Sunday, July 18, 2010
Preparing the soil blend
One of the most important factors governing the quality of a rammed earth wall is the soil blend. Rarely do we find a site soil that has the ideal characteristics required for a durable and attractive finished wall. Instead, our object is to use as high a percentage of local soil as possible and to blend it with imported admixtures as necessary to achieve that ideal ratio of clay to sand, natural binder to shrinkage control. (I recently put up a post on the Rammed Earth Works site with an in-depth discussion of soil formulations.)
At Jack's House, we actually couldn't use any of the site soil, as it was a dull gray color, too high in humus and too expansive; but we did find a good base soil from an excavation a couple miles away. We took this free import, which had a little too much clay but a good color, and blended it with an imported gravel with sand to achieve a good formulation. Based on pre-construction testing, we determined we could achieve the 600 psi strength specified by the structural engineer with only 7% cement.
To get ready for the wall building, which will take place on Monday, we blended 2 parts screened soil with 3 parts of imported sandy gravel and one part of fine sand. While blending the three materials, we added water to "condition" the clay and to get some water onto the aggregate prior to adding the cement.
The photos in this post show that blending process. You can clearly see the brown soil and the tan sand as separate in the first two photos and the mix becoming uniform in the last.