808

THE COLLIERY GUARDIAN.

April 27, 1917.

urban stop and the Vandalia Railroad. These swags
are said to have occurred one year before the mine
was abandoned, and the coal is said to lie at a depth
of 110 ft.

In Vigo county, about 1| miles north of Miami, a
number of small sinks were observed in a cultivated
field. On the opposite side of the road, in Clay county,
one of the sinks has broken through, and is about 5 ft.
deep. Other depressions were observed in the line of
these workings, but were not broken through. In
Linton, about six years ago, one side of a concrete
block house dropped 2J to 3 ft. The break extended
from top to bottom, and passed through the blocks
instead of following the joints. Two or three years ago
one section of an L-shaped school house was badly
damaged, and the front end of a store fell in. The
court records show five or six suits at Linton for
recovery of damages due to mining. The coal is
worked on the room-and-pillar system.

Longwall mining at shallow depths in Kansas, in the
vicinity of Osage City, has caused some subsidence,
but no damage has been done to sidewalks, brick
buildings, etc. The coal is from 12 to 18 in. thick, and
lies at a depth of 70 to 80 ft. Above the coal is a
light limestone, and upon it rest the upper Pennsyl-
vania strata of alternating shale and limestone.

In the south-eastern part of the State, mining is
conducted on the room-and-pillar system in coal
dipping gently towards the west. There have been
subsidences, especially near the outcrop, but no exten-
sive damage has been done.

Near Leavenworth, longwall mining is carried on at
a depth of about 700 ft. in a bed 19 to 24 in. thick.
There are no published records of subsidence. The
surface is rolling, and no damage would be likely except
to buildings, paving, pipes, or sewers. The numerous
stone and brick buildings of the State Penitentiary at
Lansing have been undermined by the State Mine, but
show no evidences of subsidence. Mines extended
under the Missouri River show no seepage of river
water.

SUBSIDENCE RESULTING FROM
MINING.*

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

(Continued from page 718.J

United States. — In Alabama little attention has
been given to the subsidence problem, owing to the
fact that many of the coal mining companies have
been operating under land to which they hold the title,
and of which the surface has relatively little value in
comparison with the coal. At least 90 per cent, of
the mining is at a depth of less than 400 ft. Some
cracks have extended to the surface, and when damage
has been caused to property not owned by the mining
companies, it has usually been possible for the mining
companies to make settlements not greatly out of pro-
portion to the damage done. At the present time,
some mining is being carried on where the cover is as
much as from 800 to 1,200 ft., and very little trouble
is being experienced.

Subsidence of the surface due to coal mining has
attracted attention in Illinois for a number of years.
In the early days of coal mining, when only the shallow
beds were mined, the surface was seriously damaged,
but in those days the price of farm lands was low, and
most of the mining was conducted in sections not
thickly populated. The first important suit for
damages that was appealed to the higher courts in
Illinois was in Sangamon county in 1880. Since that
date but few subsidence cases have been tried in the
higher courts in Illinois, most of the claims for
damages being settled by arbitration or by decisions
of the lower court.

An investigation in 1914 showed that there had been
surface subsidence in the most important coal pro-
ducing counties of the State. Twenty-four of the total
of 52 counties in which coal is mined produced 94 per
cent, of the coal mined in the State in 1913, and in
21 of these counties subsidence in various stages and
degrees of intensity was noted.

The reported damages include injury to farm lands
and buildings, to city buildings and streets, to rail-
roads and highways, and to domestic and municipal
water supplies. Large tracts of farm land in
Northern Illinois are reported to be damaged by dis-
turbance of surface drainage due to subsidence. There
has been litigation to determine the extent to which
mining is responsible for the inundation of lands adja-
cent to waterways and streams.

Few instances of injury to persons by subsidence of
the surface have been reported. Mining at shallow
depths has permitted the movement of large bodies of
surficial material, at times resulting in a rush of sand,
clay, and water into the mine, causing serious damage
to the mine. Fortunately, there have been but few
such instances of personal injury to miners from such
rushes. This may be due largely to the precautionary
steps which have been taken since the accident in the
longwall field in 1883 at Braidwood. The disaster at
Mine No. 2 of the Diamond Coal Company, near Braid-
wood, Illinois, was due to the inrush of water through
surface breaks caused by subsidence.

