NITRO-GLYCERINE COMPOUNDS.
_Nitrated Gun-cotton._--It has been shown that gun-cotton contains an
insufficient quantity of oxygen for its complete combustion. To furnish
that which is wanting, gun-cotton has sometimes incorporated with it a
certain proportion of nitrate of potash, or of nitrate of baryta. This
compound, which, it will be observed, is at once a chemical compound and
a mechanical mixture, is known as “nitrated gun-cotton.”
_Cotton Powder, or Tonite._--The explosive which is now well known as
“tonite” or “cotton powder,” is essentially nitrated gun-cotton. It is
produced in a granulated form, and is compressed into cartridges of
various dimensions to suit the requirements of practice. The convenient
form in which tonite is made up, ready to the miner’s hand, has greatly
contributed towards bringing it into favour. But irrespective of this,
the fact of its being so highly compressed as to give it a density
equal, or nearly equal, to dynamite gives it a decided advantage over
the other nitro-cotton compounds as they are at present used.
_Schultze’s Powder._--In Schultze’s powder, the cellulose is obtained
from wood. The wood is first sawn into sheets, about ¹/₁₆ inch thick,
and then passed through a machine, which punches it up into grains of a
uniform size. These are deprived of their resinous matters by a process
of boiling in carbonate of soda, and are further cleansed by washing in
water, steaming, and bleaching by chloride of lime. The grains, which
are then pure cellulose, are converted into nitro-cellulose in the same
way as cotton, namely, by being treated with a mixture of nitric and
sulphuric acids. The nitro-cellulose thus produced is subsequently
steeped in a solution of nitrate of potash. Thus the finished compound
is similar in character to nitrated gun-cotton.
_Lithofracteur._--Lithofracteur is a nitro-glycerine compound in which a
portion of the base is made explosive. In dynamite, the base, or
absorbent material, is, as we have said, a silicious earth, called
“kieselguhr.” In lithofracteur, the same substance is used; but in
addition, a mixture of nitrate of baryta and charcoal, a kind of
gunpowder, is introduced. The object of employing this explosive mixture
is to increase the force of the explosion, the kieselguhr being an inert
substance. Obviously this object would be attained if the explosive
mixture possessed the same absorbent power as the kieselguhr. But
unfortunately it does not, and, as a consequence, less nitro-glycerine
is used. Thus what is gained in the absorbent is lost in the substance
absorbed. The composition of lithofracteur varies somewhat; but its
average proportion of ingredients are the following:--
Nitro-glycerine 52·50
Nitrate of baryta 16·40
Charcoal 2·85
Sulphur 25·75
Kieselguhr 22·50
------
100·00
------
_Brain’s Powder._--Brain’s powder is a nitro-glycerine compound, similar
in character to lithofracteur. The exact composition of the base has
never been published, so far as relates to the proportions of the
ingredients. But it is composed of chlorate of potash, charcoal, and
nitrated sawdust. The proportion of nitro-glycerine never exceeds 40 per
cent. Horseley’s powder contains about the same proportion of
nitro-glycerine in a base of chlorate of potash and nut-galls.
_Cellulose Dynamite._--In Germany, gun-cotton is used as an absorbent
for nitro-glycerine, the compound being known as “Cellulose dynamite.”
It is chiefly used for primers to explode frozen dynamite. It is more
sensitive to blows than the kieselguhr dynamite.
CHAPTER III.
THE PRINCIPLES OF ROCK BLASTING.
_Line of Least Resistance._--The pressure of a fluid is exerted equally
in all directions; consequently the surrounding mass subjected to the
force will yield, if it yield at all, in its weakest part, that is, the
part which offers least resistance. The line along which the mass
yields, or line of rupture, is called the “line of least resistance.” If
the surrounding mass were perfectly homogeneous, it would always be a
straight line, and it would be the shortest distance from the centre of
the charge to the surface. Such, however, is never the case, and the
line of rupture is, therefore, always a more or less irregular line, and
often much longer than that from the centre direct to the surface. It
will be obvious, on reflection, that the line of least resistance will
be greatly dependent upon (1) the texture of the rock, which may vary
from one point to another; (2) its structure, which renders it more
easily cleavable in one direction than in another; (3) the position,
direction, and number of the joints, which separate the rock into more
or less detached portions; and (4) the number and relative position of
the unsupported faces of the rock. All these circumstances must be
ascertained, and the position and the direction of the bore-hole
determined in accordance with them, in order to obtain the maximum
effect from a given quantity of explosive. It must not be supposed,
however, that this is a labour involving minute examination and long
consideration. On the contrary, a glance is generally sufficient to
enable the trained eye to estimate the value of those circumstances, and
to determine accordingly the most effective position for the shot. In
practice, the line of least resistance is taken as the shortest distance
from the centre of the charge to the surface of the rock, unless the
existence of joint planes, a difference of texture, or some other
circumstance, shows it to lie in some other direction.
_Force required to cause Disruption._--When the line of least resistance
is known, it remains to determine the quantity of the explosive compound
required to overcome the resistance along that line. This matter is one
of great importance, for not only is all excess waste, but this waste
will be expended in doing mischief. In mining operations, the dislodged
rock is violently projected, and the air is vitiated in an unnecessary
degree; and in quarrying, stones are shattered which it is desirable to
extract in a sound state. The evil effects of overcharging, in
occasioning the formation of noxious gases, was pointed out in the last
chapter. Of course it is not possible so to proportion a charge to the
resistance that the rock shall be just lifted out, and no more; because
neither the force developed by the charge, nor the value of the
resistance can be known with precision. But a sufficient approximation
may be easily arrived at to enable us to avoid the loud report that is
indicative of wasted force.
Charges of an explosive compound of uniform strength produce effects
that vary as the weight of those charges, that is, a double charge will
move a double mass. And, as homogeneous masses vary as the cube of any
similar line within them, the general rule is established that charges
of powder capable of producing the same effects are to each other as the
cubes of the lines of least resistance. Generally, the quantity of black
blasting powder requisite to overcome the resistance will vary from ¹/₂₀
to ¹/₃₀ of the cube of the line of least resistance, the latter being
measured in feet and the former in pounds. Thus, if the rock to be
blasted be moderately strong limestone, for example, and the shortest
distance from the centre of the charge to the surface of the rock be 3
feet, we shall have 3 × 3 × 3 = 27, the cube of the line, and ²⁷/₂₅ lb.
= 1²/₂₅ lb., or about 1 lb. 1 oz., as the weight of the powder required.
If dynamite be used, and we assume it to be four times as strong as
common black powder, of course, only one-fourth of this quantity will be
required. Also if gun-cotton, or cotton-powder, be used, and we assume
its strength to be three times that of black powder, one-third only
will be needed. Again, if Curtis’s and Harvey’s new extra-strong mining
powder fired by a detonator be employed, we may assume it to be twice as
strong as common black powder fired by the ordinary means, and
consequently we shall need only one-half the quantity indicated by the
formula.
It is neither practicable nor desirable that such calculations and
measurements as these should be made for every blast; their practical
value lies in this, namely, that if the principles involved in them be
clearly understood, the blaster is enabled to proportion his charges _by
sight_ to the resistance to be overcome, with a sufficient degree of
precision. A few experiments in various kinds of rock, followed by some
practice, will enable a man to acquire this power.
