There are five principle reasons to compact soil

• Increase load-bearing capacity
• Prevents soil settlement and frost damage
• Provides stability
• reduces water seepage, swelling and contraction
• Reduces water seepage, swelling and contraction
• Reduces settling of soil

Applications

• Granular soils—since granular soils are not cohesive and the particles require a shaking or vibratory action to move them, vibratory plates (forward travel) are the best choice.
• Asphalt - Recommended
  

Vibratory Plates
Vibratory plates are low amplitude and high frequency, designed to compact granular soils and asphalt. Gasoline or diesel engines drive one or two eccentric weights at a high speed to develop compaction force. The resulting vibrations cause forward motion.  The engine and handle are vibration-isolated from the vibrating plate. The heavier the plate, the more compaction force it generates.  Frequency range is usually 2500 vpm to 6000 vpm. Plates used for asphalt have a water tank and sprinkler system to prevent asphalt from sticking to the bottom of the baseplate. Vibration is the one principal compaction effect