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

The effects of the shear strain rate (gamma) = d(gamma)/dt on the secant shear modulus Gs 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 (gamma) between 0.0002 and 0.04 %/s. For all six soils Gs increases with (gamma), such that the Gs versus log (gamma) data plot approximately along a straight line. The slope of this line is (alpha)G=strainrate shear modulus parameter, while its normalized value (alpha)G/Gs = N(gamma)-G =shear strain-rate modulus factor. For the clays tested (alpha)G ranges between 2.5 and 7.5 MPa and N(gamma)-G between 2.0 and 11.5%. For the sandy soils (alpha)G ranges between 0.2 and 3.0 MPa and N(gamma)-G between 0.2 and 6.0%. Both (alpha)G and N(gamma)-G decrease moderately with increasing (gamma)c, i.e., the largest (alpha)G and N(gamma)-G were obtained at the smallest (gamma)c. For five soils the families of the normalized modulus reduction curves (Gs/Gmax)-log (gamma)c were constructed, such that each curve pertains to a constant (gamma). It was found that (gamma) has essenlially no effect on the shape of the constant-(gamma) (Gs/Gmax)-log (gamma)c curve. A brief review of previous experimental studies is included.