SAB HOMES 6 - RAMMED EARTH CONSTRUCTIONS

An Ancient Art

Rammed earth construction is the process of building with sub soils [not top soils or ‘mud’] and compacting them into a formwork of some kind, until they are as solid as stone. It is very similar to the way in which sedimentary rock is formed in nature. Just as there are many colours of sedimentary rock, so there is a natural variation in the colouring of rammed earth – a variation that can be enhanced by the addition of iron oxide or other natural pigments.

By John Kurtz and Jim Taggart

An Ancient Art

Rammed earth is one of the earliest forms of building having originated in China around 5000BC.  Most famously it was used for sections of the Great Wall, but ancient rammed earth structures also exist in India, Africa and Europe. While most commonly found in hot arid climates, rammed earth also proved its effectiveness in the cooler wetter areas of northern Europe. It was first brought to North America by the British in the early 1800s, and several fine examples from this period are still to be found in South Carolina.
The Church of the Holy Cross in Stateburg, SC is considered to be the best example of rammed earth construction in the United States. Completed in 1852, it stands across the road from Borough House, a plantation house built partly of rammed earth in 1820. Both survived the Charleston earthquake of 1882, several hurricanes and annual average rainfall in excess of 100 inches.
In Canada, the rammed earth church of St Thomas in Shanty Bay, ON was consecrated in 1842, and has endured the extremes of temperature and heavy rains experienced in that area for more than 170 years.
Historically, rammed earth construction was only used on sites where the local soil was naturally suited to this technique. Sandy clay soils such as those along the Rhone River Valley in eastern France were found to be ideal for this purpose. Transporting soil over even a small distance to another site was impractical or impossible, making rammed earth a highly localized technology.
In North America rammed earth enjoyed a brief resurgence in popularity in the 1920s and 1930s, but after World War II a reduction in the price of other building materials saw interest decline once more.

The Revival of Rammed Earth Construction

Recent interest in rammed earth construction is due largely to its inherent green attributes. Rammed earth must be considered one of the most sustainable building technologies as it uses naturally occurring soils from the excavation of the site itself.  Moreover, the fact that subsoil rather than topsoil is used in rammed earth construction means that the latter remains available for agricultural use.
The technique is being used mostly for residential construction particularly in the south-western United States and British Columbia. The only wood [or other material] required for the construction of a rammed earth home is for the formwork and this can be re-used many times.
The contemporary practice of adding a small quantity of cement [or a natural stabilizer like lime] to the earth mixture broadens the application of the technique to areas where the natural soils are less than perfect. This means that in today’s construction market, rammed earth has become a viable technology over a much larger area. The recent completion of several prominent public buildings in British Columbia may serve to bring rammed earth into the mainstream.
Rammed earth, like masonry, is comparatively strong in compression and relatively weak in tension. Consequently it is suited to load bearing as well as non-load bearing wall applications, and historically has been used for structures up to 10 storeys in height. Rebar can be incorporated into rammed earth walls for additional tensile strength, and the addition of small quantities of Portland cement to the mix can improve both strength and weather resistance.  Rammed earth is self-finished and both interior and exterior surfaces may be left exposed.
Rammed earth walls are insect and rodent proof, mould resistant, non-combustible and provide good sound insulation. Their thermal mass contributes to temperature stabilization, and they are durable but easily repaired if damaged.
Rammed earth construction has an important role to play in addressing the global housing shortage, offering a low carbon alternative to the concrete and steel construction now predominating in the third world. It is affordable and accessible, and can be built to a high standard using only human labour. In Canada rammed earth construction is more likely to be a mechanized process, but its intrinsic environmental advantages remain.

Building with rammed earth
Traditionally, rammed earth was simply a mixture of sand, clay and gravel blended together with a small quantity [about 10% by weight] of water. Nowadays Portland cement [about 3-6% by volume] is sometimes added as a stabilizer and to improve weather resistance.
Although in some respects rammed earth construction resembles cast-in-place concrete construction, the engineering of rammed earth walls cannot be based on universal specifications and calculations because of the natural variability of the material. Instead, structural engineers and contractors who specialize in rammed earth construction rely on a few basic design principles, sample testing, and their own experience to ensure that a wall will perform as designed.
Sandy clays [with 20-30% clay content] are best suited to rammed earth construction, and blending may be required to achieve the ideal proportions. Performance of various mixtures can be verified on a project by project basis by casting them into concrete test cylinders and crushing them.
Colour variation can be achieved by using different soil mixtures in alternating layers. This gives the finished wall a subtly striated appearance when the formwork is stripped. For more dramatic colour variation, concrete colourants can be added to the earth mixture.
Like any kind of earthen construction, rammed earth walls must be lifted clear of the ground [and any possibility of exposure to standing water] on a concrete foundation. Just as it would be for a concrete wall, rebar projecting from the footing is needed to mechanically connect the wall to its foundation.
Timber or other formwork is built, much as it would be for the construction of a cast-in- place concrete wall, although the differences in material consistency and compaction method result in minor modifications. A solid rammed earth wall is usually a minimum of 8 – 10 inches thick; and where it is required to be insulated, there could be two withes of this thickness with insulation between.
The rammed earth mixture is much stiffer than wet concrete, and therefore does not impose a hydraulic load on the formwork. However, because it is laid and compacted in layers of about 6-8 inches in height, one side of the form is built in successive vertical panels to facilitate the tamping process. The forms must be strong enough to resist the lateral forces imposed when the material is compacted. The inside surface should be smooth to ensure the best wall finish, and treated with a release agent for easy removal of the formwork.
The first layer of the moist earth mixture is placed in the bottom of the form along its entire length, and compacted by pneumatic tamping. It is important to ensure that the material is properly compacted at the edges and corners. Successive layers of the moist earth mixture are added and the tamping process repeated. A bond beam, usually of concrete and mechanically connected with rebar, must be cast along the top of the wall to distribute and properly transfer roof loads.
The formwork can be stripped after a day or so, and full curing of the rammed earth takes about one week. In Canadian climates, timing is critical as the wall must not be subject to freezing temperatures during the curing process. In wet climates, substantial roof overhangs are necessary to protect the finished wall from driving rain.

Cost

The cost of rammed earth will vary by region and by contractor. One rule-of-thumb is that rammed earth costs 10 to 15% more than wood-frame construction. An internet search on rammed earth will bring lots of advice on construction techniques, and cost.

Useful web sites:

John Kurtz is a partner in Earthwall, an Ontario based company specializing in rammed earth construction. Jim Taggart is editor of SABHomes.
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