October 29, 1915.

THE COLLIERY GUARDIAN.

875

section (1-7 m. high and 1’55 m. wide) and lined with
ferro-concrete, the boards being mounted at a height of
1-15 m. above the floor. The first board was placed
103 m. from the explosion chamber, three others suc-
ceeding at intervals of 2 m. The width of the shelves
was 25 to 30 cm.; and in some cases, to increase the
quantity of stone dust used, a second shelf was mounted
25 cm. above the first. The dust used consisted of
finely-ground clay shale, weighing 0*842 to 1’1050kilogs.
per litre, as compared with 0’55kilog. for the fine bitu-
minous coal dust used in the experiments. In some
cases the explosion was started with firedamp (in order
to increase the initial force), together with 1-5kilogs. of
coal dust, the igniting charge consisting of 200 grammes
of dynamite. Coal dust was strewn, in the usual quan-
tity, throughout the whole length of the gallery, so that
the explosions were able to gather considerable intensity
before reaching the stone dust zone, and also to find
material for further propagation if the flame succeeded
in traversing the dust zone.

The experiments were carried on with two, three, and
four barriers of stone dust, beginning with a charge of
60kilogs., and increasing the quantity by 10kilogs. on
each barrier in the succeeding trials. From a number
of experiments with a charge of 200 kilogs. of dust, safety
was considered to have been attained, but confirmatory
trials dispelled this idea. However, by using a total
charge of 400 kilogs. of stone dust, arranged on four
barriers, it was found possible to arrest even the most
powerful explosion obtainable with firedamp and coal
dust. Whilst this quantity is very large in proportion to
the 100 kilogs. of coal dust strewn in the gallery (only
50 kilogs. being in front of the dust zone), it is one that
is immaterial in practice, the cost of the stone dust being
very small and the periods of renewal probably very
infrequent.

In all the experiments the boards used for holding the
stone dust -were completely smashed and destroyed. In
nearly every instance the explosion was checked in front
of the barrier (at 70, 80$ or 90 m.), and fluctuated to
and fro in the gallery several times before advancing
further. Given a sufficiency of stone dust, the flame was
extinguished in the dust zone; in some cases earlier, the
dust dislodged from the barriers by the advance wave
being drawn back along the gallery by the retonation
of the hesitating flame, and extinguishing the latter. A
special series of experiments was instituted to solve the
important problem of the temporary check sustained by
the flame between 10 and 30 m. in front of the stone
dust barrier. With this object, a wooden frame, of
triangular cross section, equal in height to the usual heap
of stone dust, was placed on the lower shelf of each of
the four double barriers, the upper shelf being left
empty. Under these conditions, the same phenomenon
was observed as when stone dust was used, the flame
(even from firedamp and coal dust) being temporarily
checked in front of the barrier (at 80 to 90 m.) before
advancing further; and in one instance the explosion
was completely arrested at 115 m., though in all the
others, after the recovery, it traversed the whole gallery
rapidly and with violence.

It being thus established that the checking effect was
due to the purely mechanical resistance presented by
the loaded barriers, experiments were made with stop
barriers, made by nailing together old boards to form
baulks, measuring 1-23 by 1’60 m., and having a total
sectional area of approximately 2 sq.m. (gallery section
2| sq. m.). These baulks were suspended vertically in
the gallery, leaving a space of 15 to 20 cm. between them
and the concrete sides, crossing laths being set up behind
them to prevent them giving way too readily. Each
baulk weighed 24kilogs., and obstructed four-fifths of
the sectional area of the gallery. The coal dust explo-
sions were started with firedamp, and the following
results were obtained :—With one stop barrier at 108 m.
from the explosion chamber, the explosion was checked
at 100 m., and then traversed the whole gallery with
great force, the flame projecting 25 m. beyond the
mouth. With two barriers (103 and 105 m.) the same
result occurred, but the flame only projected 20 m. With
a third barrier, at 107 m., the explosion was completely
arrested at 90 m.; whilst with a fourth, at 109 m., the
explosion did not extend beyond 70 m., the flame per-
sisting for a few seconds and then going out. In all
cases the barriers were shattered, the debris being
flung up to 100 m. beyond the gallery.

