1044

THE COLLIERY GUARDIAN.

June 1, 1917.

In 1913 Goldreich. published a work on the theory
of land subsidence in coal regions, and followed this
with a volume entitled “ Land Movements in the Coal
District and their Influence on the Surface.” He
criticised Jicinsky’s contribution and theory in a
number of points, taking no exception to the funda-
mental principles, but objecting to the formula. He
questioned the assumption of a coefficient of increase of
volume that would be applicable to all cases; and
pointed out the fact that the so-called “ after-slide” of
surficial material is not considered in the formula.
He accepted Jicinsky’s determination of the fracture in
the coal measures, but objected to the formulating of
general rules for determining the size of pillars.

A large portion of Goldreich’s work is given to a
discussion of the phenomena of subsidence in con-
nection with railways. The profiles of sunken railway
sections of the Ostrau-Karwm coal district show that
these profiles have a parabolic form, that the maximum
subsidences are found in the middle of these depressions,
and that the amounts of subsidence decrease almost
regularly towards the two ends of the curves until they
finally become equal to zero. This regularity of the
depressions caused Goldreich to undertake to formulate
a theory of subsidence applicable to conditions such as
those which exist in the Ostrau-Karwin district, the
most distinctive feature being the surficial bed of
plastic marl as much as 1,200 ft. thick in places. Where
the coal measures outcrop, the regularity of the surface
depressions disappears and Goldreich takes refuge in
the statement that we must depend merely upon
experience.

Goldreich’s observations developed the fact that
following the subsidence of bed rock there is a vertical
subsidence of the marl directly overlying and a lateral
after-sliding of the adjacent and outlying mar]. In
discussing the subsidence of the roof strata he
emphasised the effect of the elasticity of each stratum,
and stated that when the elasticity of the subsiding
roof strata is so great that the latter reach the floor of
the worked out room without any disturbance in the
coherence of the superimposed strata, then the volume
of these subsiding strata remains unchanged. The
subsidence of roof strata without increase of volume
will occur in the case of the extraction of thin and flat
seams. The increase of volume which takes place
during the first stage of the subsidence process is not
enduring; for in consequence of the weight of the
following roof strata the broken rock is again com-
pressed, so that at the end of the rock movement there
results a decrease of volume which is certainly not
identical with the initial increase of volume. Only by
observing the amount of surface subsidence caused by
an underground working can a basis for estimating the
coefficient of increase of volume under actual con-
ditions be obtained. When the overlying beds are
elastic there will be little increase in volume as the
movement proceeds upward; under such conditions the
term “harmless depth ” cannot properly be used.

British Theories.

Subsidences resulting from the mining of salt and
coal in the British Isles were observed at an early date
and were the cause of investigatiofi by British engi-
neers, who in general have supported the important
principles of Belgian-French theories, although certain
persons have taken exception to particular points in
these theories. Numerous observations have been made
upon subsiding areas and considerable valuable informa-
tion has been collected, the data have been correlated and
arranged, and empirical formulae have been constructed
so that adequate pillars may be left for the protection of
surface structures and property of various kinds.

Observations carried on by J. S. Dixon demonstrated
that the wave of maximum subsidence regularly followed
the advancing face, and that a wave of disturbance was
just as regularly projected in advance of it ; that is, the
wave of disturbance preceded the working face, but the
maximum subsidence followed it. Joseph Dickinson
called attention to the similarity between earth move-
ments due to natural causes and those resulting from
mining operations. He considered that “ the direction
of subsidence may be judged by analogy from the slopes
taken by faults and mineral veins. The slope of a fault
in horizontal strata averages about 1 in 3*07 from the
perpendicular, varying according to the hardness and
cohesion of the strata from about 1 in 5 in hard rock
to 1 in 3’75 in medium, and 1 in 2 5 in soft. He con-
sidered that for horizontal seams not exceeding 6 ft.
in thickness, and with strata of the average hardness of
those in Lancashire, ordinary subsidence may be taken
as extending on all sides to one-tenth of the depth,
and that to obtain security a margin should be added.
This margin is limited by some engineers to an addi-
tional one-tenth of the depth, while others add an
arbitrary amount. When the strata are softer the.
extent of the subsidence is sometimes taken as one-
sixth, or even one-fourth of the depth of the working,
while, on the other hand, for hard siliceous rock, such
as is found in South Wales, reductions are needed. He
also agrees with other writers, that in seams of moderate
inclination larger areas are required for support on the
rise side than on the dip.”

