_DOCK AND PIER WORKS._
A.D. 1831--1859. ÆTATIS 26--54.
MONKWEARMOUTH DOCKS, 1831--BRISTOL DOCKS, FLOATING HARBOUR,
1832--PROPOSED WORKS AT PORTISHEAD--NEW LOCK AT BRISTOL,
1845--PLYMOUTH GREAT WESTERN DOCKS, 1847--BRITON FERRY DOCKS,
1851--BRENTFORD DOCK, 1856--PIER AT MILFORD HAVEN, 1857.
Mr. Brunel’s dock and pier works are interesting, not only in their
general features, but also in the details of their construction; and the
plans he made for large docks at Monkwearmouth in 1831, which he carried
out on a smaller scale shortly afterwards, were among the earliest of
his independent designs.
With the exception of the gates of this dock, which are of timber, all
the dock gates Mr. Brunel constructed are of wrought iron. This material
had been employed in ship-building before Mr. Brunel adopted it for the
gates of the new lock at Bristol, and it was also beginning to be
extensively used for bridge girders.
At the same time that he introduced the use of wrought iron into dock
gates, he constructed them with a large amount of buoyancy, in order
that they might be moved easily, while being opened and shut.
The dock and pier works which he constructed are at Monkwearmouth,
Bristol, Plymouth, Briton Ferry, Brentford, and at Neyland, Milford
Haven. They will be described in this order, which is nearly that of
the dates of their construction.
_Monkwearmouth Docks._
The town of Monkwearmouth is situated at the mouth of the river Wear, on
the north side, and opposite to the towns of Sunderland and
Bishopwearmouth, which extend for about a mile along the south side. In
order to improve the entrance of the river, and to diminish the
sand-banks which lay near its mouth, piers were proposed as early as the
middle of the last century, and were partly built on both sides of the
river before the year 1800. From that date until 1831, although the
question of making docks had been considered, and designs proposed by
different engineers, no steps had been taken for their construction, and
the only works executed for the improvement of the port were the
extension and alteration of the piers already existing. In 1831 designs
for docks to accommodate the increasing traffic were made simultaneously
by Mr. Brunel and Mr. Giles. Mr. Brunel’s docks were to have been on the
north side of the river, and to have had an area of 25 acres, with
quays, warehouses, &c. Mr. Giles’s were to have been on the south side.
Neither of these schemes was approved of by Parliament; but shortly
afterwards a private company was formed for the construction of a dock
on a plan designed by Mr. Brunel, though on a much smaller scale than
his scheme of 1831, the dock being only about 6 acres in area, with a
tidal basin of about an acre and a half. The company encountered
considerable opposition from the authorities of the town of Sunderland,
but succeeded in obtaining a royal charter for the construction of the
dock. They subsequently obtained an Act of Parliament empowering them
to make the entrance from the dock to the river. The dock was
constructed, and eventually became the property of the North Eastern
Railway Company, to whom it now belongs; they have erected coal drops
along the quay, and have made it a shipping place for collieries
connected with their railway.
The work was begun in 1834, and the dock and tidal basin occupy part of
the site chosen by Mr. Brunel for his larger scheme of 1831.
The quay wall was built with a curved batter, the chord line joining the
top and bottom having an inclination of 1 in 5. The masonry was carried
up in courses, and made solid by filling every part thoroughly with
mortar. A course at the face and at the back of the wall was built up;
an abundance of mortar was then spread in the heart of the wall, and the
stones built in the mortar. Thus no crevices could be left in any part
of the work, and the back of the wall was soundly built throughout.
The entrance to the dock is 45 feet wide, with side walls of the same
profile as the quay wall. Except at the gate floors, there is a
segmental invert of dressed stone of such curvature that it is 6 feet 6
inches lower in the middle than at the side walls. The gate floors are
formed with inverts, curved to correspond with the under sides of the
gates.
The masonry of the entrance was executed within a four-sided coffer-dam,
the sides of which were slightly convex outwards. This coffer-dam was
constructed in the usual way, there being two rows of close piling with
puddle between them; and it was strengthened by internal horizontal
shores which connected the opposite sides, and by diagonal bracing. The
piles were driven until they met with so much resistance as to render it
unsafe to drive them farther. When the ground inside the coffer-dam was
excavated, it was found that the piles had been driven into sand and
gravel, and that, to enable the masonry to be built on a good
foundation, it would be necessary to excavate about 7 or 8 feet below
the piles. They were therefore driven down gradually, as the ground was
removed from the inside, until the requisite depth was obtained. The
whole coffer-dam was thus an immense caisson, the sides of which were
lowered by gradual driving, instead of being simultaneously forced down
by weights.
The masonry of the walls of the tidal basin is similar to that of the
walls of the dock; some parts of the foundation were laid by means of a
diving-bell.
