Dredging is the name given by engineers to the process of excavating
materials under water, raising them to the surface and depositing them
in barges, or delivering them through a shoot, a longitudinal conveyor,
or pipes, to the place where it is desired to deposit them. It has long
been useful in works of marine and hydraulic engineering, and has been
brought in modern times to a state of high perfection.
The employment of dredging plant and the selection of special appliances
to be used in different localities and in varying circumstances require
the exercise of sound judgment on the part of the engineer. In rivers
and estuaries where the bottom is composed of light soils, and where the
scour of the tide can be governed by training walls and other works
constructed at reasonable expense, so as to keep the channel clear
without dredging, it is manifest that dredging machinery with its large
cost for working expenses and for annual upkeep should be as far as
possible avoided. On the other hand, where the bottom consists of clay,
rock or other hard substances, dredging must, in the first instance at
any rate, be employed to deepen and widen the channel which it is sought
to improve. In some instances, such as the river Mississippi, a deep
channel has for many years been maintained by jetties, with occasional
resort to dredging to preserve the required channel section and to
hasten its enlargement. The bar of the river Mersey is 11 m. from land,
and the cost of training works would be so great as to forbid their
construction; but, by a capital expenditure of L120,000 and an annual
expense of L20,000 for three years, the depth of water over the bar at
low tide has been increased by dredging from 11 ft. to 27 ft., the
channel being 1500 ft. wide.
_"Bag and Spoon" Dredger._--The first employment of machinery for
dredging is, like the discovery of the canal lock, claimed by Holland
and Italy, in both of which countries it is believed to have been in use
before it was introduced into Britain. The Dutch, at an early period,
used what is termed the "bag and spoon" dredger for cleansing their
canals. The "spoon" consisted of a ring of iron about 2 ft. in diameter
flattened and steeled for about a third of its circumference and having
a bag of strong leather attached to it by leathern thongs. The ring and
bag were fixed to a pole which was lowered to the bottom from the side
of a barge moored in the canal or river. The "spoon" was then dragged
along the bottom by a rope made fast to the iron ring actuated by a
windlass placed at the other end of the barge, the pole being prevented
from rising by a hitched rope which caused the "spoon" to penetrate the
bottom and fill the bag. When the "spoon" reached the end of the barge
where the windlass was placed, the winding was still continued, and the
suspended rope being nearly perpendicular the "bag" was raised to the
gunwale of the barge and the excavated material emptied into the barge.
The "bag" was then hauled back to the opposite end to be lowered for
another supply. This system is still in use, but is only adaptable to a
limited depth of water and a soft bottom; it has been largely used in
canals and frequently in the Thames. At the Fosdyke Canal in
Lincolnshire 135,000 tons were raised in the manner described. According
to J. J. Webster (_Proc. Inst. C. E._ vol. 89), the first application
of steam power for dredging operations was to a "spoon & bag" dredger
for cleansing Sunderland harbour, the engine being made by Messrs
Boulton & Watt of Soho, Birmingham.
_Dredging by Bucket between Two Lighters._--Another plan of dredging,
practised at an early period in rivers of considerable breadth, was to
moor two barges, one on each side of the river. Between them was slung
an iron dredging bucket, which was attached to both barges by chains
wound on the barrels of a crab winch worked by six men in one barge and
round a simple windlass worked by two men in the other barge. The
bucket, being lowered at the side of the barge carrying the windlass,
was drawn across the bottom of the river by the crab winch on the other
barge; and having been raised and emptied, it was hauled across by the
opposite windlass for repetition of the process. This process was in use
in the River Tay until 1833.
_Bucket Ladder Dredgers._--The earliest record of a bucket ladder
dredger is contained in the first paper of the first volume (1836) of
the _Transactions_ of the Institution of Civil Engineers. This machine
was brought into use at the Hull Docks about 1782. The bucket chain was
driven by two horses working a horse-gear on the deck of the vessel. The
buckets were constructed of 5/8 in. bars of iron spaced 1/8 in. apart,
and were 4 ft. long, 13 in. deep, 12 in. wide at the mouth and about 6
in. wide at the bottom. This dredger raised about 30 tons per hour at
the cost of 2-1/2d. per ton, which covered the wages of three men working
the dredger, eight men working the lighters and the keep of three
horses. A dredger of this kind and power would only work in ballast, mud
or other soft material, but the machine was gradually improved and
increased in capacity and power by different manufacturers until it
became a very efficient machine in skilful hands, excavating and raising
material from depths of 5 ft. to 60 ft. of water at a cost not very
different from, and in many cases less than, that at which the same work
could be performed on land. With the powerful dredgers now constructed,
almost all materials, except solid rock or very large boulders, can be
dredged with ease. Loose gravel is perhaps the most favourable material
to work in, but a powerful dredger will readily break up and raise
indurated beds of gravel, clay and boulders, and has even found its way
through the surface of soft rock, though it will not penetrate very far
into it. In some cases steel diggers alternating with the buckets on the
bucket frame have been successfully employed. The construction of large
steam dredgers is now carried on by many engineering firms. The main
feature of the machine is the bucket ladder which is hung at the top end
by eye straps to the frame of the vessel, and at the lower end by a
chain reived in purchase blocks and connected to the hoisting gear, so
that the ladder may be raised and lowered to suit the varying depths of
water in which the dredger works. The upper tumbler for working the
bucket chain is generally square or pentagonal in form and made of steel
with loose steel wearing pieces securely bolted to it. The tumbler is
securely keyed to the steel shaft which is connected by gearing and
shafting to the steam engine, a friction block being inserted at a
convenient point to prevent breakage should any hidden obstacle causing
unusual strain be met with in the path of the buckets. The lower tumbler
is similar in construction to the upper tumbler, but is usually
pentagonal or hexagonal in shape. The buckets are generally made with
steel backs to which the plating of the buckets is riveted; the cutting
edge of the buckets consists of a strong steel bar suitably shaped and
riveted to the body. The intermediate links are made of hammered iron or
steel with removable steel bushes to take the wear of the connecting
pins, which are also of steel. The hoisting gear may be driven either
from the main engine by frictional gearing or by an independent set of
engines. Six anchors and chains worked by powerful steam crabs are
provided for regulating the position of the dredger in regard to its
work.
_Barge-loading Dredgers_ used formerly to be provided with two ladders,
one on each side of the vessel, or contained in wells formed in the
vessel near each side. Two ladders were adopted, partly to permit the
dredger to excavate the material close to a quay or wall, and partly to
enable one ladder to work while the other was being repaired. Bucket
ladder dredgers are now, however, generally constructed with one central
ladder working in a well; frequently the bucket ladder projects at
either the head or stern of the vessel, to enable it to cut its own way
through a shoal or bank, a construction which has been found very
useful. In one modification of this method the bucket ladder is
supported upon a traversing frame which slides along the fixed framing
of the dredger and moves the bucket ladder forward as soon as it has
been sufficiently lowered to clear the end of the well. In places where
a large quantity of dredging has to be done, a stationary dredger with
three or four large hopper barges proves generally to be the most
economical kind of plant. It has, however, the disadvantage of requiring
large capital expenditure, while the dredger and its attendant barges
take up an amount of space which is sometimes inconvenient where traffic
is large and the navigable width narrow. The principal improvements made
in barge-loading dredgers have been the increase in the size of the
buckets and the strength of the dredging gear, the application of more
economical engines for working the machinery, and the use of frictional
gearing for driving the ladder-hoisting gear. It is very important that
the main drive be fitted with the friction blocks or clutches before
alluded to.
