Soil Testing

Rammed Earth Wall Mix Design - posted by Taj

The soil that came out of the ground during pre-construction excavation and grading activities at Terra’s house is a rich-red, kaolin clay, a material that has been used effectively in earthen construction for thousands of years. The red clay has powerful binding properties that help to achieve high compressive strengths without the need for much cement. Furthermore it is hard to find a more aesthetically appealing material…

While it is not a major concern with adobe , these clay materials can be highly expansive and cause shrinkage cracking when they are used in (monolithic) rammed earth walls, which are much denser than adobe. To offset this characteristic, we used locally sourced sand and gravel in our mix design. The addition of these materials yields the benefits of providing a desirable variety of material particle sizes (an even gradation), and allowing the small fraction of cement we added to get better coverage in the mix (because the gravel and sand decrease the total surface area that cement will have to adhere to). We cast a half a dozen or so samples with varying ratios of sand, gravel and clay and compared their various aesthetics. Because we wanted to use as little cement as possible to achieve the 600 psi strength requirement specified by the project engineer, we also tested these samples for compressive strength (a relatively inexpensive test which can be performed at any materials testing laboratory). We decided a mix of 5 parts clay, 5 parts sand and 1 part gravel with about 5% cement would give us the balance we were looking for.

The last important stage in building confidence in our mix was to determine the water content that we would aim for in production. In the simplest iteration of an optimum moisture content test, we made up a small batch of our mix material and separated it into three piles. We then added differential amounts of water to these three batches of material, using our intuition to try and get a range of moisture contents surrounding what would seem to be ideal. We cast cylinders of the three mixes and compared aesthetics and compressive strength results. A mix that is too dry can create a final product with some loose areas, or <>that may be a bit dusty. A mix that is too wet may not compact as well as one that is in the optimum range, and the final product can get somewhat of a smeary-look. In the ideal range, compressive strength will be at a maximum .

The photos in this post show Taj conducting soil testing in his field laboratory, Terra setting the test block form, Abe and Junior ramming soil into the form, and the finished test block, used to evaluate moisture and compaction.