Shear Strength Wall Test Report

Test description: Shear strength of plastered 4′ x 4’/ 1.2 x 1.2 m bag wall (15″ / 38 cm thick) of weak cohesive soil with barbed wire; Materials: solid poly bags, tamped subsoils of silt and clay, lime and earthen plaster

Abstract:
Earthbag is a growing sustainable building technique that is increasingly desired in the developing world because it costs only a quarter as much as concrete block walls, is easy to learn, and does not require power tools. Because other geo-textiles perform well in hazardous areas, earthbag is desired in seismic risk areas. But structural testing is needed to determine safe plan and construction guidelines for different levels of risk.

Earthbag is usually built with cohesive earthen fill. Because it is often designed to fulfill adobe building standards, earthbag can be compared to adobe test and standards values to provide safe guidelines for use in the developing world. But to date almost all research by engineers has been limited to the structural performance of atypical sand-filled bag walls.

To explore the shear strength of a weak cohesive fill, a plastered earthbag wall portion was subjected to static diagonal compression. The earthen- and lime-plastered test wall was still stable after surpassing peak strength. With no reinforcing mesh, its peak shear strength was superior to most sand bag walls tested, and comparable to one with cement stucco reinforced with chicken wire. More rigorous testing of both weak and strong cohesive fills in earthbags is merited.

Source: Shear Strength of Earthbag Wall with Weak Cohesive Fill
All earthbag tests to date are compiled on our Earthbag Testing page.

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All the Best Earthbag Content

For several years, Kelly and I have been filtering all the best earthbag content from the Web, writing extensively on all aspects of earthbag building and organizing the information for readers. There’s now an enormous amount of information available – so much that it’s difficult to keep up with everything. That’s one reason why our sites are helpful. We gather the best information so you don’t have to spend endless hours looking for it, wasting time clicking through low quality sites, blurry videos, etc. No one else has anything close to this amount of content. Below are just a few links from EarthbagBuilding.com (the mothership) and our Earthbag Building Blog. Also note how we strive to keep all these pages up to date so readers aren’t faced with a bunch of broken links. (And it’s free.)

Earthbag Projects and Pictures
Earthbag Videos
Earthbag Articles
Earthbag Testing
Earthbag Blogs (recently updated and expanded to include earthbag blogs in Spanish)

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My Top 10 Most Popular YouTube Videos

If a picture is worth 1,000 words, then a video is worth…
Maybe that’s why my NaturalHouse YouTube channel is so popular.

2. Finished Earthbag Roundhouse
3. Cement Plastering
4. Building an Earthbag Dome
5. Hardness Test
6. Enviro Dome Fly-through
7. $300 Earthbag House
8. How to Make Square Corners
9. Bag Size
10. Custom Bags

You can watch all 91 videos at the NaturalHouse channel. (We haven’t made any videos lately since our son the videographer is grinding his way through his last year of university. I’m eager to do many more.)
The Video page at EarthbagBuilding.com has all the best videos on the Internet.
My Earthbag Building Video that covers every step of construction is nearing completion. We believe we’ve figured out the technical glitch and hopefully it should be available soon. Watch this blog for the announcement.

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Top 10 Most Popular Earthbag Blog Posts

Instructable: How to Build an Earthbag Dome by Owen Geiger

Every year we publish the most popular blog posts for the last 12 months. We’ll do that again in November on our 4th anniversary. Our goal here is to look at the most popular blog posts since we’ve started – the Best of the Bestest. Think of them as hidden gems unless you’ve read all 756 blog posts. (And if you have read them all, then you can start reading the info on our mothership at EarthbagBuilding.com. It’s all free. Enjoy.)

