to rapid and economical progress. The operations may be divided into
three series, namely: boring the shot-holes, charging and firing, and
removing the rock dislodged. Each of these series of operations may be
performed by different sets of men, and in several instances this
division of labour has been adopted. But it does not appear that such a
division leads to the most satisfactory results. The work of boring
occupies a much longer time than either of the other two series of
operations, and hence the distribution of the time is unequal. It has
been found that, generally, where all the arrangements have been well
considered, the labour of charging the shot-holes, firing the blast, and
removing the rock brought down, can be performed in about the same time
as that of boring. Thus it would seem to be more conducive to economy of
time to divide the men employed into only two sets: one set to bore the
holes, the other to perform all the subsequent operations. This division
has been adopted in numerous instances with favourable results.
Sometimes the whole of the operations have been performed by the same
set; but such an arrangement is not to be recommended. The labour of
directing the machines is of too distinct and skilled a character to be
confounded with that of removing the débris, without a strong reason for
such a proceeding, which does not appear to exist. Besides reserving a
set of men specially for this portion of the work, it is desirable to
keep the same men to the same machine, for in such a case each man gets
accustomed to the peculiarities of the machine entrusted to him, and
besides conceives a kind of affection for it that leads to careful
handling and watchful attention. In addition to the men required for the
operations referred to above, smiths will be needed to re-sharpen the
bits and to repair the machines. The amount of this labour will
obviously depend upon the number of machines employed, and the hardness
of the rock to be passed through.
EXAMPLES OF DRIVINGS.
_The St. Gothard Tunnel._--The St. Gothard tunnel is driven in five
sections. First, the “heading” is driven at the roof level 6 feet 6
inches wide, and 7 feet high. The position of the holes is shown in the
drawings on Plate IX. The number of holes at the Göschenen end is 28,
and the depth about 40 inches. The shots are fired by means of safety
fuse, the ends of the fuse being brought together at the centre. This
arrangement causes the shots to explode in the proper order of
succession. At a certain distance back from the face, is the “right
enlargement;” this is a widening of the heading to the limits of the
tunnel in that direction. Farther back is the “left enlargement,” by
which the heading is widened to the full width of the tunnel. Still
farther back is the first “bench cut,” in which one half of the floor is
blasted out to the full depth of the tunnel, and behind this again is
the second bench cut, in which the remaining half is removed. The
boring machines employed are the Dubois-François, the McKean, and the
Ferroux. The explosive agent used is dynamite. The rock is a tough
granite.
_The Hoosac Tunnel._--At the west end of the Hoosac tunnel, the system
adopted was the following. First, a centre cut was made by drilling two
rows of five or six holes each, about 9 feet apart on the face, and
converging to about 3 feet at their lower ends. The depth of these holes
was from 9 to 12 feet, according to the hardness of the rock. These
holes are numbered from 1 to 11 on Plate X. They were charged with
nitro-glycerine, and fired by electricity, Mowbray’s frictional machine
being used. As soon as the rock had been removed, the next series of
fourteen holes, numbered from 12 to 25, were drilled. These holes were
then charged and fired simultaneously like those of the first series.
When the rock dislodged had been removed, the third series of holes,
numbered from 26 to 41, were bored. This series, like the other two,
were charged, and fired by electricity. The effect of these three
blasts, which were fired within twenty-four hours, was to advance the
heading, 9 feet in height by the full width of 24 feet, to the extent of
7 feet 6 inches. The drawings on Plate XI. are: an elevation of the
fore-breast, which shows the positions of the shot-holes; a sectional
plan, which shows the directions of the first series of holes; a similar
plan, showing the directions of the second series of holes, and the
centre cut removed; and a sectional plan of the heading after the second
series have been fired, showing the direction of the third series of
holes.
The operations of taking out the “bench” were carried on at a distance
of about 170 yards back from the fore-breast. This was effected by first
drilling six holes 7 feet deep; two of these were each about 4 feet from
the face of the bench and close to the side of the tunnel, whilst two
others were each 4 feet behind these first holes, and the remaining two
holes were 8 feet from the face, 8 feet from the sides of the tunnel,
and 8 feet from each other. These were fired simultaneously, the result
being to lower the bench about 7 feet throughout the full width of the
tunnel. At a safe distance beyond this first bench cut, the same
operations were carried on by another gang of men, whereby the bench was
lowered to the floor of the tunnel, the full area of 24 feet in width by
22 in height being thus completed. The rock was a moderately tough
granite.
_The Musconetcong Tunnel._--The heading of the tunnel, shown on Plate
XII., like that of the Hoosac, was driven to the full width of the
tunnel. It is clear from theoretical considerations, and experience has
confirmed the conclusions, that the method of taking, with machine
drills, the whole width of the excavation at once conduces to rapidity
of advance, and to economy of explosive. In the example under
consideration, three tram lines were laid up to the face. The carriages
carrying the drills were run upon the two outside lines. These carriages
were simply stout frameworks of oak, each having in front three
horizontal iron bars, on which the drills were clamped in a way that
ensured easy lateral and vertical motion. After the firing of a blast,
all hands were set to shovel the dislodged rock into the middle between
the machine lines for the purpose of clearing the latter as soon as
possible to make way for the machines to be brought up for the next
boring. The lines being thus cleared, drilling was recommenced, and the
broken rock removed in wagons upon the centre line of rails. The heading
being 26 feet wide, there was ample room, and, a convenient system of
switching having been adopted, no delay was occasioned by a want of
wagons.
The system followed was that of centre cuts, and subsequent squaring up.
It consists in first blasting out an entering wedge or “key,” about 10
feet deep in this case, in the centre, and afterwards squaring up the
sides by several blasts. In the Musconetcong heading, twelve holes were
first drilled, as shown in the drawing, and marked C, A being the floor
of the heading. These holes were drilled with from 1½-inch to 2¾-inch
“bits,” in two rows of six, 9 feet apart on the face, and angled to meet
at the bottom. They were charged with 25 lb. of No. 1 and 50 lb. of No.
