TUNNELS AND TUNNELING. A tunnel may be considered an underground or sub-surface passage of any form of cross-sec tion most suitable for the purpose of construc tion or for the purposes to be served after con struction. It is ordinarily supposed to be con structed without disturbing the natural material over it or around it, although in some modern engineering works true twine's are constructed by first removing the overlying material by open excavation if on land, or by dredging if under water, then constructing the tunnels by suitable means and refilling over the completed structure: Tunnels are of all sizes as well as shapes of cross-section and may be horizontal or inclined; indeed, a shaft may be considered a vertical tunnel. When tunnels are small and built ahead of the main structure or for subor dinate purposes, as in mining, they are called headings or adits. Obviously, tunnels may be constructed for a great variety of purposes. They are commonly employed for railroad pur poses, for the passage of foot and vehicular traffic under rivers as in the city of Chicago, for sub-surface transit purposes in large cities as in the city of London, to form aqueducts in great water supply systems, as for the city of New York with the 31-mile Croton aqueduct tunnel, for ' sewer purposes, and for serving other public conveniences in modern communities.
Ancient History.— The art of tunnel build ing is one of the oldest of engineering pro cedures. The ancient Egyptians built ex tensive tunnels for tombs and temples, and nearly or quite the same thing was done in ancient India. The ancient Assyrians con structed a tunnel under the Euphrates River, after having diverted its waters to another channel, the cross-section of which was 12 feet wide by 15 feet high. They also built other tunnels. The ancient tunnels were probably invariably built in rock or other hard material in which there was not suffi cient water to give material trouble; there is no record of a true sub-aoueous tunnel being built in ancient times. Methods adapted to tunnel excavations through soft material sat urated with water have been developed only in comparatively recent times. The old Ro mans were by far the greatest engineers of ancient times and they excelled all earlier peoples in their tunnel constructions as they did in all other constructions. The 'I-fire-set method of tunneling was due to them as was also the method of sinking vertical shafts at different points on a tunnel line to afford a greater number of points of attack on the work. They built fires in their tunnels against the rock to be excavated, and, after heating it to the highest practicable temper ature, suddenly chilled it by pouring water on it and taking advantage of the resulting soft ening, cracking and disintegration. They were
also aware of the advantage of using vinegar instead of water in this method of tunneling through calcareous rocks, thus taking advantage of chemical action as well as disintegration by alternate heating and chilling. Probably the longest of the old Roman tunnels was that built to drain Lake Fucino; it was designed to have a section 6 feet by 10 feet. Forty shafts as well as inclined galleries were sunk for the construction of this tunnel miles long. The deepest shaft was about 400 feet. Most of the Roman tunnels were small, but they occasionally built large ones, like that for the highway in the Posilipo Hills between Naples and Pozzuoli, 3,000 feet long and 25 feet wide at the centre. The entrances are 75 feet wide and at the centre the height is 22 feet. Its general form, therefore, approximates the frustra of two cones with their small bases joined. This was for the purpose of concentrating the light in the central or less illumined portion.
In. the Middle Obviously the con struction of ancient tunnels was excessively slow as well as laborious, although drills, chisels and even saws, fitted with hard cutting stones like corundum for teeth, were used even by the early Egyptians. Hammers and wedges were also employed. The first appli cation of gunpowder of importance was prob ably at Malpas, France, in 1679-81, in the con struction of the tunnel on the line of the Languedoc Canal, 510 feet long, 22 feet wide and 29 feet high. The first stimulating de mand for modern tunnel construction of any great amount began with the construction of canals in France and England during the last half of the 17th century, and all of those tunnels were either in rock or dry hard ground. It was not until after 1800 that tunneling through sand and wet ground was undertaken in sufficient magnitude to be regarded as a branch of engi neering construction. A tunnel for the Saint Quentin Canal in France was built through soft soil in 1803. The width of this tunnel was 24 feet, and timbering including struts and plank ing was used to support the walls and sides prior to constructing the masonry lining. From the earliest times the necessity of lining tunnels with masonry was recognized and it was done wherever the material was of such a weak and uncertain character as to make its self sustaining qualities doubtful.