Up to the year 1877 dredgers were seldom made with buckets of a
capacity exceeding 9 cub. ft., but since that time they have been
gradually increased in capacity. In the dredger "Melbourne,"
constructed by Messrs William Simons & Co. to the design and
specification of Messrs Coode, Son & Matthews, about the year 1886,
the buckets had a capacity of 22 cub. ft., the dredger being capable
of making 37 ft. of water. The driving power consists of two pairs of
surface-condensing engines, each of 250 i.h.p., having cylinders 20
in. and 40 in. in diameter respectively, with a 30 in. stroke, the
boiler pressure being 90 lb. per sq. in. The vessel is 200 ft. long
by 36 ft. wide and 11 ft. 6 in. deep, and is driven by twin screw
propellers. The gearing is arranged so that either pair of engines can
be employed for dredging. The speed under steam is 7 knots, and in
free-getting material 800 tons per hour can be dredged with ease. On
one occasion the dredger loaded 400 tons in 20 minutes. The speed of
the bucket chain is 83 lineal ft. per minute. The draught of the
dredger in working trim is 7 ft. forward and 9 ft. aft. The efficiency
of the machine, or the net work in raising materials compared with the
power exerted in the cylinders, is about 25%. The dredged material is
delivered into barges moored alongside. Contrasting favourably with
former experience, the "Melbourne" worked for the first six months
without a single breakage. She is fitted with very powerful mooring
winches, a detail which is of great importance to ensure efficiency in
working.
The "St Austell" (Plate I. fig. 3), a powerful barge-loading dredger
195 ft. long by 35 ft. 6 in. beam by 13 ft. deep, fitted with
twin-screw compound surface-condensing propelling engines of 1000
i.h.p., either set of engines being available for dredging, was
constructed for H.M. Dockyard, Devonport, by Messrs Wm. Simons & Co.
in 1896. This dredger loaded thirty-five 500-ton hopper barges in the
week ending April 2, 1898, dredging 17,500 tons of material in the
working time of 29 hours 5 minutes.
An instance of a still larger and more powerful dredger is the
"Develant," constructed by Messrs Wm. Simons & Co., for Nicolaiev,
South Russia. She is a bow-well, barge-loading, bucket ladder dredger,
with a length of 186 ft., a breadth, moulded, of 36 ft., and a depth,
moulded, of 13 ft. The bucket ladder is of sufficient length to dredge
36 ft. below the water level. The buckets are exceptionally large,
each having a capacity of 36 cub. ft., or fully two tons weight of
material, giving a lifting capacity of 1890 tons per hour. At the
dredging trials 2000 tons of spoil were lifted in one hour with an
expenditure of 250 i.h.p. The propelling power is supplied by one pair
of compound surface-condensing marine engines of 850 i.h.p., having
two cylindrical boilers constructed for a working pressure of 120 lb.
per sq. in. Each boiler is capable of supplying steam to either the
propelling or dredging machinery, thus allowing the vessel to always
have a boiler in reserve. On the trials a speed of 8-1/2 knots was
obtained. The bucket ladder, which weighs over 100 tons, exclusive of
dredgings, is raised and lowered by a set of independent engines. For
manoeuvring, powerful winches driven by independent engines are placed
at the bow and stern. The vessel is fitted throughout with electric
light, arc lamps being provided above the deck to enable dredging to
be carried on at night. Steam steering gear, a repairing shop, a
three-ton crane, and all the latest appliances are installed on board.
The "Derocheuse" (Plate II. fig. 12), constructed by Messrs Lobnitz &
Co., is a good example of the dredger fitted with their patent rock
cutters, as used on the Suez Canal. These rock cutters consist of
stamps passing down through the bottom of the dredger, slightly in
advance of the bucket chain, and are employed for breaking up rock in
front of the bucket ladder so that it may be raised by buckets
afterwards. This system of subaqueous rock cutting plant, on Messrs
Lobnitz's patent system, was effectively employed in deepening the
Manchester Ship Canal, and removed a considerable length of rock,
increasing the depth of water from 26 ft. to 28 ft. at a cost of about
9d. per cub. yd. A full and illustrated description of this plant, and
of a similar plant supplied to the Argentine Government, was published
in _Engineering_ of August 17, 1906. An illustration of a bucket of 54
cub. ft. capacity constructed by Messrs Lobnitz & Co. is given (Plate
II fig. 11), from which some idea of the size of dredging machinery as
developed in recent practice may be obtained. In regard to the depth
of water that can be obtained by dredging, it is interesting to note
that the dredger "Diver," constructed by Messrs. Hunter & English for
Mr Samuel Williams of London, is capable of working in 60 ft. of
water. In this vessel an ingenious arrangement was devised by Mr
Williams, by which part of the weight of the dredger was balanced
while the ladder itself could be drawn up through the bucket well and
placed upon the deck, enabling a long ladder to be used for a
comparatively short vessel. The "Tilbury" dredger, also constructed by
Messrs Hunter & English, was able to dredge to a depth of 45 ft. below
the surface of the water.
_Hopper Barges._--To receive the materials excavated by barge-loading
dredgers, steam hopper barges are now generally employed, capable of
carrying 500 tons or more of excavation and of steaming loaded at a
speed of about 9 m. per hour. These hopper barges are made with hinged
flaps in their bottoms, which can be opened when the place of deposit is
reached and the dredgings easily and quickly discharged.
Good examples of these vessels are the two steam hopper barges built for
the Conservators of the river Thames in 1898. The dimensions are: length
190 ft., breadth 30 ft., depth 13 ft. 3 in., hopper capacity 900 tons.
They are propelled by a set of triple expansion engines of 1200 i.h.p.,
with two return-tube boilers having a working pressure of 160 lb.
Special appliances are provided to work the hopper doors by steam power
from independent engines placed at the forward end of the hopper. A
steam windlass is fixed forward and a steam capstan aft. The vessels are
fitted with cabins for the officers and crew. On their trial trip, the
hoppers having their full load, a speed of 11 knots was obtained, the
coal consumption being 1.44 lb. per i.h.p.
_Methods of Dredging._--In river dredging two systems are pursued. One
plan consists in excavating a series of longitudinal furrows parallel to
the axis of the stream; the other in dredging cross furrows from side to
side of the river. It is found that inequalities are left between the
longitudinal furrows when that system is practised, which do not occur,
to the same extent, in side or cross dredging; and cross dredging leaves
a more uniform bottom. In either case the dredger is moored from the
head and stern by chains about 250 fathoms in length. These chains in
improved dredgers are wound round windlasses worked by the engine, so
that the vessel can be moved ahead or astern by simply throwing them
into or out of gear. In longitudinal dredging the vessel is worked
forward by the head chain, while the buckets are at the same time
performing the excavation, so that a longitudinal trench is made in the
bottom of the river. After proceeding a certain length, the dredger is
stopped and permitted to drop down and commence a new longitudinal
furrow, parallel to the first one. In cross dredging, on the other hand,
the vessel is supplied with four additional moorings, two on each side,
and these chains are, like the head and stern chains, wound round
barrels worked by steam power. In cross dredging we may suppose the
vessel to be moored at one side of the channel to be excavated. The
bucket frame is set in motion, but instead of the dredger being drawn
forward by the head chain, she is drawn across the river by the
starboard chains, and, having reached the extent of her work in that
direction, she is then drawn a few feet forward by the head chain, and
the bucket frame being still in motion the vessel is hauled across by
the port chains to the side whence she started. By means of this
transverse motion of the dredger a series of cross cuts is made; the
dredger takes out the whole excavation from side to side to a uniform
depth and leaves no protuberances such as are found to exist between the
furrows in longitudinal dredging, even when it is executed with great
care. The two systems will be understood by reference to fig. 1, where A
and B are the head and stern moorings, and C, D, E and F the side
moorings. The arc e f represents the course of the vessel in cross
dredging; while in longitudinal dredging, as already explained, she is
drawn forward towards A, and again dropped down to commence a new
longitudinal furrow.
