building underground: New approach

I am 53 years old and looking into some ideas for a low cost living option for retirement. I have had a long time interest in building underground for a number of reasons. First, the reduced cost of heating/cooling. Secondly, it allows me to buy a smaller piece of land to live off of, thus reducing the investment and taxes on the land. Thirdly, it reduces the cost of some of the projects that I want to undertake - building an underground green house (google walipini) and also would allow me to do some fish farming more easily because of the constant temperature of the earth.

I have to admit that I am totally frustrated by the whole school of thought about building a completely underground dwelling. The current thought seems to be that the roof of an underground dwelling has to be mega reinforced in order to hold up the tons and tons of soil that will be placed on top of it. Has anyone ever thought of the idea of treating the soil on top of the dwelling so that it will support a hollow chamber built underneath? As an example, consider excavating the hole and then building a domed building at the bottom. The roof will be covered with a blanked of some substance to make it water resistance. This will be covered with a light layer of reinforced concrete, which will be covered with a layer of a soil and concrete mixture which will be covered by rammed earth, which will be covered with common topsoil. Each successive layer of material above the roof will contribute to the strength of the roof rather than having to depend only on the concrete reinforced roof. With this kind of design, the roof serves as a kind of mould to hold the materials in place while they dry. Bamboo poles could be used build the arch of the roof. I have seen some very high strength concretes now that have metal filaments inside to make them super strong, which would minimize the amount of concrete that must be used, if any at all. Or even just covering the frame with 6 feet of some kind of adobe?

Is my main idea clear? It is still likely to be expensive but, I think, this approach is more likely to give us room to use more green materials in building it.

Its an interesting idea Tom but one that will require extreme engineering and very difficult detailing (just like most living roofs). A conventional roof insulated to R100 will likely be orders of magnitude cheaper, less risky, gives a surface for rain catchment and I think would perform much better too.

From an engineering perspective, the roof has to be heavily reinforced depending on the geometry of the ceiling. It is also good practice and the general rule of thumb is to have a factor of safety that it should support 3 times the actual load it would carry. For the strongest supporting ceilings you want to avoid flat roofs because it creates stress concentrations at the edges and very high tensive stresses towards the center where it is prone to breakage. A material in tension does not carry the same potential to resist a distributed load (the weight of the stuff on top) as a material in compression. For example, Portland cement can roughly handle 8 times more weight when it is compressed compared to when it is in tension. This is the reason you want to maximize the compression by having circular arches for the strongest supporting ceilings. Now, for green roofs when compared to underground structures, it is more permissible to have flat ceilings because they are not as heavy.

On the business of being cost effective, I do not see the cost/benefit improving much compared to conventional methods by layering the concrete. I guess using an analogy, there is more than one way to screw in a light bulb whoever because the technique is not proven there are additional risks involved. Green roof homes surrounded by thermal mass from the ground are generally the most cost competitive because it can be expensive building underground, let alone satisfying the inspector's standards.

I understand what you are trying to do and it might work, but it seems like a lot of effort and a fair amount of risk.

Underground structures are certainly doable, but if you don't need to do it, it is best to avoid it. If you do build underground, get a good engineer.

If you are not ideologically opposed to using reinforced concrete, why not price one of those inflatable forms, like monolithic domes? There is a good article at their site regarding the pros and cons of underground housing. If you are applying any of the WOFATI or PAHS principles to your design, you might alleviate the cold and condensation problems, though results have been mixed with PAHS and there is no existing WOFATI yet.

As far as getting the most out of a small lot, I think going underground is a good option, if you can get a waiver on the setbacks. A sloped lot will be easier for drainage, and you might be able to do a walkout.

You can build a basement or bermed house with a heavily insulated flat or low-sloped roof and then build a patio, deck or green roof on it.

Perhaps you could utilize your setbacks as your insulated mass?

Remember that a walipini at higher latitudes will, because of the sun's angle, closely resemble an Oehler greenhouse.

In the end, what you end up doing will depend largely on the property you purchase, zoning, your physical capabilities and your budget. Oh yeah, if relevant to your situation, don't forget your spouses desires as well.

Good luck.

