This place is called Mont-Saint-Michael in France, and I’ve become fascinated by it. Why hasn’t the water after all these years worn it away? What did they do to the walls to keep them waterproof?
Non architect here, can somebody explain how this castle isn’t eroding away?
Ask /r/ArchitectureModerator removed comment
3d
Moderator removed comment
3d
Moderator removed comment
3d
As for the “waterproofing”, hydraulic lime https://www.buildingconservation.com/articles/hydraulic/hydraulic.htm
Any chance for an ELI5 on that?
The chemistry is over my head too. In extreme basic terms, hydraulic lime mortar/concrete is strengthened and kept flexible by regular exposure to water. It’s also possible to install it underwater and have it work properly.
Your Home Depot standard modern mortar will dissolve away if you try to use it underwater
Is hydraulic lime the same material the Romans used for aqueducts and other engineering projects?
Perhaps.
The Romans used volcanic ash which would crystalize and fill in gaps in the concrete and those created due to erosion.
The newly formed crystal structures were often stronger than the original concrete, to my knowledge.
Given the nature of this structure and its history I would not be surprised if this has had similar concrete used on it and replicated for use to reinforce it.
I was under the impression they didn't know exactly what the Romans used, or at least not the right proteins to replicate it. It's constituents were known but not the ratios.
Until recently, I'm nearly sure I read that they figured it out.
I also believe that the old concrete is better than what we use today, mainly because of the self healing properties you describe.
We do know, and have known how to make concrete like the Romans did for a while now, it's just not financially feasible.
We might not know the exact ingredients, especially as they changed from construction site to construction site, but we do have approximations. The main theory is that the mixing process creates calcium deposits that react with water and recrystallise, filling faults.
Also (steel) reinforced concrete is needed for our modern buildings and it doesnt really matter if the concrete "heals" with water If the steel still rusts away
Epoxy-coated rebar is pretty commonly used in wet environments to prevent this.
Epoxy coating too has its issues. When damaged, the coating causes aggressive pitting corrosion and the rebar can rust even faster that an uncoated one. It's banned in many places due to these concerns.
Stainless rebar is foolproof but costs 5 times more than regular uncoated.
The steel also expands as it rusts, creating spalling and actually damaging the concrete. Fiberglass rebar exists, but I don't know how feasible it is.
I thought the rusting rebar forms a protective layer. Does rebar actually "rust away" on human time scales?
It doesn't need to completely rust away. Once it starts to rust the rust expands and puts stress on the concrete. This causes spalling and cracking which allows more water in and causes more rust along with more damage.
MIT study 2023 January roman concrete figured out
MIT study 2023 January roman concrete figured out
https://news.mit.edu/2023/roman-concrete-durability-lime-casts-0106
It wasn't the volcanic ash that did that, it's the lime. They would mix it in a way to create little balls of lime that break open if the mortar cracks, and the water and lime then mixes and fills the hole, keeping it waterproof. It was only fully understood a year ago and the mont st Michel builders didn't know how to make it. They must have another great "concrete" because all across France there are medieval bridges with their columns built in flowing water that has held for centuries.
Ancient Rome used lime, yes, and it's really only been supplanted in the last century when Portland cement became more common. There are lots of existing (especially older) buildings still using lime. It's important to check this before repairing your old mortar, because using the wrong kind will likely cause damage to your structure.
wikipedia.org/wiki/Lime_mortar
Those are pretty incredible properties. And you’re too kind - the chemistry might be dense but I’m pretty sure it’s the several drinks I’ve had that slowed down my comprehension. The explanation is appreciated.
most concrete will set up underwater.
it can take a longer cure by not drying out which makes it stronger. proper concrete will get stronger every year over decades of time.
Afaik, the chemical process of cement goes on for a very very long time (slower and slower over time ofc) but this process requires water. That’s why you have to keep the concrete moist for the first 28 days. If it is kept submerged in water (after solidified ofc) it’ll keep curing.
Btw: The roman self-healing concrete isn’t actually that great because in order to have “self-healing”, you must have concrete that isn’t fully cured. So instead of going straight to 99% strength like modern well-made concrete do, Roman old concrete goes to 60-80%, then keeps increasing as they’re kept wet, in parallel with being worn over time. (The number % r arbitrary & made up to make a point, not real numbers)
Its basically concrete but with lime inside that will disolve into a paste when exposed to water, when disolved, it fills in the cracks and eroded places and hardens back even harder than before once dried
Not really seeing any comments here pointing out the absolute main reason why it doesn't erode. The water only comes up to the walls at high tide.
