Chapter XVI: Act 1895: has afforded a cheap and speedy remedy to all classes (4)

As dock walls, especially before the admission of water into the dock,
constitute high retaining walls, not infrequently founded upon soft or
slippery strata, and backed up with the excavated materials from
alluvial beds, into which water is liable to percolate, they are
naturally exposed under unfavourable conditions to the danger of
failure. A dock wall erected on unsatisfactory foundations is liable,
where the bottom is soft, to settle down at its toe, owing to the
pressure at the back, and to fall forwards into the dock, as occurred
at Belfast; or where the silty bottom slips forward under the weight
of the backing, the wall may follow the slip at the bottom and settle
down at the back, falling to some extent backwards, as exemplified by
the failure of the Empress basin wall at Southampton. The most common
form, however, of failure is the sliding forwards of a dock wall, with
little or no subsidence, on a silty or slippery stratum under the
pressure imposed by the backing. Thus the Kidderpur dock walls furnish
an instance of sliding forwards on muddy silt, and part of the South
West India dock walls on two underlying, detached, slippery seams of
London clay.

To avoid these failures with untrustworthy foundations, great care has
to be exercised in selecting the best hard material available,
unaffected by water, for the backing, which should be brought up in
thin, horizontal layers carefully consolidated; and where there is a
possibility of water accumulating at the back, pipes should be
introduced at intervals near the bottom right through the wall in
building it, and rubble stone deposited close to the back of the wall,
so as to carry off any water from behind, these pipes being stopped up
just before the water is let into the dock. These precautions,
moreover, are assisted by reducing the amount of backing to a minimum
in the construction of the wall, best effected by building the wall
inside a timbered trench. The liability to slide forwards can be
obviated by carrying down the foundations of the wall sufficiently
below dock-bottom to provide an efficient buttress of earth in front
of the wall, and also by making the base of the wall slope down
towards the back, thereby forcing the wall in sliding forwards to
mount the slope, or to push forward a larger mass of earth; whilst a
row of sheet piling in front of the foundations offers a very
effectual impediment to a forward movement, and, in combination with
bearing piles, prevents settlement at the toe in soft ground. In very
treacherous foundations it may be advisable to defer the completion of
the backing till after the admission of the water; but the additional
stability given to a retaining wall or reservoir dam by an ample
batter in front, is precluded in dock walls by the modern requirements
of vessels.

Maintenance of depth.

Silt accumulates in docks where the lowering of the water-level by
locking, the drawing down of half-tide basins, and the raising of the
water at spring tides, involve the admission of considerable volumes
of tidal water heavily charged with silt, which is deposited in still
water and has to be periodically removed by dredging. To avoid this,
the water is sometimes replenished from some clear inland source, an
arrangement adopted at some of the South Wales ports opening into the
muddy Severn estuary, and at the Alexandra dock, Hull, to exclude the
silty waters of the Humber. At the Kidderpur docks on the Hugli, the
water from the river for replenishing the docks is conducted by a
circuitous canal, in which it deposits its burden of silt before it is
pumped into the docks.

Equipment on quays.

In order to deal expeditiously with the cargoes and goods brought into and despatched from docks, numerous sidings communicating with the railways of the district are arranged along the quays, which are also provided with steam, hydraulic or electric travelling cranes at intervals alongside the docks, basins or river, for discharging or loading vessels, and with sheds and warehouses for the storage of merchandise, &c., the arrangements depending largely upon the special trade of the port. Though different sources of power are sometimes made use of at different parts of the same port, as for example at Hamburg, where the numerous cranes are worked by steam, hydraulic power or most recently by electricity, and a few by gas engines, it is generally most convenient to work the various installations by one form of power from a central station. Water-pressure has been very commonly used as the motive power at docks, being generated by a steam-engine and stored up by one or more accumulators, from which the water is transmitted under pressure through strong cast-iron pipes to the hydraulic engines which actuate the cranes, lifts, coal-tips, capstans, swing-bridges and gate machinery throughout the docks (see POWER TRANSMISSION: _Hydraulic_). The intermittent working of the machinery in docks results in a considerable variation in the power needed at different times; but economical working is secured by arranging that when the accumulators are full, steam is automatically shut off from the pumping engines, but is supplied again as soon as water is drawn off. Electricity affords another means for the economical transmission of power to a distance suited for intermittent working; as far back as 1902 it was being adopted at Hamburg as the source of power for the machinery of the extensive additional basins then recently opened for traffic.

Coal-tips.

At ports where the principal trade is the export of coal from neighbouring collieries, special provision has to be made for its rapid shipment. Coal-tips, accordingly, are erected at the sides of the dock in these ports, with sidings on the quays at the back for receiving the trains of coal trucks, from which two lines of way diverge to each coal-tip, one serving for the conveyance of the full wagons one by one to the tip, after passing over a weigh-bridge, and the other for the return of the empty wagons to the siding where the empty train is made up for returning to the colliery (fig. 8). Each full wagon is either run at a low level upon a cradle at the tip, then raised on the cradle within a wrought-iron lattice tower to a suitable height, and lastly, tipped up at the back for discharging the coal; or it is brought along a high-level road on to a cradle raised to this level on the tower, and tipped up at this or some slightly modified level. The coal is discharged down an adjustable iron shoot, gradually narrowed so as to check the fall; and on first discharging into the hold of a vessel, an anti-breakage box is suspended below the mouth of the shoot. When full, this is lowered to the bottom of the hold and emptied, thereby gradually forming a cone of coal upon which the coal can be discharged directly from the shoot without danger of breakage. Other contrivances are also adopted with the same object.

