Types of Gradients On Roads


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:

1. To link the places or locations which are located at different levels.

2. For smooth movement to vertical profile.

3. 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:

1. 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.

2. Drainage of water – The gradient of road provided should be steep in the areas where there is heavy rainfall.

3. Appearance – It is necessary to provide a gradient to the road for its appearance and aesthetic beauty.

4. Access to the adjoining properties.

5. Points like canals, bridges, railway crossings, etc.

6. Topography of the area or the religion where the road is to be constructed.

7. Safety of the traffic.

Types of Road Gradients

The road gradient are divided into the following categories:-

  1. Average gradient
  2. Ruling gradient
  3. Limiting gradient
  4. Exceptional gradient
  5. Minimum gradient
  6. 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.

Therefore,

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. Limiting gradient is generally adopted in hilly areas. A gradient of 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 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. Exceptional gradient is the highest grade of a gradient which 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 length.

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. 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 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.

Sr. No.Type Of TerrainRuling GradientLimiting GradientExceptional Gradient
1Plain or rolling terrain1 in 301 in 201 in 15
2Mountainous terrain ( elevation is more than 3000 m above the Mean Sea Level (MSL) )1 in 201 in 16.71 in 14.3
3Steep terrain( elevation up to 3000 m above the Mean Sea Level (MSL) )1 in 16.71 in 14.e1 in 12.5
Table – Gradients for roads in different types of terrain

Also Read –

Types Of Roads

Superelevation Of Roads

Types Of Camber In Road

Componenets Of Road


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