42
the rail engineer • February 2014
S
omething extraordinary is happening under London. Six mechanical navvies, bewildering in their efficiency, steer Crossrail’s tunnels through tangled subterranean history towards an end-point later this year. Forever hungry, Ada, Phyllis, Victoria, Elizabeth, Mary and Sophia each mine and line over 20 metres daily to tolerances measured in millimetres; verified by inertial navigation systems, scrutinised from comfy chairs. As enterprises go, this one is exceptional and yet that reality rarely impinges on our consciousness. Why don’t we awe anymore?
CGI: FOUR BY THREE
When something extraordinary happened two centuries ago - and it very often did - inquisitive crowds gathered, cheering and waving flags. Great engineers became celebrities of the day, without ever taking their kit off or eating worms in a jungle. Men of vision and tenacity - Locke, Vignoles, Stephenson, Brunel - forged national transformation. Our leaders today seem content to tinker, deciding what colour to make fag packets. Trawl through the newspaper archives and it’s clear that tunnels were sources of intrigue and wonder to the Victorians: portals into a new age. But how were tunnels constructed in a Time Before Machinery (TBM)? Keen for answers, I spent much of last summer producing a short film about it, targeted at those amongst the public with an underlying curiosity, but perhaps also of interest to the engineering fraternity. Written insight is not scarce on this subject, but generally accepted as the definitive work is Practical Tunnelling, a tome authored by Frederick Simms who secured the role of resident engineer on the South Eastern Railway in 1836, thereafter recording the tribulations encountered with Bletchingley and
CGI: FOUR BY THREE
Tunnelvision GRAEME BICKERDIKE
Saltwood tunnels. Contractor Charles Gripper published another book, Railway Tunnelling in Heavy Ground, in 1879, apparently unhappy that Simms had “rather hurried over his explanations of mining operations”. Noteworthy amongst the many papers is one exploring the seven-year construction of the first bore at Woodhead - extending for three miles between Sheffield and Manchester which had been committed to the charge of Wellington Purdon, a surveyor from Killucan, County Westmeath, who was just 23 when he took on the project in 1838. There’s little point cluttering up this space by repeating the film’s content. If you fancy investing 20 minutes of your life on the basics of Victorian tunnelling, a link to the film is provided below. Remember, the script considers a generic tunnel - it is not applicable to every one in existence. In these pages we’ll reflect in more detail on some of the challenges that had to be overcome.
Line of sight Progress with lengthy tunnels was usually expedited by sinking construction shafts, allowing them to be driven from intermediate points as well as their ends. Sighting towers, known as observatories, were erected as fixed reference points for setting out purposes; the tunnel line, and hence the shaft centres, could then be established at any point over the hill. Although substantially built, very few still stand as their valuable fabric was mostly recycled when work had advanced sufficiently. But survivors can be found at Merstham in Surrey and Bramhope on the Leeds-Harrogate line, in brick and stone respectively. Often 30 or 40 feet high, the towers housed a staircase spiralling around a central pier, the latter being entirely freestanding so as not to be influenced by any movement of the building. On top of it was a stone table supporting a transit instrument, typically incorporating a 30-inch focal length telescope which looked out through openings at the top of the tower. Provision of a basic telegraph system enabled the engineer to direct his assistant with the ranging pole.
The excavated ground had to be supported by huge timbers propped off transverse beams.