How valid is this costing of a Dover/Calais causeway - when built by the Romans?

[Andrew J Harvey]

I've estimated what it might have cost an early Roman Emperor to build a causeway from Calais to Dover and would appreciate any feedback people might have on the estimate, and how much I might be out by. And yes I am aware that the bigger question is 'why would they want to?' but I've already got that worked out.

Assumptions (provided by Google)
Average depth Dover to Calais = 45m
Length of the required causeway = 30 km
Top width = 7m

I am proposing to follow a similar approach to that which was used to by the Romans to build the artificial harbour at Caesarea Maritima. The process involved using Pozzolana concrete which hardens in contact with seawater. Barges were constructed and filled with the concrete. They were floated into position and sunk.

The stats on the barges to be constructed are based on Caligula's "Giant Ship", a very large barge, the ruins of which were found during the construction of Rome's Leonardo da Vinci International Airport in Fiumicino, Italy, in the 1950s. The barge had a length of about 95 m) and a beam of about 21m, with a depth of 13.4 m, and could carry 1,300 tons of cargo.

I estimate they would need 3,780 ships, (4 ships high x 315 ships long x 3 ships wide). I plan to sink 3 barges at the same time, side by side and chained together. I can't find any figures on how much it would cost a Roman to build a wooden barge, but the modern price of a barge of equiv size (second hand) is $2.6 million, so 3,780 ships cost $9.8 billion. Possibly less as they wouldn't have to pay for the trees.
Cost of 1 tonne cement = $75, so 1,300 tonnes of cement = $97,500
3,780 ship loads = $368,550,000
Total cost of each barges plus concrete is roughly $10 billion, which I understand may equate to approximately 200 million denarii (see

https://testamentpress.com/ancient-money-calculator.html)

Andy Goldsworthy in 'Caesar – Life of a Giant' apparently estimates the income of the republic’s treasury at 50 million denarii (200 million sesterces) before Pompey’s departure to the east. After his return (reorganization of the eastern provinces), the income increased to 135 million denarii. Caesar in 49 BCE imposed an exceptionally low tribute of 10 million denarii on Transalpine Gaul. Thus, at the time of the fall of the republic, the annual income of the Roman state was 145 million denarii. It is worth noting that in Rome you could live on 2-3 sesterces a day, where the cost of a tenement house was on average 60,000 sesterces, and the most magnificent properties could be bought for 2-20 million sesterces. On this basis, mentioning the creation of several additional provinces in Egypt, Mauritania, Cappadocia, the Danube area, Dacia, Britain and others, it is possible to estimate the revenues to the budget of the Roman Empire around 125 CE at about 210-250 million denarii.

So the total cost of the causeway is going to be close to the Empire's total annual budget, which if spread over 10 years is not necessarily impossible.

Any feedback would be appreciated.

 

[LSCatilina]

Keeping in mind the spirit of your question, I think you'd want to take in consideration some elements.

First, the logistics of it all. Who is going to work on this mega-project? It would require a considerable taskforce including skilled workers and sailors (so a servile manpower wouldn't be enough on itself) that would need to be fed and paied. Normally, this kind of work would be devolved to the army and very possibly the navy both likely from the Rhine frontier, but not only it would require a significant number to pull the project in this 10 years window but at the cost of border security and/or politics : so even if I could see some military taskforce, we're going to see a large civilian employment that have to be dealt with in feeding, paying, housing, etc.

Then, the materials : the ships are probably the least problematic in my opinion, but Roman concrete not only required skilled employees (that'd have to be carried and/or teaching) but also tuff (and preferably, puzzolane) common in Italy but not so much in Britain and Northern Gaul. Arguably, tuff could be extracted in Auvergne, requiring to create a local industry instead of the cost of carrying materials all the way (or, more probably, additionally).

Eventually, you'd have a big difference to consider between the Mediterranean coast and the Channel is that the latter was relatively more prone to storms, an issue already underlined by Caesar during his transmaritime campaigns. Of course that would definitely force in a seasonal work, but also raises the question of the cost involved not only in storms damaging a current work (and sinking ships, and harbours, and workers) but also to repair the bridge while continuing its construction (and eventual regular, expectedly costly, maintenance).

 

[Andrew J Harvey]

Keeping in mind the spirit of your question, I think you'd want to take in consideration some elements.

First, the logistics of it all. Who is going to work on this mega-project? It would require a considerable taskforce including skilled workers and sailors (so a servile manpower wouldn't be enough on itself) that would need to be fed and paied. Normally, this kind of work would be devolved to the army and very possibly the navy both likely from the Rhine frontier, but not only it would require a significant number to pull the project in this 10 years window but at the cost of border security and/or politics : so even if I could see some military taskforce, we're going to see a large civilian employment that have to be dealt with in feeding, paying, housing, etc.

Then, the materials : the ships are probably the least problematic in my opinion, but Roman concrete not only required skilled employees (that'd have to be carried and/or teaching) but also tuff (and preferably, puzzolane) common in Italy but not so much in Britain and Northern Gaul. Arguably, tuff could be extracted in Auvergne, requiring to create a local industry instead of the cost of carrying materials all the way (or, more probably, additionally).

Eventually, you'd have a big difference to consider between the Mediterranean coast and the Channel is that the latter was relatively more prone to storms, an issue already underlined by Caesar during his transmaritime campaigns. Of course that would definitely force in a seasonal work, but also raises the question of the cost involved not only in storms damaging a current work (and sinking ships, and harbours, and workers) but also to repair the bridge while continuing its construction (and eventual regular, expectedly costly, maintenance).

