122

THE COLLIERY GUARDIAN.

January 18, 1918.

MINING THIN COALS.*

The winning of thin coal seams is a class of mining
-which in the nature of things must be growing of con-
siderable interest in the older coal fields, and to the
older established companies. As the thicker and more
attractive seams approach exhaustion, the question
arises as to whether the thinner seams are to be con-
sidered as capable of being profitably . operated;
whether the coal company is to cease business; and
whether the coal field is to be abandoned. A decision
on either course of action is, to the surrounding com-
munity, a momentous one; and, bearing this in mind,
there is much to be said for the policy of “ carry on.”
Very little is known about thin coal mining possibili-
ties ; consequently, some remarks on this subject, based
on actual operation in 15 in., 18 in., and 20 in. coals,
may be of value and of interest. But there is this to
be remembered: experience in mining one thin coal is
no criterion of success in mining another. Experience,
however, can greatly add to judgment based on tech-
nical knowledge.

Principal Factors to be Considered.

In thin coal mining, the features that dominate
success are : (1) The efficient handling of the extraneous
rock overlying the seam; (2) the quantity removed;
and (3) its final disposal. The winning of thin seams
necessitates some of the rock roof being taken down in
the workings, in order that a convenient working
height may be obtained. The quantity that has to be
handled seemingly depends on : —

(а)	The thickness of the coal.

(б)	The nature of the roof.

(c)	The arrangement of the roof.

(d)	The method of operation.

flock handling, while usually shown in general cost
sheets as a separate figure in the face cost figures (/.<>.,.
so much per ton for “ brushing”), also appears as an
incidental charge in the cost per ton deduced from all
the other cost items : owing to the fact that there is
less “ payable material ” (coal) in every ton of material
handled, hauled, or hoisted.

Overcoming the Rock Factor.

The efficiency of haulage, pumping, hoisting, and
other mine operations are at a maximum in thick
seams; consequently, no extraordinary or unusual
methods can be looked for, or expected to be put in
• operation, as aids to thin coal mining, in order to over-
come the rock cost. Since useless rock is dislodged and
accumulated in mining, and, once dislodged, there is
no means of destroying it in the mine, the logical place
to effect the needed saving in expense is at the place of
its production, namely—the coal face. Two means of
effecting this economy present themselves : —

(1)	Direct Methods.—Any mining system or appli-
ance that can be used to reduce the quantity of rock
dislodged.

(2)	Indirect Methods. — Faster extraction, and
concentration of output, whereby a greater tonnage
is placed against all costs except actual mining.
Increased coal extraction may become, under some

conditions, a direct method of reducing the quantity
of rock mined, since there are roofs which will make
less waste material in the course of mining, if the
advance of the face is accelerated. Generally, how-
ever, the great advantage from quicker coal produc-
tion at the face comes from a speeding-up process of
the whole mine organisation: more coal handled
through a relatively shorter distance, at a greater
speed. The fact that quicker extraction of the seam
sometimes means profitable operation is verified in
those cases of thin coal mines where the introduction
of improved machinery and methods has not resulted
in a less cost per ton at the face over the older methods
displaced, but which, nevertheless, prove machine
mining to be a success, solely because the organisation
created to handle the machine-cut and machine-
handled coal has been so perfected as to give a sub-
stantial reduction in all other mining costs, thus off-
setting the increase due to decrease in thickness.
Organisation is, therefore, a very important matter
where machine mining is in operation.

Machine mining, as applied to the extraction of thin
coals, is the outcome of the struggle to overcome the
Jiandling of excess rock; and no other economic alter-
native is available. Mechanical coal cutters are
examples of a means to do this by indirect methods;
increased tonnage; while conveyors are a means to the
same end by direct methods: less rock made. Loaders,
when they become practical appliances, will be in the
same category as cutters.

Methods of Operation,

A discussion on mining methods, as practised daily
in the winning of thicker coals, would, ordinarily,
involve a consideration of the relative merits of “ long-
wall ” and “ pillar ” extraction—long a subject of much
dispute ; but inasmuch as it is thin coal mining that
is under consideration, this chapter will be confined to
a discussion of ways and means as specially applicable
to such seams.	/

Hand mining methods are rapidly being dis-
placed by machine mining operations; and where the
question of the successful mining of a thin coal is under
consideration, the possibilities of machine mining
should be given first and undivided attention. There
are, of course, exceptions to the almost universal suc-
cess of machine methods: conditions where hand
mining is preferable. These are mainly:___

(u) Where operation is limited by other factors,
and to such an extent that the increased capital
charges involved through the introduction of machine
mining methods cannot be offset by a sufficiently
large gross reduction in cost. This case is inten-
sified in those small mines, so situated that outside
motive power cannot be bought, and where its local

* From Bulletin No. 15, “ ThTMining of ThiiTCoal
Seams as Applied to the Eastern Coal Fields of Canada,”
by J. F. Kellock Brown. Department of Mines, Ottawa,

provision would necessarily involve a large addition
to capital.

