230

THE COLLIERY GUARDIAN.

January 29, 1915.

CURRENT SCIENCE

Subsidence and Draw.

With increasing depth of working the question of
subsidence acquires an added importance, and would
justify more consideration from the theoretical point
of view than it has received. One point which offers
itself for discussion is the relationship between
“ subsidence ” and the “ angle of pull ” or draw. In
the prior working of lower seams it has been observed
that the fractures in the upper seam caused by the
withdrawal of support slope forward over the solid coal
at varying angles depending upon local conditions. So
variable has been the experience obtained under actual
working conditions that few serious attempts have been
made to evolve a theory on the subject. In a paper
submitted at the Toronto International Geological
Congress, however, Mr. George Knox, the principal
of the South Wales and Monmouthshire School of
Mines, records several observed phenomena, and draws
some useful deductions that may form the basis for
further discussion, although the prospect of a general
accepted formula seems to be as far off as ever. His
general conclusion is 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, and the larger the proportion of
settlement resulting in subsidence the less can occur in
the form of draw, and vice versa. The more effective
is the packing the less the amount of settling that can
take place either as subsidence or draw, and as the
settlement would be likely to occur slowly the strata
would bend without fracturing. Where fracturing does
occur, the change of strain in the rock particles in that
portion of the strata which is leaving the littoral zone
and passing into the motive zone will be rapid, producing
noises known among miners as “ bumps,” “ thuds,”
“ crumps,” “bowks,” etc., and which are usually accom-
panied by sudden falls of roof and side. Beyond this
the results may be influenced by a great number of
factors, among which may be included the following :—
(1) The amount of permanent support left in the mined
area; (2) the thickness of the seam, etc., worked; (3) the
depth of workings from the surface ; (4) the method of
working adopted; (5) the direction of working in relation
to the jointing of the strata ; (6) the rate at which the
workings advance; (?) the nature of the strata overlying
the workings ; (8) the presence of faults, fissures, etc., in
the strata; (9) the permeability of the overlying rocks ;
(10) the dip of the strata; (11) the surface contour;
(12) the potential compressive forces existing in the
strata containing the workings.

Some of the above, it will be seen, are involved
in questions of practical working. With regard to
the first of the factors mentioned, it is evident, says
the author, that the greater percentage of debris put
into the mined area the less movement can take place in
the absolute roof, thus keeping the latter whole and
“ drawing” for longer distances over the solid workings.
This is an argument in favour of hydraulic packing, and
Buntzel, who gives 14 instances of packing old workings
with sand, clay, ashes, &c., in a semi-fluid state, saj s
that the subsidences varied only from 0*3 to 7'8 per
cent., of the height of the seam (as compared with
30 to 70 per cent, where the roof was allowed to fall in).
What subsidence did take place occurred without
fracturing the strata, and the angle of pull was greatly
increased.

In thick seams the total subsidence is increased and
the angle of pull is small. As the workings advance
slowly and usually irregularly, the tendency to fracture
the absolute roof is great. The fractures produced are
usually very wdde, and, the prime face being nearly
vertical, this allows the force in the motive zone to exert
itself rapidly and often in a jerky fashion, suddenly
breaking off pieces of the nether roof. This is probably
why the accident rate is usually two or three times as
high in thick seams as it is in thin seams. With
increased depth the angle of pull is likely to increase,
owing to the draw of the littoral zone against the motive
zone assisting to support the absolute roof and allowing
it to sink gradually in a slightly curved form without
fractures.

Both the method and direction of working have a
distinct bearing upon the relationship between draw
and subsidence, and if work is prosecuted parallel to
the cleat, or jointing, subsidence will be greater than
when it is carried on at an angle across the joint planes,
because fissures will be more readily produced in the
former case. Further, where workings advance rapidly
the tendency will be for the strata to bend without
fracturing. As regards the nature of overlying strata,
it has been found that where this overburden consists
of very thick hard layers of sandstone—the angle of
pull is invariably larger than where the strata are soft.
This may be due to the rapidity of the action of
subsidence in soft rocks, and to- lack of cohesion
between the rock particles permitting sudden changes
of strain to take place by fracturing. In the presence
of faults the subsidence of strata tends to draw into the
hade of the fault, which, if flat, greatly increases the
angle of pull. This explains the prevalence of gob fires
in the neighbourhood of faults in seams liable to
spontaneous combustion. Strata heavily charged with
water will no doubt move more rapidly than dry strata,
on account of the lubrication afforded by the water.
Some engineers, on the other hand, have suggested that
the draining of wet ground by underground mining
operations tends to increase the subsidence, which
would as a result reduce the draw.

The angle of inclination of the strata plays a very
important part in determining the ratio between sub-
sidence and draw. It appears to the author that in

AND TECHNOLOGY.

excavations made in fairly hard rocks the tendency
would be for the angle of pull to increase regularly
from the horizontal position to the vertical, where
almost the whole of the disturbance results in draw.

Some Coking Problems.

