What Is Road Gradient?
Road gradient is the rate of rise and fall along the length of road with respect to the horizontal. In simple words, it is the slope provided longitudinally to the road formulation along its alignment, usually to connect two points situated at diﬀerent levels.
Rise with respect to horizontal distance is called Upward gradient or positive gradient or Ascending gradient and is denoted by (+n%). Whereas, fall with respect to horizontal distance is called Downward Gradient or negative gradient or descending gradient as and denoted by (-n %).
The gradient of the road is expressed as 1 in n (1 vertical unit to the n horizontal units). Sometimes it is expressed even in percentage also. If there is a rise or a fall of 1 m in a 100 m horizontal length, then the gradient will be expressed as 1 in 100.
Why Is Gradient Provided To The Roads ?
The purposes of providing gradients on road are as follows:
- To link the places or locations which are located at diﬀerent levels.
- For smooth movement to vertical profile.
- To provide an eﬀective drainage of rainwater on roads, when the pavement of road is provided with Kerbs.
Factors Aﬀecting Gradient Of The Road
The factors aﬀecting the gradient of the roads are listed below:
- Nature of the traffic – A gentle gradient should be provided where the traﬃc on the road is usually of slow-moving vehicles like bullock carts and bicycles.
- Drainage of water – The gradient of road provided should be steep in the areas where there is heavy rainfall.
- Appearance – It is necessary to provide a gradient to the road for its appearance and aesthetic beauty.
- Access to the adjoining properties.
- Points like canals, bridges, railway crossings, etc.
- Topography of the area or the religion where the road is to be constructed.
- Safety of the traﬃc.
Types Of Road Gradients
The road gradient is divided into the following categories:-
- Average gradient
- Ruling gradient
- Limiting gradient
- Exceptional gradient
- Minimum gradient
- Floating gradient
1. Average Gradient
The vertical distance between two points divided by the horizontal distance between two points is known as the average gradient. The average gradient is generally used for the design of roads in hilly areas. The hilly areas have a relatively steeper slope.
Average gradient = The vertical distance between two points/The horizontal distance between two points.
2. Ruling Gradient
The maximum gradient within which the designer attempts to design the vertical profile of the road is known as the ruling gradient. Ruling gradient is also referred to as design gradient.
It is generally used because it gives the utmost safety at the least possible cost. The ruling gradient depends upon the following factors:
- The type of terrain.
- The nature of traﬃc.
- Length of the grade.
- Average speed of traﬃc.
- Types of vehicles, etc.
3. Limiting Gradient
Due to the topography of a particular area, sometimes a gradient steeper than the ruling gradient is provided to the roads. This type of gradient is known as the limiting gradient.
In simple words, the gradient steeper than the ruling gradient is called the limiting gradient. A limiting gradient is generally adopted in hilly areas. A gradient of the road approach to a bridge is an example of a limiting gradient.
4. Exceptional Gradient
There are a few circumstances in which it may be unavailable to provide still steeper gradients, at least for a short distance. In that case, the steeper gradients upon exception gradient may be provided.
In short, the gradient steeper than the limiting gradient is called exceptional gradient. The exceptional gradient is the highest grade of a gradient that may be used on a given road. The exceptional gradient is also known as the maximum gradient. Generally, it is adopted for short routes, of not more than 100 m lengths.
5. Minimum Gradient
The minimum required slope necessary for the eﬀective drainage of rainwater from the surface of the road is known as the minimum gradient. The minimum gradient is usually adapted where surface drainage is to be considered.
6. Floating Gradient
The descending gradient required to maintain the same speed of a vehicle, that was on the ascending gradient, without applying accelerators or brakes, is known as a floating gradient.
Gradients Of Roads In Diﬀerent Terrains By IRC
Diﬀerent gradients are provided to the roads in diﬀerent terrains. Some terrains require a steep gradient and some of them require a gentle gradient. So the gradients in diﬀerent terrains are provided accordingly. They are mentioned in the table below.
|Type Of Terrain
|Plain or rolling terrain
|1 in 30
|1 in 20
|1 in 15
|Mountainous terrain ( elevation is more than 3000 m above the Mean Sea Level (MSL) )
|1 in 20
|1 in 16.7
|1 in 14.3
|Steep terrain( elevation up to 3000 m above the Mean Sea Level (MSL) )
|1 in 16.7
|1 in 14.e
|1 in 12.5