You can walk to it at ground level at lower tides, although a decade (or 2?) they built a new access road which is above the high tide level.
It does erode of course. But it's a man-made and maintained wall that uses materials that are very resistant to erosion
They recently destroyed it because it prevented the water to surround the island. Now they built kind of a bridge which only bus can pass. Allowing the tide to fully surround the island as it was in the past.
The #1 answer is that it did erode. The dirt washed off what used to be a volcano. The monastery/ pilgrimage site and village are built on the cooled and very, very solid magma deposit left behind.
Not really a volcano, but an undeground pocket of magma that slowly crystallized to granite, and was later exposed by erosion.
Mont Royal in Montreal was formed the exact same way. Not a mini mountain but a magma plug exposed from erosion. It’s one of several in a rough chain formed as the crust passed over a magma plume
Also Morro Rock in California, part of the Seven Sisters that stretch from Morro Bay to San Luis Obispo.
You forgot jailhouse.
It's not always submerged... Sounds daft but erosion is a really slow process so only being submerged for a fraction of the day massively increases the amount of time that would take...
... Additionally with it not being submerged all the time it makes maintenance much easier, thus making the structure able to go on even longer...
Actually it is even worse for a building to be alternatively submerged then unsubmerged than always submerged. But as said below, this alternance allows maintenance.
Why’s that? Does it come down to how it’s designed? I.e. they might draw up a weaker design if it’s only going to be submerged part of the time, versus a stronger design because they know it’s going to be constantly submerged?
The moving water and cyclical thermal changes over time doesn’t seem like it would help.
Cyclical thermal changes ? It's Britanny/Normandy: never hot, never cold, always mild and rainy.
The coming and going of water submit the materials to a much greater force and pressure than stagnant water or air. Take a dirty plate after lunch and see what's faster to clean it: plunging it into water or using the tap pressure to remove bits.
I think a more apt analogy would be just dipping the plate in and out of the water, the agitation would still be enough to clean it much quicker.
The castle is on an island.
Also it's an abbey, not a castle.
Its an island. The abbey is the building at the top. The lower parts of the island are fortified which by some definitions makes it a castle
The term fortress is more adapted.
In this region, a castle woumd refer to a different kind of building (aka, a lord fortified dwelling).
This is an abbey, with a small town, the whole thing being fortified.
I mean, not that I care, but wouldn't a town with fortifications be a castle, then? Wouldn't a fort be a castle? Lots of important places would be fortified in the middle ages, doesn't make them castles. Hell, some castles wouldn't have fortifications.
It's an abbey, man.
If you want a real fight, try to sort if Mont Saint Michel is in Bretagne or in Normandie. Then, you will unleash havoc.
Ne commence pas à ce jeu.
cas·tle (n)
> a. A large fortified building or group of buildings with thick walls, usually dominating the surrounding country.
https://www.thefreedictionary.com/castle
The Mont-Saint-Michel Abbey is an abbey located within the city and island of Mont-Saint-Michel
It's so stupid whipping out dictionary definitions like this.
If we're gonna play that game, then the Vatican City is a castle...
Castles and fortified towns/cities are two different things. A city, by definition, is not a castle, even if fortified. The fortified city might have bigger fortificated structures, castles. For example, Paris, while entirely fortified, was a city. It possessed multiple castles.
Today, Carcasonne, in southern France, is a fortified city, with a castle on one side.
Also, a castle can be made of a single big fortified building, it is not always buildings behind thick walls. Lots of famous castle from the UK fall in this category and disprove the definition you used.
Mount Saint-Michel is an abbey surrounded by a fortified town, not a castle.
Also note that fortified abbey did exist, just as fortified cathedrals. Neither are castles, or fortresses, despite fortified abbey being thick wall surrounding buildings.
Its built on a rock?
Yes. It's a big lump of granite
Rocks erode
… slowly
And enven if the mont saint-michel is an island it was built in an area where there is almost no waves
👋👋
Nooo don't do that, the waves will speed up the erosion!
👋👋👋👋👋
Humans erode faster.
Granite erodes at something like 1 mm/5,000 years.
Stone erosion from weather and wind is a process on geological timescales. For limestone and sandstone, that's visible on a human scale; for igneous rock, it's barely detectable at all.