Dock extensions.

In designing dock works, it is expedient to make provision, as far as
possible, for future extensions as the trade of the port increases.
Generally this can be effected alongside tidal rivers and estuaries by
utilizing sites lower down the river, as carried out on the Thames for
the port of London, or reclaiming unoccupied foreshores of an estuary,
as adopted for extensions of the ports of Liverpool, Hull and Havre.
At ports on the sea-coast of tideless seas, it is only necessary to
extend the outlying breakwater parallel to the shore line, and form
additional basins under its shelter, as at Marseilles (fig. 5) and
Genoa (see HARBOUR). Quays also along rivers furnish very valuable
opportunities of readily extending the accommodation of ports. Ports,
however, established inland like Manchester, though extremely
serviceable in converting an inland city into a seaport, are at the
disadvantage of having to acquire very valuable land for any
extensions that may be required; but, nevertheless, some compensation
is afforded by the complete shelter in which the extensions can be
carried out, when compared with Liverpool, where the additions to the
docks can only be effected by troublesome reclamation works along the
foreshore to the north, in increasingly exposed situations.

_Dock Entrances and Locks._--The size of vessels which a port can admit depends upon the depth and width of the entrance to the docks; for, though the access of vessels is also governed by the depth of the approach channel, this channel is often capable of being further deepened to some extent by dredging; whereas the entrance, formed of solid masonry or concrete, cannot be adapted, except by troublesome and costly works sometimes amounting to reconstruction, to the increasing dimensions of vessels. Accordingly, in designing new dock works with entrances and locks, it is essential to look forward to the possible future requirements of vessels. The necessity for such forethought is illustrated by the rapid increase which has taken place in the size of the largest ocean liners. Thus the "City of Rome," launched in 1881, is 560 ft. long, and 52-1/4 ft. beam, and has a maximum recorded draught of 27-1/2 ft.; the "Campania" and "Lucania," in 1893, measure 600 ft. by 65 ft.; the "Oceanic," in 1899, 685-1/2 ft. by 68-1/4 ft., with a maximum draught of 31-1/3 ft.; the "Baltic," in 1903, 709 ft. by 75 ft., with a maximum draught of 31-3/4 ft.; and the "Lusitania" and "Mauretania," launched in 1906, 787-1/2 ft. by 88 ft.

Dimensions of entrances and locks.

The width and depth of access to docks are of more importance than the length of locks; for docks which are reached through entrances with a single pair of gates have to admit vessels towards high water when the water-level in the dock is the same as in the approach channel, or through a half-tide basin drawn down to the level of the water outside, and are therefore accessible to vessels of any length, provided the width of the entrance and depth over the sill are adequate; whilst at docks which are entered through locks, vessels which are longer than the available length of the lock can get in at high water when both pairs of gates of the lock are open. Open basins are generally given an ample width of entrance, and river quays also are always accessible to the longest and broadest vessels; but in a tidal river the available depth has to be reckoned from the lowest low water of spring tides, instead of from the lowest high water of neap tides, if the vessels in the open basins and alongside the river quays have to be always afloat.

Many years ago the Canada lock at Liverpool, the outer North lock at Birkenhead, the Ramsden lock and entrance at Barrow-in-Furness, and the Eure entrance at Havre, were given a width of 100 ft. Probably this was done with the view of admitting paddle steamers, since subsequent entrances at Liverpool were given widths of 80 and 65 ft.; whereas none of the locks in the port of London has been made wider than 80 ft., which has been the standard maximum width since the completion of the Victoria dock in 1866. The widest locks at Cardiff are 80 ft., and the entrance to the Barry docks is the same; but the lock of the Alexandra dock, Hull, opened in 1885, was made 85 ft. wide. At Liverpool, where the access to the docks is mainly through entrances, on account of the small width between the river and the high ground rising at the back, and where ample provision has to be made for the largest Atlantic liners, though the entrances to the Langton dock, completed in 1881, leading to the latest docks at the northern end were made 65 ft. wide, with their sills 3 ft. below low water of spring tides and 20-1/2 ft. below high water of the lowest neap tides, the two new entrances to the deepened Brunswick dock near the southern end, giving access to the adjacent reconstructed docks, completed in 1906, were made 80 and 100 ft. wide, with sills 28 ft. below high water of the lowest neap tides. Moreover, the three new entrances to the new Sandon half-tide dock, completed in 1906, communicating with the reconstructed line of docks to the south of the Canada basin, and with the latest northern extensions of the Liverpool docks, were made 40 ft. wide with a depth over the sill of 24-1/2 ft., and 80 and 100 ft. wide on each end of the central entrance, with sills 29 ft. below high water of the lowest neap tides, each entrance being provided with two pairs of gates, in case of any accident occurring to one pair, according to the regular custom at Liverpool. Powers were also obtained in 1906 for the construction of a half-tide dock and two branch docks to the north of the Hornby dock, which are to be reached from the river by two entrances designed to be 130 ft. wide, with sills 38-1/2 ft. below high water of the lowest neap tides, so as to meet fully the assumed future increase in the beam and draught of the largest vessels; whilst the authorized extension of the river wall northwards will enable additional docks to be constructed in communication with these entrances when required.

