Tunnel: Tunnel is an entire underground structure which enables a facility e.g. highway, motorway, railway, etc to continue its travel across an obstacle whether natural or man-made e.g. mountain, hill, water (sea, river, canal, lake, etc), residential, industrial or commercial. A tunnel is entirely enclosed / buried in the ground with its all the entries / exits open to the open-atmosphere. A tunnel follows the general survey-alignment of the facility whom it is carrying underground across the obstacle.
Why the Tunnels are constructed: (i) To provide the shortest route through an obstacle.
(ii) To provide the rapid or timely provision of facilities.
(iii) To reduce the steep gradients especially while climbing up and then climbing down rapidly i.e. in short distance.
(iv) To avoid the expensive maintenance cost in open cut area in sliding lands, sinking or unstable grounds, snow-drifting grounds, ponding, marshy, soaked, etc areas.
(v) To avoid the expensive acquisition of valuable commercial land.
(vi) To avoid the damage of built urban facilities, roads, pavements, etc.
(vii) When the depth of ground cutting exceeds 20 m.
(viii) And when the ground rises or falls abruptly for a considerable distance afterward, that it is impossible or very difficult to provide the safe drivable gradient in the road, railway, etc.
Imporatant Steps included in Tunnel Surveying: (i) Surface-Surveying usually and mainly by Traversing Method: Open-Traverse Surveying.
(ii) Transferring the Surface-Alignment i.e. direction i.e. bearing underground.
(iii) Transferring the BMs and Levels from the ground surface underground.
Surface Works includes for Tunnel Surveying: (i) Preliminary survey of 3 to 5 km along the alignment of the tunnel and about 1 km perpendicular to the longer dimension of the tunnel (this auxiliary surveying depends upon the nature and life of the project and type of topography and geology of the area).
(ii) Preparing a Plan Contour-Map with a scale of 1” with contours drawn after every 5 m interval.
(iii) Final alignment of the tunnel is assessed, evaluated and selected from the said plan.
(iv) Detailed Geological information, data and mapping is carried out as the tunnelling progress and cost of the tunnelling chiefly depend upon the nature of the ground material (including the water) encountered.
Following important information are considered to select the ROUTE of a Tunnel Surveying:
(i) Alignment of the Centre-line of the tunnel; alignment includes the bending i.e. curving pattern if a bend / curve is needed to be provided. Tunnels are designed best to be the straight in their course.
Curve Surveying for a Tunnel: A Flat or Easy, Wholly Transition Curve is designed and a maximum Deflection-Angle of 30o is provided, in the route of the tunnel.
(ii) Gradient to be adopted. A tunnel must have the zero gradient at the curving route.
(iii) Determination / designing of the exact length of the route of the tunnel.
(iv) Establishing the permanent BMs and Levels along the tunnelling route.
Transferring the Surface-Alignment underground, mainly known as “Transfer of “Horizontal Control”: It is the most difficult and important procedure:
(i) A big shaft (vertical tunnel) and a large hollow underground chamber are excavated / built first of all at a suitable location (where it is easy and practicable to do so) located on the Centre-line in the course of the tunnel.
(ii) Then the two, preferably four (if the economy / budget of the project permits) vertical or slightly inclined from the vertical, columns are fixed on the ground surface on either sides around the shaft (width of the shaft and that of the underground chamber must be {at least 10 m} wider than the designed width of the tunnel so as to manage the alignment and excavation of the two extremities i.e. side-walls of that tunnel). The said columns should be fixed in such a manner that they must represent the two marginal lines of the formation layer of road or railway formed on the ground surface.
(iii) Two (preferably four) wire-lines are then vertically suspended freely from the said columns (over the pulleys) into the shaft and chamber at the two lateral extremities. Heavy Plumb-bobs weighing at least 10 kg (so as to keep the wire-lines well taut) are suspended from the lower end of the wire-lines. Lower end of a said plumb-bob has the shape of big expanded spherical surface immersed in a container of thick oil so as to eliminate any oscillation.
(iv) Alignment of the said wire-lines suspending in the chamber in fact give the alignment of the surface margins and hence shifting it down into the chamber i.e. underground
v) Then the equipment and tapes / chains and other equipment and accessories are shifted from the surface into the chamber (an elevator or skip is provided supported from the Sinking Head-frame mounted over the shaft-mouth).
(vi) This alignment is marked permanently on the surface as well as underground.
Transferring the Levels / Elevations underground, mainly known as “Transfer of “Vertical Control”: Levels are transferred into the tunnel i.e. into the chamber from the nearest BM located on the ground surface.
(i) One fine wire is stretched horizontally across the mouth of the shaft (at surface) and same one wire inside the chamber just under and parallel to the top wire.
(ii) Another (third) wire is suspended vertically and freely into the chamber, having the above-mentioned plumb-bob at the lower end of the vertically suspended wire.
(iii) The wire which is suspending vertically into the chamber is suspended in such a manner that it must touch both the horizontally stretched wires at the two points at the same time.
(iv) Elevation of the surface wire stretched horizontally across the mouth of the shaft is found out from the elevation of the nearby BM. So the level of the said surface wire is made known.
(v) Vertically suspended wire is then marked (with paint) at the two points where it touches the surface wire and the underground wire (stretched inside the chamber). Then the said vertically suspended wire is pulled out and distance between the said two marked points is measured. This distance is deducted from the elevation of the surface-wire; it gives the elevation of the wire stretched underground and hence establishes an underground elevation known as “Level BM”.
Transferring the Levels / Elevations underground: (vi) A Centre-line (of the tunnel) is always run (centre-line is found out by measuring the accurate half way between the two demarcated marginal lines marked inside the tunnel; centre-line is marked on the ceiling of the tunnel), as the advancement of the tunnelling i.e. cutting and excavation work, is primarily carried out in the centre, then spread side-ward (BMs and Levels are fitted along the side walls of the tunnel).
(vii) Survey-lines are proceeded by Fore-sighting and Back-sighting operations. Main methods used to advance the tunnel surveying are: “Measurement of Direct Angles” and “Measurement of Deflection Angles”.
In the bending / curving tunnels, the average chord (straight line) length of the horizontal curve is 100 to 150 m.
Currently, the survey-lines are proceeded using the laser-alignment equipment.
(viii) Vertical Shafts are built (and same-mentioned procedure is carried out) after about every 1 km of length, at least after every 4 km length.
(ix) US Tolerance (permissible deviation from the designed, specified or standard value) Range for the advancement of the tunnel’s centre is 3 to 8 inches per 10 km in length of a tunnel.
Problem of staying the monuments of BMs strongly and stably in expansive or unstable soil. In this case, these monuments are anchored well in the bed-rock