coal trade bulletin - Clpdigital.org

with the rapidity of a prairie fire, and until sufficient
barriers or areas of solid coal are reached,
capable of counteracting this now tremendous
dynamic force, no available power can stop it.
Sometimes such a squeeze will destroy acres of
pillar coal and render impossible, or very expensive,
the mining of other coal lying beyond the
affected area which is only accessible through
squeezed territory. The writer has in mind an
instance where over 75 acres of pillars were
crushed so badly, in a period of less than two
weeks' time, that all hopes of ever recovering
the pillars were abandoned. Another example of
the toll a squeeze demands is the loss of life in
the Twin shaft disaster in the Wyoming region of
the Pennsylvania anthracite field, where men weie
buried dive, whose bodies never have been recovered.
These difficulties and the great expense attendant
on relaying track ancl cleaning up slate falls
with which the pillars were so frequently congested,
brought a further change in the room and
pillar method of mining, so that the pillars could
be more economically extracted, and in the event
of a squeeze starting it could be controlled, and
developed what is known as the "panel system"
of room and pillar work.
This panel system may be said to represent tbe
better practice of today. It is pertinent, therefore,
to know- what
SIZE OF PANEL,
width of entries, width and depth of rooms and
size of pillars will insure against squeezes and
yield the maximum of recovery at a minimum of
expenditure.
The factors that decide the details of pillar
work are numerous, some of which cannot, be completely
predetermined, some are unforeseen, and
others must be accepted, though often disadvantageous.
For example, the limitations of the
property may be such as to require opening it at
a point where the haulage will be on adverse
grades, room workings over a portion of the property
may be to the clip, requiring at a future date
to draw pillars up hill and possibly out of water.
Prospecting of the property, whether it is above
or below water level, is of the utmost importance.
If a shaft proposition, a careful geological study
of the strata through which the shaft is to be
sunk should always be made, in any event the
dip and strike of the seam should be determined.
Many costly errors have been made by neglecting
these all too obvious precautions. Numerous examples
might be mentioned of errors, caused as
above mentioned, that affect the cost of mining
and the percentage of recovery.
The influence of underground factors is not
so obvious. The overlying burden is often thought
to be the determining factor in regard to room
THE COAL TRADE BULLETIN. 35
widths and size of pillars, but in general it determines
the minimum size of pillars and the maximum
width of room only, which may be materially
different from the size of pillar and room width
as determined by other factors. However, a full
knowledge of the manner in which tbe
WEIGHT OF THE HOOF
tends to act should be thoroughly understood in
order that we may determine approximately what
will ba]>pen when the coal is being worked and
the pillars extracted. For an excellent analysis
of this question of roof weights, and action, the
writer would refer you to the papers of H. W. G.
Halbaum, entitled "The Action, Influence, ancl Control
of the Roof": and, "Great Planes of Strain
in the Absolute Roof of Mines." These papers
appeared in the Transactions of the North of England
Institute of Mining and Mechanical Engineers.
Briefly there are two forces acting which compose
the total roof action, a vertical force acting
Figure 1
downward and a horizontal force acting opposite
to the direction in which the workings are advancing.
The resultant of these two forces is in
a direction from over the solid coal toward the
excavated area and tends to break tbe roof over
the pillars so that one must constantly be alert
from such action. If the pillars are not mined
clean and the weight of the strata overlying the
excavated area allowed to rest freely on the bottom,
thus changing the magnitude and causing a
more horizontal direction of the resultant force
of the roof action, the tendency often will be for
the roof not to break but gradually to sag until
at some distant point in the gob it reaches the
bottom. This gradual subsidence of the roof
often will be so severe as to crush the end of the
pillars next the gob and prevent getting cars to
the point where it is desired to load them, causing
MORE UNCLEAN* MINING
and often when this stage is reached it will be