Caisson lock

A caisson lock was invented in the late 18th century as a solution to minimise the use of great volumes of water required to raise and lower canal boats through large height differences. It was normal to only raise and lower boats through small height differences of a few feet when traversing undulating terrain. A solution was required when either large height differences were encountered or water was in short supply. The Caisson (or Caisoon) was thought to be one solution. The technology of the time was not capable of achieving this type of construction economically with current building materials. This design attempted was a type of canal lock in which a narrowboat is floated into a sealed watertight box and raised or lowered between two different canal water levels. It was designed primarily as a water-saving measure, and also was an attempt to minimise construction costs compared with other engineering solutions of the time. In use it was capable of replacing up to seven conventional locks. Other design benefits were speed of boat descent/ascent, and only a little loss of water when operating compared with a conventional boat lock.

It was first demonstrated at Oakengates on a now lost section of the Shropshire Canal in 1792, where its inventor, Robert Weldon (b:?1754 to d:1810) built a half-scale model. He claimed that his design would solve the problem of water supply in dry seasons or at greater elevations, be cheaper than building aqueducts or tunnels, and be quicker to operate than the number of surface locks his design could replace. He patented his invention as the 'Hydrostatick Caisson Lock'. The full-sized box, or "trunk", would probably have displaced about 270 tonnes and weighed about 170 tonnes, including the water in it, so about 100 tonnes of ballast would have been needed to give neutral buoyancy. The box would have needed to be strong enough to withstand the pressure of 50 feet (15 m) of water i.e. about 3,000 lbf/ft² (150 kPa) gauge pressure at the bottom of the chamber.

The proprietors of the Kennet and Avon Canal Company had inspected Weldon's device and recommended it to the adjoining Somerset Coal Canal for use at Combe Hay on their new line, to overcome water supply problems there. (The Somerset Coal Canal led directly to the Kennet and Avon and it was in the latter company's interest that the new route be opened as quickly as possible.) Three such locks were proposed, each to be 80 ft (24.3 m)long and 60 ft (18.2 m) deep and containing a closed wooden box which could take the boat. This box moved up and down in the 60 ft (18.2 m) deep pool of water, which never left the lock.

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