852

THE COLLIERY GUARDIAN

May 4, 1917.

SUBSIDENCE RESULTING FROM
MINING.*

By L. A. Young and H. H. Stoek.

(Continued from page 809.J

Nature of Damage Due to Disturbance of the Overlying
Material.

The damage resulting from the excavation of
minerals may be without or within the mine, but in
this study the damage external to the mine is the
subject of investigation, the internal damage being
noted only when it occurs in connection with external
damage.

The damage external to the mine may be due to:
The vertical, or horizontal, or both vertical and hori-
zontal movement of surface material or surface struc-
tures, caused by the subsidence of the strata overlying
the excavation; surface cracks or fissures due to slips,
faults, or shear, or to the tension of the surficial beds;
pit holes or caves, formed when, instead of gradual
and more or less uniform subsidence over a large area,
the movement is localised if the excavation is at a
shallow depth; damage by water on account of the
lowering of land below the former drainage channels
or the high-water levels, and by the derangement of
artificial drainage systems, such as sewers in cities or
tiling on farms; interference with or destruction of
natural or artificial water supply; or miniature earth-
quakes occurring when large masses of rock fall over
great areas.

Within the mine, damage may result from: The
inflow of water through cracks or breaks caused by
subsidence of the strata; the inflow of sand through
breaks extending to the 'surficial material; local or

COMPRESSION

tension.

.TENSION

TENSION

Fig. 2.—Tension as a Cause of Surface Cracks.

extensive falls of roof; the failure of pillars, due to
the excessive weight of the superincumbent strata;
“ air blasts ” or “ bumps ” accompanying the sudden
collapse of pillars and the fall of large areas of roof;
or from squeezes or creeps.

Nature of Earth Movement.

Damage to structures on the surface may be the
result of either vertical or horizontal movement, or
both, and engineering observations in Europe and in
America show many interesting facts regarding the
extent, the rate, and the duration of surface move-
ments.

“ Draw ” or “ pull ” is the variation from the ver-
tical of the line of fracture of rocks that break when
the supporting bed or stratum is removed; in other
words, the variation from the vertical of the boundary
between the disturbed and undisturbed strata. In
some cases this is a well-defined plane; in -others a zone
of indefinite extent. In the case of brittle rocks, the
break will be sharp ; while in the case of more yielding
deposits, such as shales and loose soil, it may be impos-
sible to determine exactly the limits of disturbance.

Several instances of the lifting of objects on the sur-
face have been reported, but no data are available at
this time to prove definitely that either a temporary
or a permanent elevation of the surface has occurred.
Thus it is stated that at Northwich (Cheshire), where
subsidence has resulted from the removal of salt, an
elevation of certain streets has occurred. In Notting-
hamshire, where a coal seam 3 ft. 8 in. thick was mined
at a depth of 1,680 ft., there was, locally and tem-
porarily, an elevation amounting to 4 in. Also, it is
reported that an “ earth tide ” occurs in the diamond
fields of South Africa, there being noted a rise and
fall amounting to 3 in. a day; but this could have no
connection with subsidence. The engineer’s records at
the Warrior Run Colliery of the Lehigh Valley Coal
Company in Pennsylvania show that there was a lateral
movement of surface monuments as well as vertical
movement. This resulted from a squeeze in workings
extending from 500 to 1,000 ft. in depth on a coal seam
dipping approximately 30 degs.

Sags of the surface, or depressions without impor-
tant breaks or cracks, occur when the movement is due
to the bending rather than to the breaking of the
strata, and when the surficial material, without sudden
movement, accommodates itself to the new inclination
of the bed rock. Observations in the coal districts
indicate that the extent and the gradient of such sags
are influenced by the rate of advance of the working
face, particularly in longwall mining ; by the character
and amount of filling; and by the ratio between the
depth and the lateral extent of the mine workings; as
well as by geological conditions in general.

Surface Cracks.

The surface cracks and fissures that appear com-
monly when mining is carried on at shallow depths
may be due to one of several causes. As the mine roof
sags over an excavated area, the bending action pro-
duces compression in the upper part of the strata,
near the centre of the basin or sag, while around the
rim of the basin the upper strata affected are in ten-
sion, which may be sufficient to cause the surface to
break or crack. (Fig. 2.) If the movement is an
extensive one, and if the height of the surface above
the axis of bending is great, the width of the fissure
may be considerable. Fissures 2 ft. wide have been

* From University of Illinois Engineering Experiment
Station Bulletin No. 91.

noted in Illinois and in West Virginia. Fig. 3 shows
such surface cracks in Western Pennsylvania.