[Illustration: Fig. 1.--Diagram showing Moorings for Transverse
Dredging.]
_Hopper Dredgers._--In places where barge-loading dredgers are
inconvenient, owing to confined space and interference with navigation,
and where it is necessary to curtail capital expenditure, hopper
dredgers are convenient and economical. These dredgers were first
constructed by Messrs. Wm. Simons & Co. of Renfrew, who patented and
constructed what they call the "Hopper Dredger," combining in itself the
advantages of a dredger for raising material and a scow hopper vessel
for conveying it to the place of discharge, both of which services are
performed by the same engines and the same crew.
The vessel for this type of dredger is made of sufficient length and
floating capacity to contain its own dredgings, which it carries out to
the depositing ground as soon as its hopper is full. Considerable time
is of course occupied in slipping and recovering moorings, and conveying
material to the depositing ground, but these disadvantages are in many
instances counterbalanced by the fact that less capital is required for
plant and that less room is taken up by the dredger. If the depositing
ground is far away, the time available for dredging is much curtailed,
but the four-screw hopper dredger constructed by Messrs Wm. Simons & Co.
for Bristol has done good work at the cost of 5d. per ton, including
wages, repairs, coals, grease, sundries and interest on the first cost
of the plant, notwithstanding that the material has to be taken 10 m.
from the Bristol Dock. She can lift 400 tons of stiff clay per hour from
a depth of 36 ft. below the water line, and the power required varies
from 120 i.h.p. to 150 i.h.p., according to the nature of the material.
The speed is 9 knots, and 4 propellers are provided, two at the head and
two at the stern, to enable the vessel to steam equally well either way,
as the river Avon is too narrow to permit her to be turned round.
The hopper dredger "La Puissante" (Plate I. fig. 4), constructed by
Messrs Wm. Simons & Co. for the Suez Canal Co. for the improvement of
Port Said Roads, is a fine example of this class of dredger. She is
275 ft. long by 47 ft. beam by 19 ft. deep. The hopper capacity is
2000 tons, and the draught loaded 16 ft. 5 in. The maximum dredging
depth is 40 ft., and the minimum dredging depth is only limited by the
vessel's draught, she being able to cut her own way. The bucket ladder
works through the well in the stern and weighs with buckets 120 tons.
The buckets have each a capacity of 30 cub. ft. and raised on trial
1600 tons per hour. The dredger is propelled by two sets of
independent triple expansion surface-condensing engines of 1800 i.h.p.
combined, working with steam at 160 lb. pressure, supplied by two mild
steel multitubular boilers. Each set of engines is capable of driving
the buckets independently at speeds of 16 and 20 buckets per minute.
The bucket ladder is fitted with buffer springs at its upper end to
lessen the shock when working in a seaway. The dredger can deliver the
dredged material either into its own hopper or into barges lying on
either side. The vessel obtained a speed of 9-3/4 knots per hour on
trial. The coal consumption during 6 hours' steaming trial was 1.66
lb. per i.h.p. hour. Fig. 9 (Plate I.) shows a still larger hopper
dredger by the same constructors.
_Dredgers fitted with Long Shoot or Shore Delivering Apparatus._--The
first instance of dredgers being fitted with long shoots was in the Suez
Canal. The soil in the lakes was very variable, the surface being
generally loose mud which lay in some places in the sand, but frequently
more or less on hard clay. Resort was had to shoots 230 ft. long,
supported on pontoons connected with the hull of the dredger. The sand
flowed away with a moderate supply of water to the shoots when they were
fixed at an inclination of about 1 in 20, but when the sand was mixed
with shells these formed a coating which prevented the stream of water
from washing out the shoot, and even with an inclination of 1 in 10
material could not be delivered. A pair of endless chains working down
the long shoot overcame the difficulty, and also enabled hard clay in
lumps to be dealt with. One dredger turned out about 2000 cub. yds. of
thick clay in 15 hours, and when the clay was not hard it could deliver
150,000 cub. yds. in a month for several consecutive months.
Shore delivery has been successfully effected by raising the material by
buckets in the ordinary way and delivering it into a vertical cylinder
connected with floating jointed pipes through which the dredgings pass
to the shore. This, of course, can only be done where the place of
deposit is near the spot where the material is dredged. Two plans have
been satisfactorily employed for this operation. At the Amsterdam Canal
the stuff was discharged from the buckets into a vertical cylinder, and
after being mingled with water by a revolving Woodford pump was sent off
under a head of pressure of 4 or 5 ft. to the place of deposit in a
semi-fluid state through pipes made of timber, hooped with iron. These
wooden pipes were made in lengths of about 15 ft., connected with
leather joints, and floated on the surface of the water. A somewhat
similar process was also employed on the Suez Canal.
A dredger (Plate I. fig. 5), constructed by Messrs Hunter & English
for reclamation works on Lake Copais in Greece was fitted with
delivery belts running on rollers in steel lattice frames on each side
of the vessel supported by masts and ropes. It could deliver 100 cub.
metres per hour at 85 ft. from the centre of the dredger, at a cost of
1.82d. per cub. metre for working expenses, with coal at 45s. per ton,
including 0.66d. per cub. metre for renewal of belts, upon which the
wear and tear was heavy.
Another instance of the successful application of shore delivery
apparatus is that of a dredger for Lake Titicaca, Peru, constructed by
Messrs Hunter & English, which was fitted with long shoots on both
sides, conveying the dredged material about 100 ft. from the centre of
the dredger upon either side. The shoots were supported by shear-legs
and ropes, and were supplied with water from a centrifugal pump in the
engine room. This dredger could excavate and deliver 120 cub. yds. per
hour at a cost of 1.725d. per cub. yd. with coal costing 40s. per ton.
If coal had been available at the ordinary rate in England of 20s. per
ton, the cost of the dredging and delivery would have been 0.82d. per
cub. yd. for wages, coal, oil, &c., but not including the salary of
the superintendent.
An interesting example of a shore delivering dredger is a light
draught dredger constructed by Messrs Hunter & English for the Lakes
of Albufera at the mouth of the river Ebro in Spain (Plate I. fig. 6).
The conditions laid down for this dredger were that it should float in
18 in. of water and deliver the dredged material at 90 ft. from the
centre of its own hull. In order to meet these requirements the vessel
was made of steel plates 1/8 in. thick, and longitudinal girders from
end to end of the vessel, the upward strain of flotation being
conveyed to them from the skin plating by transverse bulkheads at
short intervals. The dredger was 94 ft. long, 25 ft. wide, and 3 ft.
deep, and the height of the top tumbler above the water was 25 ft.
