# Influence of Approaching Excavation on Adjacent Segments for Twin Tunnels
## 1 Introduction  
The shield tunneling method has the advantages of high safety, fast construction speed and good construction quality, so it has become the main method for tunnel construction in crowded urban areas [1; 2; 3]. For subway tunnels, twin-tunnels are more common because of two-way traffic. As compared to one single big tunnel, twin-tunnels have advantages in technical feasibility, less ground movement and smaller lining force [4]. However, it is challenging to ensure the stability of the surrounding soil and the safety of the first tunnel structure during the construction of the second tunnel [5; 6; 7]. Tunnel-ground-tunnel interactions are inherent complex especially when the clear distance is small enough [8]. The interaction mechanisms depend on the tunnel geometries, geological conditions, and the construction loadings, etc. If the clear spacing is less than the external diameter of the second tunnel, it is necessary to demonstrate the safety of construction sufficiently, which was stipulated in the standard specification for shield tunnels proposed by the Japan Society of Civil Engineers [9].  
In order to investigate the interaction mechanisms between twin tunnels, several approaches have been carried out, which include theoretical analyses, numerical simulations, and field tests.  
[PERSON] et al. [10] obtained the unloading vertical stress field beneath the new tunnel based on Mindlin solutions [11]. [PERSON] 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 [13; 14; 15; 1]. 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 [PERSON] 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 segment and the ground displacements is researched thoroughly. Finally, the optimization of successive excavation of the curved tunnel and straight-line tunnel with this project as the background is discussed. The research results provide references for the shield construction of closely spaced twin tunnels.