Kccessity for a line of communication (highway, rail way, or canal) is interrupted by a barrier of rock, and where au open cut is impracticable because of the amount of material to be removed, and also where a detour or displacement of the line of communication is undesira ble, recourse ninst be had to the tunnel. The first step to be taken in work- of this,kind is to make a geognostic examination of the rock.-forma tion to be penetrated, to ascertain its character—whether it be hard, soft, friable, gravelly, or unstable (that is, containing quick.sand)—in order from these data to determine the most suitable form for the proposed structure, the manner in which the work may most advantageously be done, and the probable length of time that will be required to complete it. Tunnelling, however, will rarely be found expedient until the depth of the cut exceeds 6o feet; and when intended for railway use, it is important that the course of the structitre should approximate as closely as possible to a straight line, to lessen the liability to accidents. Tunnel-work in rock of such a nature that it will not serve to sustain itself must be artificially supported by a lining of masonry.
line or axis of the tunnel having been established by survey upon the surface, the work is commonly carried on by driving a gallery through the hill or mountain as rapidly as possible from the two ends, the work. being facilitated, where the situation permits, by the sink ing of intermediate shafts at convenient points in the axis of the tunnel and driving headings in both directions toward the terminal galleries. By this means the lumber of points of attack is intiltiplied, and the time re quired for the completion of the structure is lessened correspondingly.
Grade and Ventilation. —With respect to grading the practice is not uniform, the plan being sometimes to grade up from both ends, and at other times to give the tnnnel an inclination in one direction. The pur pose, of course, is to facilitate veptilation and drainage. Except in the rare cases where foul air is met with, no special provision for ventilation will be found necessary, either during constrnetion or afterward, unless the length of the tunnel is considerable—according to Trantwine, about too° feet. But when this length is exceeded, such provision must be made while construction is going on, either by means of shafts or by the use of mechanical appliances for forcing air throng-h pipes to the working head ings. The substitution of compressed air in place of steam as the motive
power for actuating the drilliug-machines employed in modern engineering practice has to a large extent obviated the necessity of using special means. for supplying air, the air-compressors at the entrances serving the double purpose. Except in the case of very long tunnels, it is usually unneces sary to provide artificial means of ventilation after construction, although the shafts sunk to expedite construction are not infrequently utilized for this purpose. Tunnels with a single inclination give less trouble on this score than those which grade up from the ends. To avoid the fouling of the air of long tunnels by the locomotives, they are commonly provided with special appliances for condensing steam and smoke during the time of passage.
Mode of usual mode of excavation consists in driv ing a passage-way or heading (considerably smaller than the cross-section contemplated for the finished structure), and in maintaining this some dis tance in advance. This heading is then enlarged to full size. The mode of procedure varies according to the nature of the material to be penetrated. The current practice is indicated by Trautwine as follows: In rock the most convenient plan is to run the heading just below the top of the tun nel, which allows the workmen to drill holes conveniently in the floor for the blasting which must be resorted to in removing the rock (II 27,fig. 5). In earth, on the other hand, it is preferable to drive the headin,g along the bottom, as this affords the greatest convenience for enlarging the opening to full tunnel-size by loosening the soil and letting it fall. Again, where the tunnel is being driven through earth or other yielding materials, it is necessary carefully to protect the roof and sides of the beading and of the enlarp,ed tunnel-section as the work progresses and before the masonry lining is put in place. This is accomplished by the use of rows of vertical rough timber props and horizontal caps or overhead-pieces, between which and the earth rough boards are placed, to form temporary supports for the sides and roof of the excavation (fi/. 26, figs. The props and caps are placed in position first, and the boards are then driven in between them and the earth walls of the excavation. As the masonry lining is carried forward these temporary supports are carefully removed.