Within recent years many important tunnels have been constructed to carry rapid transit rail ways under city streets. Some of these, like the London tunnels, have been deep tunnels, but oth ers. like those at Boston, Paris, and New York, have been constructed for the most part by open cut methods at a small depth below the surface. The City and South London Railway, in London, was begun in 1886 and completed in 1890. elec tricity being ultimately adopted as the means of traction. It is 3 miles long from the city to the Swan at Stockwell, and is entirely underground. Briefly the road consists of two 10 feet 6 inch east-iron lined tunnels substantially parallel to each other, which converge into a single tunnel at the termini to permit the transfer of trains from one line to the other. These tunnels were con structed by means of the shield system. The Waterloo and City Railway in London extends from the Waterloo station of the Southwestern Railway to the Mansion House, a distance of 1 mile 4 furlongs and 150 yards. It consists of two parallel circular tunnels 12 feet 1 I inches interior diameter except at the curves, where the dimension is increased to 12 feet 9 inches. The tunnels were driven by the shield method, work having been begun in 1894 and completed in 1897. A single electric railway track in each tunnel forms a double-track railway between the termini named. The Central London Railway runs from Shepherds Bush to the Bank, a dis tance of 51 miles, the whole distance being in deep tunnel at depths varying from 60 to 00 feet. It consists, like the two roads previously mentioned, of two parallel circular tunnels. Each tunnel is feet in diameter and has a cast iron lining. The shield system of construction was employed. To be added to these tunnels are the Metropolitan District railways, parts of which are underground, begun in 1853, the Waterloo and Baker Street Railway, and the projected Euston and Hempstead Railway.
The longest of the London tunnels is the Blackwell Tunnel for street traffic under the River Thames. The total length of this work is 6200 feet, of which 30S8 feet are tunnel proper, 1370 feet are open cut roofed over, and 1742 feet are open cut without covering. The exterior di ameter of the tunnel is 27 feet. It was driven by means of a shield 27 feet 9 inches in diameter and 181/2 feet long, with two diaphragms, from horizontal partitions or stages and four vertical partitions. The lining is of cast-iron rings. Work was begun in 1892 and completed in 1897. The tunnel provides a roadway 16 feet wide and two sidewalks each 3 feet 11/2 inches wide. In Glasgow, Scotland, the Glasgow City and District Railway has a length of 3.123 miles. Of this dis tance one mile is deep tunnel, and 3433 feet are tunnel built by the open-cut method. Work was begun in 1883 and completed in 18S6. The Glas gow Central Railway has 0.S4 of a mile of deep tunnel and 3.13 miles of open-cut tunnel. The Glasgow District Svbway, miles long, con sists of two parallel circular cast-iron lined tun nels 11 feet in interior diameter. The tunnels were driven by the shield method. Construction was begun in 1891 and completed in 1894. The Metropolitan Railway of Paris, comprising alto gether some 40 miles of projected railway, com pleted in 1900 the main line from the Porte de Vincennes to the Porte Maillot and the Porte Dauphin4, 8.7 miles. This is entirely underground. In the tunnel excavation the Berlier type of shields ordinarily were used; but progress by this means was found too slow, and open, timbered excavation of the ordinary type was employed. The so-called Belgian method of tunneling was used; the arch was first built and then the side walls were erected by underpinning the arch. After the earthen core was removed the bottom arch, or invert, was set in place. At the stations, however, the side walls were first con structed and upon them the arch, or metallic roof, was built. The removal of the earth from the core then took place, as in a tunnel. and the last stage was the construction of the invert. The line is double-track electric railway throughout.
The Boston Subway, begun in 1894, is prac tically the underground tunnel of the surface electric lines entering the business district of the city of Boston. The portion of the line
completed in 1808 was 10,810 feet long, divided between double-track and four-traek tunnel. The bulk of the tunnel was built by open-eut methods, but on certain portions the shield system was employed. In 1900 work was begun on a 1M mile extension of the original line under Boston Harbor to East Boston. The East Boston tunnel has a polycentric cross-section 231/2, feet wide and 201/2 feet high and is lined with concrete masonry. The double-track subway section is approximately rectangular and 24 feet wide by 14 feet high, while the four-track section is a similar form, but twice as wide. The lining con sists of concrete side walls and brick roof arches supported by columns and 1-beams of steel.
The New Fork Subway, as contracted for in 1899, comprises some miles of line, three fourths of which is in tunnel. A four-track line runs from the City Hall to 103d Street, and there divides into two double-track lines running to Bronx Park and to Kingsbridge, respectively. The cross-section of the underground line is of different types, as indicated on the plate. In the rectangular section used for more than half of the line, the dimensions adopted for two tracks are 25X 13 feet, and for four tracks, 50 X 13 feet. The barrel-vault section. composed of a polyeentric areh, is usually 24 feet wide and 16 feet high. The circular sections are 13 feet in diameter, two tunnels being built side by side for double-track line. The work was done partly by open-cut methods and partly underground, ac cording to the depth of the line below the surface.
Among other notable tunnels the following may be briefly mentioned: Arlberg Tunnel, in Austria, begun in 1880 and completed in 1884 for a double-traek railway, 6.38 miles long. Teguix quiae Tunnel, begun in 1888 to drain the valley of ,iexieo, 6 miles long and about 14 feet wide and 14 feet high. completed in 1898. Trans Andean Railway Tunnel, through the summit of the Andes on the railway line between Buenos Ayres and Santiago, three miles long, at an elevation of 10,460 feet above the sea. First Thames Tunnel at London, England, begun in 1825 and completed in 1843. 1200 feet long with two parallel ways of horseshoe section, each 13% feet wide and 161/2 feet high. East River Gas Tunnel, between New York and Brooklyn, 2516 feet• long and 10 feet 2 inches in diameter, be gun in 1S92 and completed in 1894. Eliojak Tunnel, on the Northwestern State Railway in India, 12.870 feet long, begun in 1889 and com pleted in 1591. Strickler Tunnel, for the water supply of Colorado Springs. Colo., 6441 feet long, with a section 4 X 7 feet through the rock. Grareholz Tunnel, on the Bergen Railway, in Nor way, single track, 17,400 feet long. Busk Tunnel, on the Colorado Midland Railway, in Colorado, 9094 feet long. with a section 15 feet wide and 21 feet high through rock.
BIBLIOGRAPHY. The literature on tunneling Bibliography. The literature on tunneling which is readily available is exceedingly limited. The most Comprehensive book on the subject is Drinker. Tunneling (New York, 1874), which is now out of print. The book which ranks next in importance is Simms, Practical Tunneling (ib., 1896). The most recent book on the subject is Prelini, Tunneling (ib.. 1901), which is a thor oughly up to date text-book of some 300 pages. Legouez, De l'emploi do boudier dans la con struction des souterraines (Paris, 1897), is a comprehensive treatise on the shield system of tunneling. Among the works describing individual tunnels are: Yon Rosenberg. The Vorburg Tunnel (New York. 1887) : Walker, The Severn Tunnel (London, 1888) ; Burr. Tun neling Under the Hudson River (New York, IS85) ; Sutro, The Sutro Tunnel (ib., 1S87).
Among foreign books on tunnels the following are notable: La Dame, Les grands tunnels des .napes et du Jura ( Paris, 1889) ; Riziha, Lchrbuch der gcsamten Tanncl-Buukanst (Berlin, 1874) ; Danino, Gallcrie della dell' Appcnnino, linca Foglio (Rome and Naples, 1875). The Proceedings of the various societies and the engineering papers contain many descriptions of important tunnel works.