Engineers Australia

Overcoming the Current Density Testing Impediment to Alternative Quality Testing in Earthworks

Burt G Look


Density testing has been applied widely in earthworks quality assurance (QA), yet because of its widespread usage, this now acts as an impediment to the development of alternative methods of testing. Many incorrect inferences are made from density testing. Modern geotechnical and pavement designs are based on modulus and strength values. However, when such measurements are correlated back to density testing, a poor correlation often results. Therefore, while alternative in situ testing to density provides significant benefits to the industry, the poor correlation is often questioned, and then site personnel default to the usual density testing for quality testing of earthworks. The reason for the poor correlations includes, the depth of influence being different, with the quality and compaction being combined into one parameter (say modulus). Another rationale for the poor correlation, is the density lot measurements are normally distributed due to its low coefficient of variation, while other measurements are not normally distributed and have a large variation. A new method on matching Probability density functions (PDFs) for quality assurance has been successfully used on a large earthworks project to overcome the correlation inconsistency and is introduced. Data from several test sites using a range of alternative testing equipment are compared. One must also distinguish between a test accuracy and its precision. Traditional testing density has focused on its key benefit of precision without appreciating the poor accuracy associated with this measurement. Case studies are used to illustrate this dichotomy between traditional and non- traditional testing for QA assessment of earthworks.