THE COLLIERY GUARDIAN

AND

JOURNAL OF THE COAL AND IRON TRADES.

Vol. CXIII.

FRIDAY, MARCH 9, 1917.

No. 2932.

Cementation Process as Applied to Mining (Francois System).*

By THOMAS

The properties of cement have been taken advantage
of in many ways in dealing with problems of various
kinds in mining, and its practical use is widely known.
During the last few years, however, its field of applica-
tion in special cases has been enlarged, after a series of
trials, and it has been found possible and practical to
employ cement for conditions which previously it was
considered were unsuitable and useless. The experience
of recent years has proved that cement can be intro-
duced into almost inaccessible places, such as fissures or
cracks in strata, concrete foundations, and the like, by
admixture with water and under high pressure.

The ordinary grouting methods of running cement
into cavities either in shafts, tunnels or reservoirs are
well known, and the early attempts at the cementation
process were based principally upon the older methods
of grouting, but slightly higher pressures of several
hundred pounds per square inch were applied by means
of direct-driven force pumps.

Such pressures as were originally employed were
found in due course to be insufficient to obtain the
desired results in mining work when dealing with water
at a comparatively high pressure of 500 lb. per sq. in.;
consequently pressures up to 3,000 lb. per sq. in. have
now been successfully used.

Amongst the cases in which the cementation process
has been successfully applied may be included
specially—(1) shaft-sinking or level driving, (2) under-
ground dams, (3) underground fires, (4) defective bore-
holes, and (5) defective shaft-lining.

Shaft-sinking.

Cementation as applied to shaft-sinking consists of a
thorough treatment of the measures through which it
is desired to sink (thus rendering them practically water-
tight) by the injection of cement into all the existing
fissures or cracks in the ground previous to the com-
mencement of actual sinking operations. The first
attempt at this process was made on the Continent
some years ago, and numerous articles have been
published upon the subject.

The mode of operation may be sub-divided into two
distinct methods of application—(1) boreholes through
which the cement is introduced into the ground put
down inside the perimeter of the shaft required to be
sunk, and (2) boreholes through which the cement is
introduced into the ground put down around the outside
of the perimeter of the shaft in a similar way to the bore-
holes used in the freezing process.

The first of these two methods has the advantage of
indicating clearly the position of all water-bearing
fissures, and also gives an approximate idea of the size
and extent of such fissures, as both the quantity of
water and its pressure are registered as soon as any
fissures are encountered.

The cementation in this case is carried out in successive
stages—that is to say, a predetermined length of ground
is first of all treated, sunk through, and then lined;
after this work has been completed, an entirely fresh
set of boreholes is put down from the shaft-bottom and
inside the perimeter of the shaft. On reaching a fissure
containing water during boring, this water rises up the
borehole pipe ; but as a stop-valve is fitted on the pipe,
the water is kept under absolute control until it is dealt
with by cementation.

The second method, in which the holes are bored
outside the perimeter of the shaft, is said to have the
advantage that cementation and sinking can be carried
out simultaneously. This method, under certain con-
ditions, may be attended with satisfactory results; but,
in the writer’s opinion, where such boreholes are put
down from the surface to intersect a variety of fissu> es,
both large and small, and the injection of the cement
is carried out continually from the surface level of the
shaft and above the normal water-level of the ground,
there is less likelihood of obtaining successful results
than when the current is introduced from the shaft.
There is always a tendency for the cement mixture to
follow the line of least resistance during injection in
such a way that if the borehole intersects a large
fissure attended with several smaller ones, the most
extensive fissure would be filled with cement, whereas
the smaller fissures may only be filled in the immediate
vicinity of the borehole; and upon proceeding subse-
quently with the sinking, water would be found still to
exist in the region of the small fissures in the ground
as a result of imperfect injection.

Suitable Measures for Cementation.

The cementation process can be employed satis-
factorily in practically all kinds of rock, both porous

* Paper read before the Midland Institute of Mining,
Civil and Mechanical Engineers.