As it is a common and a convenient practice to make use of the bore-hole
as a measure of the quantity of explosive to be employed, we have
calculated the following table:--
---------+------------+----------+------------
Diameter | | |Dynamite (or
of |Black Powder|Gun-cotton| Tonite)
the Hole.| in 1 inch. |in 1 inch.| in 1 inch.
---------+------------+----------+------------
ins. | ozs. | ozs. | ozs.
1 | 0·419 | 0·419 | 0·670
1¼ | 0·654 | 0·654 | 1·046
1½ | 0·942 | 0·942 | 1·507
1¾ | 1·283 | 1·283 | 2·053
2 | 1·675 | 1·675 | 2·680
2¼ | 2·120 | 2·120 | 3·392
2½ | 2·618 | 2·618 | 4·189
2¾ | 3·166 | 3·166 | 5·066
3 | 3·769 | 3·769 | 6·030
---------+------------+----------+------------
[Illustration: FIG. 40.]
[Illustration: FIG. 41.]
[Illustration: FIG. 42.]
[Illustration: FIG. 43.]
_Conditions of Disruption._--Having explained the law according to which
the elastic gases evolved by an explosion act upon the surrounding rock,
and shown how the force required to cause disruption may be calculated,
it now remains to consider the conditions under which disruption may
take place. Suppose a block of unfissured rock detached on all sides, as
shown in plan, in Fig. 40, and a bore-hole placed in the centre of this
block. If a charge be fired in this position, the lines of rupture will
radiate from the centre towards any two, or towards all four of the
unsupported faces of the block, because the forces developed will act
equally in all directions, and the lines of rupture will be those of
least resistance. Evidently this is the most favourable condition
possible for the charge, since the rock offers an unsupported face on
every side; and it is evident that the line of rupture must reach an
unsupported face to allow of dislodgement taking place. Suppose, again,
as shown in Fig. 41, the block to be unsupported on three sides only,
and the charge placed at _h_. In this case, the lines of rupture may
run to any two, or to all three, of the unsupported faces; and hence
this will be the next most favourable condition for the action of the
charge. The greatest useful effect, however, will be obtained in this
case by placing the charge farther back at _h′_, when the lines of
rupture must necessarily run to the opposite faces _b c_, and,
consequently, the whole of the block will be dislodged. Assume another
case, in which the rock is unsupported upon only two sides, as shown in
Fig. 42, and the charge placed at _h_. In this case, the lines of
rupture must run to each of the unsupported faces _a b_. Thus, it is
evident that this condition, though still a favourable one for the good
effect of the charge, is inferior to the preceding. As rock is never
homogeneous in composition nor uniform in texture, the lines of rupture,
which, as before remarked, will be those of least resistance, may reach
the faces at any point, as at _m n_, _m′ n′_, or any point intermediate
between these. But it will be seen that the useful effect will be
greatest when these lines, radiating from the charge, make an angle of
180°, or, in other words, run in directly contrary directions, and that
the useful effect diminishes with the angle made by these lines of
rupture. Suppose, again, the rock to be unsupported upon one side only,
as shown in Fig. 43, and the charge placed at _h_. In this case, the
lines of rupture must run to the face _a_, and the condition must
therefore be considered as less favourable than the preceding. As in
those cases, the useful effect will depend upon the angle made by the
lines of rupture _h m_ and _h n_, which angle may be very small, and
which must necessarily be much less than 180°. A greater effect may be
obtained, under this condition, by firing several charges
simultaneously. If, for example, we have two charges placed, one at _h_,
and the other at _h′_, and fired successively, the lines of rupture will
run in or near the directions _h m_, _h n_, _h′ m′_, _h′ n′_, and the
portion of rock dislodged will be _m h n h′ n′_. But if these two
charges be fired simultaneously, the lines of rupture will be _h m_, _h
o_, _h′ o_, _h′ n′_, and the mass of rock dislodged will be _m h h′ n′_.
Simultaneous firing is in this way productive of a greatly increased
useful effect in numerous cases, and the mining engineer, and the
quarryman especially, will do well to direct their attention to this
source of economy. There is yet another case to be considered, in which
the conditions are still less favourable. Suppose two unsupported faces
at right angles to each other, and the charge placed at _h_, as shown in
Fig. 44. In this case, the lines of rupture will run to each of the two
unsupported faces; but as these lines must necessarily make a very small
angle with each other--for the length of the lines increases rapidly
with the angle--the useful effect will be less than in the last case. It
follows, therefore, that this is the most unfavourable condition
possible, and as such it should be avoided in practice.
[Illustration: FIG. 44.]
[Illustration: FIG. 45.]
[Illustration: FIG. 46.]
In the foregoing considerations, the holes have been assumed to be
vertical, and for this reason the unsupported face which is
perpendicular to the hole, that is, the face into which the hole is
bored, has been neglected. For it is evident that, under the conditions
assumed, the lines of rupture cannot reach this face, which, therefore,
has practically no existence. Suppose, for example, a bore-hole placed
at _h_, in Fig. 45, and the rock to be supported upon every side except
that at right angles to the hole. The forces acting perpendicularly to
the direction of the bore-hole are opposed on all sides by an infinite
resistance. Hence, in this case, either the tamping will be blown out,
or, if the forces developed are unequal to the work, no effect will be
produced beyond a slight enlargement of the hole at the base. This,
however, is a case of frequent occurrence in practice, and it becomes
necessary to adopt measures for making this unsupported face available.
Evidently this object can be attained only by so directing the bore-hole
that a line perpendicular to it may reach the face; that is, the line of
the bore-hole must make with the unsupported face an angle less than
90°. This direction of the bore-hole is shown in Fig. 46, which may be
regarded as a sectional elevation of Fig. 45. In this case, the lines of
rupture, which will run similarly to those produced in the case shown in
Fig. 43, will reach the unsupported face at _b_, and the length of these
lines, and consequently the depth of the excavation, for a given length
of bore-hole, will depend upon the angle which the latter makes with
the face. This mode of rendering a single exposed surface available is
called “angling the holes,” and it is generally resorted to in shaft
sinking and in driving headings. The conditions involved in “angling”
are favourable to the action of strong explosives.
_Example of a Heading._--To show how these principles are applied in
practice, we will take a typical case of a heading, 7 feet by 9 feet, as
shown in Fig. 47. In this case, we have at starting only one exposed
face, which is perpendicular to the direction of the driving. Hence it
is evident that we shall have to proceed by angling the holes. We might
begin in any part of the exposed face; but, as it will hereafter appear,
the most favourable position is the centre. We therefore begin at this
point by boring a series of holes, numbered 1 on the drawing. These
holes are angled towards each other; that is, the two sets of three
holes vertically above each other converge in the direction of their
lower ends, as shown in the sectional plan, Fig. 48. In this instance,
we have assumed six holes as necessary and sufficient. But it is obvious
that the number of holes, as well as their distance apart horizontally,
will be determined by their depth, the tenacity of the rock, and the
strength of the explosive used. When these holes are fired, a
wedge-shaped portion of the rock will be forced out, and this result
will be more effectually and certainly obtained if the charges be
fired simultaneously. The removal of this portion of the rock is called
“taking out the key.” The effect of removing this key is to leave the
surrounding rock unsupported on the side towards the centre; that is,
another face is formed perpendicular to the first.
