The Basics of Alignment: Definition, Features, Design, Factors for the Construction
Choosing the correct alignment of a road is not just about choosing geometry; it is also an artistic decision. As part of horizontal alignment design, uniformity of the procedure must be maintained so that it blends seamlessly into the surrounding topography. As a result, it must provide a smooth alignment that confirms natural contours, since straight or rigid alignments harm the environment, destroy natural slopes, and disrupt plant growth. Conserving existing features should be considered when selecting alignments.
Define Road Alignment
It is the position of the centerline of the road or highway that determines its alignment. Highway alignment is also known as road alignment. The alignment of the road enables the road to pass through the most economical, easy, and least conflicting path possible. In other words, it is how the road will be oriented.
Road Alignment Features
1. To reduce construction costs, the road alignment should be as short as possible. To provide higher traffic speeds and lower construction costs, the alignment should be as straight as possible.
2. The alignment of the road must be convenient for construction, maintenance, and traffic flow. At right angles to railways, other roads, and bridges, the alignment should cross them.
3. Rivers, canals, and streams must be crossed at places where their width is the smallest. In order to serve the greatest number of people, the alignment should link intermediate towns and a group of villages.
4. Natural beauty and scenic areas should not be crossed by the road alignment. There should be a minimum number of bridges, crossing culverts, and embankment places in the alignment.
5. The curve must be smooth and the gradient should be easy to follow. In order to avoid earthworks in embankments or cutting, minimal earthworks should be done.
6. A reasonable sight distance should be provided by the alignment. Water bodies, lakes, wells, monumental buildings, historical structures, etc., should be removed.
7. Ideally, the alignment should pass through areas with easy access to labor and materials for road construction. For the road to run smoothly, it must run on good soil that can bear the weight of traffic safely without damaging the surface.
8. It is important that the alignment does not pass through more expensive and cultivated land, as well as through forests.
Road Alignment Factors
Usually, roads are built to facilitate the development of an area. The road alignment should not pass through historical and cultural sites.
Road alignments are defined by mandatory points. Road alignments must pass through important towns, villages, and places of religious, social, political, and commercial significance.
It is important to select a road alignment so that longitudinal slopes do not exceed the ruling gradient. It might be necessary to deviate from a straight line in order to accomplish this.
Road alignments should be determined so that drivers have a clear sight distance.
The construction of roads becomes more economical when the alignment of the roads is straight. In this case, straight alignments should be prioritized.
Construction Materials Requirement
It is important to have easy access to construction materials. Construction costs are low because abundant materials are used.
The radius of a horizontal curve on a national or state highway must not be less than 230 meters. A better alignment is needed if this is the case.
It is also important to take into account obstructions when selecting alignments. Alignment must be improved in order to avoid marshy land, ponds, wells, graveyards, and historic, monumental, and religious sites.
Purpose & Class
Class and purpose affect the alignment of the road. In the case of national and state highways, straight alignment is preferred as much as possible, whereas deviations can be allowed on other types of roads.
Whenever fast moving traffic is present, the road alignment should be as straight as possible. If low traffic is present, the alignment may even have sharp turns.
Alignment selection should also take into account road landscaping, environmental factors, and social factors.
1. It is advisable to avoid long tangents exceeding three kilometers in length.
2. A curvilinear alignment with long curves is safer and more aesthetically pleasing.
3. This method can prevent sharp curves from occurring.
4. During alignment design, curves must be proportioned.
5. When calculating curves, you can avoid reverse and compound curves as much as possible.
6. A sufficiently long tangent accommodates the required transition length in reverse curves.
7. A three dimensional view of the road is essential for drivers.
To learn more, watch the following video tutorial.
Video Source: CE&T- Civil Engg & Technology
Sitting of Bridges
When it comes to large bridges over 300 meters, the site of the bridge should be the primary consideration. Bridge locations should be determined by the fluency of the alignment for small bridges fewer than 60 meters.
The designer should use his discretion when designing spans of 60 to 300 m, considering the importance of the road, overall economics, and aesthetics.
1. It is not a good idea to change grades too often.
2. There is no elevation change within 150 meters.
3. Curves in short valleys should be avoided.
4. Breaking back grade lines should be avoided.
5. The profile should be consistent.
6. Make sure your profile is not saw toothed.