These experiments show that even powerful explo-
sions can be arrested by the opposition of considerable
resistance. The work done by an explosion is performed
at the cost of its heat, so that if the heat be converted
into work, the explosion is stopped. This also explains
the stoppage, at some distance in front of the barrier, of
stone dust or timber, the great resistance opnosed by the
air to the expulsion of the timber by the advance wave
breaking the force of the explosion and extinguishing the
flame. The idea of converting the heat of an explosion
into work seems capable of forming the basis of a prac-
tical method tf counteracting mine explosions; and this
point is under investigation.

According to the observations made during the experi-
ments, the modus operandi of the stone dusting method
employed is as follows :—A strong blast of air is set up
in front of the advancing explosion, and scatters the dust
on the shelves throughout the sectional area of the
gallery. A considerable portion of this dust is forced
onwards from the location where it should come into
action, but nevertheless helps to fulfil its purpose by
mixing with the coal dust further on, and rendering the
latter uninflammable, as well as forming a zone of float-
ing dust which the flame cannot expel owing to lack of
fuel, even if it succeeds in passing the barriers. If,
however, sufficient dust be used, enough remains in situ
to stifle the explosion, the flame parting with its heat
to the very numerous particles of stone dust which
occupy the whole section of the gallery, as a dense cloud,
for a considerable distance. This action of the dust is

assisted by the considerable mechanical resistance pre-
sented by the barriers themselves, the explosion having
to perform a large amount of work in destroying and
expelling the dust-laden boards, and thus being deprived
of a great portion of its heat. Given a sufficient mass to
be overcome, this loss of heat alone will extinguish, or
at least dam back, the explosion, which is checked for
a moment, and then advances with diminished force, the
retardation of the flame facilitating the cession of its heat
to the particles of stone dust. Finally, this retardation
of the explosion is followed by the retonation due to the
negative pressure resulting from the cooling and reduced
volume of the gas and vapours filling that portion of the
gallery already traversed by the explosion, so that the
flame and dust cloud are driven back. Very strong
barriers destroy the force of the explosion before it
reaches them, both in consequence of the work done in
shattering the material and because the stone dust driven
back by the retonation extinguishes the flame.

The behaviour, in presence of weak explosions, of the
dusting process already practised in some pits, needs
further investigation, because of the probability of such
explosions being too feeble to dislodge the dust. The
influence of barriers at different distances from the seat
of an explosion also calls for examination, as does also
the suitability of zones in which the stone dust is merely
strewn evenly over a long distance without the use of
shelves, etc. The experiments already carried out
indicate that stone dust barriers form a useful sub-
stitute for wet zones.

SOUTH STAFFORDSHIRE AND WAR-
WICKSHIRE INSTITUTE OF MINING
ENGINEERS.

ANNUAL MEETING.

The above meeting was held at the University,
Birmingham, on the 18th inst., the President (Mr. G.
M. • Cockin) in the chair.

The annual report of the council for the last year and
the accounts and balance-sheet were adopted. The
report showed that the number of members was 157, the
receipts amounted to £286, and the expenditure to
£233.

The result of the voting for officers for the current
year showed that the following gentlemen had been
elected :—President, Mr. G. M. Cockin; vice-president,
Mr. Langford Ridsdale; new members of council, Mr.
S. F. Sopwith, Mr. W. Charlton, Mr. S. L. Thacker,
and Mr. D. Rogers.

The following gentlemen, having been approved by
the council, were elected :—As member, Mr. K. K.
Sengupta, Calcutta, India; as associate member, Mr.
W. Kean, Hall Green, Birmingham.

The President called attention to an appeal sent to
the association by the Institution of Mining Engineers
on behalf of the Red Cross Society. He said some of the
provincial institutes were raising very large sums.

It was proposed at the next meeting to discuss the
new rules drawn up by the committee in accordance
with the rules of the general institution established since
the granting of the charter.

Basement Rocks of the Bunter.

The President proceeded to read a paper on “ The
Basement Rocks of the Bunter, with Special Reference
to the Inundation at the Coppice Colliery.”