[Illustration: FIG. 47.]
[Illustration: FIG. 48.]
[Illustration: FIG. 49.]
Having thus unkeyed the rock by the removal of this portion from the
centre, it will evidently be unnecessary, except for convenience or
increased effect, to angle any more of the shot-holes. The second series
therefore, numbered 2 in the drawing, may be bored perpendicularly to
the face of the heading. When this series is fired, the lines of rupture
will all run to the unsupported face in the centre--and from hole to
hole, if the shots be fired simultaneously--and the annular portion of
rock included between the dotted lines 1 and 2 will be removed. If the
shots be fired successively, the first will act under the condition of
one unsupported face, as illustrated in Fig. 43; but as another
unsupported face will be formed by the removal of the rock in front of
this charge, the succeeding shots will be subject to the more favourable
condition represented in Fig. 42. The firing of this second series of
shots still leaves the surrounding rock unsupported towards the centre,
and consequently the same conditions will exist for the third series,
numbered 3 on the drawing, the firing of which series will complete the
excavation. Fig. 49 shows the appearance of Fig. 48 after the firing of
the central holes.
It may be remarked here that, owing to the want of homogeneity in the
rock, and to the existence of joints and fissures, the outer line of
rupture will not, in practice, run so regularly as indicated, in this
assumed case, by the dotted lines. This circumstance will influence the
position of the holes, or the quantity of explosive, in the next series,
and furnish an opportunity for the exercise of judgment on the part of
the blaster.
There exist also other circumstances which will influence the position
and the number of the holes in a very important degree, and which
therefore must be taken fully into account at every advance. One of
these is the irregularity of the face of the excavation. Instead of
forming an unbroken plane at right angles to the direction of the
heading, or of the shaft, this face is broken up by projecting bosses
and more or less deep depressions. Obviously these protuberances and
cavities will influence, in no inconsiderable degree, the lines of least
resistance; the latter being lengthened or shortened, or changed in
direction, by the presence of the former, which give existence to
unsupported faces to which the lines may radiate. These conditions must,
in every case, be taken into account when determining the best position
for the bore-hole. Of yet greater importance, is the existence of joint
planes and bedding planes. A bed of rock may be, and frequently is, cut
up by these planes into detached blocks of greater or less dimensions,
according to the more or less perfect development of the different sets.
Hence it becomes necessary, in determining a suitable position for
blasting the charge, to consider such planes as unsupported faces, and
to ascertain the direction and length of the lines of resistance under
such conditions. If a charge be placed in close proximity to one of
these planes, not only may the lines of rupture run in unforeseen
directions, but the greater part of the force of the explosion will be
lost by the escape of the gases along the plane. The same loss of force
may be occasioned by the presence of a cavity, such as are of frequent
occurrence in cellular or vughy rock. When the joint planes are fully
developed, their existence can be ascertained by inspection; but when
their development is imperfect, there may be considerable difficulty in
discovering them. In such cases, the rock should be carefully inspected,
and sounded with a hammer or pick. When a cavity is bored into, it may
be rammed full of clay, and the boring continued through the clay; or if
sufficient depth has been obtained, the charge may be placed upon the
clay, which will prevent the wasteful dissipation of the gases. As none
of the aforementioned circumstances occur under precisely similar
conditions, no general rule of much service can be laid down; they are
matters upon which the blaster must be left to use his own judgment, and
to do this effectively, it is necessary that he possess some knowledge
of the materials with which he deals.
_Economical Considerations._--Besides the important economical
considerations involved in the foregoing, there are others which claim
attention. Foremost among these is the question whether, for a given
effect, it be better to augment or to diminish the individual importance
of the shots; that is, whether it be better to diminish the number of
the holes and to increase their diameter, or to diminish their diameter
and increase their number; or, again, to diminish their diameter and to
increase their depth, or to increase their diameter and to diminish
their number and their depth. It may be readily shown mathematically,
and the results are confirmed by experience, that there is an important
gain in reducing the diameter of the shot-holes to the lowest limit
allowed by the strength and the gravimetric density of the explosive,
and increasing their depth. The gain is mainly in the direction of a
saving of labour, and it is especially remarkable in the case of machine
boring. Here again we perceive the advantage of strength in the
explosive agent employed.
The simultaneous firing of the shots offers several important
advantages. It has already been shown how one charge aids another, under
such a condition, and in what way the line of rupture is affected by it.
When the shots are fired successively, each one has to _tear out_ the
portion of rock allotted to it; but when they are fired simultaneously,
their collective force is brought to bear upon the whole mass to be
dislodged. This is seen in the diagram, Fig. 43. When deep holes are
used, the greater useful effect caused by simultaneous firing becomes
very marked. Hence electricity associates itself naturally with machine
drills and strong explosives.
_Tamping._--To “tamp” a shot-hole is to fill it up above the charge of
explosive with some material, which, when so applied, is called the
“tamping.” The object of tamping is to oppose a resistance to the escape
of the gases in the direction of the bore-hole. Hence a primary
condition is that the materials used shall be of a strongly resisting
character. A second determining condition is that these materials shall
be of easy application. This condition precludes the use of all such
devices as plugs, wedges, and forms of a similar character, which have
been from time to time proposed.
The only material that, in practice, has been found to satisfactorily
fulfil the requirements, is rock in a broken, pulverulent, or plastic
state. As, however, all rock is not equally suitable, either from the
point of view of its resisting character, or from that of convenience of
handling, it becomes necessary to consider which satisfies the two
conditions in the most complete manner.
Though it is not easy to assign a perfectly satisfactory reason why one
kind of rock substance opposes a greater resistance to motion in a
bore-hole than another, yet it is certain that this resistance is mainly
due to the friction among the particles of that substance. If a column
of solid, hard rock, of the same diameter as the bore-hole, be driven
down upon the charge, the resistance opposed by the column to the
imprisoned gases will be, neglecting the weight of the former, that of
the friction between the sides of the column and those of the hole. But
if disintegrated rock be used, not only is an absolute motion imparted
to the particles, but, on account of the varying resistances, a relative
motion also. Consequently, friction occurs amongst the particles, and as
the number of these is immense, the sum of the slight friction of one
particle against another, and of the great friction of the outside
particles against the sides of the hole, amounts to a much greater value
than that of the outside particles of the solid column against the sides
of the bore-hole. If this view of the facts alone be taken, it follows
that dry sand is the most resistant material, and that the finer the
grains, the greater will be the resistance which it offers. In practice,
however, it has been found that though the resistance offered by sand
tamping is very great, and though also the foregoing inference is true
when the tamping is lifted by the pressure of a solid against it from
below, this substance is notably inferior to some others when acted upon
by an explosion of gases. The explanation of this apparent anomaly is
that the gases, under the enormous tension to which they are subjected
in the bore-hole, insinuate themselves between the particles, and so
prevent the friction which would otherwise take place. When the
readiness with which water, through the influence of gravity alone,
permeates even closely compacted sand, is borne in mind, there will be
no difficulty in conceiving a similar action on the part of more subtile
gases in a state of extreme tension. Under such conditions as these,
there is no resistance whatever due to friction, and the only resistance
opposed to the escape of the gases is that proceeding from the inertia
of the mass. How this resistance may be very great, we have shown in the
case of air tamping. Hence, it becomes necessary to have recourse to
some other material of a composition less liable to be thus acted upon,
or to seek means of remedying the defect which renders such action
possible.