Thanks for your feedback, LSCatilina. your question about the stability of the Rhine Frontier is a good one, fortunately I can play around with that as the POD for the line is some 7000BC in the North American Continent. Your concern is helpful as I hadn't thought about that, and it opens up a couple of points I now need to consider.

You're also right regarding the Roman concrete requiring skilled employees. It also requires the correct stones to bond to as I was reading that archaeologists had found that in one location the concrete had not bonded properly with the local rock (non-Roman rock), and in addition had not been mixed properly. Can't think why not, what's difficult about standing in the middle of a barge mixing cement as other people keep adding more to the mix, and the waves keep trying to make you fall into the cement ;-)

And the question of seasonality, yes I completely missed that. And, you're right, with the causeway being initially laid down over 2000 years ago there is plenty of opportunity for maintenance to be shirked for extended periods of time, with the causeway falling into disrepair until some King or Queen decides to do something about it. That's already part of the story.

 

[Alex Richards]

Then, the materials : the ships are probably the least problematic in my opinion, but Roman concrete not only required skilled employees (that'd have to be carried and/or teaching) but also tuff (and preferably, puzzolane) common in Italy but not so much in Britain and Northern Gaul. Arguably, tuff could be extracted in Auvergne, requiring to create a local industry instead of the cost of carrying materials all the way (or, more probably, additionally).

Would Auvergne really cut that much off transportation when you've still got to get all the materials across half of modern France?

 

[LSCatilina]

Would Auvergne really cut that much off transportation when you've still got to get all the materials across half of modern France?

To be fair, the road network existed since centuries and had been perfected under the governorate of Agrippa (i.e. the various sections of the Via Agrippa) which wouldn't make connecting Auvergne to either western harbours or the Seine basin any noticeable issue, nor did it made any problem IOTL to regional products (wine, pottery, etc.) to seriously compete with Italian products. As an example, the transportation on rivers and canals of the Tendu's quarry production.

So, in itself, I'd be a viable possibility in the scope of @Andrew J Harvey's project.

Now, I think it would require to create the extraction and transportation structure : obviously, indigenous building techniques rarely used stonework except as foundation so there isn't a local work tradition to bank on (contrary to, say, shipbuilding with the Romano-Celtic boats).
It doesn't looks like Romans did extract tuff from Auvergne themselves IOTL either : but after checking a bit more in depth, there were tuff quarries in Northern-Eastern Roman Gaul, namely in Brohl and Kruft (nearby Mainz) worked for civilian and military projects in the region. (link to the study).
Would that be enough to supply and IOTL project and the causeway tough? I'm admit I'm sceptical but it would certainly serve as an additional supply with the same potential counter-points I mentioned above : use of military workforce and possibly lesser military/civilian investment in the region.

All in all, while you could bank on extraction of lesser quality tuff in the Norman Hills, I think that Romans deciding to extract high quality tuff (pouzzolana-like) from Auvergne could be a relatively easier way to both supply the building site and avoid depleting too much other sites. I'm not saying it's the best or most efficient solution, especially as I don't know what changes in this timeline, just saying it might be an fairly reasonable cop-out.

 

[Alex Richards]

All in all, while you could bank on extraction of lesser quality tuff in the Norman Hills, I think that Romans deciding to extract high quality tuff (pouzzolana-like) from Auvergne could be a relatively easier way to both supply the building site and avoid depleting too much other sites. I'm not saying it's the best or most efficient solution, especially as I don't know what changes in this timeline, just saying it might be an fairly reasonable cop-out.

Yeah I suppose that's the crux of it. We've already bracketed in the 'why' of the main project so its at least justifiable in that context.

 

[Juan Vogel]

Hard to see how they could get very far off the coast before the effects of tidal action/current/wind/etc would cause more damage than could be managed.

I'd see this causeway to be more like a breakwater than a bridge, a solid object. The longest in the UK apparently is 2.39km and it is by an old quarry. Cherbourg has longer ones from the same era, but 3.64km. I don't know the longest in the Roman Empire but I think the Ostia breakwaters were 170m. Port authorities have to spend a fair effort on maintaining breakwaters in sheltered areas so I'd hate to think how much work it would take with pre industrial age rail/lorries, or without cranes/bulldozers.

I'm also dubious they could build enough ships quickly enough. These are big ships you've sketched out, for ancient times anyway. Whilst France is huge and presumably had vast forests close to the coast/rivers at this point, I suspect they'd soon exhaust naval quality timbers close to the ports. So there would need to be a lot shipping of large trees, long distance. This alongside the material for everything else, well, you'd be putting the roads/ports/etc under great strain.

 

[Talwar]

Are conditions in the channel (currents, seafloor topography/composition) sufficient that the Romans can sink the barges with some degree of precision?

Is a 4x3 stack of barges - weight not withstanding - a stable structure?

 

[Md139115]

One more consideration is that the Romans might realize they don’t exactly want a true causeway since, you know, that kind of puts a crimp in trade from Iberia and Gaul to the North Sea if boats now have to sail up and around Scotland. You could see something built in segments, like the Chesapeake Bay Bridge (minus the tunnel bits), and then the gaps in between are filled with temporary pontoon bridges or draw bridges.

 

[Andrew J Harvey]

Thanks for everyone's reply. Appreciate the thought that went into all of them.
Andrew

 

[AndyC]

Bear in mind as well that whilst 45m is the average depth, the project difficulty will also be affected by the greatest depth. Which, depending on the route they take (because they won't know the underwater topography in depth) could be as much as 70m+.

From https://www.nature.com/articles/ncomms15101

Fig 1

Fig 2

Fig 7

Of course, the details and constraints could provide you with more plot/possibilities for things to go right or wrong