Road alignment

• The position of the layout of the centre line of the highway on the ground is called the alignment.
• There are two types of alignment:

1. Horizontal Alignment
2. Vertical Alignment

• Improper alignment of road would result in one or more of the following disadvantages:

1. Increase in construction cost
2. Increase in maintenance cost
3. Increase in vehicle operation cost
4. Increase in accident rates

 

Factors controlling alignment

• Obligatory points
• Availability of construction materials
• Traffic
• Crossing
• Economic Factors
• Geological Features
• Other Considerations
• Land Acquisition

 

Obligatory Points

• The controlling points which govern the highway alignment are known as obligatory points.
• It is divided into two parts:

1. Points through which the alignment should pass.
2. Points through which alignment should not pass.

 

Availability of construction materials

• For bringing down the cost of construction and maintenance cost it is important to have easy availability of natural building material.

 

Traffic

• Should suit traffic requirements.
• Origin and destination studies should be carried out and desire line should be drawn.
• The new rods aligned should keep in view the desire line, traffic patterns and future trends.

 

Crossing

• The provision of stream crossing, river crossing and railway crossing along the road alignment should be properly studied.

 

Economic Factor

• The alignment finalized based on all factors should also be economical.
• The initial cost, the maintenance cost, vehicle operating cost should be taken into account.

 

Geological Factors

• Alignment should be made in such a way that no serious difficulties are met from the geological point of view of the locality, e.g. earthwork, landslides.

 

Land Acquisition

• Availability of land for the construction of highway is necessary.
• If alignment should be such that considerable built up area and cultivated land area are to be acquired, it will prove to be difficult, costly and time consuming.

 

Engineering surveys for the road alignment

• The surveys carried out should be-

1. Map study
2. Reconnaissance
3. Preliminary survey
4. Detailed survey

 

Base Map Preparation

• Shows man made and natural features like rivers, hills, etc and contour lines at 15 or 30m interval.
• Shows possible alignments with obligatory points minimum number of cross drainage structures.
• By careful study of the map it is possible to have alternative routs so that further details of these may be studied later at the site.

 

Reconnaissance Survey

• Map updating- to confirm features indicated on map.
• Checking for:

1. Number of cross drainage structures.
2. High flood level (HFL).
3. Confirming length and value of gradient to IRC standards.
4. Soil Characteristics.
5. Geographical Features.

• Prepare a report on merits and demerits and profile map of scale 1:50,000.

 

Preliminary Survey

• The main objective of the survey are:

1. To survey the alternative alignment proposed after the reconnaissance and to collect all the necessary physical information and details of topography, drainage and soil.
2. To compare the different proposals in the view of the requirements of the good alignment.
3. To estimate the quantities of earthwork and other construction aspect and to workout the cost of alternative proposals.
4. To finalize the best alignment from all consideration.

 

Detailed Survey

• Ground Survey
• Soil Survey
• Hydrological Survey
• Period of construction
• Present and future traffic
• Structure design
• Construction methods
• Total cost of construction
• Return on investment