[Footnote 1: Read before the Engineer's Club, Philadelphia. Translated
from _Nouvelles Anodes de la Construction,_ March, 1890.]
By EDWIN S. CRAWLEY.
The methods of demolishing rocks by the use of explosives are always
attended by a certain amount of danger, while at the same time there
is always more or less uncertainty in regard to the final result of
the operation. Especially is this the case when the work must be
carried on without interrupting navigation and in the vicinity of
constructions that may receive injury from the explosions.
Such were the conditions imposed in enlarging the Suez Canal in
certain parts where the ordinary dredges could not be used.
Mr. Henry Lobnitz, engineer at Renfrew, has contrived a new method of
procedure, designed for the purpose of enlarging and deepening the
canal in those parts between the Bitter Lakes and Suez, where it runs
over a rocky bed. It was necessary to execute the work without
interrupting or obstructing traffic on the canal.
The principle of the system consists in producing a shattering of the
rock by the action of a heavy mass let fall from a convenient height,
and acting like a projectile of artillery upon the wall of a fortress.
From experiments made in the quarry of Craigmiller, near Edinburgh,
with a weight of two tons shod with a steel point, it was found that
with a fall of about 5.5 meters (18.04 ft.) there was broken up on an
average more than 0.113 cubic meter (0.148 cubic yard) of hard rock
per blow. The first blow, delivered 90 centimeters (2 ft. 11½ in.)
from the wall face, produced an almost imperceptible rent, a second or
a third blow applied at the same place extended this opening often to
a length of 1.50 meters (4 ft. 11 in.) and to a depth of from 90 to
120 centimeters (2 ft. 11 in. to 3 ft. 11 in.) The next blow opened
the fissure and detached the block of rock.
The application of the same system under water upon an unknown surface
would obviously modify the conditions of the experiment. Nevertheless,
the results obtained with the "Derocheuse," the first dredging machine
constructed upon this principle, have realized the hopes of the
inventor.
This dredging machine was launched on the Clyde and reached Port Said
in twenty days. It measures 55 meters (180 ft. 5 in.) in length, 12.20
meters (40 ft. 1 in.) in breadth, and 3.65 meters (12 ft.) in depth.
Its mean draught of water is 2.75 meters (9 ft. 2½ in.) It is divided
into eighteen watertight compartments. Five steel-pointed battering
rams, each of four tons weight, are arranged in line upon each side of
the chain of buckets of the dredging machine. See Figs. 1 and 2. The
battering rams, suspended by chains, are raised by hydraulic power to
a height varying from 1.50 to 6 meters (4 ft. 11 in. to 19 ft. 8 in.),
and are then let fall upon the rock. The mechanism of the battering
rams is carried by a metallic cage which can be moved forward or
backward by the aid of steam as the needs of the work require. A
series of five battering rams gives from 200 to 300 blows per hour.
[Illustration: FIG. 1.--LONGITUDINAL SECTION.]
[Illustration: FIG. 2.--PLAN]
A dredging machine combined with the apparatus just described, raises
the fragments of rock as they are detached from the bottom. A guide
wheel is provided, which supports the chain carrying the buckets, and
thus diminishes the stress upon the axles and bearings. With this
guide wheel or auxiliary drum there is no difficulty in dredging to a
depth of 12 meters (39 ft. 4 in.), while without this accessory it is
difficult to attain a depth of 9 meters (29 ft. 6 in.)
A compound engine, with four cylinders of 200 indicated horse power,
drives, by means of friction gear, the chain, which carries the
buckets. If the buckets happen to strike against the rock, the
friction gear yields until the excess of resistance has disappeared.
Fig. 3 indicates the manner in which the dredge is operated during the
work. It turns alternately about two spuds which are thrust
successively into the bottom and about which the dredge describes a
series of arcs in a zigzag fashion. These spuds are worked by
hydraulic power.
A three ton hand crane is placed upon the bridge for use in making
repairs to the chain which carries the buckets. A six ton steam crane
is placed upon the top of the cage which supports the hydraulic
apparatus for raising the battering rams, thus permitting them to be
easily lifted and replaced.
The dredging machine is also furnished with two screws driven by an
engine of 300 indicated horse power, as well as with two independent
boilers. Two independent series of pumps, with separate connections,
feed the hydraulic lifting apparatus, thus permitting repairs to be
made when necessary, without interrupting the work. A special machine
with three cylinders drives the pumps of the condenser. An accumulator
regulates the hydraulic pressure and serves to raise or lower the
spuds.
