Once the necessity of the highway assessed, the next process is deciding the alignment. The highway alignment can either horizontal or vertical and they described in detail in the following sections.
The position or the layout of the central line of the highway on the ground 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 cost. Once an alignment is xed 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 short and as far as possible be straight, but due to some practical considerations deviations may 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 economical and it can considered so only when the initial cost, maintenance cost, and operating cost is 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:
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.
The bridge can located only where the river has straight and permanent path and also where the abutment and pier can strongly founded. The road approach to the bridge should not curved and skew crossing should avoided as possible. Thus to locate a bridge the highway alignment may changed.
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.
The alignment may 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:
These have been protected by the law from being acquired for any purpose. Therefore, these points should 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 deviated not to pass through that point.
The presence of a lake or pond on the alignment path would also necessitate deviation of the alignment.
The alignment should suit the traffic requirements. Based on the origin-destination data of the area, the desire lines should drawn. The new alignment should drawn keeping in view the desire lines, traffic flow pattern etc.
Geometric design factors such as gradient, radius of curve, sight distance etc. also governs the alignment of the highway. To keep the radius of curve minimum, it may required to change the alignment of the highway.
The alignments should 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 aligned.
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