Earthquake-induced liquefaction poses a significant threat to tunnel structures. Particularly

shield tunnel crossing sandy stratums with different liquefaction susceptibilities could suffer more severe seismic damages near the soil interface. In this paper

a three-dimensional numerical study was carried out to investigate the seismic response of a shield tunnel passing through saturated sandy strata with two different relative densities. Firstly

a practice-oriented two-surface plasticity sand model was employed to model the sandy soil and was validated by shaking table experiments on a tunnel structure embedded in liquefiable soil. Secondly

a deformable force-displacement link model for circumferential joints between each successive segmental ring was employed to model the interactions between segmental rings. The approach was validated using the results of two loading experiments on model segmental linings from the literature. Finally

the 3D numerical model was established considering various relative densities of soil

peak input accelerations

and the dip angle of the interface. The results indicate that the tunnel’s horizontal displacements due to seismic excitations are coupled with the liquefaction-induced vertical uplift displacements

and the tunnel’s deformation is not simultaneous in the two soil strata

resulting in twisting distortion of the tunnel structure. The uplifts of tunnel change rapidly and are increased by the rising of the dip angle near the soil interface. Also

the bending moments suddenly change

and the shearing/tensile displacements of joints increase remarkably

which confirms that the seismic design of shield tunnel segments near the soil interface is a critical issue.