Essay: Gradients – Special consideration for hill roads

Essay details:

  • Subject area(s): Architecture essays
  • Reading time: 5 minutes
  • Price: Free download
  • Published on: July 26, 2019
  • File format: Text
  • Number of pages: 2
  • Gradients - Special consideration for hill roads
    0.0 rating based on 12,345 ratings
    Overall rating: 0 out of 5 based on 0 reviews.

Text preview of this essay:

This page of the essay has 1319 words. Download the full version above.

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

9

4.00

3.00

2. Sight Distance:

The Stopping Sight Distance is calculated from the relation,

whereV = Design speed, kmph

t = Reaction time, taken as 3 seconds

f = Coefficient of friction, assumed as 0.4

Safe Stopping Sight Distances for various speeds given by IRC are given in Table .

Table : Safe stopping sight distance

Speed, kmph

20

25

30

40

50

SSD, m

20

30

35

50

70

Overtaking Sight Distance is calculated from the relation:

OSD = 5.556 Vb + 2 S + 0.278 T Vb + 0.278 V

whereV = Speed of overtaking vehicle, kmph

Vb = Speed of overtaken vehicle kmph

= (V – 16) km/ph

S = Spacing of moving vehicles

= (0.2 Vb + 6) m.

T = Overtaking time, secs

A = Acceleration in kmph/sec.

Minimum Overtaking Sight Distance for various speeds as recommended by IRC are given in Table .

Table : Minimum OSD

Speed, kmph

30

40

50

OSD, m

90

145

210

3. Radius of Horizontal curve:

The minimum radius of horizontal curves in hill roads is calculated from the relation.

where e = Super elevation

f = Lateral friction factor = 0.15

Minimum radius recommended for various class of hill roads as per IRC are given in Table

Table : Minimum radius of horizontal curves

Type of Road

Minimum radius in metre

Mountainous

Steep hilly

No snow

Snow bound

No snow

Snow bound

National Highway and State Highways

50

60

30

33

Major District Roads

30

33

14

15

Other District Roads

20

23

14

15

Village Roads

14

15

14

15

Reverse curves are designed for a minimum radius of 30 m for the compound curves and a straight distance of 9 m between their transition ends.

4.Super elevation:

Super elevation to be provided in horizontal curves of hill roads may be computed from the relation given below for a given minimum radius of horizontal curve.

IRC has recommended a maximum value of super-elevation as 1 in 15 and 1 in 10 for hill roads of snow bound.

5.Widening of Curve:

Extra width of carriageway, We, at horizontal curve is given by the relation as,

where n is the number of lanes.

Extra widening as per IRC recommendation is given in Table.

Table : Extra widening requirement

Radius of curve, metres

14 to 20

20 to 30

30 to 60

60 to 150

above 150

Single lane:

Extra widening metres

Radius of curve, metres

1.5

30 to 40

1.20

40 to 60

0.90

60 to 100

0.6

100 to 150

Nil

above 150

Two lane:

Extra widening, metres

1.5

1.20

0.90

0.60

Nil

Extra formation width should also be provided at horizontal curves.

Minimum of 0.5 m shoulder width should be provided on either side of the carriage way.

...(download the rest of the essay above)

About this essay:

This essay was submitted to us by a student in order to help you with your studies.

If you use part of this page in your own work, you need to provide a citation, as follows:

Essay Sauce, Gradients – Special consideration for hill roads. Available from:<https://www.essaysauce.com/architecture-essays/gradients-special-consideration-for-hill-roads/> [Accessed 07-12-19].

Review this essay:

Please note that the above text is only a preview of this essay.

Name
Email
Review Title
Rating
Review Content

Latest reviews: