important feature in highway construction. The cross-section of the road at the straight portion is cambered with the crown at the center of the part of the circular curve of the road is super elevated with a uniform tilt sloping down from the outer edge of the pavement up to the inner edge.

The attainment of superelevation may be split up into two parts:

(a) Elimination of crown of the section which is cambered.

(b) Rotation of pavement to attain super elevation fully.

Superelevation Design:

The steps for the design of superelevation in India from practical considerations are given below:

Step (i):

The superelevation is calculated for 75 percent of design speed, neglecting the friction.

E =(0.75v)²/gR or (0.75V)²/127R

i.e., e = V² /225R………………………..(i)

Step (ii):

If the calculated value of ‘e’ is less than 7% or 0.07 the value so the obtained value is provided. If the value ‘e’ as per Eq. (i) exceeds 0.07 then provided the maximum superelevation equal to 0.07 and proceed with steps (iii) or (iv).

Step (iii):

Check the friction coefficient developed for the max. value of e = 0.07 at the full value of design speed, v m/sec or V kmph.

F =(v² /gR – 0.07) or (v² / 127R – 0.07)………………..(ii).

If the value of f thus calculated is less than 0.15, the superelevation of 0.07 is safe for the design speed and is accepted as the design superelevation. If not, either the radius of the horizontal curve has to be increased or the speed has to be restricted to the safe value which will be less than the design speed.

Step (iv):

The allowable speed or the restricted speed at the curve is found out by considering the design coefficient of lateral friction and the maximum superelevation.

i.e.,

e+f = 0.07+0.15 = 0.22 = v_a² /gR = v_a² / 127R

The safe allowable speed,

If the allowable speed, as found out above is higher than the design speed, then the design is adequate and provides a superelevation of ‘e’ equal to 0.07. If the allowable speed is less than the design speed, the speed is limiting to the allowable speed V_a kmph calculated above.