Determination of small-strain stiffness of Shanghai clay on prismatic soil specimen

Abstract

Although a large number of tunnels and deep excavations have been constructed in Shanghai, small-strain stiffness properties of natural Shanghai clay have rarely been reported in the literature. In this study, the degree of inherent stiffness anisotropy of natural Shanghai clay was investigated in a triaxial apparatus equipped with local strain transducers and a shear-wave velocity measurement system. Three sets of side-mounted bender elements, consisting of one transmitter and two receivers each, were installed on a prismatic specimen. Two series of triaxial tests on prismatic specimens of intact Shanghai clay were carried out under an isotropic stress state. Shear-wave velocities and hence elastic shear moduli in different planes were determined from bender element measurements. The cross-correlation method using two received signals gives rise to the most objective and repeatable results on shear-wave velocities in comparison with other commonly used methods. Intact Shanghai clay clearly exhibits inherent stiffness anisotropy in terms of its elastic shear modulus ratio (G0(hh)/G0(hv)) of about 1.2 for a mean effective stress varying from 50 to 400 kPa. The measured higher stiffness in the horizontal plane may be attributed to the stronger layering structure in the horizontal bedding plane. A unique relationship is found that relates the normalized shear moduli to the stress state in each plane by incorporating a void ratio function in the form of F(e) = e–2.6.