Engineering Wonders of the World
Volume I

232 ENGINEERING WONDERS OF THE WORLD. Fig. 2.-DIAGRAM TO SHOW HOW THE CENTRE LINE OF A TUNNEL IS TRANSFERRED FROM THE SURFACE INTO THE WORKINGS. Full lines on surface, dotted lines below ground, are carefully noted. A heavy plumbob, p r, is now suspended on the farther side of the shaft, in line with c a ; and on the nearer side of the shaft, in the same line. The plum- bobs are immersed in buckets of oil or water at the bottom of the shaft to prevent their swaying about. (Fig. 3.) The theodolite is now transferred from a to the bottom of the tunnel, and adjusted until its axis is in line with the plumb wires. As the distance from p to b (a Transferring point on the centre line) on , the surface is known, a point b below , n. , . Ground. in the heading exactly below b is easily found by measure- ment from p r. Over this point the theodolite is set up, and turned till its axis makes with the plumbob line an angle </>, equal to the angle which the tunnel line makes on the plan with the line ca produced, which is vertically over the cross heading line. Points, E E, are then fixed whereby the tunnel headings may- be guided. The exact position and direction of the tunnel line so set out are not finally deter- mined till the processes described have been repeated several times to eliminate errors. Where everything is straightforward, the work, though tedious and very trying, is not difficult. But where calculations have to be transferred down a working shaft situated in an awkward position, such as the mid-river shaft of the Waterloo and City Railway, and where direct measurements by tape are im- possible, the business becomes much more com- plicated, and an engineer has to resort to elaborate trigonometrical triangulation to fix distant points and base-lines from which to make and check his calculations. Fig. 3.—SECTION OF SHAFT AND CROSS HEADING, SHOWING PLUMB-LINES (PR, P1^) AND THEODO- LITE IN SECOND POSITION. GUIDING SHIELDS. While dealing with the mathematical side of the subject, we should anticipate a little and refer to the methods of steering a shield through the ground—first, on the “ straight; ” secondly, round a curve. “ Zero marks ” are made on pieces of wood fixed to each side of the tunnel, at about the level of the horizontal diameter, so placed, that the line joining the two marks is square to the centre line of the tunnel. Two rods, divided into feet and inches, are attached at one end to the sides of the shield, and at the other rest on the pieces of wood. As the rods move with the shield, while the