SURVEYS FOR TRACK ALIGNMENT
In a railway project from beginning till its construction, four different surveys in stages are required to be conducted. The following survey should be carefully conducted to fix a best possible alignment :
- Traffic survey
- Reconnaissance survey
- Preliminary survey or Survey for initial location
- Detailed survey or Survey for final location
Before the actual work of survey starts, study of available maps of the area should be made. This desk study helps in drawing the various suitable alignments on the available maps in the office to facilitate easy reconnaissance. For this purpose, generally, the survey maps of India or Aerial maps are used. These maps show the contours, topographical features and physical land utilization like buildings, cultivation, rives, etc.
So, tentative alignments are fixed and drawn in the office which further help in conducting various surveys in a systematic manner. The contour maps are very much helpful to the engineer for fixing gradients within the ruling gradients by providing the suitable length of line within two contours. This study of maps prior to actual survey work is known as map study or paper location.
The object of traffic survey is to make accurate determination of the potential of available traffic. This is essential for determining the viability of a new line proposed either a branch line or completely a new system, to assure reasonable return on the large investments likely to be involved. Traffic at present and future potential traffic, both passenger and goods traffic, should be considered.
This traffic can further be local as well as of through type. Therefore, the engineer should survey certain features, like the position of traffic generation sources which would furnish control points for general location of the line; the volume of traffic for selecting type of track (B.G./M.G./N.G.) construction which can justify revenues. The design should include sufficient provision for estimated growth of traffic.
For conducting the traffic survey,
These factors should be considered :
- Census of population with reference to prosperity of the people and the locality, their density and distribution.
- General resources of production which include agricultural and industrial goods. The nature and weight of different commodities, likely to be transported as a result of a new line, should be studied. The origin and destination of such commodities should be correctly surveyed and assessed.
- The general character of lands and people of communities must be considered. A rich fertile land is expected to bring a large volume of agricultural products. Mineral and forest wealth also assures the goods traffic from these sources but this traffic will diminish when their natural resources have been exhausted in due course of time. Rural centres have a slow and steady traffic but it is seasonal and fluctuates with crop and market conditions.
- A single prominent feature may be responsible for building a new line or construction of additional kilometrage as an entry of new line into places with flourishing industries, newly discovered or opened mines or oil fields, etc. should be collected which can attract the traffic.
- The general information of fairs, recreation centres, business, religious festivals, etc., should be collected which can attract the traffic.
- It is important to study the traffic history of existing tracks or road transport facilities available or likely to develop, and how much traffic they are likely to attract.
More factors should be considered :
- The position of traffic, its nature and potential, which would be served by a new line, should be finally estimated.
- The nature and volume of exports and imports, with centres of their original destination, should be studied.
- Future possibilities of the development of trade centres, industries and agriculture should be investigated.
- The station facilities should be properly located with the track alignment as the proper station location will get more business. In addition to above factors, the following practical considerations should also be taken into account.
- Changes in pricing of products, i.e., freight rates may seriously affect the traffic movements. The purchasers may change in habits and routing.
- Relocation of industries and populations, due to changing technologies, depletion of raw materials, changes in consumer habits or for purposes of national defense, may change entirely the volume of traffic moving over certain track routes.
Field surveys are carried out to collect the information on above factors and following important maps are prepared for the study –
- Topographical Maps – Showing all physical features viz. land, buildings, rivers, bridges, tunnels, existing routes, contours, etc.
- Agricultural Maps – Showing only the land, fertility value and agricultural products.
- Industrial Maps – These maps show the location of industries, their nature and further development plans.
RECONNAISSANCE SURVEY (Recce)
This is a rough and rapid inspection, both visual and instrumental, of various physical characteristics of the area to determine the suitability of different alignments marked on the available map during map study or paper location. So in this type of survey, the suitable two or three alignments are selected for further preliminary survey.
The objectives of reconnaissance survey are as follows:
- Acquire the knowledge of physical features of the country like the rivers, valleys, cultivated lands, forests, hills, existing roads, canals, etc., for selecting the proper position of alignment.
- Collect geological information regarding the following points. (a) Nature of soil, (b) Surface formation of the ground (c) Dip of the existing rocks, and (d) Hill slopes.
- Collect the information regarding availability of constructional materials, labour and sources of water as permanent facilities for the proposed alternative alignments.
- Have an idea about possible alternative alignments.
- Have an idea of rivers and streams which may cross the proposed alignments for determining suitable bridge sites and their bridging requirements.
- Locate various control points or obligatory points for getting an idea from where the alignment should pass and from where the alignment should not pass.
- Decide the maximum gradient and curvature for proposed alignment.
- Prepare rough estimates for different proposed alignments to know most economical, safe and efficient alignment.
