QUESTION: Isn't it safest for a deminer to work in a prone position?
ANSWER: Differences of opinion
over the safest position in which to excavate a detector reading
do exist. Most groups are flexible, allowing deminers to lie
down if they want to - and sometimes requiring that they lie
down if working on steep slopes (demining on slopes is always
supposed to be done uphill). This last is to improve stability.
That said, the vast majority (99.9%) of demining excavations
are made from a kneeling/squatting position.
No one works "prone".
is because working in the "prone position" is not actually possible. Try
it with your chest on the ground. You will not be able to see
the ground more than a few cm from your head when there is no vegetation
stubble, and arm movements to probe/excavate will be cramped
and clumsy. If digging to any depth, you must move over the
excavation to see what you are doing. Trying to keep your head bent back to see is very uncomfortable. People working in the prone position actually
work with their head raised and resting on one forearm. This
makes it possible to see and to work up to around 50cm in front
of them - but they usually work closer.
picture below was copied from the cover of the Cambodia MAC's old SOPs.
It shows how the deminer holds himself up and, if he is guiding
his prodder, how the length of his forearm dictates the distance
he can work away from his elbow. This is the way they
were told to work by inexperienced "experts".
face is barely 40cm from the place where his prodder enters
the ground - and the top of his head is within the 60 degree
line of the fragment cone (shown in red). Fragments of
earth, stone, roots and mine casing are ejected from a blast
mine detonation in an "ejecta-cone" that is crudely 60 degrees
from the ground.
fact, the ejecta-cone angle depends of ground-structure and
mine depth. If the mine is close to the surface the spread can
be far greater. A greater spread means less concentrated fragments,
but a wider area of risk. The fragments are generally traveling
erratically and so slow down relatively quickly whe compared with ballistic projectiles. Close up, their irregular shape and great heat make them rather nasty. Most who are aware
of the ejecta-cone prefer to have their heads further away than
they could be if they were working on one elbow. The CMAC deminer
is not even wearing a visor - but in any case the CMAC deminers
ignored the SOPs and actually worked kneeling whenever the "experts" were not there. Several who had accidents
in the kneeling position still lost their eyes because of the
inadequate industrial "safety" spectacles they had been issued for protection, but sometimes these spectacles worked. One of these victims was
even refused compensation because he was in breach of SOPs at
the time - because he was not lying down. But that has all changed in Cambodia today - where reliance on outside experts has diminished.
people are less aware of is the "blast front". Expressed simply,
when high explosive detonates, the small volume of High Explosive tries
to become a large volume of gas instantly. The interface between
the gas and the world is called the "blast front". It does not
have the same direction as the material objects ejected as fragments
and spreads low to the ground. It is moving very rapidly indeed
at first, but because it is expanding by volume, as its volume
increases the "blast front" interface slows down. In practical
terms, at 5cm the blast front from a PMN mine will often rip flesh
from a hand and may tear whole fingers away. At 30cm it will
blast front is more of a "dome" than a cone, but again its shape
is partly dependant on the mine depth and ground-structure.
In detonations of mines only just beneath the surface, you can
see the surrounding dust being sucked under the dome to fill
the low-pressure area behind the expanding blast-front.
you work in the position shown above (some call it "semi-prone"),
your head is subjected to a far greater shock-wave than if you
were kneeling and your head were further away. Kneeling does
not always mean that the head is further away - but does when
appropriate tools are in use (at least 30cm long).
prone position is a hangover from military training when demining
may be undertaken under fire, so keeping a low profile
makes sense. Humanitarian Demining is never conducted under
fire, so "open-minded" people considered other positions as
soon as Humanitarian Demining got going. A few groups disarm mines while lying down, but this does not reduce the risk to their hands (the most common disarming injury). Lying down at such times is probably only of psychological benefit... and if it makes a deminer feel more confident it may have some value.
the injury data in the Database of Demining Accidents, I can
find no evidence that "prone is safer". But since almost no
one works prone, I cannot find evidence to show that it is less
safe either. In the absence of compelling evidence, the judgment
comes down to applying common sense to what is known about small
blast events - and about the protective equipment in use.
a deminer detonates a blast mine in a kneeling position when
wearing a 5mm visor properly and using long tools, his head
and face will usually be OK. If a deminer detonates the same mine when
lying down, there is a greatly increased pressure on the head
and far faster fragments hitting the visor. 5mm visors are not
perfect: they are simply the thickest (heaviest) polycarbonate
visor that it is practical to wear. Tested at 60cm, they perform very well. At 30cm
there is a far higher risk of their penetration by ejecta and
their fracture by blast-front impact. This has been proven in tests using
real mines many times.
that wear 5mm full-face visors on a head frame and frontal apron
protection are wearing armour DESIGNED to be used in a kneeling/squatting
position. If they lie down, their armour and visors will often
anyone wanted to work "semi-prone" - they SHOULD wear an armoured helmet
and long-visor - and should wear chest and shoulder armour designed
for that position. If visors are compromised or brain
injury from shock occurs - the group management that told
deminers to put their heads so close to a blast would bear some responsibility for the unnecessary injuries incurred.
Professionals in humanitarian demining do not work "prone" except in very exceptional circumstances.