The Complete Handbook of Compactors

Chapter 1: Understanding Compactors

Every stable structure you have ever walked into, driven over, or worked beneath is resting on a foundation that was artificially densified. When an engineer specifies that a roadway subgrade must be compacted to 95 percent of its maximum dry density, they are not asking for a suggestion. They are stating a physical requirement that prevents the ground from settling, shifting, and failing over time. Compaction is the process of mechanically increasing the density of a soil or asphalt material by forcing its particles closer together, thereby removing air voids. The machines built to do this work, the compactors, are the tools of this trade. This chapter explains what these machines do, how they generate the forces that achieve this densification, and why the physics behind them is the difference between a foundation that lasts fifty years and one that cracks in the first freeze.

To understand what a compactor actually is, it helps to first understand the problem it solves. Consider a newly graded area of earth. Even after a bulldozer has smoothed the surface and the material looks solid, it still contains a significant volume of air between the soil particles. When a building sits on that ground, the weight of the structure will squeeze those air pockets out. This is called settlement, and even a few millimeters of differential settlement across a foundation slab can crack masonry, bind door frames, and shear utility connections. Asphalt is no different; a hot asphalt mat laid