Rammed Earth Buildings
Historically rammed earth walls have been used in arid environments, but most of populated Australia is subject to more persistent rain that can cause premature degrade of earthen walls. (see photo right)
For the most economic construction, load bearing rammed earth needs to be single storey with a wall height limited to 3.0 m and returns on all corners (no corner openings) to provide bracing and construction continuity. For structures outside these limits, Engineering design principles need to be applied.
The mix is compacted (rammed) into wall formwork but the result is not homogeneous like concrete, as each layer bonds poorly with the one beneath. As a result, the wall cannot sustain tension which could be caused by wind uplift, wind or earthquake lateral load or even by eccentric loading from other building elements like roofing. When rammed earth has to use its own weight to resist roof uplift, anchors have to be adequately buried into the matrix. In resisting lateral loads, the thickness of the wall has to be proportioned so no tension occurs.
Typically stabilized walls used in Australia are 300mm thick and contrast with traditional unstabilized walls in arid parts of the world where they are 280 to 600mm thick, often stone faced and unprotected by a roof.
Alternatively secondary load-bearing systems can supplement the design, such as an independent post and beam system so rammed earth is simply the cladding.
When used in more fanciful buildings, the material can be designed to carry more extreme loads by using galvanized steel reinforcing. It is limited in application though as the reinforcing affects both the homogeneity and the placement process (both handling and compacting).
Generally the building should be situated on a levelled platform so a front end loader can fill the walls, as manual lifting 2 tonnes for every metre along the building by bucket is not practical nor fast enough as the cement may partially set before it can be compacted.
The site’s foundation soil classification has cost implications as rammed earth is heavy and somewhat rigid, so it requires a heavier foundation that many other building systems. For this, as well as control of shrinkage, the walls need to have vertical control joints (articulation). The placement of these joints is dictated by structural and practical construction requirements.
Rammed earth walls are a good heat sink as they even out temperature extremes but are not very effective for insulation.
