1078

THE COLLIERY GUARDIAN.

June 8, 1917.

as the load for all practical purposes is equally distri-
buted, the curve will be a parabola with a longer or
shorter focal distance depending on the nature of the
strata. Let fig. 14 represent the section of a coal seam
from which the coal has been removed between A and
B, the roof having fallen to the irregular line ACB.
The dotted line A' C' B' will indicate the line of stress.
This, it will be seen, impinges on the coal close to the
edge at A. The stress at this point represents half the
weight of the strata overlying the span A B which is
assumed to be sufficient to crush the coal about the
point A. The integrity of the arch being destroyed,
the line of stress must seek a new position such as
DEB. Naturally this movement will be no greater
than is absolutely necessary to gain a solid footing for
the arch, which will again be so near the edge of the
coal already crushed that it will fail again in a short
while, necessitating a further adjustment of the position
of the arch. With this continuing failure and re-
adjustment we have the well-known phenomena of a
crush or squeeze advancing slowly over the workings,
destroying coal as it goes.

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Fig. 14.—Arch Stresses in Mine Roof.

If now a considerable body of ;the seam, as A H G F,
is quickly removed a “fall” may result which will
reach high above the seam, say to the line F J B, which
will cause the line of stress to move quickly and reach
the coal well back from the point F, where it is suffi-
ciently solid to give the needed support, and the
working will be said to have “ gotten ahead of the
crush,” when in fact the crushing force has gone ahead
of the working. This explains the common experience
of the relief at the working end of the pillar caused by
an extended break in the roof over the exhausted area.

Under other conditions, especially when the arching
line of stress has a wide span, thus carrying a large
amount of weight, the crushing force may prove too
much for even the solid coal well back from the end of
the pillar and cause the phenomena of crushed coal,
broken timbers, creeping floor, etc., well down the room
or stall, while the ends of the pillars will be free from
any trouble, as they carry only the small amount of
material which is below the line of stress. This con-
dition will sometimes be cured automatically by the
material falling from the top of the cavity over the
exhausted area in such a manner that the space between
the material already down and the undisturbed measures
will be filled and the opposite limb of the arch (the right
hand in the figure) will find support on this already
fallen material and thus shorten the span of the arch
and lessen the total weight.

When the break has reached the surface, this filling
takes place more rapidly owing to the fracture of the
overhanging beds along the edge of the break, and
since the arch has a new point of support for its inner
or right hand limb, the conditions are ripe for further
working undisturbed under the smaller arch.

While the general shape of the line of stress in the
cases under consideration is the parabola, for all
practical purposes the ratio between the span and the
rise or height of the arch will vary much as the material
varies in which it exists. After the fall of the first
mass the cavity grows through the crushing and falling
of the material along the top of the arch, due to the
pressure along the line of stress, and by the splitting
oft along the joint planes of the material on the sides
due to the same cause. Since the pressure along the
upper portion of the line of stress is manifestly less in
a high sharp arch than in a low flat one, the shape of
the arch in this respect may be expected to vary with
the capacity of the material to withstand this stress.
Hence, there will be a high arch in a soft material with
numerous joints and a flat arch in tough material with
fewer joints, This may be verified practically by the
examination of old drifts or tunnels where the over-
lying material has had an opportunity to fall and take
the shape due to such conditions without regard to
other influences.

If, then, the cavity in its upward progress encounters
a bed of tough resistant shale or sandstone, it may fall
so slowly that a large area may be opened by con-
tinuous mining during the delay, resulting in a heavy
weight along the line of stress due to the wide span and
crushing the coal either at the working end of the
pillar or at such point along the course of the room as
the line of stress may meet the coal. Such a crush is
not likely to find relief until the overlying measures
are sufficiently broken down to .fill the space S S, and
allow the development of a new smaller arch of stress
A B C, which, having less span and consequently less
load, will transmit less load to the point A.

According to Dr. F. W. McNair the readjustments
that take place when a pillar fails sometimes put an
enormous longitudinal thrust on the foot wall, and in
places its surface portion has buckled under such stress.
Experience seems to show that at the great depths
recently reached it is useless to expect to hold up the
hanging rock mass for a long time by any scheme of
pillars unless far too much of the lode is left in place,
and that the only feasible method is to cut away the
entire lode and permit the hanging to cave as rapidly
as it will to the point where the broken rock fills again
the whole space and redistributes the weight over the
foot wall.

