Charles George Warnford Lock.

Economic mining: a practical handbook for the miner, the metallurgist and ... online

. (page 11 of 76)
Online LibraryCharles George Warnford LockEconomic mining: a practical handbook for the miner, the metallurgist and ... → online text (page 11 of 76)
Font size
QR-code for this ebook


c These breasts are 1 fathom high and 1 to 2
£ithoms wide, and are so worked as tuways to have



FiQS. 29, 30.— Obosswobk on TmoK Veins.

finn ground on both sides of them, either solid ore as at klmn, or
^^aste stowed back in a former breast as at d. As one level is worked
out, new gangways, as at 0, are driven overhead, and the cross-cutting
is repeat^, with timbering p where necessary, and always providing
that no two breasts in the same vertical line shall be worked simul-
taneously.

Where the width and character of the ground to be s toped render



Digitized by



Google



86



ECONOMIC MINING.



the employment of either overhead or trnderhand stoping impracti-
cable, recourse is had to the system known as *' square sets" or
"Nevada timbering," which has grown out of the oonditions en-
countered in working the enormous bodies of silver ore in Australia
and America. An illustration of this system is given in Figs.
31 and 32. Sawn timber is used throughout. The uprights and
cross-pieces are 10 in. by 10 in., and stand 4 ft. 6 in. apart cdong the
course of the drive ; the cross-pieces are 6 ft. long, and the height of
the main drive and sill floor sets is 7 ft. 2 in. clear. In blocking out
the stopes, the uprights are 6 ft. 2 in., just 1 ft. shorter than those
in the main drives. The caps and struts are of the same dimensions
and timber as the sill floor. The planks used as staging are 9 in. by




-J



SIM ClXVATION



Figs. 31, 82.— Square Sets ob Nbyada Timbebino.

2i in. ; they are moved from place to place as required, and upon them
the men stand when working in the stopes and in the faces. A stope
resembles a huge chamber fitted with scafiblding from floor to ceiling.
The atmosphere is cool and pure, and there is no dust. Stage is added
to stage, according as the stoping requires it, and ladders lead from
one floor to another. The accessibility of the face is a great advan-
tage. If, whilst driving, a patch of low-grade ore is met with, it
can be enriched by taking a higher class from another face, and so
on ; any grade can be produced by means of this power of selection.

Opinions have been expressed that this system of timbering is not
secure, and that pressure from above would bring the whole structure
down in ruins. But if signs of weakening become apparent in the



Digitized by



Google



MINING AND WINNING. 87

timben, the remedy is very simple. Four or more of the uprights
are lined with planlks, and waste material is shot in from above, and
thereby a solid support is at once formed ; or if signs of crushing are
notioed, it is possible to go into the stope, break down ore, and at
onoe relieve the weight. The cost is said to be 30 per cent, less than
in the usual system, the ventilation is not impeded, and much timber
is saved under ordinary conditions.

A prominent example of the adoption of the square set system is
the well-known Broken Hill mine, in Australia, where the ore-bodies



Fia. 33.— TiMBEBiNo AT Bboksn Hilu

range from 15 ft to 316 ft. wide, and average about 105 ft. In the
early days of this mine, before the great width and friable character
of the lode had manifested themselves, the ordinary method of stoping
was adopted, but in 1887 the square set system foroed itself upon the
management. Figs. 33, 34 represent the system. Where the pressure
is light, single timbering suffices ; but under heavy pressures, false
or double sets, with diagonal struts, are neoessary, and in extreme
cases solid timber bulkheads have been built The timber preferred
is Ore^ pine from Puget Sound.

It IS not to be expected, however, that timbering carried out on



Digitized by



Googl^



88 ECONOMIC MINING.

the square set or any other system, however well designed and
executed, can of itself always withstand the pressures arising from
the excavation of such enormous ore-bodies ; and filling the stopes at
certain points from the hanging- to the foot- wall with hard dry mate-
rial, such as slag or waste rock, must sooner or later be undertaken,
to prevent the spread of fire, as well as the collapse of timbers due
directly to excessive pressure.

The opinions of mining experts differ as to the advisability of
commencing the removal of ore from the foot-wall or hanging- wall
side ; but, as a question of mining and engineering combined, Jamieson
and Howell, in a paper read before the Institution of Civil Engineers,
have no hesitation in saying that, in a lode of the character and



NOmZO.NTAk 6ECTI0II A.B.

