Mr. Wm. L. Saunders, for many years the engineer of the Ingersoll Rock
Drill Co., and hence thoroughly familiar with modern quarrying
practice, read a paper before the last meeting of the American Society
of Civil Engineers on the above subject, containing many interesting
points, given in the _Engineering News_, from which we abstract as
follows.
As a preliminary to describing the new Knox system of quarrying, which
even yet is not universally known among quarrymen, Mr. Saunders gives
the following in regard to older methods:
The Knox system is a recent invention; no mention was made of
it in the tenth census, and no description has yet been given
of it in any publications on quarrying. The first work done by
this method was in 1885, and at the close of that year 2
quarries had adopted it. In 1886 it was used in 20 quarries;
in 1887 in 44, in 1888 in upward of 100, and at the present
time about 300 quarries have adopted it. Its purpose is to
release dimension stone from its place in the bed, by so
directing an explosive force that it is made to cleave the
rock in a prescribed line without injury. The system is also
used for breaking up detached blocks of stone into smaller
sizes.
Quarrymen have, ever since the introduction of blasting, tried to
direct the blast so as to save stock. Holes drilled by hand are seldom
round. The shape of the bit and their regular rotation while drilling
usually produce a hole of somewhat triangular section. It was
observed, many years ago, that when a blast was fired in a
hand-drilled hole the rock usually broke in three directions,
radiating from the points of the triangle in the hole. This led
quarrymen to look for a means by which the hole might be shaped in
accordance with a prescribed direction of cleavage.
The oldest sandstone quarries in America are those at Portland, Conn.
It was from these quarries that great quantities of brownstone were
shipped for buildings in New York. The typical "brownstone front" is
all built of Portland stone. As the Portland quarries were carried to
great depths the thickness of bed increased, as it usually does in
quarries. With beds from 10 to 20 ft. deep, all of solid and valuable
brownstone, it became a matter of importance that some device should
be applied which would shear the stone from its bed without loss of
stock and without the necessity of making artificial beds at short
distances. A system was adopted and used successfully for a number of
years which comprised the drilling of deep holes from 10 to 12 in. in
diameter, and charging them with explosives placed in a canister of
peculiar shape. The drilling of this hole is so interesting as to
warrant a passing notice. The system was similar to that followed with
the old fashioned drop drill. The weight of the bit was the force
which struck the blow, and this bit was simply raised or lowered by a
crank turned by two men at the wheel. The bit resembled a broad ax in
shape, in that it was extremely broad, tapering to a sharp point, and
convex along the edge.
[Illustration: Fig. 1]
Fig. 1 illustrates in section one of the Portland drills, and a drill
hole with the canister containing the explosive in place. The canister
was made of two curved pieces of sheet tin with soldered edges, cloth
or paper being used at the ends. It was surrounded with sand or earth,
so that the effect of the blast was practically the same as though the
hole were drilled in the shape of the canister. In other words, the
old Portland system was to drill a large, round hole, put in a
canister, and then fill up a good part of the hole. Were it possible
to drill the hole in the shape of the canister, it would obviously
save a good deal of work which had to be undone. The Portland system
was, therefore, an extravagant one, but the results accomplished were
such as to fully warrant its use. Straight and true breaks were made,
following the line of the longer axis of the canister section, as in
Fig. 2.
[Illustration: Fig. 2.]
It was found that with the old Portland canister two breaks might be
made at right angles by a single blast, when using a canister shaped
like a square prism. In some of the larger blasts, where blocks
weighing in the neighborhood of 2,000 tons were sheared on the bed,
two holes as deep as 20 ft. were drilled close together. The core
between the holes was then clipped out and large canisters measuring 2
ft. across from edge to edge were used.
In regard to another of the older systems of blasting, known as
Lewising, Mr. Saunders says:
A Lewis hole is made by drilling two or three holes close
together and parallel with each other, the partitions between
the holes being broken down by using what is known as a
broach. Thus a wide hole or groove is formed in which powder
is inserted, either by ramming it directly in the hole, or by
puling it in a canister, shaped somewhat like the Lewis hole
trench. A complex Lewis hole is the combination of 3 drill
holes, while a compound Lewis hole contains 4 holes. Lewising
is confined almost entirely to granite. In some cases a series
of Lewis holes is put in along the bench at distances of 10
and 25 ft. apart, or even greater, each Lewis hole being
situated equidistant from the face of the bench. The holes are
blasted simultaneously by the electric battery.