When completed the dredger drew 17 in. of water. The dredgings were
delivered by the buckets upon an endless belt, driven from the main
compound surface-condensing engine, which ran over pulleys supported
upon a steel lattice girder, the outer end of which rested upon an
independent pontoon. This belt delivered the dredgings at 90 ft. from
the centre of the dredger round an arc of 180 deg. The dredger
delivered 125 cub. yds. per hour of compact clay at a cost of 1.16d.
per cub. yd. or 0.86d. per ton for wages, coal and stores. Another
method of delivering dredgings is that of pneumatic delivery,
introduced by Mr F. E. Duckham, of the Millwall Dock Co., by which the
dredgings are delivered into cylindrical tanks in the dredger, closed
by air-tight doors, and are expelled by compressed air either into the
sea or through long pipes to the land. The Millwall Dock dredger is
113 ft. long, with a beam of 17 ft. and a depth of 12 ft. The draught
loaded is 8 ft. It contains two cylindrical tanks, having a combined
capacity of 240 cub. yds., and is fitted with compound engines of
about 200 i.h.p., with a 20 in. air-compressing cylinder. The
discharge pipe is 15 in. diameter by 150 yds. long. The nozzles of the
air-injection pipes must not be too small, otherwise the compressed
air, instead of driving out the material, simply pierces holes through
it and escapes through the discharging pipe, carrying with it all the
liquid and thin material in the tanks. The cost of working the
Millwall Dock dredger is given by Mr Duckham at 1.75d. per cub. yd. of
mud lifted, conveyed and deposited on land 450 ft. from the
water-side, for working expenses only. This dredger is believed to be
the first machine constructed with a traversing ladder, as suggested
by Captain Gibson when dock-master of the Millwall Docks.
_Blasting combined with Dredging._--In some cases it has been found that
the bottom is too hard to be dredged until it has been to some extent
loosened and broken up. Thus at Newry, John Rennie, after blasting the
bottom in a depth of from 6 to 8 ft. at low water, removed the material
by dredging at an expense of from 4s. to 5s. per cub. yd. The same
process was adopted by Messrs Stevenson at the bar of the Erne at
Ballyshannon, where, in a situation exposed to a heavy sea, large
quantities of boulder stones were blasted, and afterwards raised by a
dredger worked by hand at a cost of 10s. 6d. per cub. yd. Sir William
Cubitt also largely employed blasting in connexion with dredging on the
Severn (see _Proc. Inst. C.E._ vol. iv. p. 362). The cost of blasting
and dredging the marl beds is given as being 4s. per cub. yd. A
combination of blasting and dredging was employed in 1875 by John Fowler
of Stockton at the river Tees. The chief novelty was in the barge upon
which the machinery was fixed. It was 58 ft. by 28 ft. by 4 ft., 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. Holes were bored and
charged, and when the tide rose the legs were heaved up and the barge
removed, after which the shots 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 area over which the blasting was done measured 500
yds. in length by 200 in breadth, a small part being uncovered at low
water. The depth obtained in mid-channel was 14 ft. at low water, the
average depth of rock blasted being about 4 ft. 6 in. The holes, which
were bored with the diamond drill, varied in depth from 7 to 9 ft., the
distance between them being 10 ft. Dynamite in tin canisters fired by
patent fuse was used as the explosive, the charges being 2 lb. and
under. The rock is oolite shale of variable hardness, and the average
time occupied in drilling holes 5 ft. deep was 12 minutes. The dredger
raised the blasted rock. The cost for blasting, lifting and discharging
at sea was about 4s. per cub. yd., including interest on dredging and
other plant employed. The dredger sometimes worked a face of blasted
material of from 7 to 8 ft. The quantity blasted was 110,000 cub. yds.,
and the contract for blasting so as to be lifted by the dredger was 3s.
1d. per cub. yd. A similar plan was adopted at Blyth Harbour (see _Proc.
Inst. C.E._ vol. 81, p. 302). The cost of the explosives per cub. yd.
was 1s. 4d., of boring 1s. 9d. per cub. yd., and of dredging 3s. per
cub. yd., including repairs, but nothing for the use of plant. The whole
cost worked out at 6s. 1d. per cub. yd. on the average.
_Sand-pump Dredgers._--Perhaps the most important development which has
taken place in dredging during recent years has been the employment of
sand-pump dredgers, which are very useful for removing sandy bars where
the particular object is to remove quickly a large quantity of sand or
other soft material. They are, however, apt to make large holes, and are
therefore not fitted for positions where it is necessary to finish off
the dredging work to a uniform flat bottom, for which purpose bucket
dredgers are better adapted. Pump dredgers are, however, admirable and
economical machines for carrying out the work for which they are
specially suited.
In the discussion upon Mr J. J. Webster's paper upon
"Dredging-Appliances" (_Proc. Inst. C.E._ vol. 89) at the Institution
of Civil Engineers in 1886, Sir John Coode stated that he had first
seen sand-pump dredgers at the mouth of the Maas in Holland. The
centrifugal pump was placed against the bulkheads in the after part of
the vessel, and the sand and water were delivered into a horizontal
breeches-piece leading into two pipes running along the full length of
the hopper. The difficulty of preventing the sand from running
overboard was entirely obviated by its being propelled by the pump
through these pipes, the bottoms of which were perforated by a series
of holes. In addition, there were a few small flap-doors fixed at
intervals, by means of which the men were able to regulate the
discharge. On being tested, the craft pumped into its hopper 400 tons
of sand in 22 minutes. The coamings round the well of the hoppers were
constructed with a dip, and when the hopper was full the water ran
over in a steady stream on either side. The proportion of sand
delivered into the hopper was about 20% of the total capacity of the
pump. The dredger was constructed by Messrs Smit of Kinderdijk, near
Rotterdam. In the same discussion Mr A. A. Langley, then engineer to
the Great Eastern railway, gave particulars of a sand pump upon the
Bazin system, which had been used successfully at Lowestoft. The boat
was 60 ft. long by 20 ft. wide, and the pump was 2 ft. in diameter,
with a two-bladed disk. The discharge pipe was 12 in. in diameter. The
pump raised 400 tons of sand, gravel and stones per hour as a maximum
quantity, the average quantity being about 200 tons per hour. The
depth dredged was from 7 ft. to 25 ft. The pump was driven by a
double-cylinder engine, having cylinders of 9 in. diameter by 10 in.
stroke, and making 120 revolutions per minute. An important
improvement was made by fitting the working faces of the pump with
india-rubber, which was very successful and largely reduced the wear
and tear. The cost of the dredging at Lowestoft was given by Mr
Langley at 2d. per ton, including delivery 2 m. out at sea. The
quantity dredged was about 200,000 tons per annum.
One of the earliest pumps to be applied to dredging purposes was the
Woodford, which consisted of a horizontal disk with two or more arms
working in a case somewhat similar to the ordinary centrifugal pump.
The disk was keyed to a vertical shaft which was driven from above by
means of belts or other gear coupled to an ordinary portable engine.