T. A. O’Donahue endorsed the observations of J.
Dickinson, and expressed the opinion that breaking lines
of strata may be estimated within narrow limits with
average conditions. With regard to the various angles
of draw that have been noted, he pointed out that for
safety the maximum angle for given conditions must be
taken as the limit for safety. For coal beds 6 ft. thick
and overlying strata of moderate hardness, he found
that the angle of draw is from 5 to 8 degs. beyond the
vertical; i.e , if a pillar is to be left to protect objects
on the surface, a margin of one-twentieth to one-tenth
of the depth should be left in order to provide against
the draw. With inclined strata the draw increases
roughly in proportion to the degree of inclination of the
strata. He accepted the normal theory as correct when
applied to dips of 18 to 24 degs., but only for dip
workings. When the mine workings are on the rise
the maximum draw is estimated at 8 degs. for strata
nearly horizontal, and the draw is taken to cease with
strata at an inclination cf 24 degs.

The ideas of O’Donahue are expressed in his formula
for shaft pillars as follow : M = Margin of safety, say
from 5 to 10 per cent, of the depth; D — depth of
shaft; X = distance at the seam, between two lines
drawn from a point at the surface, one fine being vertical
and the other at right angles to the seam.

Shaft pillar in horizontal strata: Radius of pillar
= M + in inclined strata: Rise side =M 4- y +
2	DI

g X; dip side =M 4- - — - X. For seams less than
6 ft. thick the size of the pillar may be decreased, while
for thick seams it is suggested that the size of pillars
determined for a 6 ft. seam be multiplied by the square
root of the thickness of the seam in fathoms.

In discussing the effect of the thickness of the seam
upon the amount of subsidence, O’Donahue called
attention to the effect of the material stowed in the
goaf or gob. Other conditions being the same, the
mining of a 6 ft. seam would result in more than twice
the vertical subsidence caused by mining a 3 ft. seam,
owing to the fact that little material is thrown into the
gob in mining the 6 ft. seam, while in mining the 3 ft.
seam undoubtedly much “ brushing ” would have to be
done and, therefore, there would be considerable material
left in the goaf or gob. Therefore, the total subsidence
per foot of coal removed will be greater in the case of
the thicker seam. He objected to the statement that
mining at depths of 1,800 to 2,000 ft. will not cause
subsidence, because careful levellings would show that the
complete removal of the coal at even greater depths
would cause a sinking of the surface. The working of
a seam, for instance 4 ft. thick, would cause the surface
to subside about 3 ft. if the seam weie not greater than
600 ft. in depth, and would cause a subsidence of from
12 to 18 in. at a depth of 2,400 ft.

H. W. G. Halbaum, in his study of the action of the
roof in longwall mining, called attention to the locking
of the roof due to the lateral thrust in great roof
sections; and he subsequently formulated the propo-
sition, that contained in the total force of the roof
action, there is a horizontal component, the action of
which is contrary to the direction of working, and the
power of which is sufficient to deflect the roof action
from the vertical fine.

In considering the cantilever idea, he likened 'the
unsupported roof strata to a huge cantilever whose load
consists primarily of its own weight, but pointed out
that the balancing of the components is unlikely, for
the resistance of ordinary coal-measure strata to com-
pression is usually greater than their resistance to
tension. The neutral surface of the cantilever must
generally lie below the half-depth of the beam. More-
over, the beam is not homogeneous. “We start with a
thick unloaded cantilever and we end with a thinner but
loaded beam ; thinner, because from the standpoint of
their efficiency, the upper layers are gone; and loaded,
because from the standpoint of their dead weight, the
upper layers remain only as a true load on the effective
beam beneath. This simultaneous thinning and loading
of the effective cantilever seems probable for several
reasons : The principal one perhaps is to be found in
the fact that the original beam is a composite beam
formed by an aggregation of smaller beams (strata) in
super-position. The whole of the composite beam is an
effective beam only so long as its several layers firmly
adhere at tbeir conterminous horizontal planes or
boundaries. As soon as the uppermost layer (or series
of layers) separates from its subjacent layer, or tends to
slide thereon, it ceases at once to form any part of the
effective cantilever, to which cantilever it must thence-
forward sustain the relation of a load only.” He calls
attention to the fact that “ when we examine the cases
of natural subsidences of the earth’s crust, we find that
the great planes of strain, in the normal case, a’e
always projected over and towards the solid (or un-
subsided) strata.”

