Main Roads WA only specifies one methodology for assessing the laboratory Density of embankment and pavement materials, WA 133.1 Maximum Dry Density.  This method can be problematic with non-plastic poorly graded materials.  During compaction as the water saturates the voids the laboratory compaction process forces the water to the top of the compaction mould where it may splash out or be removed as supersaturated material when the mould is trimmed to height.  The Moisture content taken from within the mould then under represents the moisture added to the test specimen.  This then can lead to an ill defined MDD curve, one that does not meet the requirement of moisture spacing’s.  Alternatively and commonly practised moisture is taken from mixed material prior to the compaction being performed, which does not comply with the standard and introduces a small mathematical error the data. The Austrailian MDD Standard AS 1289.5.2.1 recognises this problem in a note (see below) but does not propose a solution.

Note: If, with increasing moisture content, the wet mass of compacted soil markedly increases and then starts to decrease, the optimum moisture content probably has been straddled adequately.  For soils with low plasticity and high permeability, points wetter than optimum moisture content may not be achieved due to loss of water during compaction.

There is the opportunity to research the use of Minimum and Maximum Density AS1289.5.1.1 Designed for Cohesion-less materials to define Laboratory density.  This requires The Field compaction then to be assessed with Density Index rather than the current Characteristic Density Ratio currently used.

Current status and previous research

WARRIP Paper “Review of Compliance Systems for Subgrade and Embankment Construction – Stage 1” Section 5 Lincoln Latter, Zia Rice & Bob Andrews recommends further research into the repeatability and reproducibility of MDD determination of WA’s cohesion-less sands and include evaluating maximum – minimum density using current vibratory tables calibrated for frequency and amplitude. The desktop research could not determine sound methodology was used in earlier research and differing equipment could be a contributor to reproducibility variations.

Objectives and Outcomes

The objective would be to test the comparative relationship between MDDs as they are currently performed and Maximum and Minimum Density in Producing Density Ratio verses Density index results. Ultimately incorporating Density Index Values as the Specified method for acceptance of embankment compaction where cohesion-less materials are used.