The Somerset Coal Canal, and the world's first mildly-terrifying caisson lock


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If I tell you this is a story about coal-mining in England, and how during the industrial revolution this prompted engineering innovation to bring the necessary infrastructure to a neglected rural region, you might be forgiven for assuming we’re in the north. Indeed, when I first started thinking about what video to make next, I thought it might be nice to do one on the mere existence of the Somerset Coalfield, because it seems to me like the presence of that industry in this part of the country is relatively under-known.

I may yet still make a video on the coal mining in general, but for now, suffice it to say that south of Bristol and west of Bath was a moderately extensive coalfield, and although evidence suggests this was mined as far back as Roman times, it is in the mid to late 1700s when our story begins. This is, of course, when the industrial revolution is changing the game. Coal is being mined on a much greater scale and no longer for strictly local use - Somerset was (and is) rather lacking in factories and large industrial cities as seen rapidly growing in the north at this time. Even Bristol, the nearest major city, was fairly useless as a market for their coal, because as well as a Somerset coalfield, there is, or was, a Bristol Coalfield. Bristol had its own coal mines, which is once again ripe for a video in its own right, but for now the key point was that Somerset’s coal mines needed a way of getting their output to anybody willing to buy it. And those people weren’t nearby.

Roads at the time were severely lacking. Even today, Somerset country roads look a bit like this, so you can imagine in the 18th century they were hardly suitable for heavy use by heavily loaded freight carts. Although many mines and quarries of this era made use of wagonway and tramway systems that sort of involved rails, it’s fair to say that railways as we know them did not yet exist. The solution, therefore, was a canal.

In the 1790s the coalfield owners got together to construct the Somersetshire Coal Canal. Initial surveys occurred in 1793, Parliament granted permission in 1794 and construction began in 1795.

In an interesting side-note, one of the surveyors was William ‘Strata’ Smith, known as the “father of English geology” and credited with creating the first detailed, nationwide geological map of any country. His observations and insights regarding ‘strata’, which garnered him his nickname, largely originated from his time surveying for the Somerset Coal Canal and Somerset Coalfield mines.

The new canal was to branch from the Kennet & Avon Canal, which itself had only just begun construction in 1794, and was due to link to the River Kennet at Newbury, which could be navigated to the Thames at Reading, and from there on to London.

The plan for the coal canal was for two arms, a northern arm following Cam Brook to Paulton and a southern arm along the Wellow Brook to Radstock.

In the end, only a short stub of the southern arm was ever built. Lacking the funds for the necessary locks, the company cobbled together a tramway instead, which persisted until it was ultimately superseded by the Somerset and Dorset Railway in 1874.

The northern arm was built however, and ultimately proved quite successful. Construction was not straightforward however, because the basins at Paulton were about 135 feet (or 40 metres) above the Kennet & Avon at Dundas, and while locks were a well-established solution to elevation changes, the Coal Canal had another problem: a lack of water.

When I visited Combe Hay to see the remains of the locks, I was certainly not lacking in water. As you can see, precipitation was plentiful, which rather curtailed my enthusiasm for hanging around and shooting copious tripod footage and generous b-roll of the surrounding locale, which is partly why this video is rather low on actual video and heavy instead on maps.

Anyway, the problem with conventional locks, you see, is that they release a lockful of water downstream with every use, and as such a canal needs a constant supply of water from upstream. Here, the obvious solution would be to feed it from the Cam Brook, but existing mills had water rights, heavily restricting the amount of water the canal company could take. There was no other suitable river, lake or site for a reservoir, so the company turned to an inventor called Robert Weldon who was touting an innovative approach.

Weldon’s invention was the “Hydrostatick Caisson Lock” - here’s an engraving from his patent.

The means of operation is relatively simple, yet still far beyond my artistic abilities, so apologies for this animation. Essentially, the caisson lock is a tall, waterproofed stone chamber, filled with water, within which floats a sealed box, ballasted to have neutral buoyancy.

A boat comes along the upper level of canal, and the doors at the top are opened, the boat enters the box, and the doors are sealed. The box is then lowered to the bottom, the lower doors are opened, the boat exits, the doors are closed, and the box can be winched to the top for the whole process to repeat.

You’ll note that in the theoretical sense, at least, almost no water is lost during a cycle of this lock, which is why the Somerset Coal Canal company thought that a row of three of these locks would solve their problem rather nicely.

