Abstract

Underground structures and tunnels in urban areas constitute an important part of the transportation infrastructure. In seismic regions these structures are under earthquake induced risks. By their nature, these structures extend through very long distances. Their geometry, structural properties and also the surrounding medium may change throughout their alignment. An understanding of the effects of all these factors on the seismic behavior of underground structures is essential to cope with the problems encountered after a possible earthquake event.

Effects of the flexibility, embedment depth and input motion on the seismic behaviour of the underground structures are not fully understood. Widely used design methodologies often use simplifications and assumptions which do not represent realistic site conditions and the nature of the dynamic loads applied on the tunnels. The effects of inertia and non-linearity of the surrounding medium are often neglected. More complex numerical methods exist, but their predictions cannot be verified due to lack of experimental data.

This study is based on dynamic centrifuge experiments which were carried out on model tunnels with different shape, flexibility and embedment depth. The emphasis is given on the effects of the upwards propagating transverse shear waves in dry, loose sand. Accelerations around the tunnel, earth pressures on the lining were measured. Particle Image Velocimetry (PIV) analyses were carried out on the digital images recorded by a fast camera during the centrifuge flights. Complementary dynamic Finite Element (FE) analyses were also conducted using general finite element code ABAQUS .

Analysis of the results revealed that the shape, flexibility and the depth of the tunnel have pronounced effects on the dynamic behaviour of the tunnels and effects the accelerations around the tunnel, earth pressures and the mode of deformation of the tunnel lining. It has been shown that the current analytical methods fall short in explaining the complex nature of the dynamic behaviour of the underground structures.