SLHS 1. Lock Construction

● These items have aspects that uniquely contribute to national or world history.

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Even with modern cranes and power tools constructing a lock is a major undertaking.

(Grantham Canal, Aug 2020)

Location Of Locks

On any canal each lock is a strategic step in its course and a significant cost in capital expenditure and long-term maintenance. Its location will have been planned by the surveyor’s department. Ideally, they would have no locks but few canals start and finish at the same level (the Suez Canal being an exception, for example). Even avoiding mountains and large hills most land undulates and in order to avoid having to have locks, as much as is reasonable, the canal will follow the contours of the land. However, if they only did this they would be unnecessarily long. They must, therefore, ascend and descend using locks.

As soon as there is height change the supply of water to the top pound must be planned. In the case of the Stratford Canal this was quite a problem, hence many reservoirs were built along its path at Kingswood and Earlswood. Indeed, the reason for Kingsnorton’s guillotine lock was not to act as a lock but to prevent the Stratford Canal taking water from the Worcester & Birmingham Canal.

Just because it’s convenient to construct a lock in a particular location doesn’t mean that the ground is suitable. Experienced planning will save huge initial extra expense and troubles for the rest of the lock’s life if ground that is soft, permanently waterlogged or, worst of all, gravel is avoided.

Brick Built Locks

Brick Built Canal Locks

A huge excavation is made with the lead in and out in line with the canal. Heavy stone is laid in the base of what is to become the lock chamber. The base is then bricked over much wider than the chamber in order to act as a foundation for the walls and cills. It is dipped by about 9” along its length to match the rough outline of the hull of a narrowboat.

Two courses, side-by-side, make up the side walls. This heavy construction is necessary to prevent the walls from bowing, thence failing, due to underground water seeping from the upper pound. The inlet and outlet culverts, either on one or both sides, are constructed below what will be water level.

A lock bypass culvert, and associated weir, is built along one side.

Lock Fit Out

During the “brickies’” initial heavy workmanship another double thick brick wall is built below where the top gate is to sit (behind what will be the Fore Bay right).

Carpenters are called in; however their jobs are more akin to shipbuilding than making domestic window frames. Into a hole left in the chamber floor, and in front of this top gate wall, a giant piece of oak is set vertically (shown right, in pale brown, behind the Babbie plate).

Either side of it, and downstream of the wall, but built into it, two solid triangles, or curved triangles, of masonry (shown dark brown) are built to bolster the downward force of water seeping from the canal upstream. It’s called the Fore Bay. The overall pressure against this wall will be over 100 tons. These days they are reinforced concrete.

Horizontally, on top of this wall, the carpenters sit the Main Cill which protrudes deeply into the side walls (shown in light brown right). This is an even more massive piece of oak and forms the gate’s foundation but normally cannot be seen.

Into this, where the gate is to swing, a Cup (see right and on the next page) is set into the Main Cill. This must be precisely located as it will define how well the gate seals. On the bottom of the gate a square hole is reamed and a huge Pin (sometimes known as a Pintle or rarely a Pinsel) is inserted. These will make the gate’s fulcrum. They are made of hardened steel and, like a ball and socket joint, make the gate far easier to open and close. They keep the gate in precisely the right place.

The side walls are completed with sluice chambers where the water will enter and leave the lock. Upstream of these an oak slotted Frame is fixed which will locate with the Paddle to control water flow (winding gear is not shown).

Navy Seals

As canals developed, they found that slap-dash members of the inland navy would scrape the ends of the side walls as they came into the lock. Over time the bricks would smash, even stone crumbled with 35ton barges crashing into them, and thus the seal with the gate would be compromised. Permanent leaks not only soaked the boaters as the boat descended but a bad outpouring of water could risk sinking some craft. The answer was to install two cast iron Coins either side (shown in red above). Their strong ‘L’ shape provided an excellently flat surface, top to bottom, for the gate to bear against and robust enough to withstand continuous boater treatment. These days a 16mm neoprene strip is attached to three edges of the gate in order to perfect the seal.

Lifting the Gates In

As we’ve all seen, canal lock gates are huge and, not surprisingly, these would be the last thing to be installed. A final check of all dimensions would be made (although I’m sure that there could sometimes be muck-ups !) and then they would be transported to site. The normal method was to put stop planks just upstream of the lock to block it off and fill the finished canal. A pair of gates, one large lower gate and one smaller upper gate, would be laid on a barge and brought from the factory. A vast gantry crane would be erected first to lift each gate off the barge. The fulcrum Pins would be inserted. Then the crane would be disassembled and rebuilt across the lock and each one would be lifted into place.

Installing The Corp Cills

Once a gate was correctly aligned, with the Collar (pink above) round the Heel Post (nearest edge of the gate above), then the gate is closed and each oak Corp Cill was slid up to meet the lower edge of it and fixed in place (green above) with four enormous Cill Nails (see next page). Each are the equivalent of a door jamb and provided the seal at the bottom of the gate. Some are set into the brickwork so as to keep them exactly in place. They are an item that gets badly worn by both silt running over them but far worse damage is done by boats scraping over them either because they were over-laden and deeper than the maximum draught or the water level was unusually low. As C&RT’s other name for them suggests, they are a Sacrificial Cill to protect the Main Cill below.

A catastrophic failure is if they are split or even worse rolled forward as a boat scrapes over. This both causes a massive leak which will sink a boat and drain a pound completely and tear to pieces all the brickwork at the ends. To strengthen their mounting, they are securely fixed to the vertical cill (shown orange above) with a T-shaped Babbie (sometimes Babby) plate (shown light green/grey) with huge coach bolts (not shown).

Lock Gate Construction

This is a whole subject in its own right. See next page in our practical mini-series:  2. Lock Gate Construction

More Information

1. How A Lock Works:             YT: STEM
   

2. Glossary of boating terms:  BraunstonMarina

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