Today’s post is from Dr. Johnny Anderton of of Eternally Solar / EarthBagBuild. We’ve been discussing various aspects of their building system over the last week or so. Clearly, this is another great earthbag system to study and learn from.
“The design of the bags with the central channel allow for a lot of variation in approach to structural elements in the wall, which can be adapted depending on strength requirements.
We have used unstabilised sand only in the wall in our pilot and other projects, in both tubes and channel. The resultant structures are extremely sound and have been tested to destruction for lateral loading, way surpassing international Agrement requirements. The highly compacted sand (created through the weight of the wall above) together with the plaster skins, forms a dense laminate on either side of the lightweight core just as in a structural insulated panel (SIP). Having said that, in a cold Cape winter, we’ve found that the sand acts as an excellent insulator due to the myriad air pockets. Also has a thermal mass effect.
In seismic areas, I would use columns and horizontal stiffeners. How do you build these? Where the plan specifies a column, you end the rows of bags the specified distance apart (say 300mm). This gap in the wall is where the vertical rebar comes up from the foundation. [Ed.: See Confined Earthbag.] This vertical rebar is tied to horizontal rebar that is cast into concrete in the bag channels every 7 bag layers or so as you build the wall. These horizontal stiffeners can run into the bag wall just a short distance either side of the column, or can run right around the structure, creating a bond beam at all these levels.
Once one has built say 1.5 m of wall, you shutter on both sides of the wall, and pour concrete into the space created by the shuttering and between the wall ends. You therefore end up with a cast column and horizontal beam. These can be created anywhere in the wall, whether in a straight of a wall run or in the corners of the structure. You thereby create a very lightweight reinforced concrete ‘lattice’ integrally in the wall. This is a much more engineered approach than using barbed wire and I believe is much stronger.
It would be a good idea to use structural foam on the inside of the shuttering, so as to separate the column both thermally and structurally from the plaster skins. This means that the stiff skeleton of columns and beams is not directly connected to the flexible element (bags and sand fill) nor to the plaster skins. It is desirable to avoid having elements in the wall of different rigidity in direct contact. And this would allow the mass of the wall to vibrate at a different frequency to the skeleton, absorbing the vibrational energy.
With the appropriate engineering input, I believe that a massively strong but flexible wall can be created, which will withstand significant seismic force. And 2 story buildings or more could be created (We’ve not done anything more than single level thus far.)
Having said that, in our non seismic area, we have used unstabilised sand and no vertical or horizontal stiffeners (until ring beam or lintel height). So the wall to that upper level is purely the bags and sand, no barbed wire, steel, concrete or any other material. We don’t even used mesh in the plaster. This is how our test wall was built. Easy to build, nothing to corrode, and plenty strong. The corrosion issue concerns me, as there is always an element of residual moisture in earth walls, and any metal is bound to corrode eventually.”