DESIGN CRITERIA FOR HILL ROADS (HAIR PIN BENDS) :-

HILL CURVES:-- A road going on the outer side of a hill or spur has a convex curve which is known as a salient curve or blind curve. Inside curve at the valley of a hill and which is concave curve is known as re-entrant curve or open curve. Serpentine curves , hairpin bends, and corner ends have also to be provided in hill alignments. HAIR-PIN BENDS:-- Hair pin bends are circular curves with transition curves at each end. Compound circular curves can be provided instead of hair pin bends. The following design criteria should be adopted normally for the design of hair-pin bends (a) Min:design speed --20km/hr (b) road way width at apex (1) National /State highway:--11.5 m for double-lane and 9 m for single lane. (2) District roads :-- 7.5m (3) Village roads :--6.5 m(c) Min :-radius for inner curve :-14m (d) Min:-Length of the transition 15.0 m (e) Gradient-- max :1in 40 ;min 1 in 200 . Approach gradient should not be steeper than 1in 20 for 40 metres (f) Superelevation in circular portion of the curve to be 1in 10,this can be reduced to 1in 15 for light traffic roads.. Inner and outer edges shall be concentric with respect to centre line of the pavement. Straight length between two successive hair pin bends should be 60 m  min :excluding the length of circular and transition curves. VERTICAL CURVES OR SUMMIT CURVES:-- A curve with convexity upward is called a vertical curve and the design of the vertical curve is principally governed by the sight distance required to provide adequate visibility, unless the summit is so small as not to interfere with visibility. In practical a simple parabola curve is used instead of a circular are for a vertical curve, which joins the two gradients tangentially. Vertical curves are necessary at the intersections of different grades to smoothen out the vertical profile. The minimum radius of vertical curve depends upon the design speed of the road and the required safe stopping sight distance. Minimum radii and the lengths for different speeds has been recommended by the IRC for application.