Newbie: In some soils, can holes be permanently self supporting if protected from ground water and.. 5

[NotEngineer]

Thanks for looking at my thread. I'm not an engineer, some general advice would be greatly appreciated. In some cohesive soils, can holes (90° walls) be permanently self supporting if protected (from above and sides) from ground water and weather?

I understand if you were asked to confirm that a large hole (11x11 meters) is self supporting you'd need a large amount of data about the site geology which I don't have currently, so this is just theoretical for now.

The plan is to protect the hole from weather by covering it with a rubber membrane which is supported to be high in the middle, so to drain to the sides. Protecting the sides from running ground water is more tricky. Currently my best idea is a trench around the hole (2' away from the hole) filled with clean rubble and gravel to below the floor level of the hole. The 2' thick wall of soil between the trench and hole would act as a (not 100% effective) vapour barrier to reduce the humidity inside the hole (which has some protected ventilation).

Thanks in advance for your help!

 

[NotEngineer]

As the top soil is free draining, it's parent soil the subsoil will likely be free draining also, so perhaps even a slightly battered earthbag (rammed building earth in bags) retaining wall below the frost line and gravel filled tyres above the frost line?

 

[oldestguy]

Not an Engineer:

OK, now you explain. That would have saved a lot of discussion if presented to start with. While it may be an attractive, ecofriendly project, I would suggest that you involve an architect and an engineer, possibly several engineers. While the ideas may be interesting and progressive, there likely are unforeseen factors or events that may totally destroy the project.

I take for example here in the states the life of Frank Lloyd Wright, famous architect. He had wonderful progressive plans and managed to get most of them done. However, many were fraught with many dumb results, unexpected..

For instance your rubber roof, is it insulated? Will there be condensation on the undecided making interior very uncomfortable? The principles of vapor pressure on moisture movement and temperature differences may be very unfriendly to the interior.

Are there any mice or rats in the area? They sometimes manage to find there way into most areas and make their home in the most unlikely area, such as straw bales.

Is there ever any snow load on roofs in the area? How about strong winds?

What sort of heat is provided in cold weather? Are there any building codes and inspectors for living quarters?

Just a few things that come to mind. Good luck. You will need it.

 

[NotEngineer]

Hi again Oldestguy,

Thanks again for your input, I'm familiar with Frank Lloyd Wright especially because of his rubble trench foundation ideas. This isn't my technique by the way, I'm using a book written by a couple of who live in CA who have built bermed domes with this method already, I'm simply converting it for my climate and trying to put it underground.

I do appreciate it when any aspect of my plans is questioned, it makes me think again about the design:

Not pictured on that image is a bamboo frame supported by the domes holding up straw bales below the rubber membrane. In the rooms not in domes the underside of the straw bales and bamboo poles will be plastered with a clay plaster to avoid the condensation on rubber problem you mentioned. Inside the domes during points of high internal humidity it would settle into the earth and be released during times of low humidity (heat exchanger ventilation), or on the skylight which can be monitored and dehumidified. The bathroom will be tiled and with extra active ventilation and the kitchen will have a normal extractor fan. the floors will be sealed with linseed oil on top of a normal earth floor preparation.

The mice / rats would need to chew through the rubber (which I know they can do) enter the front door, but in the design in that image the straw bales on the sides would be available for inspection, not least by a house cat

We have very minimal snow loads and no snow most years, besides the dome shape is fantastic at distributing loads. If the design does get build underground there would be no wind loads. I'm a big proponent of composting wood chips to heat water for underwater heating.

Please feel free to let me know any other points you can think of, or if I didn't provide enough detail / you think a solution is not suitable.

Jayrod, I've just found and called up this company:

http://www.flexmse.com/

basically no difference to Earthbag building, from their pictures they've built some impressively high (and not very battered) retaining walls. On the phone he said they normally batter retaining walls from 45% to 85% and even some times vertical.

Cheers!
Mike

 

[oldestguy]

OG here once more. I see by weather conditions common to your area there can be gales of significant wind speed. Anything curved or even flat so exposed,such as a membrane roof system, is likely to get some major lift,(Bernoulli's principle I think) as happens on an aircraft wing. That is one reason why membrane flat roofs usually require a gravel covering to keep that from happening. A dome shaped roof would really "feel" it, with lift much greater than its weight. Many a conventional roof has been lifted off by strong winds. Installing earth utilizing anchors may well do the job, but there then comes connection details, as well as disturbance to that soil that must remain stable. Get used to the vibrating in the wind even if it holds.

