TUNNELLING IN TOWNS Where tunnels have to be carried through soft soil in proximity to valuable buildings special precautions have to be taken to avoid settlement. An early successful example of such work is the tunnel driven in 1886 for the Great Northern Railway Company, under the metropolitan cattle market, London. This was done by the crown-bar method, the bars being built in with solid brickwork. The subsidence in the ground was from 1 to about 31 inches. Several buildings were tunnelled under without any damage.
The District subway of Glasgow, Scotland, consists of a double tube line in the form of a loop, connecting Partick and the north ern districts with the centre of the city. It has a length of 6.5 m., and consists in large part of circular tubes, I I ft. internal diameter, built as tunnels, lined with cast iron and driven with a shield and compressed air. The line crosses twice beneath the Clyde. Work was begun in 1891 and the line opened to traffic in 1897.
London has now some 90 m. of tunnels for railways, mostly operated by electric traction. (See TRACTION, ELECTRIC.) Most of those which have been constructed since 1890, comprising some 5o m. of double tube railways, have been tunnelled through clay by the use of cylindrical shields and have linings of cast iron plates. The tubes are generally small, from io to 12 ft. in inner diameter, but shields about 23 ft. in diameter were used in con structing the stations on the Central London railway, and one 32 ft. 4 in. in diameter and only 9 ft. 3 in. long was used for a short distance on the Clapham extension of the City and South London railway. The first of the London tube railways to be built was 3.5 m., of the City and South London, from the Bank under the Thames to Stockwell, begun in 1886 and completed in 1900.
Paris has an extensive system of underground railways some 6o m. in length, portions of which were built as tunnels, and a considerable number of which were constructed under the en gineering direction of F. Bienvenue. The first line was built in 1898, from Porte Maillot to Porte de Vincennes, and other lines followed at later dates. Instead of using completely cylindrical shields and cast iron walls, as in London, roof-shields (boucliers de voute) were employed for the construction of the upper half of the tunnel, and masonry walls were adopted throughout. In general, the upper half of the tunnel was executed first and the lower part completed by underpinning.
In the case of the tunnelling near important buildings in Boston, in 1896, with a roof-shield 29 ft. 4 in. in external diameter, the vertical side walls were first made in small drifts, the roof-shield running on top of these, and the core of earth was taken out later and the invert or floor of the tunnel put in last. Each hydraulic press of the shield reacted against small continuous cast iron rods imbedded in the brick arch.
The Pennsylvania railroad tunnels crossing New York city under 32nd and 33rd streets are of unusual size and excavated through mica schist. Owing to the close proximity of large build ings and other structures, special methods were adopted for min ing the rock to lessen the vibrations from explosions.
Quite long sections of the rapid transit system beneath the streets in New York have been built in tunnels. There are about 3 m. of double tube, shield-driven tunnels in Brooklyn. Shorter sections also were built in Manhattan. A portion of the line under Flatbush avenue was built in sand with a roof shield of unusual size rolling on the completed side walls, as in the case of the Boston subway. A number of the rock tunnels disclose an exceptional size. On the Lexington avenue line, four tracks, two on each of two levels, are in one tunnel excava tion, about 32 ft. high and wide. (See fig. 8.) The upper level is carried on steel beams supported by columns at the side and centre. The concrete roof is formed of two arches carried by columns. The central portion is known as an "um brella" section, which it resembles. Beneath St. Nicholas avenue and Fort Washington avenue five of the subway stations are con structed in tunnel, four of which contain two tracks and two platforms within a single arch having a clear span of 48 feet. The width of excavation required was 57 ft. and the height 3o feet. In one case the arch carries a mezzanine platform the entire length of the station, supported by hangers from the roof arch alone.