In a paper before the International Geological Con-
gress, Prof. George Knox summarised the various
points which may be considered sufficiently well estab-
lished to form a basis for further investigations—namely:
(a) That surface subsidence invariably extends over a
greater area than that excavated ; (b) the angle of pull
is determined by the ratio between the excavated and
subsided areas ; (c) that this ratio is determined by a
large number of factors, among which may be included
the following: The amount of permanent support left
in the unmined area; thickness of the seam worked;
depth of the workings from the surface; method of
working adopted; direction of working in relation to
the jointing of the strata; rate at which the workings
advance ; nature of the strata overlying the workings ;
presence of faults, fissures, etc., in the strata; perme-
ability of the overlying rocks; dip of the strata ; surface
contour; and potential compressive forces existing in
the strata containing the workings. He concluded that
the ratio between subsidence and draw must be the
joint result of the forces liberated by the withdrawal of
support from underneath the strata in the mined area.
The larger the proportion of settlement resulting in
subsidence the less can occur in the form of draw, and
vice versa. “ The number of factors that may influence
the results produced by the settlement of undermined
strata is so great that only a wide and comprehensive
enquiry by geologists and mining engineers in those
countries where mining is conducted on a large scale
can be hoped to provide sufficient evidence to establish
a definite theory or theories to assist in overcoming
some of the more common dangers due to subsidence.”

Alexander Richardson, in a paper before the Chemical,
Metallurgical and Mining Society of South Africa, took
up the question of stresses in deep masses of rock
unsupported for hundreds of feet horizontally. “Where
the strata are unfaulted, one would be justified in con-
sidering the mass as a huge slab supported on two or
more sides or as a lever hinged at the bottom of the
workings. Over extensive areas the pressure on the
roof of an excavation, assuming the bed to be horizontal,
will become in time equal to the weight of the super-
incumbent strata ; under no circumstances is it imme-
diately so, since the overlying beds must have some
carrying strength.” m	.

(To be continued.)

LABOUR AMD WAGES.

South Wales and Monmouthshire.

No announcement has yet been made, although one was
expected, as to the difficulty at Llanhilleth Colliery, where
notices expire to-morrow (Saturday). These were given
by the men as a whole because certain of their num I er
were being discharged owing to a section of the colliery
having been worked to the boundary. The contention of
the men is that the boundary has not been reached; and
also that, in any case, the matter should have been decided
first by the Conciliation Board before notices to the r
colleagues were given.

The workmen employed at the Mynydd Maen Colliery,
Pontypool, who had been idle a week owing to a dispute
between the hauliers and management, resumed work on
Friday of last week, a settlement having been reached the
previous night.

North of England,

Further negotiations between representatives of the
Northumberland Miners’ Association and the county coal
owners, with reference to the transference of coal miners
to ironstone mining work, have resulted in an agreement
under which men may be transferred. Not many men are
required at present, and these may be obtained from those
collieries where work is limited at present. The Controller
of Mines has approved of the agreement.

The Admiralty Shipyard Labour Department is enquir-
ing as to how far it is possible to employ Northumbrian
miners, who may have occasional short spells of unemploy-
ment, in the shipyards and munition factories. It is
thought possible that miners laid idle might be given casual
employment in the manner indicated.

The executive committee of the Northumberland Miners’
Association reports that an understanding has been arrived
at in a dispute at North Wylam as to the conditions under
which hewers were called upon to put, and trusts that the
understanding will lead to a final settlement of all matters
in dispute. The committee has decided to ask each branch
what the practice is at the pit in dealing with men who
only infrequently lose a day’s work through “ sleeping in ”
or other causes. Hartford branch is to be asked for further
information regarding its complaint of part of the men
being sent to work whilst the others are idle, and whether
or not the men work in turns, so that all may get an equal
share of the work available. The committee has declined
to take any part in the National Service movement. The
owners of Seghill Colliery are to be asked to pay the cost
of an ambulance car which conveyed a man injured in the
mine to Newcastle Infirmary, and the attention of the
mines inspector is to be drawn to the evidently inadequate
arrangements at the colliery for dealing with such cases.
It has been decided to send to the Church pit branch the
reply of the Holywell branch regarding alleged violation
of the Eight Hours Act in the case of Church pit lads tem-
porarily working at Holywell pit, which reply states that
the Holywell branch was in no way responsible for the
violation, but has taken steps to stop it. The committee
expresses its complete agreement with the letter sent by
the secretary to Newbiggin branch regarding the preven-
tion by the branch of a member from tendering for a stone
canch let at the colliery, and also with regard to the
wages of a screener somewhat crippled but quite able to do
the same screening work as other men. The attention of
the Coal Owners’ Association is being drawn to a com-
plaint as to the inadequate supply of workmen’s fire coal
at Seghill Colliery.

