Definition

The word 'compact' refers to pressing things closely together, which makes the object physically smaller as it takes up less space. With this in mind, and the knowledge that soil is composed of various-sized soil grains, soil compaction occurs when these soil grains (or particles) are pressed together. When this happens, air is expelled from the soil and the compacted soil takes up less room than non-compacted soil.

To get an idea of the difference between non-compacted soil and compacted soil, picture a set of footprints left behind by someone walking over loose dirt or that pesky cat next door that keeps walking across your flower garden.

For the purpose of constructing large buildings, houses, roads, and just about any type of structure, soil is compacted deliberately and quickly with the use of heavy compaction equipment. We will discuss this equipment later in the lesson.

Consolidation is another way in which soil gets smaller; however, it results from water being removed from the soil, rather than air. Consolidation is not a part of this lesson, but it is important for you to be aware of the basic difference between compaction and consolidation.

POINTS TO REMEMBER:

• the general meaning of the word 'compact' is "to press closely together"
• in soil mechanics, it means to press soil particles tightly together by expelling air from the void space
• compaction is normally produced deliberately and rapidly and often by heavy compaction rollers
• compaction differs from consolidation, which results from extrusion of water rather than air, and is not rapid