Measurements of small-strain inherent stiffness anisotropy of intact Shanghai soft clay using bender elements

Measurements of small-strain stiffness are widely used for predicting ground movements in many developed countries such as the UK. Although an increasing number of foundations, excavations and tunnels are constructed in Shanghai, the measurements of the small-strain shear modulus of Shanghai soft clay have rarely been reported. In this study, the degree of inherent stiffness anisotropy of intact Shanghai soft clay is investigated using a triaxial apparatus equipped with Hall-effect local strain transducers and a bender element testing system. Two series of tests are carried out on intact prismatic soil specimens under an isotropic stress state. The experimental results reveal that the cross-correlation method using two received signals gives rise to more objective and repeatable results than the conventional first-arrival-time and peak-to-peak methods. The intact Shanghai soft clay clearly exhibits inherent stiffness anisotropy, as demonstrated by its elastic shear modulus ratio (G_0(hh)/G_0(hv)) of about 1.21, due to the stronger layered structure in the horizontal plane and a bonding effect. A unique relationship is found and established between the normalized shear modulus and the stress state in each plane by incorporating a void ratio function in the form of F(e)=e^-1.3.