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Structural Concerns with Rammed Earth
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Clifton Schooley is a green building professional specializing in insulated rammed earth construction and architectural design in Canada and worldwide. His vision is to create environments that are beautiful, artistic, sustainable and have a positive social impact. Clifton believes that both design and construction of a building must be intimately connected to bring maximum benefit to both people and the environment. Beyond rammed earth his ambition is to become an eco-developer and influence society on a larger scale. For more information, visit:  www.rammedearth.info

Q: How do you anchor the rammmed earth walls to the foundation? Or does the walls weight cause them to adhere to one another?

A: The walls are anchored with re-bar, the same as a concrete wall sitting on top of a footing.

Q: I am a structural engineer who may be designing a Rammed Earth structure. Can you recommend any literature that contains a design method or design examples?

A: (Leonard Jones) I'm glad to hear that you are interested in rammed earth. Rammed earth buildings exist on every continent; some of them have been in place for centuries. So, it is certainly a well-established, tried and true method of construction.

Unfortunately, there is no definitive design method for rammed earth like there is for reinforced concrete or structural steel. It is an "ad hoc" or vernacular building method that developed as people utilized the local soil to provide shelter for themselves. Soil, as you know, can vary widely from one location to another. Design and construction practices must vary as well. What I have found is a common set of design practices that seem to exist wherever rammed earth building has been done. These practice revolve around several elements:

* Foundation Design - The rammed earth walls must rest upon footings that will bear the massive weight of the earthen walls and act as
a lower bond beam to hold the building together at its base. The design of these footings follows standard engineering practice; the references I will provide below show examples of the variety of footings that have been used.

* Rammed Earth Soil Mix Design - This is the most difficult part of the design. First, a suitable soil must be located near the
building site or created by mixing two or more existing soils together. Alternatively, a suitable soil can be hauled in from elsewhere, but this adds considerably to project expense. An ideal soil will be a cohesive sand/clay mixture. The clay must be non-expansive, and some gravel of limited size is OK. Experienced builders recommend - and some building codes require - that 3% or more of Portland Cement (by weight) be added to the mix for stabilization, strength, and water resistance. Then water, roughly 10% by weight, is added to facilitate compaction and cure the cement. The only way I know to get such a mix is by trial and error. The best way to do this is to start with the local soils, make and compact test samples in concrete cylinder molds using measured amounts of materials, cure them, and then break them in a testing machine. Continue until a suitable, economical mix has been evolved. The State of New Mexico, where earthen construction is very common requires compressive strength of 300 psi for rammed earth or stabilized adobe.

* Door and Window Lintels - Must be carefully designed if they are required to bear the weight of rammed earth above their openings. Many types of lintels are used in rammed earth, as well as rammed earth arches. See the references listed below.

* Bond Beam - A bond beam must be provided at the top of the rammed earth walls as a point of attachment for the roof structure and to maintain structural integrity at the top of the walls. Reinforced concrete is normally used for this, although wooden bond beams can be used in non-seismic areas.

Some interesting construction practices have also grown up around rammed earth:

* Rammed Earth Forming - Forms for rammed earth are similar to concrete forms, but with some differences. Forms can be wood,
plywood, or metallic as long as they are properly designed Rammed earth walls are typically 18 to 24 inches thick, much thicker than typical concrete walls, so forming must be adjusted accordingly. David Easton, whose book is referenced below, as done a lot of creative work with rammed earth forming. Unlike concrete placement, hydraulic forces on the forms are not much of a factor in rammed earth. The rammed earth going into the forms will essentially be damp dirt with some cement added as opposed to a semi liquid concrete mix. However, the rammed earth forms must be sufficiently sturdy to withstand the ramming process, as the rammed earth mixture is compacted from a loose condition to a near-100% state of compaction. Ramming can be done with manual, electric, or pneumatic tampers; the last two can exert much force.