In the entrance between the dock and the tidal basin there is a pair of
gates pointing inwards, which serves to retain the water in the dock
during the fall of the tide, and there is also a pair of storm gates
pointing outwards, which protects the inner pair from the force of the
waves.
The construction of both pairs of gates is similar.
The two leaves of each pair meet at an angle of 125°. Each leaf is about
30 feet long; the bottom beams are curved to the form of the segment of
a circle; the height at the meeting-post is 27 feet, and at the heel
post 22 feet (see woodcut, fig. 17). This arrangement is, to a greater
or less extent, followed in the dock gates Mr. Brunel afterwards
constructed. By raising the pivot, the gate floor can be made of ample
strength, and the cills and heel-posts are free from mud and deposit.
The gates are constructed of horizontal beams of yellow pine timber, 21
inches thick, placed close together for a height of 12 feet above the
bottom. Above this there are beams of timber and of cast iron at
intervals. The whole is planked over on the inner side with 4-inch
planking. The heel-post and meeting-post are socketed into cast-iron
uprights, which also receive the ends of the horizontal beams. To
preserve the gate from any change of form, a diagonal iron tie-bar
extends from the top of the heel-post to the timber beams forming the
lower part of the gate.
[Illustration: Fig. 17. Monkwearmouth Dock Gates.
_Elevation. Section.
Plan.
Scale of feet._]
Nearly under each meeting-post is placed a bevelled cast-iron wheel, 18
inches in diameter, which supports part of the weight of the gate.
There are four sluices in each leaf, placed in pairs, with a small
interval between them. Each pair of sluices counterbalances the weight
of the other pair by being attached to opposite ends of a lever at the
top of the gate. A screw works in the segment of a large worm-wheel
formed on the end of the lever, and, being turned round, opens and shuts
the sluices. After the timber work of the gate had been fitted together,
it was taken to pieces, and subjected to the preserving process called
Kyanising, which consists in immersing the wood in a solution of
corrosive sublimate. This process has been so successful, that when the
gates were recently taken out for examination the timber was found to
be nearly perfect, only slight surface repairs being required in one or
two places.
The great bulk of light wood at the bottom of these gates gives them a
certain amount of flotation at all times of tide. After the gates had
been in use many years it was found that one of the wheels had been
detached for some time, but the buoyancy of the gate had prevented any
mischief resulting. The buoyancy of the lower part of the gate is
somewhat analogous to that of the air-chamber which Mr. Brunel
introduced afterwards in his wrought-iron dock gates.
_Bristol Docks._
About the year 1804 that portion of the river Avon which flows in a
serpentine course through the city of Bristol was enclosed, and the
water in it retained at a constant level, a new cut or shorter channel
being made for the river. The portion separated, called the Floating
Harbour, or Float, is about two miles long and 100 yards broad. At its
lower end it is connected with the river by the Cumberland Basin, a
half-tide basin, with two locks, and at its upper end by a feeder, which
brings the water of the Avon into it, the river in dry weather being
stopped from passing into the new cut by the Neetham Dam, About half-way
up, the Float is entered on the north side by the river Frome; and, a
little above this junction, it is crossed by the Prince’s Street
drawbridge, which divides it into two parts. About 170 feet above the
bridge the Float is connected with the new cut by another basin with a
lock, called Bathurst Basin.
Mud and other deposits had accumulated to such an extent in the
Floating Harbour, that at the end of the year 1832 the directors of the
Dock Company employed Mr. Brunel to suggest remedial measures.
In order to effect his object at the least possible cost, he proposed
certain works, together with an improved system of managing the water of
the river, so as to allow more of it to pass through the Floating
Harbour, by means of which great benefit might fairly be
anticipated.[178] He remarked that,--
By systematically following this course, the object of which is
simply to keep in continual action all the means, however small,
which can at the moment be brought to bear, and thus day by day to
remove or neutralise, or merely diminish (as the case may be), the
continual deposit which is going on--in fact, by applying a
constantly acting remedy to oppose a constantly acting evil--I have
little doubt that the formation of shoals similar to the existing
ones may be entirely prevented, or at all events that they will be
of such a nature as to be easily removed by two or three yearly
scourings, and without that time and labour which are now expended
with so little effect.
It should be observed that the yearly scourings, which became so
objectionable to the trade, were not introduced by Mr. Brunel, but were
part of the original arrangements of the docks.
After the reception of Mr. Brunel’s report, the Dock Company executed
the works which he required, namely, the Sluice, Trunk, and Drag-Boat;
but his other recommendations, as to scouring and increased supply of
water, were only acted upon to a limited extent.
* * * * *
In 1842 the Directors again asked Mr. Brunel to report, in conjunction
with Captain Claxton, upon ‘what further measures are requisite for
keeping the Floating Harbour more clear of mud than it has been for a
few years.’
