Caisson lock
Encyclopedia
Lock (water transport)
A lock is a device for raising and lowering boats between stretches of water of different levels on river and canal waterways. The distinguishing feature of a lock is a fixed chamber in which the water level can be varied; whereas in a caisson lock, a boat lift, or on a canal inclined plane, it is...
in which a narrowboat
Narrowboat
A narrowboat or narrow boat is a boat of a distinctive design, made to fit the narrow canals of Great Britain.In the context of British Inland Waterways, "narrow boat" refers to the original working boats built in the 18th, 19th, and 20th centuries for carrying goods on the narrow canals...
is enclosed in a sealed box and raised or lowered between two water levels. It was intended primarily as a water-saving measure, but also to save construction costs when compared with other engineering solutions. It was capable of replacing up to seven conventional locks.http://www.coalcanal.org/history/Rowley/tLev1.htm Other design benefits were speed of boat decent /ascent, and only a little loss of water when operating compared with a conventional boat lock, which required a volume of water equal to a boat and its cargo weight for each lock used in a flight between levels.
History
It was first demonstrated at OakengatesOakengates
Oakengates is a town in the borough of Telford and Wrekin and ceremonial county of Shropshire, England, and now forms part of the new town of Telford...
on the now lost Shropshire Canal
Shropshire Canal
The Shropshire Canal was a tub boat canal built to supply coal, ore and limestone to the industrial region of east Shropshire, England, that adjoined the River Severn at Coalbrookdale...
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 tonne
Tonne
The tonne, known as the metric ton in the US , often put pleonastically as "metric tonne" to avoid confusion with ton, is a metric system unit of mass equal to 1000 kilograms. The tonne is not an International System of Units unit, but is accepted for use with the SI...
s and weighed about 170 tonnes, including the water in it, so about 100 tonnes of ballast would have been needed to give neutral buoyancy
Neutral buoyancy
Neutral buoyancy is a condition in which a physical body's mass equals the mass it displaces in a surrounding medium. This offsets the force of gravity that would otherwise cause the object to sink...
. 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
Kennet and Avon Canal
The Kennet and Avon Canal is a waterway in southern England with an overall length of , made up of two lengths of navigable river linked by a canal. The name is commonly used to refer to the entire length of the navigation rather than solely to the central canal section...
had inspected Weldon's device and recommended it to the adjoining Somerset Coal Canal
Somerset Coal Canal
The Somerset Coal Canal was a narrow canal in England, built around 1800 from basins at Paulton and Timsbury via Camerton, an aqueduct at Dunkerton, Combe Hay, Midford and Monkton Combe to Limpley Stoke where it joined the Kennet and Avon Canal...
for use at Combe Hay
Combe Hay
Combe Hay is a village and civil parish in the English county of Somerset. It falls within the Cotswolds Area of Outstanding Natural Beauty. The parish has a population of 149.-History:Combe Hay was known in the Domesday Book as Cumb...
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.
The first lock was completed in 1797 under Weldon's supervision. The device was demonstrated to the Prince Regent
Prince Regent
A prince regent is a prince who rules a monarchy as regent instead of a monarch, e.g., due to the Sovereign's incapacity or absence ....
(later George IV
George IV of the United Kingdom
George IV was the King of the United Kingdom of Great Britain and Ireland and also of Hanover from the death of his father, George III, on 29 January 1820 until his own death ten years later...
), but was found to suffer from various engineering problems, possibly caused by the soft fuller's earth
Fuller's earth
Fuller's earth is any non-plastic clay or claylike earthy material used to decolorize, filter, and purify animal, mineral, and vegetable oils and greases.-Occurrence and composition:...
rock stratum in the area.
Method of operation
The system depended on the submerged, sealed box (the "caissonCaisson (engineering)
In geotechnical engineering, a caisson is a retaining, watertight structure used, for example, to work on the foundations of a bridge pier, for the construction of a concrete dam, or for the repair of ships. These are constructed such that the water can be pumped out, keeping the working...
", from the French for "large chest") being heavily ballasted to achieve neutral buoyancy
Buoyancy
In physics, buoyancy is a force exerted by a fluid that opposes an object's weight. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus a column of fluid, or an object submerged in the fluid, experiences greater pressure at the bottom of the...
, so it was never possible in ordinary operation to lift it to water level to allow a descending boat to float in. Instead, a masonry
Masonry
Masonry is the building of structures from individual units laid in and bound together by mortar; the term masonry can also refer to the units themselves. The common materials of masonry construction are brick, stone, marble, granite, travertine, limestone; concrete block, glass block, stucco, and...
chamber ("cistern
Cistern
A cistern is a waterproof receptacle for holding liquids, usually water. Cisterns are often built to catch and store rainwater. Cisterns are distinguished from wells by their waterproof linings...
