EXCAVATING MACHINES AND ROCK DRILLS: EXPLOSIVES AND BLASTING.
=Earth-Excavating Machines.=--Comparatively few of the labor-saving
machines employed for breaking up and removing loose soil in ordinary
surface excavation are used in tunnel excavation through the same
material. Several forms of tunnel excavating machines have been tried at
various times, but only a few of them have attained any measure of
success, and these have seldom been employed in more than a single work.
In the Central London underground railway work through clay a continuous
bucket excavator (Fig. 11) was employed with considerable saving in time
and labor over hand work. In some recent tunnel work in America the
contractors made quite successful use of a modified form of steam
shovel. These are the most recent attempts to use excavating machines in
soft ground, and they, like all previous attempts, must be classed as
experiments rather than as examples of common practice. The Thomson
machine,[4] however, can be employed in any tunnel driven through loose
soil. It occupies a comparatively small space and may easily work when
the timbering is used to support the roof of the tunnel. Steam shovel
instead may give efficient result only in the case that the whole
section of the tunnel is open at once and there are no timbers to
prevent the free swinging of the dipper handle and boom. But in tunnels
through loose soils it is almost impossible to open the whole section at
once without the necessity of supporting the roof. Consequently the use
of steam shovel in loose tunnels is very limited. The shovel, the spade,
and the pick, wielded by hand, are the standard tools now, as in the
past, for excavating soft-ground tunnels.
[4] The machine was designed by Mr. Thomas Thomson, Engineer for
Messrs. Walter Scott & Co.
[Illustration: FIG. 11.--Soft Ground Bucket Excavating Machine: Central
London Underground Railway.]
=Rock-Excavating Machines.=--At one period during the work of
constructing the Hoosac tunnel considerable attention was devoted to the
development of a rock excavating, boring, or tunneling machine. This
device was designed to cut a groove around the circumference of the
tunnel thirteen inches wide and twenty-four feet in diameter by means of
revolving cutters. It proved a failure, as did one of smaller size,
eight feet in diameter, tried subsequently. During and before the Hoosac
tunnel work a number of boring-machines of similar character were
experimented with at the Mont Cenis tunnel and elsewhere in Europe; but,
like the American devices, they were finally abandoned as impracticable.
=Hand Drills.=--Briefly described, a drill is a bar of steel having a
chisel-shaped end or cutting-edge. The simplest form of hand drill is
worked by one man, who holds the drill in one hand, and drives it with a
hammer wielded by his other hand. A more efficient method of hand-drill
work is, however, where one man holds the drill, and another swings the
hammer or sledge. Another form of hand drill, called a churn drill,
consists of a long, heavy bar of steel, which is alternately raised and
dropped by the workman, thus cutting a hole by repeated impacts.
In drilling by hand the workman holding the drill gives it a partial
turn on its axis at every stroke in order to prevent wedging and to
offer a fresh surface to the cutting-edge. For the same reason the chips
and dust which accumulate in the drill-hole are frequently removed. The
instruments used for this purpose are called scrapers or dippers, and
are usually very simple in construction. A common form is a strong wire
having its end bent at right angles, and flattened so as to make a sort
of scoop by which the drillings may be scraped or hoisted out of the
hole. It is generally advantageous to pour water into the drill-hole
while drilling to keep the drill from heating.
=Power Drills.=--When the conditions are such that use can be made of
them, it is nearly always preferable to use power drills, on account of
their greater speed of penetration and greater economy of work. Power
drills are worked by direct steam pressure, or by compressed air
generated by steam or water power, and stored in receivers from which it
is led to the drills through iron pipes. A great variety of forms of
power drills are available for tunnel work in rock, but they can nearly
all be grouped in one of two classes: (1) Percussion drills, and (2)
Rotary drills.
_Percussion Drills._--The first American percussion drill was patented
by Mr. J. J. Couch of Philadelphia, Penn., in March, 1849. In May of the
same year, Mr. Joseph W. Fowle, who had assisted Mr. Couch in developing
his drill, patented a percussion drill of his own invention. The Fowle
drill was taken up and improved by Mr. Charles Burleigh, and was first
used on the Hoosac tunnel. In Europe Mr. Cavé patented a percussion
drill in France in October, 1851. This invention was soon followed by
several others; but it was not until Sommeiller’s drill, patented in
1857 and perfected in 1861, was used on the Mont Cenis tunnel, that the
problem of the percussion drill was practically solved abroad. Since
this time numerous percussion drill patents have been taken out in both
America and Europe.
