FOUNDATIONS: MATERIALS ENCYcLOPEDIA
Applications for this system
Frost walls, including full basement walls
Can also be used as exterior and interior walls above grade
Woven polypropylene bags (grain or feed bags) or continuous polypropylene tubing
Soil, typically from site excavation, containing a good mix of gravel, sand, clay and silt
Amendments for soil mixture, if necessary. Can be graded gravel, sand, road base, portland cement, hydrated or hydraulic lime, blast furnace slag or fly ash
Barbed wire, tampers, manual and/or mechanical
Many different bag stands or chutes have been custom made to facilitate the bag loading process. None are commercially available, but most can be made quickly and easily with available materials.
Ratings Chart for Earth Bag Foundation
The ratings chart shows comparative performance in each criteria category. Click on the tabs below for detailed analysis of each criteria.
- HOW THE SYSTEM WORKS
- ENVIRONMENTAL IMPACTS
- EMBODIED CARBON
- ENERGY EFFICIENCY
- MATERIAL COSTS
- LABOUR INPUT
- SKILL LEVEL REQUIRED
- SOURCING & AVAILABILITY
- CODE COMPLIANCE
- INDOOR AIR QUALITY
- FUTURE DEVELOPMENT
Earth Bag System
The more descriptive term for earthbag construction is “flexible form rammed earth,” which gives a more accurate impression of how the system works. Woven polypropylene bags or continuous tubes are filled with a gravel-based mixture that will tamp well and solidly. As the mixture in the bag is tamped, it flattens until the bag reaches its maximum stretch, at which point it firmly contains the material and allows for tamping to a high density. The bags or tubes can be laid out in straight lines, using string lines, but can also conform to any building shape.
The fill material that is rammed in the earthbags varies widely by region, builder and code/engineering requirements. A high proportion of aggregate is always used, with the binders ranging from indigenous clay soil to hydraulic agents like hydraulic lime, fly ash, blast furnace slag or portland cement. The compacted mix creates a stable long-lasting mass that does not rely on the bag for containment once it has been compressed and cured or dried to full strength.
Earthbag foundations can be made with fill mixes that rely on the bags for long-term containment of the materials, usually graded gravel or, less frequently, sand. The bags have a long lifespan when buried, and backfill around them will both protect the bags from degrading in sunlight and provide additional restraint for the materials should the bags fail.
The bags and tubing come in a wide range of widths, from 9–24 inches (230–600 mm), so a foundation can be designed according to the stability and strength requirements of any building. A double wythe system can also be designed, using two rows of narrow bags to create an inner and outer foundation wall for wide wall systems and to allow for internal insulation strategies.
The construction methodology is the same regardless of bag size or fill type. The mix is created, moistened to the correct degree and placed into the bag or tube. When the bag contains the correct amount of mix it is tamped vigorously, manually or mechanically. The tamping process subjects the mix to a force greater than the force that will be placed on the foundation by the building loads.
The foundation wall is built up in a number of courses. The thickness of each course depends on bag size, amount of fill and degree of compaction. Typical earthbag courses range between 4–8 inches (100–200 mm) in thickness.
Between each course of earthbag, a strand or two of barbed wire is typically used to prevent the bags from sliding on top of one another in any direction. Multi-pointed wire (three or preferably four barbs) ensures that every knot is making good contact with both bags. The wire is treated like rebar in concrete, with continuous corners and overlapped joints.
Walls will sometimes be installed directly on the earthbag (with a suitable moisture break), or wooden sill plates or a thin concrete beam can be used.
With practice, an experienced crew can build courses of earthbag quite quickly and with a high degree of level and plumbness and a consistent compaction.
Environmental Impact Rating:
Harvesting — High
Polypropylene (PP) is a resin of the polyolefin family derived from crude oil and natural gas. Impacts include significant habitat destruction and air and water pollution.
Manufacturing — Moderate to High
Polypropylene is among the least energy-intensive plastics to manufacture, and a growing percentage of PP is derived from recycled sources. Impacts include significant air and water pollution. Weaving PP strands into bags is a moderately intensive mechanical process with no significant impacts.
Transportation — Moderate to High
Sample house uses 26.25 kg of bag material:
0.04 MJ per km by 15 ton truck
0.025 MJ per km by 35 ton truck
0.0065 MJ per km by rail
0.0042 MJ per km by ocean freighter
The majority of bag production is in Asia, ensuring that most bags used in North America have relatively high transportation distances. Quantity of material required is low, mitigating impacts.