C. T. Rice considers that in supporting the roof of a
stope, only that portion of the roof that is below the

line of the dome of equilibrium requires support; the
rock above this dome sustains itself. If, therefore, the
shape of this dome of equilibrium in each kind of rock
were known, it would be easy to calculate the weight of
rock hanging below the dome, and so timber the stope
as to hold up this weight. The shape of this dome is
fairly constant in each kind of rock; especially in the
same rock in the same district. Of course, slips and
joints, sudden changes in chemical composition, the dip
of the strata in sediments, and many other facts, would
affect the shape of the dome, but as long as these were
small their effect would also be small. If investigation
of the shape of this dome should suggest any formula
to determine the strength of timber necessary to
support the ground below the dome, the effect of these
joints, etc., could easily be included by the factor of
safety used.

ENGINEERING DATA AND OBSERVATIONS.

In America very few data have been collected on sub-
sidence due to mining operations, at least the data, if
collected, have not been made available for scientific
purposes.

In order that observations may be of value the
following correlated data are desirable:

(1)	The elevations of a number of points on the
surface for a period of years both prior to, during, and
following the mining directly beneath.

(2)	The position of these points with regard to
permanent stations located outside of the mining field
or upon ground which has not been or will not be
subject to the influence of the mining operations.

(3)	The position of the working face in the mine on
the various dates of survey.

(4)	An accurate location and description of the
character of the portions of the mineral deposit left
unmined.

(5)	An accurate location and a description of the
supporting materials placed in the excavated area.

(6)	The thickness and dip of the material mined.

(7)	The thickness and character of the bed immedi-
ately underlying.

(8)	The thickness, dip and character of the overlying
rocks and all available information in regard to
structure.

(9)	The thickness and character, of the surficial
material.

(10)	The quantity of water removed from the mine.

(11)	The location, extent and data of underground
movements of rocks overlying the mineral deposit.

In Europe records have been kept for many years in
various districts in order to determine the vertical
amount, lateral extent, rate and duration of subsidence.

Among the first surveys made to determine the
movement of the surface were those of Fayol. At

PlaiiafLevbl AB
; firet <LifrI

J	Section through M-H

Fig. 15.—Subsidence at Commentry Mine.

Commentry from 1879 to 1885, as shown in figs. 15 and
16, surveys were made to correlate surface movement
and the advance of the working face. The seam, which
was almost 48 ft. thick, was worked in horizontal slices
of about 8 ft. in ascending order. The thickness of rock
cover was 321 ft. Some filling of shale and sandstone
quarried on the surface was used. It was observed
that:

(1)	During the removal of the first slice, the lowering
of the surface gradually grew greater, and was further
increased considerably by the working of the second.

(2)	The area of subsidence was about four times larger
than the area worked.

(3)	The maximum sinking was 3 ft. 5 in. or one-fifth
of the height of the two slices.

(4)	The movements of the ground appeared at first
at a certain horizontal distance in advance of the
working faces, and this:? distance remained nearly
constant.

(5)	The subsidences increased during a certain time
while the working proceeded.

(6)	The second lift caused a total subsidence almost
equal to that of the first lift. This subsidence was
2 ft. 1 in. for the first and 1 ft. 11 in. for the second, in
all 4 ft.

(7)	The area of subsidence cannot be determined,
either by normals or verticals from the bed worked.

The ratio between the volume of subsidence as noted
on the surface, and the volume of excavation has been
noted for the caving system of mining on the Gogebic
range. When a slope caves, and the dome above it runs
up into sand or loose rock, the depression formed is
usually in the shape of an inverted cone; but where the
ore body is wide, or deep below the surface, the sub-
sidence usually takes the form of terraces. Sometimes
comparatively large areas will break through cleanly
and the whole surface will drop suddenly and as a unit,
but this is exceptional. After the back has once started
to cave, the surface usually sinks in terraces. In the
area under observation the deposit consisted of a lense

of soft haematite about 40 ft. in average width and
150 ft. high, with a length of nearly 1,600 ft. on the
incline, lying in a trough between a dyke of diorite and
a thick band of slate. The trough pitched 11 degs., the
hanging wall was hard jasper, and the ore was mined
first by square-set rooms and pillars, about 60 per cent,
of the ore being secured on first mining, but later the
pillars were robbed. ' The hanging wall dropped from
15 to 75 ft. and subsidence extended to the surface.