") was built with walls higher than the water level in the top pound
Canal pound
A canal pound, aka reach, is the stretch of level water impounded between two canal locks. Canal pounds can vary in length from the non-existent, where two or more immediately adjacent locks form a lock staircase, to many miles....
and itself filled completely with water, so that even at its upper position the box remained below the surface. A vertically sliding door sealed the caisson from the top pound and kept the water in.
The mechanism was operated from the top level. For a descent, the box was first wound into its upper position using a double rack-and-pinion mechanism, then drawn tightly against the frame of the opening using a ratchet
Ratchet (device)
A ratchet is a device that allows continuous linear or rotary motion in only one direction while preventing motion in the opposite direction. Because most socket wrenches today use ratcheting handles, the term "ratchet" alone is often used to refer to a ratcheting wrench, and the terms "ratchet"...
mounted on the top of the wall. The outer door was then drawn up with another rack-and-pinion. At this point the water levels in the top pound and inside the box would have been roughly equal, but as the inner door – the box door – swung outwards horizontally (like a normal single lock gate) it would not open if the outer level was to any extent higher. A small equalising cock was therefore provided. The door was opened, the boat was floated in, the doors closed and the ratchet released. Because the entering boat would displace its weight of water back into the pound, the total weight of the box was always the same and no great endeavour was needed to wind it up and down. However, the operators could release a little water into the box to assist the descent. Water pressure against the outward-opening doors kept them firmly closed and watertight.
At the lower position the process was reversed. Here the water pressure was strong enough to press the box tightly into position against the exit opening. Another rack and pinion (again operated from above) lifted the outer gate, the levels were equalised again, the inner door on the box was swung open and the boat floated out. Apart from the inevitable leakage, no water had been used in the process.
Dimensions as built
- height: 20 metres (66 ft)
- width: from 3 to 6 m (9.8 to 19.7 ft)
- length: 27 metres (89 ft)
- toothed rack: 14 metres (46 ft)
- rotation: approximately 7 minutes
Tests
- No 1: February 1798: cracks
- No 2: June 1798: success
- No 3: April 1799: success
- No 4: April 1799: success, in the presence of the Prince RegentPrince RegentA prince regent is a prince who rules a monarchy as regent instead of a monarch, e.g., due to the Sovereign's incapacity or absence ....
- No 5: April 1799: success, transport of 60 passengers
- No 6: May 1799: box jammed by a projecting stone
Abandonment
The May 1799 test, above, occurred when a party of investors was aboard the vessel and they nearly suffocated before they could be freed. Work on the second lock was suspended (the third lock had not been started) and early in the following year an inclined planeCanal inclined plane
An inclined plane is a system used on some canals for raising boats between different water levels. Boats may be conveyed afloat, in caissons, or may be carried in cradles or slings. It can be considered as a specialised type of cable railway....
, to carry boats’ cargoes in wheeled tubs, was built instead. Eventually a flight of nineteen locks on a longer alignment up the slope was constructed, with a Boulton & Watt Steam Pumping Station, capable of lifting 5,000 tons
Long ton
Long ton is the name for the unit called the "ton" in the avoirdupois or Imperial system of measurements, as used in the United Kingdom and several other Commonwealth countries. It has been mostly replaced by the tonne, and in the United States by the short ton...
of water in 12 hours, used to recirculate the water.
Other installations
In about 1817, the Regents Canal Company built one of these locks at the site of the present-day Camden LockCamden Lock
Camden Lock, or Hampstead Road Locks is a twin manually-operated lock on the Regent's Canal in Camden Town, London Borough of Camden. The sign on each of the twin locks reads "Hampstead Road Lock 1"...
, north London. Here the motivation was, again, water supply problems. The change in level is much lower than that at Combe Hay. Soon, they too substituted conventional locks. No commercially successful example has ever been built.
See also
- Canal inclined planeCanal inclined planeAn inclined plane is a system used on some canals for raising boats between different water levels. Boats may be conveyed afloat, in caissons, or may be carried in cradles or slings. It can be considered as a specialised type of cable railway....
- Boat liftBoat liftA boat lift, ship lift, or lift lock is a machine for transporting boats between water at two different elevations, and is an alternative to the canal lock and the canal inclined plane....
- Caisson (water transport)