A landfill compaction vehicle has two main functions: to spread the waste evenly in layers over the landfill, and to compact waste to reduce its volume and help stabilize the landfill. Compaction is a more efficient method to use the space in a landfill. Landfill compactors vary in size and shape. Larger landfills use larger compactors that can weigh over 100,000 Lbs. Landfill compactors are equipped with steel wheels having special teeth that penetrate the waste and deliver a focused compression point, providing compaction and increased density

http://en.wikipedia.org/wiki/Waste_compaction