In this post, we are discussing the Importance of Highway Alignment and various factors which are needed to be considered in planning the highway alignment.
The Terms which we discuss here are the most important and usually come across in highway designing and planning. As a civil engineer, you must be aware of these terms.
Once the plan of road construction is decided, the next step to work on is the highway alignment.
What is Highway Alignment?
The position of the central line of the highway or the layout of the planned highway line on the ground is called Highway Alignment.
Highway Alignment is generally phrased as two types on Highway plans
This covers the horizontal path of the road either it’s straight or curved or both. Top view of road alignment gives horizontal alignment of the road.
This deals with the gradients, slopes and levelling of the ground.
Perspective view or front view or side view of the road alignment makes you understand about Vertical alignment.
Importance of Highway Alignment:
Deciding the highway alignment is the most crucial part of road construction. Road construction involves a lot of land acquisition. Once the alignment is fixed and constructed as per plan, it is difficult to transform it because of increment in the cost of adjoining land and development of expensive structures by the side of the road. A small error in the Highway alignment enhances the cost of construction.
The ideal Highway alignment should meet the following criteria :
Look at the following image, A and B is the best alignment as it has fewer curves than a and b.
Factors controlling the Highway alignment:
1. Obligatory Points:
The control points governing highway alignment are called obligatory points. These points decide where the alignment should pass and where the alignment should not pass.
Alignment Should pass:
Roads are constructed for the development of the areas. A small deviation in alignment can be acceptable if highway passes through the towns, villages, cities etc.
If the alignment passes through the mountain, it is recommended to relook the best alternatives, either to construct the tunnel or go round the hills. The selection of the choice depends on factors like topography, site conditions and construction and operation cost
Alignment should not pass:
It is always recommended to avoid the alignment on small lakes and ponds and deviate the alignment upto some extent where required, construction of bridges on these lakes and ponds requires more funds.
The alignment should not pass through the places which are developed or having expensive buildings or high rise buildings acquisition of land at these places incurs huge initial cost and demolishing these structures takes more time. So the slight deviation in alignment is acceptable if above these points are in alignment.
The alignment should not pass through the dense forests and agricultural lands.
2. Horizontal Curve and gradient:
The Horizontal curve must be as flat as possible. The gradient should be kept as flat as possible to avoid the excessive rise and fall of the highway alignment.
3. The volume of traffic:
The volume of traffic and an increase in the traffic volume per year and peak traffic is analyzed and alignment is finalized by considering all previous data of traffic respectively.
4. Type of Traffic:
If the traffic is of vehicular type and for fast-moving vehicles, it is advised to keep the alignment as straight as possible with fewer curves. Separate lanes for fast-moving vehicles are recommended.
5. Earthwork and backfilling:
Earthwork and backfilling to level the land constitutes huge costs after the land acquisition. The alignment has deviated wherever required to avoid the excessive cutting of earthwork and backfilling.
6. Railway Crossing:
Road alignment should cross a rail lane ideally at 90Degrees.
For fast-moving lanes or national highways or expressways, it is advisable to construct the bridge over rail lane to avoid the traffic jams.
7. Radius of the Horizontal Curve:
Large curves on highways are not desirable. To maintain the comfortable and constant speed on Highways, The radius of the horizontal curve should be less than 230m. To achieve this, the alignment can be changed.
Selection of bridge site involves many factors and is finalized where the river path is the same throughout the year. The road alignment should cut the river at 90Degrees same as Railway lane.
9. Sight Distance:
To avoid accidents, minimum sight distance should always be available for the drivers. The alignment should not obstruct the visibility of drivers, especially during nights.
10. Proper Drainage:
Proper water drainage is provided at the edges of the alignment to avoid water logging during monsoon.
11. Stability of Slopes:
Special care is to be taken for road alignment in hilly areas, the problem with the hilly areas is landslides. The road should be aligned to the side of the hill which is stable. Excessive earthwork cutting and backfilling affect stability.
Straight and even road alignment are possible for flat terrain but it monotonous for driving which may lead to accidents for a sudden curve. It is recommended to keep the slight bend for every few miles to alert the driver.
The initial cost, operating cost and maintenance cost should be minimum for the finalized alignment. Avoid High embankments and deep cutting to reduce the cost of construction.
14. Formation bed:
The alignment should go through the good soil having enough strength, to achieve this a slight change in alignment is acceptable.