The pump within rested on the ground; the suction pipe was so arranged
that water was drawn in with the sand or mud, the proportions being
regulated to suit the quality of the material. The discharge pipe was
rectangular and carried a vertical shaft, the whole apparatus being
adjustable to suit different depths of water. This arrangement was
very effective, and has been used on many works. Burt & Freeman's sand
pump, a modification of the Woodford pump, was used in the
construction of the Amsterdam Ship Canal, for which it was designed.
The excavations from the canal had to be deposited on the banks some
distance away from the dredgers, and after being raised by the
ordinary bucket dredger, instead of being discharged into the barges,
they were led into a vertical chamber on the top side of the pump,
suitable arrangements being made for regulating the delivery. The pump
was 3-1/2 ft. in diameter, and made about 230 revolutions per minute.
The water was drawn up on the bottom side and mixed with the
descending mud on the top side, and the two were discharged into a
pipe 15 in. in diameter. The discharge pipe was a special feature, and
consisted of a series of wooden pipes jointed together with leather
hinges and floated on buoys from the dredger to the bank. In some
cases this pipe was 300 yds. long, and discharged the material 8 ft.
above the water level. Each dredger and pump was capable of
discharging an average of 1500 cub. yds. per day of 12 hours.
Schmidt's sand pump is claimed to be an improvement on the Burt &
Freeman pump. It consists of a revolving wheel 6 ft. in diameter, with
cutters revolving under a hood which just allows the water to pass
underneath. To the top side of the hood a 20 in. suction pipe from an
ordinary centrifugal pump is attached. The pump is driven by two 16
in. by 20 in. cylinders, at 134 revolutions per minute, the boiler
pressure being 95 lb. per sq. in. This apparatus is capable of
excavating sticky blue clayey mud, and will deliver the material at
500 to 650 yds. distance. The best results are obtained when the
mixture of mud and water is as 1 to 6.5. The average quantity
excavated per diem by the apparatus is 1300 cub. yds., the maximum
quantity being 2500 cub. yds.
Kennard's sand pump is entirely different from the pumps already
described, and is a direct application of the ordinary lift pump. A
wrought iron box has a suction pipe fitted at the bottom, rising about
half way up the inside of the box; on the top of the box is fitted the
actual pump and the flap valves. The apparatus is lowered by chains,
and the pump lowered from above. As soon as the box is filled with
sand it is raised, the catches holding up the bottom released, and the
contents discharged into a punt.
Sand-pump dredgers, designed and arranged by Mr Darnton Hutton, were
extensively used on the Amsterdam Ship Canal. A centrifugal pump with
a fan 4 ft. in diameter was employed, the suction and delivery pipes,
each 18 in. in diameter, being attached to an open wrought-iron
framework. The machine was suspended between guides fixed to the end
of the vessel, which was fitted with tackle for raising, lowering and
adjusting the machine. The vessel was fitted with a steam engine and
boiler for working and manipulating the pumps and the heavy side
chains for the guidance of the dredger. The engine was 70 h.p., and
the total cost of one dredger was L8000. The number of hands required
for working this sand-pump dredger was one captain, one engineer, one
stoker and four sailors. Each machine was capable of raising about
1300 tons of material per day, the engines working at 60 and the pump
at 180 revolutions per minute. The sand was delivered into barges
alongside the dredger. The cost of raising the material and depositing
it in barges was about 1d. per ton when the sand pumps were working,
but upon the year's work the cost was 2.4d. per cub. yd. for working
expenses and repairs, and 1.24d. per cub. yd. for interest and
depreciation at 10% upon the cost of the plant, making a total cost
for dredging of 3.64d. per cub. yd. The cost for transport was 3.588d.
per cub. yd., making a total cost for dredging and transport of
7.234d. per cub. yd. Dredging and transport on the same works by an
ordinary bucket dredger and barges cost 8.328d. per cub. yd.
Two of the largest and most successful instances of sand-pump dredgers
are the "Brancker" and the "G. B. Crow," belonging to the Mersey
Docks and Harbour Board. Mr A. G. Lyster gave particulars of the work
done by these dredgers in a paper read before the Engineering Congress
in 1899. They are each 320 ft. long, 47 ft. wide and 20.5 ft. deep,
the draught loaded being 16 ft. They are fitted with two centrifugal
pumps, each 6 ft. in diameter, with 36 in. suction and delivery pipes,
united into a 45 in. diameter pipe, hung by a ball and socket joint in
a trunnion, so as to work safely in a seaway when the waves are 10 ft.
high. The suction pipe is 76 ft. long and will dredge in 53 ft. of
water. The eight hoppers hold 3000 tons, equivalent when solid to 2000
cub. yds.; they can be filled in three-quarters of an hour and
discharged in five minutes. Mr Lyster stated that up to May 1899, the
quantity removed from bar and main-channel shoals amounted to
41,240,360 tons, giving a width of channel of 1500 ft. through the
bar, with a minimum depth of 27 ft. The cost of dredging on the bar by
the "G. B. Crow" during 1898, when 4,309,350 tons of material were
removed, was 0.61d. per ton for wages, supplies and repairs. These
figures include all direct working costs and a proportion of the
charge for actual superintendence, but no allowance for interest on
capital cost or depreciation. On an average, 20% of the sand and mud
that are raised escapes over the side of the vessel. Mr Lyster has,
however, to a considerable extent overcome this difficulty by a
special arrangement added to the hoppers (see _Proc. Inst. C.E._ vol.
188).
At the Engineering Conference, 1907, Mr Lyster read a note in which he
stated that the total quantity of material removed from the bar of the
Mersey, from the Crosby channel, and from other points of the main
channel by the "G. B. Crow" and "Brancker" suction dredgers amounted
to 108,675,570 tons up to the 1st of May 1907. "In the note of 1899
(he added) it was pointed out that the Mersey was a striking instance
of the improvement of a river by dredging rather than by permanent
works, and the economy of the system as well as the advantage which
its elasticity and adaptability to varying circumstances permit, was
pointed out.... The most recent experience, which has resulted in the
adoption of the proposal to revet the Taylor's bank, indicates that
the dredging method has its limitations and cannot provide for every
contingency which is likely to arise; at the same time, the utility
and economy of the dredging system is in no way diminished.... Having
regard to the ever-increasing size of vessels, a scheme for new docks
and entrances on a very large scale received the authority of
parliament during the session of 1905-1906 In this scheme it was
considered necessary to make provision for vessels of 1000 ft. in
length and 40 ft. in draught, and having regard to this prospective
growth of vessels it has been determined still further to deepen and
improve the outer channel of the Mersey. No fixed measure of
improvement has been decided on, but after careful survey of existing
conditions and a comparison with probable requirements, it has been
determined to construct a dredger of 10,000 tons capacity, provided
with pumping power equivalent to about three times that of any
existing dredgers. By the use of this vessel it is anticipated that it
will be possible to deal with very much larger quantities of sand at a
cheaper rate, and to 10 ft. greater depth than the existing plant
permits."
The vessel in question was launched on the Mersey from the yard of
Messrs Cammell, Laird & Co. in October 1908, and was named the
"Leviathan." Her length is 487 ft., beam 69 ft., and depth 30 ft. 7
in. Her dredging machinery consists of four centrifugal pumps driven
by four sets of inverted triple expansion engines, and connected to
four suction tubes 90 ft. long and 42 in. in internal diameter. Her
propelling machinery, consisting of two sets of triple expansion
engines, is capable of driving her at a speed of 10 knots.