* Soil Blending, Mixing, and Conveyance into the Forms - On a limited scale, this can be done manually, with hand tools and buckets. In third-world situations where large amounts of cheap labor are available, this method is predominant. Large projects
in the U.S. are sometimes done in a highly mechanized way, with bins, feeders, mixers, and conveyors moving the soil from the stockpile to the forms. However, this is not practical or affordable for the average rammed earth project. In most of these, the soil is moved, mixed, and conveyed with a skid loader or other bucket loader. In any event, site layout is a critical factor which should be considered beginning in the design process.

Here are some good references: Adobe and Rammed Earth Buildings - Design and Construction (ISBN 0-8165-1124-1) This book was primarily written by the late Paul Graham McHenry, AIA, a former Professor at the University of New Mexico. McHenry spent much of his life studying the vernacular architecture of the Southwest, particularly rammed earth and adobe. I got to know Paul a few years before his death and found him to be a heck of a nice guy, very knowledgeable and always willing to provide advice and lend a hand where he could. Later printings of the book also contains an added chapter, Structural Engineering for Earth Buildings, by Gerald W. May, PhD, also of the University of New Mexico. This chapter is somewhat general, but you may find it helpful.

The Rammed Earth House - (ISBN 0-9300-3179-2) Written by David Easton, AIA, this is, IMHO, the best overall book on the subject of rammed earth. Easton is the architect of a large number of rammed earth houses in California. He goes into considerable detail on the subjects of forming and construction methods - which you may find useful.

Another resource is the Earthbuilder's Encyclopedia. This CD-based e-book was written by experienced earth builder Joe Tibbets. It can be obtained at http://www.adobebuilder.com/earth-building-encyclopedia.html. This book contains comprehensive information on the definitions and terminology used in earthbuilding - also much information useful in design and construction. If you are interested, Tibbets also offers classes and workshops on adobe, rammed earth, and passive solar construction at various locations throughout the year. Information regarding these classes - and much other info - can be obtained from the same website.

You may also wish to review the New Mexico State Rammed Earth Building Code, which contains much applicable information. I believe that this was the first approved building code related to rammed earth. Other states and localities have used this one as a basis for their own codes. A copy of this code can be obtained from the adobebuilder.com website - ask for EarthBuilder Magazine, Issue #9...

Q: I am researching for a thesis on whether there is a conflict in using sustainable materials for construction in regions of the world that are prone to earthquakes. It seems to me that many such parts of the world are abandoning their vernacular--which may incorporate design techniques to protect against earthquakes--for reinforced concrete. Whilst I can find plenty of info on environmental building, and plenty on earthquakes and seismic construction methods, I can't find much that considers both of these topics together. Can you suggest any books, institutions or specialists in this area? I would like to find out about both traditional and modern solutions.

A: (Bruce King) This is quite a broad question. Some, even most, vernacular building technologies are really awful for earthquakes (eg the stone masonry and tile-roofed buildings that killed thousands recently in China -- and are being rebuilt as they were!). This is probably because the system feedback loop is so slow, ie the big earthquakes only hit every four or five generations, so people can't adapt as readily or cleverly as they do to rainfall, sun, heat, wind, snow, etc. A few native systems are elegantly tuned to resist earthquakes (eg the Japanese wooden pagodas), but they are the rarity.

Each material and/or building system has to be intelligently applied to the climate and seismicity of the local site. It ain't simple. You can build with earth, straw bales, concrete, or steel in such a way as to have an outstanding, durable, comfortable (in short, green) building -- or build a complete disaster. It takes experience, knowledge of materials and systems, and basic human cleverness; there really are few shortcuts. Put differently, you try to combine unrelated topics -- environmental building and seismic construction methods -- which is like asking about the relationship between passive solar building and bathroom color schemes; both must (or should) be considered, but don't necessarily affect each other.

