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 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 is steeper than the limiting gradient,whcih may be used only in exceptional situations
Design gradient<Limiting gradient<Exceptional 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.
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
- 30+R /R %
- 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%.