1. Counties with Few or No Building Codes
2. Bullet Resistance of Compressed Earth
3. Low-cost Multipurpose Minibuilding Made With Earthbags (This is my earthbag dome that almost went viral last year and got republished on dozens of blogs… see photo above.) Click here to read the free Step-by-Step How to Build an Earthbag Dome Instructable at Instructables.com.
4. Creating Earthbag House Models
5. Earthquake-resistant Earthbag Houses
6. Earthbag Rootcellar
7. Cost of Earthbag Houses
8. $2,000 Earthbag House
9. Earthbag Survival Shelter
10. Using Earthbags as Ceiling Insulation

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Seismic Resistant Gravel Bag Foundations

Seismic Resistant Gravel Bag Foundation (click to enlarge)

Yesterday’s post was about the earthquake resistance of earthbags. Today’s post illustrates what an earthquake resistant gravel bag foundation looks like and explains why this design is so effective. Gravel is an ideal material for this application because it’s durable, reasonably inexpensive, readily available in most areas, has high bearing strength and prevents wicking of moisture up in the wall. Gravel will shift or ‘give’ during seismic activity, thereby relieving stress without causing structural damage.

The gravel is contained in polypropylene bags or tubes, double-bagged for strength. The gravel bags rest directly on a typical rubble trench foundation with a French drain that removes water from under the building. Two courses of 4-point barbed wire between courses reduce slippage and provide tensile strength. Vertical rebar or bamboo pins are placed opposite each other and tied together through the wall with sturdy baling twine. Plaster mesh, fishing net or plastic fencing is tied to both sides for additional seismic resistance. Typical soil-filled earthbags are placed on top of the gravel bags once you’re safely above where moisture can cause problems.

Gravel bag foundations score highly in every category: ease and simplicity of construction, dirt cheap cost, durability (poly bags can last 500 years if protected from sunlight) and earthquake resistance. No concrete forms needed — just a few simple tools such as a shovel. I highly recommend reviewing some previous blog posts on this subject to better understand why earthbag foundations are a favorite among natural builders.
– A similar gravel bag foundation with strawbale walls successfully passed a shake table test in Nevada with forces that surpassed that of the Northridge quake in California.
– Earthbag Foundations background info
– Scoria or pumice can be used in place of ordinary gravel to create a frost-protected insulated foundation. (Frost-protected foundations save energy and greatly reduce construction costs because you don’t have to dig down to frostline.)
– Rubble Trench/Earthbag Foundation
– Earthquake-resistant Earthbag Building Details
– Low-Cost Reinforcement of Earthen Houses in Seismic Areas
– Another Earthbag Foundation Method explains an alternative way of building a foundation with gravel bags.
– Rubble Bags
– Flood and Earthquake Resistant Earthbag Foundations in Pakistan
– Reinforced Mesh Corners
– Earthbag/Stone Foundations

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Earthbags Provide Excellent Base Isolation

Our ever alert researcher into all things earthen, Patti Stouter, has found that testing has confirmed that earthbags can provide excellent base isolation to lessen the impact of earthquakes and severe winds on buildings. The Advanced Materials Research group has published a paper titled Cyclic Simple Shear Tests on Base Isolation Using Soilbags.

“Soilbags have been understood to have the effect of vibration reduction and can be used as a kind of base isolation in building foundations. In this paper, a series of cyclic simple shear tests were carried out on soilbags filled with three kinds of soils under different vertical stresses to investigate the damping and stiffness characters of the soilbags. The results show that soilbags have a relatively high damping ratio and variable horizontal stiffness so that they can be used as base isolation materials.” They go on to say that the bags can even be filled with construction waste rubble and still be effective.

Studies like this will likely put earthbag technology in the forefront of building design for safety! You can read more details about their study at the above link, and can purchase the entire paper if you want.

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Summary of Earthbag Facts

Let’s take a brief respite from castle plans, shall we? Our website traffic has hit another major spike and so I’ll take this opportunity to summarize some key facts about earthbag building.

