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between them as small as required. One of these instruments is screwed to a plank bracket close behind each plumb line, and so placed that the further can be seen through the telescope just above the other. When the slits are adjusted exactly to the plumb line wires the latter are removed and lights are placed behind the slits. Thus two fixed illuminated lines are substituted for the wires. The results of the alignment (see page 303) showed great accuracy with the use of this apparatus.
White Reflectors to Verniers.-American theodolites for ordinary field use generally have a piece of white celluloid or ivory fixed above the vernier to reflect the light on to it. This will be found most useful for underground work especially. Pieces
Handle of Lamp
Fig. 199.-Sighting Lamp.
of white glazed notepaper will do if nothing else is at hand. If the light is not vertically over the vernier, errors of reading will be caused, without these shades.
Alignment of the Totley Tunnel.-See page 286.
Alignment of the Mersey Tunnel (Fig. 200). The distance across the river was about of a mile. The base line obtainable between the plumb lines down each shaft was 6 ft. On the Liverpool side the shaft was on the centre line, but large warehouses and other buildings intervened between A and B. On the Birkenhead side the shaft was off the centre line as shown. It was therefore necessary to measure DC and DCA on the surface, transfer the line down below, range out DC, measure DC, and set off DCA down below.
All the alignment was done with a 6 in. Cooke theodolite reading to 20". When the headings met near the centre of the river
the alignment was found.
Alignment of the Mersey Tunnel.
Alignment of Dunmail Rise Tunnel on Thirlmere Aqueduct * (Fig. 201). The base line in the shaft was 6 ft. 9 in. between the plumb line wires. The length driven from the shaft to point of junction was 1,518 yds. or 675 times the base. From the south end nearly a mile -1,715 yds.—was driven, then the angle at A was turned, then nearly another
mile was driven to point of junction. To fix the position of A, 1,715 yds. had to be measured on the surface, which fell 250 ft. in this distance. This measurement was made
with rods 10 ft. long, set horizontal with a hand level, and a plumb rule similar to a mason's, specially made for the purpose.
An error of in. in the base was sufficient to account for an error of 8 in. at the point of junction. The error at the point of junction was 8 in. in line and 24 in. in level. The total length of this tunnel was nearly 3 miles, the section being 8 ft. 6 in. x 7 ft.
Examples of Errors in Alignment of Long Tunnels. -The following shows the results of the alignment in some of the longest tunnels constructed.
Minutes Proceedings Inst. C. E., vol. cxvi.
Name of Tunnel.
Croton Aqueduct, New York
Shaft 8 feet Dia
Point of Junction
45 feet Dia 66 f' deep
Error at Junction.
Levels. The longitudinal section of the surface must be levelled from end to end of the tunnel and bench marks fixed at
Fig. 201.—Alignment of Dunmail Rise Tunnel, Thirlmere Aqueduct.
convenient points close to each shaft. The levels of the bench marks must be very carefully checked before proceeding to transfer the levels underground.
Transferring Levels down Shafts. To get the levels down each shaft the best plan is to accurately determine the level of a point marked on one of the slides or guides in which the cage travels, by levelling from the bench mark established at that shaft. Another point is now marked on the guide near the bottom of the shaft and the distance between these two marks is then measured with a steel tape. When the depth of the shaft exceeds 100 ft., so that this distance cannot be measured at one operation, it must be
* Minutes Proceedings Inst. C. E., vol. cxvi.
measured in tape lengths by descending in the cage 66 ft. or 100 ft. at a time, temporary platforms being fixed in the shaft to enable the assistant to hold the end of the tape. When wire rope guides are used for the cage it becomes necessary to make the measurements on the side of the shaft itself, in which case care must be taken that the tape is held vertical at each operation. The whole measurement must be carefully repeated and the results should practically coincide if the measurements are made with sufficient
Underground Bench Marks.-Having thus obtained the level of the mark at the bottom of the shaft, the staff is held on it, the level being set up near the bottom of the shaft, and a permanent bench mark is established.
Fig. 202. Underground Bench Mark.
Iron wedges or nails driven into the side of the tunnel at a convenient height form good bench marks, and should be marked by a broad arrow in white or red paint so as to be easily found. When the centre line is fixed by pegs surrounded by brick piers as described on page 298, these may also be used as bench marks. In the case of iron-lined tunnels the flanges of the segments form convenient points for the bench marks, as shown in Fig. 202.
Checking Underground Levels and Centre Line through between Shafts.-As the tunnelling proceeds new bench marks are conveniently fixed near to the working face of the tunnel from time to time, and the levels are thus carried forward until the faces of the headings meet, when the levels may be checked through from each shaft and the error ascertained. As headings are usually driven forward a certain distance ahead of each face, any small discrepancy between the levels from each shaft may be adjusted by putting in a "junction gradient" between the already executed portions of the tunnel. This also applies to the centre line, any error in which is rectified by putting in an extremely flat curve, or when the error is inconsiderable by putting in a short straight to join up the two lines.
Setting Ribs and Profiles. In the case of brick-lined tunnels when there is an invert and side walls there are profiles
erected to guide the bricklayers in laying the brickwork. These have to be carefully set to the correct level by the engineer, and are adjusted to his direction by the foreman in charge. The operation of setting these profiles simply consists in levelling from the nearest bench mark and directing them to be raised or lowered until they are at the correct level, but it requires considerable care, as any mistake leads to very serious results. In the case of a new "break up," as the commencement of the widening out of the heading at any point to full tunnel section is called, a mistake is specially liable to be made as there is no adjacent finished work to check from. In setting profiles a level should always be taken on the corresponding part of the last executed portion of tunnel as a check.
When the tunnel is in good rock, invert and side walls are sometimes dispensed with, and in that case it is the ribs of the centring of the arch which are to be set. The ribs are supported on wedges by means of which they are raised or lowered to the desired level. Both springings of the ribs are to be levelled, and the level of the crown of the rib should also be taken.
As regards setting ribs and profiles to correct line, the centre line should be ranged out with the theodolite and points marked on the centre line opposite each rib or profile to be set. The latter are then adjusted to line by measuring the proper half width from the marks with a steel tape. After each alteration of the rib or profile for level it should be tested for line and vice versa.
Setting Segments of Iron-lined Tunnels. In the case of iron-lined tunnels each ring is bolted in succession to the last executed portion, and is therefore to a certain extent fixed by what has gone before. Any adjustment required is made by packing in between the rings with wood packing. Thus if the last ring is slightly too low or too high, or an alteration in the gradient occurs, the rectification is effected by packing in the bottom or in the top. Similarly if the last ring is slightly off the centre line, or if there is a curve in the centre line, the adjustment is effected by packing in the left side or in the right side as the case may be.
In the case of iron-lined tunnels, therefore, the engineer may test the last executed ring of the tunnel for centre line and level, and order packings accordingly. As regards testing for centre line, the best plan is to string a line across the tunnel, fasten