Clay, dried either in the sun, or, preferably, by a fire, appears to
fulfil the requirements of a tamping material in the fullest degree.
This substance is composed of exceedingly minute grains of silicious
matters, bound together by an aluminous and calcareous or ferruginous
cement. Thus constituted, there are no voids between the particles, as
in porous substances, and, consequently, there is no passage for the
gases, the substance being impervious alike to water and gas. Hence,
when this material is employed as tamping, the forces act only upon the
lower surface, friction takes place among the particles, and the
requisite degree of resistance is produced. By reason of its possession
of this property, clay is generally used as the tamping material.
In rock blasting, it is usual to prepare the clay beforehand, and this
practice is conducive both to effective results and to rapidity of
tamping. The latter consideration is an important one, inasmuch as the
operation, as commonly performed, requires a good deal of time. To
prepare the pellets of clay, a lump is taken and rolled between the
palms of the hands until it has assumed the form of a sausage, from
three to four inches in length, and of the diameter of the bore-hole.
These pellets are then baked until they are thoroughly dry, when they
are ready for use. In making them up to the requisite diameter, a little
excess should be allowed for shrinkage, since it is essential that they
fit tightly into the hole. When the charge has been put in, and covered
with a wad of hay, or a handful of sand or rubbish, one of these pellets
is inserted and pushed home with a wooden rammer. Considerable pressure
should be applied to make the clay fill the hole completely, but blows
should be avoided. A second pellet is then pushed down in the same way,
and the operations are repeated until the whole of the hole is tamped.
To consolidate the whole, light blows may be applied to the outer
pellet. It will be found advantageous to place an undried pellet
immediately above the charge, because the plasticity of such a pellet
enables it to fill all the irregularities of the sides of the hole, and
to securely seal the passage between the sides and the tamping, along
which the gases might otherwise force their way. In coal blasting, soft
shale is always used for tamping, because it is ready at hand, and heavy
shots are not required.
Broken brick constitutes a fairly good tamping material, especially when
tempered with a little moisture; but as it is not readily procurable,
its application is necessarily limited. The dust and chippings of the
excavated rock are largely employed as tamping in quarries. This
material, however, has but little to recommend it for the purpose beyond
its readiness to hand.
It now remains to consider what means are available for remedying the
defect inherent in sand as a tamping material. This constitutes a very
important practical question, because if the defect can be removed, sand
will constitute by far the most suitable material whenever the bore-hole
has a downward direction. It can be everywhere obtained at a low cost;
it may be poured into the hole as readily as water; and its application
gives rise to no danger. Obviously the difficulty will be overcome if we
can find suitable means for preventing the gases from penetrating the
sand.
The end proposed may be successfully attained by means of the plastic
clay pellet applied in the following manner. Immediately above the
charge, place a handful of perfectly dry and very fine sand. This may be
obtained by sifting, if not otherwise procurable. Upon this sand, force
firmly down with a wooden rammer, so as to fill every irregularity, a
plastic clay pellet, about four inches in length, and of the same
diameter as the bore-hole, prepared by rolling between the hands in the
manner already described. Above this pellet, fill the hole with dry
sand. The impervious nature of the clay prevents the gases from reaching
the sand, except along the line of junction of the clay with the sides
of the hole. Tamped in this way, a resistance is obtained scarcely, if
at all, inferior to that opposed by the most carefully placed dried
clay.
By the employment of a detonator, the defect due to the porous character
of sand is not removed, but its influence is greatly diminished. When
detonation is produced in an explosive compound, the full force of the
elastic gases is developed instantaneously; and it has already been
shown that, under such conditions, the resistance occasioned by the
presence of any substance in the bore-hole, even the air alone, in the
case of nitro-glycerine, is sufficient to throw the chief portion of the
force upon the sides of the hole. Loose sand, therefore, may be
successfully employed as tamping under these conditions, since its
inertia will oppose a sufficient resistance to the escape of the gases.
But though the rock may be dislodged when light tampings are used with
detonation, there can be no doubt that a considerable proportion of the
force of the explosion is lost; and hence it will always be advantageous
to tamp securely by means of the clay pellet, as already described. The
highest degree of economy is to be obtained by detonating the charge,
and tamping in this manner.
CHAPTER IV.
THE OPERATIONS OF ROCK BLASTING.
_Hand Boring._--When the positions and the directions of the shot-holes
have been determined, the operations of blasting are begun by striking a
few blows with the hammer upon the spot from which the hole is to start,
for the purpose of preparing the surface to receive the drill. In some
cases, this preliminary operation will not be needed; but generally some
preparation is desirable, especially if the surface be smooth, and the
hole be to be bored at an angle with it. For the purpose of
illustration, we will take the case of a hole bored vertically
downwards, and will suppose the boring to be carried on by double-hand.
_Boring the Shot-holes._--The surface of the rock having been prepared
to receive the drill, one man sits down, and placing the shortest drill
between his knees, holds it vertically, with both hands. The other man,
who stands opposite, if possible, then strikes the drill upon the head
with the sledge, lightly at first, but more heavily when the tool has
fairly entered the rock. The man who holds the drill raises it a little
after each blow, and turns it partly round, the degree of turn usually
given being about one-eighth of a revolution. By this means, the hole is
kept circular, and the cutting edge of the drill is prevented from
falling twice in the same place. To keep the tool cool, and to convert
the dust and chippings into sludge, the hole is kept partially filled
with water, whenever it is inclined downwards. For this reason, downward
holes are sometimes described as “wet” holes, and upward holes as “dry”
holes. The presence of water greatly facilitates the work of boring. It
has been found by experience that the rate of boring in a dry and in a
wet hole varies as 1 : 1·5; that is, it takes one and a half times as
long to bore a dry hole as to bore a wet hole. Thus, by using water, the
time may be reduced by one-third. To prevent the water from spurting out
at each stroke and splashing the man who holds the drill, a kind of
leathern washer is placed upon the drill immediately above the hole, or
a band of straw is tied round it. When the hole has become too deep for
the short drill, the next length is substituted for it, which is in its
turn replaced by the third or longest drill as the depth becomes
greater. Each drill, on the completion of the length of hole for which
it is intended, is sent away to the smithy to be re-sharpened. In very
hard rock, the drills may have to be frequently changed, a circumstance
that renders it necessary to have several of the same length at hand.
The depth of shot-holes varies from 1 foot to 10 feet, according to the
nature of the rock, the character of the excavation, and the strength of
the explosive to be used. In shafts and in headings, the depth varies
generally between 2 feet 6 inches and 4 feet, a common depth being 3
feet.
