Relating twin-tunnelling-induced settlement to changes in the stiffness of soil.

Recent research has shown that in a parallel twin-tunnel arrangement, the 2nd tunnel generates greater settlement than the 1st tunnel as the soil between the two tunnels softens. For other spatial twin-tunnel arrangements (twin tunnels with different cover depths), the pattern of settlement can be more complex and is inevitably affected by the order in which the two tunnels are constructed. Few studies have attempted to establish the relationship between the changes in soil stiffness and settlement behaviour. Hence, the settlement behaviour induced by twin tunnels is not yet well understood. A numerical model was created in this study, and a centrifuge model test was then used to verify the performance of this model. A series of comparative numerical analyses (e.g., inclined and overlapping twin tunnel arrangements) were performed to investigate the effect of the construction order and relative positions of the two tunnels. The relationship between the changes in soil stiffness and settlement behaviour was well established. The mechanisms by which the 1st tunnel affects the settlement behaviour produced by the 2nd tunnel were clarified. [ABSTRACT FROM AUTHOR]