Mr. Brunel thereupon made a report to the Directors. After having
referred to his previous reports of 1833 and 1834, he remarked that ‘the
efficiency of the whole system then recommended and adopted, and
subsequently partially carried out, depended entirely and was founded on
the supposition of the then existing mud-banks and shoals being first
removed, and the Float deepened at once to the full extent required,’
according to the plans which he had pointed out; and that ‘the
increasing the supply of water through the Float was one of those means
on which he had most insisted’ as necessary for keeping it clean and
preventing its becoming a settling reservoir. He then continued,--
The sluice at Prince’s Street Bridge, the trunk--or syphon, as it
was originally, and perhaps more correctly, called--at the
underfalls, and the drag-boat have alone been brought into
operation. These were originally intended as mere aids, which, in
conjunction with _the increased supply of water_, were expected,
after the complete deepening of the Float, to be sufficient, with
_two or three yearly scourings_, to keep it to the required depth:
these were (perhaps unfortunately) found so effective as to induce
a hope that they might be depended upon solely for the removal of
the evil. The permanent interests of the port were, I cannot but
think, sacrificed to temporary convenience: the scourings which
were required as a preliminary step to restore the Float to its
original state, or to that which was said to have been its former
state, and which is now required, were indefinitely postponed.
A material improvement being notwithstanding soon perceptible from
the first effects of the drag-boat and the removal of mud through
the trunk, the periodical scourings which formed part of the system
approved of for adoption were in a great measure given up to the
objections of the traders. The precautions actually necessary
against admitting into the Float the tide water of the Avon,
heavily charged with mud, were gradually sacrificed for the same
reasons. From all these and many other, but very similar,
circumstances no further progress has been made since the first
improvement, which was felt to be, and which unquestionably was at
the time, very considerable. For until this period it had been the
general practice to lighten all deeply laden vessels at the
entrance of the Float; and, notwithstanding this precaution, it
must be in the recollection of everybody that it was a common sight
to see several large vessels aground at various shoals in the
Float, and unable, without further discharging the cargoes, and
without great consumption of time, labour, and ropes, to get up to
the quays. For several years past the grounding of the deepest
vessels has been the exception, not the rule, but during all this
period it has been one continued and almost vain attempt to
struggle against the old difficulties with insufficient means.
* * * * *
For the removal of such deposits as will still be formed, I propose
two means; and first as regards those deposits which are
continually going on. These are formed almost entirely of mud,
which, from its want of consistency when first deposited, the great
quantity and the large surface over which it extends, as well as
the great depth of water, cannot, as I have frequently explained,
be easily or economically removed by the ordinary process of
dredging: for this reason I originally proposed the drag-boat in
conjunction with the trunk at the underfalls, and which has, so far
as it has been applied, completely answered my expectations.
I should now propose a similar arrangement for the upper float.
* * * * *
I should propose to make the additional drag-boat thus required of
rather greater power than that now in use, and to construct it so
that a chain of dredging-buckets could be hereafter attached to the
shaft; and, secondly, for the purposes of deepening the hard bottom
of the float, and of removing those banks of hard materials which
have either always existed, or have been allowed to accumulate,
dredging must be resorted to. But I should be disposed to attempt
hand dredging or spooning in the first instance; for, although the
depth of water is great, I believe the work could be executed as
cheaply, or nearly so, as by the steam dredging, as the original
outlay of capital would of course be much less, while the facility
of working at several points at once, or of moving from one berth
to another as the convenience of the trade would best allow, would
be much greater; the operation also would be much more under
command, which, taking into consideration the possibility of
undermining the foundations of the quay walls or buildings, is not
an unimportant advantage.
When the required depths are once obtained, the natural deposits
even of the harder description may probably be easily removed by
the drag-boat or by the occasional use of hand dredging.
The recommendations contained in this report were not adopted.
In 1839 Mr. Brunel, in a report to a Committee of the Council of the
City of Bristol, suggested several improvements connected with the Port,
almost all of which have since been undertaken. He proposed to
straighten and widen the course of the river, and to make new locks,
both from the river to Cumberland Basin and from that basin to the
Floating Harbour, or, as an alternative, to construct docks at Sea
Mills, a creek on the Avon, about two miles below Bristol.
