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CHAPTER VII.

tunnel alignment and setting out.

Surface Alignment.-The centre line of a tunnel must first of all be carefully ranged out on the surface from end to end. In any case for at least each portion between contiguous shafts a connected line must be ranged out on the surface extending across both shafts, before driving can be begun. In tunnelling through very high snow-clad mountain ranges it is often impossible for obvious reasons to range out the line of the tunnel on the surface directly along the centre line to be followed. In such cases the shafts or extremities of the tunnel must be connected by a triangulation or a traverse which must be conducted with the greatest possible care and checked to ensure accuracy. When the line is of great length it is not possible to measure the lengths of the lines of a traverse with the required accuracy, and in that case a triangulation is made from a carefully measured base. The alignment of the great Alpine tunnels was effected by triangulations (see page 307).

Surface Alignment over Hills. When it is possible to range out the line directly along the centre line and over hills or mountains of moderate height, very long sights of 2 or 3 miles may sometimes be necessary. In this case the ordinary small theodolite will not suffice, and a more powerful instrument must be used. The line is first of all ranged out as nearly as possible with the theodolite, and two points in this line are selected as fixed points for the final alignment. Observatories are then erected at suitable places where the slope of the surface changes, and where they will command a clear view of the shafts. A transit instrument is placed in these observatories, and by means of it the line is finally ranged out, the two fixed points on the theodolite line being taken as the base. These observatories are simply brick, concrete, or masonry piers carefully built so that the transit instrument when placed in its bearings on the piers

may be accurately adjusted to the centre line. In the transit instrument (see Figs. 187, 188) the vertical arc and horizontal limbs are dispensed with, and the telescope is levelled by means of a striding level placed across its horizontal axis.

Example of Surface Alignment over Hills: Alignment of the Totley Tunnel. The following description of the setting out of the Totley Tunnel on the Dore and Chinley Railway* is a good example of surface alignment over hills. The Totley Tunnel is the longest tunnel in the United Kingdom with the exception of the Severn Tunnel.

"The Alignment Aboveground.-The greatest precautions were taken to secure the accurate setting out of the centre line. As the longitudinal section, Fig. 185, Plate XII., shows, the profile was favourable to this work, distinct changes in the surface taking place at convenient distances, and high ground beyond each extremity of the tunnel accommodated terminal stations which could be seen from the summit observatory; there was no need to reverse the transit instrument except at that point. The line having been fixed with as much accuracy as could be obtained with a 6 in. theodolite, brick observatories were built at the extreme stations (Bradway and Sir William), and at each end of the changes of the ground surface over the tunnel. In addition to these, an observatory (No. 3 west) was also built beyond the entrance at Padley, at a level to command the heading on the 1 in 1,000 gradient; and a station was fixed at the foot of the hill beyond (No. 4 west), to enable these two points to be seen from within the heading whenever necessary.

"The observatories (Fig. 186) were built hollow of brickwork in cement and capped with stone. A large flat cast-iron plate, having a hole 6 in. wide in the centre, was let into the cap and run with cement. Upon this the transit instrument rested. A brass scale, 1 in. wide, divided into inches and twentieths of an inch, was fixed across this central hole in the plate, and a plumb line from the centre of the instrument could thus be let down through the hole in the plate to touch the side of the scale. The transit instrument was of the fixed type, with a 3 in. object

* Vol. cxvi. Minutes Proceedings Inst. C. E. Mr Percy Rickard, M. Inst. C.E., "Tunnels on the Dore and Chinley Railway." Given by permission of the Council of the Institution of Civil Engineers.

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