May 11, 1917.	THE COLLIERY GUARDIAN.	909

SUBSIDENCE RESULTING FROM
MINING.*

By Ln A, Young and H. H. Stoek.

(Continued from page 854J

INDUSTRIES AND INTERESTS AFFECTED BY
SUBSIDENCE.

Surface subsidence involves more than the question
of the present value of the land; in many instances
the fundamental problem involves the relative present
and future importance of various industries and
interests. Among the most important of these are
agriculture, transportation, and the various interests
of municipalities.

Agriculture.

In the consideration of the agricultural interests
involved, attention must be directed to the probabili-
ties of subsequent use for agricultural purposes of land
not tilled at present. In the longwall field of ‘
Northern Illinois, where it is claimed that mining has
lowered the surface so that drainage is deranged, it
is estimated that large drainage projects have cost
from 15 to 40 dols. per acre. Estimates of coal resources
of Illinois show that only 20 per cent, of the coal
occurs in beds more than 4 ft. thick, and of the total
area (37,486 square miles) underlain by workable coal
beds, 32,979 square miles do not contain coal more
than 4 ft. thick. Over this great area it is possible
that sometime mining by the longwall system may pro-
duce subsidence unless a filling system is used that is
more effective than any at present in use. This state-
ment regarding the thin coal beds in Illinois applies
as well to large areas in Michigan, Ohio, Indiana,
Kentucky, Missouri, Iowa, Kansas, and several other
States, and it is evident that the importance of the
subject of subsidence will be even greater in the
future than at present.

Transportation.

Surface subsidence may interfere seriously with
transportation, by injury to the beds of canals and
railroads and the caving of highways and streets.
Mining in Great Britain and on the Continent has
necessitated the raising of the banks and the filling of
the bottom of many canals. In some instances, canals
have been maintained on grade, while the land which
they traverse has subsided as much as 20 ft. The
necessity for protection of these interests has become
so great that laws have been enacted which require
that 30 days’ notice be given of mining under rail-
ways, reservoirs, buildings, or pipes, or within a pre-
scribed distance.

The practice regarding the protection of the right
of way of railroads has differed from time to time,
and has varied also in different countries.. The general
policy in Europe seems to be to remove all the coal if
possible, and the tendency on the Continent is to use
filling under railways in order to reduce the amount
of subsidence.

In the United States, many of the great railway
systems do not grant the right to mine coal beneath the
right of way, if the company has ever owned the coal
right. However, coal has been mined under many
branch lines and under some of the main lines of rail-
roads traversing the coal districts; and in the anthra-
cite fields of Pennsylvania there are many instances of
subsidence of railway tracks. No serious accidents
have resulted, as the railway companies have guarded
carefully all points where movement is feared. There
are no. laws regulating mining under railways in the
United States. When a pit hole or cave extends to
the surface near or under a railway track, the problem
of restoration is principally a problem of filling. Good
illustrations are found in some of the iron mines of
the Lake Superior district, where extensive filling has
sometimes been necessary to preserve the grade of
tracks, amounting in one case to more than 50 ft.

When the movement is gradual and principally a
horizontal one due to tension or compression, the pro-
blem is much different. In Germany, many observa-
tions have been made upon railway track subject to
tension or compression on account of .subsidence over
mines. In one instance, because of the crowding of
the ground toward the centre of the subsiding area,
track 150 ft. in length had to be shortened from 1 to
2 in. Rails were buckled up or to the side, and the
crowding forward of the rails and ties caused the earth
or ballast to be pushed forward or crowded up, and
an open space appeared along one side of the tie.
These spaces have been noted as much as | in. wide.
In one sag, in which the maximum subsidence was
about 3 ft. in five years, it was necessary to shorten the
rails 2-66 ft. in a total distance of 658 ft. When the
track was in tension the rails were stretched,
and at times the ends were broken. When the
principal horizontal movement is across the right of
way, the trouble is easily seen on account of the effect
on alignment.

The effect of surface subsidence upon bridges has
been repeatedly noted, and, as a remedy, English engi-
neers suggest steel construction, well-tied abutments
and wings, and plenty of height, so that there will be
sufficient clearance after the bridge has been lowered
by the removal of the coal. There has been a differ-
ence of opinion in regard to the adaptability #of
arched or girder bridges. In one instance of the
mining of a seam, 7 ft. 6 in. thick, at a depth of 216 ft.
beneath an arch of 20 ft. on the main line of a rail-
road, the arch was not damaged by subsidence. It is
conceded that arches from 50 to 60 ft. long, would not
be advisable under similar conditions.

In 1868, several bridges were built in England on
land that was known to be subsiding on account of the
mining of the coal, and special precautions were taken
to preserve these bridges. The rails were carried on
wrought iron girders and cross girders. The founda-
tion was carried deep enough to permit the construc-
tion of a concrete base 4 ft. thick. On this base was

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

laid two courses of elm planking, each 4 in. thick, on
which four courses of brick footing were built, and
on these four courses was laid a hoop iron interlaced
frame, 4| in. mesh, extending over the whole of the
abutments and wing walls. This arrangement was
repeated every four courses. Later, in some places,
the foundation sank as much as 4ft., but the whole
bridge was lowered unbroken, and it was necessary
only to lift the girders and the track to grade.