Another powerful and successful sand-pump dredger, "Kate" (Plate I.
fig. 7), was built in 1897 by Messrs Wm. Simons & Co. Ltd. for the
East London Harbour Board, South Africa. Its dimensions are: length
200 ft., breadth 39 ft., depth 14 ft. 6 in., hopper capacity 1000
tons. The pumping arrangements for filling the hopper with sand or
discharging overboard consist of two centrifugal pumps, each driven
from one of the propelling engines. The suction pipes are each 27 in.
in diameter, and are so arranged that they may be used for pumping
either forward or aft, as the state of the weather may require. Four
steam cranes are provided for manipulating the suction pipes. Owing to
the exceptional weather with which the vessel had to contend, special
precautions were taken in designing the attachments of the suction
pipes to the vessel. The attachment is above deck and consists of a
series of joints, which give a perfectly free and universal movement
to the upper ends of the pipes. The joints, on each side of the
vessel, are attached to a carriage, which is traversed laterally by
hydraulic gear. By this means the pipes are pushed out well clear of
the vessel's sides when pumping, and brought inboard when not in work.
Hydraulic cushioning cylinders are provided to give any required
resistance to the fore and aft movements of the pipes. When the vessel
arrived at East London on the 18th of July 1897, there was a depth of
14 ft. on the bar at high tide. On the 10th of October, scarcely three
months afterwards, there was a depth of 20 ft. on the bar at low
water. Working 22 days in rough weather during the month of November
1898, the "Kate" raised and deposited 2-1/2 m. at sea 60,000 tons of
dredgings. Her best day's work (12 hours) was on the 7th of November,
when she dredged and deposited 6440 tons.
A large quantity of sand-pump dredging has been carried out at
Boulogne and Calais by steam hopper pump dredgers, workable when the
head waves are not more than 3 ft. high and the cross waves not more
than 1-1/2 ft. high. The dredgings are taken 2 m. to sea, and the price
for dredging and depositing from 800,000 to 900,000 cub. metres in 5
or 6 years was 7.25d. per cub. yd. The contractor offered to do the
work at 4.625d. per cub. yd. on condition of being allowed to work
either at Calais or Boulogne, as the weather might permit. Sand-pump
dredging has also been extensively carried out at the mouth of the
ports of Amsterdam, Rotterdam and on the north coast of France by sand
dredgers constructed by Messrs L. Smit & Son and G. & K. Smit. The
largest dredger, the "Amsterdam," is 141 ft. by 27 ft. by 10 ft. 8
in., and has engines of 190 i.h.p. The hopper capacity is 10,600 cub.
ft., and the vessel can carry 600 tons of dredgings. The pump fan is 6
ft. 3 in. in diameter by 10 in. wide, the plates being of wrought
iron, and makes 130 revolutions a minute. The pump can raise 230 cub.
ft. a minute from a depth of 33 ft., which, taking the proportion of 1
of sand to 7 of water, gives a delivery of 29 cub. ft. of sand per
minute. The hopper containing 10,600 cub. ft. was under favourable
circumstances filled in 40 minutes. The vessels are excellent sea
boats.
_Combined Bucket-Ladder and Sand-Pump Dredgers._--Bucket ladders and
sand pumps have also been fitted to the same dredger. A successful
example of this practice is furnished by the hopper dredger "Percy
Sanderson" (Plate I. fig. 8), constructed under the direction of Sir C.
A. Hartley, engineer of the Danube Commission for the deepening of the
river Danube and the Sulina bar. This dredger is 220 ft. by 40 ft. by 17
ft. 2 in., and has a hopper capacity for 1250 tons of dredgings. The
buckets have each a capacity of 25 cub. ft., and are able to raise 1000
tons of ordinary material per hour. The suction pump, which is driven by
an independent set of triple expansion engines, is capable of raising
700 tons of sand per hour, and of dredging to a depth of 35 ft. below
the water-line. The lower end of the suction pipe is controlled by
special steam appliances by which the pipe can be brought entirely
inboard. The "Percy Sanderson" raises and deposits on an average 5000
tons of material per day.
_Grab Dredgers._--The grab dredger was stated by Sir Benjamin Baker
(_Proc. Inst. C.E._ vol. 113, p. 38) to have been invented by Gouffe in
1703, and was worked by two ropes and a bar. Various kinds of apparatus
have been designed in the shape of grabs or buckets for dredging
purposes. These are usually worked by a steam crane, which lets the open
grab down to the surface of the ground to be excavated and then closes
it by a chain which forces the tines into the ground; the grab is then
raised by the crane, which deposits the contents either into the hopper
of the vessel upon which the crane is fixed or into another barge.
The Priestman grab has perhaps been more extensively used than any
other apparatus of this sort. It is very useful for excavating mud,
gravel and soft sand, but is less effective with hard sand or stiff
clay--a general defect in this class of dredger. It is also capable of
lifting large loose pieces of rock weighing from 1 to 2 tons. A
dredger of this type, with grab holding 1 ton of mud, dredged during
six days, in 19 ft. of water, an average of 52-1/2 tons and a maximum
of 68-1/2 tons per hour, and during 12 days, in 16 ft. of water, an
average of 48 tons and a maximum of 58 tons per hour, at a cost of
1.63d. per ton, excluding interest on the capital and depreciation.
The largest dredger to which this apparatus has been applied is the
grab bucket hopper dredger "Miles K. Burton" (Plate I. fig. 9),
belonging to the Mersey Docks and Harbour Board. It is equipped with 5
grabs on Morgan's patent system, which is a modification of
Priestman's, the grabs being worked by 5 hydraulic cranes. It raised
and deposited, 12 to 15 m. at sea, 11 loads of about 1450 tons each
with a double shift of hands, at a cost of about 1s. 5d. per cub. yd.
of spoil, including the working expenses for wages of crew, fuel and
stores. Mr R. A. Marillier of Hull has stated that "the efficiency of
these grabs is not at all dependent upon the force of the blow in
falling for the penetration and grip in the material, as they do their
work very satisfactorily even when lowered quite gently on to the
material to be cut out, the jaws being so framed as to draw down and
penetrate the material as soon as the upward strain is put on the
lifting chain. Even in hard material the jaws penetrate so thoroughly
as to cause the bucket to be well filled. The grab is found to work
successfully in excavating hard clay from its natural bed on dry
land." It is claimed on behalf of grabs that they lift a smaller
proportion of water than any other class of dredger.
Since the beginning of the 20th century considerable advance has been
made in the use of Priestman grabs, not only for dredging and
excavating (for which work they were originally designed), but also in
discharging bulk cargo. The first quadruple dredger used by the
Liverpool Docks Board had grabs of a capacity of 30 cub. ft., but
subsequently second and third quadruple dredgers were put to work in
the Liverpool Docks, with grabs having a capacity of 70 and 100 cub.
ft. respectively. In discharging coal at Southampton, Havre, Erith,
as well as at the coaling station at Purfleet on the Thames, grabs
having a capacity of about 80 cub. ft. are in constant use. Perhaps
the most difficult kind of bulk cargo to lift is "Narvick" iron ore,
which sets into a semi-solid body in the holds of the vessels, and for
this purpose one of the largest grabs, having about 150 cub. ft.
capacity and weighing about 8 tons, has been adopted. This grab was
designed as a result of experiments extending over a long period in
lifting iron ore. It is fitted with long, forged, interlocked steel
teeth for penetrating the compact material, which is very costly to
remove by hand labour. The Priestman grab is made to work with either
one or two chains or wire ropes. Grabs worked with two chains or ropes
have many advantages, and are therefore adopted for large
undertakings.