This is easy to do.
If I were doing it, I would dig my hole in the hill side, build a monolithic concrete dome in the hole, and cover it up with the dirt. There are several examples at http://monolithic.com

You could do it with an ecoshell if you are in a more temperate climate (soil temps within the right range (like around 65F-72F) Or Insulate it with 2 inches of foam. You will never need A/C or Heat.
Just an air exchanger would do the trick.

http://www.monolithic.com/stories/underground-homes-good-or-bad
http://www.monolithic.com/stories/a-monolithic-dome-hobbit-house/photos
http://www.monolithic.com/stories/the-invisible-dome-home
http://www.monolithic.com/stories/underground-homes-good-or-bad/photos/4
http://www.monolithic.com/stories/underground-homes-good-or-bad/photos/6

Hello Tom,

Been there done that. A correction to what you have been told, there's more weight in a concrete floor than a on the roof of an underground home.

Your only real problem is roof rusting out. This is overcome by using concrete. The cheapest roof making method is leaving it round, a square house don't care what shape roof it sits under.

Construction, plastic sheeting is laid out in the hole of depth where the roof shall rest. Reinforcing rods/mesh is then laid on top which is covered by concrete. An air hose under the plastic to its centre then begins receiving air causing the concrete to bulge upward. When the desired dome is reached the pressure is held to the concrete cures. Further concrete is then spread over the dome to required thickness.

After this further excavating happens providing the living space and leaving the roof edge sitting on its earthen ledge.

Peter

Peter Mckinlay wrote:
Your only real problem is roof rusting out. This is overcome by using concrete. The cheapest roof making method is leaving it round, a square house don't care what shape roof it sits under.

Ya, but.... I want to get the most area under that roof I think I would go with a round house under too. (not going to get into the "round feels more natural/comfortable to humans" thing as I don't know from experience)

Construction, plastic sheeting is laid out in the hole of depth where the roof shall rest. Reinforcing rods/mesh is then laid on top which is covered by concrete. An air hose under the plastic to its centre then begins receiving air causing the concrete to bulge upward. When the desired dome is reached the pressure is held to the concrete cures. Further concrete is then spread over the dome to required thickness.

After this further excavating happens providing the living space and leaving the roof edge sitting on its earthen ledge.

Peter

Wow, sounds simple all right. Obviously there is more to it than that.... what kind of sheeting, is it limited to the width of that sheeting? How do you join two or more sheets? How does the reinforcing have to be laid out so it still has some effect/doesn't come apart as it is spread with the raising of the dome? What kind of concrete? What air pressure/flow rate is needed? How much of a lift can one expect to achieve (what portion of a sphere)? Oh yes, can it be engineered (Is there enough info around about the technique for an engineer to sign it off or is it just x inches of concrete and how it got there doesn't matter)? Does it have to be perfectly round or can it be oval? Could it be vaulted by only cementing the center portion of a long skinny oval? (the ends being covered with just mud) Can holes be left in it for windows/skylights or would they have to be cut later?

Obviously this could be used for above ground work too. It would be interesting to see if other materials besides concrete (cob/earthbag) could be done this way. I would expect with other materials it would be best to design a hole in the centre so the cob or whatever could slide down and remain thick. (enough)

Maybe there is a website with more info?

Hello Len,

We used builders plastic which is polythene in a roll. Overlaid the edges with aquahere glue, but not sure that needed.

Cant help much more than that other than put a soccer ball or other in the middle before air pumping to start the rise.

My personal view is if you want something to be dear as possible take it to a engineer so they can show you how complicated
they can make it as proof of their smarts.

Peter Mckinlay wrote:Hello Len,

We used builders plastic which is polythene in a roll. Overlaid the edges with aquahere glue, but not sure that needed.

I know the stuff.

Cant help much more than that other than put a soccer ball or other in the middle before air pumping to start the rise.

So the answer to most of my queries is "just do it".

My personal view is if you want something to be dear as possible take it to a engineer so they can show you how complicated
they can make it as proof of their smarts.

Point taken. The reason I asked is that people (people who have the power to make you tear things down or tear it down for you and make you pay for it) tend to notice when concrete trucks go places. I guess I could be pouring a pad... add air later. In my case at least more thought required.

Thank you for the thoughts.

Hello Len.

I understand your concern. However you cant raise a cured or even partly cured poured pad!

Engineers call the structure a dome, a seen on temples, churches etc.