Early Examples of Tunnelling.—With so many examples of natural caves and tunnels in existence it is not to be wondered at that tunnelling was one of the earliest works undertaken by man, first for dwellings and tombs, then for quarrying and mining and finally for water-supply, drainage and other requirements of civilization. A Theban king on ascending the throne began at once to drive the tunnel which was to form his final resting place, and persevered with the work until death. The tomb of Mineptah, at Thebes, was driven at a slope for a distance of 35o ft. into the hill, when a shaft was sunk and the tunnel projected a farther length of about 30o ft., and enlarged into a chamber for the sarcophagus. Tunnelling on a large scale was also carried on at the rock temples of Nubia and of India and to some extent by the Aztecs in America, and the architectural features of the entrances to some of these temples might be studied with advantage by the designers of modern tunnel portals.
Flinders Petrie has traced the method of underground quarry ing followed by the Egyptians opposite the Pyramids. Parallel galleries about 20 ft. square were driven into the rock and cross galleries cut, so that a hall 300 to 40o ft. wide was formed, with a roof supported by rows of pillars 20 ft. square and 20 ft. apart. Blocks of stone were removed by the workmen cutting grooves all round them, and, where the stone was not required for use, but merely had to be removed to form a gallery, the grooves were wide enough for a man to stand up in. Where granite, diorite and other hard stone had to be cut the work was done by tube drills and by saws supplied with corundum, or other hard gritty material, and water—the drills leaving a core of rock exactly like that of the modern diamond driil.
As instances of ancient tunnels through soft ground and requir ing masonry arching, reference may be made to the vaulted drain under the south-east palace of Nimrod and to the brick arched tunnel, 12 ft. high and 15 ft. wide, under the Euphrates. In
Algeria, Switzerland and wherever the Romans went, remains of tunnels for roads, drains and water-supply are found. Pliny refers to the tunnel constructed for the drainage of Lake Fucino as the greatest public work of the time. It was then by far the longest tunnel in the world, being more than 3.5 m. in length, and was driven under Monte Salviano, which necessitated shafts no less than 400 ft. in depth. Forty shafts and a number of cuniculi, or inclined galleries, were sunk and the excavated material was drawn up in copper pails, of about io gal. capacity, by windlasses. The tunnel was designed to be io ft. high by 6 ft. wide, but its boards at the top of the shafts are also on record. Before the advent of the railroad, tunnels were built for canals at many locations and some were of very early date. On the introduction of railways tunnelling became one of the ordinary incidents of a contractor's work. Probably upwards of 4,000 railway tunnels have been excavated, including in this list some of the longest tun nels in the world, such as the Mont Cenis, Saint Gotthard, Simplon and Loetschberg in the Alps; the Hoosac, Moffat and Cascade in mountains of the United States; and the Connaught at Rogers pass in Canada. On mountain railways, tunnels often form a large percentage of the length; on the Mexican railway in a dis tance of 66 m. there are 21 tunnels; on the Southern Pacific tunnels in 25 m., including a spiral tunnel. There are also sev eral long spiral tunnels on the Canadian Pacific railway at the Kicking Horse pass. The longest single tunnel, however, is the Shandaken tunnel, 18.1 m. in length, built by the City of New York (1917-24) as an extension of the Catskill aqueduct system for water-supply, in Greene county, N.Y.