Fio. 34.— TniBERiNa at Bboken Hoj*.

dimensions of that at Broken Hill, the correct method of opening up
the lode and sustaining the hangpig-wall, is to commence at the
hanging-wall side and carry the timbers up to it— always keeping
the base of the timber sets, in cross section, so far advanced towards
the foot-wall that the line of timbers on working faces may form a
right-angle with the hanging-wall until the foot-wall be reached.
At stated points stopes can be carried ahead of the main longitudinal
stopes across the lode from the hanging- to the foot-wall, so that the
requisite number of different ore-faces may be exposed. In any case,
the space from which ore has been removed should be filled with hard
dry material as soon as possible. Experience, and very costly ex-
perience, has clearly demonstrated that if a systematic opening of
such large and continuous ore-bodies be not carried out, serious and
dangerous settlement takes place in them, and, in some cases, great



Digitized by



Google



MINING AND WINNING, 89

of ooimtry rock become detaohed from the surrounding rock
OD the hanging-wall side, and press with enormous force on the ore
and timbers underneath. In one case, a mass of country rock, esti-
mated to weigh about 300,000 tons, broke loose from the hanging-
wall side, owing to the shrinkage of the timbers and the friable ore
andemeath. In this instance, proper attention had not been paid to
securing the hanging-wall, and the ore had been taken out chiefly
from the centre and the foot-wall side of the lode, leaving a large
mass of friable and easily compressed ore between the timbers and
the hanging-wall. The quantity of timber required in the framework
of one complete set amounts to 638*4 super, ft. for sill-floor sets, and
405*7 super, ft. for stope-sets ; the average total quantity of timber
of all sizes, including decking, shoring, lathing, false sets, &c., re-
quired per ton of ore removed being about 40 super, ft. (i. e. 12 in.
by 1 in.) per ton of ore.

The cost of mining at Broken Hill by day wages (including dead
work, trucking, and raising the ore to the siurface), per ton of ore, is
as follows : —

£ «. d.

Wages 17 2

Timber 6 10

Fnel and stores 2 10

BepaiiB and Bondries 11

Management 6

Total 18 8

Though the most experienced and skilful men have been employed
to superintend the underground operations, it has been clearly demon-
strated that timbering alone is, except in certain places and under
certain conditions, quite insufficient to support the enormous masses
of ore and the superimposed hanging-wall. Even the use of solid
bulkheads of timber of large scantling has been practically valueless ;
for, although filling the stopes with solid timber has, in certain cases,
prevented utter coUapse, it has been so costly and so blocked up the
workings, that the extraction of ore at the back of the bulkheads has
been rendered almost impossible, and other means have had to be
derised to open portions of the mine. If the mine had been opened
under the hanging-wall first, and the space from which ore was
removed had been filled with rock, probably little danger would have
resulted ; but when this style of timbering was introduced, the full
extent and character of the ore-bodies had not been accurately deter-
mined, and sometimes ore of a certain quality had to be obtained
irreepective of systematic mining, in order to ehable the output to
be maintained. Not only to relieve the pressure on the timbers, but
also to obtain certain qualities of ore more cheaply, the removal of
the cap of the lode and quarrying from the surface has been com-
tnenceo. The depth tQ which tms open-cut work will be carried
depends upon circumstances that may arise as the work proceeds ; but,
90 long as it is cheaper and safe, this work will be carried out. The
banging-wall will need to be cut away simultaneously with the
quarryixMP of the ore, and it is estimated that the quarry will thus
reqidie the removal of between 2j^ and 3 cub. yd. of rock for each



Digitized by



Googk



90 ECONOMIC MINING.

ton of ore, down to depths between 60 and 100 ft. from the sor&oe.
A large quantity of timber will also be recovered, which will go to
defray the cost of this work.

In order to maintain the hanging-wall below the level of this
open-cut work, strong and well-designed walls of waste rock will
have to be built from the foot-wall to the hanging-wall. This will
cost little, and will practically amount to replacing rock which under
any circumstances would have had to be removed.

Excellent examples of the contingencies arising in the course of
long-continued mining operations are afforded by the East and West
Vulcan mines of the Penn Iron Mining Company, Michigan, The
ore occurs here in immense bodies, lying between hard jasper-slates
and soft clay-slates; it is soft hsBmatite, and varies in thickness
from a few inches to over 100 fb. The earliest mining methods were
of the crudest sort, but the gradual increase of depth and multiplication
of difficulties have compelled successive improvements.

