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Soil, which has been disturbed or new infill, subbase and blacktop, will have small voids or air pockets which, if not compacted, will

lead to one or more problems occurring.

1. As traffic crosses the surface of an uncompacted area, the material is compressed. This leads to subsidence of the top

surface as the material fills the voids.

2. A similar situation occurs with static loads on uncompacted ground. The load (e.g. a building) will sink.

3. Materials with voids are more susceptible to water seepage, leading to erosion. Water ingress may also cause the soil to

expand during freezing temperatures and contract during dry spells. Expansion and contraction is a major cause of damage to

building foundations and normally leads to the structure requiring underpinning.

Compaction increases the density of the material and therefore increases its load bearing capacity. Reduces air voids and therefore

reduces the risk of subsidence, expansion and contraction, due to ingress of water.

Various methods have been employed in the past to specify the compaction required for various applications. The factors to consider

are, material properties, layer thickness, pressure applied, vibration and number of passess. Greater understanding of how compaction

works has lead to new compaction specifications being introduced. The most up to date specification is part of the U.K.'s NRSWA (New

Roads & Street Works Act). Civil Engineers are now adopting these specifications to ensure good compaction for all site work.

Applications/materials fall into three categories:

1. Cohesive materials (less than 20% granular) e.g clay, silt & heavy soils.

2. Granular materials (more than 20% granular) e.g hard core, sand & light soils.

3. Bituminous materials e.g asphalt (tarmac), cold lay (bitumin emulsion products).

Reasons For Compaction

Compaction Specification