Once the centreline survey activity is completed and the preliminary route is identified, it can be said in broad terms that the identified route provides opportunity for laying the pipeline. Centerline survey data is also utilised for carrying out the feasibility studies and estimation of project cost for various route alternatives of route survey so that techno-commercial viability for laying the pipeline is established and compared. However, the data obtained from the centreline survey is not sufficient enough to detail design and construct the pipeline. Therefore, detailed route survey is carried out before the start of detail engineering and construction of the pipeline project.
The main objectives of the detailed pipeline route survey are:
- To reconfirm the pipeline alignment on ground.
- Finalize crossing locations and crossing angles.
- To obtain cross-section details and topographic features at all crossings.
- To obtain detailed topographic features within 30 m on either side of centreline.
- To obtain ground levels and/or contours.
- To determine exact pipeline route length with due consideration to ground profile of the route.
- Establish survey monuments on ground along pipeline route.
- Gather details of land use pattern and soil characteristics
Pipeline route and turning points (TPs) along the pipeline route shall be located by the SURVEYOR in consultation with Company/ Engineer-in-charge considering the following:
- Avoid obstructions along the line, by ranging on ground and shifting the Turning Points if need be.
- Terrain gradient to be checked by using hand held inclinometer.
- Ensure proper angle of crossing by keeping as nearly right angle (to road/ canals/ rail/ rivers/ streams, etc.) as possible. The right angle may be laid on ground by using a chain or string lengths in ratio of 3, 4 and 5.
- To check from construction point of view and avoid objects like power, telephone and telegraph poles, walls, tube wells or such other structures falling in the strip of land, 30m on either side of pipeline alignment.
- To maintain minimum distance of 100 m between two consecutive turning points.
- Parallel Alignment: The pipeline alignment wherever runs parallel to an existing or planned under/ over ground facility will be treated as parallel alignment. For underground facilities SURVEYOR shall identify and locate them with suitable pipe locator/ cable locator. The following clearances shall be observed in case of parallel alignment defined above unless specified otherwise:
- Between existing/ planned electrical power cables/ lines and the proposed pipeline – 50.0 metres
- Between existing/ planned communication cables/ lines and the proposed pipeline – 25.0 metres
- Between existing/ planned pipeline and the proposed pipeline – 50.0 metres
- Unless otherwise stated, when the pipeline alignment runs generally parallel to a road or railway it shall be kept sufficiently clear of the Right of Way limits of the facility.
Staking of pipeline route
The pipeline defining trench centre line shall be staked by placing suitably painted marker stakes at Turning Points (TPs) and at Intermediate Points (IPs) between consecutive TPs. All Turning Points (TPs) and intermediate points are referred as Intersection Points. The pipeline centre line shall be staked on the ground as follows:
- First, the Turning Points (TPs) shall be staked on the ground. After locating and marking the TPs, the intermediate points shall be staked while measuring slack distance. The staking shall normally be done at intervals of 500 m along the centre line of the pipeline.
- The Intersection Points (IPs) shall be serially numbered from the starting point. The serial number of each Intersection Point shall be boldly inscribed on the marker stake. In addition, the Turning Point (TP) marker stake shall identify the Turning Point reference number from the starting point.
- For Intermediate Points (IPs) letter "P" shall precede the serial number of the intersection Point marker. For Turning Points, the letters "TP" shall precede the Turning Point reference number, and the letter "P" shall precede the serial number of the Intersection Point.
- The marker stakes at Turning Points (TPs) shall be referred with three reference stakes around the TP. The reference stakes shall carry the Turning Point reference number and their respective distance from the TP marker stake.
- Change in direction of line shall be marked on the TP marker stakes. In addition, direction markers near TPs and other locations shall be placed wherever necessary.
