Department of Civil Engineering

SUB-Highway Engg.

TOPIC NAME- Vertical Alignment Of Road

Semester-4^th

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BY

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ER. Ankit Joshi

(Asst Hod. CIVIL Engineering Department)

AY:2021-2022

Vertical Alignment of road

Purpose of Providing Gradient to the Roads

1. To connect the two stations or points with each other, which are located at different levels.
2. To provide effective drainage of rainwater, especially when the pavement is provided with the curbs.
3. To construct the side drains economically.
4. To make the earthwork required for the road construction economic by balancing cutting and filling.

Importance of Gradient in Roads

1. 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.
2. Gradient allows movement of the vehicle on the vertical curves smoothly.
3. The gradient also helps to drain off rainwater from the surface of the roads.
4. Gradients are very helpful on curved roads in flat terrain where drainage problem arises.
5. 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.

Factors Affecting Gradient�

1. Nature of the ground.
2. Drainage required.
3. Nature of the traffic.
4. The type of road surface.
5. The total height to be covered.
6. Road and railway interaction.
7. Safety Required.
8. Bridge Approaches.

Types of Road Gradient

The types of road gradient are as follows

• Ruling Gradient.
• Limiting Gradient.
• Exceptional Gradient.
• Average Gradient.
• Floating Gradient.
• Minimum Gradient.

Ruling 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.

Limiting Gradient

• 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.

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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.

Minimum 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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Average Gradient

• 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.

Floating 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.

Grade Compensation

Grade Compensation

While aligning a hill with a ruling gradient of 6%, a horizontal curve of radius 60 m

is encountered. Find the grade compensation

Given data,

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Ruling gradient = 6%

R = 60 m

Therefor providing a grade compensation = 1.25 %

Compensated Gradient = Ruling gradient – grade compensation

= 6.0 – 1.25 = 4.75 %

Parts of Vertical curves

It is of two types :-

1. Summit Curve
2. Sag Curve or Valley Curve

Deviation Angle N in

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Summit Curve = n1 – ( - n2) = n1 + n2

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Sag Curve = - n2 – ( +n1 ) = - ( n1 + n2 )

+n1

-n2

Summit Curve

Sag Curve

N

Parts of a Vertical curve

Summit Curve

Length of summit curve for stopping sight distance

1- When the length of the curve is greater than the sight distance ( L > SSD )

Where,

L = Length of summit curve

S = Stopping sight distance

N = Deviation angle = n1 + n2

H = height of eye level = 1.2 m

h = height of subject above the

pavement surface = 0.15m

By substituting the value of H and

H, the above equation can be written as

Length of summit curve for stopping sight distance

2- When the length of the curve is less than the sight distance ( L < SSD )

By substituting the value of H and

H, the above equation can be written as