Your heavy rains also require more than a perimeter ditch to keep from severely weakening the soil below. As a general rule, the higher the moisture content, the weaker the shear strength becomes. It is that shear strength that keeps the sides from caving in.

Again, on site advice of experienced building designers and builders will be insurance well worth it. An interesting project that you might find will get low cost services just because of that factor.

 

[oldestguy]

OG back again.

One more thought. Over here, any living area requires a building permit (most ares anyhow). The inspector here would require a professional, licensed engineer to certify as to the safety of the proposed work. In your case, the potential for collapse of the earth wall to injure or kill someone, is there and should not be taken lightly. Even working next to an earth wall sometimes is not allowed without bracing there.

The least you should be considering is some form of retaining structure, be it a gravity retaining wall or soil nail reinforced earth situation, to name a few methods.

 

[oldestguy]

More from OG.

Here is a link dealing with dome roofs.

http://www.spin.bam.de/en/spin_media/spin_wind_loading_on_shell_structures.pdf

In there I copied this diagram of some wind tunnel tests on different dome roof materials. Wheat is important is the shape of the pressures. Note pressure from outside on windward side and a significant lift at the top by those negative pressures.

 

[aeoliantexan]

Boy, did you stir up a lot of interest! As you can sense, the engineers are fascinated but nervous. Here are a few more factors to consider:

I have seen basements dug beneath homes with the walls 2 or 3 feet inside the foundation walls, vertical or slightly battered, lined with a single width of bricks or tile blocks. My daughter lives in such a home, and the basement is many decades old. The soil is loess, and the water table is about ten feet below the basement floor. This does not mean that you will be successful in your area or that success can be translated from California.

A water table above the floor level just about dooms the project.

If the excavation is exposed to freezing during construction, the surface of the cut may become unstable when it thaws. Ice lenses can form parallel to the cut surface, and when the ice thaws, the soil falls down. I have seen a deep railroad cut in loess badly damaged by this process. Where the loess was dry, it was OK, but at the lower levels, it was nearly saturated, and freezing was destructive.

Alternate drying and wetting can fracture the soil. A waterproof coating is probably needed.

Digging a trench 2 feet from a vertical face would leave the soil in a column 2 feet wide. This is easily tipped over by the modest pressure exerted by the granular backfill. Such a failure is sudden and deadly.

Yes, a circular cut is more stable, but not very stable when 11 meters wide. Existing fractures are common in clay, and can render the arch unstable.

Perhaps if you cut the drainage trench at least 8 feet away, tied the soil mass together with soil nails (bamboo?, and used woven wire mesh and gunnite on the surface, you would have a stable wall.

Hire a good soil engineer, but remember that he probably has no experience with this construction.

How about a cylindrical stone masonry wall designed as an arch?

 

[BigH]

Would you not, too, need a double way out? It is an underground home - what if a fire would block the single entrance? I've thought often of such a home - but decided if I were ever to build it, it would be above grade with domed earth on sides and over the top . . . - like a mini-hill.

 

[jayrod12]

The homes BigH is describing are quite common, but they all entail building a home above the existing ground and burying it. This is the first one I've seen that is intended to be buried in the ground. As much as I would love to be a part of a project like this, safety would trump eco-sense and I would be forcing some form of reinforced wall (whether soil or concrete, with the soil one being designed by someone who knows their stuff..... i.e. not I).

I'd be willing to consider some form of stacked battered stone retaining wall, but 11 feet is a tall one for un-reinforced.

 

[RFreund]

I have not read through all the responses but what about a segmental wall or just using geogrid. You don't necessarily need to use a precast block as a facing you could wrap the grid and try to grow something? Maybe?

EIT

http://www.HowToEngineer.com

 

[NotEngineer]

Hey Guys,

Wow what a lot of interest, very inspiring indeed! I think the most important factor deciding if this project will be pulled off to the extent where the structure is stable and safe, is how well the input from varied and experienced guys like yourselves is sought, taken on board and sternly implemented. Perhaps some of the interest in this project from a Geotech forum is due to this building method being entirely structurally earth based with geotextiles?. Concrete is fantastic for bridges, sky scrapers and countless other uses, as I'm sure you guys know concrete products are not necessary for modest structures.

