The position or the layout of the central line of the highway on the ground is called the alignment. Horizontal alignment includes straight and curved paths.
The position or the layout of the central line of the highway on the ground is called the alignment. Horizontal alignment includes straight and curved paths. Vertical alignment includes level and gradients. Alignment decision is important because a bad alignment will enhance the construction, maintenance and vehicle operating costs. Once an alignment is fixed and constructed, it is not easy to change it due to increase in cost of adjoining land and construction of costly structures by the roadside.
The requirements of an ideal alignment are
The alignment between two terminal stations should be short and as far as possible be straight, but due to some practical considerations deviations may be needed.
The alignment should be easy to construct and maintain. It should be easy for the operation of vehicles. So to the maximum extend easy gradients and curves should be provided.
It should be safe both from the construction and operating point of view especially at slopes, embankments, and cutting. It should have safe geometric features.
The alignment should be economical and it can be considered so only when the initial cost, maintenance cost, and operating cost are minimum.
Factors controlling alignment
We have seen the requirements of an alignment. But it is not always possible to satisfy all these requirements. Hence we have to make a judicial choice considering all the factors.
The various factors that control the alignment are as follows:
1.Obligatory points: These are the control points governing the highway alignment. These points are classified into two categories. Points through which it should
pass and points through which it should not pass. Some of the examples are:
o Bridge site: The bridge can be located only where the river has straight and permanent path and also where the abutment and pier can be strongly founded. The road approach to the bridge should not be curved and skew crossing should be avoided as possible. Thus to locate a bridge the highway alignment may be changed.
o Mountain: While the alignment passes through a mountain, the various alternatives are to either construct a tunnel or to go round the hills. The suitability
of the alternative depends on factors like topography, site conditions and construction and operation cost.
o Intermediate town: The alignment may be slightly deviated to connect an intermediate town or village nearby.
These were some of the obligatory points through which the alignment should pass. Coming to the second category, that is the points through which the alignment should not pass are:
Religious places: These have been protected by the law from being acquired for any purpose. Therefore, these points should be avoided while aligning.
Very costly structures: Acquiring such structures means heavy compensation
which would result in an increase in initial cost. So the alignment may be deviated not to pass through that point.
Lakes/ponds etc: The presence of a lake or pond on the alignment path would also necessitate deviation of the alignment.
2. Traffic: The alignment should suit the traffic requirements. Based on the origin- destination data of the area, the desire lines should be drawn. The new alignment should be drawn keeping in view the desire lines, traffic flow pattern etc.
3. Geometric design: Geometric design factors such as gradient, radius of curve, sight distance etc. also govern the alignment of the highway. To keep the radius of curve minimum, it may be required to change the alignment. The alignments should be finalized such that the obstructions to visibility do not restrict the minimum requirements of sight distance. The design standards vary with the class of road and the terrain and accordingly the highway should be aligned.
Economy: The alignment finalized should be economical. All the three costs i.e. construction, maintenance, and operating cost should be minimum. The construction cost can be decreased much if it is possible to maintain a balance between cutting and filling. Also try to avoid very high embankments and very deep cuttings as the construction cost will be very higher in these cases.