A paper on “ Recent Developments in By-product
Coke Practice ” was read some time ago in America by
W. Blauvelt, who is consulting engineer to the Semet-
Solvay Company, of Syracuse, New York. The paper
dealt largely with the latest developments of the
Semet-Solvay process, in which the horizontal type of
flue is employed, but some general questions of universal
interest were also touched upon. A significant feature
of the development of the retort type of oven in America
is that in almost every case the installations have been
located at the point of consumption rather than at the
coal mines, distinguishing it from the almost universal
practice in the case of beehive plants. Mr. Blauvelt
also considers that the by-product oven, although its
introduction in America was of comparatively recent
date, has made distinctly greater progress there than in
England. In the United States to-day the most modern
ovens have a capacity of 20 tons of coal per day, and
the same number of men is capable of handling more
than twice the number of ovens, turning out from 1,000
to 1,200 tons, instead of the 110 tons which represented
the production of a unit crew when the ovens were first
brought over from England.

The author referred also to the question as to whether
the heating of a by-product oven is effected by radiant
heat or by convection and conduction, leaning himself
towards the latter view. As an argument, he pointed
out that an increase in the thickness of a coke oven wall
from 2J to 4J inches does not demand in practice a
higher temperature in the flues to produce the same
result in the coking chamber; the flame as burned in
the oven flues is only slightly luminous, and it has been
shown that the radiation from a non-luminous flame is
only about one-third of that of a luminous flame.
Moreover, radiant heat is transferred in accordance
with the difference between the fourth powers of the
temperatures of the radiant and receiving bodies, while
beat is transferred by conduction proportionately to
the first power of the difference in temperatures.

In his paper, Mr. Blauvelt stated that recently high-
class coke had been made in by-product ovens from
the high-volatile coals of the Connellsville and
Klondyke districts, and some interesting details on
this point were added in the course of the discussion by
Mr. C. A. Meissner, of the United States Steel
Corporation ; he said that their experience showed that
many of these coals can be coked to the extent of 100
per cent, so as to give a good blastfurnace coke, while
in other cases suitable coke is made by the admixture
of 10 to 35 per cent, of low-volatile coals. Even certain
sections of the Pittsburg seam, which have up to now
been regarded as unsuitable for coking purposes, are
being used successfully with the admixture of low-
volatile coals up to 60 and 80 per cent.

Mr. Meissner also referred to the coking period and
stated that it had been found, as a rule, that the
shorter coking time gave the best blastfurnace coke
and that there must be no delay in pushing the coke
out of the oven after it is coked. It may be desirable
to increase the period with high-volatile coals, but the
extreme difference does not exceed two hours on the
average.

Stress Distribution in Materials.

The increased use of reinforced concrete in construc-
tion work has raised many questions respecting the
distribution of stresses in such materials, about which
too little is known. Considerable interest, therefore,
attaches to a paper, read recently before the Liverpool
Engineering Society by Mr. IL E. Lance Martin, B.Sc.,
Eng., on “ Some Experiments with Reinforced Materials
Examined by the Aid of Plane Polarised Light.”
Various attempts have been made to deal experimen-
tally with problems of this kind. The use of Rontgen
rays has been suggested and found inapplicable, since no
apparent difference can be detected in the transparency
of metals to these rays even under maximum stresses.
Sir David Brewster long ago pointed out that isotropic
substances such as glass exhibit double refraction when
submitted to strain, and he suggested that this property
afforded a method whereby the distribution of stresses
might be analysed. Glass, however, is not a convenient
material to use for this purpose, and Prof. Coker has
recently tried xylonite, with successful results. Mr.
Martin has extended the optical method to celluloid
reinforced by mild steel wires to represent reinforced
concrete beams. Such material when stressed shows
brilliant interference colour bands, the intensity of the
colour effect varying with the difference of the principal
stresses at a point, the direction of the colour bands
being parallel to the axes of these stresses. Certain
dark bands are also produced. These represent either
the position of no strain or positions in which the optical
vibration planes are parallel to the planes of polarisation.
In this way it is possible to map out the directions of
principal stress in the material. By means of a standard
test piece placed so that the axis of principal stress is
at right angles to that in the test piece, the dark field is
restored where the stresses in the standard and in the
test piece are equal. Thus the stresses may be estimated
quantitatively. Mr. Martin has thus shown that the
theories of M. Hennibique, that the compressive stress is
uniform throughout the compression area, do not accord
with his results. Prof. W. K. Hatt’s parabolic law is
also disproved, and the author confirms the straight
line law of M. Koenen, although that observer drew an
erroneous conclusion in stating that the neutral axis
was at the centre of the beam.

Lateral Pressure of Clay.

