1186

THE COLLIERY GUARDIAN

June 23, 1916,

M. Weitz based his calculations on the following

T> JO
formula :—E =	(2), but stated that this similarity

is only approximate. M. Sachlier used the formula :—
T) p

E = — - . With this formula, the thickness would

N -P

only become infinite when N = P.

After referring in minute detail to the foirmulee
adopted by several Continental engineers, the author
went on to say that in an examination of the ice wall
a great many assumptions have to be made in calcu-
lating the power of the freezing plant, but several
coefficients have been approximately determined by
experience.

Thickness of Ice Wall.

It is also possible to determine the necessary thickness
of the ice wall, though the formulee are not entirely satis-
factory. What, however, is more difficult to determine
beforehand is the proportion of rock and water. It is
necessary to assume that the conductivity for cold is the
same for the full depth of the shaft, an assumption which
is not absolutely exact, for it has been pointed out that
clay and overlying lignites are not such good conductors
as sand; and, finally, the entire theory assumes that the
freezing columns are equidistant throughout their length
—which is never correct, owing to the deviations of the
boreholes. On the other hand, only vague data exist as
to the real thickness of the ice walls. Holes drilled in the
wall to prove the thickness may cause a dangerous influx
of water. It is, therefore, very important to determine
the thickness of the ice walls, if possible, previous to the
commencement of sinking, so as to be able to concentrate
the energy of the freezing plant on the weak places. The
following experiences may be noted.

From tests made in mines it has been averred that dry
docks will allow Hertzian waves to pass through, but
that wet strata offer a resistance, which increases with
the percentage of water. Frozen ground acts like dry
rock. It is, therefore, possible to examine the thickness
of an ice wall by two different methods.

(1) Taking two freezing tubes as antennae, or feelers,
the electric wave will easily pass from the transmitter to
the receiver if the wall is thick; but if there is any break
in the wall the wave will be almost stopped.

(2) It is possible to communi-
cate electric vibrations to the
freezing columns. The amplitude
of the wave will be a large one,
if the tube is not surrounded by a
thick coating of ice, but will be
reduced if water is in the vicinity,
and becomes almost nil if the
column is surrounded by water.
As a large number of freezing
tubes are employed, the test has
to be repeated, and the weak spots
of the ice wall can be exactly
located. These experiencest are,
however, only in their infancy,
but the results already obtained
are encouraging enough to justify
the hopes that they will be pursued
further. However, many co-
efficients will have to be deter-
mined by experience, as the
various kinds of intersected rocks
have a different permeability for
Hertzian waves.



Details of the Poetsch Method.

When this method was intro-

duced it was not viewed

with favour, as one of the essential conditions for its
success was the absolute verticality of the boreholes; and
as there were no means of verifying this verti-
cality, the general idea was that the new process
was of no value beyond a depth of 40 or
45 yds. The first application of the process was
in 1883 at the Archibald lignite mine, near Schneid-
lingen, Saxony. The work was quite successful, but
took an extremely long time on account of an artificial
current which was connected with the lignite below by a
centre tube. In 1884 the process was tried at two shafts
125 ft. deep at the Emilie lignite mine, near Finster-
walde, Germany, but both there and at the Centrum
mine, Koenigswusterhausen (Brandenburg) water broke
in, through the boreholes not being deep enough, and the
shafts had to be abandoned.

The first really successful sinking by this process was
carried out at the Lens Collieries in 1890, the credit being
largely due to M. Reumeaux, the general manager of
the mines. Subsequently, the Poetsch process was largely
used in northern France for shafts from 100 ft. to 350 ft.
in depth. Among them special mention may be made of
two shafts 335 ft. deep sunk in 1884 at Vicq, for the
Anzin Mines Company.

In 1897 a shaft 460 ft. deep was sunk at Auboue, in
East France. For the first time a successful test was
made to verify the verticality of the boreholes. The
result of this experience was that the freezing process was
quickly adopted in other countries. An early contract
was made in Germany at Ronneberg by a French
company, and the number of applications in that country
exceeds 60. Great Britain, United States, Austria,
Russia, Holland, and Belgium followed. In the latter
country sinkings o>f 1,200 to 2,000ft. were made in the
new coal fields of the Campine. Such operations natu-
rally require apparatus which will correctly measure the
deviations of the boreholes, but even this question is not
entirely solved.

Bore Shaft and Freezing Cellar.