Q: I am a missionary living in the Andes Mountains of southern Bolivia (South America.) I have prior experience in building with concrete block as well as stud framing. I am thinking about using soil cement to build a house here. I came across your website as I was researching this subject. I have several questions which I would like for you to answer, if you could. What dimensions for a foundation would be needed to build a house out of soil cement? The walls would be poured soil cement 8 inches thick. Should the foundation be made in the traditional way using cement, sand, gravel, and building iron? Can a TWO story house be made using the soil cement, and if so, would it be necessary to use building iron to reinforce the walls? and what changes should be made in the foundation requirements? Could a two story house be built using the poured soil cement walls for the FIRST floor, and then use stud wall construction for the outside walls of the second floor?

A: (Bruce King) I'm afraid there are about twenty "it depends" that must accompany any answer - the type of soil (both in the wall and under the foundation), the seismic risk, the building design & style & usage, etc. You probably can build a two-story house as asked, but you need an engineer; there just are no simple (reliable) answers

Q: How do you build to prevent collapse in an earthquake?

A: (Leonard Jones) Earthquake design is indeed a concern in active seismic areas, like much of Califormia... A variety of methods have been developed to address this concern. However, I live and work in an area that has very limited seismic concerns, and I am not familiar with all aspects of the seismic design methodologies. Therefore, I would suggest that you consult the following books. These cover the subject in some detail:
The Rammed Earth House - David Easton - particularly the last chapter
Buildings of Straw and Earth - Bruce King, P.E.
Adobe and Rammed Earth Buildings: Design and Construction - Paul G. McHenry

Q: How thick do I make the first or lower part of the split level rammed earth building, the part that will be 4 or 5 feet in the ground and do I waterproof it, use some moisture barrier on the footer.... or....?

A: (Kelly) Without actually seeing your plans it is hard to answer this, but generally the thickness of an earthen wall at its base needs to be at least 1/10th of height of the wall. As for waterproofing, I would not recommend burying any earthen products even if they are stabilized...better to use a concrete foundation (or possibly earthbags) to raise the earthen part above grade.

Q: I'm planning on building an earthsheltered 3 level home in San Luis, Co in a couple of years and I've been collecting information where I can for rammed earth structures. The home will actually be divided into 3 1/2 levels with one portion fully underground, 1 section 5 feet into the ground and one above ground. My issue is that I can find no information on earth sheltered homes that doesn't require the use of concrete. Have you any ideas about rammed earth as an earth sheltered retaining wall. I'm hoping you can point me in the right direction and if you think it's a huge mistake not to use concrete that you'll let me know. I just really want to avoid concrete. The rest of the home will be earthbag.

A: (Leonard Jones) As a general rule, I would not advise the use of rammed earth with little or no cement as an underground retaining wall.  I don't believe any other experienced rammed earth person would recommend this either.  Exposure to water will cause deterioration and failure of rammed earth over time...  If the walls are supporting a roof - or more structure above, these will also fail when the supporting retaining wall fails. 

Right now, I'm (slowly) building a rammed earth house at Crestone.  I am using a rammed earth mix that contains 3-4% Portland cement by weight.  Concrete people would call this a "1-bag" (per cubic yard) mix - as opposed to the usual mix of 4 or more bags per cubic yard.  The first section of my wall has now weathered through an entire year - and there already are some signs of weathering from exposure to water. 

A little concrete for stabilization makes up for a lot of deficiencies and variations in mixing and compacting rammed earth mixes.  What I would suggest is the following...  Make some trial mixes using the earth you are proposing to use along with varying amounts of concrete.  Add water and compact the mix into a mold - just like you would do with a rammed earth wall only make a very small wall out of it.  After it has completely cured, take it out of the molds and see how strong it is.  Then soak each block in water for at least 24 hours and see if it retains strength.  Then use the mix with the least possible amount of Portland cement that does not lose strength when soaked in water.  This will enable you to utilize the least possible amount of cement in your soil mix. 

Another possibility would be to use rammed earth tires for the lowest floor.  Pack the tires with a sand/gravel mixture that will allow any water to drain down through it.  The fact that the earth is contained in waterproof tires will help it resist the effects of water from the surrounding area. 

Finally, in any event...  be sure to install a french drain around the foundation of your structure with drainage pipe routed out to daylight or to a dry well.  This will keep water from accumulating behind your walls and causing problems.  Cheap insurance!!!