2 dynamite, and fired simultaneously by electricity. The No. 1 dynamite
was used in the bottom of these centre holes; in all the subsequent
blasts in squaring up, No. 2 only was used.
As soon as the cut was out, a second round of holes was started for the
first squaring up, as shown in the drawings, where they are numbered 1,
1, 1, 1, &c. In these and in the subsequent rounds, numbered 2, 2, 2, 2,
&c., and 3, 3, 3, 3, &c., the resistance to be overcome is, of course,
not so great as in the cut. In the first and the second squaring-up
rounds, from 50 lb. to 60 lb. of dynamite was used, and, in the third
round, this quantity was increased to 80 lb. or 90 lb., the resistance
becoming greater as the roof arch falls at the sides. In this third
round, there were generally one or two additional roof holes; these are
not shown in the drawing, as their position varied, according to the lay
of the rock. The top holes in the first round are also intended to bring
down any roof not shaken by the cut, and these are therefore angled
sharply towards the centre, and bored from 12 feet to 14 feet deep. In
the plan, Plate XII., the number 3 indicates the cut holes, and 4, 5,
and 6, the squaring-up rounds. The holes of the first squaring round
were always drilled about a foot deeper than the cut holes; when
blasted, these generally brought out an additional foot of shaken rock
at the apex of the cut. The following table shows approximately the
number and the depth of the holes required, and the quantity of dynamite
used for a linear advance of 10 feet.
---------------------+------+---------+------+------+------
| | |Total | |
|No. of| Depth |Depth | |
|Holes.|of Holes.| of |No. 1.|No. 2.
| | |Holes.| |
+------+---------+------+------+------
| | ft. in. | ft. | lb. | lb.
Cut | 12 | 10 6 | 126 | 25 | 50
1st square up | 8 | 12 0 | 96 | .. | 55
2nd „ | 8 | 12 0 | 96 | .. | 55
3rd „ | 6 | 12 0 | 72 | .. | 85
Additional roof holes| 2 | {10 0} | 18 | .. | ..
| | { 8 0} | | |
+------+---------+------+------+------
| 36 | .. | 408 | 25 | 245
---------------------+------+---------+------+------+------
The cut holes being 10 feet 6 inches deep, the blast usually brought out
about 9 feet full, which, as explained above, was increased to 10 feet
in the subsequent rounds. The cross section being about 175 square feet,
in an advance of 10 linear feet, there are about 65 cubic yards of rock
to be broken; this gives on an average 0·4 lb. of No. 1 and 4 lb. of No.
2 dynamite, and a little over 6 feet of drilling per cubic yard.
The “bench” was kept from 150 yards to 200 yards back from the face of
the heading, to avoid interruptions from the heading blasts, and to
allow plenty of room for handling the wagons, and for running back the
machines to a safe distance, previously to firing. The system adopted in
removing the bench is shown on Plate XII. First, six top holes, from 12
feet to 13 feet deep, were drilled and blasted; their relative positions
are shown in the drawings, A being the centre line, B, the sides in the
enlargement, B′, the sides of the heading, C, the face of the bench, and
1, 2, 3, 4, 5, 6, the holes. These six holes lifted the greater portion
of the rock; what was left was broken by several horizontal holes. These
two sets of holes, at the top and at the bottom, gave an average advance
of about 9 feet. The following table shows, for that advance, the number
of feet drilled, and the quantity of dynamite burned.
------------+------+------+---------+---------
| |Depth | Total |
|No. of| of | Depth | No. 2
|Holes.|Holes.|of Holes.|Dynamite.
+------+------+---------+---------
| | ft. | ft. | lb.
Top holes | 6 | 12 | 72 | 62
Bottom holes| 4 | 10 | 40 | 45
+------+------+---------+---------
Totals | 10 | 22 | 112 | 107
------------+------+------+---------+---------
The sectional area of the bench being about 306 square feet, an advance
of 9 linear feet gives about 102 cubic yards of rock to be removed. The
quantity of dynamite used was therefore 1·05 lb., and the depth of
boring 1·1 foot, per cubic yard of rock broken.
Three machines were used at this bench, two on the top and one below.
The holes were commenced with 2¾-inch bits, and terminated by 1½-inch
bits. The rock was a tough syenite.
CHAPTER VI.
SUBAQUEOUS BLASTING.
_Preparation of the Charge._--It is essential to the success of
subaqueous blasting operations, that the explosive substance used should
be suitable to the conditions under which it is to be applied. This is
true of all blasting, but the requirement is frequently overlooked in
some of the operations that have to be performed under water. In
clearing a wreck for salvage purposes, gunpowder will in most cases act
more effectually than either gun-cotton or dynamite. Also, in many
cases, this compound will prove more suitable than the stronger
substances in removing obstructions in water-courses. Examples of this
will be given hereafter. But when a wreck has to be broken up, when
piles, or objects of a similar character, have to be removed, or when
rocks have to be blasted, the more violent compounds will be found to
accomplish the purpose much more effectively. Generally, it may be
stated that when it is required merely to _remove_ objects, gunpowder is
the most suitable explosive agent to employ; and that when it is
required to _break_ objects, the nitro-cotton and the nitro-glycerine
compounds are the agents whose application is likely to be attended
with the greatest degree of success.
When gunpowder is used, means must be adopted to protect it from the
water, since a small proportion of moisture is sufficient to lessen, in
a very important degree, the force developed, while a large proportion
of moisture will destroy altogether its explosive properties. It is no
easy matter, under the most favourable circumstances, to keep the water
from the charge; but when the depth of water is considerable, it becomes
very difficult to attain that object. The pressure of a considerable
“head” will force the water through substances that, without a pressure,
are sufficiently impervious. At ordinary depths, metal canisters are
usually employed to contain gunpowder. Old oil-cans are as good as
anything for this purpose. The fuse, whether safety or electric, is
passed through the cork, and the latter is luted with some waterproofing
composition. The best consists of:
Tallow 1 part.
Rosin 3 parts.
Guttapercha 4 parts.
Swedish pitch 12 parts.
