Monday, September 2, 2013

Basic Requirements of a Highway alignment on plain and hill roads


There are some basic requirements of the highway alignment in the plain and hill roads which must be fulfilled. In general the basic requirements are:

(1) Short: The alignment must be the shortest of the various alternatives available. Of course the shortest path between any two points is a straight line but the topography of the area or other factors may necessitate it do divert and take some other route, but as far as possible it should be kept minimum.
(2) Easy: Alignment should be such that the road must be easy to construct and easy to maintain or repair. If curves are of large radius and the gradient is gentle it would be easy to construct the road, rather than opposite.

(3) Safety: Safety is again the basic requirement of the highway alignment and special care must be taken to align the road in such a way that it must have the safe or minimum Sight distances and Radius of the curves, means the geometrical design features like Sight Distance, Radius of the curves and the gradient of the road must be given special attention.

(4) Economical: Road alignment must be designed to have the initial cost of construction, maintenance cost and the vehicle operation cost to a minimum. Also the locally available materials should be checked  before and it may decrease the over all cost. There must be a balance in the cutting and filling on the alignment of the road.

Some other basic requirements specially on the hill roads:
Hill roads have some other basic requirements also which govern the alignment of the hill roads:

(1) Drainage: Drainage of the road must be kept in mind and it must be insured that enough drainage structures can be built on the route. As far as possible alignment must avoid the drainage works means it must have the minimum numbers of the drainage works.

(2) Economy: Economy is governed by the numbers of the drainage works, cutting filling and the gradient.

(3) Safety: Safety is governed by the sight distance, superelevation and the design radius of the curves. It must be kept in mind that gradient must be kept below the ruling gradient. In hill roads special attention must be given to the side slopes, and thorough geological surveys must be carried out to ensure the safety while construction as well as while traffic movement.

(4) Minimum resisting length: The unnecessary rise and fall of the gradient must be minimized to reduce the cost and length of road.


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