Once the necessity of the highway is assessed, the next process is deciding the alignment. The highway alignment can be either horizontal or vertical.
Alignment
The position or the layout of the central line of the highway on the ground is called the alignment. It is an arrangement in a straight line or in correct relative positions. 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 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. A new road should be aligned very carefully as improper alignment would result in increase in one or more of the following
a) Construction cost
b) Maintenance cost
c) Vehicle operation cost
d) Accident rate
Requirements
The basic requirements of an ideal alignment between two terminal stations are that it should be:
a) Short
b) Easy
c) Safe and
d) Economical
a) Short
It is desirable to have a short (or shortest) alignment between two terminal stations. A straight alignment would be the shortest, though there may be several practical considerations which would cause deviations from the shortest path.
b) Easy
The alignment should be such that it is easy to construct and maintain the road with minimum problems. Also, the alignment should be easy for the operation of vehicles with easy gradients and curves.
c) Safe
The alignment should be safe enough for construction and maintenance from the view point of stability of natural hill slopes, embankment and cut slopes and foundation of embankments. Also, it should be safe for the traffic operation with safe geometric features.
d) Economical
The road alignment could be considered economical only if the total life cycle cost considering the initial cost, maintenance cost and vehicle operation cost is lowest.
Factors Controlling Alignment
It is not always possible to satisfy all these requirements mentioned above. Hence we have to make a judicial choice considering all the factors. For an alignment to be shortest, it should be straight between the two terminal stations. This is not always possible due to various practical difficulties such as intermediate obstructions and topography. A shortest route may have very steep gradients and hence not easy for vehicle operation. Similarly, there may be construction and maintenance problems along a route which may otherwise be short and easy. Roads are often deviated from the shortest route in order to cater for intermediate places of importance or obligatory points.
A road which is economical with lowest initial construction cost, need not necessarily be the most economical in maintenance or in vehicle operation cost. It may also happen that the shortest and easiest route for vehicle operation may work out to be the costliest of the different alternatives from construction view point. Thus, it may be seen that an alignment can seldom fulfil all the requirements simultaneously; hence a judicial choice is made 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. Obligatory points through which the road alignment has to pass are generally due to the topographic and other site conditions including natural obstructions. Some of the examples of this category include location of a mountain pass, suitable location of bridge to cross a river, presence of quarry or an intermediate town to be connected. These obligatory points necessitate deviation of the road alignment from the straight alignment with shortest or easiest path. Some of the examples are:
i) 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.
ii) 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.
iii) 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.
There are obligatory points through which the road should not pass and these locations may make it necessary to deviate from the proposed shortest alignment. The obligatory points which should be avoided while aligning a road include religious places, very costly structures, unsuitable land etc. Religious places like temple, mosque, church, grave or tomb have been protected by the law from being acquired for any purpose. Acquiring costly structures would mean heavy compensation resulting in increased cost. Marshy, peaty and water logged areas are generally unsuitable for road construction and should be avoided as far as possible. If a marshy land with peaty soil falls on the path of a straight alignment, it may be necessary to deviate the road alignment from the straight path and go around the unsuitable land or pond. The other alternative method is to resort to very expensive construction techniques. The points through which the alignment should not pass are given below.
i) Religious Places
These have been protected by the law from being acquired for any purpose. Therefore, these points should be avoided while aligning.
ii) 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.
iii) Lakes or Ponds etc.
The presence of a lake or pond on the alignment path would also necessitate deviation of the alignment.
2) Traffic
The road alignment should be decided based on the requirements of road traffic. 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, anticipated traffic flow, classified traffic volume, their growth and future trends.
3) Geometric Design
Geometric design factors such as gradient, radius of curve and sight distances also would govern the final alignment of the highway. If straight alignment is aimed at, often it may be necessary to provide very steep gradients. As far as possible while aligning a new road, the gradient should be flat and less than the ruling or design gradient. Thus, it may be necessary to change the alignment considering the design speed, maximum allowable super elevation and coefficient of lateral friction. It may be necessary to make adjustment in the horizontal alignment of roads keeping in view the minimum radius of curve and the transition curves. The absolute minimum sight distance, which should invariably be made available in every section of the road, is the safe stopping distance for the fast moving vehicles. Also, there should be enough distance visible ahead for safe overtaking manoeuvres of vehicles moving at design speed on the road. Hence the alignment should be finalized in such a way that the obstructions to visibility do not cause restrictions to the sight distance requirements.