Instead of metal canisters, indiarubber bags are sometimes used. These
are, however, more expensive than the oil-cans, and, in many cases, they
are scarcely more efficient or suitable. Small charges of gunpowder may
be put into short lengths of indiarubber tubing, so as to form a kind
of cartridge. But care must be taken to close the ends securely. The
best way is to insert a cork, or if that cannot be obtained, a
cylindrical piece of wood, and to tie the tubing to this very tightly
with twine. The ends should then be dipped into the luting composition
described above. Tubing suitable for this purpose is sold under the
designation of “blasting tubes.” For large blasts, wooden casks are the
most suitable receptacle for the charge. The casks should be well
tarred, or, if the depth of water be great, laid over with pitch applied
very hot. Great care must be taken to protect the aperture through which
the safety fuse, or the wire of the electric fuse, passes.
In blasting under water with gunpowder, only the best and strongest
qualities of that compound should be used. The extra strong mining
powder of the Messrs. Curtis’s and Harvey’s, commercially known as the
E.S.M. powder, is, of all, the most suitable. It is also highly
conducive to success to detonate the charge. If the charge be not
detonated, the enclosing vessel is ruptured when only a small proportion
of the number of grains have been ignited, and, consequently, a large
proportion of the charge is blown away into the water unburned. Were
gunpowder in blasting charges always fired by a detonation, it would
compare in its effects far more favourably with the nitro-cotton and the
nitro-glycerine compounds than it does under the circumstances
attending the common method of firing it.
When gun-cotton is used, the difficulty of waterproofing is much
lessened, but not wholly removed. Inasmuch as this compound may be
detonated in the wet state, it is not required to take those precautions
which are necessary in the case of gunpowder. But, as we have pointed
out in a former chapter, the detonation of wet gun-cotton is effected by
means of that of a small quantity of the dry substance. This quantity,
which is generally employed in the form of a cylinder, and is called the
“priming,” must be thoroughly protected from the water. For this
purpose, indiarubber tubing may be used, or, if the primer be large,
indiarubber bags. When the pressure of the water is not great, a very
efficient protective covering is obtained by dipping the primer into
melted paraffine. Care should be taken to avoid raising the temperature
of the paraffine above the degree required to melt it completely. The
primer should be placed in contact with the charge, and it is desirable
that the latter, when it can be conveniently made to do so, should
surround the former.
Charges of gun-cotton for subaqueous blasts are usually made up of discs
of a large diameter, or of slabs of a rectangular form. When, however,
the charge has to be put into a bore-hole in rock, the common cartridge
is employed.
Tonite, or cotton powder, is largely used in subaqueous blasting
operations. This substance is always applied in a dry state, and
requires, therefore, to be protected from the water. This protection it
is however, not difficult to give. Being prepared for use in a very
highly compressed state, it does not readily absorb moisture. In this
state, it is enclosed in cartridges, which are subsequently dipped into
melted paraffine. This is the form and preparation adopted for ordinary
use. For application under water, especially when the depth is
considerable, additional protection is given. For wreckage purposes,
tonite may be obtained in convenient charges, made up in suitable forms,
and sufficiently protected.
When dynamite is used, the conditions are similar to those prevailing in
the case of gun-cotton. Since nitro-glycerine is unaffected by water, no
necessity exists for protecting it from moisture. But when a charge of
dynamite is immersed in water, and not contained in a bore-hole, the
nitro-glycerine rapidly exudes. The writer once made several ineffectual
attempts to explode a charge of dynamite at a depth of 70 fathoms
beneath the surface. The cause of failure was found to be this
exudation; for subsequent experiments showed that, though the dynamite
was in the form of the ordinary parchment paper cartridges, and was
contained in a stout canvas bag, the kieselguhr retained hardly a trace
of nitro-glycerine when the charge reached the surface from that depth,
after being rapidly lowered and raised. Hence it becomes necessary to
enclose dynamite within some fairly impervious substance, to prevent the
exudation of the nitro-glycerine. Waxed linen, or fine canvas overlaid
with the composition already described, may be used as a protective
covering; for blasts in deep water, indiarubber bags and tubing are
employed. When the charge is contained in a bore-hole in rock, exudation
can hardly occur, and therefore in such cases waterproofing is
unnecessary.
For firing subaqueous blasts with safety fuse, only the guttapercha
covered kinds are suitable. Great care must be taken to render the
junction of the fuse and the detonator water-tight. A stronger detonator
is required under water than in dry ground. Electric fuses offer not
only a cheaper, but a far more certain and suitable means of firing in
water. This means is now very generally employed. When tension currents
are used, the insulation must be very good. In all cases, ample power
should be possessed by the firing machine or battery.
The shattering class of explosives are very suitable for subaqueous-rock
blasting. In many cases, their employment renders the boring of
shot-holes unnecessary, an advantage of obviously great importance. When
detached or projecting masses of rock have to be broken up, it is
sufficient to place the charges upon them. Of course, when so applied,
larger quantities of the explosive are required; but though the method
is wasteful of explosive, it is very economical of labour and time.
Even when large undetached masses of rock have to be removed, the same
method may often be successfully followed. Suppose a level surface of
rock, for example. A few heavy charges judiciously distributed over this
surface will blow out craters of a considerable radius, and more or less
fracture the rock in their immediate neighbourhood. A few other blasts
then fired between these shattered points will break up the intervening
solid portions. Sometimes the rock will be disintegrated to a
considerable depth, and so broken up generally that it may be removed by
dredging. By proceeding in this way, the whole of the rock may often be
removed without any labour of boring.
But when the rock is too tough to be removed in this way, recourse must
be had to boring, though even when boring is necessary, an occasional
“loose” shot may be found to be very efficacious.
