Structure of the Route location Process

Structure of the Route location Process:- 

In general, the approach to selecting the route for a highway can be described as a hierarchically structured decision-making process.  This very logical approach is easiest described by referring to figure 

_Region Bands (8  to 16 Km) 

Corridors  (3-10 Km) 

Route Trips  (1-1.5) Km 

Alignments  (30-50) m 

Fig: sequential structure of route location process. 

The first step in the process of route location requires fixing the end termini & then defining a region A which will include these terminal points & all conceivable & feasible routes between them. This region is then searched & several broad bands B, C, and D are selected within which it is decided to concentric further research & selections. These bands may be (8-16) Km wide depending upon the nature of the road & terrain. Within these bands, a further search may result in the selecting of corridors E, F, G & H each usually extending 3-10 Km wide or so comparisons between these corridors are made & some are omitted.

The most viable corridors usually not more than two are selected within which further search is carried out & the best route I ( for example) from routes I, J, K, L, etc. is decided.  This route again depending upon the type of road & nature of terrain could be 1-1.5 m wide. The next step then is to search this area &  locate within it one or more different alignments (each alignment will have a strip of about 30-50 m widths) containing minor geometric differences. These alignments are compared a final selection is made & this is used for design purposes. It is thus seen that the process of locating a highway involves continuous searching & selecting using increasingly more detailed information & knowledge at each decision-making stage. 

Factors influencing the selection of route at any instance includes not only such tangible as topography, soil, geology, land use,  population distribution, travel demand, use a cost structure & maintenance cost, safety, etc. but also intangible such as political, social, environmental factors as discussed under the section factors controlling highway alignments. 

Physical Survey/Engineering survey:- 

Before a highway alignment is finalized in highway project the engineering surveys are to be carried out. The surveys may be computed in four stages. The first three stages consider all possible alternatives keeping in view the various requirements of highway alignment.  The fourth one is for the detailed survey of the selected alignment.

The four stages are namely:- 

1. Map study

2. Reconnaissance

3. Preliminary surveys

4. Final locations & detail survey. 

Map Study:

A topographical map of the place where the project has to be done could be available from the department of the survey, GoN. The new maps are available on a 1:25000 to 1: 50000 scale. The main features like rivers, hills, valleys including land use are carefully shown in color. By careful study of such a map, it is possible to have an idea of several possible alternative routes so that their detail may be studied in detail on site. 

 

The probable alignment can be located on the map from the following details available:- 

1. Alignment avoiding valleys, ponds, or lakes. 

2. When the road has to pass a row of hills, the possibility of crossing through mountain passes. 

3. Approximate location of the bridge site. 

     4. When a road is to be connected between two stations, one of the top & other on the foot of the hill, then the alternative route can be  suggested keeping in view the permissible gradient, say ruling gradient. 

Thus from the map study, alternative routes can be suggested. 

Reconnaissance:

a field survey party (group) inspects a fairly broad stretch of land along the proposed alternative routes of the map in the field survey using a simple instrument like Abney level. Some of the details to be collected during reconnaissance are: 

 

1.     Valley, ponds, marshy land, ridge, hills, permanent structures & other obstructions along the route are not available on the map. 

2.     Approximate value of gradient, length of gradient & radius of curves of alternative alignments 

3.     Number & types of cross-drainage structure, maximum flood level & natural ground level. 

4.     Soil type & geological features 

5.     Sources of construction material 

6.     For hill road geological formations, types of rocks, the dip of strata, seepage flow, etc. 

7.     When the area is vast or terrain is difficult reconnaissance may be done by aerial survey. 

As a result of the reconnaissance, a few alternative alignments maybe chosen for further study. 

Preliminary surveys:- 

Objectives: 

1.     To survey the various alternative alignment & collect necessary physical information & details of topography, drainage & soil.

2.     To compare the different proposals because of the requirements of a good alignment. 

3.     To estimate the number of earthworks & construction aspects & workout on the cost of alternatives proposals

4.     To finalized the best alignment from all considerations. 

Methods: (Conventional approach) 

1. Preliminary traverse: primary or secondary traverse following the line recommended by reconnaissance. Length is measured by chaining or tachometry. 

2. Topographical Features: all geological & man-made features along the traverse & certain width on either side is surveyed &  plotted. 

3. Leveling work: leveling work is also carried out to give centerline profiles & typical cross-sections. For contour mapping, cross-section level is taken up to 100 to 200 meters intervals in terai, 50 meters in rolling & 30 meters in the hilly region 

4. Drainage study & hydrological data collected to estimate type, number & approximate size of drainage structures, vertical alignment is also based on that. 

5. Soil survey: it helps in earthworks, slopes, stability of the material, subsoil & surface drainage requirements pavement type &  approximate thickness. 

6. Material survey: a survey of naturally laying materials. 

7. Traffic survey: it is used in deciding the number of traffic lanes, width, pavement design & economic evaluation, O-D surveys is  carried out 

8. Determination of final centerline. 

Final location & detail survey: 

The alignment that is finalized by the preliminary survey is located on the field by establishing a centerline. The next detailed survey is carried out for collecting the information necessary for the preparation of the plan & construction details for the highway project. The data are; 

- Topographical detail -Soil profile -Drainage