TUNNEL, in civil engineering, is an arched passage formed underground to conduct a canal or road on a lower level than the natural surface. Long tunnels are usually made through hills in order to avoid the incon venience and loss of power occasioned by conducting a canal, road, or railway over elevated ground, and also the enormous ex pense of such an open excavation as would be necessary in order to preserve the requi site level. Those of less extent are frequently constructed to avoid the opposition of land owners, or to afford uninterrupted passage under a road, canal, or river.
Rocky strata, if the stone be of a nature to work freely, are usually the cheapest for tunnelling, owing to the absence of lining, and the power of saving labour by the use of gunpowder. Tunnelling in clay is frequently attended with formidable difficulties which render it very expensive. It is, when tough, a difficult material to remove, blasting being of no use, and spades and pick-axes being almost inapplicable. In such cases hatchets may be used to advantage, but cross-cut saws answer best. Tunnels formed through chalk are often impeded by faults or cavities filled with wet gravel or sand, which pour a flood of semifluidrnatter into the excavation as soon as they are cut into. The irruption of such loose materials, as well as of water alone, has in many cases occasioned difficulties almost in surmountable. Loose sand is perhaps the most difficult stratum that can be met with in tunnelling, but it has been in several instances successfully passed through.
Short tunnels are occasionally excavated from the ends only, but those of considerable length are usually formed by sinking vertical shafts, about nine feet in diameter, down to the level of the tunnel, and excavating in each direction from the bottom of each shaft, until the several parties of workmen meet in the intermediate portions. By this means the work can proceed at any required number of points or faces, so as to bring the execution of the tunnel, may be its length, within a moderate period of time. The num ber of working shafts in a given. length of tunnel is determined by the nature of the ground and the time allowed for excavation: Besides the working-shafts there are usually small air-shafts of three or four feet diameter to prevent the accumulation of foul air in the workings of the tunnel. In very long tunnels
one or more large shafts are desirable for the purpose of ventilation, and also to admit some degree of light. When a complete brick lining is required, the bottom is 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 The Thames Tannel.—Many projects have been started for making a tunnel under the Thames ; but the only one put into successful operation is that for which an act was obtained in 1824, with Mr. (afterwards Sir M. I.) Brunel as the engineer. Operations were commenced early in 1825 by the construction of a shaft 50 feet in diameter, at a distance of 150 feet from the south side of the river, at a point about two miles below London Bridge. A similar shaft was sunk on the north side of the river. The excavation foi the tunnel was an oblong square, 38 feet wide and 22 feet 6 inches high, presenting a sectional area of 855 square feet. The shield by which the excavation was effected consisted of twelve massive frames of iron, placed side by side, and capable of being slid forward for a short distance independently of each other. The whole apparatus may be compared to a massive cofferdam laid on its side, and capable of being moved forward by the action of screws abutting against the end of the completed brick-work, which followed it up closely. Each frame of the shield consisted of three stories, each of which formed a cell large enough for one man to work in conveniently. The ar rangements for supporting the wet clay of the Thames bed while the brickwork was being prepared, were of a very ingenious description.
The excavation of the tunnel was com menced in January, 1826, in a stratum of clay ; but several irruptions of the river took place ; one, in 1828, was so serious in its consequences that the Company did not re sume operations till 1835. The tunnel is 1200 feet in lend between the two shafts on the opposite banks of the river. It was not until 1843 • that the Thames Tunnel was opened for foot passengers.
This grand undertaking has altogether failed to realise the commercial anticipations of its projectors. It has been, and still is, little more than a penny ' lion' of London ; nor is it easy to see any probability that it will acquire trading importance.