After noting another system used to a limited extent, and not to be
commended, viz., the use of inverted plugs and feathers (the plugs and
feathers being inserted as a sort of tamping which the blast drives
upward to split the rock), Mr. Saunders continues in substance as
follows:
It is thus seen that the "state of the art" has been
progressive, though it was imperfect. Mr. Sperr, in his
reference to this subject, made in the report of the tenth
census, says: "The influence of the shape of the drill hole
upon the effects of the blast does not seem to be generally
known, and a great waste of material necessarily follows."
This was written but a few years before the introduction of
the new system, and it is doubtless true that attention was
thus widely directed to the conspicuous waste, due to a lack
of knowledge of the influence of the shape of a drill hole on
the effect of a blast. The system developed by Mr. Knox
practically does all and more than was done by the old
Portland system, and it does it at far less expense. It can
best be described by illustrations.
[Illustrations: Figs. 3, 4, 5, 6]
Fig. 3 is a round hole drilled either by hand or otherwise, preferably
otherwise, because an important point is to get it round. Fig. 4 is
the improved form of hole, and this is made by inserting a reamer,
Figs. 5 and 6, into the hole in the line of the proposed fracture,
thus cutting two V-shaped grooves into the walls of the hole. The
blacksmith tools for dressing the reamers are shown in Fig. 7. The
usual method of charging and tamping a hole in using the new system is
shown in Fig. 8. The charge of powder is shown at C, the air space at
B and the tamping at A. Fig. 9 is a special hole for use in thin beds
of rock. The charge of powder is shown at C, the rod to sustain
tamping at D, air space at BB, and tamping at A.
[Illustration: Fig. 7]
Let us assume that we have a bluestone quarry, in which we may
illustrate the simplest application of the new system. The sheet of
stone which we wish to shear from place has a bed running horizontally
at a depth of say 10 ft. One face is in front and a natural seam
divides the bed at each end at the walls of the quarry. We now have a
block of stone, say 50 ft. long, with all its faces free except
one--that opposite and corresponding with the bench. One or more of
the specially formed holes are put in at such depth and distance from
each other and from the bench as may be regulated by the thickness,
strength and character of the rock. No man is so good a judge of this
as the quarry foreman who has used and studied the effect of this
system in his quarry. Great care should be taken to drill the holes
round and in a straight line. In sandstone of medium hardness these
holes may be situated 10, 12 or 15 ft. apart. If the bed is a tight
one the hole should be run entirely through the sheet and to the bed;
but with an open free bed holes of less depth will suffice.
[Illustration: Fig. 8, 9]
The reamer should now be used and driven by hand. Several devices have
been applied to rock drills for reaming the hole by machinery while
drilling; that is, efforts have been made to combine the drill and the
reamer. Such efforts have met with only partial success. The perfect
alignment of the reamer is so important that where power is used this
point is apt to be neglected. It is also a well known fact that the
process of reaming by hand is not a difficult or a slow one. The
drilling of the hole requires the greatest amount of work. After this
has been done it is a simple matter to cut the V-shaped grooves. The
reamer should be applied at the center, that is, the grooves should be
cut on the axis or full diameter of the hole. The gauge of the reamer
should be at least 1½ diameters. Great care should be taken that the
reamer does not twist, as the break may be thereby deflected; and the
reaming must be done also to the full depth of the hole.
The hole is now ready for charging. The powder should be a low
explosive, like black or Judson powder or other explosives which act
slowly. No definite rule can be laid down as to the amount of powder
to be used, but it should be as small as possible. Very little powder
is required in most rocks. Hard and fine grained stone requires less
powder than soft stone. Mr. Knox tells of a case which came under his
observation, where a block of granite "more than 400 tons weight,
split clear in two with 13 oz. of FF powder." He compares this with a
block of sandstone of less than 100 tons weight "barely started with
2½ lb. of the same grade of powder, and requiring a second shot to
remove it."
It is obvious that enough powder must be inserted in the hole to
produce a force sufficient to move the entire mass of rock on its bed.
In some kinds of stone, notably sandstone, the material is so soft
that it will break when acted upon by the force necessary to shear the
block. In cases of this kind a number of holes should be drilled and
fired simultaneously by the electric battery. In such work it is usual
to put in the holes only 4 or 5 ft. apart. The powder must, of course,
be provided with a fuse or preferably a fulminating cap. It is well to
insert the cap at or near the bottom of the cartridge, as shown in
Figs. 8 and 9.
