Traditional tunnelling is a vague term that can lead to confusion. Often it refers to hand mining regardless of the tunnel lining, in some countries such as Japan it can refer to excavation by semi-mechanised TBM as opposed to a closed shield, and sometimes it can refer to the means of support during excavation.

With regards to hand mining, it is never used for excavations of serious length in modern tunnelling, purely for economical reasons in an age of mechanisation. Tunnel finishing work on the periphery with a spader, or interfacing a new tunnel into an old system, is more likely for the space might be too small, or too awkward for a small hydraulic excavator to be used effectively.

Other factors limit its work even for small-scale, restricted use. The most obvious are the health and safety implications of having men working directly on the ground. This opens workers up to falls of ground; possible working in close proximity to heavy machinery; manual operation of hydraulic equipment with the burden of lifting it, possibly over head, opening the miner up to risks of falling equipment or balance issues that can endanger fellow miners. The risks are there to be seen.

Vibration in hand mining
When Tunnels last looked into handwork in a technical focus specifically targeted at hand mining, the focus was immediately shifted to Reynaud’s Phenomenon; the infamous and most apparent symptom of Hand Arm Vibration Syndrome – White Finger.

This is an issue that has not so much divided the UK tunnelling industry as placed the Health & Safety Executive (HSE) on one side, and experienced contractors on the other. The prevailing thought seems to be that while vibration is a risk, it is not one that applies overly to the tunnelling industry. When tunnellers use a hand tool, it is for short periods and not for prolonged exposure times as are experienced by road breakers or ship riveters.

However, the legislation is in place and was made an enforceable matter around ten years ago in the UK by the HSE. Elsewhere in the world, ISO 5349 on mechanical vibration is frequently used to provide limit values for hand miners, and sometimes worker protections are symptom-led (HAVS progresses increasingly quickly with further exposure).

A science of experience
London Clay has possibly been the most extensively used geology for the development of soft ground timbering in the world. Work on the London Underground has run in stages for nearly 150 years, from the days in the immediate aftermath of the Crimean War to the present. In this time it has seen linings of brick, steel and concrete. A working understanding of controlling the face and supporting the excavation during work has developed over many decades of observation.

With the advent of mechanised tunnelling, soft ground tunnelling has been given over to TBM excavation with an immediately erected and grouted precast concrete ring. The temporary support of timber is not needed in such an environment, or so runs the common line of thought. There is a possible risk that the knowledge gathered over the years will be lost, or at least not understood first hand by a new, computer-aided generation of tunnellers.

Timbering
The term timbering, as implied, comes from the timbers that were traditionally used as the material for the supports. This was due to the suitability of timber as a lightweight material with "excellent" load-bearing capabilities. Structural steel sections have also been used where loads and stress were particularly high.

A new guide on traditional timbering
The British Tunnelling Society (BTS) is in the process of publishing a new guide for traditional timber work in soft ground tunnelling. Written by retired contractor Colin Mackenzie, it delves into the main points of temporary tunnelling support in London Clay, identifying the particular challenges that render the techniques given suitable for many other types of clay. It does not delve into long-term support, highlighting the plastic behaviour of London Clay that develops as a squeezing load over decades. Temporary supports, covering ‘short-term’ loads, are designed to the requirements of support for "days, weeks and, very occasionally, months".

The guide also covers typical component speci¬fications for the various timber section sizes, and diagrams drawn in the traditional style relating to projects over the past 50 years, with full procedure details listed beside.

Author
Colin Mackenzie is a retired civil engineer with forty years of experience contracting in the UK. He earned a degree in civil engineering from University of Aberdeen in 1961.

He moved to Mitchell Construction working on the Awe Hydroelectric Project in Argyll, Scotland before
travelling to London in 1964 to work for John Mowlem on the Victoria Line. Over the next 24 years he reached the position of director of civil engineering responsible for tunnelling activities and worked across the UK before moving to Amec Civil Engineering in 1988 as director responsible for tunnelling, where he remained until his retirement in 2001.

In his career he has received the Telford Gold Medal and the James Clark Medal from the Institution of Civil Engineers, as well as the Sword of Honour from the British Safety Council. He is a former chairman of the BTS and held a position on its committee for some nine years.

British Tunnelling Society interest
The BTS was formed in 1971 as a learned society of the Institution of Civil Engineers to provide a forum for discussion on tunnel related matters. In its own words, it also publishes industry guidelines and codes of practice, conducts design and construction training courses.

Traditional timbering techniques that emerged from decades of underground construction in London Clay, for the London Underground metro system, are no longer widely used. Mackenzie has written the Timbering Guide from a position of experience, and the BTS plans to publish the resulting document as an aid to the new generation of tunnellers who are perhaps less familiar with the traditional methods, concerns and requirements.

Modern developments
Mackenzie points to a loss of separation of piecework payments between less demanding machine work and more demanding handwork.

There is pressure, he says, to drive handwork payments even higher, increasing the cost of traditional working methods "at the very time that modern developments such as compact electro-hydraulic excavators, lighter structural support components and spheroidal graphite ductile cast iron segmental tunnel lining are available to make the work less arduous and more cost effective".

Speaking to Tunnels, he adds, "A good rate is possible with the system – the spheroidal graphite cast iron segments deal with the final loading. It is not like sprayed concrete lining where you have a primary and secondary lining"