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- Hybrid-Adobe (Uses Cement and Paper or other added materials in the mix, instead of straw.)
- Rammed Earth
- Rammed Block
- Earth Bag
- Earth Tube
- Earth Ship (Rammed Tire)
- Earth Covered Roofs (Sod)
- Dirt floors
You may want to read as a prerequisite Green? my blog post on the green building trend.
This is some images of an earth sheltered design. The infil between columns in this design is earthbag, though it might be rammed block, cinder block, log, concrete and other methods. Warning this is a concept that has not been fully engineered.
The above list all the types of earthen construction of which I have read about thus far. What type of earth? It depends, but most of the methods call for a proportional mixture of clay to sand. All would be without loam, which is the more decayed plant like material. Gravel and stones would not make up much of a mixture if any.
Earth may need to be stabilized. This means that it would need to be made stronger, glued together better and so that it can support more compression or weight without falling apart or mashing. This also helps to prevent cracking. Stabilizers add abrasion resistance and erosional resistance.
Wheat straw (which can be found sold as fodder or bedding for horse stalls, at the time of this writing for $4.50 a bale, bale being 8″x16″x32″) would be the most famous choice as a stabilizer in some types such as adobe or cob. Cement would be another stabilizer for types such as rammed earth or rammed block. Lime might be another, for example in “Roman Cement” lime is mixed with clay. Fly Ash (ash from burned coal or coke) could be another as well. Asphalt yet another. And there are other types of stabilizers that I most likely have not mentioned in this article. Much of the advancement in stabilization of earth has come from the civil engineering in road building in recent years.
The exact construction of a given type would be shaped by the humidity, rainfall, snowfall, ice and freezing for the location of the structure. I suppose the driest most arid climates would be most suited, however cob has been used in very wet climates such as the British Ilse. If the designer applies enough thought he may be able to use any of the earthen methods in virtually any climate. The equation is simple. The more wind, rain, moisture that is present the more stabilized and protected the earthen structure must be made to be. None of the earthen structures are as durable as baked clay bricks or concrete.
Why use earthen methods of construction?
- Thermal mass properties. (It stores heat and cold well, which is good for the energy efficiency of a dwelling.)
- Economics (It could be near dirt cheap all depending on the situation)
- Readily available (Can be gathered from the building site itself or from nearby)
- You have access to very cheap or near free or maybe even free labor?
- Playing in the mud is fun? Well for boys, of course men have pricier toys and projects.
- Maybe you have a taste for military field style construction?
Adobe would be made from sun dried bricks of mud and straw. The mud should have the proper proportions of sand and clay. Too much clay and it shrinks and cracks easily. Too much sand and it falls apart. Adobe walls are usually about a foot and a half thick. Bricks are made to be about 40 lbs each so that they may be easily carried. Cob is very similar to adobe except that it is not sun dried and cures more like concrete. Cob walls are about 1.5 feet thick as well and are built from the ground up by packing globs of the cob mixture onto the top of the construction over and over until it is completed. Thick paddles are used to tamp and pack the outside of the wall as you go up. Earth bag, earth tube and Earth Ship all use normal loose soils. Ratio of sand to clay is not as critical. It is packed in place in a near dry state. It is packed into tires with a sledge hammer. Soil is shoveled into polypropylene earth bags, or long earth tubes. (Recently at a company called “White Bag Company” in North Little Rock, Arkansas I purchased 1000 polypro bags 14″x22″ in size for $160 UV Rating of 1500hours, UV rating means number of hours in full sun that the bags will take without falling apart, and yes they will if not covered in some fashion such as by plaster or earth or mud.) In the case of tubes, the tubes are run in layers coiled in a circular fashion. For earth bag and earth tube walls barbed wire is used between layers for reinforcement.Earth bags are usually staggered like bricks.
Hybrid-Adobe uses adobe mix without the straw, but adds other materials such as paper, cement, glass shards etc. Not much to say here, but I will direct you to a web site–Hybrid Adobe dot com And they have a book which I have yet to buy and read, so I can’t really recommend it just yet.
Rammed earth is stabilized with cement or possibly lime and/or fly ash. If cement it is near 10% mixture of cement to earth. Rammed earth is moistened to a bread dough consistency then packed into forms that resemble concrete forms. Rammed blocks are the same but use a machine to press the blocks. Fernco Metal has several block pressing machines. The cheapest is $1200 and is manually operated. Blocks are similar in size and shape to adobe blocks. Rammed blocks would be stronger and more stable than adobe. Rammed earth does not include straw.
Earth covered roofs are roofs with strong frame structure supporting them, near 180 lbs per square foot strong. These roofs can be steep but need to have at least a slight slope. This next list shows the layers in an earth roof top to bottom.
- Sod (Grass)
- 6 inches of soil
- Old used carpets to protect water proof membrane.
- Water proof membrane such as a 40 mil pond liner.
- Old used carpets to protect water proof membrane.
- 4 to 6 inches of Styrofoam insulation.
- Tar paper
- Plywood or planking.
- Structural frame (roof support beams, rafters, joist)
I felt I at least needed to mention a gabion. A gabion is a box, cage container which is filled with loose earth or rock. The gabion container itself merely acts as a retainer. Metal wire boxes might be used. Bamboo, or wicker containers have been used in the past. Most people have noticed these as hex poultry fence wire made into boxes and filled with fairly large stones. I have seen this used as retaining walls and privacy walls, and for erosion control. The Earthship wall is basically as set of gabion bricks where a tire is used as the “cage” to hold the earth. In earth bag the bag or tube acts as the gabion.