He also proposed to construct a large pier at Portishead. The rise of
tide is there sometimes 45 feet, and the velocity of rise or fall as
much as 10 feet per hour. There is also a great deposit of mud by the
Severn. Mr. Brunel considered that these circumstances rendered a fixed
structure undesirable, and he therefore recommended a floating pier. He
said:--
I propose two or three vessels of 300 or 200 feet of length, built
of iron, as the material cheapest and best adapted to the purpose,
of 16-feet or 20-feet draft of water, and about 30 feet beam,
moored close stem and stern, so as to form one continuous floating
body. Any steamboat or other vessel alongside will of course be on
the same level as the pier; the passengers, on disembarking, will
at once be on a level platform or deck, under shelter, where the
luggage or goods can also be placed; and the communication with the
shore will be effected without steps.... Such a pier would afford
stowage for almost any quantity of coals, fresh water, and general
goods, which could be stored here for embarkation.
In 1847, after an Act had been obtained for making a railway from
Bristol to Portishead, with a pier at the latter place, Mr. Brunel
designed the pier on the plan described in his report of 1839; but the
project was not carried out.
* * * * *
As has been already mentioned, the communication between the lower part
of the Floating Harbour and the river Avon is through the Cumberland
Basin; between this basin and the river were two locks, made at the time
that the Floating Harbour was constructed.
Owing to the increased size of merchant vessels, it had long been in
contemplation to enlarge the entrance. At the time when the ‘Great
Britain’ was built, the northern lock was so narrow that a portion of
the upper masonry had to be removed in order to give room for the ship
to pass from the basin to the river on a spring tide. It was then felt
that the enlargement of one of the locks could no longer be delayed, and
Mr. Brunel was asked to adapt the narrower or southern entrance lock to
the passage of the largest vessels.
Between the two locks was a pier, from which vessels were guided, and
the gates opened or shut. The elongation of the lock was limited by the
length of this pier, as it could not be extended towards the river
without diminishing the area of entrance, nor could it be extended
upwards without lessening the area of Cumberland Basin. Mr. Brunel,
although hampered by this restriction, succeeded in obtaining a lock of
considerable length. He constructed the gates of a single leaf, and
placed the upper gate outside the lock so as to shut against the upper
end of the middle pier, and to swing back when opened into a recess in
the side wall of the Cumberland Basin. He thus avoided the necessity of
finding room on the pier for the machinery to open one of the leaves of
the upper gate. Had the gate been in two leaves, the lock would have
been shortened from 30 to 40 feet. At the lower end he placed the gate
as near the river as possible; and, lest the end of the middle pier
should not be strong enough to withstand the pressure, he secured the
quoin stones, against which the gate closed, by horizontal wrought-iron
bars at different levels, built into the side wall of the lock.
The lock is 262 feet in length between the gates, and 54 feet wide at
the narrowest part.
The masonry is of plain character, all the part below the ashlar coping
being of ordinary fitted rubble of great thickness, solidly built with
hydraulic lime mortar. The ground behind the wall consisted of a wet
silty clay, causing a great pressure against the masonry. The under part
of the body of the lock is formed to a semi-oval cross, section.
The works were commenced by the construction of coffer-dams at each end.
In 1846, when the masonry was approaching completion, a very high tide
took place, and a portion of the upper dam gave way. As some work still
remained to be done at the sill and apron of the lower gate, Mr. Brunel
decided to make a brick dam in the middle of the lock, where the masonry
had been completed. This brick dam was a horizontal arch built on the
bottom of the lock, up to the level of the water in the Floating
Harbour. The abutments were formed by the masonry of the lock walls,
which was notched to receive the bricks of the arch rings. The dam was
28 feet high, only 8 feet thick at the bottom, and 3 feet thick at the
top. It was set in Roman cement, and was completely water-tight. It was
easily and rapidly made, and the cost was small, as compared with what
would have been the cost of repairing the upper dam.
In this lock the chief point of interest consists in its being the first
in which wrought-iron gates were introduced, these gates being at the
same time made buoyant.
Floating caissons had been previously used at the entrances to graving
docks, and in similar situations; indeed at Bristol, a caisson had long
been employed at Prince’s Street Bridge, to separate one part of the
Floating Harbour from the other. The buoyant gates of the Bristol Docks
differ essentially from these vessels, inasmuch as, instead of requiring
to be floated into their places, they turn on a hinge, and do not rise
or fall vertically.
The gates are provided with wheels, but only a small part of the weight
rests on them, as the gates are rendered buoyant by large air-chambers,
formed in the lower part of them.
The upper and lower gates are alike in construction and dimensions, so
that it is only necessary to describe one of them. (See woodcut, fig.
18.)
The extreme length of the gate is 58 feet, and the extreme height 29
feet. In the middle it is about 10 feet wide, the width diminishing to 3
feet at the top. In plan it is curved to resist the pressure of the
water. The gate when closed is not at right angles to the direction of
the length of the lock, but is at an angle of about 12°. The length is
thus scarcely increased, while the travel in being opened and shut is
much reduced. The top is at the level of the water in the Floating
Harbour; so that, when the tide falls, the water in the Cumberland Basin
may be retained at the same height as in the Float.