A percussion drill consists of a cylinder, in which works a piston
carrying a long piston rod, and which is supported in such a manner that
the drill clamped to the end of the piston rod alternately strikes and
is withdrawn from the rock as the piston reciprocates back and forth in
the cylinder. Means are devised by which the piston rod and drill turn
slightly on their axis after each stroke, and also by which the drill is
fed forward or advanced as the depth of the drill-hole increases. The
drills of this type which are in most common use in America are the
Ingersoll-Sergeant and the Rand. There are various other makes in common
use, however, which differ from the two named and from each other
chiefly in the methods by which the valve is operated. All of these
drills work either with direct steam pressure or with compressed air.
Workable percussion drills operated by electricity are built, but so far
they do not seem to have been able to compete commercially with the
older forms. No attempt will be made here to make a selection between
the various forms of percussion drills for tunnel work, and for the
differences in construction and the merits claimed for each the reader
is referred to the makers of these machines. All of the leading makes
will give efficient service. It goes almost without saying that a good
percussion drill should operate with little waste of pressure, and
should be composed of but few parts, which can be easily removed and
changed.
_Drill Mountings._--For tunnel work the general European practice is to
mount power drills upon a carriage moving on tracks in order that they
be easily withdrawn during the firing of blasts. Connection is made with
the steam or compressed air pipes by means of flexible hose which can
easily be attached or detached as the drill advances or when it is moved
for repairs or during blasts. Two, four, and sometimes more drills are
mounted and work simultaneously on a single carriage. In America it has
been found that column mountings have been more successful for tunnel
work than any other form. The column mounting made by the
Ingersoll-Sergeant Drill Co. is shown in Fig. 12. In using this form of
mounting no tracks or other special apparatus is required; it is not
necessary, as is the case with the carriage mounting, to remove the
débris before resuming operations, but as soon as the blasting has been
finished and the smoke has sufficiently disappeared the column can be
set up and drilling resumed.
[Illustration: FIG. 12.--Column Mounting for Percussion Drill:
Ingersoll-Sergeant Drill Co.]
_Rotary Drills._--Rotary drilling machines, or more simply rotary
drills, were first used in 1857 in the Mont Cenis tunnel. The advantages
claimed for rotary drills in comparison with percussion drills are: (1)
That less power is required to drive the drill, and the power is better
utilized; (2) once the machines work easily they do not require
continual repairs, and (3) in driving holes of large size the interior
nucleus is taken away intact, thus reducing work and increasing the
speed of drilling. Rotary drills are extensively used for geological,
mining, well-driving, and prospecting purposes; but they are very seldom
employed in tunnels in America, although successfully used for this
purpose in Europe. The reason they have not gained more favor among
American tunnel builders is due to some extent perhaps to prejudice, but
chiefly to the great cost of the machine as compared with percussion
drills, and to the expense of diamonds for repairs. Those who advocate
these machines for tunnel work point out, however, that under ordinary
usage the diamonds have a very long life,--borings of 700 lin. ft. being
recorded without repairs to the diamonds.
[Illustration: FIG. 13.--Sketch of Diamond Drill Bit.]
The form of rotary drill used chiefly for prospecting purposes is the
diamond drill. This machine consists of a hollow cylindrical bit having
a cutting-edge of diamonds, which is revolved at the rate of from two
hundred to four hundred revolutions per minute by suitable machinery
operated by steam or compressed air. The diamonds are set in the
cutting-edge of the bit so as to project outward from its annular face
and also slightly inside and outside of its cylindrical sides (Fig. 13).
When the drill rod with the bit attached is rotated and fed forward the
bit cuts an annular hole into the rock; the drillings being removed from
the hole by a constant stream of water which is forced down through the
hollow drill rod and emerges, carrying the débris with it, up through
the narrow space between the outside of the bit and the walls of the
hole. There are various makes of diamond drills, but they all operate in
essentially the same manner.
