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Earth Sheltered Housing


Nigel Bridgman-Elliot
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I'll build something when I have enough money, so I won't be investigating forever, just whatever is the best I can find and afford at the time.

 

Listening to some of the experts about concrete/rebar and why it sometimes fails, its still a bit of a black art. I've seen old concrete buildings with no apparent failings, and also ones that you can litterly push over :-)

 

I do like stone built, but its not very waterproof :-) (Not unless you include lighthouses..)

 

Whilst concrete could be said to be not very environmentally friendly in the short term, in the long term with a structure that doesn't need knocking down and rebuilding every few decades, your save material in the long term which could well offset the unfriendly nature of doing it with concrete in the first place.

 

I like to over engineer design to allow for future issues, rather than planning on the once in a hundred years design.

 

 

I've also an interest in archaeological stuff (Went on a dig when I was younger and doing a course on the subject.) and the Scottish castles with fused stone walls I find intriguing, are they by design or accident..

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Yup, there could always be an earthquake, but in a simple dome the concrete would always (with no earthquake) be in compression, as in an arch, so as a first guess it'd seem to me that an earthquake would need to make accelerations of more than 1g to induce any tension in the concrete at all? That's a fairly hefty shake.....

 

There could be some wilder post tensioned designs, but that would not seem economical for a private house? The over-engineered solution would seem to be to not flatten the domes too much, get good foundations, and pour on plenty of concrete - not many Roman arches get knocked down by earthquakes ;)

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when reinforcing concrete, however you do it, the best safeguard against the reinforcing "rotting" is to give it plenty of cover...

Absolutely, and in this case there is no problem providing enough.

 

 

For an idea of how to build earth houses that will stand the test of time...look no further than Jarlshof.

Again, absolutely so. I often point out that the northern isles were building round house designs right from the beginning. It is a good solution and is infact indigenous to Shetland.

 

I'm not really keen on wooden structures because of the fire hazard point of view as the main reason.

As paradoxical as it may sound, wood is actually one of the best materials from a fire safety point of view. Its problem for an earth sheltered structure is more to do with the large deflections and long term creep. Mind you that does not rule it out. The latest concept for the Scatness grass roofed dome proposal uses wood

 

and having the internal blocks replacable

I don't think that is likely to be a good design requirement.

 

 

... in a simple dome the concrete would always (with no earthquake) be in compression, as in an arch...

That should generally be the objective, and we have nice ways now of designing geometries which are always tension free. Still need some reinforcement to control the cracking of course. With the sprayed examples chain-link fencing mesh does the trick nicely.

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Wood also has issues with getting it from sustainable forests, and often glues used being less than than human friendly. (Yes I know they have passed tests, but most now have health concerns, and I really don't want to fill new homes with a slow acting poision.)

 

Modern compressed wooden beams made like chipboard are again a concern as I've heard of them failing due to glue failure and wood rotting, not to mention how people like to make holes in them for plumbing/electric and then wondering why they fail :-)

 

 

I'd be interested to hear of the arguments for wood V concrete when it comes to fires.

 

 

I'd also be interested to hear why the thought of replacable internal blocks isn't a good design approach.

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...often glues used being less than than human friendly.

Yep, this is also a hassle with the resins used for fibre composite construction. I've been trying for years to source a cheap (comparable to epoxy or just polyester) bio-resin which can be used for construction. Various firms are producing such resins for other purposes but none seem to be targeting construction yet.

 

...wooden beams ... people like to make holes in them for plumbing/electric and then wondering why they fail :-)

This, lamentably, also happens with steel more often than it should. Not much of an issue with concrete though :-)

 

I'd be interested to hear of the arguments for wood V concrete when it comes to fires.

My mentioning of wood's very good fire resistance was not to argue that it would be better than concrete for an earth dome. Rather, to remind that wood structures are not the fire susceptible joke that most laypeople believe. After the fire you can't use them again, but in terms of how many minutes the structure remains safe, due to the charring the members remain structurally intact for a long time.

 

 

I'd also be interested to hear why the thought of replacable internal blocks isn't a good design approach.

Well I might not have grasped your proposed fabrication/installation/maintenance sequence, but I can't see how it would be possible to have them removable after construction, while still being able to benefit from them properly during installation and during service. My main interest in using such panels is as lost-formwork which is fully integrated with the monolithic concrete which is sprayed or poured on top. Once built it should be the mass concrete which is doing all the work. By doing it so they can be removed (and remember they will not be able to move inwards unless there is some complicated jointing strip) everything becomes very messy (structurally), but there is no apparent advantage gained.

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I did come across a lost-formwork approach a while ago (Looks around at notes, notices OS is in state of reinstall and makes note to look again after everything is put back..) though I seem to recall it used steel panels. One could use concrete instead I imagine, but the concern of steel rebar failure so often seen in concrete structures concerns me, especially as no one is really sure why they fail. Hence the thought that having removable panels would allow replacement in the future if/when they degrade.

 

There is additional concerns I have with concrete meeting concrete, that of outgasing which might be an issue, as I've seen that with concrete that has something on its surface, tarmac for example where it bubbles up. So the panels may need to have channels to allow the gases to escape.

 

Then there is the issue of joints between pours, again this area appears not to be fully understood and is another area of concern to me.

 

I got interested in the idea after looking at lighthouse and tunnel construction approaches.

