Earthbag construction

Superadobe

While Gernot Minke, the German professor of earthen architecture, first developed a technique of using bags filled with pumice to build walls, it was architect and builder Nader Khalili who popularized the methods and architecture of earthbag construction (particularly for residential buildings).

Khalili called his technique superadobe, because he filled the bags with moistened adobe soil. Many examples of his work can be seen at the Cal Earth Institute in Hesperia, CA, which he established in 1986 to popularize his ideas about natural construction techniques. Khalili pioneered code approval of earthbag domes for high seismic risk regions. Several books and videos have been produced by the institute to demonstrate his methods, however a number of other individuals and groups now offer training workshops.

Writers

Although Joseph Kennedy probably invented the term earthbag (as well as contained earth), Paulina Wojciechowska wrote the first book on the topic of earthbag building in 2001: Building with Earth: A Guide to Flexible-Form Earthbag Construction. Kelly Hart developed a massive online database of earthbag information that encouraged idea sharing and development. Kaki Hunter and Doni Kiffmeyer worked on a variety of projects after studying with Khalili. They aptly called earthbag 'flexible form rammed earth'. Their 2004 book, Earthbag Building : the Tools, Tricks and Techniques is available for purchase as an easy-to-translate ebook.

Free online booklets have been developed by different authors. Owen Geiger and building designer Patti Stouter have written many that address issues of using earthbag for different cultures, climates, and levels of hazard as well as testing soils.

A 2011 e-book by Owen Geiger, Earthbag Building Guide: Vertical Walls Step-by-Step, provides complete photo illustrations of the process and discussions of new techniques.

Promoters

Many like Akio Inoue, from Tenri University in Japan and Scott Howard of Earthen Hand have tested and built buildings. Kelly Hart has shared information freely on his internet sites. With Owen Geiger of GRISB.org he encouraged earthbag's development into different culturally and climatically-appropriate shapes.

By early 2010 earthbag buildings had been built in many countries around the world, and the spring 2015 earthquakes in Nepal revealed excellent performance from dozens of earthbag buildings near destroyed buildings. Special reinforcement techniques are being defined for straight-walled earthbag buildings in medium and high seismic risk regions.

Dr. John Anderton of South Africa has tested a triple channel bag version that reduces the slumping problems inherent in non-cohesive fill material like sand, and pioneered work in a narrow wall contained sand system which he calls E-khaya.

Pacheco pioneered the use of lighter HDPE mesh tubing or hyperadobe. This process can be less expensive and simpler where seismic risk levels do not require barbed wire between all courses.

Engineer Nabil Taha developed the first general specifications for reinforcement appropriate for the highest seismic risk zones. Organizations building in Nepal are currently working with other engineers to improve and refine reinforcement options.

Additional engineering tests are needed to further understand and refine reinforcement for different situations. BSI keeps summaries of university and in-house testing regarding earthbag's structural properties.

Construction method

The basic construction method usually begins by digging a trench down to undisturbed mineral subsoil. This trench can then be partially filled with cobble stones or gravel to create a rubble trench foundation. In high seismic risk regions a reinforced concrete footing or grade beam may be recommended.

Above that, several rows of contained gravel are used: doubled woven bags (or tubes) are filled with gravel and placed into the trench and one or two courses above grade to form a water-resistant foundation. Each successive layer will have one or two strands of barbed wire placed on top. This digs into the bag's weave and prevents slippage of subsequent layers, and also resists any tendency for the outward expansion of walls (especially with domes).

The next row of bags is offset by half a bag's width to form a staggered pattern. Bags can be pre-filled with material and hoisted up, or bags or tubes can be more simply filled in place. The weight of this earth-filled layer pushes down on the barbed wire strands, locking the bag in place on the row below. A light tamping of the bags or tubes serves to consolidate the moist clay-containing fill and creates interlocking bags or tubes of a strength between adobe and rammed earth. The same process continues layer upon layer, forming walls. A roof can be formed by gradually sloping the walls inward to construct a dome. Traditional types of roof can also be added.

Container types

The most popular type of bag is made of solid-weave polypropylene, used around the world to transport rice or other grains. Polypropylene is chosen for its low cost and its resistance to water damage, rot, and insects. Tubes of the material are often available from manufacturers who sew them into bags.

Organic/natural materials such as hemp, burlap or other natural-fiber bags (like "gunny sacks") can be used; however, since these may rot, they should only be used with fills containing a significant proportion of clay.

Tubes of soft crocheted or stiff extruded mesh can also be used.

Terminology

Earthbag construction has grown into a varied family of techniques. Since each variation in fill and containment has different strength and reinforcement requirements, more accurate terminology is needed. Contained Earth (CE) implies the basic technique. Damp, cohesive, tamped bag fill is used, and bonds strongly with barbed wire or other inserted materials when it cures.

Contained earth is not sandbags. Contained Sand (CS) uses sand fill or any fill too dry or with poor cohesion that performs structurally like sand bags. The 'soil bags' that civil engineers use for horizontal footings or some inclined surfaces are similar. CS must be built with solid-weave fabric bags and have good protection from fabric damage. Contained sand needs more vertical reinforcement than contained earth.

Some builders use narrow bags of contained sand as wall infill. Structurally, alternative loose fills (like rice hulls) will create a wall more similar to contained sand than to contained earth.

