Structural Integrity of Insulated Earthbag Vaults

This blog post provides additional information about the insulated earthbag vaults that I’ve been discussing the last few weeks. (Use the search engine in the upper right corner and enter “insulated earthbag vaults” to read prior posts.)

As explained from the very beginning “We get quite a few enquires about vaults. So far we’ve always cautioned against building earthbag vaults except for small entranceways, corridors between domes and the like, because they are inherently unstable if you just stack earthbags into a vault shape. (There’s a tendency for the bags to slide horizontally.) The proposed design shown here resolves the stability issues. This vault building method is very strong, simple, low cost, superinsulating and extremely fast and easy to build.”

Features that add strength to this vault design:
– small size: 12’ wide or less
– base of earthbag wall is below grade. This keys the walls into the earth to resist outward thrust.
– heavy earthbags are low in the wall; lightweight insulated earthbags are high on the wall
– lower earthbags (or tubes) are pinned together with steel rebar to prevent slippage
– the rebar is overlapped and joined at the top with galvanized tie wire
– the rebar frame is covered with welded wire remesh and expanded metal plaster mesh
– lengths of twine are attached to the rebar, which is used to secure the insulated earthbags
– the rebar and mesh is plastered to create a ferrocement shell (see below)
– add plaster with workers on both sides of the wall who apply counter pressure with their trowels or apply with a mortar sprayer to prevent air pockets
– mesh can be added on the exterior for additional strength (best to tie the inner and outer layers of mesh together through the wall)
– interior and exterior of vault is plastered for added strength and to protect bags against the elements
– use a roofed vault in rainy/snowy climates for protection against the elements

More factors to consider:
– stable shape: vault walls create an inverted V-shape much like a tipi or swing set that lend support to each other (or a triangle shape if the reinforcement in the floor slab joins the wall reinforcement)
– stable shape: catenary shape or pointed vault shape portrayed on my blog posts. The most dangerous situation would be an overly large vault that’s flat on top and/or building in earthquake, hurricane or tornado zones.
– horizontal ties between walls create an A-shape to resist horizontal thrust (also used to support an optional loft)
– ribs and buttresses can be added for increased strength
– rebar spacing can be increased and additional layers of remesh and plaster mesh added for increased strength if necessary
– rebar can be placed on both sides of the wall and tied together for maximum strength
– insulated earthbags improve comfort, but also add strength by protecting the ferrocement from thermal expansion
– thick interior plaster adds thermal mass that stabilizes indoor temperatures

Note the similarity in shape between vaults and boats. The vaults I’m describing are based on the proven strength of ferrocement. Ferrocement boats have been made for decades. According to The World of Ferro-Cement Boats “Ferciment [French term for ferrocement] boats built before 1855 are still in existence and at least one is still afloat.” This method isn’t limited to small boats. “It is the cheapest and easiest form of construction for boats over 25 ft.” Obviously boats must be very strong to resist the pounding forces of waves for decades. A modest sized (12’ wide or less) vaulted home would experience far less stress than this unless it’s in an earthquake, hurricane or tornado zone.

From Wiki: “Ferro concrete has relatively good strength and resistance to impact. When used in house construction in developing countries, it can provide better resistance to fire, earthquake, and corrosion than traditional materials, such as wood, adobe and stone masonry. It has been popular in developed countries for yacht building because the technique can be learned relatively quickly, allowing people to cut costs by supplying their own labor.”

From the National Conference on Ferrocement: “Ferrocement, a thin structural composite material, exhibits better crack resistance, higher tensile strength to weight ratio, ductility and impact resistance than conventional reinforced concrete. A team of researchers at The National University of Singapore has made an effort to popularise ferrocement as construction material through research and development since early 1970.”

Perhaps the most definitive site on ferrocement is, now available in multiple languages. They provide many free resources, including a guide on ferrocement house construction.

The Ferrocement Educational Network provides a lot more free information, as well as Steve Kornher at FlyingConcrete, who’s famous for his fabulous ferrocement designs. Steve goes into great detail about building ferrocement vaults. I believe the earthbag vaults I’m describing would be even more durable than Steve’s due to the second ‘skin’ of plaster that greatly reduces the risk of rusting in the steel reinforcing.

Ferrocement is accepted by building codes. Here are some resources used by design professionals from the Ferrocement Society. American Concrete Institute (ACI) first produced the state of the art report on ferrocement ACI 549R-97 in 1982 and the Guide for the Design, Construction and Repair of Ferrocement, ACI 549-1R-93 in 1988. Now 549.1R-93: Guide for the Design, Construction & Repair of Ferrocement (Reapproved 2009) supplements these earlier publications. Additional resources include Ferrocement and Laminated Cementitious Composites by Professor Naaman and the International Ferrocement Society’s Model Code, both published in 2000.

In conclusion, earthbag building, including earthbag vaults is a relatively new and evolving technology. Use common sense and do the necessary research. Don’t build a large vault without professional engineering. Don’t get in over your head. Start small and learn the basics on a simple building.

Additional sources:
Auroville Building Centre
(government sponsored ferrocement research in India) (Dr. Nabil Taha, Ask the Expert about Ferrocement, and developer of reinforced earthbag building)
Ferrocement: Applications in Developing Countries
– Javier Senosiain, the architect who designed The Nautilus house, used 2” ferrocement to create the sweeping curves (but apparently there’s no insulation)
Ferro-Cement (a good introductory article)

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