The débris which accumulates at the bottom of the hole must be removed
from time to time to keep the rock exposed to the edge of the drill. The
removal of this sludge is effected by means of the tool called a
“scraper.” If the sludge is in too liquid a state to allow of its ready
removal by this means, a few handfuls of dust are thrown in to render
the mass more viscous. The importance of keeping the bore-hole clear of
sludge, and of shortening the time expended in using the scraper, has
led, in some localities, to the adoption of means for rendering the
sludge sufficiently viscous to adhere to the drill. When in this state,
the sludge accumulates around the tool rather than beneath it, the fresh
portion formed pushing the mass upward till it forms a thick coating
upon the drill throughout a length of several inches. When the tool is
withdrawn from the hole, this mass of débris is withdrawn with it; in
this way, the employment of a scraper is rendered unnecessary. This mode
of clearing the bore-hole is commonly adopted by the Hartz miners, who
use slaked lime for the purpose. This lime they reduce to the
consistency of thick paste by the addition of water, and they store it,
covered with water, in a small tin box, which they carry with them to
their work. To use this paste, they take a piece about the size of a
walnut, dilute it with water, and pour it into the bore-hole. This lime
paste is, for the purpose intended, very effective in friable rock,
especially if it be of a granular structure, as sandstone. As the grains
of sand resulting from the trituration of such rocks have no more
tendency to adhere to each other than to the drill, each of them becomes
covered with a coating of lime, which causes them to agglutinate into a
viscous mass possessing sufficient adhesiveness to enable it to cling to
the tool in the manner described.
When the hole has been bored to the required depth, it is prepared for
the reception of the charge. The sludge is all carefully scraped out to
clear the hole, and to render it as dry as possible. This is necessary
in all cases; but the subsequent operations will be determined by the
nature of the explosive, and the manner in which it is to be used. If
black powder be employed in a loose state, the hole must be dried. This
is done by passing a piece of rag, tow, or a wisp of hay, through the
eye of the scraper and forcing it slowly up and down the hole, to absorb
the moisture. If water is likely to flow into the hole from the top, a
little dam of clay is made round the hole to keep it back. When water
finds its way into the hole through crevices, claying by means of the
“bull” must be resorted to. In such cases, however, it is far more
economical of time and powder to employ the latter in waterproof
cartridges. Indeed, excepting a few cases that occur in quarrying,
gunpowder should always be applied in this way. For not only is a
notable saving of time effected by avoiding the operations of drying the
hole, but the weakening of the charge occasioned by a large proportion
of the grains being in contact with moist rock is prevented. But besides
these advantages, the cartridge offers security from accident, prevents
waste, and affords a convenient means of handling the explosive. It may
be inserted as easily into upward as into downward holes, and it allows
none of the powder to be lost against the sides of the hole, or by
spilling outside. These numerous and great advantages are leading to the
general adoption of the cartridge.
_Charging the Shot-holes._--When the hole is ready to receive the
explosive, the operations of charging are commenced. If the powder be
used loose, the required quantity is poured down the hole, care being
taken to prevent the grains from touching and sticking to the sides of
the hole. This precaution is important, since not only is the force of
the grains so lodged lost, but they might be the cause of a premature
explosion. As it is difficult to prevent contact with the sides when the
hole is vertical, and impossible when it is inclined, recourse is had to
a tin or a copper tube. This tube is rested upon the bottom of the
hole, and the powder is poured in at the upper end; when the tube is
raised, the powder is left at the bottom of the hole. In horizontal
holes, the powder is put in by means of a kind of spoon. In holes that
are inclined upwards, loose powder cannot be used. When the powder is
used in cartridges, the cartridge is inserted into the hole and pushed
to the bottom with a wooden rammer.
If the charge is to be fired by means of a squib, a pointed metal rod,
preferably of bronze, of small diameter, called a “pricker,” is placed
against the side of the bore-hole, with its lower pointed end in the
charge. The tamping is then put in, in small portions at a time, and
firmly pressed down with the tamping iron, the latter being so held that
the pricker lies in the groove. The nature of tamping has been already
fully described. When the tamping is completed, the pricker is
withdrawn, leaving a small circular passage through the tamping down to
the charge. Care must be taken in withdrawing the pricker not to loosen
the tamping, so as to close up this passage. A squib is then placed in
the hole thus left, and the charge is ready for firing.
If the charge is to be fired by means of safety fuse, a piece
sufficiently long to project a few inches from the hole is cut off and
placed in the hole in the same position as the pricker. When the powder
is in cartridges, the end of the fuse is inserted into the cartridge
before the latter is pushed into the bore-hole. The fuse is held in its
position during the operation of tamping by a lump of clay placed upon
the end which projects from the hole, this end being turned over upon
the rock. The tamping is effected in precisely the same manner as when
the pricker is used.
If the charge is to be fired by electricity, the fuse is inserted into
the charge, and the wires are treated in the same way as the safety
fuse. When the tamping is completed, the wires are connected for firing
in the manner described in a former chapter.
In all cases, before tamping a gunpowder charge placed loose in the
hole, a wad of tow, hay, turf, or paper is placed over the powder
previously to putting in the tamping. If the powder is in cartridges, a
pellet of plastic clay is gently forced down upon the charge. Heavy
blows of the tamping iron are to be avoided until five or six inches of
tamping have been put in.
When gun-cotton is the explosive agent employed, the wet material which
constitutes the charge is put into the shot-hole in cartridges, one
after another, until a sufficient quantity has been introduced. Each
cartridge must be rammed down tightly with a wooden rammer to rupture
the case and to make the cotton fill the hole completely. A length of
safety fuse is then cut off, and one end of it is inserted into a
detonator cap. This cap is fixed to the fuse by pressing the open end
into firm contact with the latter by means of a pair of nippers
constructed for the purpose. The cap, with the fuse attached, is then
placed into the central hole of a dry “primer,” which should be well
protected from moisture. When an electric fuse is used, the cap of the
fuse is inserted in the same way into the primer. The primer is put into
the shot-hole and pushed gently down upon the charge. As both the dry
gun-cotton and the detonator may be exploded by a blow, this operation
must be performed with caution.
Cotton-powder or tonite requires a somewhat different mode of handling.
It is made up in a highly compressed state into cartridges, having a
small central hole for the reception of the detonator cap. This cap,
with the safety fuse attached in the way described, or the cap of the
electric fuse, is inserted into the hole, and fixed there by tying up
the neck of the cartridge with a piece of copper wire placed round the
neck for that purpose. The cartridge is then pushed gently down the
shot-hole, or, if a heavier charge is required, a cartridge without a
detonator is first pushed down, and the “primed” cartridge put in upon
it. No ramming may be resorted to, as the substance is in the dry state.
When dynamite is the explosive agent used, a sufficient number of
cartridges is inserted into the shot-hole to make up the charge
required. Each cartridge should be rammed home with a moderate degree
of force to make it fill the hole completely. Provided a wooden rammer
be employed, there is no danger to be feared from explosion. A detonator
cap is fixed to the end of a piece of safety fuse, and, if water tamping
is to be used, grease, or white-lead, is applied to the junction of the
cap with the fuse. A “primer,” that is, a small cartridge designed to
explode the charge, is then opened at one end, and the detonator cap, or
the cap of the electric fuse, is pushed into the dynamite to a depth
equal to about two-thirds of its length, and the paper covering of the
primer is firmly tied to the cap with a string. If the cap be pushed too
far into the dynamite, the latter may be fired by the safety fuse, in
which case the substance is only burned, not detonated. With an electric
fuse this cannot occur. The same result ensues if the cap be not in
contact with the dynamite. The object of tying in the cap is to prevent
its being pulled out. The primer thus attached to the fuse is then
pushed gently down upon the charge in the shot-hole. It should be
constantly borne in mind that no ramming may take place after the
detonator is inserted.