An engineer should bear in mind the following factors in reconnaissance survey :
- The reconnaissance survey should be done for the whole area influencing the railway project, particularly for wide belt on either side of the general direction of alignment rather than for a line only.
- All the possible alignments marked on the map, during map study, should be examined and improvements made if necessary.
- All the intermediate points should be very carefully fixed so as to attract maximum traffic and hence more revenue; less construction problems and hence economy in construction. To achieve this, the following points should be considered :
(a) Rivers should be crossed at right angles and at those places where approaches are sound and approach banks are not very high.
(b) Mountain passes should be so located that they could be reached without steep gradients or deep cuttings.
(c) A tunnel may also be proposed if very economical.
(d) The station site should be located within 2 km from the existing town or village and at level stretch of land.
4. The various elements governing the alignment such as gradient, curvature, rise and fall, traffic, etc. already discussed should be thoroughly studied.
The following instruments are used for conducting the reconnaissance survey:
- A Prismatic Compass – This is used to obtain the magnetic bearings of the proposed routes in different directions.
- An Abney Level (or Hand Level or Clinometer) – This is used to determine the gradients or slope angles of the ground upto an accuracy of 10 minutes.
- A Barometer and a Thermometer – This combination is used for accurate determination of altitudes and temperatures of the various points.
- A Pair of Powerful Binoculars or a Telescope – These are used for inspecting the distant physical features to get an idea of the topography of the country.
- A Pedometer – This is used to have rough idea of the distance.
At the end of reconnaissance survey, a map with 6 m to 15 m contour intervals showing all the physical features and two or three suitable alignments is prepared. Out of these alignments, best one is selected further on the basis of preliminary survey.
PRELIMINARY SURVEY (Initial Location Survey)
The object of this survey is to determine the details of different alternative routes as found and drawn in reconnaissance survey and at the same time economies of different routes are studied.
This preliminary survey should be conducted with great precision as the selection of final alignment is based on this survey.
Working of preliminary survey is as follows :
- First of all, a traverse survey in a belt of about 100 to 150 m width on either side of the centre line, with a transit instrument is done.
- A techometer is used for plotting the main features ; a chain, prismatic compass and levelling instrument are used for fixing details.
- Finally drawings and details are prepared for each alignment with respect to following information :
(a) Length of alternative routes,
(b) Various possible gradients, along the alignment.
(c) Quantity of earthwork.
(d) Maximum heights and lengths of embankments and cuttings.
(e) Characteristic of rivers to be crossed such as H.F.L., L.W.L., flow direction, stream profiles, cost of boring encountered and difficulties in alignment.
(f) Geological information like soil type, rocks and slips, if any.
(g) Details of existing structures viz. bridges, tunnels or culverts.
(h) Details of canal crossing, if any.
(i) In case of level crossing with road, the angle of crossing, longitudinal sections of road for 300 m on either side of crossing and traffic intensity on road.
(j) Availability of materials, labour, drinking water facilities etc.
(k) Details of various affected properties with details of owners to help in acquisition of land.
(l) The climatic conditions of the routes traversed.
4. In the end, a comparative study of various results is made with relative merits and demerits. The route, which is most economical and best from all considerations of an ideal alignment, is selected and plotted on the map.
Various instruments used in preliminary survey are as follows:
- Theodolite – for traversing.
- A Tacheometer – for plotting main features.
- A Dumpy level – for drawing the longitudinal sections and cross-sections.
- Plane table – for plotting interior details.
- A prismatic compass – for magnetic bearings of routes and main points.
The choice of survey instruments will largely depend upon the character of country. In recent times various electronic survey instruments like Total Survey Station are used for collecting all types of survey data. This survey plates can be processed by use of computer soft wares for preparation of various design drawings.
DETAILED SURVEY (Final Location Survey)
The object of final location survey is to transfer or refix the final location of alignment from paper to the ground, in order to carry out the ground survey of this alignment in detail. Before getting the sanction of a railway project, a detailed survey is necessary. Moreover, this survey gives all the required data to the construction engineer such as levels, bench marks, reference pillars, physical features, their measurements, etc. for construction of the track.
The survey work is carried out as below :
- The centre line of the alignment is marked on the ground by means of fixing about 15 cm long pegs at every 30 metres interval and about 60 cm stout pegs at every 300 metres chainage. All the points, where a change in direction occurs, tangent points of the curve and also the intersection points, are distinctly marked on the ground. Masonry pillars are built around gaps at the tangents points and at around 1500 m chainage. A number of bench marks are established at chainage less than 800 metres to ease the process of checking of levels and to provide gradients. The levels are also marked on the pillars surrounding the pegs, known as reference pillars.
- The centre line of waterways of culverts, starting and ending points of bridges, the centre line of tunnel, and position of station buildings, station yards, signal cabins, etc., should be clearly marked, on the ground.
