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Following the recent news about the Sau reservoir in Spain, the draught revealed an 11-century stone church which has been under water for 70 years since 1962 when the reservoir was built.

How is it technically possible, that the stone building survived 70 years under water without serious structural damage and has not collapsed? Why didn't the mortar gluing the stones dissolve throughout the years and the movement of the water did not disassemble the building?

Before:

enter image description here

Now:

enter image description here

See more picture of the church here.

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    $\begingroup$ I helped a guy build a house once and he wondered, "Does the mortar stick the blocks together or keep them apart?" That was forty years ago and I'm still not sure. $\endgroup$
    – Transistor
    Mar 22 at 14:22
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    $\begingroup$ Besides this, "the interior of the bell tower was plastered over before submersion, to prevent collapse" - en.wikipedia.org/wiki/Old_church_of_Sant_Rom%C3%A0_de_Sau $\endgroup$
    – Pete W
    Mar 22 at 14:55
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    $\begingroup$ @PeteW "the interior of the bell tower was plastered over before submersion, to prevent collapse" sounds to me like the most plausible answer to my question :) $\endgroup$ Mar 22 at 15:13
  • $\begingroup$ @PeteW Regarding the "modern cements vs. old mortar" - I have always thought that we started adding cement to the mortar only "recently", and that the old mortars contained only slaked lime. And I somehow understood that the lime-only mortar is water-soluble, whiles the cement is not. Am I wrong? $\endgroup$ Mar 22 at 15:17
  • $\begingroup$ @HonzaZidek - It's not my field, to be clear. I'll remove previous comment to avoid confusion. $\endgroup$
    – Pete W
    Mar 22 at 20:32

3 Answers 3

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See the rubble around it? that's other buildings that were less robustly put together that fell apart. And you can see the first floor of another tower that did collapse.

Older building techniques don't rely on the mortar holding the structure together, instead it's the interlocking bricks and structure of the arches holding each other up.

That reservoir would have been fairly still water, so there is no movement of water to push it around.

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The first part of the answer should be that basically neither stone nor mortar are soluble in water, so stone, mortar or concrete structures can last for quite long in water even if they weren't conceived to be underwater. In fact, there are a lot of stone or concrete structures conceived to be underwater that are fine after decades or centuries - for example, bridge pillars and foundations, dams or piers.

The second part of the answer is that Sant Romà de Sau church has been actively maintained since it was first covered by water in 1962. For example, this article from 2012 mentions that restoration works had been carried 15 years before (that is, about 1997). Sant Romà de Sau is a cultural and touristic landmark and is cared for as such. Without that maintenance the church would be in worse condition nowadays. The remaining of the village doesn't benefit from such an status, in addition to be less water resistant.

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    $\begingroup$ They actively maintained it when it was under water? $\endgroup$
    – DKNguyen
    Mar 24 at 6:47
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    $\begingroup$ They maintain it when the level of the reservoir drops. There have been a handful of serious droughts since 1962, which are both the reason and the opportunity for maintenance works. I'd would say that often the efforts are proactive: when water level drops enough to enter the church by foot people start going there and they need to maintain the church to prevent blocks from falling on a tourist's head. $\endgroup$
    – Pere
    Mar 24 at 12:12
  • $\begingroup$ Oh, so it turns into a tourist sight whenever there is a drought? See now! Limited time only. $\endgroup$
    – DKNguyen
    Mar 24 at 17:50
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As Pere mentioned stone structures can last a long time underwater. There are many buildings around the world that appear during drought conditions. Here are some pictures of the Temple of Quichula, a 16th century Catholic Church.Quichula
The Temple of Quechula was built in 1564 and abandoned in 1776. It was submerged in 1966 after a dam was built and occasionally reappears. It’s still pretty structurally intact.
partially submerged This one hasn’t had any maintenance, and was already a ruin when submerged.

What surprises me about the Sant Romà de Sau church is the roof. It must be supported by wooden beams and that still looks solid.

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