_Boring under Water._--The percussive drills, one of which, the
Darlington, was described in a former chapter, may be used effectively
under water. Compressed air is used as the motor fluid. The tripod
stand, having its legs weighted to give it stability, is generally the
most suitable support. These drills need the immediate attention of a
diver. Sometimes the boring is carried on by hand from the deck of a
vessel or from a raft provided for the purpose. The following
description will give a general notion of the operations involved in
subaqueous boring:--
The working vessel having been moored over the rock by means of
mooring-lines attached to buoys placed about 50 yards from each quarter
of the vessel, the diver descends and selects the most suitable position
for the blast; he then signals, by a certain number of pulls upon his
signal line, to have the drill and stand lowered to him. This being
quickly done by means of a steam derrick, he guides the drill-stand to
its place, and finally fixes it in position by means of its adjustable
legs. This being done, he signals for air to commence drilling.
It has been found that the drill can be worked in a rapid current as
well as in slack water. This allows the operations of drilling and
blasting, by a proper division of time and labour, to be conducted in an
extremely rapid tidal current, so that the principal work of the diver,
in inserting charges for blasting and slinging stone, may be done near
the periods of slack water, while the drilling may be advantageously
continued during the period of rapid flow. In a rapid current, the
stoppage of the drill for the purpose of “spooning out” the hole becomes
unnecessary, as the motion of the drill works up the débris to the mouth
of the hole, whence it is sucked out and carried off by the current in a
dark stream, like the smoke from the funnel of a locomotive. In a
sluggish current, or during slack water, the hose of the air-pump is
sometimes introduced, and air forced into the bore-hole to create a
current of water, by which means the hole is cleared more thoroughly
than by the most careful “spooning out.”
As soon as the hole is drilled to the required depth, the drill is
stopped; the diver then fastens the derrick chain, which is lowered to
him for the purpose, to the drill-stand, and signals to hoist away,
whereupon the machine is quickly hoisted on deck.
After having examined the hole and cleared away any débris remaining at
the bottom, the diver comes to the surface, and taking in his hand the
charge contained in a water-tight cartridge, and provided with its
electric fuse to which a sufficient length of insulated wire is
attached, returns with it, and inserts it into the drill hole, carefully
pressing it to the bottom with a rod. The tamping, if any is used, is
then inserted above the cartridge, and the diver comes up.
The working vessel having been quickly hauled by the mooring-lines to a
safe distance by means of capstans worked, whenever practicable, by the
steam-engine, the wires are attached to the machine, and at the signal
“all ready” the charge is fired.
The working vessel is then hauled back to her position, and as soon as
the water becomes sufficiently cleared of the dark muddy matter stirred
up by the blast, to enable the diver to see in it, he descends and
examines the result.
If the blast has been effective, he signals for the stone chains to be
lowered to him; which being done, he proceeds to sling the large pieces
of broken rock, one after another, as they are hoisted up and deposited
on deck. All the pieces large enough to sling having been thus removed,
he signals for the tub and shovel, and upon their being lowered to him,
proceeds to shovel into the tub the small fragments, and to have them
hoisted up and piled on deck, until the surface of the rock is
sufficiently cleared to place the drill for a new blast.
_Submarine Rocks._--The following brief account of the removal of the
“Tower” and the “Corwin” Rocks from the Narrows, at the entrance of
Boston Harbour, U.S., from the pen of J. G. Foster, is instructive as
illustrating the method of procedure in submarine blasting, and as
showing the unfitness, for work of that character, of the slow-burning
explosives:--
“Tower Rock,” being the smaller of the two, was selected as the one to
be first removed. Its horizontal dimensions being only 50 by 26 feet, it
was estimated that one large central charge surrounded by five or six
others, all in large and deep drill-holes, would be able to rend the
rock into pieces.
The working vessel, the sloop “Hamilton,” of 70 tons, was moored over
this rock on the 30th of July, 1867, and the new submarine drilling
machine, designed for this work, by Mr. Townsend, the contractor, was
placed in position and tried.
Several imperfections were found at the first trial, which prevented its
efficient working. While these were being remedied, a trial was made of
surface blasts, placed in and around the rock in the positions most
favourable to their action. These proved to be entirely without effect.
No seams or breaks were made by them in the smooth surface of the rock.
As soon as the submarine drilling machine was perfected, it was put in
operation, and successfully worked. The central and the surrounding
holes were drilled to depths varying from 2 to 8 feet, each hole being
3½ inches in diameter. These were well charged with black blasting
powder, and tamped with sand. In some holes, the charges produced no
visible effect, the tamping being blown out like the charge from a
cannon. In others, a crater was formed, but with a radius only about
one-half the line of least resistance. The holes that were intact were
then deepened, and new ones drilled; these were charged with Dupont’s
sporting powder. The result was much better, but not what was desired.
The pressure of the water, from 23 to 33 feet in depth, seemed to
diminish largely the ordinary explosive effect of gunpowder upon rock,
as seen in blasts in the open air.
Trial was then made of the patent safety blasting powder, manufactured
by the Oriental Company of Boston, the proportions of the ingredients
having been modified to increase its strength for this especial use.
This produced the desired effect. The rock was rent in pieces; and by
drilling additional holes and continuing large charges of the powder,
the rock was finally reduced to the required depth.
To smooth off its upper surface and break down the sharp projecting
points, large surface charges of sporting powder were employed. These
accomplished the result to a limited extent, but not completely. A large
15-inch shell was then placed in a crevice near the centre of the rock
and fired. Its explosion swept the rock completely, breaking down and
levelling the projecting points.
The work upon this rock occupied eight weeks. In that time, 80 tons of
stone had been blasted out, hoisted up, and deposited on shore,
attaining the required depth of 23 feet at mean low water. About 70 tons
of small fragments were suffered to remain on the bottom around the
rock, where they had been thrown by the blasts, and where they could do
no harm.
The cost per ton of the quantity hoisted up and deposited on shore was
64·93 dollars, no account being taken of the quantity blown, in small
fragments, into deep water.
“Tower Rock” having been entirely removed to the required depth, the
moorings of the working vessel were at once removed to “Corwin Rock,”
and work commenced upon it on the 1st of October, 1867. This rock was
found to be much more difficult to blast, on account of its extremely
tortuous lamination, its great toughness, and the presence of a great
number of iron pyrites.