The first was the introduction of the square-set system of
timbering, which, as already described, consists in filing the space
exhausted in the ore-body with a series of frame cubes. These cubes
are constructed with white pine timber of excellent quality, 12 to
15 in. sq., framed into squares of 7 ft. from centre to centre. There
is great value in this system of timbering as related to strength,
facility in erecting, and its adaptability to all thicknesses of ore-
deposits or variations in hanging- or foot-walls. When the walls are
tolerably firm and not easily softened by exposure to the moist
atmosphere of the mine, it possesses great strength ; but where the
hanging-wall is of soft clay-slates, softening rapidly on exposure, this
svstem with its large timbers affords only temporary support. When
tne crush begins, the upright posts lose their vertical position, and
many of the timbers are reduced to splinters. Collars and cross-
braces are sometimes used to arrest a squeeze; but a brief respite
only is secured in this way. The flexure of a soft hanging^waU
indicates the early crushing of timbers in the exhausted portion of
the leveL It becomes then a struggle on the part of the miner to
remove as much as possible of the ore out of the creeping levels before
the final crush comes to close out all mining operations. It has thus
been found that for certain qualities of mine-walls this elaborate
system of timbering fulfils its ofiice satisfactorily, but that under a
softening hanging- or foot-wall it is not reliable for a sufficient time
to afford opportunity for exhaustive mining in each level, or to pro-
tect ways to the adjoining leveL The depth to which it can be safely
carried depends on the f^mness of the hanging- and foot-walls more
than on the increased pressure from depth, although the latter is also
an important factor.

The adoption of the method known as " rock-filling " was ochd-
pelled by the conditions, already described, in the West Vulcan mine.
At the eighth level the ore is 600 ft. long and the average thickness
about 25 ft. The shaft is 665 ft. deep to the ninth leveL The
timbering, mainly after the Nevada system, has been carried to the
eighth level. Bock-filling is now being used in the ninth level.

Figs. 35-37 show the method of this filling. From the main



Digitized by



Google



MINING AND WINNING. 91

shaft a a drift cuts the ore-body h and 25 fb. into the foot-wall c of
firm jasper-alates. From this point d a rook-tnnnel e is driven in the
foot-wall c east to shaft /. Along this rook-
tunnel e ports g are made at intervals of
about 100 ft. into the ore-body 6. From these
ports the mining of the ore begins on the
bottom of the ninth level.

The first cut of the ore is mined about

8 to 10 ft. high, and the spaces from which

the ore has been removed are filled with

rock. This rock-filling ib follows the mining

^'^^'^^J^^ and affords absolute safety and inflexible sup-

^' port to the walls of the mine. The broken



^

5



1

8



Digitized by



Google



92 ECONOMIC MINING.

rock for filling is conveyed down the winzes ^, and the ore through
the shutes t, which are built up as fast as the filling is made
upwards.

It will thus be. seen that the tunnel e in the foot-wall secares
complete safety to the main ways of each level ; it is out of the crush
range in any event. The winzes li are also ventilators, and greatly
improve the sanitary conditions of the mine.

The ore-body 6 is under this system attacked by the miners on
double face at each port gf, from the main rock level or tunnel e in
the foot-wall. The mining by sections upwards is simply a repetition
of the first 8-10 ft., the rock filling following the mining work and
occupying the spaces from which the ore has been removed as rapidly
as room for the mining operations will permit. Some timbering will
be required occasionally in this rook-filling, especially in sustaining
the filling in upper levels, as it is approached from below, in removing
the last cut of ore.

This rock-filling system has not been adopted on the score of
economy over the Nevada system of timbering, but has been com-

Selled by the necessities of the case. Sufficient work has not been
one to afford a reliable statement of the relative cost of the two
systems. Nor would such a statement at any time form a permanent
l^u9is of comparison, because the forests of this region are being ex-
hausted, with consequent effects on the market prices of timber, while
the cost of breaking up and shooting waste rock into the mine
fluctuates but little. The price paid for timber and timbering at
West Vulcan was 1«. 7 Jd. per ton of ore mined. The cost of rock-
• filling, with attendant consumption of timber per ton of ore mined,
was 7d. a ton for rock-fiUine and 34. a ton for timber used therewith.
It may be pointed out that me rock-filling affords a permanent supx>ort,
which is not liable to decay as is the timbering system, and will not
require renewals during the progress of the mine-workings.

