THE COLLIERY GUARDIAN

AN®

JOURNAL OF THE COAL AND IRON TRADES.

Vol. CXVI.

FRIDAY, AUGUST 2, 1918.

No. 3005.

Cementation Process Applied to Mining.*

By A. H. KRYNAUW.

During recent years experience has proved that
cement can be introduced, under pressure, into the
minutest fissures and cracks in rock strata, as well
as cracks and interstices in masonry and concrete
constructions, and made to set there. This practice
of introducing cement grout under pressure has
evolved to that known to-day as the Francis
cementation process. An essential condition in the
introduction of cement into fissures and cracks is that
the injection should be done under a considerable pres-
sure, the object being, firstly, to overcome the contra
pressure of water present in the fissure; secondly,
for the purpose of forcing the cement as far as pos-
sible into the minute cracks; and, thirdly, for the
purpose of squeezing out the superfluous water from
the cavity which is being filled with cement pulp,
and thus leave the cement in a condition most suit-
able for its rapid and efficient setting.

Conditions of Setting.

Cement milk, when allowed to set ordinarily under
water, does so only very slowly and reluctantly, but
when the same milk is subjected to pressure, such
as produced by a specially constructed cementation
pump, the cement sets hard within 24 hours. The
setting of cement is chiefly due to the inter-crystallisa-
tion of hydrated calcic silicate and hydrated calcic
aluminate, just the necessary amount of water being
taken up to complete the crystallisation; the crystals
grow together and form a hard mass. Now, when a
superabundance of water is present, such as in the
mixture used in cementation work, these crystals
ordinarily seem reluctant to inter-crystallise; conse-
quently the setting of the cement is very slow and
incomplete. The condition most suitable for hard
setting in the shortest time, when the cement is
pumped under pressure into fissures, etc., appears to
be somewhat as follows: First of all, the constricted
exits of the fissures are choked with cement pulp,
which forms a filter bed on which successive layers
of cement are deposited, the surplus water being
squeezed out through the cement filter already formed
in the peripheral areas of the fissure or cavity. As
more cement solution is injected the pressure gradually
rises, and goes on rising, owing to the. fact of the
water having to be squeezed through an increasing
thickness of cement filter. The successive layers of
cement eventually fill the fissure or cavity completely,
leaving the cement in a condition suitable for com-
plete and good setting. Pressures of 1,000 lb. to 4,000 lb. ‘
per sq. in. are used, and for this purpose a specially
designed pump is employed, of the double ram hori-
zontal type with a 12 in. stroke, the steam end having
a diameter of 14 in., and the water end a plunger
diameter of If in. to 3 in., depending on the final
pressure likely to be required. As the wear on the
valves is great, hardened steel balls are used.

Application of the Cementation Process.

In the following cases cementation has been success-
fully applied : —

1.	In greatly minimising water difficulties in shaft
sinking or tunnelling through water-bearing strata,
faults and dikes.

2.	In sealing and rendering water-tight the concrete
tubbing or lining often used in circular shafts.

3.	In sealing open fissures through which water is
flowing.

4.	In damming back water leaking through broken
ground, in the vicinity of faults and dikes.

5.	In rendering underground dams water-tight.

6.	In rectifying defective boreholes.

7.	In rendering impervious the foundations of
surface dams or making solid the rock and concrete
foundations of engines, etc.

Shaft Sinking.

The process as applied to shaft sinking consists of
a treatment of the strata, through which the shaft
is to be sunk, with cement grout, which is injected
into the fissures and cracks of the rock mass, thus
sealing them and rendering the ground practically
free of water. The method of carrying out the work
is as follows: A system of drill holes is put down on
the periphery of the shaft, and through these holes
a thin cement mixture is introduced, which finds its
way into the cracks and crevices, rendering them
impervious to water. There are two methods of carry-
ing this out. In the Portier method the holes are
put outside the perimeter of the shaft, and it is claimed
that this has the advantage of permitting the sinking
of the shaft simultaneously with the injection of the
cement. This is a very strong point, in that it practi-
cally overcomes the interruption and delay to the

* From paper submitted to the monthly meeting of the
Chemical, Metallurgical and Mining Society of South
Africa on May 18.

sinking of the shaft while conducting the process. On
the other hand, the Portier system is attended by the
following disadvantages: —

(a) The possible deflection of the boreholes into
the shaft, rendering them useless for the
subsequent injection of cement.

■(b) The danger to sinkers, when applying great
pressure in the boreholes, owing to the possi-
bility of rupturing the sides of the shaft.

(c) The boreholes may intercept several fissures,
but the mouth of the hole being above the
water level, it is not possible to tell which
fissure is the chief water carrier, so that
when injection takes place the tendency
will be for the cement to take the course of
least resistance into the larger fissures,
while the smaller ones, though probably of
considerable importance, are only sealed in
the vicinity of the hole itself.

In the Francis system the boreholes through
which the cement is introduced are put down, within
the limits of the shaft, arranged on the perimeter

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at various distances apart, depending on the size of
shaft and thickness of rock strata to be treated. The
detailed method of procedure is indicated by the
following description of the work being done at the
Daggafontein and Brakpan mines : —

Daggafontein Sinking.

