Compaction is a mechanical process with which air is expelled from the voids in soil to produce a closely packed soil mass for better unit weight to provide better bearing capacity and stability to foundation soil. In designing foundation, sometimes different stabilization methods are used introducing mechanical power or chemical agents to improve shearing characteristics of soil. Compaction with or without application of chemical agents are also used in stabilization process. However, we will now concentrate on compaction process.
Foundation engineers have to assume a bearing capacity of compacted soil to determined foundation sizes and other parameters. To estimate improvement of capacity of soil, they often define relative compaction. Relative compaction is measured in percentage. This is a ratio of field dry density to maximum dry density. The maximum dry density can be determined in laboratory with standard proctor test or modified proctor test. The expression is
In case of cohesive soils, using pneumatic-tyred roller or sheep-foot roller, a relative compaction of about 95% can be achieved. In case of heavy clay pneumatic-tyred roller is not used, the later roller is only used in this purpose. In case of moderate cohesive soils, around 95% relative compaction can be achieved using the pneumatic tyred roller with addition of inflation pressure of the roller of 600KN/m2 or greater.
In case of cohesionless soil relative compaction of 100%, even sometime greater than that can be achieved using vibratory rollers, pneumatic tyred rollers and other types vibratory equipment.