Installation — Negligible
Harvesting — Negligible to High
Site soil fill will have negligible impacts.
Aggregate and virgin hydraulic binders (if required) are mechanically extracted from quarries and can have low to high impacts on habitat and ground and surface water contamination and flow.
Manufacturing — Negligible to High
Site soil fill will have negligible impacts.
Aggregate is mechanically crushed and has moderate impacts for fuel use for machinery and dust dispersion.
Virgin hydraulic binders like lime and portland cement are fired at extremely high temperatures and have high impacts, including fossil fuel use, air and water pollution and greenhouse gas emissions.
Recycled hydraulic binders like fly ash and blast furnace slag are the by-products of industrial processes that have high impacts, but these can be mitigated to some degree by diverting these materials from landfill.
Transportation — Negligible to High
Sample house uses 15,616 kg of fill material:
23.4 MJ per km by 15 ton truck
14.7 MJ per km by 35 ton truck
Site soil will require no transportation. Locally obtained soils will have negligible to low impacts.
Aggregate is typically sourced nearby the project site, and will have low to moderate impacts depending on distance traveled.
Hydraulic binders are often sourced nearby the project site, but may have to travel long distances.
Installation — Negligible
Waste: Negligible to Low
Biodegradable/Compostable — All natural soil material.
Recyclable — Polypropylene bag material, barbed wire offcuts.
Landfill — Cement and/or lime containers.
Chart of Embodied energy & carbon
Energy Efficiency: Very Low
A rammed earth foundation has very little thermal resistance. In cold climates, it will need to be properly insulated in order to contribute to an energy-efficient building. Insulation strategies can vary depending on the style of foundation, the climate and the type of insulation used (see sketches of four styles of earthbag foundations: 4-ft frost wall with insulated slab, 2-ft perimeter beam with skirt, double wythe with internal insulation and basement). If the design for the building has accounted for potential heat loss through the earthbag foundation it can easily be part of a well-designed, thermally appropriate structure in a wide range of climates.
In some areas, insulative aggregate may be available in the form of pumice, volcanic rock or other “expanded” minerals. Depending on the type of aggregate and the loads imposed on the foundation, high percentages of these aggregates can result in a foundation with reasonable strength and thermal characteristics.
Material costs: Low
Soil, aggregate, bags and barbed wire are all relatively inexpensive. Site preparation costs are similar to other comparable foundations.
Labour Input: High to Very High
Those used to the mechanical mixing and placement of concrete into formwork may find the amount of physical effort involved with earthbag daunting. However, a large part of the labor required to build concrete foundations is in the construction and removal of the formwork that holds the liquid concrete. Because the bags are the formwork for earthbag, this labor-intensive step is eliminated. Once lines are laid for the foundation, earthbag construction begins immediately. When compared this way, the labor balance becomes much more favorable. As there are currently no mechanical means for filling bags or tubes with mix, all work is manual.
Powdered binders are high in silica content, and are dangerous to breathe. Wear proper breathing protection.
Skill level required for homeowners: Negligible to Low
Earthbag building is very simple in practice, and the skills required can be picked up relatively quickly. The process is quite forgiving, as it’s possible to correct for errors on a subsequent course. Only the final course needs to be completely level, and most crews will have the methodology developed by then.
It definitely helps to have at least one experienced earthbagger on a crew to get started. One person can usually direct an entire crew until everybody understands the process. If nobody has previous experience, it’s worth looking into workshops or other training opportunities before commencing with a foundation.
Sourcing & availability: Easy to Difficult
Obtaining large quantities of bags/tubes can be difficult. Farm co-ops or grain and feed stores will have new bags, and their customers will have used bags. Bag printers will sometimes have misprinted bags that are given away or sold below cost. Bag manufacturers and printers will have rolls of tube, and may be willing to sell full rolls. Otherwise, rolls of tube will have to come direct from the manufacturer in Asia, or their North American distributor.
Fill materials are typically easy to source. Grades of aggregate will vary by region, but easily tamped mixtures are required for many purposes and are available everywhere. The road-building industry relies heavily on compacted aggregates for road base, and finding out what is being used locally for this purpose can help determine what you should be using in your earthbag mix.