Surface blasts were also tried upon this rock at the outset, in hopes
that, by being placed in the most favourable positions between the sharp
ridges of the rock, they might break them down. These, however, like
those upon Tower Rock, entirely failed to produce any noticeable effect,
even when they contained four and five hundred pounds of the best
sporting powder. The drilling machine was therefore called into
requisition as before, and used continuously till the completion of the
work.
On account of the extent of this rock, a different plan of operations
for its removal was adopted. One side of the rock most favourable for
blasting was selected, and a row of holes drilled parallel to the edge,
and at a distance from it equal to the depth of the holes, which was
taken to extend 1 foot below the required level, 23 feet at mean low
tide. After blasting out these holes, a new line of holes was drilled
parallel to the former line, or to the “face” left by the blasts, and
these also were blasted out; then a third line, and so on, progressing
regularly across the rock, continually blasting it off in parallel
blocks, extending downward a little below the depth required.
The advantages of this mode of operation were that it enabled the blasts
to act laterally, in which direction they were the most powerful; and
the rock was left, after each series of blasts, with a nearly vertical
side, or “face,” in which the presence of seams could be more readily
detected, and the character of the strata observed, so that the most
favourable positions could be selected for the next blasts.
Sometimes the craters, following the strata, ran under, or left an
overhanging “face,” in which case a large charge placed under its
projecting edge, usually had the effect of throwing off the overhanging
portion, and sometimes of dislodging large masses.
After the rock had been in this way blasted entirely across, and to the
general depth required, a careful survey was made, the soundings being
taken in lines from 5 to 10 feet apart, and at right angles to each
other, the lower end of the sounding pole being placed by the diver
alternately upon the highest and the lowest points.
This survey showed that although more than the required depth had been
generally attained, yet many points projected above this level by
distances varying from 2 to 14 inches.
To remove these, large surface charges were again tried, but with the
same ineffective result. Their only effect was to pile up the sand and
small fragments of stone into irregular windrows on the surface of the
rock. Small holes had, therefore, to be drilled at each of these points
to blast them off. This occupied much more time than could reasonably
have been expected; so that it was not until two months’ labour had been
expended, that all the points were finally reduced to the required
level.
_Obstructions in Water-courses._--The removal of obstructions from
water-courses often leads to much subaqueous blasting. Trees that have
fallen into the stream are most effectively broken up by charges of
gunpowder fired by a detonation. The success of the operation will,
however, be greatly dependent upon the judicious placing of the charges.
Brickwork may also be very effectively dealt with by charges of
gunpowder. But stone masonry and blocks of rock may be more effectively
broken up by gun-cotton, tonite, or dynamite. For work of this
character, electrical firing offers great advantages, for, besides its
convenience, it allows of several charges being exploded simultaneously,
a condition that is always favourable, and in many cases essential, to
success.
The following highly interesting and instructive account of the removal
by blasting of some obstructions in certain rivers in India is given by
Lieut. A. O. Green, R.E.
[Illustration: FIG. 51.]
[Illustration: FIG. 52.]
He, in company with some assistants, left Calcutta for Maldah on the 8th
of April, 1874, where they commenced work on the following day upon the
wreck of a large county boat, which lay on the top of a tree in
mid-stream, as shown in Fig. 51. Soundings were taken over and around
this tree, which was found to be about 3 feet 6 inches in diameter at
its base. The gunpowder intended to be used in these operations not
having arrived, three 5-lb. charges of gun-cotton were made up; it was
thought that under the 15-feet head of water, these would have been
sufficient to break the tree in half. The gun-cotton was in the form of
compressed discs, 2½ inches in diameter, and 2 inches thick, each disc
weighing about 5 ounces. These discs were filled into a tin cylinder to
within about 4 inches of the top. An electric fuse, with wires attached,
having been securely pushed into the hole in the centre of the top disc,
the empty space above was filled up, with first a layer of sawdust, and
then a layer of plastic clay, well rammed. The whole was then painted
over, and the upper end tied up in a covering of waxed cloth, the holes
through which the fuse wires passed being carefully luted. The charge,
thus made up, is shown in section in Fig. 52. It was fired by means of
a dynamo-electric machine.
The first charge produced but little effect; a second failed from the
case not being water-tight; a third charge was more effective, as it
lifted the tree and the boat partially out of the water. The positions
of these gun-cotton charges are indicated by circles on the figure. The
next day, two charges of gunpowder, of about 70 lb. each, were placed
under the boat, these charges being lashed on to the snag by the divers.
These charges consisted simply of common oil-tins, carefully cleaned and
painted over with red-lead paint. The bunghole was closed by a wooden
plug, bored through to allow the fuse wires to pass. This plug, after
being inserted, was coated over with a waterproofing compound. The
effect of the two charges was to completely demolish the boat. Another
charge of 50 lb. removed the tree underneath. The positions of these
gunpowder charges are indicated by squares in Fig. 51.
The next obstruction met with was a sand bank caused by a boat which had
broken in half and then sunk. The sand nearly covered the boat, so that
there was little else to operate upon. A charge of 80 lb. (one large
charge being considered preferable to two or three small ones in getting
rid of the sand), placed close to the part of the boat that was visible,
made a considerable crater, and a second charge of 80 lb. was placed in
a much more favourable position, as nearly all the boat was removed
except portions of the bow and stern, which required two separate
charges of 50 lb. each before they disappeared. In half an hour, the
whole of the sand bank had been washed away by the stream, and there was
from 3 to 4 feet of water over the spot where before the sand was high
and dry out of the water. The removal of this obstruction was dangerous,
owing to the nearness of the boat to the surface, the consequent small
resistance offered to the projection of its pieces through the air, and
the largeness of the charges used. Had, however, small charges been
used, it is more than probable that the small craters made by them would
have become too quickly filled up again to have been of any good in
facilitating the placing in position of subsequent ones.
The following day, a large mango tree, about 4 feet 6 inches in
diameter, was destroyed by two 50-lb. charges, which broke it up into
three pieces, easily removable ashore.
