What Is the Gradient of Road?
It is defined as the rate of rising or falls along the length of the road with respect to the horizontal is known as Gradient of Road.
In another word, it is the longitudinal slope provided to the formation level of the road along its alignment.
It is generally expressed in 1 in n ( where 1 is a vertical unit to n is a horizontal unit)
It is also expressed in percentage.
Gradient = (Vertical distance / Horizontal distance) x 100
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Purpose of Providing Gradient to the Roads
• To connect the two stations or points with each other, which are located at different levels.
• To construct the side drains economically.
• To make the earthwork required for the road construction economic by balancing cutting and filling.
Importance of Gradient in Roads
• The gradient is the most important part of the construction of roads. It is essential to give 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.
• The gradient also helps to drain off rainwater from the surface of the roads.
• Gradients are very helpful on curved roads in flat terrain where drainage problem arises.
• Before finalizing the gradient of the road, it is important that the construction cost, vehicular operation cost, and the practical problems that may arise on the site also have to be considered.
Effect of Gradient on Roads
• The effect of gradient on the vehicular speed is considerable. This is particularly important in roads where the proportion of heavy vehicles is significant.
• Due to less sight distance at uphill gradients, the speed of traffic is generally controlled by these heavy vehicles. Due to which the operating costs of the vehicles are increased, and the capacity of the roads will have to be reduced.
• Due to high speed between heavy and light vehicles, and between uphill and downhill gradient accidents abound in the gradients.
Factors Affecting Gradient
• Nature of the ground
• Drainage required
• Nature of the traffic
• The type of road surface
• The total height to be covered
• Road and railway interaction
• Safety Required
• Bridge Approaches
Irc Recommendations for Gradient
IRC had specified desirable values for different types of terrains.
Nature of Area
Plain or Rolling Area
1 in 30
1 in 20
1 in 15
1 in 20
1 in 16.7
1 in 14
1 in 16
1 in 14.3
1 in 12.5
Types of Road Gradient
as per below six types of road gradient.
1. Ruling Gradient
2. Limiting Gradient
3. Exceptional Gradient
4. Average Gradient
5. Floating Gradient
6. Minimum Gradient
• The gradient, which is usually adopted while making the alignment of the road, is known as the ruling gradient.
• The ruling gradient is used for designing the road because it gives maximum safety at minimum cost.
• The gradient, which is steeper than the ruling gradient, is known as the limiting gradient. In some situations, we cannot adopt the ruling gradient, where we have to use a limiting gradient.
• It is usually used in hilly terrain and rolling terrain.
• The topographical condition of a place compels adopting the steeper gradient. It is also known as the maximum 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.
• Exceptional gradients are a very steeper gradient that is 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.
• The total rate of rising or fall between any two points along the alignment of the road divided by the horizontal distance between two points is known as an average gradient.
• The gradient on which a motor vehicle, moving with a constant speed, continues to descend at the same speed without any application of power or brakes is known as a floating gradient.
• The minimum desirable slope which is essential for the effective drainage of rainwater from the surface of the known as a minimum gradient. It is usually adapt where surface drainage is important.
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