[Illustration: Fig. 18. Bristol Dock Gate.
_Elevation._ _Section A. B._ _Section C. D._
_Plan._
_Scale of feet._]
The air-chamber is formed by two water-tight decks of wrought-iron
plating, one at the level of half-tide, the other a short distance above
the bottom of the gate. Above the deck forming the top of the
air-chamber, the water, as the tide rises, flows freely into the
interior of the gate, through openings in the face next the dock, so
that when the water is level with the top of the gate, the part above
the air-chamber is full of water, which flows out again if the level of
the water falls.
In ordinary working the gate only needs to be opened or shut when the
water is above the level of half-tide, and therefore at these times the
whole of the air-chamber is immersed. To whatever height the water rises
above this level, the buoyancy remains almost the same, the only change
being caused by the displacement of the iron of the upper part. This
displacement, when the water is level with the top of the gate, amounts
to about six tons.
The size of the air-chamber is so arranged that when the water is level
with the top of the gate it is just afloat; and at half-tide, when the
water is at the level of the top of the air-chamber, there is a weight
of about six tons on the wheels.[179]
The gate is provided with a sluice, by which water may be admitted into
the air-chamber, or allowed to escape when the water outside is at a
lower level; there is also a pump, by which leakage water may be
extracted. The volume of the air-chamber may thus be altered at will,
and the buoyancy may be modified so as to counteract the effect of the
weight of any mud which may be deposited upon the decks of the gate.
There are in each gate two very large double sluices, which are used for
working the lock, and for lowering the water in the Cumberland Basin to
meet the tide. They are also used for scouring away the mud. The
shutting pieces of the gates, which bear against the granite masonry and
form a water-tight joint, were made of Honduras mahogany, a very durable
wood for the purpose. The timber is bedded in creosoted felt and bolted
to the gates; and the pieces are still sound, after the lapse of more
than twenty years.
Underneath the gate are two wheels, slightly conical, 3½ feet diameter
and 1 foot wide, which travel on level cast-iron rails let into the
masonry of the gate floor. There is no heel-post such as is usual in
dock gates, but the gate is hinged to the masonry by wrought-iron
collars and a wrought-iron pin about 8 inches diameter, which is passed
through them. Any portion of the weight of the gate which is not
supported by the flotation of the air-chamber is borne entirely by the
wheels, the arrangement at the hinge being merely for the purpose of
retaining the gate in its position and guiding it in opening or
shutting. The gate is moved by chains, which are attached to the barrels
of powerful crabs, and conveyed through passages formed in the masonry;
at the lower ends of these are chain rollers or broad sheaves, round
which the chains pass. The sheaves are fluted on the circumference, to
ensure their turning readily. When the gates are nearly afloat, they can
be moved with great ease; but at the time of their construction it was
considered essential to provide machinery sufficiently powerful to open
and close them at low water, when the whole weight of each gate, nearly
one hundred tons, rests on the wheels. The crabs and chains were
therefore made much stronger, and were more difficult to move, than if
they had been merely designed to work the gate under ordinary
conditions.
* * * * *
The gates were constructed in Bristol at the Great Western Steamship
Works in 1847. After the lower gate had been tested and proved to be
water-tight, it was launched, and floated with the front surface nearly
level. The positions it would assume under different conditions had been
calculated beforehand. Before it was fixed, the gate was made to float
nearly upright by the admission of water; it was then towed to its
place, and brought into correct position. The hinge-pin was dropped
through the collars, and by admitting water into it, the gate was sunk,
so that it rested upon its wheels. This operation had to be performed
at high water, which only lasted for a short time.
In fixing the upper gate, the water was kept up to the proper level by
the lower gate, and therefore there was no need to do the work quickly.
_Plymouth Great Western Docks._
As it was considered probable that, on the completion of railway
communication, mail packets and other large ocean steamers might make
Plymouth their port of departure, a company was formed in 1846 for the
construction of a dock in Mill-bay, a large inlet in Plymouth Sound near
the entrance to the Hamoaze.
The bay was already protected from the prevailing winds by a pier at its
eastern side constructed by Mr. Rendel.
It was decided to form a wet dock and graving dock at the inner end of
the bay, and to make quay walls along the side of the outer part, to
join the existing pier. A floating pier for large vessels afterwards
became part of the scheme. In 1847 preliminary trials were made as to
the best means of excavating in deep water the limestone rock of which
part of the bottom of the bay consisted; some quay walls were also
constructed and made use of by the shipping.
At the end of 1851 a contract was entered into for the execution of the
whole of the works remaining to be completed; the most important of
these were the entrance, the graving dock, and the completion of the
floating basin.
The progress of the undertaking was much facilitated by Mr. Brunel
giving his sanction to the proposal of the contractors to form a stone
and earth embankment across the mouth of the bay, instead of employing
the usual timber coffer-dams. This embankment, which completely answered
its purpose, was finished by the middle of 1853.
