Page 432 - Advanced Mine Ventilation

Mine Sealing and Recovery 401

Temporary stoppings are also built in a mine fire emergency situation to protect
people engaged in firefighting. They are made of a wooden frame with brattice cloth
or belting on top [1]. Another variation of a temporary stopping is a parachute stopping
developed by Kissell [3]. They look similar to the parachute used to jump off an
airplane and are made of impermeable fabric. They can be installed in minutes and
are mostly used during rescue and recovery operations.

24.1.2.2 Permanent Stoppings/Seals
Permanent seals are mostly used to isolate old/abandoned workings in a coal mine.
They may not be explosion proof but are generally much stronger than temporary stop-
pings. They are typically made of masonry/concrete blocks with occasional steel rein-
forcing. US regulations [2] require them to be strong enough to withstand:

1. 50 psi overpressure when the atmosphere in the sealed area is monitored and maintained
inert. Here inert means that the oxygen content is less than 10% and methane concentration
is either less than 3% or greater than 20%. Each seal will have two sampling pipes in it. One
pipe shall extend about 15 ft into the sealed area and another will extend to the center of the
first connecting crosscut inbye the seal.
2. 120 psi overpressure if the sealed area atmosphere is not monitored and is not maintained
inert but it is not explosive.
3. An overpressure greater than 120 psi if the sealed area is not monitored and is likely to
contain an explosive/flammable gas mixture: methane between 4.5% and 17% and oxygen
above 17% throughout the entire sealed area.
These stoppings to withstand 120 psi are typically masonry stoppings about 4 ft in
thickness and are hitched in the roof and floor. No steel reinforcing is used.
Using the equation for failure in a pressurized cylindrical vessel, we can check if
this is adequate.

Pd
s ¼ (24.1)
4t
where s is the tensile strength of the vessel wall in psi, P is the inside pressure behind
the seal, d is the entry/vessel diameter, and t equals thickness of the seal, in feet.
Typical safe tensile strength for:
Masonry ¼ 100 psi
P ¼ 120 psi;
2
d ¼ 10 ft (75 ft area of tunnel)
120 10
Hence, t ¼ ¼ 3 ft.
100 4
A 4 ft thick seal is, therefore, adequate to withstand 120 psi.
24.1.2.3 Explosion-Proof Stoppings

If a sealed area is likely to contain an explosive gas mixture and it is not monitored,
the seals must be designed to withstand an internal pressure up to 640 psi (refer to