Though, with the exception of Southampton and Dover, other British ports do not aim, like Liverpool, at accommodating the largest Atlantic liners at all times, the depths of the sills at the principal ports have been increased in the most recent extensions. Thus at the port of London the sills of the first lock of the Albert dock were 26-1/2 ft. below high water of neap tides, and of the second lock adjoining, 32-1/2 ft. deep; whilst the sills of the lock of the Tilbury docks are 40-1/2 ft. below high water of neap tides. Moreover, in spite of the great range of tide at the South Wales ports on the Severn estuary, the available depth at high water of neap tides of 25 ft. at the Roath lock, Cardiff, was increased in the lock of the new dock to 31-1/2 ft.; the depth at the entrance to the Barry docks, opened in 1889, was 29-1/2 ft., but at the lock opened in 1896 was made 41-1/3 ft.; whilst a depth of 34 ft. has been proposed for the new lock of the Alexandra dock extension at Newport, nearly 10 ft. deeper than the existing lock sills there. Similar improvements in depth have also been made or designed at other ports to provide for the increasing draught of vessels.

The length of locks has also been increased, from 550 ft. at the Albert dock, to 700 ft. at Tilbury in the port of London, from 300 ft. to 550 ft. at Hull, and from 350 ft. to 660 ft. at Cardiff. The lock at the Barry docks is 647 ft. long, though only 65 ft. wide. A lock constructed in connexion with the improvement works at Havre, carried out in 1896-1907, was given an available length of 805 ft. and a width of 98-1/2 ft., with a depth over the sills of 34-3/4 ft. at high water of neap tides.

Entrances to docks.

Entrances with a single pair of gates, closing against a raised sill
at the bottom and meeting in the centre, have to be made long enough
to provide a recess in each side wall at the back to receive the gates
when they are opened, and to form a buttress in front on each side to
bear the thrust of the gates when closed against a head of water
inside. A masonry floor is laid on the bottom in continuation of the
sill, serving as an apron against erosion by water leaking between or
under the gates, and by the current through the sluiceways in the
gates, when opened for scouring the entrance channel or to assist in
lowering the water in a half-tide dock for opening the gates (fig.
19). A sluiceway in each side wall, closed by a vertical sluice-gate,
generally provided in duplicate in case of accidents and worked by a
machine actuated by hydraulic pressure, enables the half-tide basin to
be brought down to the level of the approach channel outside with a
rising tide, so that vessels may be brought into or passed out of the
basin towards high water. The advantages of these entrances are, that
they occupy comparatively little room where the space is limited, and
are much less costly than locks; whilst in conjunction with a
half-tide basin they serve the same purpose as a lock with a rising
tide. Vessels also pass more readily through the short entrances than
through locks; and as entrances are only used towards high water,
their sills need not be placed so low as the outer sills of locks to
accommodate vessels of large draught. On the other hand, they are
accessible for a more limited period at each tide than locks; and they
do not allow of the exclusion of silt-bearing tidal water, and
therefore necessitate a greater amount of dredging in the docks, and
especially in half-tide basins, for maintenance. Entrances, however,
at large ports are frequently supplemented by the addition of a lock
at some convenient site, rendering the ports accessible for the
smaller class of vessels for some time before and after high water, as
for instance at Liverpool, Barry, Havre and St Nazaire. A small basin
with an entrance at each end--an arrangement often adopted--is in
reality, for all practical purposes, a lock with a very large
lock-chamber. An entrance or passage with gates has also to be
provided at the inner end of a large half-tide basin like the basins
adopted at Liverpool, to shut off the half-tide basin from the docks
to which it gives access, and maintain their water-level when the
water is drawn down in the basin to admit vessels before high tide.

Reverse gates pointing outwards are sometimes added in passages to
docks and at entrances, to render the water-level in one set of docks
independent of adjacent docks, to exclude silty tidal water and very
high tides, and also to protect the gates of outer entrances in
exposed situations from swell, which might force them open slightly
and lead to a damaging shock on their closing again.

Locks at docks.

Locks differ from entrances in having a pair of gates with
arrangements similar to an entrance at each end, separated from one
another by a lock-chamber, which should be large enough to receive the
longest and broadest vessel coming regularly to the port. These dock
locks are similar in principle to locks on canals and canalized
rivers, but are on a much larger scale. The lock-chamber has its water
raised or lowered in proportion to the difference in level between the
water-level in the dock and the water in the entrance channel, by
passing water, when the gates are closed at both ends, from the dock
into the lock-chamber or from the lock-chamber into the entrance
channel, through large sluiceways in the side walls, controlled, as at
entrances, by vertical sluice-gates. In this way the vessel is raised
or lowered in the chamber, till, when a level has been reached, the
intervening pair of gates is opened and the vessel is passed into the
dock or out to the channel. Generally the upper and lower sills of a
lock are at the same level, a foot or two higher than dock-bottom; and
the depth at which they are laid is governed by the same
considerations as the sill of an entrance. Vessels longer than the
available length between the two pairs of gates can be admitted close
to high water, when the water in the dock and outside is at the same
level, and both pairs of gates can be opened. When the range of tide
at a port is large, and the depth in the approach channel is
sufficient to allow vessels to come up or go out some time before and
after high water, and also where the water in the dock is kept up to a
high level from an inland source to exclude very silty tidal water, it
is expedient to reduce the cost of construction by limiting the depth
of the excavations for the dock, and consequently also the height of
the dock walls, to what is necessary to provide a sufficient depth of
water below high water of the lowest neap tides, or below the
water-level to which the water in the dock is always maintained, for
the vessels of largest draught frequenting the port, or those which
may be reasonably expected in the near future. The upper sill of the
lock is then determined by the level of dock-bottom; but the lower
sill is taken down approximately to the depth of the bottom of the
approach channel, or to the depth to which it can be carried by
dredging, so as to enable the lock to admit or let out at any time all
vessels which can navigate the approach channel. Thus, for instance,
the outer and intermediate sills of the lock at the Barry docks are 9
ft. lower then the upper sill.