A horizontal bed, 3 ft. thick, was being mined at a
depth of about 100 ft. The overlying strata are
largely shale and clay. Longwall mining had per-
mitted the surface to sink, and, at various points at
which the rock cover was thin, cracks and breaks
extended for some .distance up into the surficial
material. In February 1883 there had been a heavy
fall of snow, followed by a thaw and rain. On
February 15 vast sheets of water were standing on the
prairie, and on the following day a number of the
miners did not go to work, as they feared that the
water would break through into the mine. At 11 a.m.
on February 16 there occurred a cave, which per-
mitted a great inrush of water from the surface. The
flow of water cut out a larger inlet to the mine, and
in a short time all of the low points on the roadways
were filled with water, so that escape was impossible.
In three hours the entire mine was filled, and the
water rose to the surface; 61 men and boys failed to
escape before the mine was flooded.

In Indiana a few squeezes have been reported in the
mines near Evansville. At one mine operated under
the Ohio River at a depth of 260 ft. below the river
bottom no trouble from the overlying river was
reported, and at another mine operated at a depth of
about 300 ft. no loss of coal due to squeezes was
reported when about 55 per cent, of the coal was
removed. Local squeezes occurred, but were stopped
by a barrier pillar, and the coal was reached from the
next set of parallel entries.

Probably the greatest damage from subsidence in
Indiana has been in Clay county, over the upper and
lower block coal beds. In one instance, in which there
was from 20 to 40 ft. of cover, consisting of shale with
2 to 6 ft. of clay and soil on top, the overlying material
was so yielding that an outline of each pillar or stump
could be traced on the surface. After a period of
20 years these sinks are said to have evened up, leaving
little or no trace upon the surface. Over recent work-
ings succeeding rooms can be traced on the surface by
pit holes or sinks. In some .cases the strata have broken
through to the surface, and the depth of the hole is
the same as the thickness of the coal, that is, about
5 ft. Local residents state that within five years farm
land again becomes tillable, and in 20 years the depres-
sions have disappeared. This does not apply to large
swags which cover a number of adjacent rooms, and
which in some cases must be drained. A good example
of these swags is found at West Seeleyville in the field
north of the inter-urban stop and between the inter-

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

Fig. 1.—Relation of Surface Cracks to
Underground Workings.

In the Georges Creek region, the Pittsburg coal seam
varies from 6 ft. 6 in. to 9 ft. 10 in. in thickness, and,
when the pillar coal is removed, falls occur which
extend to the surface. Fig. 1 indicates the position
of surface breaks due to the removal of pillars at a
depth of 170 ft. over an area 300 ft. by 350 ft., the
thickness of coal averaging 8 ft. The first surface
break occurred between rooms Nos. 1 and 2, and was
about 70 ft. from the barrier pillar. The average
angle of fracture from the vertical is 22 degs. 30 mins.
The break along the barrier pillar at the top of the
rooms was at an angle of 14 degs. from the vertical,
while the break along the left-hand pillar of No. 4
room was nearly vertical. The conclusion has been
drawn that, until a pillar fall extends to the surface,
the fracture is conical in shape, but as the pillar line
extends down the rooms beyond the first surface
break, the strata fracture on a nearly vertical line.

At Lexington, Missouri, the mining of 20 in. of coal
at a depth of 160 ft. has caused subsidence amounting
in places to the full thickness of the coal. No serious
damage has resulted.

In the Joplin district extensive caves of the surface
have resulted from the mining of large bodies of zinc
ore at shallow depths, but no detailed study of subsi-
dence has been made. During the year 1915 a number
of mills were damaged through the tailings piles fall-
ing into the excavations. One cave-in resulted in the
death of several men in the mine by drowning, and it
seems inevitable that there will be many more caves
in the district, particularly in the sheet deposits where
small pillars are left.

In the Flat River district large areas have been
mined at depths up to 700 ft. The beds are practi-
cally horizontal, and have good roofs. In some mined-
out areas where the pillars have been removed and
slabbed, the back has come in, extended over an area
of from one to three acres. In two such caved areas
that have been examined, it has been found that in
each case a natural arch has been formed, and the
caved material has nearly filled the opening to the
back. The largest of the caves of this type has run

up about 100 ft. into the back, which leaves about
4U0 ft. of undisturbed formation above it.

Coal mining in Oklahoma has caused surface dis-
turbance at a number of places. On September 4,
1914, a serious squeeze occurred in Mine No. 1 of the
Union Coal Company at Adamson, which resulted in
the death of 13 miners, complete loss of the mine, and
minor surface damage. The mine was opened on the
pillar-and-room system. Throughout this mine tlie
lower seam is 4 ft. tnick, and 45 ft. above it is a seam
2 ft. 3 in. thick, not worked. The beds dip about
30 degs. Rooms were turned on 33 ft. centres, tne room
pillars being not more than 9 to 12 ft. wide, and in
places much less. The roof was a sandy snale, 28 ft.
thick, and above it an equal thickness of hard sand-
stone and a little fireclay. The squeeze came compara-
tively quickly, completely closing the slope. A number
of cracks appeared on the surface, but no serious sur-
face damage was done.