After the charge the usual thing to do is to insert tamping. In the
improved form of hole the tamping should not he put directly upon the
powder, but an air space should be left, as shown at B, Fig. 8. The
best way to tamp, leaving an air space, is first to insert a wad,
which may be of oakum, hay, grass, paper or other similar material.
The tamping should be placed from 6 to 12 in. below the mouth of the
hole. In some kinds of stone a less distance will suffice, and as much
air space as practicable should intervene between the explosive and
the tamping. If several holes are used on a line they should be
connected in series and blasted by electricity. The effect of the
blast is to make a vertical seam connecting the holes, and the entire
mass of rock is sheared several inches or more.
The philosophy of this new method of blasting is simple, though a
matter of some dispute. The following explanation has been given. See
Fig. 10.
[Illustration: Fig. 10]
"The two surfaces, _a_ and _b_, being of equal area, must receive an
equal amount of the force generated by the conversion of the explosive
into gas. These surfaces being smooth and presenting no angle between
the points, A and B, they furnish no starting point for a fracture,
but at these points the lines meet at a sharp angle including between
them a wedge-shaped space. The gas acting equally in all directions
from the center is forced into the two opposite wedge-shaped spaces,
and the impact being instantaneous the effect is precisely similar to
that of two solid wedges driven from the center by a force equally
prompt and energetic. All rocks possess the property of elasticity in
a greater or less degree, and this principle being excited to the
point of rupture at the points, A and B, the gas enters the crack and
the rock is split in a straight line simply because under the
circumstances it cannot split in any other way."
Another theory which is much the same in substance is then given, and
after some general discussion of the theory of the action of the
forces under the several systems, the paper continues:
The new form of hole is, therefore, almost identical in principle with
the old Portland canister, except that it has the greater advantage of
the V-shaped groove in the rock, which serves as a starting point for
the break. It is also more economical than the Portland canister, in
that it requires less drilling and the waste of stone is less. It is,
therefore, not only more economical than any other system of blasting,
but it is more certain, and in this respect it is vastly superior to
any other blasting system, because stone is valuable, and anything
which adds to the certainty of the break also adds to the profit of
the quarryman.
It is doubtless true that, notwithstanding the greater area of
pressure in the new form of hole, the break would not invariably
follow the prescribed line but for the V-shaped groove which virtually
starts it. A bolt, when strained, will break in the thread whether
this be the smallest section or not, because the thread is the
starting point for the break. A rod of glass is broken with a slight
jar provided a groove has been filed in its surface. Numerous other
instances might be cited to prove the value of the groove. Elasticity
in rock is a pronounced feature, which varies to a greater or less
extent; but it is always more or less present. A sandstone has
recently been found which possesses the property of elasticity to such
an extent that it may be bent like a thin piece of steel. When a blast
is made in the new form of hole the stone is under high tension, and
being elastic it will naturally pull apart on such lines of weakness
as grooves, especially when they are made, as is usually the case in
this system, in a direction at right angles with the lines of least
resistance.
Horizontal holes are frequently put in and artificial beds made by
"lofting." In such cases where the rock has a "rift" parallel with the
bed, one hole about half way through is sufficient for a block about
15 ft. square, but in "liver" rock the holes must be drilled nearly
through the block and the size of the block first reduced.
A more difficult application of the system, and one requiring greater
care in its successful use, is where the block of stone is so situated
that both ends are not free, one of them being solidly fixed in the
quarry wall. A simple illustration of a case of this kind is a stone
step on a stairway which leads up and along a wall, Fig. 11. Each step
has one end fixed to the wall and the other free. Each step is also
free on top, on the bottom and on the face, but fixed at the back. We
now put one of the new form of holes in the corner at the junction of
the step and the wall. The shape of the hole is as shown in Fig. 12.
[Illustration: FIG. 11.]
It is here seen that the grooves are at right angles with each other,
and the block of stone is sheared by a break made opposite and
parallel with the bench, as in the previous case, and an additional
break made at right angles with the bench and at the fixed end of the
block. Sometimes a corner break is made by putting in two of the
regular V-shaped holes in the lines of the proposed break and without
the use of the corner hole. A useful application of this system is in
splitting up large masses of loose stone. For this purpose the
V-shaped grooves are sometimes cut in four positions and breaks are
made in four directions radiating from the center of the hole as shown
in Fig. 12. In this way a block is divided into four rectangular
pieces.
[Illustration: FIG. 12.]
