Compaction, in other word means compressing the soil material. This process is important because it reduces the volume of the total mass of the soil which consists of soil particles, water content & air by application of pressure from different ways like rolling, tamping and vibration.
Compacted fill material are those loose soil that are compacted in layer during the backfilling. The importance of compacting the fill material is to increase the shear strength, decreases its compressibility and decreases its permeability. The fill material can be categorized in two classification of soil which have different ways to effectively compact the material during the backfilling. The two classification are cohesive soil and cohesion less soil.
Cohesive soil are the type of soil that are fine-graded, low-strength and easily deformed soils. These material have significant cohesive strength and can exhibit plasticity. According to OSHA, cohesive soil means clay (fine-grained soil), or soil with a high clay content. Cohesive soil do not crumble, can be excavated with vertical side slopes, and is plastic when moist. Cohesive soil is hard to break up when dry, and exhibits significant cohesion when submerged.
Cohesive soils include clayey silt, sandy clay, silty clay, clay and organic clay. Cohesive soils require high amplitude, high impact forces to squeeze and mold the material together, therefore are effectively compacted by the use of impact such as rammers or tamping rollers.
Cohesion less soil are the relatively clean sands and gravels that remain pervious even when well compacted. Cohensionless soil are those soil particles that is hard to stick together. Cohesion less soils pass through a #200 sieve, which has an opening size of 0.075 mm. Soil cohesion is closely related to soil consistency.
This type of soil can be coarse which contains stones, gravel or fine like sand or silt. In cases of cohensionless soil, the best way to effectively compact is by vibration. Excessive compaction can even lead to separating of soil particles rather than letting them stick together to remove the void content which is not good backfill.