Essay: Gradients – Special consideration for hill roads

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Gradient:

It is the rate of rise or fall of a road level along its length. It is expressed as a percentage rise or fall or a rate of rise or fall with respect to horizontal distance.

A gradient of 1 in 20 or 5 % represents that there is an ascending or descending of road profile by one meter for every twenty metres. As the gradient on the road is not very steep the length along the road is taken as a horizontal length.

Grades are expressed as the horizontal distance along the length of the road.Longitudinal grades are almost permanent features of the highway. This is the case in urban and built up areas where no changes can be incorporated easily.

The design of a longitudinal grades depends on following factors:

i)Anticipated traffic

ii)Site conditions

a)Drainage

b)Appearance

c)Approach to adjacent property

d)Safety aspects

iii)Railway – intersections

iv)Bridges and culverts

v)Design speed

vi)Sighting line and sight distance.

1.Types of Gradient:

Gradients are classified as:

i)Ruling gradient

ii)Limiting gradient

iii)Exceptional gradient

iv)Minimum gradient

i)Ruling Gradient:

It is the desirable upper limit of gradient adopted in the normal course of design. Adoption of ruling gradient should balance the cutting and filling of earth work which will give an economical design. Different factors which are to be considered in the choice of ruling gradient is:

Type of terrain

The length of the grade

The speed

Pulling capacity of vehicles and

The presence of horizontal stretches of road.

Indian Roads Congress has recommended gradients for roads in different terrians which are specified in the Table below.

Table : Gradients for roads in different terrains (CRC)

Type of terrain

Rolling gradient

Limiting gradient

Exceptional gradient

Plain or rolling

3.3% (1 in 30)

5.0 % (1 in 20)

6.7 % (1 in 15)

Mountanious terrain and steep terrain having elevation more than 300 m above mean sea level

5.0 % (1 in 20)

6.0 % (1 in 16.7)

7.0 % (1 in 14.3)

Steep terrain up to 3000m height above mean sea level

6.0% (1 in 16.7)

7.0 % (1 in 14.3)

8.0 %(1 in 12.5)

ii)Limiting Gradient:

It is provided when topography is not permitting to provide the ruling gradient,due to economy. Length of continuous grade of stretch of road steeper than ruling value should be limited. It happens in rolling and hilly terrains to go in for limiting gradient but it has to be separated by straight stretches or road with easier grade frequently.

iii)Exceptional Gradient:

In some ground conditions it will be inevitable to provide a gradient steeper than limiting gradients such gradients are referred to as exceptional gradient.Such situations may arise in approaches to causeways, near hair – pin bends,etc. In a hilly terrain short stretches of grades, steeper than the maximum gradient may have to be resorted to reach an obligatory point on a fixed route.

The exceptional gradients have several adverse effects, viz., more fuel consumption, more friction losses, reduces engine efficiency, increased wear of road surface, etc. Hence it should be adopted only in very essential field conditions.

iv)Minimum Gradient:

A road with less gradient or level may not be in a position to drain easily. The surface water may be drained to the side drain due to the camber. A longitudinal slope is needed to drain the water on the surface and from the side drains.

It is essential to have a certain minimum gradient on roads from drainage point of view provided topography permits.The minimum gradient depends on the factors like rainfall, runoff, type of soil, topography and other site conditions.

2.Grade Compensation:

In addition to gradient if there is horizontal curve then there will be added resistance to traction.

In such cases, the total resistance due to grade and curve should not exceed the resistance due to the maximum value of the gradient specified.

For design purposes, the maximum gradient may be considered as that of ruling gradient. In some special cases the limited gradient may be used.

The gradient should be reduced in situations where sharp horizontal curves are present so as to compensate for the loss of tractive effort due to the curve. This reduction in gradient at the horizontal curve is called the grade compensation.

It is provided to offset the extra tractive effort involved at the curve.

Grade compensation (%) = subject to maximum value of 7.5 / R,

where,

R is the radius of the circular curve in metres.

As per IRC, there is no need for grade compensation for gradients flatter than 4.0 %.

Special consideration for hill roads:

Geometric design of hill roads are different from those in plains. Main reasons for the difference are the topography and other problems.The design speed should be different. Recommendations of Indian Roads Congress (IRC) for hill roads are discussed below.

Design Speed:

For hilly regions design speeds as per IRC recommendations are as follows.

Table : Design speed in hilly region

Design speed km / hr

National Highway and State Highway

Major District Roads

Other Roads

Village Roads

Minimum

50

40

30

25

Maximum

60

50

40

35

Cross – Section Elements of Hill Roads:

As a general rule, geometric features of a highway except cross – sectional elements do not lead to stage construction.In the case of hill roads, features like grade and curvature cannot be changed at a later date easily. It may be very expensive and may not be possible at all.

A careful design should be made regarding alignment and to some extent cross – section elements.Development to cross-section should be decided very carefully as it involves various additional construction like retaining walls, breast walls, catch water drains etc.

1. Formation Width:

Formation widths and pavement widths as per IRC are given in Table .

Table : Formation widths

Type of highway

Land width

Formation width in metres (excluding side drain and parapets)

Carriage way metres

Normal

Exceptional

National of Highway

24

18

8.80 to 6.25

7.0

State Highway

24

18

8.80 to 6.25

3.72

Major District Road

18

15

4.75

3.75

Other District Road

15

12

4.75

3.75

Village Road

9

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