Engineers Australia

Relative compaction: In search of a rational method for specificationBaden Clegg Award winner

Brook Ewers


The primary purpose of compacting engineered fills is to expel water and air from the soil matrix to achieve an increase in stiffness, thereby reducing the likelihood of post construction settlement.

Engineers and earthwork contractors are accustomed to adopting ‘relative compaction’ as a means of compliance testing. Specifications relating to earthwork construction control are commonly expressed as:

γd field / γd max x 100 > R (%) (1)

where γd field is the material density of the material measured in the field , γd max is the maximum dry density obtained from a known energy input and R(%) is the required relative compaction. Despite being widely accepted, relative compaction does not have any direct correlation with the known properties of a material. Hence, there is no rational method for selecting an acceptable percentage R(%) for a particular purpose (Gue and Liew, 2001).

The intention of this paper is to the investigate the rationale behind the perceived need within the construction industry to specify a minimum density ratio R(%) and consider what rational recourse a civil engineer or technician has when a compaction specification cannot be met.