C: As far as the buried walls go I was planning on using 2 bags of portland just to be on the safe side. 

(Leonard Jones) This will probably work out OK unless there is silt or expansive clay in your soil...  I'd do some trial mixes just to be sure....  

When I say concrete I mean slab or poured concrete.

Concrete has its place...  Like in roads, bridges, large buildings, etc. etc.  where reliable, consistent, high strength is critical...  I don't think that very many things in residential construction fit in this category.   

I'm thinking I'll be using a rumble trench foundation.

Good!!  If well built, this can be an excellent alternative to a concrete foundation...

I'm not going to be stacking any other stories on top of each level but the idea is a living roof on all levels so yes, stability is a huge issue.

I also like living roofs...  I'm planning to use a bit of it in my Crestone house...  The only issue is the weight and making sure that the underlying structure will support it under all conditions...  But the issues can be handled with proper engineering.   

My concerns mainly flow toward rammed earth reacting to frost heave and water.

You need to make sure that the soil you use for rammed earth has little to no expansive clay (like bentonite) If it is mostly gravel and sand, the potential for trouble will be minimized...  

Q: I am in north Texas and am preparing to build a rammed earth garage and eventually a rammed earth home. I would like to use a rubble trench foundation with a concrete grade beam, stem wall and slab. I have VERY sandy soil. The garage walls will be ten feet total and twelve inches wide. Our frost level is about six inches here. My question is this: How deep and how wide should the trench and grade beam be to support the weight of this structure?

A: (Leonard Jones) It is feasible to build a rammed earth wall on a foundation that consists of a rubble wall topped by a grade beam.  I would make sure that the grade beam was 12" thick and at least as wide as the wall is thick.  Use plenty of re-bar to reinforce the grade beam.  I would make the rubble wall at least 18" top to bottom, preferably 24" (without having more soils and site information, that is)

My experience tells me that it will be difficult to build a rammed earth wall only 12" thick.  You could either try to do a cast earthen wall with a significantly wetter soil mix - or you could increase the thickness to 18" or so.

It would be worth building a section of test wall per your desired specs just to see how it comes out and to practice your building techniques.  Then you'll have a better idea how things will go.

Q: I am a landscape architect working on a LEED Platinum project for a corporate campus expansion in Houston, Texas. We are considering using existing site soils which have a high clay and sand content for the side walls of our detention volume. As the detention plane tilts, the walls will appear to grow out of the earth, from 0' to a max height of 6'6". The planted detention area will be dry most of the time but will fill up with 3-1/2' of water during the average two year storm event and up to 6' during a 100 yr storm event. The 2 yr storm volume is designed to be filtered and drained within 48 hours. Our question is, how will rammed earth walls perform structurally over time in these fluctuating saturation/dry periods? Your expert advice is much appreciated to help us understand if this is an appropriate material!

A: (Leonard Jones) I'd suggest that the first thing to do is to check the clay to ensure that it's not expansive.  If it is, then the walls are likely to fail after only a few wet/dry cycles. 

Every soil or mix of soils has it's own unique character, as I'm sure you already know.  So it's really hard to state any definitive rules or guidelines.  You pretty much have to work your way through by just seeing what works.  An interesting experiment in this case would be to conduct some testing in a tank - like a cattle tank.  Build some forms inside the tank, mix up your rammed earth, and ram some scale model walls.  Let them cure for a couple of weeks, then subject them to wet/dry cycles by filling and emptying the tank. 

In a case like this where the walls are not required to bear more than their own weight, it might work out pretty well.  It would be worth experimenting with various percentages of stabilizer, both Portland cement and asphalt emulsion to see what works best.  Also, it would be useful to try out various types and amounts of linseed oil and/or concrete sealant to keep the water from damaging the surface of the rammed earth.

Q: I am in north Texas and am preparing to build a rammed earth garage and eventually a rammed earth home. I would like to use a rubble trench foundation with a concrete grade beam, stem wall and slab. I have VERY sandy soil. The garage walls will be ten feet total and twelve inches wide. Our frost level is about six inches here. My question is this: How deep and how wide should the trench and grade beam be to support the weight of this structure?

