coal trade bulletin - Clpdigital.org

36 THE COAL TRADE BULLETIN.
found that neither the setting of props nor their
removal will affect the action of the roof.
In general it may be said that so far as the
weight of the roof is concerned the following
dimensions of the pillars apply:
Depth of Size of Width of
Cover. Pillars. Rooms.
300 15 ft. 20 ft.
500 20 ft. 20 ft.
700 25 ft. 20 ft.
900 30 ft. 20 ft.
1200 35 ft. 20 ft.
Ordinarily the pillars within certain limits are
made of uniform size, in the same mine or group
of mines.
By reference to the above table it will be seen
that the size of the room pillars required undeineath
a ravine where the cover is only 150 feet
should be 15 feet by 80 feet ancl underneath a
mountain top where the cover is 175 feet the size
of the pillars should be 20 feet by SO feet. As
a matter of fact all are somewhat larger than the
maximum size required according to the table, for
the reason that the top immediately overlying the
seam falls, and the pillars should not only be large
enough to support their unit weight while rooms
are driving, but also at the time the
PILLARS ARE 11EINO ROBBED
back. Changing room spacing would involve a
further complication conducive to losses of coal
in that the small pillars would be robbed back
more quickly than the larger ones and a very
irregular breakline would result, causing some
pillars to extend far back into the gob, and be
crushed.
An error that is sometimes made in drift mining
is to suppose that because workings are near the
outcrop and the cover light, wide rooms on narrow
centres may be driven. While it is true that
the cover is light, as a rule it is so badly weathered
that it has little strength and before mining
has proceeded very far the rooms will become
congested with slatefall, splitting of the pillars
become imperative; the dead weight resting on
the wings of the pillars soon crushes them and
a squeeze may result causing large losses of coai.
Probably no one factor contributes more to determining
the working size of the pillars than
the nature of the top and bottom immediately adjacent
to the coal. Its behavior, especially that
of the top, under the forces acting during* robbing,
the weathering effects of the air, the effect of temperature
changes upon the top and its ability to
withstand tension, compression and shear are all
of prime importance. If the roof is of a material
that cannot resist these agencies, it will
fall very probably before the pillars are robbed,
cleaning up slate falls become a daily occurrence
and so great are the
DELAYS AND EXPENSE
occasioned that the cost becomes prohibitive, unclean
mining is certain, a squeeze comes on and
much good coal is lost resulting in only a partial
recovery. On the other hand, if the top is a
strong sandstone the impression is common that
very wide rooms may be driven and only small
pillars are necessary. Because of the great
strength of the top and the absence of slate falls,
the time of starting the robbing is a matter that
receives very little attention.
Under either a very strong top, or a. very weak
top, large pillars and narrow rooms are imperative
and the robbing should follow within a reasonably
short time after completion of the room.
An example of honest effort to obtain the
coal in the pillars, shows that wide, deep rooms
were driven on narrow centres, under very weak
to]) which was known not to stand weathering
well and fell for a distance of from five feet up to
15 feet and more. The rooms were driven during
the years 1898-99 and under the system then practiced
at the mine, robbing did not follow. Extracting
the pillars was started in the year 1907.
All of the rooms were congested with slate falls
and the work was started by slabbing. By this
method the miner was continually retreating from
his solid work and placing himself in danger from
the bad roof overhead. When the miner was
ready to come back with the remaining portion of
the pillar, it was often found to be so badly
crushed that only a partial recovery could be
obtained and frequently
SLATE FALLS
would close the space in. Splitting the pillar
from the entry to the aircourse above proved to
be a failure because the wings could not support
the load coming upon them and the timber cost
of attempting to recover the pillars in this way
became prohibitive. Finally in an honest effort
to recover the coal, if at all possible, counter haulways
were driven across the pillars and the falls
met in crossing the rooms cleaned up. Then
short splits were driven up the pillars from the
counter haulway, and the wings quickly robbed
back before they became so weakened that their
recovery was impossible. This resulted in the
maximum of recovery under the circumstances.
at an expense for this coal, for handling slate falls,
repetition of track-laying, and timbering, and
other items that have overdoubled the normal
cost of production at the mine.
The maximum of recovery can never be obtained,
at a minimum operating cost, by splitting
pillars and then loading out the wings if the roof
is bad or the robbing falls follow quickly after
the removal of the coal, for the reason that the robbing
fall on the wing prevents clean mining of
the other wing or a sufficient quantity of timber