Experience has shown that the damage to a bridge
will be least if the workings (longwall) approach it
broadside. The working face will pass under the struc-
ture much more quickly with that plan of working,
and there will be probably less difference in elevation
between the ends of the structure at any stage of the
subsidence.

In the construction of the Hull and Barnsley Rail-
way across the South Yorkshire coal field, which it
traversed for 12 miles, the problem of supporting
bridges was of great importance. Owing to the great
value of the coal beds, the plan of reserving coal pillars
was given up. W. Shelf ord advocated the separation
of the bridge masonry into parts which could subside
independently of each other, but should have the
materials in each part bonded together. Several
bridges were designed on this principle with abut-
ments and wings separated only by a straight joint of
mortar, which was concealed by a pilaster. A large
bridge built in 1884 after this plan subsided 3 ft. in
1891. The wing walls separated from the abutments,
but the abutments themselves were uninjured and sub-
sided bodily, so that they were only 3 or 4 in. out of
plumb. When subsidence had ceased, the wings were
repaired and the bridge was again placed in service.

The effect of subsidence upon railroad tunnels has
been noted previously, particularly in the construction
of the Merthyr tunnel in Wales, and the Greentree
tunnel at Pittsburg, Pennsylvania.

Municipalities.

Many towns in Europe and America have been
damaged by subsidence caused by mining. The
damages to property in municipalities may include: —

Injury to Streets, Side Walks, and Transportation
Lines. — When pit holes or caves occur, it becomes
necessary to fill until subsidence has ceased, and then
re-construct the street upon the most satisfactory
grade. When there is horizontal movement, due to
tension or compression rather than caves, the streets,
kerbing, and side walks may be crushed or heaved, or
there may be tension great enough to cause serious
cracks. This trouble has become so severe in certain
German cities that in the sections where compression
occurs the gutters and kerbs are laid so as to have
elastic and waterproof joints. When large gaps are
left in construction between kerbstones, they are
covered with strips of sheet iron about 2 in. wide.
In order to prevent the over-turning of kerbing, due
to compression occurring transversely, the flagging is
made narrower than the side walks, and a strip of
material that will permit compression is laid between
the flagging and the kerb. Coherent paving, such as
asphalt, cement, and concrete, is not used because it
would be cracked or crushed.

Injury to Buildings, Towers, and Chimneys.—This
may be due to caves, or to tension, compression, or
twisting. Large high buildings suffer more than low
buildings covering but little ground. Masonry and
concrete structures are damaged more than those of
wood.

E. Kolbe has pointed out the various factors and
conditions with which one must deal in preserving
buildings upon land which has subsided, as follow: —
A building may sink wholly or in part into a surface
break; it may stand upon the edge of a break and be
suddenly and violently twisted or wrenched and
shaken; it may be located in the mining area and may
be subjected to the earth movement and be damaged
by the jamming of the adjoining houses; a building
lying over the mined area may sink slowly in the sub-
sidence basin without undergoing greater damage than
being placed in an inclined position; or it may suffer
on account of the shock resulting from a fall of roof in
the mine.

Cracks in brick buildings particularly around and
between windows generally follow the joints of the
mortar, as these offer the least resistance. When cut
stone window sills and lintels are used, the fracture
naturally follows upward around the stone without
cracking it. In long brick or tile walls without open-
ings, as, for example, walls surrounding estates, there,
may be three types of fractures in relation to direc-
tion : —The fracture may go perpendicularly up the
wall and break the stone coping; it may extend
diagonally away from the plane of the crack in the
ground following the joints in the brickwork; or it
may extend along the joints of the brickwork diagon-
ally in the same general direction as the plane of frac-
ture in the ground. The second type is of most
frequent occurrence. The same three types of frac-
turing are characteristic also of high enclosing walls,
partition walls, and fire walls and chimneys.

Buildings may be damaged by side movement in
which structures are crowded upon each other. When
the mortar in masonry walls is cracked, the arches
over doors and windows fail, and increased pressure
is thrown upon adjacent sections of the structure.
When buildings are located over the edge of. a pillar
or on the side of a trough caused by subsidence, the
cracks may extend in step fashion diagonally across
a masonry wall. Secondary stresses may cause addi-
tional cracks in other directions. The cracks extend
in the same general direction as the cracks in the
ground.

In Germany, where subsidence has been anticipated,
large buildings have been erected in sections from 60
to 120 ft. long, and these sections have been reinforced
in all directions by rods and plates, so that they will
withstand both tension and compression. The joints
between the sections have been caulked with suitable
material, or protected with a covering. When build-
ings are not of great value, European engineers have
removed the coal as rapidly as possibly and completely

if possible, advancing the working face in a direction
at right angles to the axis of the most important struc-
ture. When such precautions were used, the working
of two 4 ft. seams of coal at a depth of 600 to 780 ft.
in England caused practically no damage to two rows
of 120 cottages.