The rotary drill principally employed in Europe in tunneling is the
Brandt. The cutting-edge of the Brandt drill consists of hardened steel
teeth. The bit is pressed against the rock by hydraulic pressure, and
usually makes from seven to eight revolutions per minute. Some of the
water when freed goes through the hollow bit, keeping it cool, and
cleaning the hole of débris. A water pressure of from 300 to 450 lbs.
per square inch is required to operate these drills. Rotary rock-drills
may be mounted either on carriages or on columns for tunnel work.
Several machines have recently been constructed for the purpose of
breaking the rock in tunnels without blasting, but they did not meet the
approval of tunnel engineers. One of these machines is provided with
numerous electric torches, which are applied to the rock at the front.
By suddenly chilling the rock with a stream of cold water the stone will
crumble away. Another machine was tested, with little success, in the
excavation for the New Grand Central Depot in New York. On the face of
this machine there is a multitude of chipping drills revolving on four
arms and driven by air pressure. They attack the rock and chip it into
fragments, which are carried away by an endless band.
EXPLOSIVES AND BLASTING.
When the holes are once drilled, either by hand or power drills, they
are charged with explosives. The principal explosives employed in
tunneling are gunpowder, nitroglycerine, and dynamite.
=Gunpowder.=--Gunpowder is composed of charcoal, sulphur, and saltpeter
in proportions varying according to the quality of the powder. For
mining purposes the composition employed is 65% saltpeter, 15% sulphur,
and 20% charcoal. It is a black granulated powder having a specific
gravity of 1.5; the black color is given by the charcoal; and the
grains have an angular form, and vary in size from ¹⁄₈ in. to ³⁄₈ in.
Good blasting powder should contain no fine grains, which may be
detected by pouring some of the powder upon a sheet of white paper. The
force developed by the explosion of gunpowder is not accurately known;
it depends upon the space in which it is confined. Different authorities
estimate the pressure at from 15,000 lbs. per sq. in. in loose blasts to
200,000 lbs. per sq. in. in gunnery. Authorities also differ in opinion
as to the character of the gases developed by the explosion of
gunpowder, a matter of vital concern to the tunnel engineer, since they
are likely to affect the health and comfort of his workmen. It may be
assumed in a general way, however, that the oxygen of the saltpeter
converts nearly all of the carbon of the charcoal into carbon dioxide, a
portion of which combines with the potash of the saltpeter to form
carbonate of potash, the remainder continuing in the form of gas. The
sulphur is converted into sulphuric acid, and forms a sulphate of
potash, which by reaction is decomposed into hyposulphite and sulphide.
The nitrogen of the saltpeter is almost entirely evolved in a free
state; and the carbon not having been wholly burnt into carbonic acid,
there is a proportion of carbonic oxide.
=Nitroglycerine.=--Nitroglycerine is one of the modern explosives used
as a substitute for gunpowder. It is a fluid produced by mixing
glycerine with nitric and sulphuric acids; it freezes at +41° F., and
burns very quietly, developing carbonic acid, nitrogen, oxygen, and
water. By percussion or by the explosion of some substances, such as
capsules of gunpowder or fulminate of mercury, nitroglycerine produces a
sudden explosion in which about 1250 volumes of gases are produced. The
pressure of these gases has been calculated at 26,000 atmospheres, or
324,000 lbs. per sq. in. Nitroglycerine explodes very easily by
percussion in its normal state, but with great difficulty when frozen;
hence, in America, at the beginning of its use, it was transported only
in a frozen state. When dirty, nitroglycerine undergoes a spontaneous
decomposition accompanied by the development of gases and the evolution
of heat, which, reaching 388° F., causes it to explode. Notwithstanding
the enormous pressures which nitroglycerine develops, it is very seldom
used in its liquid state, but is mixed with a granular absorbent earth
composed of the shells of diatoms. The fluid undergoes no chemical
change by being absorbed, and explodes, freezes, and burns under the
same conditions as in the fluid state.
=Dynamite.=--The credit of rendering nitroglycerine available for the
purposes of the engineer by mixing it with a granular absorbent is due
to Albert Nobel of Stockholm, Sweden, who named the new material
dynamite. The nitroglycerine in dynamite loses very little of its
original explosive power, but is very much less easily exploded by
percussion, and can be employed in horizontal as well as vertical holes,
which was, of course, not possible in its liquid state. Dynamite must
contain at least 50% of nitroglycerine. Some manufacturers, instead of
using diatomaceous earth, use other absorbents which develop gases upon
explosion and increase the force of the explosion. These mixtures are
classed under the general name of false dynamites. A great many
varieties of dynamite are manufactured, and each manufacturer usually
makes a number of grades to which he gives special names. Dynamite for
railway work, tunneling, and mining contains about 50% of
nitroglycerine; for quarrying about 35%, and for blasting soft rocks
about 30%. It is sold in cylindrical cartridges covered with paper.
