While pneumatic compactors are typically used on granular or mixed soils, they are rarely used on cohesive soils. Instead, a vibratory rammer or sheepsfoot roller works better on silt, clay, or other cohesive soils with fine grains that clump or become sticky when wet and are difficult to crumble when dry. The impact force causes a shearing effect that squeezes out the air and excess water. The rammer should be used on cohesive soils in narrow areas (4 inches to 24 inches wide) or adjacent to structures, while the trench or sheepsfoot roller is better in larger trenches and
open areas.