A: (Leonard Jones) 1)  It is feasible to build a rammed earth wall on a foundation that consists of a rubble wall topped by a grade beam.  I would make sure that the grade beam was 12" thick and at least as wide as the wall is thick.  Use plenty of re-bar to reinforce the grade beam.  I would make the rubble wall at least 18" top to bottom, preferably 24" (without having more soils and site information, that is)

2)  My experience tells me that it will be difficult to build a rammed earth wall only 12" thick.  You could either try to do a cast earthen wall with a significantly wetter soil mix - or you could increase the thickness to 18" or so.

3)  It would be worth building a section of test wall per your desired specs just to see how it comes out and to practice your building techniques.  Then you'll have a better idea how things will go.

Q: If I used barbed wire (superadobe style) in a rammed earth structure would it reinforce the wall strength substantially, be redundant, or actually weaken the walls?

A: (Kelly) Most rammed earth walls are thick enough that the barbed wire buried in it would likely have little effect, although I suppose that in an earthquake it might keep the wall from fracturing and actually falling apart. I believe that most rammed earth buildings these days use a reinforced concrete bond beam on the top, which tends to serve the same function.

Q: I would love to do a rammed earth home. I have two issues to resolve, the first is can rammed earth be used on a remodel? If so, how would one marry the two types of construction? Additionally I assume by looking at the Q&A on this website that rammed earth homes can have a basement provided it is cement. Can it support a full basement?

A: (Leonard Jones) I don't know why rammed earth couldn't be used in a home remodeling or expansion project.  However, it may be difficult to butt up sections of rammed earth directly against sections of other types of construction.  What I think I'd do is to end the rammed earth walls a foot or so away from the existing structure, then connect them with a section of frame construction.

It would also be possible to have a basement with concrete walls.  However, the cement wall must provide full support for the rammed earth wall at its base.  This would require either very thick concrete - or some special reinforced concrete construction.  Very thick walls would be prohibitively expensive - special design and construction would be expensive as well - but perhaps less so.  The only way to know for sure would be to pay for the design, then get contractor bids.

Q: Do you have any suggestions on adding onto a single wide mobile home with rammed earth? We also would like to add a greenhouse and rammed earth additions to living space? Has this ever been done before? We have no building restrictions that will prevent us from doing so.We would like to do so as dirt cheap as possible!

A: (Kelly) People have expanded mobile homes in many ways, and it is certainly possible to so with rammed earth. Most mobiles are poorly insulated, so any additions might well help with this. While earth is obviously dirt cheap, making a rammed earth building is very labor intensive, usually requiring heavy equipment, so it may not be so easy or cheap to actually accomplish. Also, in most climates, it is best to insulate the rammed earth with some rigid insulation on the outside, which then needs to be protected with a plaster...adding to the expense.

Q: I would like to design a rammed earth house for my studio this semester, however one of the building's requirements is that it be raised above the ground 4'-6" with a partially surrounding wood deck. Is it possible to lift a rammed earth building that high off the ground, and if so, what type of foundation would be recommended?

A: (Leonard Jones) Well, anything is possible I suppose. But - there is little point in this. The only sort of foundation that could support rammed earth walls 4'-6" off the ground would be concrete walls the same thickness as the rammed earth. Which would be grossly expensive and excessive.

Q: Is it possible to use earthbags for grade beams on a rubble trench foundation and build rammed earth walls on top of them?

A: (Kelly) I think that it is theoretically possible to do this, but I am not sure that I would recommend it. Rammed earth require considerable pressure and compaction, as well as a perfectly solid foundation to rest on. I would prefer to rely on a concrete grade beam for this purpose; earthbags might be stressed too much or shift some over time.

