Site Evaluation

Evaluating the building site for an earthbag structure follows all the same criteria as any other structure. If you are on a flat plain with decent drainage or a southern slope, you have an ideal opportunity to partially bury or berm your structure. Fairly stable soils and sandy soils are ideal for buried structures.

4.18: Penny Pennel's 36'- Diameter Bermed Earthbag Kiva Southern, AZ.


French drain installation details.

porous filter fabric porous filter fabric

French Drain Detail

line bottom of trench with sand to support pipe and pro -tect fabric from

4" diameter perforated pipe with holes on sides sloped to daylight,, dry well, or holding tank etc.

line bottom of trench with sand to support pipe and pro -tect fabric from

4" diameter perforated pipe with holes on sides sloped to daylight,, dry well, or holding tank etc.

When planning to "dig in," bury, or berm an earthbag structure, avoid sites located in an area with a high water table, flood plains, natural drainage areas ("dry" washes and intermittent streams), bogs, swamps, highly expansive clay soils, and steep slopes prone to mudslides and rock falls. Even with all these careful considerations, a bermed structure will require additional earthwork around the perimeter of the structure to ensure proper drainage. The installation of a French drain or swales will divert water around and away from the structure. Extra care must be taken to ensure adequate drainage exists around the entire perimeter of a buried earthbag building (Fig. 4.19a, b& c). (For more information about French drains and swales, consult the Resource Guide at the back of this book).

fold filter fabric over topof cobbles fold filter fabric over topof cobbles

fill around pipe with cobbles 2" - 3" and larger to provide lots of air space for water to drain easily to bottom of trench

4.19 b pile more cobbles and rocks on top of exposed filter fabric water collects in trench water flows down hill through rocks. Fabric filters out sediment. Water fills up bottom of trench, enters holes through sides of pipe. Pipe channels water to desired location away from building. Plant vegetation on hillside slopes to inhibit fbw-of sediment into trench

Earthquake Resilient Foundations

History has shown us that a foundation can be constructed of individual stacked units like rock and brick just as successfully as a poured concrete foundation. A poured concrete foundation relies on rigidity as a means to provide stability in regard to earthquakes. We can try to overcome nature through resistance or we can go with the flow and flex right along with her. The latter has been the design preference of choice for thousands of years throughout the Middle East and Asia in the most active earthquake regions of the world.

As far as earthbag building is concerned, Nader Khalili, innovator of the earthbag method, had tests conducted on his workshop structures to simulate earthquake movement. The tests were done in accordance with ICBO standards for a structure in earthquake Zone 4. This is recognized as the highest earthquake zone in the United States. Test results far exceeded the limits set by the ICBO, and in fact, the testing apparatus began to fail before any deflection was observed in the buildings tested.

Tests conducted at the University of Kassel in Germany conclusively prove that in comparative studies of square and round rammed earth structures, round structures show much higher stability in earth quake impact tests. The report went on to state that "it is advantageous if the resonant frequency of the house does not match the frequency of the earth movement during an earthquake." This implies that heavy houses built with solid construction (as in the case of earthbags) should not be attached to a rigid foundation. Light houses (such as frame construction) perform better attached to a solid foundation. In an earthquake, buildings are mainly affected by the horizontal acceleration created by the movement of the earth. A massive structure, independent of the foundation is able to move independently of the foundation. Therefore, in an earthquake, the ground stresses transmitted to the foundation are not carried through to the building. A light-weight structure relies on its elasticity to counter the seismic forces on the foundation it is attached to.

If you intend to build an earthbag building in a high seismic zone, intensive research should be undertaken, and a full knowledge (or at least a structural engineer's blessing) of what is required should be employed. It's probably a good idea to use a round design, too. Designing buildings for earthquake resistance is beyond the scope of this book, but as more research is done in this area, indications are that earthbag buildings could prove to be a low-cost, low-impact alternative to present day conventional seismic construction practices.


Structural Design Features for Earthbag Walls

Every construction medium incorporates specific design principles to get the optimal performance from the material being used. Timber, stud frame, and post and beam incorporate diagonal bracing and crossties to provide shear strength. The dimensions of the lumber and spacing dictate load-bearing capabilities, etc. No building material is immune to natures governing principles. Even rock is affected by frost heave.

Earthbag building is still in its infancy, so it remains open for exploration. The design principles that we have incorporated into earthbag building are simple, common sense strategies inspired by FQSS, observation of successful indigenous building techniques, and some of the current provisions to adobe building codes, especially those of the state of New Mexico where they have a long-standing tradition of building with earthen materials.

A list of the fundamental structural principles for building vertically plumb earthbag walls is as follows (note that domes are in a category all to themselves). (Refer to Chapter 11).

5.1: Linear, freestanding walls require lateral support (buttressing) every 12 feet on center.

Earthbag Building Plans

5.2: Various shapes that provide lateral stability — buttressing for linear walls or curves.

Earthbag Buildings

5.3: Buttresses, corners, intersecting earthbag or stud-frame wall, and sufficient curves are all considered effective forms of lateral support in a structure.

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