But you may also note that this system involves the boat being sealed into a box inside a box with hundreds of tons of water pressure bearing down on it, in the 1700s, well before any underwater tunnels or submarines had ever been built, and think this is crazy, terrifying and surely what put paid to this whole scheme.

In fact, there’s no good reason for the boatman to remain on board; it seems reasonable to assume the intention was for crews to get off while the boat was raised or lowered through the caisson lock with nobody on board, and therefore in no danger at all. However, for reasons that frankly escape me, the company decided to undertake “manned voyages” during testing anyway.

The first test, in February 1798, failed due to cracks in the waterproofing of the chamber. Three subsequent tests were successful though, and in April 1799 the lock successfully transported 60 passengers.

In May, however, a party of investors were being transported when the box jammed on a projecting stone, and they were nearly suffocated before they could be freed.

While this makes a dramatic story to explain the failure of the caisson locks, it’s not entirely clear if this was solely or even primarily responsible. The local geology also played a part, apparently proving quite unsuitable, the “fuller’s earth” of the local area was not strong enough to buttress the immense water pressure coming out from the chamber. This is what led to the buckling of the chamber and the projecting stone and hence the jammed caisson and the trapped investors. So, construction of the second caisson lock had been begun but was never completed, and the third was never even begun.

Unfortunately I can’t show you the remains of the caisson locks, because essentially nothing remains of them today, and what little traces there may be lie on private land.

They were located near the small Cotswold village of Combe Hay. Let’s zoom in our map a little.

We’ll start with showing the Cam Brook, flowing west to east, and then since this whole story is about conquering the elevation change, we’re going to need some elevation contours. I don’t really have the patience to draw proper contours, but I guess if I do this… and then this… and yeah, that’ll do.

So, circa 1797, one stretch of canal exists to the west of here, coming in at roughly the 75 metre contour, while another exits to the east, somewhere between the 35 and 40 metre contours, to join the Kennet and Avon.

The aforementioned caisson lock, built and tested in 1798-99, was roughly here. Evidence suggests the partially built second one was directly in line down the hillside. For a time it was unclear if the third continued this direct line, but it seems unlikely as this would actually have descended too far into the Cam brook’s valley. Evidence suggests it was actually intended to be built further east, the other side of the small tributary at Rowley Bottom.

Speaking of which, you may wonder why this stream was not the answer to their water shortage issues, but streams this small tended to run dry in summer, when water was needed most, so they were considered pretty useless.

Anyway, as we know, the caisson lock experiment failed, so instead the company built an inclined plane, which opened in 1801. Once again, there is nothing left to see of it, so I will illustrate with some images of Foxton Inclined Plane instead. To be clear, this was built a century later, over a hundred miles away, and had a different mechanism of operation, but, y’know, it gives you the general idea. While Foxton was designed with great water-filled caissons that the boats could sail into and then be transported up and down the slope, the inclined plane at Combe Hay was a rather more low tech solution.

Coal had to be unloaded from boats and packed into wagons attached to a circular rope; gravity did the job of lowering them, and the empty wagons being hauled upwards as a result, somewhat like a ski lift. The coal then had to be re-packed into different boats at the bottom.

Needless to say this was a major inconvenience, adding lots of time, effort and breakage to the equation, so it was only ever intended as a temporary measure. And even then, it didn’t completely close the gap. To reduce the height difference the incline had to cover, the company built 3 conventional locks, and to feed these water-hungry locks they constructed a weir on the Cam brook, diverting some of its flow down a feeder channel into the lower reach of the canal. Below the third lock, this was returned to the brook, to keep the mill-owners happy.

While the inclined plane handled the cargo temporarily, the company pressed on constructing the rest of the canal link with a traditional flight of locks. A further 19 were needed, to be precise, to add to the previous three, and they opened in 1806.

The lower reach was blocked off at the bottom, but the weir and feeder channel were kept. The lower reach constituted a highly useful reservoir of water, with the additional advantage that silt settled into the blocked-off far end of it; the only problem being it was near the bottom of the flight of locks, and that water was needed at the top.

The solution was, as so often in this era, a steam engine. Specifically, a Boulton & Watt pumping station, which lifted the water some 40 metres to an extension of the upper reach, from where it could feed the top of the flight of locks.

Water coming down the Cam Brook therefore found itself dancing a complex spiral, from the weir pond into the lower reach reservoir, then pumped up the hill into the upper reach, before looping around Rowley Bottom as it tumbled down 22 locks, before finally returning to the brook.