Q: I am a retired architect living in France. I want to design my own rammed earth house but have no experience in such construction. We have contacted a 'macon' with experience in the technique who has given us an estimate but with so many recommendations that make it seem more like a post and beam timber construction with rammed earth infilling! What I need is some guidelines on, for instance: how long can a wall 16" thick and 8'-10' high be without extra intermediate support in the way of timber posts or projecting piers? How can I end a wall without massive timber to restrain it? Where can I find such information?

A (Kelly): Most rammed earth buildings that I have seen are happily self-supporting, without the need for corner posts or intermediate timbers. The rule of thumb is that the width of an earthen wall needs to be at least 1/10th of the height, so a 16" thick wall should be able to be self-supporting up to about 13 feet high. For extra security, you can always pour a reinforced concrete bond beam at the top. There are a number of books listed on the rammed earth page  that should provide additional information.

Q: I am looking for an example of the structural design and considerations for a continuous grade beam over a rubble trench. There is not a lot of information concerning rubble trenches nor grade beams soil supported "over a rubble filled trench" available. My interest is for preliminary design considerations, limitations and projecting cost. I need to determine the rule of thumb.

A: (Nabil Taha) There is not a rule of thumb when it come to engineering. However, if you increase the width of the rubble foundation, say 1 feet wider than the grade beam on each side, this will reduce the stresses on the soil and help you to add more load on the grade beam. The depth of the rubble foundation must be double the projection of the of rubble foundation beyond the grade beam, in this case it would be 2 feet.

Would a 36" wide rubble trench (compacted fill) increase the bearing area of 24" grade beam to allow increase in allowable design load?

Yes

Or would the soil displace laterally and cause settling?

Not if the rubble foundation is confined between the native undisturbed soil.

Q: How do you anchor the rammed earth walls to the foundation? Or does the wall's weight cause them to adhere to one another?

A: The walls are anchored with re-bar, the same as a concrete wall sitting on top of a footing.

Q: How would you remodel a rammed earth home?

A: As for remodeling, I'll assume you are referring to changing wall configurations. With a good design in the first place, remodeling will not be necessary. People are usually remodeling due to a poor initial design, opening up rooms to make an open floor plan etc. But if you would want to do such a thing it would be like making a modification to a concrete home, you would have to cut through the walls with concrete saws. This would not be cheap or desirable aesthetically. I am an advocate of permanent design with a purpose and believe in making a new house instead of modifying existing ones.

Q: Our house is rammed earth. The bathroom has a leaking pipe under the built-in bath. It's caused water damage to the walls - about 1m x1m. The external wall is damp with a hollow sound when knocking. Also the outer layer of earth crumbles away easily. Damage is at ground level & we have a concrete base. How do you go about fixing the walls after we pull out the bath and let the walls dry out?

A: Your walls are unstabilized rammed earth because there should be no crumbling. I recommend finding the person who made your house walls and get the recipe for the wall mix from them so you can match the repair as close as possible. Ideally they or another rammed earth builder can help with the repairs so you can get the moisture content of the mix right. Mix up the quantity of mix you will need. Spray the affected area with water in a spray bottle. Just make sure the area is moist. This will prevent sucking of moisture out of your new mix when applied to the wall; it gives a better bond. Get some blocks of wood like 2x4s and some plywood and start packing the mix in one handful at a time. Note you are not using any type of formwork, just freestyle packing. It will take 2 people for this size of operation. Start at the bottom and work your way up the wall. The mix will want to shift around, so don't pack too hard. Use the 2x4's on end and on there side as needed. One person can hold a piece of plywood up against the finished packed area as you move up, it will prevent shifting of the packed material. A lot of this will all become self evident during the process. Be sure to work in shade during this process and also shade the area for a few days after you are finished so it can cure slowly. A note for other people, always use a conduit around piping, essentially this is a double pipe. It protects in case of a leak and it allows for future retrofitting or repair.

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I specifically disclaim any warranty, either expressed or implied, concerning the information on these pages. Neither I nor any of the advisor/consultants associated with this site will have liability for loss, damage, or injury, resulting from the use of any information found on this, or any other page at this site. Kelly Hart, Hartworks, Inc.

 

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