Methods of providing superelevation

Superelevation is provided by rotating the pavement cross-section over the transition length (or partly on the tangent before the curve if there is no transition). The pavement is first changed from a cambered section to a flat or one-way section, and then to the full superelevation. The change should be gradual (rate 1 in 150 to 1 in 60, depending on the terrain).

1. Eliminating the crown (removing the adverse camber)

• Outer edge rotated about the crown: the outer half is rotated about the crown until it is in line with the inner half (the normal camber is removed); then the whole pavement is rotated to give the required superelevation.
• Diagonal crown (crown shifting) method: the crown is gradually shifted towards the outer edge until the whole width has a one-way slope. It needs less change of edge level but is rarely used.
• Done over the first part of the transition length.

 normal:   /\        flat:  ____    full e:     ___/
          /  \              ----           ___/

2. Rotating about the centre line

• The pavement is turned about the centre line: the outer edge is raised and the inner edge lowered by equal amounts, each by half of the total difference.
• Advantages: the centre-line level stays fixed (profile unchanged), and the earthwork is balanced on the two sides. Mostly used on plains.
• Disadvantage: the inner edge drops below the ground profile, which may cause a drainage problem.

   outer edge up   ____.-----   
   centre fixed ---+
   inner edge down ----'

3. Rotating about the inner edge

• The inner edge is the fixed pivot and the outer edge is raised; the centre line also rises.
• Used where drainage is critical or the inner edge level cannot be changed (e.g. in cuttings, on hill roads).
• Disadvantage: the centre-line profile is changed and the outer edge is raised more, which affects levels and appearance.

   outer edge up  ______----.
   centre line up    .--'
   inner edge fixed ---

4. Rotating about the outer edge

• The outer edge level is kept fixed and the inner edge is lowered; used where the outer edge profile must be kept (for appearance, or on a curve on a hillside).

Remarks

• The pavement is generally rotated about the centre line on plain roads and about the inner edge in drainage-sensitive cases.
• If no transition curve is provided, about two-thirds of the superelevation run-off is placed on the tangent and one-third inside the circular curve (IRC). The extra widening is introduced over the same length.