A few days later, a large trunk of a tree, about 3 feet in diameter, was
removed with two 50-lb. charges; but the depth of water over it was so
small that a large portion of the trunk was thrown a considerable
distance on shore. The next day a large tree which had formed a sand
bank was very successfully removed by a charge of 50 lb. placed among
the roots, it being considered that a smaller charge than 50 lb. would
not have effected the purpose. Opposite to Kásimpore, a boat was removed
with a charge of 50 lb. placed in the centre up-stream, which entirely
demolished it, the pieces being all dragged ashore. At Mootyá, a large
cotton tree, the wood of which is extremely tough, was found with many
large branches projecting out of the water. A charge of 70 lb. tied
under the tree at the springing of the branches effectually broke it up,
and the pieces were all hauled to land. Three miles farther down the
river, an attempt was made to destroy another large cotton tree with a
similar charge, but it only broke it into three pieces, and two more
charges of 50 lb. each were necessary to clear it away effectually. This
tree was, if anything, slightly larger than the last, i. e. from 3 to 4
feet in diameter, and there was less water over it.
Farther on, the party came across a collection of three or four trees,
with their branches interlaced, lying on a sand bank near Alumpore
Dáldah; these were sufficiently broken up by a 70-lb. charge to make
them easy of removal by coolie labour. Opposite to the village, another
awkward snag, in the shape of a large tree sticking up in 30 feet of
water, was destroyed by tying a 70-lb. charge at its base. A charge of
50 lb. of powder under this head of water, or even a smaller amount,
might have been sufficient, but as the work had to be done quickly, not
much account was taken of a few pounds of powder more or less, provided
the object was attained. At Gomashtapore, a large tree, branches and
all, was found in 25 feet of water, lying in the channel under the bank.
The current here was considerable, and some difficulty was experienced
in placing the charges. One charge of 70 lb. broke the tree in half;
another of 50 lb. at the springing of the branches broke them up; and
another of the same weight got rid of the roots. Below Gomashtapore, a
large mango tree was demolished with 60 lb. of powder. A short distance
farther on, a bad bamboo snag was met with. These bamboo snags, which
were merely the roots of the bamboos with perhaps a dozen or so whole
ones left, gave much trouble. Fig. 53 gives an idea of what these snags
are like. It was found impossible to place a charge underneath this one,
so an opening was prized between the bamboos, and a charge of 70 lb.
rammed down pretty well into the middle. This cleared the whole of it
away and opened the channel.
[Illustration: FIG. 53.]
At Chandpore, at a re-entering angle of the river and in a place
peculiarly dangerous to navigation during the rains, was an enormous
banyan tree (_Ficus Indica_), the main trunk of which, to judge from the
branches, must have been at least from 12 to 15 feet in diameter. An
approximate measurement was made with a pole, but any such measurement
can only have been a very rough one.
[Illustration: FIG. 54.]
[Illustration: FIG. 55.]
The trunk was lying in deep water, but the branches, more like an
accumulation of large trees, were lying stretched out for a considerable
distance over the bank, covering an area of more than 80 square feet. A
charge of 200 lb. of powder was made up in an indiarubber bag, and
placed by the divers in about 28 feet of water, well under the trunk of
the tree. The effect of this was to split the trunk up into several
pieces, each of which subsequently required separate removal. A 70-lb.
charge was next fired under two of the largest pieces in 18 feet of
water, and this broke them up completely. Having now run out of all the
cases for powder, three charges of gun-cotton, similar to the first,
were made up, and fired separately, each placed under a good thick
branch, about 8 feet in girth. The effect of all three was prodigious;
seemingly greater than that of the 70 lb. of gunpowder. As there were no
more cases left, and time was precious, some common earthenware ghurrahs
were obtained from the village as a makeshift. These held about 20 lb.
of powder; the fuse was placed in the centre in a disc of gun-cotton,
and the neck was closed up with damp earth, white-lead paint, &c., just
in the same way as the gun-cotton charges had been. A rope for lashing
them to the obstacle was securely fastened round the neck, and the fuse
wires were tied under, this lashing, leaving a small loop towards the
fuse free, so as to avoid any chance of a strain being brought on the
fuse in lowering the charge. Figs. 54 and 55 show the arrangement of
this charge. The first one tried had but little effect when placed
under a branch of the tree in deep water, and it was accordingly
determined to wait for cases from Calcutta; but after waiting five days
without their appearing, three more of these charges were tried, and
this time with very excellent results. They were indeed so satisfactory,
that the same evening four more were made up and fired. The first under
a mango tree a little farther down the river. This broke it in half,
throwing one part high and dry on shore, and the other into deep water.
The other three were fired under the remaining branches of the banyan
tree with very good effect, cutting them away.
It is more than probable, observes Lieut. Green, that the good results
obtained with all these ghurrah charges were entirely due to the
gun-cotton disc inside causing the gunpowder itself to detonate, so that
the thinness of the envelope was of little moment in determining the
force of the explosion.
The tin cases having arrived, the rest of the powder was made up into
five charges of 48 lb. and three ghurrah charges of 20 lb each. About
four miles farther down the river, there was an old peepul tree lying in
mid-channel, with several of the branches above water. Two tins, one
placed under the springing of the branches and the other under the
roots, blew away the lower branches on which the tree was resting, and
it sank slightly in the water. A ghurrah was next fired under the trunk
with splendid results, the tree disappearing entirely except one branch,
which required another small charge to remove it. The trunk of this tree
was nearly 8 feet in diameter, but of soft stringy wood.
On returning to camp, a small charge of 2 lb. of gun-cotton was made up
in a section of bamboo, and used against the banyan tree with very good
effect, and a ghurrah charge demolished the last branch but one. The
next day 1½ lb. of gun-cotton in a piece of bamboo finished the last of
this enormous tree.
After clearing away several more trees, the foundations of an old
factory, which had slipped into the stream, were removed by introducing
two charges of 1½ lb. of gun-cotton, in the ends of two bamboos, well
into the crevices of the masonry under the water.
Another obstruction consisted of a row of old piles, about 15 inches
square, stretching across the river below the surface of the water. Six
of the most dangerous of these were removed from the dry season channel
with ghurrah charges tied to the foot of the piles.
