Preparing for Intelligent Compaction – The Importance of Degree of Saturation when Measuring Dry Density with Stiffness Indexes

Increasingly, the earthworks industry seeks to assess compaction through stiffness indexes, as it is both easier to measure, and is relatable to road design parameters such as soaked CBR. This interest in stiffness is not addressed in a conventional Proctor based approach to compaction. Furthermore, there is an often-held misconception that additional compaction improves stiffness. However, as shown in this paper, and by others, it is clear that stiffness is not related to density in a simple way. During a recent compaction study undertaken at the University of Sydney, it has been shown that monitoring degree of saturation ($S_r$) is essential for understanding the relationship between small shear modulus ($G_o$) and dry density ($\rho_d$). Silty sand samples were prepared at differing moisture ratios before undergoing multi-staged compaction. First arrival shear wave velocity was measured via Bender Element (BE) to produce $G_o$ at a range of $\rho_d$. Results presented in this paper show $G_o$ increases with increasing $\rho_d$ at a constant $S_r$ rather than constant water content ($w$). Analysis indicates that additional compaction past the critical $S_r$ range of 50% will reduce stiffness response Go at constant $w$, and that when compacting across the typical range of $S_r$ of 40-70% $G_o$ is nearly constant and largely independent of $\rho_d$.