BLANDFORD.

and non-porous, but is of no value in the case of a
surface quicksand. Amongst the various kinds of
strata which have been treated are limestones, sand-
stones, gritstones (including Pennant rock), chalk, red
sandstone, and red marls (porous new red sandstone).

The first trial of cementation with the internal
method of boring was carried out at the Mine de Maries
in 1908 upon a disused shaft that had collapsed in 1854,
had run in, and had filled with water. The crater
formed by this collapse at the shaft top was 92 ft.
(28 m.) in diameter. A strong concrete raft was con-
structed, through which boreholes were put down, and
cementation then commenced, the object being to
solidify the broken material and to fill the fissures in
the ground at the points where feeders of water existed.
The strata in this case consisted of chalk, and the
results obtained were highly satisfactory. The cementa-
tion was carried out under the direction of and on the
plans made by M. Deffieux, chief engineer and agent to
the company.

Amongst other work on the Continent successfully
completed since 1908 may be mentioned a shaft sunk
through limestone by cementation at Forte-Tailee;
sinking through blue marlstone at Zeche Ewald, in
Westphalia; and sinking through limestone and clay at
Ottange, Lorraine.

Examples of Cementation.

Cementation has been successfully applied for shaft-
sinking in Great Britain in the following districts :—

Lanarkshire.—Two rectangular shafts were sunk at
Kingshill Colliery through coal measures consisting of
sandstones and shales. In this case cementation was
commenced at a depth of 300 ft. after previous sinking
with pumps. As the shaft-lining is timber, in
accordance with the usual practice in such shafts, and
in the absence of any form of watertight lining, the
water remaining after the treatment of the ground is
collected and raised to the surface.

Northumberland.—Two shafts were sunk at the
Plenmeller Collieries, and were completed by cementa-
tion from a depth of 420 to 780 ft., the previous sinking
in the upper length of the shafts having been carried
out with the aid of sinking pumps and cast iron tubbing,
whereas the remaining portion of the shafts was lined
with concrete. The measures sunk through here were
in the little limestone series, and consisted of limestone,
sandstones, and shales.

Durham.—At Blackball Colliery a large feeder of
water was successfully dealt with by means of a concrete
plug fixed in the shaft under water, and after cement a-
tion this enabled sinking to be resumed.* At Fishburn
Colliery two shafts were first treated by cementation
and then sunk through limestone.

Yorkshire.—At Hatfield Main Colliery the cementa-
tion process was decided ripon for sinking through the
heavily-watered porous new red sandstone, and was
commenced on the lines which had so far proved
successful in ordinary stratified measures. In this case,
however, it was soon discovered that in these measures
the ordinary cementation process alone was of no
service, it being found practically impossible to introduce
cement into the ground to any useful purpose, as it was
washed out by the pressure of water. Instead, however,
of discarding the process altogether, the work was
persevered with until finally all difficulties were over-
come by the introduction of a chemical process of
precipitation which facilitated the introduction of the
cement into the ground. Two chemical solutions were
employed, the properties of which produced,, when
mixed, the formation of a gelatinous precipitate which,
by sealing the pores of the sandstones in the walls of
the fissures and the loose materialin the fissures, permitted
of the introduction of the cement. The chemicals used
in this instance were silicate of soda and sulphate of
alumina, and the most efficient precipitation effect was
obtained, by experience, in the degree of concentration
of the respective solutions. The lining of these two
shafts consists throughout of ferro-concrete. The writer
does not wish, however, to enter more closely into the
details of this work, as it is a subject for a special
paper by itself, and one which will no doubt be given to
the institute at a later date. At Bossington Colliery
cementation was effectively applied during the sinking
through water-bearing limestone.

Warwickshire.—At Coventry Colliery, after proceeding
with the sinking by the aid of pumps down to approxi-
mately 550 ft., with heavy feeders of water, cementation
was decided upon. It has been continued successfully
throughout measures consisting of red sandstone and
red marls to a depth of approximately 1,200 ft. Below
this depth in coal-measures the process was found to be

* See Colliery Gutvrdian, June 5, 1914, p. 1237.

necessary, and was successfully employed to a depth of
approximately 1,650 ft. from the surface, at which depth
water was found to exist at a pressure of 560 lb. per
sq. in. In this case also the lining through the treated
ground consists of ferro-concrete, and the chemical
process has been adopted in order to enable the cement
to be introduced.