The foundations for the sill and side walls at each end of a lock, and
also for the side walls and invert commonly enclosing the lock-chamber
at the sides and bottom, are generally constructed simultaneously with
the dock works, under shelter of a cofferdam across the entrance
channel, and in the excavations kept dry by means of pumps. The
foundations under the sills and adjacent side walls are carried down
to a lower level than the rest, and if possible to a water-tight
stratum, to prevent infiltration of water under them owing to the
water-pressure on the upper side of the gates; or sometimes one or two
rows of sheet piling have been driven across the lock under the sills
to an impermeable stratum, to stop any flow. The foundations for the
sills consist usually of concrete deposited in a trench extended out
under the adjoining side walls. The sill, projecting generally about 2
ft. above the adjacent gate floor over which the gates turn, is built
of granite; and the same material is also used for the hollow quoins
in which the heelpost, or pivot, of the dock gates turns, and which,
together with the sills, are exposed to considerable wear. The side
walls of the lock-chamber are very similar in construction to the dock
walls; but they are strengthened against the loss of water-pressure in
front of them when the water is lowered in the chamber by an inverted
arch of masonry, brickwork or concrete, termed an "invert," laid
across the bottom of the chamber along its whole length, against which
the toe of each side wall abuts and effectually prevents any forward
movement. The side walls also, alongside the gates at each end, abut
against a thick level gate floor and apron, and, moreover, are
considerably widened to provide space for the sluiceways and gate
machinery.

The new Florida lock (fig. 20), forming the main entrance through the
new approach harbour and tidal harbour to the Eure dock and other
docks of the port of Havre, is the largest lock hitherto constructed.
It has an available length of chamber between the gates of 805 ft., a
width of 98-1/2 ft., and depths over the sills of 15-3/4 ft. at the
lowest low water of spring tides, 23-1/2 ft. at low water of neap
tides, 35 ft. at high water of neap tides, and 40-1/2 ft. at high
water of spring tides. Owing to the alluvial stratum at the site of
the lock close to the Seine estuary, of which it doubtless at one time
formed part, the foundations for the sill and side walls or heads at
each end of the lock were executed by aid of compressed air. The
foundations for these heads were carried down to an impermeable
stratum by means of two bottomless caissons, filled eventually with
concrete, 213-1/2 ft. long across the lock and 105 ft. wide in the
line of the lock at the upper end, and 206-3/4 ft. long and 116-1/2
ft. wide at the lower end, to a depth of 18 ft. below the sill at the
upper end, and 41 ft. at the lower end, owing to the dip down seawards
and southward of the water-tight stratum. These caissons were provided
for their sinkage with temporary dams of masonry closing the opening
of the lock at the extremities of each caisson, enabling the gates to
be subsequently erected under their shelter. The junctions between the
foundations of the heads and the adjacent foundations were effected by
small movable caissons carried down in recesses provided in the buried
caissons. The connexions with the adjacent quay walls were
accomplished by two supplementary side caissons at the end of each
head; and the north side wall of the lock was founded by means of
seven bottomless caissons sunk by aid of compressed air, on account of
the proximity of the tidal harbour on that side. The south side wall
was founded for a length of about 200 ft. at its western end in an
excavated trench kept dry by pumping; but the greater portion was
founded in a dredged trench in which bearing piles were driven under
water, on which the masonry was built in successive layers, about
3-1/4 ft. thick, in a movable caisson 93-1/2 ft. long and 37-3/4 ft.
wide; whilst a bottomless caisson, left in the work, was employed for
founding about 100 ft. of wall at the eastern end. The bed of concrete
also, 10 ft. thick, forming the floor of the chamber, was carried out
for 82 ft. at the western end in the open air, and the remainder in
the same movable caisson as used for the south wall. Two sluiceways on
each side running the whole length of the lock, differing 6-1/2 ft. in
level, communicate with the lock-chamber through openings in the side
walls, 67-1/4 ft. apart, and provide for the filling and emptying of
the chamber.

Dock gates.

The gates closing the entrances and locks at docks are made of wood or
of iron. In iron gates, the heelpost, or a vertical closing strip
attached to the outer side of the gate close to the heelpost, the
meeting-post at the end of each gate closing against each other when
the gates are shut, and the sill piece fitting against the sill are
generally made of wood. Wooden gates consist of a series of horizontal
framed beams, made thicker and put closer together towards the bottom
to resist the water-pressure increasing with the depth, fastened to
the heelpost and meeting-post at the two ends and to intermediate
uprights, and supporting water-tight planking on the inner face (fig.
21). Iron gates have generally an outer as well as an inner skin of
iron plates braced vertically and horizontally by plate-iron ribs, the
horizontal ribs being placed nearer together and the plates made
thicker towards the bottom (figs. 22 and 23). Greenheart is the wood
used for gates exposed to salt water, as it resists the attack of the
teredo in temperate climates. As cellular iron gates are made
water-tight, and have to be ballasted with enough water to prevent
their flotation, or are provided with air chambers below and are left
open to the rising tide on the outer side above, the gates are light
in the water and are easily moved; whereas greenheart gates with their
fastenings are considerably heavier than water, so that a considerable
weight has to be moved when the water is somewhat low in the dock and
the gates therefore only partially immersed. On the other hand, wooden
gates are less liable than iron gates to be seriously damaged if run
into by a vessel.