Contained Gravel (CG) uses fill of any aggregate larger than coarse sand. CG can function as concurrent footing and footing drain material. CG can use either solid-weave or mesh containment.

Modular CE is the traditional earthbag material which is built in solid weave poly-propylene bags or tubes. Walls rely on attachment between barbed wire barbs and/ or added pins between courses.

Solid CE is a more accurate term for hyperadobe, developed by Fernando Pacheco of EcoOca in Brazil. A pliable knit raschel mesh tube is used, the same material used in packaging fruit. This mesh allows the earthen fill to solidify between courses as long as the fill is built damp. Solid CE has higher shear and out of plane strength than modular CE of the same soil, and may allow the use of wire mesh in addition to or in place of barbed wire.

Bag-fill materials

Generally inorganic material is used as filler, but some organic material (such as rice hulls) can be used if a strong matrix like wire mesh reinforces the plaster. Filled with soils containing 5- 50% clay, like reject fines, road base, or many subsoils, earthbags tamp into solid structural units but cannot withstand prolonged soaking. Subsoils with clay mold tightly and cure attached around the barbed wire barbs. This may contribute significant and needed tensile strength to walls. Sands, stone dust and gravels can survive prolonged flood conditions, but most require special bracing during construction as well as some form of structural skin. Included chunks of cement mortar between bags may be appropriate to hold barbed wire to bags if clay or cement-stabilized fill is not used. Sand fill may be appropriate for several courses to provide a vibration damping building base, but becomes unstable in ordinary bags above 60 cm- 1m (24- 39") heights. Cement, lime or bitumen stabilization can be used to allow earths with clay to withstand flooding or to allow sands to be used in traditional bags with a non-structural plaster skin, but because earthbag walls are usually 38 cm (15") thick a large amount of stabilizer is needed.

Thermal insulating properties are an important consideration, particularly for climates that experience temperature extremes. The thermal insulating value of a material is directly related to both the porosity of the material and the thickness of the wall. Crushed volcanic rock, pumice or rice hulls yield higher insulation value than clay or sand. Untreated organic materials that could decay should not be used as part of a structural wall mass, but can be used as infill. Scott Howard and others have suggested adding crushed bottles, shredded bags, or other plastic trash as insulating filler in earthbags.

Thermal mass properties of the fill material are also an important consideration, particularly for climates that experience high temperature fluctuations from night to day. The thermal flywheel effect can make massive earth walls ideal for mild climates. Materials like clay or sand also have excellent heat retention characteristics and, when properly insulated from the home's exterior, make a logical choice for incorporating a passive solar building design in cool climates. Such a design has the capability of keeping a home's internal temperature stable year-round.

Forming the house

Various types of roofs may be used, including earthbag extensions of the wall which create barrel vaulted or domed roofs, although vaulted roofs of much size are difficult to achieve with earthbags. Earth domes are very inexpensive to build, but waterproofing them is a complex or expensive process for humid regions. Recently rectangular and round buildings are being built with roofs of wood or metal structure to mimic local architectural styles.

Windows and doors can be formed with a traditional masonry lintel or with corbeling or brick-arch techniques, usually on temporary forms. Light may also be brought in by skylights, glass-capped pipes, or bottles which are placed between the rows of bags during construction.

Finishing

To prevent damage to the bags from UV rays or moisture, it is necessary to cover the exposed outer surfaces of the bags with an opaque material. There are many possibilities of what material to use, including cement-based stucco, or lime or earthen plaster. This will need to meet any waterproofing requirements, but such requirements can also be met by using additives in the bag-fill material. Some buildings use a planted-earth "living roof" ("green-roof") to top the structure, while others use a more conventional framing and roof placed atop earth-bag walls.

Environment friendly

It's easy to posit that earthbag construction uses the least energy of any durable construction method. Unlike concrete, brick or wood, no energy is needed to produce the necessary materials other than gathering soil. With on-site soil being used, practically no energy is expended on transportation. And unlike rammed earth construction, no energy is required to compact the soil (although some tamping is still required). The energy-intensive materials that are used — plastic (for bags & twine), steel wire, and perhaps the outer shell of plaster or stucco — are used in relatively small quantities compared to other types of construction. The buildings last a long time; however, when they are no longer useful they may simply erode with no serious threat to the environment, or even be recycled into new earthbag-constructed buildings.

Use in Disaster Areas

Earthbag building techniques were also explored in Sri Lanka after the 2004 tsunami. Multiple earthbag construction projects have been completed in Haiti, most of these after the earthquake. First Steps Himalaya and other charities had built more than 50 earthbag buildings in Nepal prior to the April 2015 earthquake. As these were obviously still standing near devastated villages, local builders flocked to ongoing earthbag training opportunities, including those by Good Earth Nepal, which are leading to official Nepal building code acceptance of this technique.

Colonization of the Moon

Khalili proposed using the techniques of earthbag construction for building structures on the Moon or other planets. Currently, it is quite expensive to lift a positive-mass payload from Earth. Thus, Khalili's techniques would seem to be an ideal solution as the requisite supplies would consist of lightweight bags and a few tools to fill them. He specified that such bags would probably have pre-sewn "hook and loop" (i.e. Velcro) fastener strips in lieu of barbed wire.