Accuracies in Measurement
SURVEYOR shall incorporate corrections to the linear ground measurement due to standard errors (and variations from standard temperature and/or pull in case of chain survey). The error for angular closure for the work shall not exceed one minute per station and for linear measurements it shall be read to the nearest 0.001 m. The error on closure for measurements on vertical distance to establish bench marks shall not be more that 24√k millimetres (where 'k' is the linear distance in km). The observations for measurement of vertical distances on bench marks shall be read to accuracy of the nearest 5 mm and for Intersection Points and other points along the pipeline route and at crossings to the nearest 10 mm. The error of misclosure in vertical distance shall be distributed linearly.
Azimuth control shall be maintained by observations of the Sun or Polaris at intervals of 15 - 20 km by closing the traverse on existing control points. These observations of bearings together with deflection angles shall be recorded in survey notes. Azimuth misclosure based on bearings with observed angles at Turning Points shall be equally distributed over the number of stations observed in between. Accuracy of azimuth observations shall be acceptable if the three deductions agree within one minute when Sun is used for Azimuth observation. In case of stars/Polaris observation, computed values should agree within 10 to 15 secs. This check is not mandatory when DGPS system is used for establishing location/ co-ordinates.
Chaining shall be continuous in the direction of survey. The true bearing of all straights shall be observed and recorded. Data regarding the nature of terrain, viz. sandy, stony, vegetation, etc. and type of ground shall also be recorded along with chainages of change points. Distance measurement shall be made by Electronic Distance Measurement (EDM) equipment (or by using 50 m steel tape or 30 m chain). However, in case of abrupt slope change the tape/chain shall be used and it shall be straightened parallel to the probable grading. Distance between Intersection Points (IPs) staked along the pipeline route should be measured and recorded. In addition, distance between level points shall also be measured and recorded. Check on distance measurements shall be by Stadia method at the time of levelling.
Measurement of Horizontal Angles
Horizontal angles are measured to indicate the change in direction of alignment and specify the horizontal bend at the Turning Points. Theodolite, reading direct to 10 seconds or better, shall preferably be used. Angles shall be measured clockwise from back station to fore-station. Mean of two readings i.e. one on face left and other on face right shall be taken as the horizontal angle. The line at both ends shall be tied to the grid control system being used for end facilities. True bearing at the beginning, end and at every 15 - 20 km shall be observed to keep a check on errors in angular measurements.
Continuous profile of the proposed pipeline route shall be established from the reduced levels taken.
- at the starting point,
- at all Turning Points (TPs),
- at all Intermediate Points staked on the ground,
- at all points on the pipeline route where there is a change in slope.
All levels shall be with respect to Mean Sea Level (MSL). When the terrain is flat, reduced level shall be additionally recorded along the pipeline route at 100 m interval and when the terrain is undulating, observation of reduced level shall be made at a sufficient number of points so as to give an accurate plotting of the ground profile along the route.
For road and railway crossings
: the reduced levels shall be recorded at an interval of 5 m along the pipeline alignment and at closure intervals wherever there is a change in slope within the entire width of the Right-of-Use of the road/railway.
For river/ stream/ canal crossings
: levels shall be taken at intervals of 5 m up to 30 m beyond the highest banks on both sides. Levels shall be taken at closer intervals, if there is a change in slope. In right-of-use having slope across the pipeline alignment, as encountered in hilly areas, Ghat regions, ravines and other similar areas, cross-sections at 50 m interval and for a length of 30 m on either side of the pipeline alignment shall be observed and recorded.
For major water crossing sites, cross section as above shall be observed at both banks.
Method of RISE and FALL shall be used to compute reduced levels of various points. Check on computation shall be made by using the following formula:
[ Back site ‒ Fore Site ] = ( ∑ Rise ‒ ∑ Fall ) = ( First Reference Level ‒ Last Reference Level )
Maximum misclosure shall not exceed 24√k mm where `k' is the distance in km. Maximum length of line of sight shall not exceed 100 m.
Co-ordinates and Grid
Differential Global Positioning System (DGPS) is used to determine co-ordinates of all intersection points and facilities along pipeline route. Accuracy of co-ordinates shall be within 0.5 metre. Non-differential or autonomous GPS are not accepted.
