2.4 Engineering surveys for highway location or alignment

  

A. Map Study

The topographical map is studied to find out the possible routes of the road. The following information are obtained from the map study: 

1. Alignment avoiding valleys, ponds, and lakes.
2. When the road has to cross a row of hills, a mountain pass may be a suitable alternative.
3. Approximate location of the bridge site.
4. Need of connecting important villages and towns.
5. a Suitable number of cross drainage works.

B. Reconnaissance Survey

A reconnaissance survey is done to examine the general character of the area for the purpose of determining the most feasible routes for further more detailed investigation. Simple Survey Instruments are used in the reconnaissance procedure such as a compass, Abney level/altimeter, pedometer, etc.  Following are the information obtained from the reconnaissance survey: 

1. Valley, pond, lakes and other features that were not present in the topographical map.
2. A number of cross drainage structures, High Flood Level (HFL), Natural Ground Level (NGL).
3. Values of the gradient, the length of gradients and the radius of the circular curve.
4. Soil type along the routes from field identification tests and observation of the geological features.
5. Sources of construction materials.

C. Preliminary Survey 

 In this step, the alternative routes which are proposed after a reconnaissance are surveyed in detail using some advanced instruments like a level machine, chain/tape, theodolite, etc. The objectives of the Preliminary Survey are listed below: 

1. To collect necessary physical information and details of topography, drainage, and soil.
2. To compare different proposals in view of the requirement of good alignment.
3. To estimate the quantity of earthwork.
4. To finalize the best alignment.

Two Methods of Preliminary Survey: 

i)Conventional Method: 

The procedure for the conventional approach are listed below: 

1. Traverse: The traverse is run from the starting point to the endpoint by setting out various control points. Both primary traverse and secondary traverse may need to be run.
2. Levelling work: The levelling work is carried out along the centre line or the proposed road. The levelling work is used to estimate the volume of the earthwork. Both L-section and X-section are carried out.
3. Topographical features: All geographical and man-made features are surveyed and plotted which are along the traverse and for a certain width on either side.
4. Drainage Studies and Hydrological data: The number of cross-drainage structures is estimated during the preliminary survey.
5. Soil Survey: The soil survey is conducted in working out details of earthwork, slope, and stability of materials, subsoil and surface drainage requirements and the type of pavement requirements.
6. Material Survey: The location of construction materials need to be known.
7. Traffic Survey: Survey regarding the number of lanes, roadway width, and pavement design need to be done.
8. Determination of final centre line: After completion of all the above mention steps and calculating the amount of earthwork, the final centre line is determined.

ii)Modern rapid approach: 

The procedure of the Modern rapid approach are listed below as: 

1. Taking aerial photographs with required lateral and longitudinal overlaps.
2. These photographs are then examined under stereoscopes and control points are selected for the establishment of the traverse.
3. The spot levels and contour lines may be obtained from the stereo–pair observations.
4. Photo interpretation method is used to grab information on the geological features, soil conditions, drainage requirements, etc.

D. Final Location and Detailed Survey

Location: 

The centre line of the road which is finalized in the preliminary survey is then located in the field by establishing the centre line. Major and minor control points are then established on the ground and the central pegs are driven, checking the geometric design criteria. If necessary, the modification of the final location can be altered. 

Detailed Survey: 

1. Temporary Bench Marks (TBM) are fixed at all underpass structures and drainage structures.
2. Levels along the final centre line should be taken with great importance as these data are required for vertical alignment, earthwork calculation, and drainage details.
3. A detailed survey is carried out to enable the drawing of the soil profile up to the depth of 1.5-3m below the Ground Line and twice the height of the finished embankment in the case of the high embankment.
4. The data during the detailed survey should be elaborated on and completed for the preparation of the plans, designing, and estimation of the project.