Virgin binders are available from masonry supply stores and well-stocked building supply yards. Recycled binders like slag or fly ash may be easily available, or may require extra effort to obtain. If a local concrete batching plant is adding recycled binders to their mixes, they should be willing to sell the binder in bulk.
Barbed wire is easy to obtain from farm, fencing or hardware stores. The barbs should be at least three- and preferably four-point, but never two-point.
Durability: High to Very High
The durability of earthbag foundations has not been proven by the test of time. As a relatively recent form of construction, there aren’t any historical examples upon which to base durability parameters.
However, rammed earth construction without the poly bags as formwork has a long history of durability. In climates where rammed earth has proven to be viable, earthbag using a soil mixture can be expected to have a similar or longer lifespan.
Where soil mixtures are not deemed durable enough, the addition of binders creates a concrete-like material inside the bags and this can be expected to share high durability expectations with other concrete materials.
In an area where rammed earth has little history, and even where the materials in the bag lack sufficient binder or are soil-based, an earthbag foundation can be expected to be quite durable as long as the bag material is protected from UV radiation. Polypropylene has shown itself to be very persistent when buried and the bags should maintain their integrity for a long time, continuing to contain the fill for decades or even centuries.
Alternative compliance applications will need to be based on accurate load calculations and engineering principles, along with the small amount of study data that currently exists. Soils engineering principles and data are highly applicable and can provide the basis for justification. A mixture that is adequately tamped and has a good degree of internal cohesion can be shown to be feasible in most conditions. It is highly advisable to discuss the earthbag option with code officials and find out whether or not they are willing to consider it, and under what conditions, before proceeding with plans to use an earthbag foundation.
It may seem obvious, but it can be worth pointing out to code officials that all buildings with concrete foundations sit on a bed of tamped gravel beneath the footings, so codes already accept the use of restrained, tamped fill for structural purposes.
Indoor air quality
Earthbag foundations will have no direct impact on indoor air quality. A well-built foundation can help keep the floors and walls of the building dry and prevent other IAQ issues.
Resources for further research
Geiger, Owen. “Earthbag Building: Earthbag Building Guide.” Earthbag Building: Earthbag Building Guide. N.p., n.d. Web. 13 Apr. 2013.
Hunter, Kaki, and Donald Kiffmeyer. Earthbag Building: The Tools, Tricks and Techniques. Gabriola Island, BC: New Society, 2004. Print.
Wojciechowska, Paulina. Building with Earth: A Guide to Flexible-form Earthbag Construction. White River Junction, VT: Chelsea Green, 2001. Print.
Khalili, Nader, and Iliona Outram. Emergency Sandbag Shelter and Eco-village: Manual—How to Build Your Own with Superadobe/Earthbag. Hesperia, CA: Cal-Earth, 2008. Print.
Khalili, Nader. Ceramic Houses and Earth Architecture: How to Build Your Own. Hesperia, CA: Cal-Earth, 1990. Print.
Simplicity, low cost and effectiveness make earthbags attractive. Earthbag foundations are relative newcomers to construction, though their use in civil engineering projects and flood control provides some performance basis. There is a lot of room for the development of earthbag foundations into a more refined, more widely accepted system.
New research is ongoing into the strengths of different mixes, which should help with code compliance issues. As the system becomes more widely used, new tools and techniques are sure to be developed that will streamline the process. Mixing and pouring concrete foundations used to involve large amounts of labor input that have, over time, been replaced with mechanical devices. The same could easily happen to earthbag foundations, making them even more attractive than they already are.
Tips for a successful earthbag foundation
1. Placement of materials to be mixed should facilitate easy delivery to all points of the foundation
2. Don’t lay string lines directly on the foundation lines, as the bags will nudge the string constantly. Instead, lay out lines that are a couple inches wider than the foundation and measure into the bags.
3. A sturdy loading stand will make the job much faster. The resources listed below describe various loading stand options.
4. A practiced team can move quickly and create a very level surface. As you are learning, don’t worry about every course being perfectly level. In the end, only the top course matters and you can make corrections on successive courses. A transit or laser level makes the job much more accurate.
5. Fill a sample bag to determine the height of each course to plan the number of courses and quantity of bag material required.
6. Secure the bag material well in advance to ensure supply and sizing.