(b)	* Where the roof conditions and character of
the strata are so inherently weak that they will not
stand the greater roof stresses demanded in longwall
working.

A roof that can be described as “ mealy,” is gener-
ally unsuitable, and extremely troublesome in machine
workings. The cost of excavating a machine, buried
nightly, within perhaps a short stretch of a few yards
at one particular point, runs away with any advan-
tage there may be from machine installation. In
extreme cases, the laying out of cutter walls might also
become doubtful where timber was scarce and costly,
and where none was recoverable in the workings. Water
troubles may cause a normally good roof to become
unsuitable for machine longwall mining.

(c)	Where the mine is situated in such an isolated
region that skilled and steady labour is very diffi-
cult to obtain, and to retain, a series of “ stops and
starts,” operation and suspension — occurring
through machine hands leaving unexpectedly,
operations thus coming to a periodic standstill until
the deserters are replaced by other men—might mili-
tate severely against machine mining.

In those cases where conversion to machine work
means a change to entirely new and untried method*,
and in face of a passive or maybe active labour oppo-
sition, the expense of trying to educate the men to the
new ways may completely discount the benefits of the
suggested change.

The introduction of machine mining was the first
step towards bringing the potential value of the thin
coals up to their present economic value. It is not
many years since a 24 in. seam was considered the
lowest workable thickness; now, machine mining takes
out seams down to 12 in. The coal cutter was the first
phase in this successful innovation, and the one that is,
as yet, the most developed. While, to-day, machine
methods are applicable to all systems, and the various
modifications thereof, it is in thin coals—at least in
the longwall method—that the greatest field of appli-
cation will be found, because: —

(a)	With thin coals, machine mining must come to
mean in the’ future, not only undercutting by
mechanical appliances, but, in addition, the entire
series of operations necessary to the getting of the
coal by power. This is the logical development of
applied power to offset the increase in wages and
other costs brought about through mining thin coals.
And that bejng so, the longwall system is the one
that offers the greatest tonnage within the smallest
area, and consequently provides the maximum of
continuous work with the minimum of lost time.
Conditions under which machines can be kept going
steadily for eight hours getting coal are preferable
to conditions where the work in each place is a series
of separate operations occurring intermittently
during the day.

(b)	With thin coals, under any system whereby the
machine changes from place to place, the amount of
coal obtained or work done in each place becomes
very small, since the seam is low, and consequently
the proportion of time spent to the tonnage cut, in
changing from position to position, automatically
rises. While this fact is not so evident nor so serious
in mining the thicker coals, it becomes very pro-
nounced in the case of the smaller seams.

(c)	Concentration of work is also an additional
feature of longwall working, whereby the problem of
handling and attending to machines and to their
outputs is simplified and manipulated at a'decreased
cost rate.

Roof Troubles.

While longwall mining seems the natural path along
which thin coal operations will develop, the sum total
of other considerations may be, and frequently is,
adverse to the introduction of this method. The
adverse factors of greatest importance are: —

(1)	Roof troubles.

(2)	Inexperienced labour.

This roof question has a recurring frequency that
emphasises its importance in determining not only the
method but also the manner of working. It is worth
remembering, however, that this question can be viewed
from another standpoint, which is exemplified in say-
ing that it is perhaps not the condition of the roof that
makes the operation successful or otherwise, but the
method of mining. Practice has proved this to be so ;
some cases where a coal worked by longwall hand
mining had barely successful results, when turned into
machine sections were complete failures, because the
quantity of rock then handled became too great.
Changing the direction of the advance of the face
through 90 degs. produced fairly satisfactory results,
since the rock taken down was much less. These
attempts were all made under the same roof; the only
changes introduced were differences in the method of
mining. It was the method of working in this case,
therefore, that determined the behaviour of the roof.

The study of the different forms of roof, and their
relation to the various modifications of machine or
hand, 11 longwall,” or “ pillar and stall” methods of
working, is a wide subject, and no two cases seem
alike. Four hypothetical cases can be mentioned: —

(1) A roof with heavy pressure and 11 short
break ”—meaning thereby a superimposed rock
which is continually exerting a heavy pressure
through another rock immediately over the coal, and
which latter rock has, of itself, little adhesion—is
inimical to longwall working, since the quantity of
waste rock dislodged is far greater than any reduc-
tion of operation cost could overcome. The imme-
diate overhead rock being of very fine character,
runs, on disturbance, like “ meal.” Such a roof
may cause a seam to be unworkable under any con-
ditions ; but is particularly adverse to machine
mining, since even vibration may cause a break. The •
“cleats,” “cracks,” or “breaks” in this rock are
so well developed and equalised in three directions

(extra well-defined stratification and two sets of
cleats close together at right angles), that only a
continuous solid support would hold it up.