The formation of surface cracks by tension is well
demonstrated by an occurrence in Ashland, in the
anthracite district of Pennsylvania. The crack (fig. 4)
extended for a distance of about a quarter of a mile,
and was from 1 in. to 6 in. wide, causing considerable
damage to property. The vertical distance to the first
coal seam was over 800 ft., and later development
showed that the crack did not extend to the coal.
The coal along the outcrops on both the Holmes and
the Mammoth seams had been removed, and it is pre-
sumed that the crack was due to tension resulting from
the settling of the overlying beds into the worked out
portion.

The importance of the effect of surface beds upon
draw or pull has been pointed out by Mr. A. Sopwith.
According to his observations, the following classifica-
tion of overlying beds may well be made: —

(1)	Measures consisting of fairly equal proportions of
rocky and argillaceous beds, and containing thick beds
of sandstone.

(2)	Measures including a small proportion of rocky
beds, say, 15 per cent., and only thin beds of sandstone.

(3)	Variations between these two.

In the first case, the edge of the subsidence will
follow or lie over the excavation, and in the second case
it will lie over the solid coal. In the third case the
draw will vary between (1) and (2).

Kay has emphasised the serious effects which may
result from the “ pull over ” or draw. In his opinion,
this may cause much greater damage than the actual
downward movement. “ The strata appear to bend
over the goaf in a curve of radius depending on the
depth, and thereby subject the strata overlying the
recently worked area to a strain (rendered passive
from the movement of the face), coincident with the
progress of the working face, and, owing to its great
radius and slow movement, doing very little damage to
surface structures of ordinary character, as a rule.”
If the advance of the face is stopped, buildings over
the line of the face may be seriously damaged.

Mr. R. E. Cooper called attention to the absence of
pull where the overlying beds include strong layers of
limestone, shale, and sandstone. Surface cracks may
be due to shear. Cracks caused in this way generally
are parallel, but they may constitute more than one
system. If there are two systems of fissures, generally
the openings due to one system are larger and more
regular than those due to the othei- system.

Cracks may be caused by sliding of surficial material,
particularly where the topography is rough. The
shifting of beds of clay may cause subsidence, and
form a sag or basin, around the perimeter of which
tension cracks will appear.

Pit Holes or Caves.

When the mining is carried on at a shallow depth,
where there is very little solid rock cover, or when the
roof fails under shear, the movement frequently causes

Fig. 3.—Surface Cracks in

Western Pennsylvania.

a sharp break in the surface, forming pit holes or
caves. Fig. 5 is from a photograph of pit holes in
Indiana. In this case, coal 6 to 7 ft. thick had been
mined at a depth of about 100 ft. The overburden
consisted of about 10 ft. of shale and 90 ft. of drift.
Such holes may be caused by the surficial material
running into the mine entries beyond the point at
which the break actually occurred. This type of dis-
turbance is the cause of much damage to the anthra-
cite mines of Pennsylvania. In regions where the sur-
face is valuable for agriculture and for building sites,
pit holes are frequently a serious problem, because the
cost of filling may be great. Subsidence of filled
material is likely to continue for some time, and the
value of such filled ground for building sites is gener-
ally low.

Effect of Unwatering Surficial Beds.

Considerable discussion has been aroused by the sug-
gestion that the unwatering of water-bearing beds of
clay, marl, and sand may result in subsidence, when
no mining has.been done. German engineers have
had to contend with heavy beds of marl overlying the
coal, and have made a number of observations upon
the effect of unwatering the surficial beds. There is a
difference of opinion, but possibly the majority of the
German engineers have thought that unwatering will
cause subsidence. It was held by many that when the
surficial beds are drained by boreholes or excavation,
there is a reduction in volume of the beds, and that
sinking of the surface will result. The mining industry
was held responsible for surface damage, simply
because it was acknowledged that unwatering had
taken place.

In studying the subsidences about Essen in 1866 and
1868, von Dechen came to the conclusion that the sub-
sidences and surface cracks were not directly the

result of the coal workings, but that they were caused
by the partial drying of the chalk marl and greensand
overlying the coal measures, which was caused by
unwatering through the mines, boreholes, and wells.
He also pointed out that there was a shrinkage in
volume in the chalk marl, due to the dissolving of car-
bonate of lime in the marl.

