Highway Alignment and Surveys

Alignment: The position or layout of the centerline of the high way on the ground is called ‘Alignment’.

Basic requirements of an alignment are

• Short
• Easy
• Safe
• Economical

Factors controlling highway Alignment:

                                                       
Obligatory Points: There are control points through which alignment has to pass and not to pass.

>>The road bridge across a river can be located only at a place where the river has a straight and permanent path, away from bends, where good foundation start is available.

> > Alignment should pass along a hill side pass (if possible) so that a tunnel or heavy cutting is avoided, bridge
Traffic: Origin and destination studies are conducted not preferred and “desire lines” are drawn.
Geometric design: Gradient, Radius, Sight distance etc.,
Economy: Balance in cutting and filling.
Other considerations: Drainage, hydrology, political and topography.

Engineering surveys for highway Alignment:

• Map Study
•  Reconnaissance
• Preliminary survey
• Final location and detailed survey

Map Study: Gives a rough guidance of the routes to be further study in the field. Reconnaissance: To examine general characteristics of the area with a view to select possible alternate alignments. Instruments used are simple in nature like Abney level, Tangent clinometer, Barometer etc
Preliminary Survey : Aim is to finalize the mos jtbeali ment out of various alternative alignments. All modem equipment like Theodolite are used.

The primary survey is involved with the following steps:

* Primary traversing

* Topographical features

* Leveling works

* Drainage studies and hydro logical features

* Data soil survey

* Material survey

* Traffic survey

*Determination of final center line

 Final location and detailed survey:

* Alignment finalized in the design office during primary survey is to be first located on the field

* All detailed topographical, hydro logical and geo technical data are collected. Longitudinal and cross sections are drawn. Detailed soil survey is carried out to know soil profile. Soil sampling is to be done 1.5 to 3.0 m below ground line or finished grade line of the road whichever is lower.

Drawings required:

·         Key Map: Shows the proposed and existing roads and important places to be connected.

·         Index Map: Shows the general topography of the area.

In submersible areas, the formation level of road should be 0.6 m above H.F.L (High Flood Level).

General Form for providing a over bridge or under bridges for a N. H across a rail level:

• If the product of number of gate closures and the intensity of traffic on the highway in tones per day exceeds 50,000.
• When the level crossing is with in shunting limits of a railway station.