OG: Thanks again! Your first point I must admit I didn't even consider! I assumed due to the aerodynamic shape and almost grade level of the waterproofing membrane and skylights that wind wouldn't be a problem. Polypropylene mechanically tightened strapping will now hold the skylights in place with tonnes of dirt, I'm adding this detail (and others) to a new CAD elevation sketch which I'll post here once completed. This solution should cope with considerable vibration. Regarding uplift to the waterproofing membrane, you paralleled an airplane wing, which are of course smooth, I haven't mentioned but I intend to cover the waterproof membrane in a thin (1-3") of soil to sustain succulents such as sedums which will provide a very rough surface. Do you think this will be enough to counteract the uplift effect?

Due to the complications mentioned of having a wall of undisturbed earth to the outside of the domes I'm not yet decided if the waterproofing membrane will simply be applied to the outside of a more load bearing insulation, such as volcanic rock in polypropylene sacks stacked against the domes. The earth could then press against the domes from the outside which have proven themselves (in other projects) able to distribute / support such a load. The amount of horizontal load will be clearer on the new elevation sketch once completed.

That said I'm still very interested in the eco solutions for retaining walls, as not having the earth loaded onto the domes does have advantages. Below is an image from

http://www.flexmse.com/

who basically are branding Earthbag building. I phoned the guy up and asked him directly what the difference is between their earth walling system and earthbag building, there's basically no difference, what do you think? Does this count as a gravity retaining wall?

I will employ a local engineer who specialises in earth building. Thankfully in the UK (not sure about the US) we have third part approved inspectors (who don't work for the local building control office) who can sign off a building. This means I can find one who specialises in earthen building methods and who hopefully has good Geotech experience.

 

[NotEngineer]

Hey aeoliantexan: Yeah I'm very pleasantly surprised about the level of interest! Thanks for your advice. Hokie mentioned loess and said that's the only soil type he knows of which naturally stands vertically, that soil type would make things easier, though the UK is a lot wetter so this may not be the case this side of the pond. Oh yeah, a high water table would definitely put a dampener on the idea of an underground earthen house. Happily I don't have that problem. Out of interest, for some one who does want to build underground in a high water table area, do you think Bentonite/earth slurry walls 6' around the perimeter would be sufficient to hold back the water? So the hole could be excavated?

Earth building is really not a technique for freezing weather in general, but thanks for that insight. I like the idea of bamboo soil nails but am trying to avoid concrete such as gunnite, is there an eco alternative?

Hey BigH! well spotted, I'm considering applying for a relaxation on that "multiple exits" build control requirement, as earth buildings can't catch fire and the furniture will be earth sculpted. I did consider that type of earth bermed construction and love those houses. If underground proves infeasible I'll go back to that idea.

Jayrod: Good to hear you'd consider an earth retaining wall, with the calcs done by some one who knows what they're doing (also not I!).

Hey RFreund! Geogrid sounds like a very simple and low cost option, can you give more of a description on how this would be applied? Or a link.

Cheers!
Mike

 

[cvg]

straw bales don't catch on fire? what about the kitchen or other furnishings? generally bedrooms require egress directly to the exterior of the building, not to an interior hallway.

 

[oldestguy]

OG again:

Your questions are good and some form of answer may come here, but not necessarily sufficient considering unknowns (to us).. For instance the bagged slope covering shown is more of an erosion prevention system, against a slope that probably has good internal strength itself. In any case an experienced geotech should be consulted to be sure of a safe design.

As to a barrier around the facility to cut off lateral flow of water, well yes that works, but nothing can prevent that water from sneaking around under that barrier short of founding it on very impermeable material. Again, with these questions coming up, it shows the need for help there with your project. As to some vegetation on a dome preventing lift, I'd not take a chance, since a strong wind will peel of coverings once it gets under the edge. I'd not do any plan using details not tried and proven elsewhere. Should I repeat that? From an 86 year old who has seen a lot of failures.

 

[BigH]

OG - isn't that 68?

 

[oldestguy]

I feel that at times, but sorry, it ain't 68

 

[oldestguy]

A final note. I should ahve read this before, but one part of the rules here reads:

Homeowner "do-it-yourself" posts are strongly discouraged. This is not a purpose of the Eng-Tips.com fora and non-engineers should not substitute asking questions on this site for obtaining appropriate professional assistance.