=Storage of Explosives.=--In driving tunnels through rock large
quantities of explosives must be used, and it is necessary to have some
safe place for storing them. In many States there are special laws
governing the transportation and storage of explosives; where there is
no regulation by law the engineer should take suitable precautions of
his own devising. It is best to build a special house or hut in one of
the most concealed portions of the work and away from the tunnel, and
protect it with a lightning-rod and from fire. Strict orders should be
given to the watchman in charge not to allow persons inside with lamps
or fire in any form, and smoking should be prohibited. The use of
hammers for opening the boxes should be prohibited; and dynamite,
gunpowder, and fulminate of mercury should not be stored together in the
same room. A quantity of dynamite for two or three days’ consumption may
be stored near the entrance of the tunnel in a locked box, the keys of
which are kept by the foreman of the work. When dynamite has been frozen
the engineer should provide some arrangement by which it may be heated
to a temperature not exceeding 120° F., and absolutely forbid it being
thawed out on a stove or by an open fire.
=Fuses.=--When gunpowder is used in tunneling it is ignited by the
Blickford match. This match, or fuse as it is more commonly called,
consists of a small rope of yarn or cotton having as a core a small
continuous thread of fine gunpowder. To protect the outside of the fuse
from moisture it is coated with tar or some other impervious substance.
These fuses are so well made that they burn very uniformly at the rate
of about 1 ft. in 20 seconds, hence the moment of explosion can be
pretty accurately fixed beforehand. Blickford matches have the objection
for tunnel work of burning with a bad odor, especially when they are
coated with tar, and to remedy this many others have been invented.
Those of Rzika and Franzl are the best known of these. The former has
many advantages, but it burns too quickly, about 3 ft. per second, and
is expensive; the latter consists of a small hollow rope filled with
dynamite.
Blickford matches cannot be used to explode dynamite, the use of a
cartridge being required. These cartridges are small copper cylinders
containing fulminate of mercury. They may be attached to the end of the
Blickford match, which being ignited the spark travels along its length
until it reaches the copper cylinder, where it explodes the fulminate of
mercury, which in turn explodes the dynamite. Blasts may also be fired
by electricity, which, in fact, is the most common and the preferable
method, because several blasts can be fired simultaneously, and because
the current is turned on at a great distance, thus affording greater
safety to the workmen.
The method of electric firing generally employed in America is known as
the connecting series method, and consists in firing several mines
simultaneously. The ends of the wires are scraped bare, and the wire of
the first hole of the series is twisted together with the wire of the
second hole, and so on; finally the two odd wires of the first and last
holes are connected to two wires of a single cable or to two separate
cables extending to some safe place to which the men can retreat. Here
the two cable wires are connected by binding screws to the poles of a
battery, or sometimes to a frictional electric machine. The current
passes through the wires, making a spark at each break, and so fires the
fulminate of mercury, which explodes the dynamite.
Simultaneous firing by electricity by utilizing the united strength of
the blasts at the same instant secures about 10% greater efficiency from
the explosives. Another advantage of electric firing is that in case of
a missfire of any one of the holes there is slight possibility of
explosion afterwards, and the place can be approached at once to
discover the cause.
=Tamping.=--Tamping is the material placed in the hole above the
explosive to prevent the gases of explosion from escaping into the air.
Tamping generally consists of clay. When gunpowder is used the clay must
be well rammed with a wooden tool, and paper, cotton, or some other dry
material must be placed between the moist clay and the powder. When
dynamite is used it is not necessary to ram the tamping, since the
suddenness of the explosion shatters the rock before the clay can be
driven from the hole.
A few experienced men should be appointed to fire the blasts. These men
should give ample warning previous to the blast in order that all
machinery and tools which might be injured by flying fragments may be
removed out of danger, and so that the workmen may seek safety. When
all is ready they should fire the blasts, keeping accurate count of the
explosions to ensure that no holes have missed fire, and should call the
workmen back when all danger is over. In case any hole has missed fire
it should be marked by a red lamp or flag.