Though the new system is especially adapted to the removal of heavy
masses of rock, yet it has been applied with success in cases where
several light beds overlie each other. In one such instance 10 sheets,
measuring in all only 6 ft., were broken by a blast, but in cases of
this kind the plug and feather process applies very well, and the new
system, when used, must be in the hands of an expert, or the loss will
be serious.
Referring again to our stone step, let us imagine a case where this
stairway runs between two walls. We have here each step fixed at each
end and free only on the top, the bottom, and one face. Let us assume
that there is a back seam, that is, that the step is not fixed at the
back. In a quarry, this seam, unless a natural one, should be made by
a channeling machine. In order to throw this step put of place it must
be cut off at both ends, and for this purpose the V-shaped holes are
put in at right angles to the face. It is well, however, to put the
first two holes next the back seam in a position where the grooves
will converge at the back so as to form a sort of key, which serves a
useful purpose in removing the block after the blast. In quarries
where there are no horizontal beds a channeling machine should be used
to free the block on all sides and to a suitable depth, and then the
ledge may be "lofted" by holes placed horizontally.
Where "pressure" exists in quarries, the new system has certain
limitations. After determining the line of "pressure" it is only
practicable to use the system directly on the line of thrust, or at
right angles to it. It is much better, however, to release the
"pressure" from the ledge by channeling, after which a single end may
be detached by a Knox blast. It is well to bear in mind that the holes
should invariably be of small diameter. In no case should the diameter
of a hole be over 1½ in. in any kind of rock. This being the case, the
blocks of stone are delivered to the market with but little loss in
measurement. It is a noticeable fact that stone quarried by the new
system shows very little evidence of drill marks, for the faces are
frequently as true as though cut with a machine.
A further gain is the safety of the system. The blasting is light and
is confined entirely within the holes. No spalls or fragments are
thrown from the bast.
The popular idea that the system is antagonistic to the channeling
process is a mistaken one. There are, of course, some quarries which
formerly used channeling machines without this system, but which now
do a large part of the work by blasting. Instances, however, are rare
where the system has replaced the channeler. The two go side by side,
and an intelligent use of the new system in most quarries requires a
channeling machine. There are those who may tell of stone that has
been destroyed by a blast on the new system, but investigation usually
shows that either the work was done by an inexperienced operator, or
an effort was made to do too much.
A most interesting illustration of the value of this system, side by
side with the channeler, is shown in the northern Ohio sandstone
quarries. A great many channeling machines are in use there, working
around the new form of holes, and when used together in an intelligent
and careful manner, the stone is quarried more cheaply than by any
other process that has yet been devised.
To a limited extent the system has been used in slate. The difficulty
is that most of the slate quarries are in solid ledges, where no free
faces or beds exist; but it has been used with success in a slate
quarry at Cherryville, Pa., since 1888. Among notable blasts made by
this system are the following: At the mica schist quarries, at
Conshohocken, Pa., a hole 1½ in. in diameter was drilled in a block
which was 27 ft. long, 15 ft. wide and 6 ft. thick. The blast broke
the stone across the "rift," only 8 oz. of black powder being used. At
the Portland, Conn., quarries a single blast was fired by electricity,
15 holes being drilled with 2 lb. of coarse No. C powder in each hole,
and a rock was removed 110 ft. long, 20 ft. wide and 11 ft. thick,
containing 24,200 cu. ft., or about 2,400 tons, the fracture being
perfectly straight. This large mass of stone was moved out about 2 in.
without injury to itself or the adjoining rock.
Another blast at Portland removed 3,300 tons a distance of 4 in.
Seventeen holes were drilled, using 2 lb. of powder in each hole, the
size of the block being 150 × 20 × 11 ft. In a Lisbon, O., quarry a
block of sandstone 200 ft. long, 28 ft. wide and 15 ft. thick was
moved about ½ in. by a blast. This block was also afterward cut up by
this system in blocks 6 ft. square. A sandstone bowlder 70 ft. long,
average width 50 ft., average thickness 13 ft., was embedded in the
ground to a depth of about 7 ft. A single hole 8 ft. deep was charged
with 20 oz. of powder and the rock was split in a straight line from
end to end and entirely to the bottom. A ledge of sandstone open on
its face and two ends, 110 × 13 × 8 ft., was moved by a blast about 3
in. without wasting a particle of rock, 8 holes being used, drilled by
three men in just one day, and 15 oz. of powder being used in each
hole. A sandstone ledge, open on the face and end only, 200 × 28 × 15
ft., containing 84,000 cu. ft. stone, was moved ½ in. by 25 holes,
each containing 1 lb. of powder.
* * * * *