ALIGNMENT OF TUNNEL

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ALIGNMENT OF TUNNEL

The work contain the alignment of tunnel of centre line on surface and tranfer it to tunnel and good levelling work on surface and inside the tunnel can be achieved.

1)First a preliminary  location survey is done followed by very precise resurvey of line

2)As tunnel excavation is carried out from each end face as also from several intermediate shaft faces,minute accuracy in survey of centre line is essential

3)When tunnel is short,ordinary engineer transit,properly handled gives close and satisfactory results

4)When tunnel is very long it is very necessary to have a tunnel transit and is fitted with striding level by which transverse axis can be made truly horizontal

5)Levelling work is carried on in the standard way except for fact,that where steep slopes are encountered extra care are taken

6) Horizontal distance are measured by 

1. Stepping
2. Inclined sights

Transfering centre line into the tunnel for alignment

This is  a work  needing the utmost care and accuracy in execution as otherwise the centre line worked from opposite faces will not meet and deviations in centre line will cause overbreaking and additional expenditure especially when the material tunneled through happens to be hard rock occur due to Alignment of tunnel

Initially plumb bob weighing 22 lbs  are lowered into tunnel shafts from the surface from opposite faces of shaft to transfer two points from centre line on the surface vertically down .The plumb bob are suspended by piano wires passing over the grooves cut on pipe fixed across the mouth of shaft.The wire of each plumb bob is wound over a reel and released to required length to bottom of shaft .Once the bottom is reached,these plumb bobs are replaced by a pair of extra heavy ones made of angle iron to keep wires steady.The line joining these two piano wires is fixed and extended by a theodolite placed in shaft  on to a point on roof of heading and indicated on spuds or dog fixed to the roof .A corresponding point is similarly located on the roof of opposite face.The center line is then marked on floor of the shaft with reference to roof points and is extended trough the tunnel.

Transfer of tunnel grade for Alignment of tunnel

1. The heading contain short tangents tangents to the curved line like ab,bc,cd etc.
2. To locate the centre line offsets from these tangents are set off
3. When heading has proceeded upto x y align points like n on the tangents.Set off the calculation offsets n o to intersect the centre line at o on the heading 
4. Centre line has to be transferred thus after each blasting of heading face

Alignment of Tunnel

INITIAL SURVEY FOR SETTING TUNNEL WORK

1. Preliminary setting of tunnel on topographical survey of Indian maps.

2. Marking tunnel obligatory points.

3. Driving lines between obligatory points.

4. Marking points with concrete pillars.

Investigation at time of construction

1. Heading : It is a part of tunnel c/s excavated for small length. It may be top, bottom and side.

2. Driving Drift : Excavated for entire length of tunnel.

• Heading give information at the time of construction while drift give complete information prior to construction of tunnel.
• When length of tunnel is small, center line located by means of theodolite.
• When the length of tunnel is sufficient, triangulation is used for locating center lines.

Tunnel alignment and grade 

1. 0.2% grade for proper drainage.
2. Straight alignment should be provided.
3. Side drain have slope of 1 in 500.
4. ventilation more effective if grade is in one direction.
5. In underground compound leveling BS & FS must be made same.
6. Grade in tunnel should be less than surface grade.
7. For tunnel greater than 300m length grade provided below 75% of ruling gradient.
8. In order to maintain desire shape of tunnel c/s is check at interval of 2m to 3m.