Three-dimensional numerical investigations of new Austrian tunnelling method (NATM) twin tunnel interactions

A series of systematic, three-dimensional coupled finite element analyses was carried out to investigate the multiple interactions between large parallel hypothetical twin tunnels constructed in stiff clay using the new Austrian tunnelling method. Special attention was paid to the influence of lagging distance between the twin tunnel excavated faces (LT) and the load-transfer mechanism between the two tunnels. It is found that LT has a stronger influence on the horizontal movement than on the vertical movement of each tunnel, and it significantly affects the shortening of the horizontal diameter of the tunnels. The change of pillar width appears to be an approximately linear function of LT. The location of the maximum settlement is offset from the centerline of the pillar, and the offset increases with a range of LT values. The magnitude of the maximum settlement is independent of LT, however. As LT increases, there is a transfer of load from the lagging (right) tunnel to the leading (left) tunnel, resulting in an increase in the bending moment in the left tunnel but a decrease in the bending moment in the right tunnel. There is a corresponding increase in the axial force at the left springline of the left tunnel and a decrease in the axial force at the right springline of the right tunnel. Due to the twin tunnel interactions, the distributions of pore-water pressures are highly nonsymmetrical at both tunnels.Key words: NATM, tunnel, interaction, three-dimensional, numerical