Dock gates are sometimes made straight, closing against a straight
sill (figs. 20 and 23); and occasionally they are made segmental with
the inner faces forming a continuous circular arc and closing against
a sill corresponding to the outer curves of the gates (fig. 22), or by
means of a projecting sill piece against a straight sill (fig. 21).
More frequently the gates, curved on both faces, meet at an angle
forming a Gothic arch in plan, and close by aid of a projecting piece
against a straight sill, which in the Barry entrance gates is modified
by making the outer faces nearly straight (fig. 19), giving an unusual
width to the centre of the gates. The pressures produced by a head of
water against these gates when closed depends not only on the form of
the gates, but also upon the projection given to the angle of the sill
in proportion to the width of the lock, which is known as the rise,
and is generally placed at a distance along the centre line of the
lock, from a line joining the centres of the heel-posts, of about
one-fourth the width. With straight gates, the stresses consist, first
of a transverse stress due to the water-pressure against the gate,
which increases with the head of water and length of the gate; and
secondly, of a compressive stress along the gate, resulting from the
pressure of the other gate against its meeting-post, which is equal to
half the water-pressure on the gate multiplied by the tangent of half
the angle between the closed gates, varying inversely with the rise.
Though an increase in the rise reduces this stress, it increases the
length of the gate and the transverse stress, and also the length of
the lock. By curving the gates suitably, the transverse stress is
reduced and the longitudinal compressive stress is augmented, till at
last, when the gates form a horizontal segmental arch, the stresses
become wholly compressive and uniform in each horizontal section,
increasing with the depth; and the total stress is equal to the
pressure on a unit of surface multiplied by the radius of curvature.
Though the water-pressure is most uniformly and economically borne by
cylindrical gates, they are longer, and encroach more upon the lines
of quay with their curved recesses than straighter gates; and,
consequently, Gothic-arched gates are often preferred. Straight gates
afford the greatest simplicity in construction.

Gates in wide entrances or locks are generally supported towards their
outer end by a roller running along a castiron roller-path on the gate
floor (figs. 19, 21 and 22), as well as by the heelpost, fitted over a
steel pivot at the bottom, and tied back against the hollow quoins at
the top by anchor straps and bolts, on which the gate turns. In some
cases, by placing the water ballast in iron gates close to the
heelpost, a roller has been dispensed with, even, for instance, at the
wide entrance at Havre (fig. 23). The gates are opened and closed,
either by an opening and a closing chain for each gate, fastened on
either side and worked from opposite side walls by hydraulic power, or
by a single hydraulic piston or bar hinged to the inner side of each
gate (figs. 19 and 20). The latter system has the advantages of being
simpler and occupying less space in the side walls, of avoiding the
slight loss of available depth over the sill due to the two closing
chains crossing on the sill when the gates are open, and especially of
keeping the gates closed against a swell in exposed sites.

Caissons for docks.

A sliding or rolling caisson is occasionally placed across each end of
a lock in place of a pair of dock gates, being Caissons drawn back
into a recess at the side for opening docks. the lock. As a caisson
chamber has to be covered for over to provide a continuous quay or
roadway on the top, a lowering platform is supplied to enable the
caisson to pass under the small girders spanning the top of the
chamber, or the caisson is sunk down sufficiently (fig. 24). The
caisson is furnished with an air chamber to give it flotation, which
is adjusted by ballast according to the depth of water. The advantages
of a caisson, as compared with a pair of gates, are that the gate
recesses, gate floor, hollow quoins and arrangements for working in
the side walls are dispensed with, so that the lock can be made
shorter, and the work at each head is rendered less complicated. The
caisson itself also serves as a very strong movable bridge, and
therefore is often preferred at dockyards to dock gates. By
improvements in the hauling machinery, a caisson can open or close a
lock as quickly as dock gates; the caissons at Zeebrugge lock, at the
entrance to the Bruges ship canal, are drawn across the lock or into
their chamber by electricity in two minutes. A caisson is specially
useful in cases where there may be a head of water on either side, as
then it takes the place of two pairs of gates pointing in opposite
directions, or for closing an entrance against a current. A caisson,
however, requires a much larger amount of material than a pair of dock
gates, and a considerable width on one side for its chamber, so that
under ordinary conditions gates are generally used at docks.

A ship caisson, so called from its presenting some resemblance in
section to the hull of a vessel, occupies too much time in being
towed, floated into position, and sunk into grooves at the bottom and
sides of an entrance for closing it, and then refloated and towed away
for opening the entrance again, to be used at entrances and locks to
docks (fig. 25). Being, however, simple in construction, taking up
little space, and requiring no chamber or machinery for moving it,
this form of caisson is generally used for closing the entrance to a
graving dock, where it remains for several days in place during the
execution of repairs to a vessel in the dock. A ship caisson only
requires the admission of sufficient water to sink it when in position
across the entrance to a graving dock; and this water has to be pumped
out before it can be floated, and removed to some vacant position in
the neighbouring dock till it is again required. Like a sliding or
rolling caisson, it provides a bridge for crossing over the entrance
of the graving dock when in position.