Gun-cotton and tonite require a light tamping. This should consist of
plastic clay; or sand may be used in downward holes. The tamping should
be merely pushed in, blows being dangerous. A better effect is obtained
from dynamite when tamped in this way than when no tamping is used. In
downward holes, water is commonly employed as tamping for a dynamite
charge, especially in shaft sinking, when the holes usually tamp
themselves. But in other cases, it is a common practice to omit the
tamping altogether to save time.
_Firing the Charges._--When all the holes bored have been charged, or as
many of them as it is desirable to fire at one time, preparation is made
for firing them. The charge-men retire, taking with them the tools they
have used, and leaving only him of their number who is to fire the
shots, in the case of squibs or safety fuse being employed. When this
man has clearly ascertained that all are under shelter, he assures
himself that his own way of retreat is open. If, for example, he is at
the bottom of a shaft, he calls to those above, in order to learn
whether they be ready to raise him, and waits till he receives a reply.
When this reply has been given, he lights the matches of the squibs or
the ends of the safety fuse, and shouts to be hauled up; or if in any
other situation than a shaft, he retires to a place of safety. Here he
awaits the explosion, and carefully counts the reports as they occur.
After all the shots have exploded, a short time is allowed for the fumes
and the smoke to clear away, and then the workmen return to remove the
dislodged rock. If one of the shots has failed to explode, fifteen or
twenty minutes must be allowed to elapse before returning to the place.
Nine out of ten of the accidents that occur are due to these delayed
shots. Some defect in the fuse, or some injury done to it, may cause it
to smoulder for a long time, and the blaster, thinking the shot has
missed, approaches the fuse to see the effects produced by the shots
that have fired. The defective portion of the fuse having burned
through, the train again starts, and the explosion takes place, probably
with fatal consequences. Thus missed shots are not only a cause of long
delays, but are sources of great danger. Accidents may occur also from
premature explosion. In this case, the fuse is said to “run,” that is,
burn so rapidly that there is not sufficient time for retreat.
[Illustration: FIG. 50.]
When the firing is to take place by means of electricity, the man to
whom the duty is entrusted connects the wires of the fuses in the manner
described in a former chapter, and as shown in Fig. 50. He then connects
the two outer wires to the cables, and retires from the place. Premature
explosion is, in this case, impossible. When he has ascertained that all
are under shelter, he goes to the firing machine, and, having attached
the cables to the terminals, excites and sends off the electric current.
The shots explode simultaneously, so that only one report is heard. But
there is no danger to be feared from a misfire, since there can be no
smouldering in an electric fuse. The face may, therefore, be approached
immediately, so that no delay occurs, and there is no risk of accident.
Moreover, as all the holes can be fired at the moment when all is in
readiness, a considerable saving of time is effected. It is essential to
the success of a blast fired by this means that a sufficient charge of
electricity be generated to allow for a considerable loss by leakage. If
Siemens’ large dynamo-machine be used, the handle should be turned
slowly till a click is heard inside, and then, not before, the cable
wires should be attached to the terminals. To fire, the handle must be
turned as rapidly as possible, a jerky motion being avoided. As
considerable force is required, the machine must be firmly fixed. If a
frictional machine be used, care must be had to give a sufficient number
of turns. As this kind of machine varies greatly, according to the state
of the rubbing surfaces and the degree of moisture in the atmosphere, it
should always be tested for a spark before firing a blast. In this way
only, can the number of turns required be ascertained. It is important
that the discharging knob should be pushed in, or, as the case may be,
the handle turned backward, suddenly. A slow motion may be fatal to the
success of a blast. In testing Bornhardt’s machine, the handle should
always be turned forwards; but in firing, half the number of turns
should be given in one direction and half in the other. The following
table shows the number of turns required for a given number of André’s
fuses with Bornhardt’s machine. The first column, containing the least
number of turns, may be taken also for Julian Smith’s machine as
manufactured by the Silvertown Company with the modifications suggested
by W. B. Brain.
FIRING TABLE FOR FRICTIONAL MACHINE.
-----------------+----------------+----------------+----------------
| When the | When the | When the
| Machine sparks | Machine sparks | Machine sparks
| with 10 Turns. | with 12 Turns. | with 14 Turns.
-----------------+----------------+----------------+----------------
Fuses in Circuit.|Number of Turns.|Number of Turns.|Number of Turns.
-----------------+----------------+----------------+----------------
4 | 12 | 15 | 17
5 | 12 | 15 | 17
6 | 14 | 17 | 20
7 | 16 | 19 | 22
8 | 18 | 22 | 25
9 | 20 | 24 | 28
10 | 22 | 26 | 31
11 | 24 | 28 | 34
12 | 25 | 30 | 35
13 | 26 | 31 | 36
14 | 27 | 33 | 38
15 | 28 | 34 | 39
-----------------+----------------+----------------+----------------
NOTE.--If the machine does not spark with 14 turns, the rubber should
be taken out and brushed.
Places of refuge, called man-holes, are often provided in headings for
the blaster to retire into; these man-holes are small excavations made
in the sides of the heading. Sometimes it is necessary to erect a shield
of timbers in the heading for the protection of the men; such a shield
is frequently needed to protect machine drills from the effects of a
blast. In Belgium, it is a common practice to provide man-holes in the
sides of a shaft as places of retreat for the men; these holes are
called _caponnières_. Instead of caponnières, a hollow iron cylinder is
sometimes used as a protection to the men. This cylinder is suspended in
the shaft at a height of a few yards from the bottom, and is lowered as
the sinking progresses. The men climb into this cylinder to await the
explosion of the shots beneath them.
The workmen, on returning to the working face, remove the dislodged
rock, and break down every block that has been sufficiently loosened.
For this purpose, they use wedges and sledges, picks, and crowbars. And
not until every such block has been removed, do they resume the boring
for the second blast. Sometimes, to facilitate the removal of the rock
dislodged by the shots, iron plates are laid in front of the face in a
heading. The rock falling upon these plates is removed as quickly as
possible, to allow the boring for the succeeding blast to commence. It
is important, in the organization of work of this character, that one
gang of men be not kept waiting for the completion of the labour of
another.
MACHINE BORING.--In machine drilling, the operations necessarily differ
somewhat in their details from those of hand boring, and, in some cases,
other methods of procedure will be adopted more suitable to the
requirements of machine labour. It may even be, and in most cases indeed
is, inexpedient to follow closely the principles which lead to economy
of the explosive substance employed, since the more restricted
conditions under which machine power may be applied may point to more
important gains in other directions. Thus it may be found more conducive
to rapidity of execution to determine the position and the direction of
the shot-holes rather to satisfy the requirements of the machine than
those of the lines of least resistance; or, at least, these requirements
must be allowed to have a modifying influence in determining those
positions and directions. For it is obvious that holes cannot be angled
with the same ease when a machine drill is used, as they can when the
boring is executed by hand.
_Boring the Shot-holes._--It has already been remarked that the
exigencies of machine labour render it impracticable to follow closely
the principles which lead to economy of labour and material in blasting.