It is eroding away. However there are these thing called repair and maintenance
This should really be the top reply and not the pedantic “it’s on an island lol”
Speak for yourself, I'm here for pedantic
I actually enjoy pedantic and informed answer on Reddit.
Most recent one is when someone came to explain about hurricane Beryl. How it's name starts with B because storm season's peak is on August, not June. How it's incredibly strange it can withstand wind sheer. Etc.
I mean it isn't clear what OP means. Are they talking about the bottom of the outside walls specifically? Then yes it's repair and maintenance. Are they asking about how the whole hill the town is built on isn't eroding away? It's an island that has existed for millennia and would still exist in more or less the same shape regardless of whether a town was built on it or not.
To me OP's question sounds more like the latter, so I think the "it's build on an island" answers are not pedantic and make more sense.
Maintenance and repair is an equally pedantic answer lol
Eroding but very slowly then, it’s built on a granite island with granite from a second islànd. And the shape of the bay makes very high or low tides (I think it’s the one of the most extreme in the world) but even at high tide he is barely immerged, and anyway to waves are powerful, but very small
Read this in the voice of Chick Hicks' crew who says "you see those round things? they're called tires."
I' no architect, but from my understanding:
- Erosion really works when there are no humans involved. Humans fight back, you can see this in many shorelines around the world.
- If I understand correctly, there are many pictures of this place with no water at all. Iirc this is "high tide" and is not all year around, so water is not really "allways working".
- In this pic there is really not much movement or waves in the water. I assume this may be because the sirface is quite flat, and it level is not really too "high". I asumme erosion is bigger where waves are bigger and constant.
It's a shallow tidal inlet, this is the tide at it's peak. It's a calm body of water with low wave action. Most of the day the rocks below the finished wall are visible, these are maintained against erosion.
That causeway is build up from what it was, it used to be submerged by the tide back when castle fortifications were important.
Low tides a calm sea = very very very low erosion
This. It’s not in open water it’s in a tidal pond where seawater flows in but not past or over. It’s built on a rock outcrop that has survived thousands of years without a castle on top of it.
It actually is, just very very slowly.
It is eroding. It’s just also being maintained at great expense.
Because castles are built out of magic by dragons.
It’s a monetary/church actually but yeah.
I guess churches are built out of jeezuz magic or something then.
Or feudal peasants scared of hell by the clergy.
That’s why Puff lived by the sea.
I always tell my clients. You can do anything. It just needs the right amount of money…and magic. But I probably can’t make it levitate.
85 comments and nobody has pointed out yet that Mt St Michel is not, and has never been, a castle. It was built and used as an abbey.
I was doubting my knowledge of my own country when I saw castle in the title XD
Yet St Michael’s mount in Penzance in Cornwall - aka the English version of this just over the water, IS a castle!
There's probably a county surveyor in Avranches or Saint-Lô who can tell you all about this.
They probably won't answer you if you write in English. At the least you need to use Google Translate.
Ultimately you should be like, "my friend, I look forward to visiting you to discuss this because I find this so interesting - is there anyone else I should speak to?" Be prepared for boozy lunches.
Went there years ago, iirc the causeway created the opposite issue where sediment was building up slowly turning the island into a peninsula. They’ve built a nice bridge out to it since.
Erosion is a very slow process and the castle was built atop an island so it would erode at a similar rate to the island rock below!
Breakers built around it. The foundation made from Coquina shell. Finally, internal cisterns divert water through select chambers, allowing water to freely flow in and out making the foundation last with minimal maintenance on the cisterns only.
No wave action here to erode things
Figure Three on this page makes it fairly clear - it is built on rock, just surrounded by water.
https://bluemarbleearth.wordpress.com/2019/09/05/why-can-mont-saint-michel-withstand-the-tides/
The bay around it has altered a lot over time though - sediment deposited by the tide can shift fairly easily based on changes in currents.
Being submerged doesn't always mean erosion. Erosion requires friction from sediments.
It is build on a little island consisting of Muscovite Peraluminous Granite, a very durable type of stone. During the 100- year war (1337-1453) the island was fortified with walls, also made of granite, adding an extra layer of protection against erosion.
It is not a castle ; it is Mont-Saint-Michel Abbey in Normandy.
