Method of Obtaining Superelevation:
Introducing superelevation on a horizontal curve in the field is an important feature in construction. The full super-elevation is attained by the end of transition curve or at the beginning of the circular curve.
The attainment of superelevation may be split up into two parts:
- Elimination of crown of the cambered section
- Rotation of pavement to attain full superelevation
i) Elimination of crown of the cambered section
This may be done by two methods:
In this method, the outer half of the cross-slope is rotated about the crown at a desired rate such that the surface falls on the same plane as the inner half and the elevation of the center line is not altered.
The outer half of the cross-slope is brought to level or horizontal at the start of the transition curve or at tangent point (T.P.). Subsequently the outer half is further rotated so as to obtain uniform cross-slope equal to the camber.
This method has a drawback that the surface drainage will not be proper at the outer half.
In this method, the crown is progressively shifted outwards, thus increasing the width of the inner half of cross-section progressively. This method is not usually adopted as a portion of the outer half of the pavement has increasing value of negative super-elevation.
ii) Rotation of pavement to attain full super-elevation
When the crown of the camber is eliminated, It does not signify that superelevation desired at that section is attained.If desired superelevation is more than camber/ cross sections for the payment section will have to be rotated for the till the desired superelevation is attained.
Desire superelevation can be attained by two method:
By rotating the pavement cross section about the centre line, depressing the inner edge & raising the outer edge each by half the total amount of superelevation required with respect to centre.
By rotating the pavement cross section about the inner edge of the pavement raising both centre as well as the outer edge of the pavement such as the outer edge is raised by the full amount of superelevation desired.
|Rotation about center line||Rotation about inner edge|
|It may with interfere drainage system.||No interference with drainage system.|
|Centre line remain unchanged.||Centre line is elevated.|
|Earth work is balanced in cutting & filling.||Filling is required.|
In cases where a transition curve cannot be provided for some reason, 2/3 of superelevation may be attained at the straight portion before the start of the circular curve and the balance 1/3 is provided at the beginning of the circular curve.
The superelevation is introduced by raising the outer edge of pavement at a rate not exceeding (1 in 150) in plain & rolling terrain and (1in 60) mountainous & steep terrain.
Radius of Horizontal Curve
Horizontal curve of highways are designed for the specified design speed (Ruling & Min).
However if this is not possible due to site restriction, the horizontal curve may be redesigned.
On the basic of speed Radius of Horizontal Curve is classified into following:
1.Ruling Minimum Radius(RMR)
V= Ruling design speed (kmph)
2.Absolute Minimum Radius(AMR)
V’= Ruling design speed (kmph)
3.Radius beyond which no superelevation is required (RBNSN):
It is the radius, corresponding to which camber serve the purpose of superelevation.
e=camber, f=0, V=V0.75
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