The works now proceeded steadily until they were completed, and at the
end of the year 1856 the dock was opened.
The middle of the embankment was cut through, and an entrance channel
dredged to a level of 8 feet below low water. The remainder of the dam,
being partly protected by masonry walling, was used as a quay, and
served for the protection of the outer basin.
Mr. Brunel had prepared the foundation, and had intended to build a pier
to shelter the entrance gate from the sea, and to assist in pointing
long vessels. The want of this shelter was felt in the gale of October
1857, when the gates were thrown down. In order to avoid any future
casualty, storm gates, or framed struts reaching down to low water
level, were placed immediately behind the entrance gates, so as to
support them against heavy seas.
The dock, which is of oblong shape, has an area of 13 acres, and a
length of quay wall of 3,490 feet. The greater part has a depth of 22
feet below high water at spring tides, or 16 feet at neap tides; the
remainder is 4 feet deeper, or the same depth as the channel in the
outer basin.
The walls are 8½ feet thick at bottom, and 3½ feet at top, and are built
to a curved batter. They reach generally to a depth of 5 feet below low
water, and rest on concrete 12½ feet thick, carried down until a rock
foundation is reached, which in some places is as much as 40 feet below
the bottom of the dock.
To accommodate the larger paddlewheel steamers, the entrances to the
basin and graving dock were made 80 feet in width.
The dock is entered through a short passage, the sill being 13 feet
below low water. The masonry of this entrance has curved battering
sides, with a segmental invert.
The entrance is closed by a pair of wrought-iron buoyant gates, which
meet at an angle of 127°. Each gate is 48 feet long, and weighs
seventy-five tons, the breadth being 8 feet in the middle, curving to 2
feet at the ends. There are six horizontal decks and four vertical
bulkheads. The depth at the heel-post is 22 feet, and at the
meeting-post 35 feet (see woodcut, fig. 19).
[Illustration: Fig. 19. Plymouth Dock Gates.
_Elevation._ _Section A. B._
_Plan._
_Scale of feet._]
In each gate is an air-chamber, the top of which is at half-tide level;
and its volume is such that when the gate is wholly immersed there is a
small downward pressure.[180]
Under each gate, near the meeting-post, is placed a wheel, which
supports part of the weight. This wheel is so arranged that it can be
easily removed for repair. The heel-posts are of cast iron, planed and
ground in place against the polished surface of the granite hollow
quoins so as to form a water-tight joint.
There are large scouring culverts behind the side walls of the entrance;
but for the purpose of regulating the level of the water in the dock,
and of discharging a large volume of water readily, without having to
overcome the friction of ordinary sluice valve faces, each gate is
furnished with a cylindrical valve, of the following description:--
From an opening in the side of the gate next the dock there is a large
curved pipe or sluice-way, which terminates inside the body of the gate,
with a circular horizontal orifice about 5 feet in diameter. The opening
is covered by a short length of vertical pipe of the same diameter,
reaching above high water, the bottom edge making a water-tight joint.
This pipe can be raised or lowered by a screw at the top of the gate.
When raised a short distance it allows the water from the dock to flow
out between the bottom of the movable pipe and the orifice of the
sluice-way into an isolated compartment of the gate, and to escape by an
opening provided in the outer face. The movable pipe is guided by
rollers, and from the construction the pressures on it are balanced.
The entrance to the graving dock, 80 feet wide, is closed by a pair of
gates of the same dimensions and construction as those of the entrance.
This dock is 380 feet long, 92 feet wide, and has a depth of 28 feet
over the sill.
* * * * *
The floating pier was erected in Mill-bay in 1852, to accommodate the
steam-shipping trade. It consists of a pontoon with a bridge leading to
it.
The pontoon is a large wrought-iron vessel of nearly rectangular
cross-section, 300 feet long, with 40 feet breadth of beam, and a depth
of 17½ feet. It is loaded so as to draw 10 feet of water, and is capable
of storing 4,000 tons of coals. To connect the pontoon with the shore
there is an iron bridge, working upon hinges, in two spans of 125 feet
each. The connection between the two spans was supported on a timber
pier, and was made so as to be movable vertically, and capable of being
adjusted by a crab and counterbalance weights. By means of this
arrangement a uniform gradient over the whole length of the bridge could
be obtained at any time of tide, for the convenience of heavily laden
carts passing to and from the pontoon.
_Briton Ferry Docks._
In 1846 a company was formed to establish docks in Baglan Bay at Briton
Ferry, near the mouth of the river Neath; but nothing was done until
1851, when the necessary powers were obtained under an Act of
Parliament. From the vicinity of the proposed docks to the South Wales
and Vale of Neath Railways, a large amount of trade was anticipated; and
in 1853 an Act was obtained for the South Wales Mineral Railway, which
was intended to terminate at the docks, and was expected to bring a
large traffic in coals and other minerals.