At the end of the Suez Canal next to the Red Sea, the bottom consists
of various conglomerates containing gypsum, sandstone and sometimes
shells. It was upon a bed of this nature that the machine was first
put to work. The mean depth of water, originally 8.25 meters (26 ft. 3
in.), was for a long time sufficient for the traffic of the canal; but
as the variations in level of the Red Sea are from 1.8 to 3 meters (5
ft. 11 in. to 9 ft. 10 in.), the depth at the moment of low water is
scarcely adequate for the constantly increasing draught of water of
the steamers. Attempts were made to attack the rocky surface of the
bottom with powerful dredges, but this method was expensive because it
necessitated constant repairs to the dredges.
[Illustration: FIG. 3.--DREDGE MOVEMENT.]
These last, although of good construction, seldom raised more than 153
cubic meters (200 cubic yards) in from eight to fifteen days. Their
daily advance was often only from sixty to ninety centimeters (about 2
to 3 ft.), while with the "Derocheuse" it was possible to advance ten
times as rapidly in dredging to the same depth. The bottom upon which
the machine commenced its work was clean and of a true rocky nature.
It was soon perceived that this conglomerate, rich in gypsum,
possessed too great elasticity for the pointed battering rams to have
their proper effect upon it. Each blow made a hole of from fifteen to
sixty centimeters (6 in. to 2. ft.) in depth. A second blow, given
even very near to the first, formed a similar hole, leaving the bed of
the rock to all appearances intact between the two holes. This result,
due entirely to the special nature of the rock, led to the fear that
the action of the battering rams would be without effect. After some
experimentation it was found that the best results were obtained by
arranging the battering rams very near to the chain of buckets and by
working the dredge and battering rams simultaneously. The advance at
each oscillation was about 90 centimeters (about 3 ft.)
The results obtained were as follows: At first the quantity extracted
varied much from day to day; but at the end of some weeks, on account
of the greater experience of the crew, more regularity was obtained.
The nature of the conglomerate was essentially variable, sometimes
hard and tenacious, like malleable iron, then suddenly changing into
friable masses surrounded by portions more elastic and richer in
gypsum.
During the last five weeks at Port Tewfik, the expense, including the
repairs, was 8,850 francs ($1,770.00) for 1,600 cubic meters (2,093
cubic yards) extracted. This would make the cost 5.52 francs per cubic
meter, or $0.84 per cubic yard, not including the insurance, the
interest and the depreciation of the plant.
After some improvements in details, suggested by practice, the machine
was put in operation at Chalouf upon a hard rock, from 1.50 to 3
meters (4 ft. 11 in. to 9 ft. 10 in.) thick. The battering rams were
given a fall of 1.80 meters (5 ft. 11 in.). To break the rock into
fragments small enough not to be rejected by the buckets of the
dredge, the operations of dredging and of disintegration were carried
on separately, permitting the battering rams to work at a greater
distance from the wall face. The time consumed in thus pulverizing the
rock by repeated blows was naturally found to be increased. It was
found more convenient to use only a single row of battering rams. The
production was from about seven to eleven cubic meters (9.2 to 14.4
cubic yards) per hour. Toward the close of September, after it had
been demonstrated that the "Derocheuse" was capable of accomplishing
with celerity and economy the result for which it was designed, it was
purchased by the Suez Canal Company.
During the month of September, an experiment, the details of which
were carefully noted, extending over a period of sixteen days, gave
the following results:
Crew (33 men), 140 hours. 2,012.50 francs $402.50
Coal, @ 87.50 francs ($7.50) per ton 787.50 francs 157.50
Oil and supplies 220.00 francs 44.00
Fresh water, 16 days 210.00 francs 42.00
Sundries 42.50 francs 8.50
---------------- ---------
Total expense for removing 764
cubic meters (999.2 cubic yards), 3,272.50 francs $654.50
Average, 4.28 francs per cubic meter ($0.65 per cubic yard).
This result cannot be taken as a universal basis, because after a
year's use there are numerous repairs to make to the plant, which
would increase the average net cost. This, besides, does not include
the cost of removal of the dredged material, nor the depreciation, the
interest and the insurance.
It should be added on the other hand, however, that the warm season
was far from being favorable to the energy and perseverance necessary
to carry on successfully experiments of this kind. The temperature,
even at midnight, was often 38° C. (100.4° F.). Still further, the work
was constantly interrupted by the passage of ships through the canal.
On an average not more than forty minutes' work to the hour was
obtained. Notwithstanding this, there were extracted at Chalouf, on an
average, 38.225 cubic meters (50 cubic yards) per day without
interrupting navigation. At Port Tewfik, where there was much less
inconvenience from the passage of ships, the work was carried on from
eight to eleven hours per day and the quantity extracted in this time
was generally more than 76 cubic meters (99.4 cubic yards).
In most cases the system could be simplified. The engine which works
the dredge could, when not thus employed, be used to drive the pumps.
The propelling engine could also be used for the same purpose.
The results obtained at Suez indicate the appreciable advantages
arising from the application of this system to the works of ports,
rivers and canals, and ever, to the work of cutting in the
construction of roads and railroads.
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