Mr. Chas. Alderson has been re-elected chairman of the
Joint Committee for the Northumberland Coal Trade. At
the committee’s most recent meeting, the following award,
dated March 24, 1917, was received with reference to filling
after mechanical coal cutters in the Beaumont seam at
Dinnington Colliery:—At and above 3 ft. 8 in., lOd.;
3 ft. 4 in., lid.; 3 ft., Is.; 2 ft. 8 in., Is. Id.; 2 ft. Gin.,
Is. 2d.; 2 ft. 4 in., Is. 3d.; 2 ft. 3 in., Is. 4d.; 2 ft. 2 in.,
Is. 5d.; 2 ft. 1 in., Is. 6d.; 2 ft., Is. 7d.; 1ft. Ilin.,
Is. 8d.; 1ft. 10in., Is. 9d.; 1ft. 9 in., Is. lOd.; 1ft. 8 in.,
Is. lid.; and 1ft. 7 in., 2s. Filling after coal-cutting
machines shall be cavilled for quarterly, as in the case of
hewing. Measurements for the purpose of ascertaining
tonnage price to be paid are to be taken in each gateway.
Then the average height on the whole of the gateways on
the coal filling face shall be the height on which the tonnage
price is paid. The coal fillers are not to be under any
obligation to take up coal left by the coal cutter. All
kirvings shall be properly raked from the cut and thrown
back from the face, a space of not less than 2 ft. to be
left along the face for an airway. Stone canches are to be
kept up within 4 ft. of the face where possible. No filler
is to be required to commence filling in a gateway where a
coal cutter is working, or until it has passed the centre of
the next gateway. Ramble money is to be paid to men
who fill from a gateway in which there may be ramble
money due. Each gateway is to carry 11 yds. of coal,
04 yds. on each side of gateway centre, or as near as may
be, except where the seam, including bands, is high enough
for the tub to travel along the face. Coal fillers are to
work single as far as is practicable. Wet is to be accord-
ing to colliery custom. An award with reference to the
Harvey (or Beaumont) seam at Netherton Howard pit
stipulates that payments for working this seam shall be
continued upon the systems of bargains existing at the
time the arbitration was entered into, and so long as the
minutes of undertaking between the two associations con-
tinue in force, this being without prejudice to the estab-
lished customs and practice of the county coal trade.
Ellington workmen claimed for lamp cabin men to be put
back to the hours per shift they had been working before
the manager made a change, and claimed also extra pay
for extra time worked since. The Joint Committee decided
that it had no jurisdiction as to the alteration of hours, but
that each man was entitled to be paid according to the
hours per shift at which he was set on until he received
. notice to terminate his engagement.

Federated Area.

Negotiations between the Lancashire and Cheshire Coal
Owners’ Association and the Amalgamated Society of
Railway Vehicle Builders, Wheelwrights, Carpenters and
Mechanics have resulted in an increase in the wages and
a revision of the war bonus paid to colliery tradesmen and
semi-skilled workers. The new terms accepted by the
men are an advance of 7s. per week in wages, with a
revised war bonus of 5s. per week.

With the object of obtaining an award for the Derby-
shire colliery enginemen and firemen, which has been made
to the Notts enginemen and firemen by Mr. W. Mackenzie,
K.C., and now in operation in Notts, the general secretary
(Mr. S. W. Rowarth) and other officials, with the execu-
tive committee of the Derbyshire, Notts and Midland
Counties Colliery Enginemen, Motormen, and Firemen’s
Union, met the Midland Counties colliery owners at Derby,
but the owners refused the application from the Derby-