The dock works were begun soon afterwards, but the earthwork was not
completed, nor the construction of the gate commenced, until the year
1858. The docks were completed, after Mr. Brunel’s death, by Mr.
Brereton, and were formally opened on August 22, 1861.
They consist of an outer tidal basin of about 7½ acres, and an inner
floating basin of about 11 acres, with a depth of water of 27 feet at
spring tides, and 16 feet at neap tides. The two basins are connected by
a passage or entrance of 50 feet in width, with curved battering walls
and an invert, closed by a gate of a single leaf. An important advantage
of single gates is that the sill and quoins may be in one plane, and
that the troublesome and costly fitting of the hollow quoins is avoided.
The sill was laid 6 feet below what was then the low-water level, as it
was thought that future improvements might reduce the bar at the
entrance of the river Neath to that level. This has already been nearly
accomplished.[181] The total length of the docks is a little over half a
mile, and the average breadth is about 400 feet. They are connected with
the South Wales Railway by branch railways.
With the exception of the walls dividing the two basins near the
entrance gate, the sides of the dock are not constructed with masonry
quay walls, in the ordinary manner, but are formed in a very inexpensive
manner of slopes pitched with furnace slag, obtained from the copper
smelting works on the Neath river, with jetties at intervals for the
shipping. Besides being suitable for the soft clay in which the dock is
made, this plan is specially adapted for mineral traffic, as the work of
loading or discharging a cargo of minerals can only be properly carried
on at the point where fixed machinery is provided for the purpose. This
machinery may be placed as conveniently on projecting jetties as on a
dock wall. The traffic, which consists mainly of coals and metals, is
accommodated at the jetties, which are furnished with cranes for loading
and unloading the vessels employed in the metal and iron ore trade, and
with tipping-frames, which discharge the coals into the ships. These
cranes and tipping-frames are worked by hydraulic machinery.
In order to facilitate the entrance of vessels from the river Neath to
the tidal basin, and to protect it from the sea, two pier-heads were
built, one at each side of the basin at the point where it joins the
river. These piers are of timber piling and framework, filled in with
copper slag; the entrance between them is 300 feet wide. They were
constructed after Mr. Brunel’s death.
The gate is a wrought-iron buoyant gate, with five vertical partitions
or bulkheads and six decks. The length is 56 feet; the depth in the
middle is 31 feet 6 inches, and at the sides 26 feet 6 inches. The
breadth in the middle is 9 feet, and is curved to 2½ feet at the ends
(see woodcut, fig. 20).
[Illustration: Fig. 20. Briton Ferry Dock Gate.
_Elevation._ _Section A. B._
_Plan._
_Scale of feet._]
The air-chamber, which is similar to that of the Plymouth gates, is
placed so that the top is at the level of high-water neap tides, about
half-way up the gate.
There are two sluices at low-water level, each having an area of 8
square feet.
The entrance invert being subject to the influx of sand from the outer
basin, and to the deposit of coal rubbish dropped into the dock, Mr.
Brunel decided in this gate not to use wheels, but to make the hinge
and heel-post strong enough to carry the whole weight of the gate, even
if it were unsupported by the buoyancy of the air-chamber.
The heel-post is a massive piece of cast iron; the bottom part is bored
out, and into it is fitted a cast-iron cylindrical pin, 1 foot 6 inches
diameter and 7 feet 6 inches long. On this are ground discs of steel,
lubricated with oil, whereon the gate rests and turns. Thus the surfaces
exposed to friction are above the sand or grit at the bottom of the
dock. The lower end of the pin fits into a cast-iron socket fastened to
the masonry, and is prevented from turning round by being made
hexagonal. The sides of the hexagon have sufficient play to enable the
gate to adjust itself, when shut, to the masonry sill, so as to be
water-tight. The top of the heel-post works in a brass bush, 18 inches
diameter and 15 inches broad, enclosed in a massive wrought-iron collar,
which is strongly fastened by anchor chains to the masonry. After the
gate was completed the strength of the hinge was tested by moving the
gate before the water was admitted into the dock. The only resistance to
the motion of the gate is the slight friction at the hinge.
No coffer-dam was used in the construction of this work, but advantage
was taken of a large bank of slag and earth enclosing a portion of the
site of the dock. This was extended and raised, and a sea dam formed.
The dam was cut through when the works were completed, and a channel
dredged to the depth of 6 feet below low water.
_Brentford Dock._
In 1855 an Act was obtained for making a dock on the Thames at
Brentford, and a railway to join the Great Western Railway at Southall.