_Graving Docks._ - Provision has to be made at ports for the repairs of vessels frequenting them. The simplest arrangement is a timber gridiron, on which a vessel settles with a falling tide, and can then be inspected and slightly cleaned and repaired till the tide floats it again. Inclined slipways are sometimes provided, up which a vessel resting in a cradle on wheels can be drawn out of the water; and they are also used for shipbuilding, the vessel when ready for launching being allowed to slide down them into the water. Graving or dry docks, however, opening out of a dock, are the usual means provided for enabling the cleaning and repairs of vessels to be carried out.

A graving dock consists of an enclosure, surrounded by side walls
stepped on the face, and paved at the bottom with a thick floor
sloping slightly down from the centre to drains along the sides, long
enough to receive the longest vessel likely to come to the port. Its
entrance, at the end adjoining the dock, is just wide enough to admit
the vessel of greatest beam, and deep enough over the sill to receive
the vessel of greatest draught, when light, at the lowest water-level
of the dock (figs. 26 and 27). Graving docks are constructed of
masonry, brickwork or concrete, or formerly in America of timber; they
should be founded on a solid impervious stratum, or, where that is
impracticable, they should be built upon bearing piles and enclosed
within sheet piling, to prevent settlement and the infiltration of
water under pressure below the dock. Keel blocks are laid along the
centre line of the dock, for the keel of the vessel to rest on when
the water is pumped out; and the vessel is further supported on each
side by timber shores supported on the steps or "altars" of the side
walls, which are lined with granite or other hard stone, or blue
bricks, or, when constructed of concrete, with a facing of stronger
concrete, to enable these altars to withstand the wear and shocks to
which they are subjected. Steps and slides are provided at convenient
places at the sides to give access for men and materials to the bottom
of the dock; and culverts and drains lead the water to pumps for
removing the water from the dock when the entrance has been closed,
and to keep it dry whilst a vessel is under repair. Culverts in the
side walls of the entrance enable water to be admitted for filling the
dock to let the vessel out. Graving docks are generally closed by ship
caissons; but where they open direct on to a tidal river, and there is
some exposure, gates are adopted, or sometimes sliding caissons.

The dimensions of graving docks vary considerably with the nature of
the trade and the date of construction; and sometimes an intermediate
entrance is provided to accommodate two smaller vessels. The sizes of
some of the largest graving docks are as follows: Liverpool, Canada
dock, 925-1/2 ft. long, 94 ft. width of entrance, and 29 ft. depth at
the ordinary water-level in the dock; Southampton, 851-3/4 ft. by 90
ft., and 29-1/2 ft. depth at high-water neaps (figs. 26 and 27);
Tilbury, 875 ft. by 70 ft. by 31-1/2 ft.; and Glasgow, 880 ft. by 80
ft. by 26-1/2 ft.

_Floating Dry Docks._--Where there is no site available for a graving
dock, or the ground is very treacherous, floating dry docks, built
originally of wood, but more recently of iron or steel, have
occasionally been resorted to. The first Bermuda dock towed across the
Atlantic in 1869, and the new dock launched in 1902, 545 ft. by 100
ft., are notable examples. Water is admitted into the pontoon at the
bottom to sink the dock sufficiently to admit a vessel at its open
end; and then the water is pumped out of compartments in the pontoon
till the vessel is raised out of water. It is only necessary to find a
sheltered site, with a sufficient depth of water, for conducting the
operations. (L. F. V.-H.)

DOCKET (perhaps from "dock," to curtail or cut short, with the diminutive suffix _et_, but the origin of the word is obscure; it has come into use since the 15th century), in law, a brief summary or digest of a case, or a memorandum of legal decisions; also the alphabetical list of cases down for trial, or of suits pending. Such cases are said to be "on the docket." In commercial use, a docket is a warrant from the custom-house, stating that the duty on goods entered has been paid, or the label fastened to goods, showing their destination, value, contents, &c., and, generally, any indorsement on the back of a document, briefly setting out its contents.

DOCK WARRANT, in law, a document by which the owner of a marine or river dock certifies that the holder is entitled to goods imported and warehoused in the docks. In the Factors Act 1889 it is included in the phrase "document of title" and is defined as any document or writing, being evidence of the title of any person therein named ... to the property in any goods or merchandise lying in any warehouse or wharf and signed or certified by the person having the custody of the goods. It passes by indorsement and delivery and transfers the absolute right to the goods described in it. A dock warrant is liable to a stamp duty of threepence, which may be denoted by an adhesive stamp, to be cancelled by the person by whom the instrument is executed or issued.

DOCKYARDS. In the fullest meaning of the word, a "dock-yard" (or "navy yard" in America) is a government establishment where warships of every kind are built and repaired, and supplied with the men and stores required to maintain them in a state of efficiency for war. Thus a dockyard in this extended sense would include slips for building ships, workshops for manufacturing their machinery, dry docks for repairing them, stores of arms, ammunition, coal, provisions, &c., with basins in which they may lie while being supplied with such things, and an establishment for providing the _personnel_ necessary for manning them. But in practice few, if any, existing dockyards are of so complete a nature; many of them, for instance, do not undertake the building of ships at all, while others are little more than harbours where a ship may replenish her stores of coal, water and provisions and carry out minor repairs. Private firms are relied upon for the construction of many ships down to an advanced stage, the government dockyards completing and equipping them for commission.