Over a large portion of the Cannock Chase coal field in
Mid-Stafford shire the coal measures are concealed by beds
of gravel, sand, pebbly sandstone, and conglomerates, which
form the hunter beds, the lower group of the triassic forma-
tion. These rocks afford many features of interest to the
geologist, they contain elements of commercial value, and
they are a source of danger at times to the mining industry.

No attempt has ever been made to correlate or define the
sequence of these beds. The layers of sand, gravel, and
conglomerate are extremely irregular, and constantly thin
out. The conglomerates are composed chiefly of quartzite
pebbles, cemented by lime and iron into a solid mass, the
pebbles being very variable in size. But it is impossible
*o tell from its appearance whether any bed is high up or low
down in the series, although as a rule the larger the pebbles
the lower the position is likely to be. Besides quartzites,
the conglomerates contain a certain percentage of pebbles
and fragments of sedimentary rocks of earlier geological
ages, such as those of the carboniferous, devonian, and
silurian and cambrian periods. In such fragments are
frequently found the fossils of the particular period. In
some places these fossils are much more abundant than in
others, i.e., at the Satnall Hill Quarries, the Wolseley Park
Quarries, and on Stile Cop. at which places I have found
very fine specimens in considerable quantities. It may be
that where these derived fossils occur in such abundance
there exists a definite tone in the conglomerate beds, and
that similar conditions observed elsewhere might enable
us to trace a continuation of fossil evidence and prove a
means of correlation.

The porous condition of the beds generally tends to the
storing up of vast quantities of water, and the fact that
these rocks are spread over a great area of country enables
them to absorb and retain a large percentage of the annual
rainfall. It is not, however, generally the case that the
water so retained constitutes -what may be called a dangerous
condition, but it is always necessary in mining beneath these
rocks to regard such conditions as being possibly dangerous.
As a general rule, the coal seams lying beneath the over-
lying hunter beds have been worked by the longwall system,
and where this is the case the overlying strata has sub-
sided so completely that breaks may sometimes be traced
on the surface in parallel lines. There are many places
where such subsidences have taken place without any incon-

venience arising from dangerous quantities of water
penetrating into underground workings. It is therefore
evident that the bunter beds are not throughout water-
bearing, indeed, much of the great deposit may be
comparatively dry, but the risk of water being present—
possibly in dangerous quantities—must always be borne in
mind for the following reasons :—

A : Variableness.—The formation of the beds is never
constant. The conditions under which they were laid down,
sometimes under the influence of rapid flowing torrents,
sometimes of sluggish streams, caused the current bedding
which is a characteristic feature. Great thicknesses of sand-
stone may suddenly thin out and give place to coarse
pebbles. There is no regularity of bedding, in one place
the beds may consist of loose porous sands, and in another
they may be formed of impermeable conglomerates so hard
set that blasting will hardly affect them.

B : Line of Saturation.—The area occupied by the bunter
beds on Cannock Chase is roughly about twenty-seven square
miles, and taking the infiltration at 40 per cent., and
the annual rainfall, 33 inches, the quantity of water
annually absorbed by the rocks amounts to about 2'21 million
tons, or 1,300 tons per acre. Generally speaking, it is the
lowest beds which act as a reservoir for this stored up
water, and the previous portion having become charged, the
residue is given off in springs and streams. The site of
these springs indicates the “ line of saturation,” which is
usually found at about 400 ft. above sea level. It follows,
therefore, that the lower the horizon at which the beds are
disturbed below the “ line of saturation,” the more likely
■is water to be found in large quantities.

C : Unconformability.—Previous to the deposition of the
triassic formation, the coal measures were subject to a vast
amount of faulting, folding, and denudation, and suffered
enormously from erosion and detrition. Indeed, so much
was this the case that the upper coal measures, so prominent
a feature in the sequence of North Staffordshire, are in the
Cannock Chase coal field almost entirely absent. This
unconformability introduces an element of great uncertainty.
It is therefore of vital importance to safety in mining
operations that the relative position to Ordnance Datium
which the floor of the bunter beds occupies should be known,
and also that the amount of cover remaining between that
floor and the workable seams of coal should be ascertained.