SURVEYOR shall submit a procedure for set-up & utilization of DGPS. All co-ordinates shall be indicated in UTM grid as well as geographic grid. In case more than one survey party is deployed, each party shall use same reference grid and DGPS system. Following information are required for the DGPS proposed by the Surveyor:
- GPS Receiver Model used
- Differential Receiver Type
- Guaranteed accuracy in metres
Levels and Contours
In addition to the elevation along the centreline of the pipeline route, ground levels within 30 m wide corridor on either side of route alignment shall also be obtained and recorded using Total Station along with the co-ordinates (in UTM grid) of each level point. All such level points shall be plotted in the planimetry portion of the alignment sheets. The levels shall be taken in such a manner that contour can be drawn with a contour interval of 5 m. Electronic data, in a format compatible with commercially available Digital Terrain Modelling (DTM) software shall be produced.
As a minimum following drawings are prepared during detailed engineering survey:
- Detailed Route Map (Scale 1:50,000)
- Additional Route Map for Hilly, Ghat and ravinous regions (Scale 1:15,000)
- Alignment sheets covering
Crossing Details of Road/ Railway/ Stream/ Canal or Utility Crossings (Horizontal scale 1:100 and vertical scale 1:100)
Crossing details for River Crossings of width
- Right-of-Use Planimetry in UTM grid (Scale 1:2500 along the line & 1:2500 across the line)
- Ground Profile (Horizontal scale 1:2500 & vertical scale 1:250)
Cross Section for sloping Right-of-Use (horizontal scale 1:100, vertical scale 1:100)
- upto 250 m width (Horizontal scale 1:200, vertical scale 1:200)
- more than 250 m to 500 m (Horizontal scale 1:500, vertical scale 1:500)
- more than 500 m width (Horizontal scale 1:1000, vertical scale 1:1000)
Crossing drawings are prepared using same horizontal and vertical scales as indicated above. However, in case one crossing cannot be accommodated in single sheet additional sheets may be used.
Pipeline route map shows all features including, but not limited to roads and railroads, canals, streams, lakes, rivers, villages, towns, and cities that are located within a distance of 5 km from the pipeline centre-line on either side of it. For the entire region, contours are plotted on the route map at 20 m contour interval. Additional information like cultivated areas, barren land, areas prone to flooding, rocky areas and forests including access paths/roads to Right-of-Use is also shown on the route maps. Additionally for areas which are undulating such as hilly areas, Ghat regions, ravines, and other areas, Pipeline route map to a scale 1:15,000 is drawn over a distance of 1.0 km from the pipeline centre-line on either side of it. For such areas, contours are plotted at 10 m contour interval.
Right-of-Use Planimetry drawings shows all objects within 30 metres on either side of the Pipeline in plan and spot level & contours at 5m interval.
The angle of crossing is mentioned for all rail, road, river, stream, canal and utility crossings. For rail, road, river, stream and canal crossings wider than 10 m, the distances at the start and at the end of the crossing from the nearest IP is also mentioned. For crossings less than 10 m, the distance of the centre line of crossing from the nearest IP is given.
For all river, stream and nala crossings, the level of water at the time of survey, high flood level recorded in last 100 years and the approximate surface velocity of the flowing stream (observed & recorded during survey) is reported in the survey drawings. Also, the general nature of the surface soil (soft/hard, normal soil or rock/boulders) at the bed and banks of the river/stream/nala is observed and mentioned in the drawings.
In addition to the drawings listed above, detailed survey report is also prepared indicating:
- Details of survey methodology followed
- Details of equipment used
- Brief Route Description
- Salient features of the route such as
Print out of information required to be submitted as data base including:
- Total length of pipeline
- State wise distribution of pipeline route
- Terrain details and land use pattern summary
- No. of crossings (state wise & chainage wise)
- Details of River crossings
- State wise extent of forest/ reserve forest/ environmentally sensitive area
- Significant elevation changes along the pipeline route with chainages
- Nature of soil (sandy, clay, rocky, marshy, etc.)
- Level & contour information for DTM
- Pipeline Profile data
- Turning Point (TP) detail