The dock has an area of about 3½ acres.
The works were begun in July 1856, and were completed, and the dock
opened, three years later.
The walls are founded in the London clay, which here underlies a bed of
gravel of some thickness; from this there was a considerable influx of
water.
The chief peculiarity of the dock is the form of construction adopted
for the sides. Piers of brickwork, 10 feet long and 2 feet 3 inches
thick, are placed at right angles to the sides of the dock at intervals
of 26 feet. The backs of these piers are connected by horizontal arches,
carried up with a curved batter. The piers are about 20 feet high, and
arches are turned upon them, which support the front part of the quay,
and meet the horizontal arches at the backs of the piers. Thus the sides
of the dock consist of a series of vaults, arched over at the top, and
also at the back towards the pressure of the earth.
[Illustration: Fig. 21. Brentford Dock Gate.
_Elevation._ _Section A. B._
_Plan._
_Scale of feet._]
The thickness of the horizontal arches which form the bulk of the wall
is only 3 feet, but these are so strengthened by the piers in front,
that a wall strong enough to resist the pressure of the earth behind it
was obtained by means of a very small quantity of brickwork.
Along one side of the dock the piers are 31 feet long, in order that
coal barges may lie with part of their length in the vaults between the
piers while their cargo is being put on board. By this arrangement the
barges have their longest dimension at right angles to the side of the
dock, and a much greater number can be accommodated than if each
occupied a space alongside the quay wall. The contents of the coal
trucks are tipped into the barges through sloping shoots.
The entrance has a clear width of 30 feet, and is closed by a single
wrought-iron buoyant gate, which, like the Bristol gates, is, when shut,
not quite at right angles to the entrance.
The gate is 33 feet long, 19 feet high, 2 feet 6 inches wide at the
middle, curved to 1 foot 6 inches at the sides, and weighs sixteen and a
half tons. It is divided into compartments by four decks and two
vertical bulkheads. The air-chamber occupies the whole space below one
of the decks, 7 feet 6 inches above the bottom (see woodcut, fig. 21);
and there are two sluices, each having an opening of 4 feet by 2 feet.
This gate, like that at Briton Ferry, has no wheel under it, the weight
being carried upon the pivot.
In order to avoid side strains upon the pivot and top collar, a
counterbalance arm is fastened on the top of the gate. This is formed of
two cast-iron girders, bolted together and enclosing weights between
them. The ends of these girders project beyond the heel-post over the
quay, as in canal lock gates, and carry the machinery by which the gate
is turned, as there are no crabs, chains, or chain rollers. Instead of
these, a cast-iron circular rack is fixed on the top of the masonry, in
which a pinion works, turned by gearing fixed to the end of the
counterbalance.
This gate turns with remarkable freedom, and the current of water
running into the dock on a spring tide opens it completely.
_Pier at Neyland, Milford Haven._
The South Wales Railway was originally intended to terminate at
Fishguard, on the north coast of Pembrokeshire, with the view of
securing a large quantity of Irish traffic, the distance across the
channel to the Irish coast at Wexford being only 60 miles, less than the
distance from Holyhead to Kingstown.
It was, however, ultimately decided to form a terminus, which would
accommodate the ordinary Irish traffic, and would not require such an
extensive outlay on harbour works as would have been necessary on the
northern coast.
With this object, the inlet or natural harbour of Milford Haven was
examined, and the South Wales Railway was carried to Neyland Point,
opposite Pembroke, where the position is sheltered, and there is deep
water at all times of tide for the largest vessels.
The pier at Neyland, or New Milford, which was made in 1857, consists of
a timber viaduct, with a pontoon at the end, 150 feet long and 42 feet
beam, loaded so as to draw about 7 feet. There is a depth of 16 feet
alongside it at low water, and it is connected with the shore by a
landing bridge. The pontoon is made of wrought iron, and has three
transverse and two longitudinal bulkheads. It is moored by chain cables,
which pass through two large hawse pipes, extending from the bottom
nearly up to the deck, with cast-iron mouthpieces at their lower ends.
The cables passing through these are anchored firmly to the ground at a
considerable distance from the pontoon.
The pontoon was intended to be the centre of several others, which were
to be moored in deeper water.
The rise of tide being sometimes as much as 25 feet, it was necessary
that the landing bridge should be of considerable length, in order that
there should be a moderate inclination at all times of tide. It is
accordingly made in one span of 205 feet. It consists of two plate-iron
side girders, of the uniform depth throughout of 14 feet, and width of 2
feet 6 inches. These are placed 12 feet 6 inches apart, the roadway
being between them. The ends of the girders which rest on the pontoon
are provided with cast-iron wheels, 1 foot 6 inches in diameter.
The pier has since been extended by additional pontoons, which were
those used in the floating of the Saltash Bridge.