(2)	As the distance between the “ cleats ” or
“ breaks ” increases, and one set becomes more
developed in one direction than another, the roof
acquires more stability, and cohesive power to stand
strain in one direction. In such cases, an alteration
of direction of working would cause a distinct differ-
ence in the behaviour of the roof. A pillar section,
going in either direction, might be very successful,
owing to the natural solid supports in the form of
pillars being correctly placed. This case still sup-
poses the existence of heavy pressure, but with the
immediate coal roof steadily gaining a strength of
its own.

(3)	The case of the roof with the “short break,”
but without heavy pressure above, as in case (2),
provided the immediate roof is not too broken, is
largely favourable to machine longwall; for the stone
produced can be used for building purposes, packs,
and pack walls, while pressure is neither so great, nor
so sudden, as to disturb the stone walls before they
take the weight. There are conditions here also
where the working again determines the roof con-
ditions. For example, a roof under wThich a face
is advancing at the rate of only 18 in. per day, hand
mining may prove difficult to hold, owing to the
pressure getting time to weigh on the artificial sup-
ports, and perhaps getting out of control; but if a
machine is installed which advances the face from
4 ft. to 6 ft. per day, the rate of progress is trebled,
and the pressure has somewhat less time to exert its
troublesome and costly influence before a new expo-
sure of roof is made, under which the miner works
in greater safety.

(4)	'Machine longwrall is most successful where the
roof has great adhesive power, and little immediate
direct pressure, but it is also conceivable that
adhesion might be so great as to produce no packing
stone, and be a cause of additional trouble through
excessive falls’ and bumps: subsidences that take
place when the long stretch of unsupported roof and
unbroken strata behind the coal face becomes too
great to be sustained by artificial means, and which,
in falling, crushes through roads right up to the
line of the workings.

Between these four cases there are innumerable vari-
ations of all sorts and conditions; hence it frequently
happens that* roof possibilities can only be judged,
and final success arrived at, by a process of trial
and elimination.

Economy of Machine Mining.

Machine mining helps to attain better control of the
roofs, as witness the conditions in any old hand-mined
longwall working, set out without any regard to roof
conditions. Naturally, through various troubles, cer-
tain places fall behind others ; some pillars are built
solid, timber is left standing in the waste. From all
these causes trouble arises. One miner says his coal is
hard, and therefore demands extra rates; another fills
nothing but small coal; falls are numerous, and seem
to occur regularly at one or two points; more timber-
ing is wanted at each road-head. At other points the
pavement has to be lifted. In other words, due to the
over-riding weight of the strata above, lines of force
are produced which run any way, and local pressures
come into existence at certain points, whose position is
determinable by the relationship between the line of
the coal face, the distance apart of the roads, and the
areas open, supported, and unsupported. The man in
charge of such a section usually blames the roof, and
in this contention is quite correct; but he should go
behind this statement, and realise that it is his method
of working that caused the roof to act in such a
manner.

The introduction of machine walls and conveyor
operations demands straight lines in the coal face, and
that the necessary supports be placed at stated inter-
vals, running both ways. Regularity becomes the
fashion, with the result that equal pressure lines—
unknown under former conditions—develop parallel to.
or at right angles to, the line of the coal, and can be
taken advantage of in operation. The cohesive
strength that is in the roof becomes a help instead of a
hindrance; safety is increased; for if you know what
a roof will do, you can, within limits, make your pre-
parations accordingly. No too roofs are alike: in
weight exerted, in the type and direction of break, or
in the swing of the loosened strata; but once regu-
larity of working is established, they may be expected
to exert their several powers in the same way.

Labour Considerations.

Inexperienced labour in machine or hand longwall
mining is, in certain cases, so great a drawback that it
is often a reason for the non-adoption or condemnation
of longwall methods. In communities and districts
where one class of mining has been in vogue for years,
and where the facilities for spreading knowledge of
modern mining work are poor, .there is usually bred a
race of workers who are more or less passively, and
sometimes actively, opposed to the introduction of new
methods. Insular industrial circles are all, more or
less, afflicted by the antediluvian doctrine, “ What was
my father’s way is good enbugh for me ” ; and where
this is the case in mining, the introduction of
machinery or any alteration of system will involve not
only consideration of the whole question of the organi-
sation of the work, but also the steady propagation of
/information on the subject. In the capacity of the
workers to appreciate this technical knowledge, and
on their willingness to learn it, lies a great deal of the
future success of machine and longwall methods. There
are mining districts where, for this c a use, it takes
dozens of years before a new idea can be grafted on to
the native mind. Previous to that psychological
moment, it was banned as a “ foreign ” notion. Con-
ditions such as these require infinite patience much
tact, and a steady educational effort, before the results
desired are attained.