Gradient and types of gradient for highway construction

 Gradient 

The rate of rise or fall of a road surface along the length of the road is termed as gradient. It is usually expressed as a ratio of 1 vertical in'n horizontal (i.e. 1 in n). For example if a road surface falls 4 m in a horizontal distance of 200 m measured along the length of the road, the road gradient is 1 in 50. Some times it is also expressed as percentage. Gradient is always necessary to drain off the surface water easily from the side drains. It should be remembered that surface water can be drained off to the side drains by providing camber to the road surface but a certain longitudinal slope (Gradient) is essentially required to drain the water along the side drains (easier flow of open drain water to some gutter, ditch or intercepting chamber). Gradient of a road depends upon the following factors:

 () Nature of traffic : For slow moving traffic like Tongas and Bullock carts, Steeper pa e: gradient is avoided. 

(ii) Natural Topography of area : To avoid unnecessary cuttings or excessive fillings, sometimes steep gradients are provided in areas with steep undulations. 

(iii) Rainfall : More the rainfall in the locality, steeper is the gradient to be provided to drain the surface water through side drains. 

(iv) Safety considerations : Excessive steep gradients are unsafe.

⁵)  General appearance : Minimum gradient always improves the appearance of road than flat or level road. 

Types of Gradients 

various types of gradients which are provided in different portions of a road length are : 

1) Maximum or limiting gradient

 2) Ruling gradient

 3)Minimum gradient

 ( iv ) Exceptional gradient 

1) Maximum gradient : A gradient which should never be exceeded in any part of a road is called maximum gradient or limiting gradient . Thus it is the steepest gradient which is to be permitted on the road surface as steeper gradients cause great unconvenience to slow moving traffic . Its value has been fixed as 1 in 15 for hill roads and 1 in 20 for roads in plain . Such gradients are not provided for long stretches of road .

 ( ii ) Ruling gradient : The permissible gradient provided to the road alignment which can be easily overcome by any type of traffic without much fatigue is termed as ruling gradient . It also does not cause uneconomical fuel consumption of vehicles for long distances of this gradient as in case of limiting and exceptional gradients . IRC recommends ruling gradient of 1 in 20 for hill roads and i in 30 in plains . 

3) Minimum gradient : It has already been discussed that for eficient drainage of surface water from roads a certain minimum gradient or longitudinal slope is essential . " This essential longitudinal slope which must be provided to the roads for drainage purpose is termed as Minimum Gradient . " Its value is generally fixed as I in 200 as per IRC recommendation but the minimum gradient also depends upon nature of ground , type of soil and rainfall in that particular area . For concrete drains , a minimum gradient of 1 in 500 is sufficient , 

( iv ) Exceptional Gradient : In some extra ordinary conditions , to avoid deep cuttings or excavations and for economical considerations , it becomes a necessity to provide a still steeper gradient than the limiting gradient . " This steeper gradient which exceeds maximum or limiting gradient in extra ordinary circumstances is termed as Exceptional gradient . " However , exceptional gradient should be strictly limited for short stretches only not exceeding 100 metre in one go . 

RELATION BETWEEN CAMBER AND GRADIENT 

Both camber and gradient are provided to drain off the rain water from the surface of road . Steeper camber helps in draining the rain water quickly where frater camber helps in smooth flow of traffic . It is generally misconceived that on steeper gradients the camber can be reduced . But in fact the camber should always be increased on steeper gradients , otherwise the rain water will travel a longer distance almost parallel to the centre line of road. Generally, gradient should be equal to half of the camber for an efficient drainage and smooth traffic flow.