=Nature of Explosions.=--When the explosives are ignited a sudden
development of gases results, producing a sudden and violent increase of
pressure, usually accompanied by a loud report. The energy of the
explosion is exerted in all directions in the form of a sphere having
its center at the point of explosion, and the waves of energy lose their
force as the distance from this central point increases. The energy of
the explosion at any point in the sphere of energy is, therefore,
inversely proportional to the distance of this point from the center of
explosion. In the vicinity of the center of explosion the gases have
sufficient power to destroy the force of cohesion and shatter the rock;
further on, as they lose strength, they only destroy the elasticity of
the material and produce cracks; and still further away they only
produce a shock, and do not affect the material. Within the sphere of
energy there are, therefore, three other concentric spheres: the first
one being where cohesion is destroyed, the second where elasticity is
overcome, and the third where the shock is transmitted by elasticity.
When the latter sphere comes below the surface, the gases remain inside
the rock; but when the surface intersects either of the other two
spheres, the gases blow up the rock, forming a cone or crater, whose
apex is at the point of explosion, and which is called the
blasting-cone. The larger the blasting-cone is, the greater is the
amount of rock broken up; and the object of the engineer should,
therefore, always be so to regulate the depth of the hole and the
quantity of explosive as to secure the largest possible blasting cone in
each case. Experiments are required to determine the most efficient
depth of hole, and quantity of explosive to be employed, since these
differ in different kinds of rock, with the position of the rock
strata, etc.; but in ordinary practice, the depths of the holes are
made from 2 to 3 ft. in the heading and upper portion of the tunnel,
when drilled by hand; and from 6 to 8 ft. when drilled by power drills.
In the lower portion of the profile, the holes are made deeper, from 3
ft. to 4 ft. when drilled by hand, and exceeding 6 ft. when drilled by
power. The distance of the holes apart should be about equal to the
diameter of the blasting-cone; as a general rule it is assumed that the
base of the blasting-cone has a diameter equal to twice the depth of the
hole. The following table gives the average number of holes required in
each part of the excavation for the St. Gothard tunnel in which the
heading was excavated by machine drills while the other parts were
excavated by hand drills:
NO. OF PART.[5] NAME OF PART. NO. OF HOLES.
1. Heading 6 to 9
2. Right wing of heading 3 to 5
3. Left wing of heading 3 to 5
4. Shallow trench with core 2
5. Deepening of trench to floor 6 to 9
6. Narrow mass of core to left 3
7. Greater mass of core to left 6 to 9
8. Culvert 1
--------
Total section 30 to 43
[5] The location of the parts numbered is shown by Fig. 14, p. 36.
The quantity of explosives required for blasting depends upon the
quality of the rock, since the force of the explosives must overcome the
cohesion of the rock, which varies with its nature, and often differs
greatly in rocks of the same kind and composition. The quantity of
explosives required to secure the greatest efficiency in blasting any
particular rock may be determined experimentally, but in practice it is
usually deduced by the following rules: (1) The blasting force is
directly proportional to the weight of the explosives used, and (2) the
bulk of the blasted rock is proportional to the cube of the depth of the
holes. It is usually assumed, also, that the explosive should fill at
least one-fourth the depth of the hole.
The following table gives the depth of holes and amount of dynamite used
at each advance in the Fort George Tunnel illustrated on page 135.
+-------------------+------------+--------+-------+----------+
| ORDER OF FIRING. | KINDS OF | DEPTH. |CHARGE.| KIND OF |
| | HOLES. | | | DYNAMITE.|
+-------------------+------------+--------+-------+----------+
|Bench { 1st round|4 grading |3′ to 5′|50 lbs.|40% climax|
|Holes { |5 bench |9′ 6″ |45 „ |40% „ |
| { 2nd round|6 trimming |3′ to 9′|42 „ |40% „ |
| | | | | |
|Heading { 3d round |8 center cut|9′ |56 „ |60% „ |
| Holes { 4th round|8 side |8′ |48 „ |40% „ |
| { 5th round|6 dry |8′ |36 „ |40% „ |
+-------------------+------------+--------+-------+----------+