466 DREDGING to form but a small impediment. Fig. 2 is a section of the bore holes, and fig. 3 a plan in which the inner dotted circles repre sent the cliametejs of the broken spaces at the level of the bottom "Water Level FIG. 2. of dredging. The cartiidges were formed, in the ordinary way, with canvas, and fired with Bickford s fuse. The weights of powder used for bore-holes of 4 feet, 4 feet C inches, and 5 feet were respectively 2 ft, 3 ft, and 4 Ib. The effect of the shot was generally to lift the pipes which were secured by ropes to the Y I! I/ A s y s >; Y V V
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10 FEET
Fro. 3.
rafts a few inches. Mr Edwards says that not one in a hundred
shots missed fire, and these shots were generally saved by the fol
lowing singular expedient : The pointed end of an iron bar,
inch diameter, was made red hot, and, being put quickly through
the water, and driven through the tamping as rapidly as possible,
was, in nine cases out of ten, sufficiently hot to ignite the gun
powder and fire the shot.
The cost of each shot is calculated as follows :
Use of material ..................................... 1
Labour .............................................. 03
Pitched bag for charge ........................... 00
3 Ib of powder at 5^d ............................ 1
15 ft. of patent fuse at r of a penny ........
Pitch, tallow, twine, coals, &c .................
Cost per shot 7
Each shot loosened and prepared for dredging about four cubic
yards, so that the cost of blasting was Is. 9d. per yard. The cost
of dredging the material, after it had been thus prepared, was 2s.
3d. , making the whole charge for removing the marl 4s. per cubic
yard. "
One of the most recent successful combinations of
blasting and dredging was that completed in 1875 by Mr
John Fowler of Stockton at the river Tees, to whom the
writer is indebted for the following particulars. The chief
novelty was in the barge upon which the machinery was
fixed. It was 58 feet by 28 feet by 4 feet, and had eight
legs which were let down when the barge was in position.
The legs were then fixed to the barge, so that on the tide
falling it became a fixed platform from which the drilling
was done. The holes were bored and charged, and when
the tide rose the legs were heaved up and the barge removed,
after which the shota were discharged. There were 24
boring tubes on the barge, and that was the limit which
could at any time be done in one tide. The surface over
which the blasting was done measured 500 yards in length
by 200 yards in breadth, a small part of that surface being
uncovered at low water. The depth obtained in mid-
channel was 14 feet at low-water, the average depth of rock
blasted being about 4 feet 6 inches. The holes, which were
bored with the diamond drill, varied in depth from 7 to 9
feet, the distance between them being 10 feet. Dynamite
in tin canisters fired by patent fuse was used as the ex
plosive, the charges being 2 Ib and under. The rock is
Oolite shale of variable hardness, and the average time
occupied in drilling 5-feet holes was twelve minutes.
The dredger raised the blasted rock, the cost for blast
ing, lifting, and discharging at sea being about 4s. per
cubic yard, including interest on dredging and other plant
employed. The dredger sometimes worked a face of blasted
material of from 7 to 8 feet. The quantity blasted was
110,000 cubic yards, and the contract for blasting so as to
be lifted by the dredger was 3s. Id. per cubic yard.
Dredging in Exposed Situations. In some cases dredg
ing has to be conducted in exposed situations such as the
deepening of the " flats " at Londonderry and the bar at
Carlingford. Messrs Stevenson found that dredging at the
Foyle could not be conducted when the height of the waves
exceeded 2| feet ; and Mr Barton at Dundalk so far con
firms this, as he estimates a swell of 2 feet as the highest
to work in.
Dredging on the River Clyde. An important point con
nected witla this subject is the cost at which dredging may
be done when conducted on a large scale. This, of course,
must depend on the character of the stuff to be raised and
other circumstances ; but the following information, kindly
communicated by Mr James Deas, the engineer to the
Trustees of the Clyde Navigation, cannot fail to be both
interesting and iiseful.
Mr Deas says truly that the Clyde Trustees employ pro
bably the largest dredging fleet of any trust in the kingdom,
in maintaining and still deepening and widening the river
to meet the ever-increasing demands of the shipping trade.
In the year 1871, for example, 904,104 cubic yards, or
about 1,130,000 tons, were dredged from the river, of which
689,560 cubic yards were carried to sea by steam hopper
barges, and 214,544 cubic yards deposited on land by
means of punts. Of this 904,104 cubic yards, 345,209
cubic yards were deposit from the higher reaches of the
river and its tributaries, and from the city sewers, and
558,895 cubic yards new material. The total cost for
dredging and depositing was .35,448, or about 9 41 pence
per cubic yard.
Owing to the difference in power of the dredging
machines employed, and the character of the material
lifted, the cost of dredging varies much. In 1871 the
most powerful machine, working 2420 hours, lifted 430,240
cubic yards of silt and so,nd at a cost of 2 60 pence per
yard ; and this was deposited in Loch Long, 27 miles
from Glasgow, by steam hopper barges, at 5 46 pence per
yard. On the other hand, another dredger, working 2605
hours, lifted only 26,720 cubic yards of hard gravel and
boulder clay, at the cost of 20 "8 pence per cubic yard,
which was deposited on the alveus of the river at the cost
of 17 46 pence per cubic yard; another, working 18312-
hours, lifted 122,664 cubic yards of silt, sand, and sewage
deposit, at the cost of 5 67 pence per cubic yard, which was
deposited on land at the cost of 16 40 pence per cubic
yard; and another, working 2233 hours, lifted 65,160
cubic yards of till, gravel, and sand, at the cost of 5 89
pence per cubic yard, which was deposited on the alveus
of the river at the cost of 9 83 pence per cubic yard.
The total quantity dredged from the river during the