In hand boring, economy is gained by reducing to a minimum the number of
holes and the quantity of explosive substance required. But in machine
boring, economy is to be sought mainly in the reduction of the time
needed to accomplish the driving.
Attempts have been made to assimilate the methods of machine boring to
those adopted for hand labour, but the results have not been
satisfactory. On the contrary, the conditions determining the position
and the direction of the holes relatively to the production of the
greatest useful effect have been wholly ignored in favour of those which
determine the most rapid boring. This system has been attended with more
satisfactory results. Another system, partaking of both the preceding,
is widely adopted, and hitherto the best results have been obtained from
this, which may be regarded as a compromise between conflicting
conditions. Thus we have three systems of executing machine boring: one
in which a single machine is used upon a support capable of holding it
in any position, so as to be able to bore at any angle, and in which the
holes are placed according to the lines of least resistance, as in hand
boring. A second, in which several machines are fixed upon a heavy
support, allowing but little lateral or angular motion, and in which the
holes are placed at regular intervals apart, and bored parallel, or
nearly parallel, with the axis of the excavation, irrespective of the
varying nature of the rock, and the lines of least resistance. And a
third, in which it is sought, by the employment of one, two, or at the
most three machines, upon a simple and light support allowing the
position and direction of the machine to be readily changed, to satisfy
in some degree the two sets of conditions determining the two former
systems, by placing the shot-holes as far in accordance with the lines
of resistance as the exigencies of a fairly rapid handling of the
machine will allow.
In the first of these systems, the necessity for extreme lightness in
the machine is unfavourable to its efficient action, and the great
length of time consumed in changing the position of the machine, so as
to comply with the conditions of resistance in the rock, render it
impossible to attain a much higher rate of progress than is reached by a
well-regulated system of hand boring. With such a result, there is
nothing to compensate the first cost of the machinery, or in any way to
justify its adoption. In the second system, the time consumed in
removing and fixing the machines is reduced to a minimum, and the chief
portion of the time during which the machines are at the working face
is, consequently, occupied in actual boring, a circumstance that is
highly favourable to machine labour. Hence the rate of progress attained
by this system is greatly in excess of that accomplished by hand labour;
and this superiority has led to the adoption of the system in several
important cases, and has also led many to regard it as the most
favourable to the exigencies of machine drilling. But as the holes are
bored to suit the requirements of the machine, quite irrespectively of
the resistance of the rock, their positions and directions are very
unfavourable to the action of the explosive. This circumstance
necessitates a much greater number of holes to ensure the fracture of
the rock around each charge, and hence the time saved in shifting the
machines is in part lost in extra boring; besides which, the consumption
of powder is enormously increased. It would, therefore, appear that the
full advantages of machine boring are to be obtained from the
intermediate system, if carried out in accordance with all the
conditions of the case.
Assuming that we have a machine and a support of such dimensions,
weight, and construction as to be capable of being readily placed in
position, it is evident that we shall still require a much larger number
of holes than would be needed if the boring were performed by hand,
because they are not placed wholly in accordance with the lines of least
resistance. In some parts of the heading, indeed, these lines will have
to be wholly neglected, in order to avoid the loss of time involved in
shifting the supports; for the principle upon which an intermediate
system is based is to fulfil the requirements of the lines of least
resistance, when that can be conveniently done, and to neglect them,
when such fulfilment would involve a considerable expenditure of labour
and time.
In this way, the time both for fixing and removing the machines and of
boring is reduced to a minimum, and thus two conditions favourable to
rapid and economical progress is ensured. It is evident that when this
system is followed, the face will not require the same number of holes
at each blast. Another circumstance operating to increase the number of
shot-holes is the desirability of bringing down the face in fragments
small enough to be lifted without great difficulty. When the rock is
completely broken up, the labour, and, consequently, the time of
removing it after each blast, are lessened in an important degree. Hence
there will be an advantage in placing the shot-holes sufficiently close
together to ensure the fracture of the mass between each. These
circumstances render it necessary to bore a large number of holes when
the work is done by mechanical means. The boring of the additional holes
reduces the superiority of machine over hand labour, and the additional
quantity of the explosive required augments the cost of the work. To
counterbalance these disadvantages, the shot-holes should be bored deep.
It has already been pointed out that when a hole is once started with a
machine, the rate of progress is immensely superior to that attained in
hand boring, and to profit by this advantage, the hole should be
continued to as great a depth as practicable. This is sufficiently
obvious, since it amounts to increasing the proportion of the whole time
consumed that is occupied in actual boring; for as it is in the
rapidity of the operation of boring alone that the superiority of
machine labour exists, it is plain that the longer the proportion of the
time so occupied, the more marked that superiority will be. Thus, by
increasing the depth of the holes to the farthest practicable limit, we
approximate as much as possible to the condition most favourable to
machine boring. The intermediate system, therefore, which takes full
advantage of this means, will lead to the best results. To recapitulate
the main points of such a system; it should follow the lines of least
resistance when that can be conveniently done, and neglect them when the
fulfilment of their requirements would occasion a considerable
expenditure of time; and to counterbalance the disadvantages of machine
boring, it should employ shot-holes of as great a depth as is
practicable.
Supposing such a system in use, it now remains to consider the
operations of boring, and the subsequent operations of charging, firing,
and removing the rock dislodged by the blast. Of the method of executing
the boring, little remains to be said. It may, however, be well to
direct attention to the necessity of keeping the holes clear of the
débris. To ensure this, the bits should be chosen of a form suitable to
the nature and the structure of the rock, and the hole kept well
supplied with water. When the hole becomes deep, it should be cleared
out with a scraper during the time of changing the bit, and in very
argillaceous rock it may become necessary sometimes to withdraw the
tool, and to remove the accumulation with the scraper. When the débris
does not work out freely, its escape may be facilitated by giving a slow
motion to the tool, and then suddenly changing to a rapid motion. When
several machines are employed, the maximum number that can be applied
with advantage is one to the square yard of working face. The absolute
number of holes required in any case, will, of course, depend upon the
tenacity of the rock and the development of the joint planes, and also,
in some degree, by the lines of fracture due to the preceding blast. The
same circumstance will determine the distribution of the holes. Leaving
minor variations out of account, however, the same distribution will be
adhered to throughout the driving.
The manner of distributing the holes over the face of the heading may be
varied according to the judgment of the engineer in charge; that is, the
general features of the distribution to be adopted may be chosen to suit
the requirements of the machines and their supports. Also, it should be
noted that one method of distributing the shot-holes will require a less
number of them than another. Some examples will be found on Plate IX.,
where there are represented the Göschenen end of the St. Gothard tunnel;
the Airolo end of the same tunnel; the face of a stone drift driven at
Marihaye; that of a similar drift at Anzin; and that of a drift of the
same character at Ronchamp; the latter three examples being typical of
the distribution adopted in the French collieries.
The same mode of unkeying the face is adopted with machine as with hand
boring. Generally, two parallel rows of holes, from two to five in a
row, are bored in the middle of the face or fore-breast, the rows being
from 18 inches to 30 inches, according to the strength of the rock,
apart on the surface, and angled so as to be from 9 inches to 15 inches
apart at the bottom. These shots unkey the fore-breast; and it is
greatly conducive to a successful accomplishment of the operation, to
fire these shots simultaneously. Sometimes, when dynamite is used,
another method is adopted. A hole is bored horizontally in the centre;
at about three inches distant, are bored three other holes at an equal
distance apart. These latter are heavily charged with dynamite, the
centre hole being left empty. When these charges are fired, the rock
between them is crushed, and a large hole made. The lines of fracture of
the subsequent shots run into this hole. In this case, it is even more
desirable than in the preceding to fire the central shots
simultaneously.
