Soil compaction is generally handled by two different types of compactors. The vibratory plate type shown here is designed for granular soils and pea gravel. A vibratory pulse is sent down through the soil and returns as an echo once it hits a firm base. For this reason the passes are generally limited to a 4" lift. If you put more than 4" of loose fill in at a time the compaction force will suffer dramatically and you won't have a solid base. If you need to add 12" of fill just make sure you put in 4" at a time and compact in between.
For cohesive soils, such as clay, or for lifts greater than 4", a vertical rammer compactor is utilized. Vertical rammers are like high-speed "foot stompers" that can handle loose fill up to 12" at a time. They use a narrower shoe and are best used for backfilling around foundations or for working in trenches. The drawback to the vertical rammer is that it tends to move concrete pouring forms due to the stomping action and it doesn't leave a smooth flat finish suitable for flatwork. If you are pouring concrete and need to fill up a 3' hole it is suggested that you use the vertical rammer to fill the majority of the hole and then finish off with the vibratory plate. For more info on the vertical rammer please click here to access that page.
The vibratory plate compactors are quite heavy so it will take 2 men to lift them. They fit well in the back of vans, pickup trucks, SUVs, or small trailers but don't go into most car trunks.