Three-dimensional numerical investigation of ground settlement caused by piggyback twin tunnels

Abstract

Induced transverse ground surface settlement during the excavation of shallow piggyback twin tunnels is examined in this study. A series of three-dimensional parametric finite element analyses were carried out to investigate the settlement behavior and interactions between piggyback twin tunnels excavated in clay using mechanized excavation method. Various geometric parameters and construction sequences play important roles in the interaction between twin tunnels and on the induced ground settlement and internal forces in the linings of both tunnels. Analyses were carried out for five cover-to-diameter (C/D) ratios, three possible construction sequences and four pillar distances (P). The total settlement of the excavated twin tunnels as well as the settlement induced solely by the excavation of a new second tunnel, are both presented along with anticipated shear strains under various excavation sequence scenarios. The observed data trends from this study are generally consistent with the data available in the literature. One of the key findings, among others from this study is that construction of twin or multi-tunnels with a small number of steps induces relatively low settlement compared to tunnel construction with a high number of steps. Furthermore, the effects of C/D and P/D ratios on the magnitude of induced settlement and the characteristics of settlement trough are identified. In addition, two design charts have been proposed to calculate the settlement induced from a new second tunnel excavation.