Grade of Roads-Highway Geometric Design

Grade of road is and important factor in the design of roads.Gradient is the slope of the road in the longitudinal direction of the road

Types of Gradient

Rulling gradient or Design gradient

Rulling gradient of design gradient is the gradient given to a road, which does not affect the tractive power of the vehicle.

Limiting Gradient

Limiting gradient is the maximum gradient steeper than rulling gradient,which may be used in restricted length ,where providing rulling gradient is not possible.Limiting gradient is done in the economical point of view.

Exceptional gradient

Exceptional gradient is steeper than the limiting gradient,whcih may be used only in exceptional situations

Design gradient<Limiting gradient<Exceptional gradient

Minimum gradient

A minimum gradient is done in the longitudinal direction of the road for the drainage purpose.There should be a minimum gradient in the drainage for the flowing of water.

Grade Compensation

When a vehicle is moving through a horizontal curve.If there is gradient also,the tractive resistance is more because the final resistance is due to the gradient and the curve.So in order to compensate the resistance due to the curve,the gradient of the road is reduced to some amount in the curve.This is called grade compensation

The main advantage is to overcome the tractive resistance due to the curve. Thus grade compensation can be defined as the reduction in gradient at the horizontal curve because of the additional tractive force required due to curve resistance

The value for grade compensation is

1. 30+R /R %
2. 75/R %

Compute the minimum value out of two and that is value for grade compensation

Compensated gradient=Given gradient-Grade compensated

If the compensated gradient is less than 4%,then the compensated gradient can be kept as 4%.