Daggafontein No. 2 Shaft is a seven-compartment
rectangular shaft, 49 ft. long by 9 ft. wide, and was
sunk from the surface to 175 ft. in dwyka conglomerate
containing numerous cavities and fissures, giving a
fair amount of water, but not of such quantity as
to seriously hinder sinking. At a depth of 354 ft. the
syenite dike of the far East Rand was encountered,
and the shaft sunk in it to its present depth of 500 ft.
At this stage the water dealt with amounted to 21,000
gallons per hour, and the management decided to
put down a pilot borehole to ascertain the nature
of the strata ahead. This hole intercepted the dolomite
series at a depth of 12 ft. below the shaft bottom,
and at a depth of 165 ft. a considerable amount of
water was encountered. In view of the probability
of large quantities of water being met with in the
dolomites, shaft sinking was stopped, and the adop-
tion of the Francis cementation process decided
upon. From the data available the thickness of the
dolomite formation was estimated at 200 ft., the direc-
tion of the boreholes being set out accordingly, so
as to permit of a single stage treatment for the
full width of the strata. Unfortunately, the thickness
of the dolomites proved to be considerably more than

anticipated, being a total of 345 ft., which distance
was considered too much for treatment in one stage.
A distinct open parting exists at the contact of the
syenite dike and underlying dolomite. This was
clearly demonstrated, for when drilling through the
dolomite contact the feed water was forced out of
holes previously drilled. At a depth of 35 ft. the
first feeder was met with containing sufficient water
to cause a flow of 250 gallons per hour out of a 1| in.
drill hole. A second and larger fissure was struck at
a depth of about 130 ft., and as each hole intercepted
water, cement grout was injected, the progress of
the injection being observed by the rise in pressure
indicated on a pressure gauge. When a pressure of
2,000 lb. per sq. in. was reached, pumping was stopped
and the cement allowed to set. In all 55 tons of
cement were used, and as the depth of the holes is
about 180 ft., sinking can now be continued for that
depth.

Brakpan Sinking.

Brakpan No. 3 Shaft is a 20 ft. diameter circular
shaft, and from the surface to 162 ft. is sunk in
dwyka conglomerate. At a depth of 170 ft. the syenite
dike was met, and sinking was continued through this
dike and for 6 ft. into the underlying dolomites. At
this point a sudden inrush of water occurred which
flooded the shaft to a depth of 80 ft. in a very short
while, the suddenness of the inrush causing the death
of a native. The method to be applied at this shaft
will be similar to that employed at Daggafontein,
except that in this case the shaft had in the first
instance to be dewatered by pumping and baling, and
when sufficiently dry a concrete plug about 5 ft. thick
was placed on the shaft bottom, with a release pipe
to take off the water while laying the concrete matt.
After allowing the concrete time to set, holes will be
put down and the treatment carried out in a manner
similar to that already described. In this instance,
however, the dolomite beds being only 140 ft. thick,
one stage treatment will be sufficient. Even had this
inrush of water occurred at a much greater depth
with considerable pressure, and in such quantity that
the shaft could not be conveniently dewatered, the
concrete plug could be put at the shaft bottom with
no great difficulty, advantage being taken of the water
rising in the shaft to its natural head.

Tunnelling.

When required to drive a tunnel through a water
zone, the method adopted is very similar to that just
described for shafts, and the results obtained at the
East Rand Proprietary Mines in this connection have
been very satisfactory. Two different examples of
this work were conducted, the first being a case where
the water was unexpectedly struck in the 30 Level
West Drive. In this instance a 12 ft. brick dam was
built with certain pipes arranged through it to allow
of the water escaping while building the dam and
to permit of cementation being subsequently carried
out. It should be noted that at the West Shaft,
1,500 ft. away from this point, a borehole had struck
water of the same pressure, suggesting the same source
of water as in this drive, and as proof of this when
injecting at this dam some cement came out of the
borehole in the West Shaft, clearly showing how well
the cement at times will travel.	(

Colloidal Injection.

During cementation work at the Hatfield Main
Colliery, it. was practically impossible to force the
cement into the fissures in the old red sandstone forma-
tion. The cement would not travel any distance,
but set immediately in the vicinity of the boreholes.
Mr. Francis hit upon the idea that the trouble was
due to the fissures being all more or less filled with
sand and dirt, the product of the sandstone, and
that when forcing the cement into these fissures the
sand would act as a filter bed and the cement would
immediately pack into a mass. To overcome this,
solutions of silicate of soda and sulphate of alumina
were separately injected into the fissures. These
chemicals react on each other and form in the fissures
a gelatinous or colloidal substance, which being forced
through carry away the sand, leaving the fissures open
and clear to receive the cement. It also seems to
line the walls of the fissure with a thin smooth layer
of chemicals, and thus acts in the manner of a lubri-
cant which nullifies the frictional resistance of the
walls of the fissure.

Sealing Ferro-Conorefe Shaft Lining.

In shaft sinking, particdiarly circular shafts, a
lining of ferro-concrete or bricks is generally adopted,
and in this connection it frequently becomes desirable
to dam back water behind the lining. As this water
may be under a considerable pressure, the problem
is no easy one unless a process such as the Frangois
method is adopted. In this instance a number of
weep pipes are built in with the lining, which allows
the escape of the water while the lining is being con-
structed, thus relieving the “ green wall ” of the water
pressure. After allowing the lining sufficient time
to set, the weep pipes are in turn connected to the
cementation pump, and cement grout is injected
behind the lining and into the fissures, thus sealing
them up. Should the lining of the shaft not be of