At Bent Colliery J. S. Dixon selected a line at right
angles to the advancing workings and as nearly as
possible on the level course of the coal. Stations were
put in every 100 ft. and afterwards every 50 ft. The
excavation was 5 ft. 6 in. highland the overlying strata
were allowed to fall and fill in. The surface was prin-
cipally boulder clay.

The pillars were removed for a distance of 240 ft.
back from the solid coal on January 21, 1882, and no
subsidence of the surface had ensued. On May 27,1882,
the levels showed the maximum subsidence to have been
180 ft. at station 1650, which was 145 ft. back from the
face, and the draw extended to 60 ft. in advance of the
working face. On November 14, 1882, the face was

Section through C-D.

Fig. 16.—Subsidence at Cqmmentry Mine

610 ft. from the solid, and on April 15,1883, it was 750 ft.
from the solid ; on November 27, 1883, it was 1,060 ft.
distant; and on October 23, 1884, the removal of the
pillars had been completed for some months, and the
face was 1,230 ft. from the solid. On June 17, 1885,
the workings had been in the same position for about a
year. On December 4, 1885, it was found that sub-
sidence had practically ceased and the draw had not
altered.

The conclusion arrived at is that subsidence from the
removal of coal in this case attains its maximum towards
the centre of the excavated space, and gradually
decreases in each direction. The maximum subsidence,
4 ft., was 73 per cent, of the thickness of the coal, and
the average, 3*76 ft., was 68 per cent. The wave of
maximum subsidence regularly followed the working
face at an average distance back of 186 ft., or 1 ft.
horizontal for each 3J ft. perpendicular. The permanent
lengths of the draw may be taken as 100 ft. on one side
and 83 ft. on the other. At these points the depth of
the coal was 650 and 646 ft., representing a draw of
1 horizontal for each 7T4 ft. perpendicular on the
average. The coal dips at 1 in 20.

Surveys were made at the South Kirby Colliery in
order to determine the extent and amount of surface
movement due to the removal of a shaft pillar. No
observations were made until two years after the mining
of the pillar was commenced. The seam in which the
pillar was removed lies at a depth of 2,108 ft., is 3 ft. 9 in.
thick, and dips 1 in 18. Above the coal is 1 ft. of clod.
Some movement of the surface was noted before the
surveys were made. At depths of 1,600 and 1,800 ft.
occur the Beamshaw seam of 3 ft. and the Barnsley
seam of 9 ft. which had been worked previously. The
unusually large ratio of subsidence (3’47 ft.) to the total
thickness excavated in removing the pillar (4’75 ft.) is
attributed to the failure of the shaft pillar in the over-
lying Barnsley seam.

Levellings made by C. Snow at the Hickleton Main
Colliery showed that subsidence was evident 433 ft. in
advance of a rapidly advancing longwall face, and that
total subsidence occurred 666 ft. back from the face, the
amount of subsidence being 4 5 ft. At the edge of the
shaft pillar, 500 ft. back from where the greatest sub-
sidence occurred, the subsidence was 1’28 ft. Subsidence
extended for a distance of approximately 600 ft. over
the shaft pillar.

For a period of 16 years surveys were made at the
Teversal and Pleasley Collieries by J. Piggford, but
details of the surveys have not been published. The
angle of draw or fracture was estimated to be approxi-
mately 16 degs. from the vertical and toward the
unworked coal. The depth of the coal seam was
approximately 600 ft. and the coal was from 5 to 6 ft.
thick.

W. Hay reported that at Shirebrook Colliery, where
the coal lies at a depth of from 1,500 to 1,700 ft., dips
1 in 24, and is 5 ft. thick, when the longwall face was
240 ft. from Stuffynwood Hall, cracks were noted in the
surface, the direction of fracture varying as much as
15 degs. from the direction of the coal face. Fig. 17
shows the angle of fracture in section. When the
working face was almost vertically beneath, the cracks
had attained their maximum width and thereafter com-
menced to close. When the face had advanced 300 ft.
farther, the walls of the buildings had assumed prac-
tically their normal position. The maximum subsidence
was 1’75 ft. and the minimum 1’34 ft., the average being
practically 30 per cent, of the total height of excavation.

Levels extending over a period of five years were
taken by S. R. Kay on the surface of a portion of two
collieries mining at depths of 360 ft. and 990 ft. Where
the levels were run, the surface was fairly level, the
strata were nearly horizontal, and were free from faults
of any magnitude. The strata consisted of alternating