the soil type - that is, grain-size distributions, shape of the soil grains, specific gravity of soil solids, and amount and type of clay minerals, present - has a great influence on the maximum dry unit weight and optimum moisture content. it also has a great influence on how the materials should be compacted in given situations. compaction is accomplished by use of heavy equipment. in sands and gravels, the equipment usually vibrates, to cause re-orientation of the soil particles into a denser configuration. in silts and clays, a sheepsfoot roller is frequently used, to create small zones of intense shearing, which drives air out of the soil.