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Some of the largest and most impressive dams, like Hoover or Glen Canyon, are built entirely out of concrete.

But in this scenario, Hoover is an enormous wall of granite and/or marble bricks glued together by Roman concrete (which is tougher and less porous than modern concrete, or "Portland Cement").

In a Life After People, how long could such a dam stand before crumbling under the will of erosion?

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  • $\begingroup$ Tough is a relative term, concrete with reinforced steel may not stop a wrecking ball because it doesn't have a hardness of 7 or 8 on Mohs scale, it is capable of handling being bend, twist and what have you it doesn't even bulge when being spitted with millions tons of saliva rich water. $\endgroup$ – user6760 Oct 31 '15 at 13:06
  • $\begingroup$ Who said anything about concrete being reinforced? The Romans never reinforced their concretes, and that's one of the big keys to the buildings' survival. $\endgroup$ – JohnWDailey Oct 31 '15 at 17:30
  • $\begingroup$ I don't think it's going to matter--it's going to die from a failure of it's supports before it wears away whether it's made of concrete or brick. $\endgroup$ – Loren Pechtel Nov 1 '15 at 0:29
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The best estimate for modern dams is they will last for centuries before finally crumbling. The most likely failure mode for a modern concrete dam is actually the reinforcing steel, which will corrode in between 200 – 500 years without constant upkeep. Earthen dams, or ones made with a rubble core, don't have that limitation, but will likely also fail after several centuries of use without upkeep.

The issue for large dams over long periods of time is that eventually the basin behind the dam will silt up, and the volume of the reservoir will gradually decrease. If the flow of water into the reservoir remains constant, then the dam will eventually be overtopped and a waterfall will appear where the dam was, gradually eroding away the upper portion of the dam. There is also the issue of the pressures on the base of the dam; they were calculated for a particular weight of water, rather than silt (which is also being compacted over the centuries as more and more is deposited at the base of the dam.

For the sort of dam you are proposing, there is the additional issue of the dam being made of separate elements (rocks or bricks) which are held together by a combination of their mass and sealed with Roman cement. This sort of construction has thousands of distinct stress points, and the shifting pressure of the reservoir behind it over the centuries will gradually change the stresses on the structure in ways the designers may not have anticipated. As well, when the dam is overtopped, it will be far easier for the water to wash away a large structural element (one of the bricks or rocks), creating a channel which will amplify the flow of water and create a greater scouring force, eroding the cement and causing more elements to fail.

There are clearly a lot of different elements at play which make an exact calculation difficult. If the builder was in a hurry and didn't lay the courses of bricks or rocks properly, then the structure will be weak and may collapse with a high rain season or snow melt. If the cement wasn't allowed to cure properly, then you have another weakness. If the weather patterns change and the dam is called to hold back more water than it was designed for, then you have a pretty obvious problem. And of course, without regular inspections and maintenance by competent engineers, the dam will not last as long as it could.

So I would suggest that your dam will probably last for at least 200 years with no maintenance and excluding exceptional conditions like earthquakes or massive changes to the weather.

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  • $\begingroup$ 2-500 years is a huge gap. Out of curiosity, where did you get those numbers? $\endgroup$ – HDE 226868 Nov 1 '15 at 0:05
  • $\begingroup$ just guessing that it is "200-500" not two years to five hundred years. $\endgroup$ – NadjaCS Nov 1 '15 at 4:45
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There do not appear to be any pure brick dams anywhere in the world; everything is rubble- or earth-core masonry. Looking at a sampling of those:

  • Kodiveri Dam is still going after almost a thousand years. No indication of how much repair work has been needed, though.
  • Gilboa Dam lasted about 80 years before decaying to a point where it was judged unsafe.
  • Aswan Low Dam is still going after 113 years, but it's protected against flooding by Aswan High Dam upstream.
  • Assiut Barrage dates from the same period, and has undergone major repairs every 40-50 years.
  • Roosevelt Dam, probably the closest thing to your brick Hoover Dam, lasted 80 years before being re-worked as a concrete dam.

In the absence of maintenance, I'd say that your dam is likely to last 50-100 years before being breached.

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  • $\begingroup$ Bricks are porous! A brick dam will be bricks holding a non porous earth (clay?) core in place. This works quite well for Victorian railway cuttings though these not holding back a lake. $\endgroup$ – nigel222 Nov 1 '15 at 23:44
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A dam made of bricks won't work.

Brick walls are good at taking stress if form of vertical compression. But in a dam, a wall needs to withstand lateral deformation due to water pressure. Brick-walls are not very good at this. The binding force between bricks and mortar would be a weak-point.

So, "how long could such a dam stand before crumbling under the will of erosion"? I would give it a few minutes.

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  • $\begingroup$ Bricks can also take the stress of horizontal compression, and you can turn the shear forces from water pressure into compression forces by building an arch dam. $\endgroup$ – Mark Jan 20 '16 at 2:23

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