An old well that had fallen bodily into the water was afterwards met
with. The position of this well is shown in Fig. 56. A charge of 4 lb.
of gun-cotton completely destroyed it.
Near Azimgunge, the trunk of a very large peepul tree was found sunk in
deep water. It was so large that it was thought necessary to place a
100-lb. charge underneath it; this charge broke it up completely, but
two small charges of 20 lb. each were subsequently required to remove
the pieces.
[Illustration: FIG. 56.]
Later on, a well, similar to the one previously destroyed, was met with.
The brickwork was remarkably good and about 3 feet thick, and the mortar
was excellent. One charge of 4 lb. of gun-cotton broke it up into large
pieces; but it took another similar charge, and two charges of 20 lb. of
gunpowder to destroy it completely. On the same day, two trees were
removed with ghurrah charges, which had been used throughout, for small
charges, with unvarying success.
At a place called Farrashdangah, there was a very bad obstruction in the
river, caused by the remains of an old bathing ghat and bridge having
been cut out from the bank by the water getting underneath the masonry.
Both were projecting about 3 feet above the water, and in the rainy
season they formed the centre of a very nasty and dangerous whirlpool,
in which many boats had, according to the Executive Engineer of the
Nuddea Rivers Division, been lost. There was an immense mass of masonry,
but no means of getting a charge placed underneath it; so a charge of
100 lb. of powder was placed close alongside it in about 15 feet of
water. This shunted the mass bodily over and underneath the water. Two
50-lb. charges were next placed underneath the mass, and these shattered
it all up, except one piece, which was got rid of with a fourth charge
of 20 lb. placed well underneath it. The Executive Engineer wishing that
the wing-wall of the bridge, which was on dry land during the dry
season, might be removed as well, a small hole was made at the foot of
the visible portion of the brickwork, and a charge of 2 lb. of
gun-cotton was introduced into this, and fired with only a tolerable
effect, the brickwork being cracked for a distance of 3 or 4 feet from
the centre of the charge. A hole was next dug down about 5 feet at one
side of the wing-wall, and a charge of 4 lb. of gun-cotton well tamped
was fired. The tamping was blown out, and the wall foundations cracked
a good deal. The excavation was now deepened to 6 feet, and a hole made
under the brickwork big enough to contain a 100-lb. charge. It was then
well tamped up and fired. Its effect was excellent. All the brickwork of
the wing-wall was got rid of, and a crater about 30 feet in width at the
top blown out in the point of the bank that was required to be removed,
and which was one of the chief causes of the whirlpool, so that the next
rise of the river was sure to carry it all away. The following day an
old pucka ghat opposite to Berhampore was entirely broken up with three
20-lb. charges, and an enormous quantity of old bricks were thrown into
the river.
The last operation undertaken consisted in the blowing up of a very
large ghat opposite to the Nawáb of Moorshedabád’s palaces. The river
during successive rains had cut into and underneath the steps of the
ghat, bringing down large masses of it into the river, where they formed
most dangerous obstacles to navigation. The work was necessarily carried
out in a very rough way, for want of the proper tools. Deep excavations
were made under the three largest masses of masonry, at about 25 feet
apart, and into these were introduced three 50-lb. and one 20-lb.
charges of powder. These charges were well tamped, connected up in
divided circuit, and fired simultaneously. All the masonry was broken up
completely, so as to be easily removable afterwards by coolie labour,
which was all that was required.
The conclusions to be drawn from the foregoing notes are, that large
trees lying in shallow water require charges of 50 lb. of gunpowder and
upwards for their effectual removal; but that where there is plenty of
water, and the trees are not very large, 20 lb. is sufficient.
For these small charges, it has been seen that the common earthenware
ghurrah answers admirably, and under similar circumstances it would
undoubtedly be advantageous to use them, as they are inexpensive, and
obtainable in nearly every Indian village.
The charges used might, in many cases, at first have been no doubt made
smaller with advantage, both for safety and economy; but as speed was
the great object, these were not so much thought of.
For the removal of masonry under water, it is not necessary to place the
charge underneath the mass, which is often impossible; a large charge
alongside it being generally quite sufficient to break it up pretty
effectually where there is sufficient head of water. Smaller charges can
of course be easily used afterwards, whenever required, and for these
small charges, gun-cotton is very effective, as it can be easily
introduced, in the end of a bamboo, into holes and crevices where it
would be impossible to get any but the smallest charges of gunpowder.
INDEX.