In shaft sinking, if the strata are horizontal or nearly so, it is usual
to unkey from the centre, as in the heading. But if they be highly
inclined, it will be better to unkey from one side of the excavation.
The water which flows into the workings must be collected into one
place, both for convenience in raising it, and for the purpose of
keeping the surface of the rock clear for the sinkers. The depression
caused by the removal of the key serves to collect the water, and, on
that account, it is called “the sump.” Into this sump, the tub dips, or,
when pumps are used, the suction pipe drops. When the strata are highly
inclined, the water gravitates towards the dip side of the excavation,
and it becomes, therefore, necessary to place the sump in that
situation. The unkeying of the rock from this direction is, moreover,
favourable to the effect of the shots. In putting in the shot-holes, it
is well to avoid, as far as possible, terminating them in, or nearly in,
a bedding plane, because when so terminated, the force of the charge
expends itself along this plane. The position and the direction of the
holes will, however, be determined in some degree by the character of
the support used for the drills, and by other conditions of convenience.
_Charging and Firing._--The operations of charging the holes and firing
the shots demand particular attention when machine labour is employed.
It has been pointed out in the foregoing paragraphs that holes bored by
machine drills cannot be placed or directed strictly in accordance with
the requirements of the lines of least resistance; but that, on the
contrary, these requirements can only be approximately complied with,
and in some cases must be wholly neglected. To compensate in some degree
this defect of machine labour, the strength of the charges should be
varied according to the resistance which they will be required to
overcome. That is, the principles of blasting described in a former
chapter, which cannot be complied with by the borer, should be strictly
followed by the blaster in apportioning his charges. By this means, a
great saving of the explosive compound may be effected, and that without
difficulty or loss of time, if the blaster be intelligent and understand
his work. A glance will be sufficient to show what charge a given hole
of a known depth will require, and as cartridges of different sizes are
ready at hand, no delay is occasioned in making up the charge. The holes
in the centre, which are intended to unkey the face, require, of course,
the heaviest charge, since the conditions are there most unfavourable to
the effects of the explosion. And the more complete is the unkeying
resulting from this first explosion, and the more fractured and jointed
is the rock surrounding the cavity thus formed, the more may the charges
placed behind these unsupported faces be reduced.
As economy of time is, in machine boring, the chief end to be attained,
the tamping should be done with dried clay pellets previously prepared.
This material gives the greatest resistance, and thereby ensures the
maximum of useful effect; and if prepared beforehand, in the manner
described in the preceding chapter, the time consumed in tamping will be
reduced to a minimum. An abundant supply of such pellets should always
be ready at hand. In downward holes, such as are used in shaft sinking,
the plastic clay pellet and sand may be employed. This tamping may be
put in very rapidly, and, in all but very shallow holes, it is very
effective. When it is desired to use sand tamping in horizontal holes,
and holes bored in an ascending direction, the sand should be made up in
paper cartridges. The tamping employed in the St. Gothard tunnel
consisted of sand prepared in this manner. At the Mont Cenis tunnel, an
argillaceous earth was similarly prepared in paper cartridges for
tamping.
Firing the charges also affords an occasion for the exercise of
knowledge and judgment. A skilful determination of the order in which
the charges are to be fired will in a great measure compensate the ill
effects of badly-placed holes. The firing of a shot leaves the
surrounding rock more or less unsupported on certain sides; and it is
evident that to profit fully by the existence of these unsupported
faces, the succession of explosions must be regulated so that each shall
have the advantage of those formed by the preceding shots. This
condition can be wholly fulfilled only by simultaneous firing; but when
the firing is to take place successively, the condition may be
approximated to by regulating the succession according to the
indications observed on a careful inspection of the rock. Before firing
the charges, the blaster should consider the relative positions of the
holes, the stratification and jointing of the rock, the fissures caused
by the preceding blast, and any other circumstances that may influence
the results. The charges intended to unkey the face will be fired first,
and those in the concentric series will be then fired, in the order
determined upon, by means of different lengths of fuse. The series will
follow each other from the centre outwards. When a large number of shots
regularly placed in series have to be fired, a convenient practical
means of ensuring the successive explosion of the series, in the case of
the whole being lighted simultaneously, consists in bringing the fuses
from all the shot-holes together to one point at the centre. This method
of regulating the length of the fuses was adopted at the St. Gothard
tunnel.
It is obvious that the acceleration of the labour of excavation, which
has been effected in so remarkable a degree by the introduction of
machine drills and strong explosives, may be still further promoted by
the adoption of electricity as the firing agent. The advantages of
firing a number of shots simultaneously, some of which have already been
pointed out, are great and manifest. In the case of a driving, for
example, when all the holes have been bored and charged, and the
machines withdrawn, it is clearly desirable to blast down the face as
quickly and as effectively as possible. If the whole of the shots can
be fired at once, the time is reduced to a minimum, and, consequently,
the maximum of progress in a given time is ensured. Electricity affords,
indeed, the most convenient, the most effective, and the most safe means
of firing blasts. Hofrath Ritter von Pischof, the Austrian Chief
Inspector of Railways, in one of his reports, says:--“A greatly
increased amount of work and a notable saving of cost are effected when
the shots can be so disposed and fired as to mutually aid one another.
These results are obtained by employing electricity as the firing agent.
The experience which has been gained at the Büchenberg cutting, where
electrical firing has been extensively adopted, has shown that, when
properly employed, this means allows, in comparison with the ordinary
methods, twice the amount of work to be performed in a given time. It is
therefore highly desirable to adopt electrical blasting whenever it is a
question of economy of time and money.”
_Removing the dislodged Rock._--As the removal of the rock brought down
by the blast consumes a large proportion of the time saved by machine
boring, it becomes necessary to provide means for reducing this loss to
a minimum. The most important of these means is a suitable provision for
the rapid removal of the machine to a place of safety, and a
conveniently designed and well-laid tramway, upon which the rock may be
quickly run out without confusion and its consequent delay. The number
of wagons required to remove a given cube of rock may be readily
ascertained, and sufficient provision should be made for the transport
of these to “day” in the most rapid succession. The wagons should be of
such dimensions as to be capable of being handled without great
difficulty; the importance of this condition will be understood when the
frequency of derailments is borne in mind. The shovelling up of the
rubbish is greatly facilitated by laying iron plates in front of the
face to be brought down previously to the firing of the blast. This
expedient is often adopted in important drivings. It has also been
remarked that the dislodged rock can be more rapidly removed when it
exists in small blocks. Thus there will be an advantage in placing the
charges and in regulating their strength so as to completely break up
the rock. Another matter of importance in the arrangements for the rapid
removal of the rock brought down by the blast, is the proportioning of
the number of hands employed to the requirements of the case. This
number will increase with the size of the blocks to be lifted, the
distance to be run over, and the want of suitability in the _matériel_
employed.