Caissons – to include or not?
A dictionary check shows that the word ‘caisson’ is an old French word meaning ‘box’. The word
has several uses.
In some forms of interior decoration a caisson is a sunken panel in a door or ceiling. In
military usage it refers to an ammunition box or a vehicle designed to carry ammunition. In
relation to water it is applied to water tight boxes used for a variety of purposes. A
caisson dam is used to create a working environment in which the caisson structure is sunk
below water level and the water then pumped out so that workers can build structures that will
eventually be underwater.
The Brunels used caissons to build the first tunnel under the Thames. Water was kept out by
pumping air under high pressure into the sealed caisson. It was an extremely hazardous working
environment and caused serious health problems for all who entered it.
In dockyards and waterways a caisson lock refers to a large sealed box that is designed to be
sunk across a channel to hold water in or hold it back. It is a form of lock gate.
In Chatham Naval Dockyard there were several such caissons. There was a caisson across each of
the 2 channels connecting the 3 basins, and each of the graving docks was sealed off by a
caisson.

When empty of ballast the caissons float, and can be moved about comparatively easily. Once
in position across the channel or dock entrance valves are opened to allow water into the
ballast chamber and the caisson sinks to the bottom. As it sinks it forms a seal between the
walls of the dock. When used to close off a dock water can then be pumped out of the dock to
create a dry dock.
The later versions of the caissons at Chatham were fitted with air valves so that water could
be forced out of the ballast chamber by pumping in air at 100psi. Once empty the caisson could
be moved aside to allow ships to enter or leave the dock.
In Chatham Dockyard the 2 caissons across connecting channels, and those across graving docks
connected to the basins, also carried roadways and railway tracks across the channels and docks.
This did not apply to the caissons for the graving docks that connected to the river since the
railway tracks ran along the dockside at the inner end of these docks.
My dilemma
When I began work on this history of movable bridges I spent some time trying to establish a
definition of what I – and a handful of supporters – would count as a movable bridge. The
definition that we eventually agreed on is –
A movable bridge is a structure which has been designed to have two alternative positions and
which can be moved back and forth between those positions in a controlled manner. The three
primary purposes of movable bridges are as a form of defense, to allow conflicting flows of
traffic to pass through a crossing point or to move traffic across a waterway.
I excluded lock gates, partly because the main purpose of a lock gate is to be a water barrier.
Many lock gates do provide a walkway across the top – sometimes this walkway may have become a public
right of way – but the lock gate is nonetheless designed mainly as a barrier to water movement.
There is also the problem of just how many lock gates there are on canals, managed rivers and
in dockyards – there must be thousands. I had also dismissed caissons from the study on the
same principle.
However, after talking to Chris Scott, Alan Rayner and Eric Cross – all former tradesmen at
Chatham Dockyard and now volunteers with the Chatham Dockyard Historical Society – I am having
doubts about the exclusion of caissons from my study.
The caissons in Chatham dockyard were designed primarily as water barriers, but they were also
a planned and systematic part of the transport infrastructure within the dockyard, making it
possible to move trains and heavier road vehicles around within the docks. The caissons were
in effect semi-submersible movable structures.
If I decide to include the Chatham dockyard caissons as movable bridges then I also will have
to search for and add many more around other dockyards – perhaps adding hundreds of new
structures to the study.
If I decide not to include these and other caissons is my study truly comprehensive?
Alan and Chris have supplied me with a thick file of records of the caissons at Chatham, for
which I am very appreciative. I will be taking a more detailed look at those notes in the
near future so that I can write a more detailed account to include in a general description
of the dockyard. In the meantime I will consider whether or not to include these and other
caissons to my definition of a movable bridge.
Comments are always welcome!