road alignment

Road Alignment | 15 Principles & Factors Controlling Road Alignment | Steps in Design of Road Alignment

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Road alignment is the positioning of the centerline of the highway or road. It is also called as highway alignment. Simply, it is the direction through which the road will pass.

Road alignment provides proper guidance to pass the road through the alignment which is most economical, easy to construct and free from conflicts.

There are two types of road alignment.

a. Horizontal Alignment

It includes straight stretches, curves, etc.

b. Vertical Alignment

It includes upward or downward gradients.

 

Result of Improper Road Alignment

Improper road alignment leads to the following:

a. Increase in the rate of accidents.

b. Increase in construction and maintenance cost.

c. Increase in the vehicle operating cost and also leads to the discomfort of the users.

d. The decrease in durability and strength of the road.

 

Basic principles of road alignment 

1. The alignment of the road should be as short as possible to give the economy in the cost of construction.

2. The alignment should be as straight as much as possible which provides higher speed to the traffic and lower cost of construction.

3. The road alignment must be easy for the construction, maintenance and traffic operation.

4. The alignment should cross the railway lines and other roads and bridges at right angles.

5. It should cross the rivers, canals or streams etc., at a place where its width is minimum.

6. The alignment should serve the maximum population by connecting intermediate important towns and a group of villages.

7. The road alignment should not pass through regions of natural beauty and scenery.

8. The alignment should be such that it crosses the minimum number of bridges, crossing culverts and embankment places.

9. It must give a smooth curve and easy gradient.

10. It should be such that minimum earthworks in embankment or cutting are done.

11. The alignment should provide good sight distance.

12. It should be free from obstruction like ponds, lakes, wells, monumental buildings and historical buildings etc.

13. The alignment should run through such places where materials of road construction and labour are easily available.

14. As far as possible, it should run on good soil having the good bearing capacity to bear loads of traffic safely without any damage to the road.

15. The alignment should not passes through more costly and cultivated land. It must also avoid forests.

 

Factor Controlling Road Alignment: 


1. Class & Purpose

The alignment of the road is affected by class and purpose. National and state highways between two stations must be aligned straight as much as possible whereas, in the case of other types of roads, a deviation may be allowed where it is found necessary.

 

2. Obligatory Points

Obligatory points define the way through which road should pass and through which road should not be passed. Road alignment should not be passed through historical places, cultural places, etc. The roads are usually built for the development of the areas. Therefore, road alignments must necessarily pass through important towns, group of villages and places of religious, social, political and commercial importance.

 

3. Type of Vehicular Traffic

The alignment should be selected according to the type of vehicular traffic. In the case of fast-moving traffic, the road alignment must be straight as much as possible. In the case of low traffic, the alignment may even have sharp turns.

 

4. Gradient

The alignment of roads should be selected such that longitudinal slopes are not steeper than the ruling gradient. To achieve this, the alignment might need a deviation from the straight line.

 

5. Horizontal Curves

In the case of national and state highways, the radius of the horizontal curve must not be less than 230 m otherwise the alignment must be improved.

 

6. Sight Distance

The alignment of roads must be decided such that more and clear sight distance is available for drivers of the vehicles.

 

7. Obstructions

Obstruction also affects alignment selection. For example, the alignment should be improved to avoid marshy land, ponds, wells, graveyards, historical monumental and religious sites etc.

 

8. Economical Factor 

The road construction becomes economical when the road alignment is straight. So, straight alignments should be given priority.

 

9. Availability of Construction Materials

Construction material should be easily available. Abundant material leads to higher cost of the material. This directly affects the economical condition.

 

Steps in Design of Road Alignment

There are four major steps in road alignment. They are:

a. Map Study

The first step of road alignment is map study. Generally, geographical and contour maps are studied. This helps to know the position of natural features like ponds, rivers, lakes, hills etc. This also helps to learn about different alternative alignments.

 

b. Reconnaissance Survey

Reconnaissance is a rough survey done by visiting the field. Positions of natural features in the field are cross-checked as indicated in the map. If some features are not indicated in the map then such features are included. It also includes the collection of data like several cross drainage structures, high flood level,  soil characteristics, etc. Report of selected alignment is also prepared that includes the merits, demerits and profile map of scale 1:50,000.

 

c. Preliminary Survey

The main objectives of the preliminary survey are:

1. To analyze the different alternatives to the road alignment.

2. To estimate the cutting and filling required in the construction.

3. To survey the alignments proposed by reconnaissance survey.

4. To finalize the alignment among different alternatives.

 

d. Detailed Survey 

It is the final step before starting the construction work. In this detailed study of the following is done:

1. Soil Study

2. Hydrological Survey

3. Construction cost and time

4. Return on investment

5. Present and Future Traffic

 

Precautions To Take While Selecting Road Alignment

The following precautions should be taken while selecting road alignment:

a. Only alignments following the SESE rule should be selected.

( Note: SESE rule indicates

S = Short

E = Easy

S = Safe

E = Economical)

b. As far as possible, alignments should be free from problems like landslide, rockfall, conflicts, etc.

c. Several alternative alignments should be prepared and the best alignment should be chosen among them.

d. If a large amount of land acquisition has to be done then it is better to go with an alternative because it leads to more conflict & cost requirement.

 

Read Also: Earth Road

Read Also: Flexible Pavement

 

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