Proper surveying is required for construction of new railway line between two places. Several stages are involved to propose suitable alignment for new railway line.
Stages in Surveying for New Railway Line
Various steps involved in railway line surveying are as follows.
- Validation of New railway line construction
- Marking of tentative alignments
- Reconnaissance survey
- Selection of Good alignments
- Preliminary survey of alignment
- Final alignment survey
- Final survey report
Viability of New Railway Line Construction
When there is a demand for new railway line, the first and foremost point is to justify the viability of railway line by taking some important points into consideration. Those are
- The total number of people living in the area across which a railway line is proposed are taken into record.
- The population habits and living standards, economic conditions are observed to make sure that the department will earn revenue by this line.
- To record the number of bridges, culverts, tunnels etc. a topographical map of that area should be studied.
- To study the alignment and gradients, contour map of that area should be studied.
- To connect all the industries by railway line, industrial map of that area should be studied.
- New railway line laid should not affect the valuable land in agriculture so, agricultural map should also be studied.
- Presence of agricultural, natural, industrial resources should be recorded.
- Presence of religious places, business centers, markets etc. should be noted.
- An estimate should be made on amount of revenue that may accumulate from passengers, goods, etc.
Marking of Tentative Alignments
Once, justification is shown to construct a new railway line then the next step is marking of different alignments which are likely to support the new railway line. But when marking is done there are some considerations should be followed as follows.
- The alignment marked should be as short as possible.
- The alignment should pass through less valuable lands and it should not pass through high value lands like through center of villages or cities, valuable structures etc.
- The alignment should be as straight as possible and number of curves should be minimized.
- The alignment should be marked on ridge lines to eliminate the earth filling cost. So, if there is any valley or depressions then it is better to avoid them. If it is necessary to mark across them, then proper attention is taken on this area in reconnaissance survey.
- The alignment should not pass through religious places, temples, churches, mosques, burial ground etc.
- The alignment should cross the river perpendicularly.
Among the marked alignments, only one alignment is finally needed. So, reconnaissance survey is performed in which so much data is collected from every alignment. The information obtained is
- Using prismatic compass find out the magnetic bearings of lines of alignments. And find the distances by stepping. Both measurements should be recorded in the field book.
- The surrounding area of alignment about 100 meter radius is studied thoroughly and locate the positions of different objects.
- Presence of water table level along the alignment is determined using boring.
- Soil survey should be done along the alignment.
- Note down the number of bridges, culverts etc. and their details along the alignment.
- Note down the number of crossing points like roads, rivers, etc. along the alignment.
- Note down the number of curves present along the alignment.
- Determine the slope of ground along alignment using abney level.
- Construction material availability and its transportation way should be observed.
- Availability of labor and places for them to live, needs etc. should be observed.
- If Depressions and high summits are unavoidable then, note down the details of those.
- Note down the past recorded values of rainfall, discharge by the rivers etc. by considering past 10 year period.
- If the alignment is passed through a private property or land or structure, then record its details along with owner details. Then the owner will receive compensation from the government.
- An index map is prepared for all the alignments with details like no. of curves, bridges, approximate longitudinal section etc. which will help to select the good alignment in the next stage.
Selection of Good Alignments
When reconnaissance survey is completed for all the marked tentative alignments, some good alignments are selected in this stage. Its selection is done based upon the following points.
- A good alignment is always short and economical.
- A good alignment can generate considerable amount of revenue.
- A good alignment consists minimum number of bridges or culverts in its way.
- Earth filling or earth excavation is as minimum as possible along good alignment.
- Alignments consisting vertical curves should be eliminated.
- Construction material and labor availability is as near as possible.
- A good alignment should connect all the important and rush areas in that region.
- Location of station yards should be easily reachable for Passengers.
- Cost of construction and expected revenue should be estimated along the good alignment.
Preliminary Survey of Alignment
After selecting good alignments, then it’s time to decide only one alignment finally. But before this final step, preliminary survey is conducted for all the good alignments. The preliminary survey consists:
- A pillar is constructed at the starting point of alignment and this point is connected to nearby GTS benchmark using fly leveling.
- Longitudinal leveling with an interval of 20 or 40 meter is carried out along the alignment.
- Cross leveling with an interval of 100 meter is carried out.
- Magnetic bearing of each line of traverse is noted in level book.
- A route survey map is prepared which give the details of 100 m land on both sides of alignment. This can be done by plane table surveying or prismatic compass surveying.
- At every 2km or with regular interval permanent bench marks are established.
- Bearing capacity of soil and water table level is recorded.
- A map is prepared for the marked station yards using plane table surveying.
- At river crossings, the details of river like its cross section, width, water level, HFL, scour depth etc. are noted for a distance up to 1000 meters on the both sides crossing with an interval of 100 m.
- The river bed is bored to find out the depth of foundation required.
- With all the recorded readings, a drawing is prepared for the whole alignment.
- For the drawings, an approximate estimate sheet is prepared for each alignment. Which includes earth works, compensations, cost of culverts, bridge structures etc.
Final Survey Alignment
After completion of preliminary survey, final alignment among all good alignments is selected. This is purely based on the economy and final alignment is economical than the others. But before its approval, final location survey is conducted for one final time which involves in the following steps.
- Masonry pillars are constructed along the center line of alignment with an interval of 1000m. The pillar position can define the width required for railway track. In between these pegs are provided with an interval of 30m.
- Station yards are marked at required points.
- Level crossings, culverts etc. are marked.
- Bridge provision places are marked with these pillars.
- Intersection points and tangent points of curve are marked.
- Compensation of properties is estimated for the final time and marked those places.
Final Survey Report
When final survey alignment is completed then a report is prepared for the final alignment and it is submitted for approval. The report includes
- Project introduction
- Necessity of project
- Justification of final alignment
- Details of final alignment like its length, area, number of bridges, culverts, crossings etc.
- Estimation sheet
- Specification details
- Revenue expected
- Recommendation of project
Along with the report some maps with suitable scale are to be attached and they are
- Map of the area through which alignment will pass
- Route survey map
- Longitudinal section of alignment
- Cross section of alignment
- Map of station yards
- Drawings of culverts, bridges, tunnels etc. to be constructed along the alignment.
- Drawings of station buildings, yards etc.