At a general rule, excavation is started with a larger
diameter for a short depth, in order to receive the inside
guide tubes. This chamber will also receive the brine
mains. The depth of this cellar depends to a large
extent on the method of boring employed. If several

sets of boring apparatus are employed, more especially
in deep shafts, this excavation is the first operation, and
is surrounded by a brick wall which will also support
the headgear. If, however, the boring apparatus can
do the work quickly, for shallow depths it is not always
essential to make this cellar, though there must be a
derrick which will travel round the outside of the shaft.
In that case the cellar will only be needed for the
circular mains, and will not be required until the bore-
holes are completed. This system was employed at
one of the Bruay collieries, where the ground was frozen
to about 450 ft. For very deep shafts the same method
has also been used. For example, at the Levant-du-
Flenu Colliery, Belgium, two derricks were employed
for a shaft which had to be frozen to 760 ft.

On the Continent the advantage of this system is that
very large and high headgear is not required, but in
Great Britain the Mining Regulations demand a detach-
ing hook, which necessitates the pulleys being always
fixed at a considerable height. Where there is a water
level in close proximity to the surface, the bore shaft
should only be very shallow, but, as a general,rule, it
is from 10 to 13 ft. in depth. There are, however, cases
in which it is sunk to greater depths, the No. 9 shaft of
the La Louviere and Sars Longchamp collieries,
Belgium, being 140 ft. in depth.

As the freezing cellars are for the reception of the
brine mains, they are insulated from the shaft by means
of a wooden skirting, which makes it an enclosed
chamber, or sometimes by means of a circular brick wall,
which will eventually surround the headgear. But as
the whole weight of the headgear is carried by this inner
wall, and the boring excavations will considerably
weaken the foundation of the wall, the latter may be
incapable of giving the necessary support to the head-
gear. Of the Flines-les-Raches Colliery in North
France, where the natural water level was at about
6 ft. above the ground, the brine mains were placed in
the brick chamber above the ground, in order to
re-establish the equilibrium of the water which filled it
and made a kind of tank.

Boring Methods.

Several systems of boring have been employed—rods
(with or without free-fall), water-flush, and rope boring.

.jm
JW ■ 'f
w •,•11 ■'j

Fig. 2.—Bkine Mains and Connecting Pipes.



Quick boring apparatus should be adopted for deep
shafts, but it has been found that the vibrations set up
in the hole floor by excessive boring speed is unfavour-
able to the verticality of the boreholes. An attempt
has been made to overcome that defect by erecting the
boring engines on solid concrete foundations outside the
freezing cellars altogether, but though this method facili-
tates the removal of the apparatus for each hole, it
does not obviate altogether the excessive vibration
caused by high speed. The Liegeois Colliery in
North Belgium probably holds the record for speed
in boring, but the holes were a great deal out of vertical,
necessitating a number of supplementary boreholes.

Distribution of the Boreholes.

The freezing tubes are spaced round the shaft at a
distance of not more than 3 ft. from the side of the shaft,
and about 3 ft. 4 in. apart. For deep shafts the former
distance has to be increased to allow a very large ice wall
to be made and for th© deviations from the vertical not
exceeding -1 per cent. Beyond this limit, it is generally
necessary to bore supplementary holes, which, however,
is not always easy, it being very difficult to make a
straight hole in ground which has been softened by
borings in the vicinity. In the early application of the
freezing process in France, the freezing tubes were also
inserted in the inside of the shaft, to he sure of finding
the hard ground at their base, but where it was, however,
found that too many masses of ice were created, which
exercised great pressure on each other and fractured the
freezing tubes. At No. 3 shaft of the Dourges Colliery,
only one freezing tube was inserted in the centre, and at
Vicq the shafts of the Anzin Colliery Company in 1894,
the central tubes were completely suppressed, since which
time they have only been used in very exceptional oases.
At the shaft No. 6 of the Maries Colliery the base of
frozen ground was not very firm, and had to be solidified
in the centre of the shaft. In this case the greater part
of the centre tube was also- insulated.

As a rule, central holes are bored if the ground con-
tains a certain amount of clay, to allow an outlet for the
water which is found between the circular ice wall and
the two horizontal planes of watertight clay. The holes
round the shaft must penetrate into the solid or water-
tight strata below.

Freezing Columns : Ascending Tubes.

In the early stages it was thought that copper tubes
would give better results than iron or steel, because of
their conductivity. Comparative tests have proved that
this effect is only transient, and disappears after five
days. In the early application of the freezing process,
the casing tubes of the boreholes were used for the circu-
lation of cold brine, but these tubes could not be made
watertight, and have since been superseded by tubes
with closed welded bottoms. The outside diameter is
generally about 5 in.

