The number of working shafts in a given length of tunnel is deter mined by the nature of the ground and the tirno allowed for excavation. They should be so near together ns to allow the tunnel to be excavated and lined, if brickwork be requisite, for a length equal to one-half of the diatanco between two shafts, at least two months before the time appointed for the completion of the tunnel ; that time being desirable to allow for accidents, for building the tunnel-frontn, ballasting and laying the railway, &c. The shafts are usually 9 feet in diameter in the clear, and lined with brickwork 9 inches thick, laid in cement ; ail the bricks being laid as Maulers, or with their ends towards the centre of the shaft ; but the substance of the brickwork must vary with circumstances, and engineers are by no means uniform as to the dimen sions of their shafts. Those of the Box tunnel are mostly 25 feet in diameter, while some tunnels have been excavated with shafts of only 3 or 4 feet. Down to a depth of 50 or 60 feet the earth may be removed by means of a simple roller or winch worked by two men ; and when the depth is greater a horse-gin may be erected to draw up the loaded skips or buckets. The depth at which the horse-gin be comes preferable is however dependent in some degree upon the nature of the strata to be excavated; because when they are hard, the men above may have to wait occasionally for the coming up of the skip; and it is better to have only two men at a stand-still instead of a horse, a horse-boy, and a banksman or skip hinder. The roll or winch is, at the commencement of the shaft, supported by four bars or cilia laid across each other on the ground; and the excavation is continued nutil the earth exhibits signs of weakness. A wooden curb or riug about 3 inches thick, and as wide as the brick lining is to be, is then laid in, and upon this the brickwork is laid. When the lining of this portion of the shaft, which is frequently but half a brick, or 41i inches in thickness, is completed, the excavation is recommenced, and is carried down in a vertical line even with the inner surface of the brick work, which is then supported by the earth left under the curb, which may be further supported if necessary by diagonal timber props. When the excavation has been carried so much deeper that the ground again appears weak, a second curb is inserted in a groove cut in the earth, and the ground between the two curbs is divided into four, six, or eight vertical masses, of which one or two are removed to a depth equal to the thickness of the brickwork. The wall is then built up in its place, and a further portion of earth is removed, and so on until the lining is complete. When the shaft is carried down to the full depth, the miners begin to excavate laterally by forming a heading or driftway along the level of the upper part of the tunnel. Sometimes such a drift is formed throughout the whole length of the tunnel before any part is opened out to the full size ; but in other cases it is made in short portions, little exceeding the lengths in which the exca vation of the tunnel itself is carried on, which may vary, according to the ground, from S to 15 feet. In the former case the driftway, which is about 4 feet wide and 5 feet high, affords a satisfactory test of the strata to be passed through, and of the probable quantity of water to be met with, for which it may in some cases serve as an edit or drain ; and for these reasons such a heading is occasionally formed before letting the contracts for the tunnelling.
In addition to the working shafts, the contractor is usually allowed to sink any number of small air-shafts of 3 or 4 feet diameter, as may be necessary to prevent the accumulation of foul air in the workings of the tunnel ; provided that no such shaft shall open into a public road, or be within 50 feet of a working shaft. These are formed in a similar manner to the working shafts, and both are finished at their lower ends by resting upon a cast-iron curb or ring imbedded in the masonry of the roof of the tunnel, and at the upper ends, after the works are completed, by building them about 10 feet above the surface, and coping'them with stone. In very long tunnels one or more large per manent shafts are desirable for the purpose of ventilation, and also to admit light, so as in some degree to lessen their gloom. In the Kilsby
tunnel, which is between a mile and a quarter and a mile and a half long, there are two such ventilating shafts, 60 feet in diameter and about 100 and 132 feet deep respectively. These were built from the top downwards, in the manner above described, in portions 10 feet deep, and from 6 to 12 feet wide. Lecount gives the following directions for the brickwork of such a shaft, if made in unfavourable ground : The tunnel Itself at the point of intersection should be of stone. The lower part of the shaft, to the height of 46 feet, should be 3 feet thick ; the next 17 feet, 2 feet 8 inches thick ; the next portion.of the like extent, 2 feet 3 inches ; and the uppermost 16 feet, 1 foot 10 inches. The bricks should be laid in alternate courses of headers and stretchers, and each brick should be well flushed up. The top may be finished with a stone coping and an iron railing, and protected by an iron fence wall, to prevent the risk of anything being thrown down the shaft either by accident or design. In some tunnels a large oblong ex cavation, called an eye, is introduced in lieu of a circular ventilating shaft. In the Bishopton tunnel there is such an eye 300 feet long ; and there are two similar openings in the Glasgow tunnel of the Edinburgh and Glasgow railway.
After the completion of the driftway, either through the tunnel or to the length of is single stage only, the miners excavate the tunnel to its full dimensions, beginning by cutting downwards, and propping up the earth with timbers as they proceed ; those whieh support the roof being at such an elevation as to allow the centering to be set up and the brick arch to be built beneath them. The bars which immediately support the earth, extending from the top or outside of the completed brickwork to a framework placed against the face of the excavation, are called side bare, and are in favourable earth required at the upper part of the tunnel only ; but in bad ground they are required some times as low as the springing of the inverted arch which forms the bottom of the tuoneL When a complete brick lining is required, the invert Ls the part first built, and it is completed by a course of stone laid along each side, at the point where the side walls spring from it These courses of stone, which are marked a, a, a, a, in the subjoined :uts, figs. 1 and 2, consist of blocks about three feet long, well bedded a mortar, upon a few courses of brickwork laid as a footing. The Bide walls are next raised, with such a batter or curvature as may enable them best to sustain the pressure of the external earth ; and when they are raised to the level of the springing of the arch, beams of wood, called cells, are laid across the tunnel and built in with the brickwork. Upon these cis the trusses of the centering are set up, and adjusted with wedges to the proper height. Laggins, or pieces of wood stretching longitudinally from one centering to another, are then added, and upon these the bricks are laid. In seine cases the cills are supported upon trestles, instead of being built into the walls; and where this is not the case, the holes left in the masonry must, after the removal of the cilia, be carefully filled up. The excavation should always be made as nearly as possible of the size and shape of the intended masonry ; and as the building proceeds, every cavity left out side the brickwork should be carefully filled up. Well-pounded clay may for this purpose be rammed under the invert, and almost any other material may be used for the sides and roof, The ramming of the sides may be performed after the laying of every second course of brickwork, and that above the arch as frequently as convenient. The timbers used to support the superincumbent earth are in most cases removed as soon as the arch is completed ; but in very bad ground it is sometimes necessary to leave them imbedded in the earth. When the work has proceeded so far that the excavations from the two adjoining shafts are within about fifty yards of each other, if no drift way have been previously made through the tunnel, it is advisable to drive a heading through the intervening earth, to insure a perfect junction of the two shifts.