Elk City mining news. (Elk City, Idaho) 1903-1913, 1906-03-10
Air Blasts IN Mines.

miners in the Lake Superior copper re-
gion have left work because earth
quakes or air-blasts have been So fire.
quent and so severe as to endanger
their lives. In several mines there
have been a. series of shocks of more
than ordinary intensityis and even at
surface houses have been shaken and
disorder created in the china pantry.
AS rule, these disturbances. are sensi-
tive to atmospheric conditions; it has
been noticed that they begin shortly
midnight, to continue to less degree
during the following daytime.
This !, is an interesting subject, the
more so because it is not well under
stood. Similar occurrences under
ground have been recorded in the Hill
grove district, New South Wales, and
in the Mysore mines, India. Miners
usually impute these rock explosions
to earthquakes, to gases and to the
little men, the gnomes who, accord
ing to folklore, are supposed to delve
underground when the workmen leave
the mine. These air blasts are not
really explosions, but they are due to
the bursting of rock under great pres
sure and the disturbance to the air of
the mine produced by the shock of dis
ruption. They characterize mines
where the rock is under particular
strain by reason of big cavities created
by STOPPING, causing large masses of
rock to exist under unbalanced stress.
1t is the custom to support the hang
ing wall by means of portions of lode
which are left standing, to do duty as.
pillars; these pillars hold the ground,
until the pressure from above becomes
too much for them. Then they do not
yield gently, but, like cubical block
under testing machine, they break
suddenly with explosive violence. The
bursting force is due to external and
not to internal conditions, as has been
pointed out by Mr.. F. w. Sperr. pro-
fessor of mining in the Michigan College
of Mines. When the pressure has be-
come more than the pillar of rock can
bear and the moment of rupture has
arrived, even the scratch of pick will
preeipitate a crash When this hap
penS,. the walls of the stope will vibrats
and.an earth-tremor is transmitted to
surface. Where the rock will stand
and the walls yield slowly, so as to.
close the excavations made by mining
there are no air blasts or similar dis
turbances. Where large areas of roof
are left unsupported in ..fiat veins, it
sometimes happens that the hanging
finally falls suddenly, and then, acting
like piston, it forces the air out of
the old Slopes with all the violence of a
gas explosion. But it is the former
and not the latter case that obtains
near Houghton. In such mines as the
Quincy, where the copper lode dips at
81 and an area several thousand feet
in both directions has been excavated
for width of to 20 feet, there is a
big 'overhang' and an excessive pres
sure, which is suddenly released when
one of the pillars in the old slopes gives
way. 1t is question whether it would
not be better to rob the pillars and
permit the hanging to come down and
key itself. In New South Wales, in
mines similarly affected, the rock is a
silicious slate, which, after exposure to
the air, splits and kicks with violence
although the more serious explosions
occur when bridge is being removed
below alevel. Mems faces have been
badly cut by flying splinters of rock.
1t is held by Mr.. j. B. Jaquet, the
Chief Inspector If Mines in, New South
! Waless that the kicking is due to the
fact that the slate is so brittle that il
breaks rather than bends; it resists
pressure as long as possible and ther
bursts, this being FASTENED by the shock
of blasting or hammer-work. The
large explosions are due to the same
general cause, namely, general con
dition of strain due to the removal Of
rock by mining. 1t is noticed that the
disturbances at Hillgrove have gained
in severity as the mine has deepened
in the neighboring Sunlight mine these
troubles are unknown. because the
workings are in slate of more yield
ing character. 1t has been recon-
mended that in Kicking ground the
pressure be taken up gradually by the
use of soft hsad-boards in conjunction
with stulls and the filling of Slopes
i when thew anntnach the Laval ohoua