DESIGN OF HILL CURVES AND SAFETY MEASURES FOR PROVIDING PROTECTIVE RELIEF, REQUIRED TO AVOID ACCIDENTAL RISKS.

Road curves are mainly of two types--Horizontal curves and the vertical curves. A horizontal curve is a curve in plan provided to change the direction of an alignment, where as a vertical curve in elevation is provided at change of gradient. There are two classes of vertical curves (a) convex or summit curves and (b) concave or valley curves, also known as sag curves. The design of a summit curve is governed mainly by the sight distance, required to provide adequate visibility, unless the summit is so small, less than 0.90 mt, above the lowest point of the curve, as not to interfere with visibility. In practice a simple parabola curve is used instead of a circular arc for a vertical curve, which joins the two gradients gradually.when two slopes meet, the change from one to the other should be gradual. This is achieved by using a vertical curve to unite the two gradients. Simple curve is a single circular arc connecting two tangents whereas a compound curve is formed by two adjoining simple curves. Hair pin bends are circular curves with transition curves at each end. Compound circular curves can be provided instead of the hair pin bends, and it is necessary to follow design criterion required to adopted normally for the design of hair pin bends for which straight length between two successive hair pin bends should not be less than 60 mt minimum, excluding the length of circular and transition curves. In case of the village roads length of transition should not be less than 15 mt whereas superelevation in circular portion of the curve to be taken 1 in 10 however this can be reduced to 1 in 15 for light traffic roads.

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.