Later investigations led the German engineers to
change their views upon the effect of unwatering.
Graff made tests, and showed that drainage does not
cause any changes in volume in gravel, sand, and
quicksand. He concluded that subsidence will not
result from unwatering.if no solid material is carried
away mechanically.

Tests made in the laboratories of the United States
Bureau of Mines at Pittsburg have shown that
materials flushed with water do not compress nearly
so much as the same materia] if dry. This would
seem to indicate that by unwatering the strata of a
mineral deposit, damage may be caused to the sur-
face, even though no solid material is carried away.

F. Bernardi holds that the drying of beds of sand
does not cause a decrease in volume or a reduced bear-
ing power. He reached this conclusion because in
“ water-soaked sand strata, the grains of sand rest
upon grains of sand, and the weight of the surface is
carried by these grains of sand resting upon one
another, and not by the water.” If the drying of
sand causes a decrease in volume, the wetting of sand
should cause an increase.

Of the Austrian engineers, Rziha held that unwater- -
ing may cause subsidence, but the later writers, as
Jicinsky and Goldreich, who have had a better oppor-
tunity to make observations, hold that no movement
occurs if the water does not carry away any solids
mechanically or in solution. Data on the shrinkage of
beds of -loam and clay have been assembled by
R. Dawson Hall, who stated that a clay slime, 200 ft.
thick, will reduce to 50 ft. and less, as a result of
drainage, and though such a result is rare, yet the
figures suggest what an action drainage has in
shrinkage of roof coverings of mines, and how even
clays of great age may lose bulk by mining operations,
and let down the rock or surface with its buildings
above them. A flint clay drying in air will shrink in
all directions 5 per cent., so that it will measure
linearly only 95 per cent, as much as before shrinkage.
The loss in drying is 14-26 per cent., and this, if the
clay were plastic, so as to give laterally with freedom,
would reduce the thickness of the bed 14 ft. 3 in. in
every 100 ft. of depth of measure.

Effect on Drainage.

In the prairie lands and the river bottom lands of
the coal fields of the Middle West, the complete
removal of the coal from horizontal beds at compara-
tively shallow depths has been attended with the pro-
blem of the drainage of the surface. Over large areas
of prairie land there may be almost no natural
drainage, and if the mining of several feet of coal
permits the uniform subsi-
dence of the surface, large
sheets of water may stand for
a number of months over the
subsided land, thereby greatly
reducing its value for farming
purposes. In many instances
the value of the land for farm-
ing purposes exceeds greatly
the value of the coal in the
ground at the present leasing
rates.

Satisfactory artificial drain-
age has been provided in such
flat prairie land by the laying
of drain tiles at considerable
expense. Subsidence m a y
seriously disturb this tiling,
and may make the entire
drainage system of little or no
value. In a district such as
the Mississippi Valley, where
the streams are bordered by
extensive bottom lands that
are little, if any, above
the high-water line, it is
claimed that surface subsi-
dence may materially increase the area flooded at a
time of high water, and may even produce areas that
are continually under water, or are too wet for farm-
ing purposes.

Effect on Water Supply.

Subsidence of strata generally results in the forma-
tion of cracks and fissures in the rock, which may be
sufficient to permit the escape of -water from a water-
bearing bed which may have been the source of the
water supply of a community or of an industry ; thus
the fissuring of the rock beneath gravel beds may
permit the drainage of the beds which have been the
source of water.

N umerous wells and cisterns have been damaged per-
manently by subsidence due to mining. Instances of
only a temporary loss of water in wells have been noted
in Illinois, Oklahoma, Maryland, and Pennsylvania,
the wells furnishing the normal supply of water after
subsidence has ceased if below the wells there are beds
of such texture that the fissures will close tightly
enough to hold water.

Subaqueous Mining.

The subaqueous mining of coal and other minerals
may shatter the overlying strata, and permit an inrush
of water which will destroy life and property. A
number of valuable mineral deposits have been opened
at the edge of the ocean, and from time to time the
workings first made on the shore portion have been
extended seaward until the mining of the undersea
portion by a safe method has become the chief problem
in the undertaking.

Much attention has been given to the study of
pillars and of subsidence, owing to the vital necessity
of mining in such a manner that water may not enter
the mine. Particulars regarding the working of coal