The first step to any project is to source out locally available soils that are suitable for RE walls.  It is one of the most important and enjoyable parts of a project.  Samples may be taken from the building site itself if promising soils are evident.   A profile of soils is developed often with two or three different soil components and from that a preliminary soil mix is developed that will meet the strength requirements of the structural engineering and the appearance characteristics desired by the client.   Cement is an essential component to the mix in quantities that are determined by the local climate, the minimum engineered strength, the unique characteristics of the subsoil and the degree of exposure to weather the walls are subject to.  A series of mix samples are rammed into cylinders, cured, and then taken to an independent testing lab for compressive strength tests.  Soil strengths for our projects have ranged from 2500 psi to over 7000 psi.