Stress-Ratio-Based Interpretation of Modulus Reduction and Damping Curves

Modulus reduction and damping values are commonly plotted against cyclic shear strain amplitude (${\gamma }_c$), and the resulting curves are known to depend on mean effective stress ($p^{\prime }$), plasticity characteristics, strain rate, and number of loading cycles. The dependence on $p^{\prime }$ is potentially problematic for undrained effective stress analysis where excess pore pressure may develop during loading. This paper presents a new concept in which normalized modulus reduction ($G/G_{\text{max}}$) and damping ($D$) values are plotted against stress ratio ($\eta$) rather than ${\gamma }_c$. Relations developed for sand, clay, and peat are found to be essentially pressure-independent when $G/G_{\text{max}}$ and $D-D_{\text{min}}$ are plotted versus $\eta$,whereas all three are pressure-dependent when plotted versus ${\gamma }_c$. This finding is potentially useful for undrained effective stress analysis where $p^{\prime }$ may change during loading, and provides a new approach for interpreting laboratory tests in future development of $G/G_{\text{max}}$ and $D$ curves.