What is Alignment of Road ?

The position occupied by the center line of a road in plan is called alignment of road. A new road should be aligned very carefully as the cost of construction, maintenance, safety and ease in travel etc. depends much upon the alignment of road.

Once the construction of the road is completed, the alignment of the road cannot be changed. It is difficult to change the alignment after the completion of road construction.

Due to increase in cost of land and construction of costly roadside structures, it difficult to change the road alignment post the construction. Hence road alignment should therefore, be carefully selected and located.

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Requirements of an ldeal Alignment of Road :

The following are the basic requirements of an ideal road alignment.

• Easy
• Short
• Economical
• Safe
• Utility
• Natural aspects

Easy :

The alignment must be easy in construction, maintenance and traffic operations. The alignment should be easy for the operation of vehicles with easy gradients and curves to meet this requirements.

Short :

In between two terminal station the alignment should be as short as possible. Short alignment provide economy in the cost of construction, maintenance and transportation.

The alignment should be as straight as possible to meet this requirement.

Economical :

The alignment should be economical in its cost of construction, maintenance and traffic operations.

Safe :

The alignment should be safe for traffic operation. To fulfill this requirement, the alignment should be safe enough for construction and maintenance from the view point of stability of natural hill slopes, embankment and cut slopes and foundation of embankments.

Utility :

The alignment should offer maximum utility by serving maximum population by connecting intermediate important towns and group of villages.

Natural Aspects :

The alignment should pass through regions of natural beauty and scenery to have good natural aspects.

Factors Affecting Alignment of Road :

Various factors affecting the alignment of the road are as follows :

• Need of traffic
• Purpose and class of road
• Obligatory points
• Curve
• Gradient
• Sight distance
• Number of drainage crossing
• Railway and river crossing
• Obstruction
• Formation bed
• Earthwork
• Availability of labour and material
• Existing right of way
• Lengthy straight route
• Aesthetical aspect.

Need of Traffic :

Alignment should suit need of traffic. For fast moving traffic, it must be as straight as possible. For slow moving traffic, like bullock cart and other it may have sharp bend.

Purpose and Class of Road :

The alignment should be selected according to the purpose and class of road. The national highway connecting two important towns should be kept perfectly straight as far as possible.

On the other hand alignment of other category of road can be deviated when straight alignment is not feasible.

Obligatory Points

The alignment should pass through obligatory points such as intermediate important towns, group of villages and area of commercial, political, military and social importance.

Hence to connect obligatory points alignment may be changed.

Curve :

Curves must be as flat as possible. It may be necessary to make adjustment in the horizontal alignment of roads keeping in view the minimum radius of curve and the transition curves.

Gradient :

While aligning a new road, the gradient should be flat and less than the ruling or design gradient. Thus in order to avoid excessive fall or rise the alignment is to be changed.

Sight Distance :

The minimum sight distance, which should be available in every section of the road, is the safe stopping distance for the fast moving vehicles.

Also there should be enough distance visible ahead for safe overtaking operations of vehicles moving at design speed on the road. Hence the alignment should be finalized in such a way that it should provide good sight distance.

Number of Drainage Crossing :

The alignment should have minimum number of drainage crossing. To have maximum number of drainage crossing alignment must be changed.

Railway and River Crossing :

The alignment should cross river or the railway line at right angles. In case of river crossing point, it should fulfill the essential requirements of a good bridge site.

Obstruction :

Alignment should be free from obstruction. Hence alignment can be changed to avoid well, lake, pond, historical and religious, buildings etc.

Formation Bed :

Alignment should run on good soil as far as possible. Hence to fulfill this requirement alignment can be changed.

Earthwork :

The alignment should have less earthwork. Hence to avoid excessive cutting or filling, the alignment must be changed.

Availability of Labour :

The alignment should provide proximity to labour and material required for the construction of road. Hence to make construction and maintenance of the road economical, alignment of road is changed.

Existing Right of Way :

The alignment should make use existing right-of way. Hence to satisfy the requirement, alignment is changed in order to curtail the cost of land acquisition.

Lengthy Straight Route :

The alignment should not be monotony caused due to lengthy straight routes. Hence alignment must be have slight bend to break the monotony and to keep the driver alert.

Aesthetical Aspect :

The alignment can be changed in order to pass the road through regions of natural beauty and scenery.

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Survey Operations During Fixing Alignment Of Road

The following survey operation should be done with great accuracy during fixing the alignment of the road

(a) The center line of the road is transferred from the map on the ground, by using transit theodolite and fixing the stakes at 50 m intervals in plain terrain and at 20 m intervals in hilly terrain along the center line in the direction of progress of the survey.

All the angles at intersections are to be measured using transit theodolite by using double reversal method.

(b) The angles of intersections at the road crossing with the center line should be measured with the help of a transit theodolite.

(c) The reference points for the center line should be located carefully so that during construction of the road they will not be disturbed. These reference points should be properly describe and well located so that they will be helpful for final drawings.

(d) The distance measurement along the center line of the road including curved portion should be done continuously.

(e) The starting and end points of all the curve namely beginning transition curve, circular curve should be fixed and provided with references.