TUNNELS FOR CONVEYING WATER From very early days, water has been conveyed in aqueducts formed by tunnelling, and some early examples have been men tioned. Generally such tunnels were of small cross-section, but many of them were of great length. They also generally followed the grade of the flowing water and where valleys were encoun tered they were crossed by aqueducts of stone arches, of which examples still exist. Examples of modern tunnelling in connection with water-supply may be found in many sections of the world. Where necessary to obtain water for the supply of large cities it sometimes must be brought from great distances and the sources of supply are generally in mountainous districts.
Both the largest and longest aqueduct tunnels are those supply ing water to the City of New York. They also present the novelty of sinking to great depths below the hydraulic grade line and rising again to that line to pass beneath river gorges, which are crossed at many points and also to pass beneath the entire length of Manhattan island. This introduces an internal or bursting pres sure in the tunnel, which is called "a pressure tunnel." The Croton aqueduct, from the Croton dam to the gate house at 135th street, constructed 1885-90, forms, in its completed state, a tunnel 31 m. long and includes. an inverted siphon or pres sure tunnel, passing beneath the Harlem river. A number of short sections were built in open excavation and then covered over.
The Catskill water-supply system, which extends from the Catskill mountains to New York city, a distance of about 160 m., consists in large part of true tunnels and includes both the largest tunnels for such purpose, and the longest tunnels for any purpose in the world. All are concrete lined. There are 25 tunnels on the hydraulic grade, with a total length of 32 m., including the tunnel
beneath the Shandaken mountains, 18.1 m. long, about 4o m. south-west of Albany, N.Y. It has a "horseshoe" section II ft. 6 in. high by Io ft. 3 in. wide and is the longest continuous true tunnel in the world for any purpose. Seven shafts were used in constructing this tunnel. There are also seven pressure tunnels crossing beneath river valleys, including that beneath the Hudson river from Storm King to Breakneck mountain, at a depth of 1,114 ft. below sea-level. It is 14 ft. internal diameter and 3,022 ft. between the shafts on either side of the river. (See fig. 9.) The total length of the Catskill aqueduct pressure tunnels exclu sive of the city aqueduct tunnels is 17 m. and they are from 14 ft. to 17 ft. 6 in. in diameter. The city aqueduct tunnel extends from Hillview reservoir, beneath the Harlem river and nearly the entire length of Manhattan island and the East river, to shafts in Brook lyn, a distance of 18 miles. It is the second longest tunnel in the world, and the longest carrying water under pressure. To provide resistance to the bursting pressure of the water and also water tightness, it is placed in the rock from Zoo to 75o ft. below the surface of the streets beneath which it was built. It was driven from 25 shafts, averaging 4,00o ft. apart. The work of con structing the Catskill aqueduct system, including the reservoirs, the city tunnel and the Shandaken tunnel, occupied from i9o5 to 1926.
In 1920, a tunnel 4.5 m. long, 9 ft. wide and 9.5 ft. high was begun in the Alps of Switzerland, to convey water to an hydro electric plant. The Nevada irrigation tunnel is 4.1 m. long and has a cross-section 9 by 9 feet. It was completed in May 1928 at a cost of $1,5oo,000.