These screening machines are primarily for production earthbag builders and other natural builders who want to mechanize the building process for maximum efficiency. They are useful for screening soil for earthen plaster, earthen floors, straw/clay and, of course, soil for earthbags. One advantage is the ability to utilize soil from the building site, which would offset the purchase or rental cost of the soil screening machine/attachment. The screened soil would be ready for filling earthbags. The rubble can be used in rubble trench foundations, under floors and as fill material.
Soil screening machines are usually not needed for earthbag building. Typical clayey/sandy soil can often be used as is to fill the bags. Or you can use road base or crusher fines, which have been processed in a gravel yard. This is my preferred approach because using a readily available material that’s already been processed greatly reduces labor, speeds construction and eliminates the need for a screening machine.
More soil screening videos:
http://www.youtube.com/watch?v=lEnZFqpLGkc (note how he is standing and shoveling from the wrong side — either that or build a lower soil screen)
Sandbag Machines (covers a wide range of machines for automating all aspects of earthbag building)