Tuesday, September 3, 2013

Engineering Surveys and the location of highway alignment

Location of the highway alignment is done after carrying out survey of the area, these surveys are called Engineering Surveys. 

We have to locate an alignment which fulfill the basic requirements like the path must be short, safe, economic, easy and useful. To check all these basic requirements we can carry out the

Engineering Surveys in the following phases:
  1.  Map Study
  2.  Reconnaissance Surveys
  3.  Preliminary Surveys
  4. Location Surveys
  •  Map Study: 
This is the first step of the Engineering survey, using a topographic map of the area under consideration, which can be availed from the Survey of India, we can propose different alternatives of the road alignment. 
This topographic map in general have a contour interval of around 30 m to 40 m. 

We can get the details of the natural and artificial features of the area using the topographic map, and accordingly we can suggest a numbers of alternatives for the road alignment. These routes are further studied in the Reconnaissance survey.
  •  Reconnaissance Survey: So in the second phase/step a survey team is headed to the area under study with the minor surveying instruments like Abney level, Tangent Clinometer etc. to do a rough survey of the area under study. 
The rough survey is done along the alternatives proposed in the map study and feasibility of the road alignment is checked along the different routes. 
Some of the routes may be cancelled out or they may be changed if they appear to impossible in this study. So finally they will have a set of routes which are to be further studies in the next step.
  • Preliminary Survey: In this step the alternative routes which are proposed after a rough survey in the second step are surveyed in details using some advanced instruments like levels, chain and theodolite. Aerial Photogrammetry is best suited for this type of survey.                                                                
      All the necessary details to carry out the comparative study of the different routes are collected and then finally we have to decide one alignment best suited for the alignment of the road. 
Here various details are found out along the stretches of the routes, which can also be found using the aerial techniques by taking photographs along the routes and then further processed to find out the final details of the area.                         
Different kind of surveys like, Soil investigations, cross sectioning and profiling, marine surveying, hydrology data collection, obligatory points, industries and  population surveys are necessary along the routes and only then it is possible to have a fair comparison of the different routes. 

So finally one among all of them is chose and drawings are prepared on the sheet which will show its alignment to be shifted on to the ground.

  • Location Survey: 
In this fourth phase of the Engineering Survey for the highway location, we have a drawing of the alignment and we have to go through the further two processes:
   (a) Location      (b) Detailed Survey

  •  Location:- Location of the center line of the road is done with very much precise instrument like Theodolite and Chain using the drawing prepared or the details gathered in the third step(i.e. Preliminary survey). This is done by staking the ground with the stakes inserted at the intervals of 50 m to 100 m in the plain area, 50 m to 75 m in the rolling terrain and 30 m to 50 m in the hills and steep terrain.                                                                                  
Pegs may driven at all the control points. At the curves control points, starting of the transition curve, starting of the circular curve and terminal of the circular curve and the terminal of the transition curve the pegs/stakes are driven into the ground to firmly locate these control points. Bench marks are located at and interval of 250 m and they are necessarily located at the sites of the cross drainage works.                                                        
  •   Detailed Survey: In this part we have carry out the detailed study of the final route using some very precise instruments like Theodolite and Chain to gather all the necessary data for the final estimation, design and preparing drawings using which the construction can be started. A detailed project report is to be prepared and all the necessary data is collected to prepare that report.
 So, the profiling, cross-sectioning and soil investigation are carried out very precisely. CBR values are also found to find out the design thickness of the pavement. After collecting the data a final drawing a report is prepared which concludes the highway planning part. 
Relevant books:

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