Staffordshire.—Here again, in red measures comprising
red sandstone and red mails, the utility of cementation
has been demonstrated at the Holditch sinking in North
Staffordshire down to a depth of 600 ft. One shaft was
sunk through w^ater-bearing measures by the aid of
pumps and tubbing ; but cementation and ferro-concrete
lining were adopted for the sinking of the second
shaft.

Kent.—The water in the chalk measures has been
dealt with by cementation, and its value in these
measures (which are similar to those of the north of
France) is recognised.

Wales. — At the Cwm sinking of the Great Western
Colliery Company, cementation was adopted after
previous sinking with pumps. The measures here are
Pennant rock, and a concrete lining was employed. The
results were highly satisfactory, and water at a pressure
of 350 lb. per sq. in. was kept under control and
closed off.

Shaft-lining.

Since the introduction of cementation into this
country, many improvements have been made upon the
older types of shaft-lining, and where the process is
employed for the treatment of virgin ground before
sinking, concrete as a subsequent lining has been largely
used. Two methods in particular have been employed,
as under:—

(1)	That of lining a shaft with specially-moulded and
interlocking concrete blocks prepared on the surface.
After sufficient time has been allowed for the blocks to
mature, they are built into the shaft, and are backed up
in some cases with concrete and in others with
ordinary rubble filling. In dry ground where it is
necessary to close off any water by means of such
lining, this method is quite satisfactory.

If, however, as is usually the case, it is necessary to
seal off the small percentage of water remaining after
treatment of the ground, and which is found to exist
during sinking, then trouble may be experienced in the
endeavour to close off this water and to make the shaft
dry. The chief reason is that, even though a con-
siderable thickness of concrete is put in behind the
lining blocks, and although such blocks may be strongly
reinforced, there is a tendency for the water to find its
way through the concrete filling when existing under
pressure, despite the relief pipes which may be fixed in
the concrete. The result is that pressure then exists
upon the exterior face of the blocks, and this will
almost certainly cause displacement or rupture.

Where ordinary rubble filling is placed behind lining
blocks, cement can afterwards be'injectedinto this filling
wuth the object of forming concrete; but this may prove
to be a tedious process and expensive in cement, and if
the water which it ig desired to shut off exists under
pressure, it may be found difficult to obtain a water-
tight finish.

(2)	The second method of utilising concrete, which
has been attended with considerable success, is that of
the construction of reinforced lining with mass concrete
in the shaft, such concrete being carefully rammed
solidly into position. Removable steel facing-plates are
built up as the walling proceeds, and these retain the
concrete until it sets; the plates are taken away after-
wards, and leave a perfectly circular, smooth concrete
wall. In this case pipes are also fixed for the con-
veyance of any remaining water from the rock to the
shaft during the process of walling, and after sufficient
time has elapsed for the concrete to set, this water is
shut off by injection.

The design of reinforcement and thickness of this
type of wall can be arranged to suit the pressure which
it is required to withstand, and consequently with
increasing depth and pressure its use has been proved
to compare favourably in cost with that of cast iron
tubbing. A valuable feature is that if at some time in
the life of a shaft any earth-movements should take
place which should cause cracks in the lining, very little
water would probably be met with on account of the
previous cementation of the ground surrounding the
shaft; whereas in the case of cast iron tubbing, if any
failure should take place, it would be necessary to deal
with the whole quantity of water which was met with
during the sinking through the length where such
failure occurs.

In addition to this, the fixing of a solid mass of
concrete into a shaft as a lining forms an absolutely
close bond with the rock itself, and all irregularities
formed in the shaft sides during sinking are tightly
filled and by subsequent injection made practically
watertight, so that the concrete forms a solid mass with
the rock itself.