Engineering Wonders of the World
Volume I

THE TUBE RAILWAYS OF LONDON. 231 means the introduction of some complicated curves. As a matter of fact, the methods adopted require, so far as calculations are concerned, the use of but elementary trigonometry, coupled with the greatest care in taking all measurements—not only underground, where errors can so easily be made in the darkness of the tunnel, but also on the surface, where observations are much hindered by passing vehicles and human beings. The difficulties incurred can be appreciated fully only by those who have had to do the work under such trying conditions. SETTING OUT A TUNNEL. The first task of the engineer is to determine the route to be followed by referring to an ordnance survey map. Then, armed with steel tapes, level, theodolite, and other appa- ratus, he proceeds to run a series of traverse lines over the actual surface, preferably along Fig. 1.-DIAGRAMMATIC PLAN OF TRAVERSE LINES (T T) CARRIED ROUND OBSTACLES (o o). At each angle a disc is let into the surface of the ground, to enable the lines to be found by theodolite when required. the pavements of the streets under which the intended tunnels are to be constructed. Each traverse line extends for such a distance as can be commanded conveniently by observa- tion instruments from one end. At each extremity of the line metal discs are let into the pavement, and marked with a punch in the centre to indicate the points between which measurements are to be made subse- quently, in order to determine the respective lengths of the line. To enable the lines to be plotted on a plan, the angle between each pair is accurately measured with a theodolite set up over the punch-mark on the disc at the point where the two lines intersect. The measurement of distance is done with a 100 feet steel tape. As the length of the tape varies slightly with the temperature of the air, due allow- ance is made for this when recording the readings. Also, since the surface of the pave- ment is more or less uneven, and the discs at the end of each traverse line may be at dif- ferent levels, it is necessary to take the level of the ground at the end of each 100 feet tape measurement, or at any intermediate points where a change in the evenness of the surface occurs, so that the oblique distances measured between the several pairs of discs may be converted by calculation into their true hori- zontal distances. These facts are mentioned to show the need for exactness. A plan, generally drawn on a scale of 30 feet to the inch, is next prepared, showing the traverse lines and all details of pavement, kerbs, building - fronts, etc., The that may be necessary. To pjan this plan are added the posi- tions of the shafts, stations, and tunnels, and particulars of the relations between the traverse and tunnel lines, as the first afford the basis for transferring the second below ground. Then follows the actual transference of the tunnel centre line from the plan to the site of the work itself. This, of course, cannot be done until the shaft, say, at the station site has been sunk to full depth and a cross head- ing has been driven from it to a point on the path of the future tunnel. The operation will be more easily under- stood with the help of Fig. 2. tt is a tra- verse line. To the left of it is a shaft, s, which has been sunk, and a cross heading, h, driven from it. At a convenient point, A, on tt, a disc is let into the surface, and the angles made by t t with AC, a line crossing the shaft,