Deep level underground

Deep level underground refers to constructions 100 metres or more below ground and not using the cut and cover method, especially train stations, air raid shelters and bunkers and some tunnels and mines. Cut-and-cover is a simple method of construction for shallow tunnels where a trench is excavated and roofed over with an overhead support system strong enough to carry the load of what is to be built above the tunnel.

Although some deep mining took place as early as the late Tudor period (in the North East, and along the Firth of Forth coast) deep shaft mining in Britain began to develop extensively in the late 18th century, with rapid expansion throughout the 19th century and early 20th century when the industry peaked. Before 1800 a great deal of coal was left in places as extraction was still primitive. As a result, in the deep Tyneside pits (300 to 1,000 ft deep) only about 40 percent of the coal could be extracted. The use of wooden pit props to support the roof was an innovation first introduced about 1800.

Before any plans were made for transit systems with underground tunnels and stations, several railway operators had used tunnels for freight and passenger trains, usually to reduce the grade of the railway line. Examples include Trevithick's Tunnel from 1804, built for the Penydarren locomotive, the 1829 Crown Street Tunnel at Liverpool and the 1.13 miles (1,820 metres) long 1836 Lime Street Tunnel also at Liverpool, of which a part is still used today making it the world's oldest used tunnel.

The first urban underground railway was the Metropolitan Railway, which began operations on January 10, 1863. It was built largely in shallow tunnels (see more at cut and cover) and is now part of the London Underground. It was worked by steam trains, and despite the creation of numerous vents, was unhealthy and uncomfortable for passengers and operating staff. Nevertheless, its trains were popular from the start and the Metropolitan Railway and the competing Metropolitan District Railway developed the inner circle around central London (completed in 1884) and an extensive system of suburban branches to the northwest (extending into the adjoining countryside), the west, the southwest and the east (mostly completed by 1904).

Liverpool James Street railway station together with Hamilton Square underground station in Birkenhead are the oldest deep level underground stations in the world, London's underground stations were just below the street surface built via the cut and cover method. The stations were so deep they required lifts to access easily, this gave another world's first in having the first lift accessed stations. The lifts were hydraulically operated. For the first deep-level tube line, the City and South London Railway, two 10 feet 2 inches (3.10 m) diameter circular tunnels were dug between King William Street (close to today's Monument station) and Stockwell, under the roads to avoid the need for agreement with owners of property on the surface. This opened in 1890 with electric locomotives that hauled carriages with small opaque windows, nicknamed padded cells.

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