The Mont-Saint-Michel and its bay – has been a UNESCO World Heritage Site since 1979, and is managed by the Centre des monuments nationaux.
This is no castle, it’s an abbey!
It's on an island, and they're both made of stone.
Where's the parking lot and Walmart?
Behind the camera.
Church footings and foundations aren’t ever submerged. The surrounding wall protects the island. If the soils in the island are bedrock, there would be almost no erosion ever.
The water rarely goes that high at Mt St Michel. To the left in that picture there is a sizable parking lot and a paved road leading to all this which is slightly raised and usually doesn’t get covered like this.
The land that sides the road at least half way out has lots of low lying shrubs and plants. The area halfway to the island is sandy but usually walkable.
As others have said it is constantly maintained as it’s a major patrimony site of the Ministry of Culture and UNESCO.
Been five times. It’s amazing and peaceful. You can stay overnight in one of the few hotels in the island itself.
Millions and millions of dollars per year in maintenance expenses.
Another comment mentioned it, but I went to the wiki and followed one of the citations. It was an article talking about how the bay it's in has been building up a lot of sediment and most of the time it's not even surrounded by water except at spring high tides a few times a year.
I'd assume all the amazing photos you see of it is during those events. Just google image search the same place but add "low tide" and you can see how little water there is around it. I even saw a photo of a guy standing not even waist deep at high tide.
I have no answer for you, just here to say it's an amazing place. Went there last year and it's well worth a look around. The tide was out for our visit so it was just sandy all around.
It is simply incredible, I admire those who did it!
The tide and the river bring more material than it takes away. We actually have the opposite issue and do maintenance to keep it a bay and avoid sand overflow. Keep in mind it isn’t very deep on a very long distance
You has answer, so pedantic french reporting in. It's not a castle but an abbey. Mont Saint Michel to be precise
Cause it's fucking french. We don't do weak ass erosion.
This is in a tidal basin. At low tide, you can walk to it. Not too much crashing waves.
So far. Money has solved this problem.
Maybe people have already said it, but too lazy to check it so first thing first: it's Le Mont-Saint-Michel in Normandie (France). If you look for it on Google you will see it's not always sugmerged. Most of all it's very "muddy", so not as much erosion as we could think of, although there is some. As people already said, what limited erosion there is has been maintained to this day.
They just removed a whole bunch of sand so that it stays an island.
It's the remnants of a mountain range from 300 million years ago which were the size of the Himalayan range. So erosion did it's job.
French qualities
Magic
Was exactly there 2 days ago ;)
Just to add in here, the bay is constantly cycling seawater and freshwater, has extreme high and low tides, as well as a circular flow, it's a really particular ecosystem. People still cross on foot with guides, because you must be trained to know what to look for, and the first thing they teach you is how to get out of quicksand. The new bridge allows the tidal currents to pass and flushes sand away from the base wall naturally.
I don’t want to beeeeeee a crumbling crumbling crumbling castleeeee
Wall building in/along water is not at all uncommon. Why are you singling this out?
It’s built by the frenchies. That’s the secret.
Dragon eggshell stucco.
Belt
A drop of the sun fell down upon it and it's blessed with extremely long hair... I mean life...
ask an engineer
wall
It’s under constant maintenance.
It is, it's just well maintained
Every time the tide goes out, they build a new wall with sand.
Is this place what inspired that level from the latest Snipe Elite game, because it looks an awful lot like it.
Yes it's the same place
Amazing place. Didn’t appreciate seeing this as a kid!
I'm puzzled, why would you think it would erode any more than any other building. It has a wall at the sea as do many structures that sit at the sea. The only difference is this one at high tide has water all around it. The rest of the buildings are just like any other building any other place in a marine environment
I am sure over the centuries there has been work done to its seawall for upkeep. Just guessing ,but I would imagine just again, as any place that is placed near the ocean with moving tides, needs periodic maintenance and buffeting
Let's see how it looks in a million years
Oh, it’s actually floating you see.
rocks
French wine
Piggybacking on people saying it’s not always in water, if you look at it on google maps, it’s not in water then. And even it’s “street view” you can see the water is receded and exteriors are accessible.
Because of engineers.
its a monastery
Because it isn't wet
Mont Sant Michel is an abby not a castle
That seems like hogwarts, so probably magic 🧙
Mt. St. Michel
Interestingly, there's a smaller (but quite beautiful) counterpart across the channel in Cornwall https://stmichaelsmount.co.uk/plan-your-visit/
It’s Mont-Saint-Michel. Not Michael.
