Wednesday, February 8, 2012

Approval! (well, nearly)

Peter sent a note off to the structural engineer last week, and we’ve had initial feedback and it’s positive!  He doesn’t have an objection to using rammed earth for the inner structural wall, so now we’re just waiting for his revised specification so we know what reinforcement and structure is necessary.

So, it’s full steam ahead this weekend to finish my first set of formwork and get stuck into a test wall.  I’ve been doing a lot of research this week into the construction methods, soil composition, formwork design, etc. and I’m starting to get a feel for what I’m facing.

Typically, the soil composition is around 30% clay/silt, and 70% sand/gravel for unstabilised earth, or with up to 10% cement to stabilise the mix. Since ours will be a load-bearing wall, I’m expecting to use cement.

According to, there are a number of simple on-site tests for soil suitability:

smell test can determine if there is organic matter in the mixture; pure loam is odorless. The texture of the soil when tasted can also reveal the basic soil composition. High clay content produces a sticky or floury sensation in the mouth.

(not sure I’m going to eat our soil to find out…)


wash test, done by rubbing a damp soil sample between the hands, reveals how sticky the soil is. If the hands can be cleaned by rubbing them, the soil is silty. If the hands require water to be cleaned, the soil is clayey. If, when rubbing the hands together, grains are felt, the soil has a high sand content.

sedimentation test visually displays soil composition in stratified layers. A soil mixture is stirred with high amounts or water in a glass jar. The content is allowed to settle revealing the soil composition. The largest particles settle first, on the bottom, and the clay settles last, on top with the organic material above the clay.

ball dropping test displays the binding strength of specific soils. A ball of soil with a 4 cm diameter is dropped half a metre onto a hard, flat surface. If the ball shows few cracks and generally maintains its shape, flattening only slightly, the soil has a high binding force. This means that the soil has a high clay content and must be thinned with sand. If the ball completely falls apart and disintegrates on impact the binding force is too low and the soil should not be used for construction. If the ball displays deep cracks or moderately falls apart the soil can be used without amending the soil much.

The soil that I’ve excavated when preparing the site seems to be pretty suitable (at least, to these admittedly inexperienced eyes) but I’ll perform a few of these tests over the weekend and then we’ll know.

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