Proper soil compaction can avoid many pavement problems by increasing load-bearing capacity, decreasing water seepage, and minimizing soil settlement. The goal of compaction is to pack as many soil solids as possible into a given space. Achieving that goal requires an understanding of the following three interrelated factors:

SOIL TYPE: Most topsoils fall into the category of organic soils. They must be removed and replaced because they cannot be adequately compacted. Granular soils contain primarily gravels, sands, and silts and can be compacted to a high density. Cohesive soils contain very fine, platelike clay particles that have a strong attraction to water to other clay particles. The ease with which cohesive soils can be compacted depends on their moisture content and plasticity.

SOIL MOISTURE CONTENT DURING COMPACTION: Dry soils usually do not compact well. The best or optimum moisture content for compaction is the one that produces the highest dry density.

COMPACTION EQUIPMENT: Most machines compact soil through vibration or a ramming action. Vibration agitates the particles, reducing friction at the contact surfaces and allowing the granular material to settle into a denser configuration with fewer air voids.