Finding the Right Soil / by Watershed Materials

Clay/sand ratio has the greatest contributing effect on how well an earth wall will perform.  Traditionally, for raw rammed earth, that ratio has been established as 30% clay and 70% sand. 

When using cement as a stabilizer, clay content can be reduced, in some cases and with high stabilization rates, clay (and other fines) can be as low as 8% to 10%, depending on numerous factors (uniformity of gradation, plasticity, particle shape, and parent rock).
Unlike earlier times, when the building material was nearly always harvested on or near the construction site, today we have access to a wide range of importable mineral soils and admixtures.  Formulating a blend of soils capable of achieving optimum structural performance is our objective.

To do this, we begin by looking at the underlying soil on the building site itself.  A review of the boring logs from the geotechnical report will yield valuable data:  gradation, USCS soil type, and in some cases a plasticity index.  We have found that most site soils can be used in some proportion to create a useable formulation.  Using site soil has several advantages:  reduced cost of importing materials, increased LEEDs points, color continuity with the local geology, reduced off-haul costs, and reduced carbon emissions from construction.

The gradation report (also called a sieve analysis) identifies how much of a given soil is fine particles, those passing the 200 mesh screen.  The Plasticity Index is an indicator of how much of those fine particles are “clayey”.  Clay particles help to bind together the soil matrix.  If the gradation indicates more than 25% passing 200, the addition of sand will likely be required.  High clay soils also benefit from small gravel as supplemental amendments.  

If boring logs and site investigation indicate utter unsuitability, or if there is no excavation planned for the site, it is possible to source a portion of the wall building material in other ways.  Excavating contractors, pool contractors, or other general contractors frequently have excess material they need to move off site.  Phone calls or scouting trips can be productive, as is the old “Clean Fill Wanted” sign.  

For the required amendments to site or other free material, you can start the search for a suitable sand or gravel amendment at the local masonry or landscape supply yards.  Coarse sands with a good distribution of particle sizes are usually better than fine or uniform sand.  Cracked or crushed gravel is better than “pea” or river gravel because of its angularity.  Color and cost are also important considerations. 

For a small project and for all of the required pre-construction testing, the search can end at the supply yard.  For larger projects where several truck loads of amendment will be required, you will be able to negotiate a better price dealing directly with the quarry.  

Finally, if site soil is unsuitable and free clean fill is unavailable, or if screening and processing is impractical, then purchasing and importing all of the wall building material from a quarry is the logical choice.  Cost, travel distance, color, wall density, required stabilization and geo-regionalism will dictate which quarry to use.