A few quick facts:
– Successful completion of an earthbag/strawbale shake table test in Nevada applied forces that surpassed that of the Northridge quake in California.
– Seismic tests in California showed zero deflection on earthbag domes with forces that maxed out the testing equipment.
– Engineer and code approved earthbag designs are now available for seismic and non-seismic areas through Precision Structural Engineering, Inc. This had been a major barrier in the past, and we’re glad to see this service on offer.
– A master’s degree thesis showed earthbag walls have 10 times the bearing capacity of stud framed walls.
– Sand bags are the material of choice for storing ammunition in war zones due to their ability to absorb blasts (if they’re strong enough for this, they’re definitely strong enough for housing).
– Around a dozen earthbag projects are taking off in Haiti. Some of have been completed and the groups involved have decided to build more earthbag structures.
– Earthbag is comparable to rammed earth, which can last for thousands of years. See Ancient Rammed Earth Structures.
– Earthbag consists of compacted earth, which has now been shown to be resistant to 50 caliber bullets.
– Compressed earth walls withstood impact testing from a black powder cannon that’s comparable to forces from F5 tornadoes.
– There are various ways of stabilizing earthbags and/or filling lower courses with gravel to make them extremely resistant to water damage, including floods.
– For cold climates, bags can be filled with insulation or earthbags can be covered with a layer of insulation.
– A Federal Highway Administration report estimates the half life of polypropylene bags (sand bags) may exceed 500 years in benign environments.
– Earthbag is faster and more efficient than building with adobe, cob, rammed tires and rammed earth.
– Building with sand bags is around 250 years old if you include the original burlap structures built by militaries.
– Countless millions of poly sand bags are used every year to hold back floodwaters because they’re simple to use and effective.
– An earthbag wall passed a recent wind test with out-of-plane dynamic pressures up to 30 psf and quasi-static pressure up to 60 psf.

This list barely scratches the surface as far as identifying all the benefits of earthbag building. There are now over 730 blog posts available on every aspect of building with bags — foundations, plastering, domes, greenhouses, rootcellars, you name it, it’s here. Every topic can be searched from the search engine near the top of the page. You can also search this blog and our main site (which is by far the largest repository of all things related to earthbags) with our new search engine at EarthbagBuilding.com.

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Gernot Minke – Building With Earth

Special thanks to Eugene in Russia for the heads up on this great find. This earth building book by Gernot Minke’s is one of the most informative books on the subject, and now it’s available as a free download. How can you beat that? Thank you Gernot Minke for all you’ve done in support of the earth building movement.

From the Internet Archive, where you can legally download the book:

One of the most complete and up to date (2010) handbooks around this subject available. This volume is loosely based on the German publication Das neue Lehmbau-Handbuch (Publisher: Ökobuch Verlag, Staufen), first published in 1994 and now in its sixth edition. Of this publication a Spanish and a Russian edition have also appeared. While this is first and foremost a technical book, the introductory chapter also provides the reader with a short survey on the history of earth architecture. In addition it describes the historical and future roles of earth as a building material, and lists all of the signifi- cant characteristics that distinguish earth from common industrialised building materi- als. A major recent discovery, that earth can be used to balance indoor climate, is explained in greater detail. The book’s final chapter deserves special mention insofar as it depicts a number of representative earth buildings from various regions of the world. These constructions demonstrate the impressive versatility of earth architecture and the many different uses of the building material earth. Kassel, February 2006 Gernot Minke

Source: Internet Archive

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Get the Basics Right

Make sure you get the basics right so your earthbag home is safe, sound and durable. As obvious as this seems, the most fundamental earthbag building principles sometimes get lost in the forest of information. That was one reason for writing the Earthbag Building Guide – to focus on the most important steps and demonstrate the best techniques in the right order of construction to avoid mistakes. A $20 investment in this book can easily save hundreds of dollars or more in wasted time and effort. (Same is true with just about any subject – knowledge is power.)

