The Importance of Compaction – The process of compaction in pavement and subgrade material is a method of mechanically induced stress applied repetitiously to achieve a desired density to promote both short and long term performance.

The basic concept for compaction is to ‘work’ the material to remove effectively remove air voids resulting from the loosely placed material until achieving a relative compaction of soil.

From a technical perspective, the resulting design thickness of a pavement is relative to the nominal compaction standards adopted for each material, i.e. where a CBR 80% material is used as a base course, it is so on the provision that the testing compaction meets compaction during construction. If this level of compaction cannot be achieved, it can pose significant risk to short term and long term performance on the basis of the following:

  1. Increased permeability – A lack of compaction can accelerate the pavement deterioration on the basis of increased permeability from moisture ingress. It can also attribute to greater capillary rise moisture movement.
  2. Consolidation potential – A particular problem in fine-grained cohesive soils the repetitious wheel loading can result in the voids being ‘squeezed’ out of the material which can be worsened if excessive moisture is present.
  3. Strength – As outlined above, the characteristic strength of a material is determined by testing undertaken at a nominal compaction, if this compaction isn’t achieved, it is likely that it will result in a reduced bearing capacity

The methods of determining the insitu compaction at the time of construction vary between materials, however generally follow a similar concept.

As constructed compaction is typically determined by a compaction test, using one of two methods:

  1. Sand Replacement Test – the removal of a test specimen from the compacted area followed by the replacement of fine grained sand. Effectively calculating the volume of sand replaced. The mass of the sample is then weighed in a laboratory environment to calculate the density. The level of compaction is subsequently measured from the ratio of the insitu density to the maximum dry density of the material – typically only used on granular material and soils.
  1. Nuclear Density Test – Emits and receives gamma radiation (using both direct transition and back-scatter methods) to determine the insitu density and moisture content using count data. This data is then correlated against the calibrated ‘standard density block’ in which a relative compaction is determined – can be used on a range of unbound and bound materials.

Given the importance of compaction in road building, suitable compaction methods and plant should be employed with consideration given to the specific nature of the material.

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