What are railway tunnels and why are they necessary?

What is a railway tunnel?

In general terms, a railway 'tunnel' is a special type of Bridge (Overline). Most tunnels are longer (from a railway perspective) than the other types of Bridge (Overline). However, length is not the key determining factor: the key difference is that a tunnel allows the natural or human-modified land surface to continue across or 'over' the railway in a relatively unconstrained and uninterrupted form. In some cases, this is achieved by creating a cutting, covering it with a structural (load-bearing) lid, then reinstating the landscape over the top. This type of tunnel is often referred to as a 'cut-and-cover' tunnel. However, in most cases within the SCRCA, the tunnel was created by 'tunnelling' (i.e. burrowing) below the ground surface. Some tunnels are created using a combination of the two techniques.

For the purposes of the SCRCA Project, there are four key location types / location variants associated with tunnels:

  1. Tunnel Mouth: the entrance / exit portals at either end of the tunnel.
  2. Bridge (Tunnel): the underground structure linking the two tunnel mouths.
  3. Access Shaft: the vertical shafts created to facilitate the construction of the tunnel (and then, in some cases, to provide ventilation on a permanent basis).
  4. Spoil Tip: the mounds of excavated rock and earth that are still clearly visible beside many of the tunnel mouths and access shafts.

Why were tunnels needed?

Railways work best when they are reasonably flat and level. However, the natural land surface of north-west England takes the form of a series of high hills intersected by deep valleys. The surveyors tasked with finding a route for the Settle-Carlisle Railway soon noticed that four major valleys (Ribblesdale, Dentdale, Garsdale and the Eden Valley) could be used for much of route. There is a natural pass connecting Garsdale and the Eden Valley and this provided a usable route between these two valleys at Ais Gill Summit (the highest point on the line). However, there was no easy (naturally occurring) way of passing from Ribblesdale into Dentdale or from Dentdale into Garsdale. Going over the intervening hills was not an option because the ground between the valleys was too high and too steep. Going around the hills was not an option because it would have made the route too long and taken it too close to routes used by competitors. The only option left was to go through the hills, which involved digging and blasting two very long (and very expensive) tunnels, namely Blea Moor Tunnel (2,629 yards long) and Rise Hill Tunnel (1,213 yards long).

In addition to the two major tunnels, there are eleven shorter tunnels on the Settle-Carlisle Railway. These were constructed to help keep the line reasonably straight (an important consideration for any high-speed railway), to reduce the amount of land needed for the railway, to avoid the problems associated with very deep cuttings, and to placate influential landowners concerned about access issues, noise and other negative aspects of the new railway.

How were tunnels built?

Brief details of the some of the contractual obligations relating to Blea Moor Tunnel are provided in the "BATTYE MOSS VIADUCT AND BLEA MOOR TUNNEL" section of the indenture for Contract No. 1 (signed on 13th August, 1869).

We are also very fortunate in having a number of excellent first-hand accounts relating to the construction of the Settle-Carlisle Railway. The following in particular provide vivid accounts of the construction of Blea Moor Tunnel and Rise Hill Tunnel:

Also, the spoil tips provide us with a fascinating (and highly visible) insight into the construction process. This is explored in detail in the article 'What are spoil tips and how were they formed?'.

Acknowledgements

Text and photographs by Mark R. Harvey (© Mark R. Harvey, 2017).

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