Wild's single chain half-tine grab works entirely with a single chain,
and has been found very useful in excavating the cylinders in Castries
harbour. Upon experimenting with an ordinary grab a rather curious
condition of things was observed with respect to sinking. On
penetrating the soil to a certain depth the ground was found as it
were nested, and nothing would induce the grab to sink lower. Sir W.
Matthews suggested that a further set of external tines might possibly
get over this difficulty. A new grab having been made with this
modification, and also with a large increase of weight--all the parts
being of steel--it descended to any required depth with ease, the
outside tines loosening the ground effectually whilst the inside
bucket or tines picked up the material.
_Miscellaneous Appliances._--There are several machines or appliances
which perhaps can hardly be called dredgers, although they are used for
cleansing and deepening rivers and harbours.
Kingfoot's dredger, used for cleansing the river Stour, consisted of a
boat with a broad rake fitted to the bow, capable of adjustment to
different depths. At the sides of the boat were hinged two wings of
the same depth as the rake and in a line with it. When the rake was
dropped to the bottom of the river and the wings extended to the side,
they formed a sort of temporary dam, and the water began to rise
gradually. As soon as a sufficient head was raised, varying from 6 to
12 in., the whole machine was driven forward by the pressure, and the
rake carried the mud with it. Progress at the rate of about 3 m. an
hour was made in this manner, and to prevent the accumulation of the
dredgings, operations were begun at the mouth of the river and carried
on backwards. The apparatus was very effective and the river was
cleansed thoroughly, but the distance travelled by the dredger must
have been great.
In 1876 J. J. Rietschoten designed a "propeller dredger" for removing
the shoals of the river Maas. It consisted of an old gunboat fitted
with a pair of trussed beams, one at each side, each of which carried
a steel shaft and was capable of being lowered or raised by means of a
crab. An ordinary propeller 3 ft. 6 in. in diameter was fixed to the
lower end of the shaft, and driven by bevel gear from a cross shaft
which derived its motion by belting from the fly-wheel of a 12 h.p.
portable engine. The propellers were lowered until they nearly reached
the shoals, and were then worked at 150 revolutions per minute. This
operation scoured away the shoal effectively, for in about 40 minutes
it had been lowered about 3 ft. for a space of 150 yds. long by 8 yds.
wide.
A. Lavalley in 1877 designed an arrangement for the harbour of Dunkirk
to overcome the difficulty of working an ordinary bucket-ladder
dredger when there is even a small swell. A pump injects water into
the sand down a pipe terminating in three nozzles to stir up the sand,
and another centrifugal pump draws up the mixed sand and water and
discharges it into a hopper, the pumps and all machinery being on
board the hopper. To allow for the rising and falling of the
vessel--either by the action of the tide or by the swell--the ends of
the pipes are made flexible. The hopper has a capacity of 190 cub.
yds., and is propelled and the pumps worked by an engine of 150 i.h.p.
From 50 to 80 cub. yds. per hour can be raised by this dredger.
The "Aquamotrice," designed by Popie, and used on the Garonne at Agen,
appears to be a modification of the old bag and spoon arrangement. A
flat-bottomed boat 51-1/2 ft. long by 6-1/2 ft. wide was fitted at the
bow with paddles, which were actuated by the tide. Connected with the
paddles was a long chain, passing over a pulley on uprights and under
a roller, and a beam was attached to the chain 14 ft. 8 in. long,
passing through a hole in the deck. At the end of the beam was an iron
scoop 2 ft. wide and 2 ft. 6 in. deep. When the tide was strong enough
it drew the scoop along by means of the paddles and chains, and the
scoop when filled was opened by a lever and discharged. About 65 cub.
yds. of gravel could be raised by the apparatus in 12 hours. When the
tide failed the apparatus was worked by men.
The Danube Steam Navigation Co. removed the shingle in the shallow
parts of the river by means of a triangular rake with wrought-iron
sides 18 ft. long, and fitted with 34 teeth of chilled cast iron 12
in. deep. This rake was hung from the bow of a steamer 180 ft. long by
21 ft. beam, and dragged across the shallows, increasing the depth of
water in one instance from 5 ft. 6 in. to 9 ft., after passing over
the bank 355 times.
A combination of a harrow and high pressure water jets, arranged by B.
Tydeman, was found very efficacious in removing a large quantity of
mud which accumulated in the Tilbury Dock basin, which has an area of
about 17 acres, with a depth of 26 ft. at low-water spring tides. In
the first instance chain harrows merely were used, but the addition of
the water jets added materially to the success of the operation. The
system accomplished in six tides more than was done in twelve tides
without the water jets which worked at about 80 lb pressure per sq.
in. at the bottom of the dock.
Ive's excavator consists of a long weighted spear, with a sort of
spade at the end of it. The spade is hinged at the top, and is capable
of being turned at right angles to the spear by a chain attached to
the end of the spear. The spade is driven into the ground, and after
releasing the catch which holds it in position during its descent, it
is drawn up at right angles to the spear by the chain, carrying the
material with it. Milroy's excavator is similar, but instead of having
only one spade it generally has eight, united to the periphery of an
octagonal iron frame fixed to a central vertical rod. When these eight
spades are drawn up by means of chains, they form one flat table or
tray at right angles to the central rod. In operation the spades hang
vertically, and are dropped into the material to be excavated; the
chains are then drawn up, and the table thus formed holds the material
on the top, which is lifted and discharged by releasing the spade.
This apparatus has been extensively used both in Great Britain and in
India for excavating in bridge cylinders.
The clam shell dredger consists of two hinged buckets, which when
closed form one semi-cylindrical bucket. The buckets are held open by
chains attached to the top of a cross-head, and the machine is dropped
on to the top of the material to be dredged. The chains holding the
bucket open are then released, while the spears are held firmly in
position, the buckets being closed by another chain. Bull's dredger,
Gatmell's excavator, and Fouracre's dredger are modifications with
improvements of the clam shell dredger, and have all been used
successfully upon various works.
Bruce & Batho's dredger, when closed, is of hemispherical form, the
bucket being composed of three or four blades. It can be worked by
either a single chain or by means of a spear, the latter being
generally used for stiff material. The advantage of this form of
dredger bucket is that the steel points of the blades are well adapted
for penetrating hard material. Messrs Bruce & Batho also designed a
dredger consisting of one of these buckets, but worked entirely by
hydraulic power. This was made for working on the Tyne. The excavator
or dredger is fixed to the end of a beam which is actuated by two
hydraulic cylinders, one being used for raising the bucket and the
other for lowering it; the hydraulic power is supplied by the pumps in
the engine-room. The novelty in the design is the ingenious way in
which the lever in ascending draws the shoot under the bucket to
receive its contents, and draws away again as the bucket descends. The
hydraulic cylinder at the end of the beam is carried on gimbals to
allow for irregularities on the surface being dredged. The hydraulic
pressure is 700 lb. per sq. in., and the pumps are used in connexion
with a steam accumulator.