D : Faulted Areas.—The faults met with in the coal
measures were mostly formed in the carboniferous period,
but there are faults which have been formed subsequent
to that period, and these are a source of danger. Such
faults being common to the coal measures and the bunter
beds, may act either as barriers and pound up the water
within certain limits, or they may act as the conductors of
water to the measures below.

E : Washouts.—There are places in the coal field where
the coal measures have been completely denuded, at any
rate, down to a level below the lowest workable seam, and
the spaces occupied by masses of gravel which may possibly
contain large quantities of water.

Such being some of the reasons accounting for large accu-'
mutations of water, it becomes of first importance to ascer-
tain the relative level of the base of the water-bearing strata.
I have in preparation a chart showing most of the known
sections of the bunter beds on the Cannock Chase coal field.
The base of these is found to occupy regular lines of contour
dipping to the north. This chart may be extended or revised
as more information becomes available. The practical
benefit of realising the danger line cannot be too strongly
insisted upon; the failure to understand it may involve the
loss of valuable lives.

It may be of interest to refer here to some of the circum-
stances attending the flooding of the Coppice Pit, belonging
to the Earl of Shrewsbury and Talbot, on February 15, 1908.
The Shallow coal seam at this colliery was being worked by
the longwall system in an easterly direction along a face of
220 yds. in length, rising 10 in/per yard. This face of
working was separated from the principal workings of the
colliery bv a fault rising west 140 ft., towards which fault
the face was advancing. On the eastern side of this fault
the old workings had encountered a washout, and for a
distance of 600 vds. both the Deep and Shallow seams had
in places worked right up to the washout, and had stripped
the gravel beds. In these places of contact with the gravels
a feeder of water had been found varying from 100 to 200
gals, per minute, the levels being in one place 169 ft. O.D.,
and at another 215 ft. O.D. No apprehension of danger was
felt by the manager in again approaching the same line of
washout at a point 200 yds. distant, although the fact that
the level of the working face was in this case 55 ft. O.D.
mav not have been fully appreciated. Up to the moment of
the accident taking place no indication whatever of danger
from flooding had been given, the roof and coal were dry,
and the floor merely damp. The night foreman’s evidence
was that he had examined all the stalls, and on returning to
make his second examination, found one stall weighting so
badly that he could not travel along the face, but returned
down the jig: soon after a fall of roof took place, followed
bv a sudden rush of water, drowning three men and flooding
the workings. The quantity of water at first given off was
estimated by the manager to be 14.000 gals, per minute, and
this filled the colliery to the shaft, up which it steadily rose
with a lessening come of water until the adit was reached
84 ft. from the surface (380 O.D.). and it commenced to flow
down the adit in Tune 1911. At the time of writing the
flow has decreased to about 170 gals, per minute.

A Government enquiry was held on April 22, 1908, pre-
sided over by Hugh Johnstone, Esq., H.M. inspector of
mines, when the following explanation was suggested to
account for the unusual occurrence :—At the base of the
bunter rocks there is a marl bed, which at this place is about
15 ft. thick, sufficiently strong and impervious to hold up a
vast quantity of water; thus after the shallow coal had been
removed, a dangerous reservoir was formed overhanging the
waste until the fatal break occurred, when the stored-up water
escaped. A curious feature of the case is that the water
originally tapped by the old workings encountering the wash-
out flowed perfectlv clear from the gravel beds. But the
water which inundated the colliery on February 15, 1908,
was strongly impregnated with ochre, and this continued to
be the case for a year or two, when the water gradually
become quite clear again.

In all cases where the bunter beds overlie the coal measures
too much care cannot be taken to ascertain the relative levels
of the base of the hunters and the coal faces being worked
beneath them. Where there is any uncrtainty on this point,
and especially where it is known that water exists in uncer-
tain quantities, it is suggested that boreholes should be put
down to determine the position, or that, as an alternative,
pillars of coal should be left to prevent subsidence, or that
some method of hydraulic packing should be adopted.