FACTORS AFFECTING ROAD ALIGNMENT
1) Availability of construction materials :
For bringing down the costs of construction and maintenance of highways, it is necessary to have easy availability of natural building materials like stones, gravel and sand along the highway.
The road alignment having minimum carting problems of construction materials is generally preferred.
2) Obligatory Points :
The controlling points which govern the highway alignment are known as obligatory points.
They are mainly responsible for the deviation of highway from its straight location.
The obligatory points may be divided into two categories :
i) Points through which the alignment is to pass :
~ Bridge Site
~ Important town or city
~ Mountain Pass
Obligatory points through which the road alignment has to pass may cause the alignment to often deviate from the shortest or easier path.
The road bridge across a river can be located only at place where the river has straight and permanent path and where the bridge abutments and piers can be properly founded.
While aligning road between two stations, it may often be desirable to con nect some of the important intermediate towns, villages or other places.
When it is necessary to cross mountains or high ridges, the various alternatives are to cut a tunnel across or to go round the hills or to deviate until a suitable hill pass is available.
ii) Points through which the alignment should not pass :
The obligatory points which should be avoided while aligning a road are :
~ Religious places like temple, mosques, church, grave, tomb, etc.
~ A lake, pond, water logged area
~ Monumental places
~ Valley etc.
3) Low grades and casy curves :
The highway alignment should be such that low grades and easy curves are obtained and for achieving this purpose, it sometimes becomes necessary to sacrifice the directness of road and to increase its length.
The highway alignment should be such that the quantities of earth work in cutting and filling in longitudinal section as well as transverse section are balanced.
4) Crossings :
The provisions of stream crossing, river crossings and railway crossings along the road alignment should be properly studied.
The availability of a suitable bridge site along the road alignment is very important.
5) Geological Features :
The highway alignment should be made in such a way that no serious difficulties are met from the geological features of the locality such as stability of earthwork, landslide, etc.
6) Proper drainage :
The provisions of proper and sufficient drainage is necessary for the stability of road and hence, the road alignment should take into consideration the factors of subsurface water level, seepage flow and high flood level.
7) Traffic :
The alignment should suit traffic requirements.
Origin and destination study should be carried out in the area and the desire lines be drawn showing the trend of traffic flow.
The new road to be aligned should keep in view the desire lines, traffic flow patterns, and future trends.
8) Land Acquisition :
The availability of adequate land for the construction of highway is necessary.
If the road alignment is such that considerable built up area and cultivated land area are to be acquired, it will prove to be difficult, costly and time consuming.
The highway alignment should be such that least problems of land acquisition are involved.
9) Economic Factor :
The alignment finalised based on the above factors should also be economical.
In working out the economics, the initial cost, the maintenance cost and vehicle operation cost should be taken into account.
10) Miscellaneous :
i) The road alignment should avoid any foreign territory.
ii) The long straight stretch of road in flat area may be given slight bend to break the monotory.
iii) The road should have good visibility under all the climate conditions.
iv) The political and strategic considerations may necessiate the deviation of road alignment.
v) The alignment should not pass through the highly populated area.
vi) Lowlying areas should be avoided in the alignment.
vii) The alignment should not pass through snowy area and dense forest.