The freezing tubes must be quite free from any cracks
or fissures. They must be strong enough to resist the
pressure of the ice (which may exceed that of the
ground), and therefore it is nece-ssary to test each tube,
before insertion, at a pressure of at least double that
of the water which is contained in the ground. The
nipples must be longer than would be necessary
for air tubes, because of the enormous strain put on
them during freezing, and as the different strata are
not all solidified in the same time, two parts of the
freezing columns may be strongly secured by the ice
wall, while the middle portion continues to contract.
To meet this contingency, Gebhardt placed several
stuffing boxes in the column, but it is impossible to see
whether they are watertight, and in many cases leak-
age of brine has occurred which made dangerous holes
in the ice wall. The writer thinks it would be better
not to extend the freezing tubes to the bottom of the
boreholes but to suspend them, thus allowing for their
contraction when the freezing has commenced.

Inner Tubes : Descending Tubes.

These are about 2 in. in diameter, and are inserted in
the larger tubes, to direct the cold brine to the bottom.
In the early application of the freezing process, the
bottom end of the tube was closed by a wooden plug,
and the brine had an outlet through several small holes
in the walls of the tube, but these are liable to choke
with impurities or rust, and the result may be a com-
plete interruption of the brine current. It is therefore
advisable to have the lower end about 2 ft. from the
bottom of the outer tube.

Brine Mains.

When all boreholes are provided with outer and
inner tubes,, cappings are fitted to the outer tubes, to
connect both sets of tubes, through pipes of lead or
steel, to two cast iron or steel mains, one acting as a
distributor, and the other as a collector (fig. 2). These
are, as a rule, arranged in circles, but for a square
shaft would be laid in a rectangle. Each main has a
number of flanges corresponding to the freezing tubes,
and each flange has a valve to regulate the brine circu-
lation. It is essential to know whether the brine is
running through each tube, and also if the circulation
of the brine is regular. Should one of the tubes be
obstructed, this will be indicated by the head or capping
being free from rime, since when the circulation is in
order these cappings quickly get covered with ice. It
is advisable to provide the cap with a small cock, which
can be opened from time to time to measure the temper-
ature of the outflowing brine, and also to release any
air imprisoned in the brine circulator.

Choice of Circulating Medium.

The selection of the refrigerating liquid depends upon
the temperature to which the ground has to be frozen.
As a general -rule, a solution of calcium or magnesium
chloride is adopted. With the former it is possible to get
down to — 25 degs. Cent. ( —13 degs. Fahr.), but with a
saturated -solution a temperature of — 40 degs. Cent.
(— 40 degs. Fahr.) can be obtained. This process is not
free from drawbacks, -as the salt settles easily by
crystallisation. With a solution of magnesium chloride,
temperatures between —40 and — 45 degs. Cent.
( -40 and — 49degs. Fahr.) can easily be attained; but
it has been suggested -that this class of brine will affect
the metal of the freezing tube. For very low tempera-
tures denatured alcohol is used, by which a temperature
of — 75 degs. Cent. (— 103 degs. Fahr.) can be obtained
before the freezing point is -reached. As these three
liquids dissolve ice it is absolutely necessary that the
joints of the freezing columns should be perfectly water-
tight, to prevent the risk of holes being formed in the
ice wall. Non-freezing oil could be adopted to avoid this
risk, but until recently the high price of this product
prevented its adoption.

Rofrig .rating Machinery and Medium.

The systems used for the production of cold are
absorption and compression machines, and -the media are
ether, ammonia, or carbonic acid g-as. The writer thinks
that an ether compressor was employed in 1864 in Wales,
but since that time the freezing industry has made con-
siderable progress and ether i-s no longer used. In the
early days of the Poetsch method, absorption machines
of the Carre system were used, but compressors of the
Fixa-ry, Linde, and Oseinbruck systems or other similar
types are now preferred.

For very low temperatures carbonic acid machines are
convenient, but for the purpose of shaft freezing they
are more difficult to supervise than ammonia compressors.

The cooling tanks, or evaporators, are insulated as
much as possible. At Llay Main Collieries they are
placed in an insulating chamber (fig. 3) an arrangement
which has the advantage of easily maintaining the brine
at a low temperature, should the plant be stopped for any
reason. For the brine and water circulation, centrifugal
or plunger pumps are used, the latter being generally
preferred for the brine, as they allow for -the removal of
any obstruction .in the columns, and can retard the speed
of the whole plant, when less cold is required after the
complete formation of the ice wall. The freezing plant
is connected with the brine mains by a double pipe line.

The Freezing Process.

If the brine pipes and mains are carefully insulated
the running of the compressors must be regulated so that