There are other important factors in connection with the general
application of this system of rock-filling, e. g. the location of the
shafts or slopes to the mines. The first planning of these slopes or
skip-ways placed them in the ore-body, requiring large pillars of or©
for their maintenance. Where the ore is of moderate thickness
(12-15 ft.) this method is not so open to criticism; but when the ore-
body is 20-100 ft. thick and rather soft, it has serious drawbacks in
the great pillars of ore which must be set apart for this purpose, and
in their tendency to crumble, especially in the event of a creep or
crush, which follows in a greater or less degree in exhaustive minin g.
The use of skip-ways in the foot-walls, or t£e sinking of shafts in the
hanging-walb, are matters of the greatest importance in assuring safe
and economical mining. It has been shown that slopes or skip-ways
in the ore-body are open to serious objections in large deposits of ore.
It is quite possible to sink the slopes or skip-ways in the foot-waU,
especially when this is firm. They could be driven in the foot-wall,
and a sufficient distance under the plane of the ore, say 10-20 fU, «o
as to assure a permanent rock tunnel under all conditions of creep or
crush. This will have some exceptions, as where the ore-bodv is
much flexed and pitching ; but there are many deposits where a slope



Digitized by



Google



MINING AND WINNING. 93

or skip- way could be driven in the foot- wall of an ore-deposit, and be
readilj made to eon form to the flexures met, in ordinary cases, in
these foot-walls. This would be costly at first, but it would ultimately
lie fonnd to assure great safety, and permit exhaustive mining, since
no pillars would be required for its protection.

West Vulcan mine exhibits an example of approaches by slope or
skip-ways, partly in foot-wall slates, and a vertical shaft in the soft
slates of the hanging-wall. The location of the main shaft in the
L&Dging-wall was the result of several conditions bearing on the
ultimate economy of mining its ore. It is near the siding of the rail-
road, a£bFding a ready way for delivering the ore into railroad cars,
and also for receiving coal by the same way for the boilers, and
timber for the mine. This location also reduced largely the height
of the pumping column, and the shaft was sunk in soft slates rapidly
and cheaply. Aside from these special conditions, the locating of a
shaft in Uie hanging-wall cannot be commended, as there is generally
some shifting in the hanging-wall ground, endangering imaft and
machinery.

It has been a matter of discussion whether the usual method of
Tiiining by beginning operations at or near the surface and working
downwards, is the best plan. It is true that it afifords a ready out-
put of ore, and a quick return of money ; but, unless permanent rock-
ways are established, it involves increasing expenditure in the down-
ward workings. It is submitted by Fulton that in deposits of ore of
moderate thickness, a slope could be cut in the ore to the bottom of
the deposit, and workings commenced there,
entirely exhausting the ore in the progress
of the working upwards. The exhausted
spaoee below would afford a ready place for
mining refuse, and could, if necessary, be
supplemented with additional rock-filling.
Even should the hanging-wall swell or
buckle, no serious injury could result, as
such a crush would be arrested at each leveL

The objections to the rock-fiUing system
ire the amount of dead work entailed, and
the inconveniences and destruction of tim-
t^ring owing to the ground sinking un-
jvenly under foot.

The system of mining in use at the Iron
Fifiver mine, Menominee region, is extremely
^ell adapted to ores hard enough to stand
>ver the width of the vein. A section
kcross a stope (Fig. 38) shows the method

>i work. The ore a is taken down in an fio. 38.— Workikg on Hard
overhead stope 6 running the width of the Vein.

rein firom the hanging-wall c to the foot-

iv^ll d for any desirable length, and for a height of say 12 ft. A
imber-drift e is then built along the floor of the stope, and the
KftlMioe / of the stope is packed with waste sent down from the
fxr&ce through winzes previously sunk or upraised at intervals of



Digitized by



Google



94 ECONOMIC MINING.

about 60 ft. in the length of the stope. A " mill " or " shnte " ^ is
carried up every 50 ft., along the level to run the ore down ; it is
about 4 ft. sq. inside, and is built of round, rough hardwood sticks.
The spaces between the sticks when they do not fit closely are filled
with pieces of plank, and the inside of the " mill " is lined with hard-
wood planks spiked on to the side timbers. These planks are easily
replaced when worn. The packing is levelled over as close to the
backs of the stope as is convenient for working, and is planked A over,
to keep the ore from mixing with the filling. The " mill " is carried
up berore the filling as high as this is to go ; and when the fiUine is
levelled off, a few large sticks are laid across the mill, leaving
intervals large enough to throw the ore down, but so narrow as to
prevent the falling in of a man or a block of ore that would choke up
the outlet from the milL This method is extremely satisfactory
where the ore is strong enough to stand without timbering across
the vein. It requires no timber, except for the level and the mills,
and a few planks, while permitting the extraction of nearly all the
ore. The cost of filling at Iron River was only 6 Jd. a ton on the ore
got out, but on the average this figure would be much exceeded.

