What is Road Gradient? Know the Effects, Types, Representation, and Grade Compensation

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The vertical alignment of a road consists of vertical gradients and vertical curves. Road Gradient is an important part vertical alignment.

While aligning a highway it is necessary to follow local topography or profile of land throughout the length. Topography of land is not necessarily straight all the time it can have slopes with varying magnitude.

Constructing road on such profile will result in very bad driving experience and even can result in accidents so to provide smooth vehicle movement the changes in elevation can be smoothened by vertical curves. By road curves I mean road gradient and vertical curves.

In this article we will take a detailed look at road gradients, its types, effects, representation, grade compensation.

What is Gradient of Road?

Road gradient is the rate of rise or fall along the length of the road with respect to the horizontal. It is expressed as a ratio of 1 in x(1 vertical unit to x horizontal) also gradient can be expressed in percentage.

Road gradient

Types of Road Gradient

Following are the different types of road gradient

  1. Ruling Gradient
  2. Limiting Gradient
  3. Exceptional Gradient
  4. Minimum Gradient

1) Ruling Gradient

Image showing Road with ruling road gradient

The ruling gradient or design gradient is the maximum gradient within which the engineer attempts to design the vertical profile of the road.

Ruling gradient is also known as Design gradient as it is the maximum gradient the designer can adopt in designing vertical profile.

Ruling gradient depends on terrain, length of the grade, speed, pulling power of the vehicle and horizontal curve.

The ruling gradient is fixed in such a way that the maximum power developed by the engine is equal to the power required to overcome the resistance to the motion on the grade at design speeds.

2) Limiting Gradient

Image showing Road with limiting road gradient

A gradient steeper than the ruling gradient which may be used in restricted length were restricting the gradient within the ruling gradient is not possible.

As the Limiting gradient is greater than Ruling gradient it is also known as Maximum gradient.

It is provided when restricting the gradient in ruling gradient resulting in enormous increase in cost of construction.

On rolling and hilly terrain, limiting gradient may be frequently adopted but the length of limiting gradient should be restricted.

3) Exceptional Gradient

Exceptional gradient is gradient steeper than ruling gradient and limiting gradient and provided only if the situation is anavoidable.

The exceptional gradient should be limited for short stretches not exceeding about 100m at a stretch.

4) Minimum Gradient

It is provided along the length of the road for drainage point of view. Camber takes care of the lateral drainage but for longitudinal drainage, it is necessary to provide some slope for smooth flow of water.

A gradient of 1 in 500 is sufficient for concrete drains and gradient of 1 in 200 for open soil drains.

Effects of Road Gradient

Road gradient can influence the vertical speed, acceleration, deceleration, stopping distance, sight distance and also vehicular movement in the way explained below

  • Long steep gradient affects vehicular speed very hard and it is worst in the case of heavy vehicles.
  • In uphill gradients speed of whole traffic in mostly governed by big vehicles and due to slope and restrictive sight distance speed of heavy vehicles is already very slow.
  • As a result of this operating cost of vehicle increases and capacity of road reduces.
  • Due to speed difference in heavy and light vehicles, restrictive sight distance and uphill downhill slopes accidents can happen.

Gradient of Different Roads as per IRC Specifications

IRC code has given following recommendation for the gradients. The table shows road gradient in hilly areas, maximum gradient of road in India.

TerrainRulingLimitingExceptional
Plain and Rolling3.35.06.7
Mountainous terrain5.06.07.0
Steep terrain
a) Upto 3000m above mean sea level.
6.07.08.0
Steep terrain
b) Above 3000m above mean sea level.
6.07.08.0
Table No 1: Gradient of Different Roads as per IRC Specifications

Representation of Road Grade

Road gradient can be expressed in ratio as 1 in x i.e. 1 vertical unit to x horizontal unit, and in percentage as N i.e. N in 100.

An ascending gradient or positive gradient is denoted as +n and the descending gradient or negative gradient is denoted as -N.

Deviation Angle

Deviation angle is the angle which measures change in direction when two different grades meets.

Deviation angle is given by algebraic difference between two grades that is given below

[n_1 - n_2] = n_1+ (- n_2) = α_1 + α_2

Grade Compensation

When there is horizontal curve in addition to the gradient, there will be increased resistance to traction due to both gradient and curve. It is necessary that in such cases the total resistance due to grade and curve should not normally exceed the resistance due to maximum value of the gradient specified.

Then a sharp horizontal curve is to be introduced on a road which has already the maximum permissible gradient, then the gradient should be decreased to compensate for the loss of tractive effort due to the curve.

This reduction in gradient at the horizontal curve is called grade compensation.

As per IRC limitations grade compensation is not necessary for gradients flatter than 4% and thus when applying grade compensation correction, gradients needs not to be reduced beyond 4%

Grade Compensation Formula

Grade compensation is calculated from following formula= (30+R)/R

Grade Compensation= { { 30 + R } \over R }
grade compensation formula

where, R is the radius of curve in m

FAQs on Road Gradient

What is gradient of road?

Road gradient is the rate of rise or fall along the length of the road with respect to the horizontal. It is expressed as a ratio of 1 in x(1 vertical unit to x horizontal) also gradient can be expressed in percentage.

What is ruling gradient for mountainous roads?

Ruling gradient for mountainous road is
a) For elevation up to 3000m above mean sea level= 6%(1 in 16.7)
a) For elevation more than 3000m above mean sea level= 5%(1 in 20)

What is pusher gradient?

Pusher gradients are gradients on which extra engine is required to pull the train. These gradients are constructed only when there is need of heavy cutting and filling.

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