When the structures are important, and it is esti-
mated that the damage caused by subsidence will
exceed the value of the coal, pillars may be left or
filling introduced to prevent or reduce the subsidence.

The problem of protecting important public build-
ings has received serious attention in Scranton, Penn-
sylvania. In several instances buildings have been
erected on reinforced concrete piles constructed upon
the rock underlying shallow coal beds which had been
worked by the pillar-and-room method and of which
the roof had fallen or seemed likely to do so. An
inspection of the conditions of mining beneath the city
and school properties indicated that some coal had
been mined under most of the buildings, and that in a
number of instances mining had been carried on in
several beds. Several of the buildings were damaged
by subsidence, the estimated damage being approxi-
mately 4*7 per cent, of their assessed valuation.

Injury to Water, Gas, and Steam Lines.—This type
of damage is not unusual in communities in which
mining has been carried on extensively. The crack-
ing of water mains has caused damage not only
through the direct injury to the main and the tem-
porary failure of the water supply, but also through
the escaping water, which in a number of instances has
flooded buildings, washed out foundations, and
destroyed streets, roads, and earthen structures. Fires
have resulted from the escape of gas from broken gas
mains. Necessity has brought about the use of
expansion and compression joints of various types for
preventing or reducing the damage to such lines. The
need for frequent inspection of such pipe lines has
made it important that they be laid in tunnels or large
conduits.

Injury to Sewers and Sewage,Plants.—Sewer lines,
as well as steam, water, and gas mains, may suffer
from subsidence, but in the case of sewer lines the
difficulties are even greater, since these lines are gener-
ally constructed of materials which are less able to
resist tension and compression, and a change in eleva-
tion of part of the line may render the entire system
useless. For example, at Ravensthorpe, in the Calder
Valley, sewage works constructed in 1874, which lay
on the verge of a colliery leasehold, had remained
intact for 24 years. In 1897 the effluent outlet sub-
merged 15 in. below the ordinary level of the stream
into which it discharged. The settling tanks were
cracked across the centre, and the tank sewer had
settled considerably. . These settlements arose from
getting a 20 in. seam of coal, besides the dirt, about
150 to 160 ft. deep, and the boundary of the worked
coal terminated in or near the sewage workings. In
the same year, a similar disturbance took place at the
Castleford sewage works in the same valley. Complete
re-levellings of the three roads intersecting the land
were taken, and proved an average settlement of 3-3 ft.
throughout nine-tenths of the 12-5 acres of sewage
land, without the surface being broken. In this case
the getting of coal, 4 to 4| ft. thick, at a depth of
603 ft., was the cause. The contour was singularly
constant, the new section being almost parallel with
the original section. The strata here were the shales
and sandstones of the coal measures, overlaid by the
marls and limestones of the permian formation.

Geological Conditions Affecting Subsidence.

The behaviour of the measures overlying the mineral
■deposit which is being worked depends to a large
degree upon the physical character and the structure
of the measures themselves. In a paper by Mr. G.
Knox before the International Geological Congress,
attention was called to the various geological condi-
tions which influence the effect of underground mining
upon the surface, as follow: —The general character
of the overlying strata; the presence of faults, fissures,
etc. ; the dip of the strata ; the direction of the work-
ings with regard to the jointing of the strata ; the
compressive strength of the rocks of the various over-
lying beds; the bearing power of the underlying beds;
and the angles at which rocks break when stressed.

Geological conditions must be studied in each dis-
trict, as no generalisations can be made which will
apply without reservation to all mining fields. The
measures overlying a flat seam may be made up of
various beds of sedimentary rocks, and in places may
include sheets or beds of intrusives. The physical
character, as well as the thickness of each bed, may
vary over different parts of the same mine, and there
may be faults, fissures, rolls, etc., which greatly
influence the supporting power of the bed, as well as
the manner in which the weight of the bed itself is
distributed upon the underlying supports. Unless the
thickness and the character of the beds have been
proven, and unless it is known definitely that the beds
are fairly uniform throughout the field under consider-
ation, it" will be impossible to formulate even approxi-
mate rules and theories regarding subsidence which
will be useful in the study of the problem of surface
support.

Mineral Deposits.

Physical Character. — Before considering the over-
lying and the underlying beds, it will be well to note
some of the conditions in the deposit being worked
which may greatly influence the problem of surface
support. The physical character of the material
being mined, and of that part, if any, of the deposit
which is left in the form of pillars or of filling, must
be considered. The texture and the structure of the
rock left in pillars is of great importance in deter-
mining the burden the pillars will carry, and in affect-
ing the stability of the pillar after it has been sub-
jected to the action of explosives in the adjacent por-
tion of the deposit, and after it has been exposed to
the action of the atmosphere and water. In many
coal mines, owing to the friability of the coal, it has
been necessary to reduce the charges of powder used