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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.

As the economy develops rapidly, newly built tunnels in congested urban areas are becoming more and more common. Based on similarity theory, this study conducted a laboratory model test to investigate the soil–structure responses induced by the construction of a new tunnel in limited space which was formed by the existing underground infrastructures. The soil movement, convergence of the tunnel and distribution of the soil stress were explored. The test results revealed that the limited space could have a significant shielding effect on the soil movement. The maximum ground subsidence and the range of ground settlement induced by the construction of the tunnel in limited space were 60% and 40%, respectively, smaller than those induced by the construction of a tunnel at a “greenfield” site. The deformation of the tunnel was also restrained distinctly. Moreover, a soil settlement trough appeared below the bottom slab of the underground structure. The findings can shed some light on the surrounding responses induced by tunnelling in limited space in soft clay and can offer the valuable guidance for the similar projects.

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