For working in large soft ore bodies, Bothwell thinks it would be
found in every way advantageous to work the vein out from the top
downwards. He would drive the main levels a in the vein h (Figs.
39, 40), and most conveniently on the foot-wall c, d being the hanging-
wall. As the ground is undisturbed, it should be feasible to keep
open a level the width of an 8-ft. set, no matter how soft the ore, thus
avoiding much dead work. At suitable intervals the vein would be
cross-cut e, and stoped out the width of one set, up from one level to
the next above. However soft the ore, it should be possible, even
with rather light timber, to hold a stope only 8-9 ft. wide running
across the vein in the solid well-drained ore. This will take the
place of winzes and cross-drifts at much less cost, and will serve as a
pocket or shute / to hold the ore, which can be drawn thence into the
cars li below ; or the mill or shute /, through which the ore is sent
down, can be built say %\ by 4 ft. in the clear, of round hardwood
sticks lined with hardwood plank. By a little care in packing round
it, this can probably be held in the filing ^ as a waste mill, through
which filling may be sent down from above ; or the mill can be cot
in the foot-wall. When the cross-stope reaches the upper worked-out
ground, longitudinal stopes t, one or two sets in height and one set in
width, are driven to half the distance between the cross-stopes,
leaving between them intervals or pillars two sets wide or more if
the ground permits. These longitudinal stopes are timbered lightly,
having to stand but a very short time, and being in the solid undia-
turbed ore, with the filling from above resting on each side of them
on solid ore. When the mid-distance between the cross-stopes is
reached, the back-stoping commences by taking out the ore on each
side of the stope t to the width of a set, or half the pillar left between
the longitudinal stopes t supporting tiie " gob " roof while doing so,
and laying lagging poles or slabs across the floor of the stope as the
work pro^eds. Any waste rock or material available or desirable
may be thrown back in the packing, and when a space the area of one



Digitized by



Google



MINING AND WINNING. 95

or two sets is worked out on eacli side of the last sets of the longi-
tudinal stope t, the temporary timbering is drawn, and the " gob "
roof^ with the lagging previously laid under it, is allowed to drop on
the bottom of the stope lagged to receive it. Light poles, and even
brushwood, will serve for thus keeping the ore from mixing with the
wsuite. It will probably be found possible, as well as advantageous
in many cases, to drive these longitudinal stopes t two sets high, and
draw back the upper one a little in advance of the lower, and as they
can be driven out at any point in the cross-stope, a whole horizontal
slice or section of the ore-body, no matter what its thickness, can be
opened out, say two sets in height, at the same time.



Figs. 39, 40.— Wobkoyq DOWirwABDe ok Soft Orb-bodies.

The advantages claimed for this system are : (1) that all the work
is simply stoping; (2) all levels and stopes that have to be timbered
aie in solid, undisturbed ore, and being only one " set " in width, are
easily held, and require but light timber, while much of this is drawn
and saved in letting the roof down ; when the stope comes up to the
filling, this has only to be supported over one set of timber, while it
rests on the solid ore on each side ; (3) the filling follows the ore
^wn, and as long as this occurs, the cave on the surface can be con-
stantly filled from " borrow pits " much cheaper than by sending the
filling below ; (4) it is possible to obtain practically all the ore in the



Digitized by



Google



96 ECONOMIC MINING.

vein, and to get it free irofm mixture with waste ; (5) oaves or cnii^ei
are impossible ; (6) more ore can be extracted from a given amonnj
of ground in a given time.

Another example of working in soft ore-bodies is that at I^o-w
Moor, Virginia. The ore is generally soft, and all drifts require clc«

timbering. The hanging-wall c
(Fig. 41) is a band of brdcen flini
and clay, and the foot-wall h ij
sandstone. Main levels c are driven
in the vein alon^ its strike, 60—80
ft. apart vertically, starting fron



Online LibraryCharles George Warnford LockEconomic mining: a practical handbook for the miner, the metallurgist and ... → online text (page 11 of 76)