An unloading apparatus was designed by Mr A. Manning for the East &
West India Dock Co. for unloading the dredged materials out of barges
and delivering it on the marsh at the back of the bank of the river
Thames at Crossness, Kent. A stage constructed of wooden piles
commanded a series of barge beds, and the unloading dredger running
from end to end of the stage, lifted and delivered the materials on
the marsh behind the river wall at the cost of 1 d. per cub. yd.
_Dredging on the River Scheldt below Antwerp._--This dredging took place
at Krankeloon and the Belgian Sluis under the direction of L. Van
Gansberghe. At Melsele there is a pronounced bend in the river, causing
a bar at the Pass of Port Philip, and just below the pass of Lillo there
is a cross-over in the current, making a neutral point and forming a
shoal. After dredging to 8 metres (26.24 ft.) below low tide, in clay
containing stone and ferruginous matter, a sandstone formation was
encountered, which was very compact and difficult to raise. A suction
dredger being unsuited to the work, a bucket-ladder dredger was
employed. The dredging was commenced at Krankeloon in September 1894 and
continued to the end of 1897. A depth of 6 metres (19.68 ft.) was
excavated at first, but was afterwards increased to 8 metres (26.24
ft.). The place of deposit was at first on lands acquired by the State,
2.17 m. above Krankeloon, and placed at the disposal of the contractor.
The dredgings excavated by the bucket-ladder dredger were deposited in
scows, which were towed to the front of the deposit ground and
discharged by a suction pump fixed in a special boat, moored close to
the bank of the river. The material brought by the suction dredger in
its own hull was discharged by a plant fixed upon the dredger itself. In
both instances the material was deposited at a distance of 1640 ft. from
the river, the spoil bank varying in depth from 2 to 7 metres. The water
thrown out behind the dyke with the excavated material returned to the
river, after settlement, by a special discharge lock built under the
dyke. After 1896 the material was delivered into an abandoned pass by
means of barges with bottom hopper doors or by the suction dredger. One
suction dredger and three bucket-ladder dredgers were employed upon the
work, and a vessel called "Scheldt I." used for discharging the material
from the scows. Four tugboats and twenty scows were also employed.
The largest dredger, "Scheldt III.," was 147.63 ft. long by 22.96 ft.
wide by 10.98 ft. deep, and had buckets of 21.18 cub. ft. capacity.
The output per hour was 10,594 cub. ft. This dredger had also a
complete installation as a suction dredger, the suction pipe being 2
ft. diameter. The fan of the centrifugal pump was 5.25 ft. diameter,
and was driven by the motor of the bucket ladder. The three bucket
dredgers worked with head to the ebb tide. They could also work with
head to the flood tide, but it took so long a time to turn them about
that it was impracticable. The work was for from 13 to 14 hours a day
on the ebb tide. The effective daily excavation averaged 4839 cub.
yds. Each dredger was fitted with six anchors. The excavated cut was
164 ft. wide by 6.56 ft. deep. "Scheldt III." was capable of lifting a
mass 9.84 ft. thick. The suction dredger "Scheldt II." was of the
multiple type, and is stated to be unique in construction. It can
discharge material from a scow alongside, fill its own hopper with
excavations, discharge its own load upon the bank or into a scow by
different pipes provided for the purpose, and discharge its own load
through hopper doors. The machinery is driven by a triple expansion
engine of 300 i.h.p. working the propeller by a clutch. Owing to the
rise and fall in the tide of 23 ft. the suction pipe is fitted with
spherical joints and a telescopic arrangement. The vessel is 157.5 ft.
by 28.2 ft. by 12.8 ft. The diameter of the pump is 5.25 ft. The wings
of the pump are curved, the surface being in the form of a cylinder
parallel to the axis of rotation, the directrix of which is an arc of
a circle of 2.62 ft. radius with the straight part beyond. The suction
and discharge pipes are 2 ft. diameter. A centrifugal pump is provided
for throwing water into the scows to liquefy the material during
discharge. The dredger, which is fitted with electric lights for work
at night, is held by two anchors, to prevent lurching backwards and
forwards; it can work on the flood as well as on the ebb tide, and can
excavate to a depth of 42.65 ft., the output depending upon the nature
of the material. With good material it can fill its tanks in thirty
minutes. To empty the tanks by suction and discharge upon the bank
over the dyke takes about fifty minutes, depending upon the height and
distance to which the material requires to be delivered. The daily
work has averaged eighteen hours, ten trips being made when the
distance from the dredging ground to the point of delivery is about 1
m. When the dredged material is discharged into the Scheldt, a
quantity of 5886 cub. yds. has been raised and deposited in a day, the
mean quantity being 4700 cub. yds. When the distance of transportation
is increased to 2-1/2 m., six voyages were made in a day, and the day's
work amounted to 3530 cub. yds.
_Gold Dredgers._--Dredgers for excavating from river beds soil
containing gold are generally fitted with a screen and elevator.
[Illustration: FIG. 2.--Diagram showing Action of Lobnitz Gold Dredger.]
They have been extensively designed and built by Messrs Lobnitz & Co.
(fig. 2) and also by Messrs Hunter & English.
The writer is indebted to the _Proceedings_ of the Institution of
Civil Engineers, and especially to the paper of Mr J. J. Webster
(_Proc. Inst. C.E._ vol. 89), for much valuable information upon the
subject treated. He is also indebted to many manufacturers who have
furnished him with particulars and photographs of dredging plant.
(W. H.*)
Plate I.
[Illustration: FIG. 3.--Barge-loading dredger, "St Austell," constructed
for the British Government by Wm. Simons & Co.]
[Illustration: FIG. 4.--Stern-well hopper-dredger "La Puissante," by Wm.
Simons & Co. Length 275 ft., breadth 47 ft., depth 19 ft.]
[Illustration: FIG. 5.--Dredger constructed for the Lake Copais Co. by
Hunter & English.]
[Illustration: FIG. 6.--Light-draught dredger, with delivery apparatus
working round an arc of 210 deg., by Hunter & English.]
[Illustration: FIG. 7.--Twin-screw sand-pump dredger, "Kate," built for
the East London Harbour Board by Wm. Simons & Co.]
[Illustration: FIG. 8.--Twin-screw hopper-dredger, "Percy Sanderson,"
built for the European Danube Commission by Wm. Simons & Co.]
[Illustration: FIG. 9.--Twin-screw grab-dredger, "Miles K. Burton,"
built for the Mersey Docks and Harbour Board by Wm. Simons & Co.]
[Illustration: FIG. 10.--Hopper-dredger, "David Dale," with buckets of
54 cub. ft. capacity (see fig. 11) built for the North Eastern Railway
Company by Lobnitz & Co.]
Plate II.
[Illustration: FIG. 11.--BUCKETS OF 5 AND 54 CUBIC FEET CAPACITY
COMPARED.
The latter, the largest ever made, were for the hopper-dredger "David
Dale" (Plate I. fig. 10), built by Lobnitz & Co.]
[Illustration: FIG. 12.--MODEL OF ROCK-CUTTING DREDGER, "DEROCHEUSE."
Built for special work on the Suez Canal by Lobnitz & Co. Length 180
ft., breadth 40 ft., depth 12 ft.]