The completion of the shaft enables the workings to be commenced on each side, the excavated material can be hoisted to the surface, and building material lowered down. When the tunnel works are finally finished, the lining of the shaft should be carried up until it is 15 or 20 feet above the level of the surface of the ground, and a dome-shaped iron grating placed on the top as a protection against stones or other articles which malicious persons might attempt to throw down the shaft.
Some shafts are only intended for the temporary purpose of lifting the excavations from below, or lowering building materials
down, and when the work is completed they are filled in again and closed. These service shafts are generally made square in section, and are merely lined with wood. Strong vertical timbers are placed at the four corners, to which horizontal double cross-pieces are bolted, thick planking being placed vertically at the back of these cross-pieces to support the sides of the excavation.
The heading of a tunnel is a narrow passage or gallery cut through from end to end of the works in the direction of the centre line. Where there are shafts, the cutting of the heading can be pushed on from several points, and be completed much more rapidly than when the working is restricted to the two ends. Headings are usually made just sufficiently large for the miners to work, say about 5 feet 6 inches high by about 3 feet wide, the object being rather to expedite the driving of the driftway than to remove large masses of material. They must be set out with great accuracy, and be constantly checked as the driving is in progress. When completed from end to end, the centre line can be checked throughout, and the course actually taken compared with the course intended. If there has been much variation in the narrow pioneer pathway, either in line or level, the amount of the divergence must be rectified when ranging the final centre line for the full-size excavation.
Tunnels cannot always be delayed until the heading is cut through for the entire length. In many cases the heading, the full-size excavation, and the permanent lining have all to be carried on at the same time, but as the work of the heading is smaller in extent, that portion of the operations can usually be kept well in advance of the others. The critical moment arrives when the headings from opposite directions meet, as any deviation or want of coincidence must be adjusted in the portion of the tunnel still remaining to be opened out to full size. Some tunnels of moderate length have been constructed without any heading at all, the excavation being taken out to the full dimensions from the commencement.
The heading of a tunnel assists not only in the correct alignment of the work, but furnishes at the same time an accurate knowledge of the strata passed through. It is also of service for ventilation, communication, and drainage.
In some cases the heading is driven at the bottom of the tunnel section, as in [Fig. 211], and in others at the top, as in
[Figs. 202] and [204]. Many of the earlier tunnels were constructed on the former system, while of late years the latter method has been very largely adopted. The bottom heading may perhaps in some instances be more efficacious for drainage, but it is very liable to be frequently choked up when taking out the excavation to the full size, and the lower surface is much cut up by the movement and conveyance of materials. Another disadvantage arises from the necessity of removing such a large amount of the cutting approaching the tunnel entrance before a beginning can be made to the bottom heading. The top heading has the advantage that it requires less removal of open cutting previous to its commencement, and, being high up in position, there is less chance of its being stopped up by falling material, the finished excavations being carried out on the sides and below the heading.
Where the headings are cut through solid rock, stiff shale, or compact chalk, little or no supports are necessary, but where they pass through clay or loose material, timbering will be required for sides, roof, and floor. Rough round poles, about 6 inches in diameter, are generally used for verticals, and are firmly secured to transverse sole-pieces, and on the top of these verticals strong transverse top-sills are fastened by means of rough tenons or checks. Strong boards are inserted at the back of this framework to keep the earth from falling into the working. The distance apart of the verticals will depend upon the description of material excavated; in very soft places they will have to be placed very close together, but where fairly sound and tenacious they may be placed at about 3-foot centres. The excavated material must be conveyed away to the entrance of the heading in small hand-trucks running on planks or light rails.
The widening out of the excavation to the full size will be a repetition on a large scale of the work carried out in the heading, with the difference that, the exposed surfaces being of so much greater extent, extra care and precautions must be taken with the framework and shoring of the timbering.