|Tunnel at Crystal Palace, photographed on 13th September 2009.
photograph by Gregory Beecroft
|In engineering terms, a tunnel is normally regarded as a
structure with overburden, such as soil or rock. However, there are some
structures which are strictly speaking bridges, but are treated by the railway
as tunnels. This is usually because they impose operating or inspection and
maintenance constraints similar to those of a tunnel. Examples include
buildings on rafts over the railway, for example at Woolwich, and wide bridges
with restricted clearance, as at Fulwell.
When a cutting would need to exceed 60 feet in depth, it was usually more economic to construct a tunnel. Particularly difficult ground conditions or a requirement for a large volume of material for embankments, might make a deeper cutting preferable. Some tunnels were not necessary, but had to be constructed to meet the requirements of influential landowners.
Although South East England is low-lying, it is quite hilly. If any type of major structure typifies the Southern, it is tunnels, as the following comparison shows.
Southern tunnels more than a mile in length are listed below. All except Penge Tunnel are through the North or South Downs. There are few lengthy tunnels on former London & South Western Railway lines, the longest (apart from the Waterloo & City Line) being Honiton Tunnel at 1,345 yards.
There are much longer tunnels on High Speed 1. London Tunnel 1, between St Pancras International and Stratford International is 7,543 metres long (up) and 7,529 metres long (down). This is exceeded by London Tunnel 2, between Stratford International and Dagenham, at 10,120 metres. The Thames Tunnel is 3,121 metres long and the North Downs Tunnel is 3,199 metres long.
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This page was created 3 April 2010