Key earthbag principles:
– Earthbags are not just bags of dirt or sand. Proper earthbags have enough clay in the soil to bind the aggregates together into a solid block.
– Use moist soil, not loose, dry soil.
– Earthbags are tamped solid, much like rammed earth. Once dry, they are similar to giant bricks.
– Avoid slumping corners by pre-tamping the soil (this is explained in my book)
– Overlap bags in a running bond, including at corners or use tubes
– Use barbed wire between courses for tensile strength and to prevent slippage
– Build straight, plumb and level (for rectilinear structures)
– Build uniform, smooth curves if building in the round
– Protect walls with a good foundation and a good roof with adequate roof overhang
– Raise the building site so water flows away from the building.
– Add reinforcing where necessary, especially on long, straight walls and at door and window openings
– Protect the bags from ultraviolet (UV) damage if your project will take more than a few weeks

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Earthbag Shear Test

Here’s Patti Stouter’s report on her earthbag shear test.

The quick way to find out how to build strong enough for earthquakes? Hundreds of thousands of dollars for earthbag testing to university engineering departments. I’m helping one US university seeking grants just for earthbag, and I’m looking myself as well. Email me at simple_earth@yahoo.com to get involved.

The slow way? Wait for individual students to chose some facet to research, and hope their tests reflect building techniques accurately. Sand bag construction has received some excellent testing recently. This should pave the way for more knowledge of bags filled with cohesive soil containing clay. But what do I say today to emails from Haitians who need to build there now?

Recently I tested a 4’ long x 4’ high (1.2 x 1.2 m) wall portion of plastered earthbags filled with a rather low strength soil containing clay. This 15” (38 cm) thick wall with 2 strands of low-tensile strength barbed wire between each course resisted more than 2200 pounds (10 kN) shear force before the plaster was seriously damaged. It resisted 2700 pounds (12 kN) maximum before it began to deform.

Results of earthbag shear test

My test wall is still sitting there, holding itself up, unlike adobe that cracks all its mortar joints and begins to fall apart. When pushed, my wall is still stiff. Also unlike the recent sand bag tests, earthbags did not move apart under stress.

But without reinforcing steel or mesh or cement stucco my wall was at least as strong as unreinforced adobe. For my wall’s square face area it seems it withstood at least twice as much force in a static test as the unreinforced adobe for the New Zealand earthen building standards withstood in cyclic (pulsed) dynamic testing.

I’m no engineer. My test is not highly accurate, or repeated three times to confirm results. And it almost broke our simple equipment.
But with stronger earthen fill, reinforcing mesh in the plaster and vertical rebar, it looks to me like earthbag can cheaply be much stronger than reinforced adobe. My test was much stronger than sand bags, and about as strong as 11” (28 cm) wide sand bags with chicken wire and cement stucco.

I used diagonal compression to mimic the shear force of a perpendicular wall moving in an earthquake. As happened in the sand bag tests, the wall end far from the pressure compressed a little (mine 4%). The edges of openings seem more vulnerable than continuous walls. That’s why the NZ standards doesn’t let you use short walls for bracing – 4’ or 5’ 10” minimum depending on the wall height.

Why did my bags start to shift? My earthbags formed firm, solid blocks under tamping that easily held up a man’s weight spanning 10 inches. They held a 3 inch nail securely. They didn’t crumble when jumped on.

But I played the devil’s advocate and used a soil that when settled in water showed about 8% clay and more than half silt. And the clay wasn’t a terribly strong one. This fill might be strong enough for non-hazardous areas. My cured bags remained solid blocks in the wall after testing. But they crack at the corners when kicked, and crack badly when dropped from about 3’ high (a test used on adobe blocks). So in the test, the barbed wire points flaked a conical depression in the soil loose, and the wall shifted under sideways pressure.

If someone can offer me stronger testing equipment I’d be happy to build a test wall with stronger fill. I’d love to see how much stronger mortar anchors make it, and vertical rebars inside the wall and outside it. I hope that university students will explore all this soon. The sooner we know, the more safe buildings in the world.

See our newly updated Earthbag Testing page for details.

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