Influence of Approaching Excavation on Adjacent Segments for Twin Tunnels

Abstract

This paper aims at influence analysis of approaching excavation on adjacent segments for twin tunnels with variable clear spacing located in Beijing, China. A series of numerical simulations have been conducted to investigate the influence of approaching excavation on ground displacements and mechanical responses of segments. What's more, on-site monitoring was conducted at the position of the minimum spacing between the twin tunnels. It is found that the deformation of the measured segment presents a pronounced asymmetric ovalization when the second tunnel approaches the measured cross-section. During the approaching excavation of the second tunnel, the normal force increases gradually, whereas the bending moment first decreases and then increases. The maximum increments of internal forces in the measured segment are both located at the regions near the second tunnel, in which the springline and shoulder of the segment are mainly subject to the effect of the circumferential compression and axial tension. The approaching excavation reduces the horizontal displacements of the ground between twin tunnels and increases the ground settlements as well as the horizontal displacements of the ground on both sides. The actual tunneling case of first along the curved trajectory and then along the straight-line trajectory can reduce internal forces of the segment.Liang et al. [10] obtained the unloading vertical stress field beneath the new tunnel based on Mindlin solutions [11]. Kong et al. [12] obtained the analytical solution of the displacement field of twin tunnels by using the Schwartz alternating theory. The ground settlements during the construction of twin tunnels were predicted by the experimental and numerical methods [1,[13][14][15]. The influence of the interaction between twin tunnels on the horizontal and vertical ground displacements will induce the variations of structural forces during the approaching excavation [16].The field measurement on the strain (or stress) increments in the segment induced by the approaching excavation were carried out by Gao et al. [9]. Unfortunately, comparative and parametric analyses have not been implemented via numerical or analytical methods [17][18][19][20][21]. The finite element modeling was performed to predict the influence of approaching excavation on the ground displacements and the structural forces in the segment of twin tunnels [22][23][24]. However, the construction loadings (e.g., friction force, jacking force, grouting pressure, etc.) were not simulated in detail. Note that the lateral spacing between twin tunnels in the above investigations is constant along the tunneling direction, while this condition of equal spacing is often subject to other factors and cannot meet in actual engineering.In this paper, twin tunnels with variable clear spacing is taken as a case study. The meticulously and completely real-time monitoring on the adjacent segment has been carried out. Furthermore, the influence of the tunnel approaching excavation on the internal forces of the...

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