Dirt floors are made by layering and tamping. Gravel is used below for good drainage, then sand, then earth. The stabilization of the earth is increased the closer you get to the surface. Finally the floor can be waxed with bees wax. Also straw may be used in the layers for reinforcement.
Walls are plastered with either a cement type plaster, stucco or a more breathable lime plaster, and possibly in some cases a mud plaster. Cement plaster can trap moisture between the plaster and the wall and should be used carefully. Plasters can be reinforced with fibers or animal hairs. A lath may be needed to hold the plaster in place or for reinforcement of the plaster. A lath is a rough grid like surface by which the plaster can adhere too. This can be chicken wire or specially made lath. In times gone by it could have been wooden lath. In waddle and dob it was made from thin branches woven together.
Now I will talk about some of the book resources available. Most of the adobe books I found (and there are quite a few on adobe) seemed to be very good all the way around. One that I found was on the repair or restoration of old adobe churches and homes. The one book on Cob I found was very good and covered earth floors and a Cob bread and pizza oven. Rob Roy has a book called “Earth Covered Shelter” which is a must have. I have seen one on earth bag listed below but have not bought it yet, though it looks very nice and comprehensive. There are 3 books on Earth Ships written by a hippie environmentalist architect. Those houses are architecturally sound and are simple to construct. However I feel that the tire wall thing is kind of risky in any climate but a very arid desert like climate, especially the way the author uses the ground itself in places and merely plasters over the ground. I have the engineering text book on Soils and Foundations and have so far found it to be invaluable source of general info, even if you don’t work the formulas, as with many college engineering level books. It goes a long way in letting you understand exactly what kind of dirt you are dealing with.
However after much thought I have found ways in which the tire walls could be constructed in very wet climates and be made to be waterproof. This adds to the construction time, cost and complexity. For example the walls would have to extend all the way to the grade over the entire U module, and the base of the walls would need to be above grade. You would need very good gravel drainage, possibly a pond liner and french drains to protect the wall and berm from water penetration. Also something like a cavity wall could be constructed between the tire walls and the berm or embankment (hill side). A cavity wall is merely two walls with some space between them.
None the less I personally would consider using tire walls for any non living space such as storage, shop space, garage space, barns, sheds, animal shelters, retaining walls etc. in wet climates. In wet climates for living spaces I would go with 2 to 3 foot thick walls made of Brick, Block, Rock, Concrete, Earthen or Earthen core with any of the other methods as a box or perimeter or surface to contain the earth core. Also remember that with a wall of this size the footing will need to be very huge, maybe 2 feet deep and 8 feet wide. The U shape of the Earth Ship module can be more square in shape but this might reduce the strength of the design somewhat. Reinforcement bar may need to be added to strengthen the design so that the U Shape for non-tire walls would be comparably strong to that of the tire walls.
One particularly interesting document I found was produced for the Peace Corp. It is called “Handbook for building homes of earth”. It goes into some very good detail on soils and stabilization of soils. It goes into great detail in telling you how to determine clay content. It covers how to test compressive strength. How to test for abrasion resistance and erosional resistance with a spraying test. I found this as a PDF on the web, but also it can be bought as a book. Earth adobe blocks or cob walls end up having about a 150 to 200 psi strength. Rammed blocks maybe 350 to 400 psi strength. Compare that to concrete block of 2000 psi, or baked brick of 4000 psi, or concrete at 6000 psi or granite stone at 15,000 psi or steel at 25,000 psi. Wood from the top of the post pressing downward is about 10,000 psi. Soils can be any psi up to hundreds of psi. Usual soil strength might be 50 to 70psi. Of course moisture in soil changes a lot and makes a huge difference. More moisture typically reduces compression strength. Earth walls are wide however which means they spread more roof weight over a larger area. An earth wall that is 1.5 feet wide will support nearly 43,000 lbs per linear foot. A concrete block wall 8 inches thick will support 191,000 lbs per linear foot. An earthship tire wall (3 foot thick) would support 100,000 lbs per linear foot or more. So you can see that the earthen walls, though being the weakest in pounds per square inch, do have strength for supporting roof structures.
By the way would anyone know what the EPA thinks about using trash or tires as a construction material? EPA will mandate strict disposal of waste tires. I think though if the tires were used in construction they then become a construction material and not waste. And a last note, remember that anything slightly underground or earthen in wet, humid climates may sweat, or collect dew or otherwise be moist. Therefore in living spaces or environmentally controlled spaces de-humidification would be necessary. The good news is that the energy cost for de-humidification will be half that of the standard A/C and you might get grey water which can water plants or even drinking quality water from some dehumidifiers. Eco Blue is one such dehumidifier. Passive dehumidification can be achieved by setting containers of rock salt here and there. The rock salt can be cooked to remove moisture and then reused. Damprid sells some rock salt type dehumidification products that let water in and will not let it escape back out.
About earth roofs and berms: One way to protect from water penetration is pond liner. The other is Bentonite or other clay layer. You can get clay in sheets I think. If you are really serious use clay on top of the pond liner. Or clay directly beneath and on top of a pond liner. Clay can actually self heal if it gets a small hole in it.