_Great Britain._--Previous to the reign of Henry VIII., the kings of England had neither naval arsenals nor dockyards, nor any regular establishment of civil or naval officers to provide ships of war, or to man them. There are, however, strong evidences of the existence of dockyards, or of something answering thereto, at very early dates, at Rye, Shoreham and Winchelsea. In November 1243 the sheriff of Sussex was ordered to enlarge the house at Rye in which the king's galleys were kept, so that it might contain seven galleys. In 1238 the keepers of some of the king's galleys were directed to cause those vessels to be breamed, and a house to be built at Winchelsea for their safe custody. In 1254 the bailiffs of Winchelsea and Rye were ordered to repair the buildings in which the king's galleys were kept at Rye. At Portsmouth and at Southampton there seem to have been at all times depots for both ships and stores, though there was no regular dockyard at Portsmouth till the middle of the 16th century. It would appear, from a curious poem in Hakluyt's _Collection_ called "The Policie of Keeping the Sea," that Littlehampton, unfit as it now is, was the port at which Henry VIII. built

"his great _Dromions_
Which passed other great shippes of the commons."

The "dromion," "dromon," or "dromedary" was a large warship, the prototype of which was furnished by the Saracens. Roger de Hoveden, Richard of Devizes and Peter de Longtoft celebrate the struggle which Richard I., in the "Trench the Mer," on his way to Palestine, had with a huge dromon,--"a marvellous ship! a ship than which, except Noah's ship, none greater was ever read of." This vessel had three masts, was very high out of the water, and is said to have had 1500 men on board. It required the united force of the king's galleys, and an obstinate fight, to capture the dromon.

The foundation of a regular British navy, by the establishment of dockyards, and the formation of a board, consisting of certain commissioners for the management of its affairs, was first laid by Henry VIII., and the first dockyard erected during his reign was that of Woolwich. Those of Portsmouth, Deptford, Chatham and Sheerness followed in succession. Plymouth was founded by William III. Pembroke was established in 1814, a small yard having previously existed at Milford.

The most important additions yet made at any one period to the dockyard and harbour works required to meet the necessities of the British fleet were those sanctioned by the Naval Works Acts of 1895 and subsequent years, the total estimated cost, as stated in the act of 1899, being over 23-1/2 millions sterling. The works proposed under these acts were classified under three heads, viz. (a) the enclosure and defence of harbours against torpedo attacks; (b) adapting naval ports to the present needs of the fleet; (c) naval barracks and hospitals. Under the first heading were included the defensive harbours at Portland, Dover and Gibraltar. Under heading (b) were included the deepening of harbours and approaches, the dockyard extensions at Gibraltar, Keyham (Devonport), Simons Bay, and Hong-Kong, with sundry other items. Under heading (c) were included the naval barracks at Chatham, Portsmouth and Keyham; the naval hospitals at Chatham, Haslar and Haulbowline; the colleges at Keyham and Dartmouth; and other items.

Great Britain possesses dockyards at Portsmouth, Devonport, Chatham, Malta and Gibraltar, each in charge of an admiral-superintendent, and at Sheerness and Pembroke in charge of a captain-superintendent, together with establishments at Ascension, Bermuda, Simons Town (Cape of Good Hope), Queenstown (Haulbowline); Hong-Kong, Portland, Sydney and Weihaiwei. The Indian Government has dockyards at Bombay and Calcutta. The medical establishments include Ascension, Bermuda, Cape of Good Hope, Chatham, Dartmouth, Deal, Gibraltar, Haslar, Haulbowline, Hong-Kong, Malta, Osborne, Plymouth, Portland, Portsmouth, Sheerness, Sydney, Yarmouth, Yokohama and Weihaiwei.

The arrangements for the administrative control of the dockyards have varied with those adopted for the regulation of the navy as a whole. (See ADMIRALTY ADMINISTRATION; and NAVY: _History_.) At the present time, whether at home or abroad, they lie within the province of the controller of the navy (the third lord of the board of admiralty); and the director of dockyards, whose office, replacing that of surveyor of dockyards was created in December 1885, is responsible to the controller for the building of ships, boats, &c., in dockyards, and for the maintenance and repair of ships and boats, and of all steam machinery in ships, boats, dockyards and factories. The director of naval construction, who is also deputy-controller, is responsible, not only for the design of ships, but for their construction, in the sense that he approves great numbers of working drawings of structural parts prepared at the dockyards. But the director of dockyards is the admiralty official under whose instructions the work goes on, involving the employment and supervision of an army of artisans and labourers. Instructions, therefore, emanate from the admiralty, but the details lie with the dockyard officials, and in practice there is a considerable decentralization of duties.

The chief function of a dockyard is the building and maintaining of ships in efficiency. The constructive work is carried out under the care of the chief constructor of the yard, in accordance with plans sent down from the admiralty. The calculations for displacement, involving the draught of water forward and aft, have already been made, and, in order to ensure accuracy in the carrying out of the design, an admirable system has been devised for weighing everything that is built into the new ships or that goes on board; and it is astonishing how very closely the actual displacement approximates to that which was intended, particularly when the tendency of weights to increase, in perfecting a ship for commission, is considered.

