TUNNEL. Tunnels are passages constructed under ground to carry roads, railways, canals, or streams of water. Tunneling, which has long been in use for roads, and aque ducts, has of late received a great development in the construction of railways. At the pres ent time there are believed to be upward of 80 in. of railway tunnels in areat Britain in constant use for the passage of trains; and, as their cost averages from £45 to £50 per yard, a total of about seven millions sterling has been expended in their construction. In tunnels of considerable length, as the progreSs made by working from the two ends would be very slow, it is considered advisable to commence the work from many points of its length; for this purpose, shafts or pits are made at these points down to the level of the tunnel. Of these shafts, sonic are temporary, and only kept open during the progress of the work; others are permanent, and for the purpose of ventilating the tunnel when in use. These shafts have to be large enough to allow the ascending and descending skips or buckets containing the excavated materials to pass one another. For the temporary shafts, an elliptical shape is found to give the greatest room for this purpose at the least expense. Square shafts are to be avoided, on account of the difficulty of excavating the corners in rocky strata. As the shaft descends, its sides are lined with timber-planks, supported by strong timber-frames, about 5 ft. apart. The permanent shafts, when the material is not of rock sufficiently solid, are lined with brick-work or masonry, built in lengths, as the shaft proceeds downward. These per manent shafts are generally made circular in section, and it is found better to place them 3 or 4 yards from the side of the tunnel, communicating with it by a small passage. This is convenient in the construction, and also is a useful refuge for work men subsequently during the passage of trains. These shafts are generally made about 10 ft. diameter. They are sunk a few feet below the floor of the tunnel, to form a pit for the collection of the water from the workings, which is hauled to the top in barrels or buckets. The raising of the excavations and the water, and the lowering of
building materials, and of the workmen, is done by a windlass, a horse-gin, or steam power, according to the extent of the work. On the completion of the shaft, the tunnel is commenced in both directions from its bottom; and in the case of ordinary rock, it is found convenient to commence by making a small alit, or passage, along from shaft to shaft, through the whole length of the tunnel; this is made six or seven ft. high, and the top of it placed at the level of the top of the tunnel. When this is completed, the cor rect center line is marked out in it thoughout the tunnel; the edit is then enlarged to the shape and size of the arch of the tunnel, which is built in, and then the excavation is com pleted, and side-walls built up to underpin the arch. In cases where the material is soft and full of water, the full section of the tunnel is generally carried ft rward at once, and in such cases an invert has to be built between the side-walls, to withstand the tipwarfl pressure, as the pressure of soft material has the character of a fluid pressure, and presses the tunnel on all sides. The excavation is then done in lengths of aout 24 ft., which is trimly secured with poling-boards and larch bars, and securely shored; the centers are then set, and the brick-work built up. The timber bars are generally drawn out when the brick-work is carried up, and the holes they leave rammed tight with clay; but they have sometimes to be built in. When the quantity of water is very great, an edit is driven through the tunnel, at the level of its floor, before the work is begun, to allow the water to run off.
Tunne:s are generally made straight, but sometimes they are curved. this is done that they may pass under the lowest pert of the bill, in order that the shafts may he as short as possible. They are frequently constructed on steep gradients, but as the trains experience some resistance from the air in passing through them, it is advisable not to make them so steep as the gradients in the open air.