Cause its built on a quite river
“Liquid hot, magma”
It’s not a castle but an abbey with a village, so it is always inhabited. It is interesting tho to note that it’s not sinking into quicksand
It’s a half iles, when low tide not an ile, hight tide iles abd only way to join it it swim or boat
Is that the one from the walking dead? Either way its quite beautiful ;D
An architect wouldn't know, ask an engineer.
This Normand monument is build on a rock (granite) in a bay with very high tidal variation. As such, the water is actually quite lower than on this picture much of the time, and it's very easy to maintain.
Also the whole bay struggle to erode away its own sand and was actually silting up a lot for 50 years when they added a concrete pathway to the Mont in the XXth century
Because it is not a castle
It's not a castle, and erosion is a very long time process so with a good amount of maintenance you can fix it.
The Mont Saint Michel gain a lot of money with tourism so they can afford the heavy maintenance.
The outer walls have been built in the past 200 to 300 years or so,
it has not eroded away because they are recent and have been in constant used ever since they were built,
also because the French Govt used as a prison during the French Revolution,
also because it has been used as a tourist trap since it got reclaimed from the French Govt after the end of the 1st Republic
I've been there. The seas are quite calm there because the island is on a huge expanse of sandy strand which is either dry or wet depending on the tide. Waves can't get very high under those conditions
The blessing of God and Saint Michel /joke
C'est beau, c'est français.
This is a leucogranite island, this is eroding but not at the human time scale. Concerning the walls and human building, there is maintenance. However the bay is very flat, there isn't big waves on the mont saint mich Mich.
It's obviously protected by the power of the Archangel Michel
Never heard of Venice?
Calling Mont Saint Michel “this castle” barbaric
"Pas mal, non ? C'est français 👍"
Tax lot of tax
Holy power of maintenance
Mont Saint-Michel is a beautiful place. The tide goes down and you are only able to reach it then. Also, when low tide, there is no water surrounding it. I'm sure that's when they do their maintenance.
C'est un monument classé donc il est régulièrement rénové
Maintenance. This concept has been long forgotten in many quote modern unquote countries, with rusting bridges and crumbling roads
Afaik they had to remove sediment to keep it as an island. Did also myself a walking tour around it during low-tide. And dont forget to eat an omelette at Mme Poule.
Magic!
First, it's built on rock. This article seems good but it's in French https://planet-terre.ens-lyon.fr/#:~:text=Toute%20la%20zone%20de%20la,bloc%20Nord%20vers%20le%20Sud. Link doesn't work, the title is "Le Mont-Saint-Michel et sa baie, une longue histoire géologique"
They didn’t get the memo about Climate Change
It stands afloat without tilting to any direction thanks to the eternal bickering between Bretons and Normands each claiming it as part of their region.
Vous aimez? C'est Français. 😎
Because is a french castle 💪🏻🇫🇷❤️
Build by angels a very long time ago!!!
They maintain it with the money of its 3 millions visitors/year
magic
Build years before power tools, my opinion built by a more knowledgeable civilization, maybe even God's angels.
Made in France.
Cheese water helps
Maybe they used some witchcraft or aliens helped build it.
Sky hooks
The castle is build on a island
French engineering ;-)
La qualité française !!!
Been there several hundred years, makes you wonder about rising sea levels.
Not a castle here but an abbey.
It’s not a matter of architecture really, but more a matter of geology. But the main reason is because it’s harder to erode rocks than it is to erode sand or soil. Additionally it’s positioned in a river mouth rather than off a normal bit of coastline, this means there’s less waves and the water passing it out toward the ocean is freshwater which is less prone to causing erosion than salt water.
Not an architect but i was here like two days ago and I will say this photo kind of gives a false impression of the abbey. For most of the day it’s surrounded by grasslands, but later on the tide comes in, so it’s not just constantly submerged
I’ve been fascinated with this place as well since I saw it in one of my favorite conversation movies. The water recedes on a cycle so that you can even walk to it sometimes. I wonder what the foundation is—a rock?
French engineering
I think it's because it's french
When it come to building things, we french are kinda okay.
Not a castle.
Disney Fairytale Magic
smile in french
À la vitesse d'un cheval au gallop!
Because of Saint Michel
Walls
4 missing replies
It is, they just give it maintenance