These locks, at last, offer some visible remains for me to film and show you. In particular, locks 11 through 15 are in pretty decent condition, and have a public footpath running alongside. I might technically have been trespassing to jump down into the locks themselves, in which case I apologise, but I didn’t jump any fence, or damage any crop or livestock, so hopefully no harm done.

I should point out the condition as seen here is not their natural state after a century of abandonment. Since the year 2000, the charitable Somersetshire Coal Canal Society has been gradually working toward restoration of the canal, and their work parties have cleared out dumped rubble and rubbish, cut back shrubs and brambles, and so on.

In fact, at the far west of the canal, near Paulton and Timsbury, they have re-watered about a kilometre of canal. I was hoping to stop by there to grab some illustrative footage, but as you can see the weather was not on my side on this day, so you’ll just have to take my word for it, or check out the society’s website (which I must also credit as a major source of information for this video). Separately, the far eastern end of the canal, where it meets the Kennet & Avon, was restored into a quarter-mile long marina known as Brassknocker Basin in the 1980s.

Between the two, much of the canal is in a similar sort of state to what you see here: very derelict but clearly visible, mostly not built over and tantalisingly restorable-looking. A complete end-to-end restoration remains an aspiration for the society, but there are some major obstacles in the way, in a literal physical sense alone… like this railway embankment for example.

For most of the 1800s this canal was shifting 100,000 tons per year; it peaked In 1838 in terms of weight of cargo transported, with 138,403 tons. Perhaps surprisingly, the Somerset coalfield continued production until 1973. The canal didn’t last nearly as long, however. As was so often the case in the Victorian era, the railway proved an ultimately fatal opponent.

Railways first reached this general area with the Great Western Mainline in 1841, and other intercity lines appear on this map over the years, which I won’t go into detail, but it was not until 1854 that railways entered the coalfield. That year the Bristol & North Somerset Railway opened their line from Frome to Radstock, which rapidly began eroding the commercial viability of the canal company’s southern arm, still operated as a tramway. As mentioned earlier, the Canal company gave up and sold off the route to a company, in this case the Somerset and Dorset Railway, who opened their line in 1874.

A year earlier, in 1873, the Bristol and North Somerset had opened their line up to Bristol, with a further branch line to Camerton built in 1882, which encroached on the canal’s northern arm. Profits collapsed, and when the pump at Dunkerton failed it was not replaced, resulting in inadequate water to operate the locks. By 1894 the company was bankrupt, by 1898 the canal was closed and in 1904 it was sold to the Great Western Railway.

They used part of the canal route to link the aforementioned Camerton branch line back to the mainline, which is why lock 16 has been obliterated by this railway embankment.

However, as we’ve just seen, the canal company deliberately concentrated almost all the 40m of elevation change into this small area near Rowley Bottom, so that efficiency of water usage could be maximised. Railways, on the other hand, are obliged to spread their elevation change as gradually as possible along their route.

Thus, if we trace the path of the railway on our zoomed map it’s no surprise to see that it actually hardly overlaps with the canal at all here.

And the railway has, in turn, long since closed. So who knows - will this flight of locks ever one day be restored and see narrowboats, loaded this time with tourists rather than coal, passing through? I’m personally a little sceptical because, well… it just doesn’t really go anywhere. I don’t mean to be rude; the Somerset coalfield region today is a very attractive rural area where you can enjoy some nice walks and lovely pubs and so forth.

But it doesn’t connect to any other canal, nor does it even go to any moderately sized town like Radstock where pleasure boaters could connect to other forms of transport at the end of a trip. The canal essentially just stops in the middle of nowhere. So it’s hard for me to imagine there is enough demand to justify the surely enormous cost and effort that would be required to transform these relics to working use.

In the meantime, they remain a reminder of the industrial history of the era, and the rather under-known engineering claim to fame held by this small Cotswold valley, site of the world’s first and, as far as I can tell, essentially only ever operational caisson lock.

That’s all for this video.

As always, thanks to the photographers and wikipedians who have made their stuff available under Creative Commons licenses or similar, which I have drawn upon for this video. Special thanks to the Somersetshire Coal Canal Society for their highly informative website.

If you enjoyed this video, perhaps you’d also enjoy my previous one on the Winford aqueducts, a Victorian civil engineering scheme to bring drinking water from Somerset’s hills to the city of Bristol. And why not subscribe in case I make any more of these.