Safety and safety planning - Tunnel Engineering: Basics and Additional Services for Design and Construction - page 315

formation of hazardous materials should be used and care should also be taken that there
is adequate excess oxygen. If explosives in powder form are used, care has to be taken that
these do not become damp or become compacted during loading.
Another reduction of harmful substances can be achieved through meticulous filling of the
shots, particularly with the use of water-stemming ampoules, and adequate spraying of the
muck pile, where the nitrous gases collect.
Adequate ventilation must always be provided to ensure adequate thinning of the hazard-
ous gases and dust (see also Volume I Section 8.2).
Drilling into the remains of holes from the previous round. The risk of an accident caused
by the detonation of explosive residues remaining in a hole from the previous round can
primarily be countered by always loading the priming cartridge at the end of the hole. If this
is done, it can be practically ruled out that subsequent drilling encounters explosive residues.
Explosive residues in the muck pile. The provision of stemming, particularly with stem-
ming screens for contour holes, can practically rule out the ejection of cartridges. The
muck pile should nonetheless be inspected for any explosive residues before loading and
carting away.
Loading charges before the completion of drilling. Unintended drilling into an already
charged hole can be ruled out by always maintaining a safety margin between the drill and
the nearest charged hole, and this should be at least one hole depth.
Firedamp. In the presence of methane, unintended detonation of an explosive mixture of air
and methane - it only takes a spark to ignite it - must be avoided. All tunnels, in which the geo-
logical conditions do not permit the occurrence of firedamp to be ruled out in advance, should
be provided with continuously functioning monitoring instruments to detect any potentially
explosive concentrations. When 50 % of the lower explosion limit for methane is detected
(corresponding to 2.5 % concentration in the air), not only blasting work but also all works
should be stopped until appropriate ventilation has reduced the reading to below this limit.
In addition, in cases where the occurrence of firedamp has to be expected, only safety
blasting explosives (and also the corresponding detonators with copper casings) should be
used. These do indeed have the disadvantage of less explosive effect, but cannot detonate
firedamp along with the blast. As an additional measure, the priming cartridge should be
loaded last and all holes should be carefully sealed by stemming.
Stray currents. The danger of early detonation due to stray current is primarily countered
by using highly unsusceptible detonators.
Rails, pipes and electrical conductors running parallel to them should be electrically connected
at a defined spacing, normally every 50 m, and earthed. The firing circuit and the firing cable
should be separated from rails, pipes etc. by an adequate distance, normally at least 50 cm.
Lightning. When electric detonators are used near the surface, all blasting work should
be stopped when a thunderstorm is approaching and already loaded charges should be
disconnected.
Working from both ends. When the two tunnel drives are near enough to represent a risk,
normally within 25 m, the miners in the opposing drive should be informed before blasting
so they can leave their workplace and find a safe location. When the two drives are still
nearer, normally within 10 m, excavation should only take place from one side.