Strain-Rate Effect on Soil Secant Shear Modulus at Small Cyclic Strains

The effects of the shear strain rate $\dot{\gamma }=d\gamma /\mathrm{dt}$ on the secant shear modulus $G_s$ of three clays and three sands at small cyclic shear strain amplitudes ${\gamma }_c$ under simple shear loading conditions are described. The amplitude ${\gamma }_c$ varied between 0.0003 and 0.02% and $\dot{\gamma }$ between 0.0002 and 0.04 %/s. For all six soils $G_s$ increases with $\dot{\gamma }\text{,}$ such that the $G_s$ versus $\log \dot{\gamma }$ data plot approximately along a straight line. The slope of this line is ${\alpha }_G$=strain-rate shear modulus parameter, while its normalized value ${\alpha }_G/G_s=N_{\dot{\gamma }-G}$=shear strain-rate modulus factor. For the clays tested ${\alpha }_G$ ranges between 2.5 and 7.5 MPa and $N_{\dot{\gamma }-G}$ between 2.0 and 11.5%. For the sandy soils ${\alpha }_G$ ranges between 0.2 and 3.0 MPa and $N_{\dot{\gamma }-G}$ between 0.2 and 6.0%. Both ${\alpha }_G$ and $N_{\dot{\gamma }-G}$ decrease moderately with increasing ${\gamma }_c\text{,}$ i.e., the largest ${\alpha }_G$ and $N_{\dot{\gamma }-G}$ were obtained at the smallest ${\gamma }_c\text{.}$ For five soils the families of the normalized modulus reduction curves $(G_s/G_{\max })-\log {\gamma }_c$ were constructed, such that each curve pertains to a constant $\dot{\gamma }\text{.}$ It was found that $\dot{\gamma }$ has essenlially no effect on the shape of the $\mathrm{constant}-\dot{\gamma } (G_s/G_{\max })-\log {\gamma }_c$ curve. A brief review of previous experimental studies is included.