4) Economics
The alignment finalized based on the above factors should also be economical. While working out the economics, the factors to be considered are,
i) initial construction cost of the road,
ii) regular and periodic maintenance cost of the road and
iii) vehicle operation cost in future years.
While trying to decrease the initial construction cost, either the future road maintenance cost or vehicle operation cost or both of these may increase considerably. Therefore, while carrying out economic analysis, it is essential to work out overall economics based on life cycle cost of the road project and not consider the initial cost of the road project only.
5) Other Considerations
Various other factors which may govern either the horizontal or vertical alignment of the road are drainage considerations, hydrological factors, political considerations and monotony. The vertical alignment is often guided by drainage considerations. The sub-surface water level, seepage flow and high flood level are the factors to be kept in view, while deciding the highway alignment.
Types of Alignment
1) Horizontal Alignment
Horizontal alignment in road design consists of straight sections of road, known as tangents, connected by circular horizontal curves. It is the design of the road in the horizontal plane. It consists of a series of tangents (straight lines), circular curves and transition curves. It should provide safe travel at a uniform design speed.
2) Vertical Alignment
Vertical alignment is the longitudinal section and it consists of straight grades joined by vertical curves. Vertical alignment specifies the elevations of points along the roadway. Once the road is aligned and constructed, it is not easy to change the alignment due to increase in cost of adjoining land and construction of costly structures by the road side, as the land value increases manifolds once the road is opened to traffic.
Principles of Highway Alignment
The alignment of a highway is a three-dimensional problem measured in x, y, and z coordinates. This is illustrated, from a driver’s perspective, in Fig.1. However, in highway design practice, three-dimensional design computations are cumbersome and the more important thing is the actual implementation and construction. As a consequence, the three-dimensional highway alignment problem is reduced to two two-dimensional alignment problems, as illustrated in Fig. 2. One of the alignment problems in this figure corresponds roughly to x and z coordinates and is referred to as horizontal alignment. The other corresponds to highway length (measured along some constant elevation) and y coordinates (elevation) and is referred to as vertical alignment. The horizontal alignment of a highway is referred to as the plan view, which is roughly equivalent to the perspective of an aerial photo of the highway. The vertical alignment is represented in a profile view, which gives the elevation of all points measured along the length of the highway.
Special Considerations while Aligning Roads on Hilly Areas
During alignment of hill roads, special care should be taken on the following points which pertain to the hill roads.
i) Stability of Hill Side Slopes
While aligning hill roads, special care should be taken to align the road along the side of the hill which is stable. A common problem in hill roads is that of landslides. The cutting and filling of earth to construct roads on hill-side causes steepening of existing slopes and this affect its stability of the hill slopes.
ii) Drainage of Surface and Subsurface Water Flowing from the Hill Side
Numerous hill-side drains should be provided for adequate drainage facility across the road. But the cross-drainage structure being costly, attempts should be made to align the road in such a way that the number of very expensive cross drainage structures is kept minimum.
iii) Special Geometric Standards for Hill Roads
Different sets of geometric design standards are followed on hill roads with reference to gradient, curves and speed, and they consequently influence the sight distance, radius of curve and other related features. The route should enable the ruling gradient to be attained in most of the length, minimizing steep gradients, hair pin bands and needless rise and fall.
iv) Resisting Length
The resisting length of a road may be calculated from the total work to be done to move the loads along the route taking the horizontal length, the actual difference in levels between the two stations and the sum of ineffective rise and fall in excess of floating gradient. In brief, the resisting length of the alignment should have kept as low as possible. Thus, the ineffective rise and excessive fall should be kept minimum.
0 comments:
Post a Comment