Types of Road Gradient in Highway engineering
What is Road Gradient?
The gradient is the rate of rising or fall of the road along its alignment is called a Gradient on Road. It is expressed as the ratio of the difference in height of its extremes to the horizontal length between them. Thus if a road ascends or decends one metre for every 30 metres, the gradient is, said to be 1 in 30 gradients.
Gradients are not very steep as to make an appreciable difference between the horizontal distance and the distance along the sloping length of the alignment, the distance may be measured along the length of the road and assumed as a horizontal distance.
Longitudinal grades once provided in the road will form permanent features especially in built-up areas.
Longitudinal gradient depends on:-
- Topography of the country.
- The characteristics of the traffic.
- Drainage.
- Access to adjoining property.
- Road and railway intersection and bridge approach.
- Appearance.
- Safety.
Importance of Gradient in Roads
It is essential to provide a properly required gradient to the road along the length of its alignment with respect to Horizontal. Gradient allows movement of the vehicle on the vertical curves smoothly.
Types of road gradient –
The gradient can be classified into the following types :-
- Maximum gradient.
- Minimum gradient.
- Average gradient.
- Rulling gradient.
- Floating gradient.
- Exceptional Gradient
Maximum Gradient –
It is the steepest gradient which is to be permitted on the road and which on no account is to be exceeded. Maximum Gradient is also called Limited Gradient.
It is determined by the pulling power of the animal or the automobile vehicle using the road. It eliminates heavy earthwork and voids long detours.
Minimum Gradient –
A minimum gradient of 1 in 330 for concrete road and 1 in 250 for other types of roads, is desirable for the purpose of drainage.
Minimum gradient maintenance cost is less. In such a case, there should be no potholes or depression which may encourage stagnation of water. Mainly it is used in city road.
Average Gradient –
The average gradient is the total rise or fall between any two points chosen on the alignment divided by the horizontal distance between the two points. The determination of the average grade is useful in carrying out the first paper location or preliminary survey and gives the approximate length of the proposed alignment.
Exceptional Gradient
The Gradient, which is steeper than the limiting gradient, is known as the exceptional Gradient. This type of Gradient is generally used in an extraordinary situation where shorter lengths of the roads are available.
The exceptional gradient is very steeper gradient used in the unavoidable situation like in the mountainous region and terrain. But the Drawback of an exceptional gradient is that it required more fuel consumption and has more friction losses.
Rulling Gradient –
It is the permissible gradient in the alignment of the road. This gradient is such that vehicle whether animals are drawn or power-driven can overcome the long length of this gradient without much fatigue or government uneconomic to consumption of petrol or other power generating materials.
The gradient depends on the type of transport that will use the road, and naturally upon the type of road surfacing provided.
Floating gradient –
If a motor vehicle is traveling at a constant speed and meets descending grade in which it will continue to travel at the same speed without any tractive effort on the part of the vehicle, then such a gradient is known as a floating gradient.
Purpose of providing gradient to the roads
- To connect the two station or points with each other.
- Provided effective drainageof rain water.
- To construct the side drains economically.
- To makes the earth works required for the road construction economic cutting and filling.
- Gradient allows movement of the vehicle on the vertical curves smoothly.
Factors Affecting Gradient
- Drainage required
- Nature of the traffic
- Nature of the ground
- Types of road surface
- Total height to be covered
- Railway interaction and Road
- Safety required
- Bridge Approaches