Appliances for firing blasting charges, 42
Auxiliary tools, 17
Batteries, firing, 62
Beche, 21
Bichromate firing battery, 62
Bits, borer, 31
Blasting gear, sets of, 22
---- sticks, 51
---- subaqueous, 164
Borer-bits, 31
Boring under water, 170
---- the shot-holes, 128, 142
Bornhardt’s firing machine, 57
Brain’s powder, 105
Bull, 20
Cables, 53
Cellulose dynamite, 105
Charging and firing, 150
---- the shot-holes, 132
Chemical compounds, 81
Claying iron, 20
Conditions of disruption, 110
Connecting wires, 52
Cotton powder, 103
Darlington drill, 26
Detonation, 95
Detonators, 54
Dislodged rock, removal of, 154
Disruption, conditions of, 110
---- force required to cause, 107
Division of labour, 155
Drills, dimension of, 6
---- form of, 4
---- hand, 1
---- hardening and tempering, 10
---- making and sharpening, 7
---- sets of, 13
Drivings, examples of, 157
Dubois-François carriage, 39
Dynamite, 100
---- composition of, 87
Electrical firing, advantage of, 153
Electric fuses, 47
---- tension fuse, 50
Example of a heading, 115
Examples of drivings, 157
Explosion, force developed by, 72
---- heat liberated by, 67
---- gases generated by, 69
---- nature of, 64
Explosive agents, nature of, 76
Firing batteries, 62
---- blasting charges, appliances for, 42
---- by electricity, 138
---- machines, 55
---- machine, Bornhardt’s, 57
---- ---- induction coil, 61
---- ---- Mowbray’s, 59
---- ---- Siemens’, 60
---- ---- Smith’s, 58
---- points of the common explosive agents, 102
---- table for frictional electric machine, 140
---- the charges, 137
Force developed by an explosion, 72
---- developed by gunpowder, 88
---- required to cause disruption, 107
Fuse, safety, 45
Fuses, electric, 47
Gases generated by an explosion, 69
Gun-cotton, 99
---- constitution of, 81
Gunpowder, 97
---- composition of, 80
---- force developed by, 88
Hammers, 14
---- patterns of, 15
Hand boring, 128
Heading, example of, 115
Heat, action of, in firing, 92
---- liberated by an explosion, 67
---- measure of, 66
---- specific, 66
Hoosac Tunnel, 158
Induction firing coils, 61
Joint and bedding planes, 118
Jumper, 3
Labour, division of, 155
Line of least resistance, 106
Lithofracteur, 104
Machine boring, 142
---- rock-drills, 23
Machines, firing, 55
Means of firing the common explosive agents, 92
Measure of heat, 66
Mechanical mixture, 76
Mowbray’s firing machine, 59
Musconetcong Tunnel, 159
Nature of an explosion, 64
Nitrated gun-cotton, 103
Nitro-glycerine, constitution of, 86
Obstructions in water-courses, 178
Operations of rock blasting, 128
Preparation of subaqueous charges, 164
Principles of blasting, 106
Rammer, 20
Relative strength of gunpowder, gun-cotton, and dynamite, 91
---- ---- of the common explosive agents, 88
Removing dislodged rock, 154
Rock-drill supports, 34
Safety Fuse, 45
St. Gothard Tunnel, 157
Schultze’s powder, 104
Scraper, 18
Shot-holes, boring, 128, 142
---- charging, 132
Siemens’ firing machine, 60
Silvertown firing battery, 62
Sledges, 14
---- North of England, 17
---- North Wales, 17
---- South Wales, 16
Smith’s firing machine, 58
Some properties of the common explosive agents, 97
Some varieties of the nitro-cellulose and the nitro-glycerine
compounds, 103
Squibs, 44
Steel, hardening and tempering, 9
Stemmer, 20
Sticks, blasting, 51
Stretcher bar, 37
Subaqueous blasting, 164
---- charges, preparation of, 164
Submarine rocks, removal of, 173
Swab-stick, 19
Tamping, 121
Tonite, 103
Water, boring under, 170
Water-courses, obstructions in, 178
Waterproofing composition, 165
Weight of explosive in bore-hole, table of, 109
Wires, connecting, 52
LONDON: PRINTED BY WM. CLOWES AND SONS, STAMFORD STREET AND CHARING
CROSS.
[Illustration: Plate I.
SET OF COAL BLASTING GEAR.
_Fig. 1._
_Fig. 2._
_Fig. 4._
_Fig. 5._
_Fig. 3._
E & F. N. Spon. London & New York.]
[Illustration: Plate II.
SET OF SINGLE HAND STONE BLASTING GEAR.
_Fig. 6._
_Fig. 7._
_Fig. 8._
_Fig. 9._
_Fig. 10._
_Fig. 11._
E & F. N. Spon. London & New York.]
[Illustration: Plate III.
SET OF DOUBLE HAND STONE BREAKING GEAR.
_Fig. 12._
_Fig. 15._
_Fig. 13._
_Fig. 14._
_Fig. 16._
_Fig. 17._
_Fig. 18._
_Fig. 19._
E & F. N. Spon. London & New York.]
[Illustration: Plate IV.
THE DARLINGTON MACHINE DRILL.
_Fig. 20._
_Fig. 22._
_Fig. 21._
_Fig. 23._
_Fig. 24._
_Fig. 25._
E & F. N. Spon. London & New York.]
[Illustration: Plate V.
STRETCHER BAR.
_Fig. 26._
E & F. N. Spon. London & New York.]
[Illustration: Plate VI.
MACHINE DRILL SUPPORTS.
_Fig. 27._
_Fig. 28._
E & F. N. Spon. London & New York.]
[Illustration: Plate VII.
MACHINE DRILL SUPPORT.
_Fig. 29._
E & F. N. Spon. London & New York.]
[Illustration: Plate VIII.
DUBOIS-FRANÇOIS MACHINE DRILL CARRIAGE.
_Fig. 30. Elevation._
_Fig. 31. Plan._
_Fig. 32._
_Raising and lowering Screw._
_Fig. 33._
_Back support for the Drill._
E & F. N. Spon. London & New York.]
[Illustration: Plate IX.
S^{T}. GOTHARD TUNNEL.
_Göschenen End._
_Airolo End._
E & F. N. Spon. London & New York.]
[Illustration: Plate X.
EXAMPLES OF HEADINGS.
_Heading, Marihaye._
_Heading, Anzin._
_Heading, Ronchamp._
HOOSAC TUNNEL--_West End._
E & F. N. Spon. London & New York.]
[Illustration: Plate XI.
HOOSAC TUNNEL.
_Centre Cut._
_Side Cuts._
_Squaring the Heading._
E & F. N. Spon. London & New York.]
[Illustration: Plate XII.
MUSCONETCONG TUNNEL.
_The Heading; Plan._
_The Bench._
_Sectional Elevation._
_The Bench, Plan._
_The Heading,_
_Elevation._
E & F. N. Spon. London & New York.]
Transcriber’ Notes
Inconsistent and unusual spelling, hyphenation, use of accents and
lay-out have been retained, except as mentioned below.
There are two sets of numbered illustrations: numbered illustrations
in the text, and numbered illustrations in the plates in the back of
the book.
Calculations and tables have been copied verbatim although some
contain calculation errors.
Some reference letters in the text are missing from the illustrations.
Changes made:
Tables and illustrations have been moved to between paragraphs.
Some minor typographic errors have been corrected and some missing
punctuation has been added silently.
Page 40: plate ~V~ changed to plate V.
Page 109: Curtiss’ and Harvey’s changed to Curtis’s and Harvey’s.
Page 166: Curtis’s and Harvey changed to Curtis’s and Harvey’s.