Tunnelling

 

Underground Working Parties

Typically there would be 4 tunnellers working at the face (one of whom would be resting), plus 2 tunnellers mates handling trolley, air pumping and preparation of timber.  Generally infantry working parties were employed to remove the spoil from the shaft heads and dispose of it.  (One foot run of subway created 70 sandbags of spoil).   At any time a Tunnelling Coy would have several digging parties working towards each other.  A common pattern was that in each Section three of the four shifts would be in line, each for 4 days, and one shift resting or on light duties out of line for 4 days.  Shifts normally worked underground for 8 hours in every 24 hours.  The 16 hours ‘off’ included moving into and out of the line.

Tunnel Construction

This was influenced by many factors. As a rule of thumb a Tunnelling Coy would drive a subway, including side chambers, at about 20ft (6m) per day.  Rates over double this were achieved on occasion.  The British miners tried to strike a balance between sufficient space to work efficiently and minimizing the spoil to be removed.  Fighting tunnels were generally between 4ft 6 ins and 5ft 2 ins high (1.4m to 1.6m) and 2 ft 6 ins to 3 ft wide (0.8m to 0.9m).  There were though many variations depending on circumstances and tunnelling companies.  For example the New Zealand Tunnelling Company preferred more head room.  It was rare to construct crawl only tunnels, though sometimes in emergency (usually in clay) the miners would burrow what they termed ‘rabbit holes’.  French miners tended to smaller tunnels than the British.  Dimensions of German tunnels were similar or slightly larger than the British.

British practice was to use no more support than essential.  In clay this did entail extensive timbering but in chalk, supports were only used at weak points or in fractured ground close to mine blows.  In Flanders, where deep tunnels were driven through ‘Blue Lias’ (a form of clay) it was sometimes necessary to use steel girders to resist expansion pressure.  German practice was to ‘close timber’ most tunnels regardless of support requirements, a procedure that slowed down their progress.

When silent digging the chalk face was softened with water before pieces were prised out with a spike.  This was a very slow process and rarely used.

Locating Enemy Tunnels

Ground is a good conductor of sound so by listening to sounds of enemy working and then taking compass fixes from several underground positions and obtaining an intersection it is possible to locate enemy activity.  The most effective instrument for this was a French device known as a geophone.  This comprised two pressure sensitive diaphragms with a tube leading from each to the ear of the listener.  These would be moved until equal pitch was obtained in each ear.  A bearing taken at right angles to the line between the two diaphragms would show the direction of the sound.  Listeners (usually officers or Senior NCOs) were selected for their near perfect aural capability and trained for about six weeks at a special ‘listening school’.  The procedure was remarkably accurate, and experts could even use it to determine differences of elevation.  Numerous methods were employed by both sides to confuse or mislead the other, or to draw the opposing miners into an underground ambush.  Listeners were involved in a very deadly game of bluff.

 

Explosives

Guncotton and gunpowder was used in the early mines.  However from early 1915 the British employed ammonal.  This is a compound of 65% ammonium nitrate, 15% TNT, 17% coarse aluminum and 3% charcoal.  It is an inert slow lifting explosive.   To explode it has to be ‘hit’ by a powerful detonation wave, usually provided by guncotton primers.  These in turn have to be initiated by detonators containing highly sensitive fulminate of mercury.  Firing was usually with an electrical circuit.  For some very large mines  gelignite boosters were used. 

Ammonal was normally decanted from 50lb tins into 25lb rubberized bags clamped with wooden slats.  Most of the ammonal found in the Vimy tunnels by the Durand Group is still in near prime condition.

Intensity

In June 1916 (the peak month), along the line of the British front, the British fired 101 mines or camouflets; the Germans fired 126; a total of 227 in the month.

This equals one mine every three hours

Hazards

More miners were probably killed and wounded by shelling and harassing fire on the surface than underground.  The greatest subsurface killer was carbon monoxide (CO) poisoning, a consequence of gases from the mine and camouflet explosions.  Of course many were killed by being crushed, buried or entombed.  CO is particularly invidious being odourless, colourless, and slightly lighter than air.  It combines with the haemoglobin of the blood to displace oxygen, and also acts on the central nervous system.  The effect is not immediately felt.  A person exposed to even a very small proportion in the air becomes drowsy, and will then suddenly collapse.  Unless quickly supplied with oxygen death inevitably follows.

To give warning of the presence of CO caged canaries and mice would accompany miners working underground.  Both have a very high metabolic rate and are very sensitive to carbon monoxide and other gases.  Some men even had their own personal mice that they kept in a pocket.  The canary’s claws were always clipped so that on becoming unconscious they would fall off their perch.  Usually they revived on return to good air and it was customary to ‘retire’ them as pets after a half dozen or so occurrences.  Incidentally an escaped canary could compromise security in areas where mining was not suspected by the enemy and there are several recorded cases of artillery or mortars being used to ‘extinguish’ escaped birds.

The miners in the deep tunnels were rarely troubled by war gases, but had to take exceptional precautions against ingress of gases (such as CO) from mine explosions which could leak a considerable distance into tunnels through cracks in the chalk.  Consequently the deep mining systems were compartmentalised with gas doors which could be quickly released to seal an area.  Pockets of CO still exist in the chalk today.

Ventilation was used extensively to combat CO and to provide fresh air into the tunnel systems.  Methods varied, ideally employing an updraught process that provided natural ventilation.  At gallery headings air was hand pumped in along metal pipes.

A very extensive rescue system was set up and each Army had a Mine Rescue School.  As a general rule of thumb, in mining areas, there was a mine rescue station to every 1000 yards of front.  Rescuers where equipped with a 'Proto' breathing system and the Officers carried electric torches.

Water and flooding was also a serious problem and a lot of manpower effort (usually from attached infantry) had to be used for hand pumping – although power driven pumps were used when practicable.  Sudden flooding was sometimes a risk, and men were on occasion drowned, particularly when trapped and rescuers could not get to them in time.  It was not however one of the major hazards.

 Casualties

181 mining officers lost their lives including 9 from the Canadian Coys, 12 from the Australian and 3 from the New Zealand.   This is approximately 30% of front line strength.  It is not known how many soldiers of the Tunnelling Coys died but around 3,000 would seem a fair estimate.  Probably about one third of the casualties were incurred underground.