The ship having been built to her launching weight, the duty of putting her into the water devolves upon the chief constructor of the yard, and failures in this matter are so extremely rare that it may almost be said they do not occur. As soon as the ship is water-borne the responsibility falls upon the king's harbour master, who has charge of her afloat and of moving her into the fitting basins. When the ship has been brought alongside the wharf, the responsibility of the chief constructor of the yard is resumed, and the ship is carried forward to completion by the affixing of armour plating (if that has not been done before launching), the mounting of guns, the instalment of engines, boilers, and electrical and hydraulic gear, and the fitting of cabins for officers, mess places for men, and storerooms, and a vast volume of other work unnecessary to be specified. In regard to the complicated details of guns and torpedoes, the captains of the gunnery and torpedo schools have a function of supervision. The captain of the fleet reserve also closely watches the work, because, when the heads of all departments have reported the ship to be ready, she has to be inspected by the commander-in-chief at the port, and then passed into the fleet reserve as ready for sea, and there the captain of the fleet reserve is responsible for her efficiency. Other important officers of a dockyard are the chief engineer; the superintendent civil engineer, who has charge of the work involved in keeping all buildings, docks, basins, caissons, roads, &c., in repair; the naval store officer, who has charge of most of the stores in the dockyard; and the cashier of the yard, whose name sufficiently expresses his duties.

The system of conducting business at the dockyards is analogous to that which prevails at the admiralty. There is personal communication between the officers responsible for the work, and facilities are afforded for coming to rapid decisions upon matters that are in hand, and the operations are conducted with an ease which contributes much to efficiency. In 1844 the custom was introduced of all the principal officers of the dockyard meeting at the superintendent's office at 9.30 A.M. every day, to hear the orders from the admiralty and discuss the work of the day. But this system of "readings" was abolished at the beginning of 1906, the naval establishments inquiry committee considering that the assembling of the officials was unnecessary since the communications after reception are copied and sent to the departments concerned.

The police force necessary in a dockyard is in some cases supplied from the London metropolitan police, and is under the orders of the superintendent of the yard for duties connected with it, and under the commissioner of police for the discipline and disposition of the force. The charges are, of course, paid by the admiralty, and the system answers well.

_United States._--The shore stations under control of the Navy Department (see also ADMIRALTY ADMINISTRATION), and collectively known as naval stations, are under different names according to their nature. Of those called _Navy Yards_, and intended for the general purpose of sources of supply and for repairs of ships, there are within the United States eight in number. Two of them are on the Pacific coast, situated on Puget Sound, at Bremerton, Washington; and at Mare Island, near San Francisco. The other six are on the Atlantic coast, and are situated at Portsmouth, N.H.; Boston, Mass.; Brooklyn, N.Y.; Philadelphia, Pa.; Washington, D.C.; and Norfolk, Va. There are also naval stations at Port Royal and Charleston, S.C.; Key West and Pensacola, Fla.; New Orleans, La.; Guantanamo, Cuba; Culebra and San Juan, Porto Rico; Honolulu, H.I.; Cavite, P.I.; Tutuila, Samoa; and Island of Guam, in the Ladrones Islands. The floating dock Dewey, having a lifting capacity of 18,500 gross tons with a free-board of 2 ft., was stationed in the Philippine Islands in 1906.

Besides these, there are important naval stations established for special purposes, which in some cases are also available for ports of supply and for repairs. These are: the U.S. Naval Academy, Annapolis, Md., for the instruction of naval cadets; the training stations at Newport, R.I., and Yerba Buena Island, Cal., for the instruction of apprentices; the proving ground at Indian Head, Md., on the Potomac river, where all government-built ordnance is tested; the War College at Newport, R.I., for the instruction of officers; the torpedo station at Newport, for the instruction of officers and men in torpedoes, electricity and submarine diving; the naval observatory at Washington; and the marine post at Sitka, Alaska. Coaling depots have been established at Honolulu, Pago Pago, Samoan Islands, and at Manila, P.I. Naval hospitals are located at the Portsmouth, Boston, New York, Philadelphia, Washington, Norfolk and Mare Island yards; at Las Animas, Colo.; at Newport, R.I.; Canacao, P.I.; Sitka, Alaska; and Yokohama, Japan.

The commandant of a navy yard and station, who is usually a rear-admiral, is its commander-in-chief. His official assistants are called heads of departments. The captain of the yard, who is next in succession to command, has general charge of the water front and the ships moored there, and of the police of the navy yard; it is his duty to keep the commandant informed as to the nature and efficiency of all work in progress. The equipment officer has charge of anchors, chains, rigging, sails and the electric generating plant. The other heads of departments are the ordnance officer, the naval constructor, the engineering officer, the general storekeeper, the paymaster of the yard, the surgeon and the civil engineer. The clerks and draughtsmen employed by these officers are appointed under civil service rules, and their employment is continuous so long as funds are available. The foremen are selected by competitive examination, and their number is fixed. In the employment of mechanics and labourers, veterans are given preference, after which follow persons previously employed who have displayed especial efficiency and good conduct. The rates of wages are determined semi-annually by a board of officers, who ascertain the wages paid by private establishments in the vicinity of the navy yard. Eight hours constitute the legal work day. When emergencies necessitate longer hours the workmen are paid at the ordinary rate plus 50%.