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i had steel lintels in the old house and they were shot bowing causing the brickwork to crack up. they ust have been 50 years old though. ive seen lots of concrete flaking fro the rods in it but i suspect it was done on the cheap. a lot seeed to be from the 70s must have been bad for building.

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but in a simple dome the concrete would always (with no earthquake) be in compression

 

no, only the concrete under the arch/dome is in compression

the stuff at the top(outside) of the arch/dome is in tension hence the need for steel...

 

I do like stone built, but its not very waterproof

 

so a 3ft thick pointed stone wall isn't going to be very water proof?

I beg to differ....if you're doing it these days it's not going to be difficult to include a waterproof membrane in the design to overcome any ingress of damp. not to mention the insulation properties of such a thick wall.

(you'll see plenty of these when you visit, hundreds of years old and still standing)

 

and having the internal blocks replacable

I'm with EM here, why do you need the internal blocks replaceable?

for one I have witnessed bad block in Shetland, the only ones affected were on the outer skin...the inner ones(even in the dwarf walls) were intact yet they were all from the same "affected" batch of blocks

and secondly, if you're going to all the trouble of getting the concrete right on the outside you're not going to need them replacing, chances are you're not going to need them at all wiht a mass concrete structure anyway!

 

no where is safe from earthquakes...

when amyone say we were safe from having earthquakes? what I asked was when was the last one to hit us that did any significant damage like the one Joannie mentioned....

 

The safest way to avoid problems with reinforced concrete is to take advice from the experts like BCA etc, design you're building with best cover to the steel and use additives like DCI and make sure the quality of the concrete is good enough for the job at hand and source your steel from a good supplier too so you don't end up with any inferior materials.

 

After that if the building fails, it's hardly your fault...you've taken all the precautions you can just don't let the 200IQ get in the way and make more out of it than you really need to.

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I haven't personally encountered issues with rebar and concrete, as I've only built things with it a few times. But I've visited, poked prodded, asked owners of thousands of concrete buildings where such problems have occured. (I have seen that if the rebar is too close to the surface this can cause the problem, or if that point in the structure is under a lot of repeated stress, eg. wind blowing it from side to side.)

 

 

It would be nice to build in stone, but I'm not too sure about if one could make all those joints actually water tight. (Though I could understand if you shaped the blocks the right way so that they interlocked that might help provide a tight seal.) As they appear to solve the issues with lighthouses, but there might be a difference there between a structure above ground and getting wet and one underground and under pressure from water to get in. (It would certainly be interesting to hear from lighthouse designers/owners..)

 

I'd be interested to hear more about how practical stone might be, though you'd still need an insulation layer on the outside.

 

 

Again I've seen and heard of many waterproof membrane failures, such that using them gives me great concern. (The clay one appears good mind you.) You also have the issue of quality control when building in that if someone misses a spot.. (And that is the main reason why I want to do all the building work myself, as I've heard just too many horror stories of getting someone else to do the work and skipping on quality control. Though obviously when I come to build them on a large scale, I'll have to have hard working inspectors keeping an eye on the labour force.)

 

 

Its hard to know which experts really know their stuff when it comes to concrete buildings, as so many of the experts was behind the poor buildings we see nowdays, especially those towerblocks..

 

As a kid I grew up surrounded by experts in this and that, and during my life have met many more experts, but often they disagree and its really hard to tell them apart. Often the only good way of doing that is to publically debate a particular solution so that shortcomings in a design can be spotted by someone and pointed out as in error.

 

 

I really don't want to build another 1970's towerblock design and then tell people 20 years later I did my best, I really want to make sure I pushed the envelope and took the cautious route which might well be more difficult and costly, but in the end could see a product with a greater chance of success in years to come.

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no, only the concrete under the arch/dome is in compression the stuff at the top(outside) of the arch/dome is in tension hence the need for steel...

Fraid not. There are basically two possibilities. Either the geometry is determined such that the structure experiences pure compression under dead load (a compressive funicular membrane), or not. In the latter case it is a shell and will have to deal with bending under dead load. When such bending causes tensile stress in domes, depending on the geometry, it can turn up above but will almost invariably occur somewhere underneath.

 

In both cases it is necessary to check the configuration using full shell analyses for the live load situations to ensure both compressive and tensile stresses are ok, but for a thick shell the dead loads entirely dominate.

 

For a short-span situation like this there is no need whatsoever to use a bending resistant shell so all the issues Nigel has with rebar etc just do not crop up. All the problems with concrete shells have been associated with thin shells being either the wrong shape, or poorly constructed. I don't know of any problems with funicular thick shell buildings ever.

 

One thing which hasn't been specified is the symmetry of the proposed buildings. This is a key decision affecting the whole design economics. In the past an axisymmetric geometry would no doubt be planned in order to make repetitive fabrication cheaper. I hope that this is not intended because architecturally (both aesthetic and functional) free-form plan shapes are much nicer. Structurally better too and they also allow buildings to better fit into the existing landscape, especially as earth sheltering works best on hillsides.

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Nigel: I doubt if stone would work in an underground situation, I was thinkin more for the overground part of the build ;)

 

 

I'd be interested to hear more about how practical stone might be, though you'd still need an insulation layer on the outside.

you still wouldn't need insulation on the outside tho, maybe in the roof ;)

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