- The demarcating lines of the alignment, according to land width, site of stations, yards etc., should also be marked.
- Levelling is done along the alignment with the help of precise levels at 30 m, and cross-section are taken at 90 m interval but at closer intervals in case of steep slopes. Longitudinal and cross levelling is done to ascertain the final gradients of alignment.
- The magnetic bearings of each tangent should be taken at every curve in case of flat-country terrain.
6. The following data is collected for the detailed survey:
- On major bridges, cross-sections are taken at bridge-site and at 0.8 km to 1.6 km upstream and down-stream of bridge-site. In case of minor bridges, cross-sections are taken at bridge-site and at 90 to 150 m upstream and down-stream of bridge site.
- The longitudinal sections of the bed of the stream or water course are taken upto the two extremes of cross-sections on up stream and down stream of water course. The levels, H.F.L. and L.W.L., are shown on the longitudinal sections taken.
- The record of floods in the past i.e. flood history data should be collected and a drawing for river-banks along with the contours on it be prepared.
- The current velocities and depth of scour of the river should be measured.
- The discharge of the river should be determined with the help of velocity of flow and catchment area (either by inspection or by survey map) for that locality.
- Sub-soil boring is carried out along the alignment. The number of bore-holes depend upon the length of bridge. For large bridges, boring at every pillar position and at banks is carried to a depth more than the depth of foundation. In case of small bridges, three borings one at centre and two borings, one at each bank, are carried out to a depth of 15 metres. The data of boring is useful during the construction and maintenance.
- The history of river along with its characteristics such as nature, character, frequency of floods, duration of flood etc. should be recorded.
- The properties of materials, at bed and banks of the river, should be recorded, to assess their erosition characteristics.
7. After collecting the data, the following details should be worked out before starting construction :
(a) Total discharge to be considered, factors affecting discharge, various formulae used and variations in their result.
(b) Calculation and fixation of the waterway.
(c) The type of foundations recommended to be used with reasons and depth of foundations.
(d) Length of bridge and span arrangement adopted with reasons.
8. For detailed survey following instruments are used :
(a) A Theodolite
(b) A Precise level or Dumpy level.
(c) A steel type etc.
In recent days Total Survey Station, an electronic instrument is used for survey work, the data of which can be processed using computer softwares for preparation of construction drawings.
SURVEY DRAWINGS AND PROJECT REPORTS
After conducting all types of surveys, the following construction drawings should be ready to start actual construction work:
(i) General overall map of that area of the country which has been traversed. This is prepared on scale : 1 cm = 20 km. (i.e. 1:20,000,000)
(ii) Index map. scale : 1 cm = 2.5 km. (i.e. 1:2,50,000)
(iii) Index plan and section. Scales : 1 cm = 640 m (Horizontal)
1 cm = 12.5 m (Vertical)
(iv) Detailed plan along the alignment. This consists of longitudinal section and plan.
Scale : 1 cm = 50 m. (Horizontal) (i.e. 1:5000)
1 cm = 5 m. (Vertical) (i.e. 1:500)
(v) Contoured plans and longitudinal sections of all bridges with span 9 m or more to a suitable Scale.
(vi) Plans of station yards : Scale 1 cm = 50 m.(i.e. 1:5000)
(vii) Detailed drawings of buildings and bridges : Scale 1 cm = 1 m. (i.e. (1 : 100)
(viii) Plans of level crossings or grade separated crossings. Scales : 1 cm = 50 m. (i.e. 1:5000)
(ix) Details of all the important features, such as bridges, rivers, roads, canals, air bases, etc. lying within 300 m on either side of the centre line, should be supplied, and drawings should be prepared to a suitable scale.
At the end of the railway project, the engineer must prepare the project report under these headings :
- Introduction. Historical and geographical background.
- Alignment Elements. Gauge, obligatory points, gradients, curves, length and levels of different points.
- Alignment. Basic requirements and factors to be considered for good alignment.
- Alternate Routes for Alignment. Their merits and demerits with economic considerations.
- Proposed Alignment. Detailed survey, preparation of drawings and collection of information, etc.
- Construction. Standard of construction, depending upon type and potential of traffic, locomotive performance and economic considerations, must be described and discussed.
- Conclusions and Recommendations.
- Estimation of Railway Project. Finally an estimate of project, considering all capital investments and working expenses, should be prepared.
The state-of-art of surveying in railway projects is very important. Money spent in survey-work is non-recoverable and every effort should, therefore, be made to conduct survey work as precisely as possible. The four important types of surveys include Traffic Survey, Reconnaissance Survey. Preliminary (or Initial location) survey and detailed (or final location) survey. The instruments used in surveying should be very accurate. Based on the above surveys detailed maps are prepared and information collected is presented in form of drawings and a project report.