The question of stability of clay is one which often
occurs both in engineering and in mining practice.
Engineers have generally relied upon mathematical
formulae based upon the theoretical angle of repose, but
these apply only to granular rocks, and are not
theoretically applicable to clays, which seem rather to
behave in the same way as a heavy fluid. The problem
of clay pressures has recently been studied experimen-
tally as a result of considerations arising during the
construction of the monolith foundation work at
Rosyth dockyard, and a paper was read recently before
the Institution of Civil Engineers by A. L. Bell upon
this subject. Rankine based his well-known formula
upon the law represented by the equation q — pn tan.
—where pn is the normal pressure on the plane of shear,
q is the resistance to shear, and </> is the angle of repose.
But as the value of the angle of repose for clay is
variously given as any angle between 1 deg. and 45 degs.,
depending upon the nature of the clay, this affords
ittle help in practical work, such as calculating the
supporting power of clay foundations. The author has,
therefore, studied the question experimentally, and
proposes new formulas suitable for the design of
retaining walls in clay. He has supported his theo-
retical deductions by making direct observations of
pressure within a large mass of clay by placing pressure
gauges in the side of a monolith sunk into a clay bed.
Although written purely from an engineering stand-
point, it is possible that the paper may throw additional
light upon such phenomena as “ creep ” in coal mines.
It is a well-known fact that there has been a high
percentage of failures in works constructed in clay, and
many of these may be attributable to too close a reliance
upon formulae which are not properly applicable, and r
some of which have already been abandoned by general /
consent as having but little practical value.	/

------------------------- q

THE GERMAN AM AUSTRIAN COAL
AHD IRON TRADES.

We give below further extracts from German ,
periodicals that have reached us, showing the course of ■■
the coal and iron trades in Germany and Austria:—

Shortage of Coal in Germany.

The Vossiche Zeitung states that there is a shortage
of coal in Germany. The Coal Syndicate has announced
that—presumably owing to the fact that so many
skilled miners have been called to the front—the yield
of the pits under its control is now only from 50 to 60
per cent, of what it was before the war. Most of the
important customers declare, however, that they must
insist on the fulfilment of their supply contracts, for
either they are working on behalf of the Army or they
are compelled by public considerations to keep their
factories running. The Coal Syndicate suggests that
coke, of which there are ample stocks in the country, be
used as a substitute for coal, but the manufacturers
reply that their furnaces are constructed to burn coal,
and, moreover, a special kind of coal. The Vossiche
Zeitung points out that it is unfortunate that this
should be the case, and urges the factory owners to
alter their plant in such a way that coke also can be
used as fuel.

Coal, Coke, and Briquette Traffic in Ruhr Harbours.

No particulars are available with regard to the
deliveries to these harbours by rail, &c., in November.
Outward shipments to Coblenz and places higher up
river 549.308 tons (deficit as compared with November
1913, 32,440 tons); places below Coblenz, 14,486 tons
(deficit 9,174 tons) ; Holland, 183,054 tons (deficit
378,530 tons) ; Belgium, 65,724 tons (deficit 233,597
tons); France, nil (deficit 23,779 tons); other destina-
tions, 8,611 tons (deficit 23,443 tons). Total shipments
from Ruhrort, 542.817 tons (960,328 tons in November
1913); Duisburg, 118,704 tons (290,706 tons); Hochfeld,
134 tons (28,492 tons); Rheinpreussen, 66,834 tons
(95,580 tons); Schwelgern, 48,881 tons (78,114 tons);
Walsum, 43,812 tons (68,917 tons); total, 821,182 tons
(1,522,137 tons), a deficit of 700,955 tons. Total ship-
ments during the first four months of the war, 2,949,642
tons, a deficit of 4,222,381 tons, or 58'87 per cent.

Coal Syndicate Report for November.

Total coal raised 5,753,293 tons (7,801,848 tons in
November 19.13), or 239,721 tons (337,377 tons) per
working day ; calculated distribution 4,600,119 tons
(6,036,509 tons;, or 191,672 tons (261,038 tons), being
65'29 per cent. (88 90 per cent.) of the participation.
Total coal distribution of the syndicated pits, 5,936,390
tons (2,203,398 tons), or 247,350 tons (333,072 tons) per
working day. Deliveries, including local sales, miners’
house coal, and supplies to pits’ own ironworks:—Coal
3,827,765 tons (5,023,897 tons), or 159,490 tons (217,250
tons) per working day; coke, 1,023,294 tons (1,508,402
tons), or 34,110 tons (50,280 tons) per working day;
briquettes, 360,086 tons (340,908 tons), or 15,004 tons
(14,742 tons) per working day. As compared with
October, the coal output shows a decline of 288,216 tons,
the calculated distribution one of 66,965 tons, the total
distribution of coal a diminution of 113,745 tons, the
distribution of coal for Syndicate account a fall of
68,282 tons, the total distribution of coke one of 15,904
tons, and for account of the Syndicate 18,250 tons, the
only increase being in the case of briquettes, namely.
31,469 tons on the whole and 31,126 tons for Syndicate
account. The pits with which the Syndicate has a
selling agreement produced 375,522 tons (451,901 tons
in November 1913), and had a total distribution of
•■>71,751 tons (423,342 tons), including 130,109 tons
(173,582 tons) for Syndicate account; total coke distri-
bution 120,518 tons (114,309 tons), of which 83,280 tons
(93,123 tons) were for account of the Syndicate. For