Because of the continuous moving of the wires, the fixing of the points on the roof of the tunnel is very troublesome, and the operation should be repeated by different men at different times before the points are permanently fixed.
Fig. 5.—Method of Transferring the Center Line down Side Shafts.
When the shaft is placed at one side of the tunnel the pillars or bench marks are placed normal to the center line on the edges of the shaft as shown by [Fig. 5]. Between the points A and B a wire is stretched, and from it two plumb bobs are suspended, as described in the preceding case; these plumb bobs establish a vertical plane normal to the axis of the tunnel. The excavation of the side tunnel is carried along the line BW until it intersects the line of the main tunnel, whose center line is determined by measuring off underground a distance equal to the distance BO on the surface. By setting the instrument over the underground point O, and turning off a right angle from the line BO, the center line of the tunnel is extended into the headings.
Curvilinear Tunnels.
—There are various methods of locating the center lines of curvilinear tunnels, but the method of tangent offsets is the one most commonly employed.
At the beginning the excavation is conducted as closely as may be to the line of the curve, and as soon as it has progressed far enough the tangent AT, [Fig. 6], is ranged out. At B a point is located over which to set the instrument, and the distance AB is measured for the purpose of finding the ordinate of the right angle triangle OAB. Now OA = r, AB = d, and φ = angle ABO. Then: Tang. φ = rd.
Fig. 6.—Method of Laying Out the Center Line of Curvilinear Tunnels.
Doubling the value of φ and making the angle ABC = 2 φ, the line BC will be fixed and the point C located by taking AB = BC. On BC the ordinates are laid off to locate the curve. Prolong CB so that CD = CB. Then the portion of the curve CF is symmetrical with CE, and the ordinates used to locate EC may be employed to locate CF, by laying them off in the reverse order.