There are no records available'showing the amount
of surface subsidence, but the surface cracks have been
located fairly accurately. Assuming that the under-
ground break extended to the 10th level, there was
about 700 ft. of cover, and the angle of break may be
calculated for the cracks farthest from the mouth of
the slope. It has been assumed that the large crack
over the east side resulted when the 7th east entry was
lost. Similar cracks on the west may be correlated
with the underground movement on the 6th west and
9th west entries.

Mr. George S. Rice, Chief Mining Engineer, United
States Bureau of Mines, says in regard to this acci-
dent:—“ What happened was almost inevitable with a
strong roof and increasing depth, where so large a
percentage of the coal had been extracted in the
advance work, and the pillars left standing. Esti-
mating from the map, about 75 per cent, of the coal
had been taken out by the entries and rooms. As a
result, in the lowest level of the mine there was a load
of over 3,000 lb. per sq. in. on the 25 per cent, of coal
which remained in the entry and room pillars. This
is more than bituminous coal can sustain. Therefore,
I am inclined to think that the main support of the
overlying strata had been carried by arch stresses, the
arch being buttressed on one side by the strata near
the outcrop and on the other by the dipping strata.
Then when tne fracture occurred at the latter buttress,
it threw the entire weight on the mine pillars, causing
them to be crushed. The surface cracks, reported by
reliable witnesses to have occurred prior to the collapse,
running parallel but in advance (horizontally) of the
lowest level, indicated that a sheer fracture had
occurred in a plane roughly at right angles to the plane
of the dipping beds, and when this fracture extended
laterally to a sufficient distance, it formed a slip plane
which permitted the entire weight of the overlying
strata less friction to be thrown upon the pillars,
resulting in the collapse of the mine.”

In the anthracite field of Pennsylvania, the city of
Scranton extends entirely across the coal field, a dis-
tance of five miles, and for the same distance along the
valley. Beneath a portion of the city are 11 important
coal beds, having an average aggregate thickness of
58 ft. It has been estimated that during the 75 years
of active mining under the city, 177,000,000 tons of
coal have been produced. This output represents a
volume of 198,000,000 cu. yds., an amount in excess of
the total estimated excavation in connection with the
Panama Canal. In the early years of mining accu-
rate maps of the mines were not made and preserved,
and a number of the coal companies made little effort
to columnise the pillars in the various coal beds that
were worked. These conditions have made it difficult to
study the problem, and to provide adequate and prac-
tical remedies.

While subsidence has occurred in many parts of the
city, it was estimated in 1912 that not more than 15
per cent, of the area of the city was threatened.
Although surface subsidence had damaged property
within the city limits prior to August 29, 1909, the
public in general gave the matter little connected
attention. This was due largely to the fact that the
mining companies hold deeds which permit them to
remove the coal without liability for damage to the
surface. On the date mentioned, surface subsidence
caused serious damage to a school building, which for-
tunately was not in use at that season of the year.

A Commission was appointed to investigate the
physical causes of mine caves and the legal responsi-
bility therefor, and the report of this Commission was
published in 1912 as United States Bureau of Mines
Bulletin No. 25. Considerable attention was given to
“ flushing ” methods of filling, and the report contains
a chapter by N. H. Darton, entitled “ Notes on Sand
for Mine Flushing in the Scranton Region.” An
appendix includes the results of tests to determine the
compressive strength of anthracite, and of tests of
various kinds of materials for supporting the roof in
mine workings. The United States Bureau of Mines
has also published bulletins on: 11 Hydraulic Mine
Filling: Its Use in the Pennsylvania Anthracite
Fields” (Bulletin No. 60), and on 11 Sand Available
for Filling Mine Workings in the Northern Anthra-
cite Basin of Pennsylvania” (Bulletin No. 45).

The Pennsylvania State Anthracite Mine Cave Com-
mission has investigated the physical conditions and
legal rights of surface support, its report being printed
in the Journal of the Pennsylvania Legislature for
1913 (vol. 5, page 5947). The Commission also con-
ducted a series of tests, on supporting materials; at the
Bureau of Mines Laboratory in Pittsburg, in co-opera-
tion with the United States Bureau of Standards.

The 1913 session of the Pennsylvania General
Assembly enacted the Davis Mine Cave Law, which
provides for the protection of public highways, streets,
etc., and also provides for the creation by municipali-