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  The Analysis of Excavation Sequence Effect of Overlapped Shield Tunnel in Sandy Cobble Ground

  The mechanical behavior of the overlapped region of shield tunnel in the sandy cobble ground is complicated, which differs from other constructing ways under common geological conditions. Combined with the construction of overlapped shield tunnel in the Baicao Road interval of Chengdu second Metro, the 3-D finite difference model was built. The overlapped tunnels mainly go through loose sandy cobble ground which is poor in stability and uniformity due to the filling sand among particles. Both of these disadvantages lead to the loosened, deformation and uneven settlement in case of disturbance. Under this geological condition, the excavation sequence is of great importance to the tunnel construction design and safety. This paper considered two construction schemes, i.e., top-down(means excavating the upper tunnel first) and bottom-up (means excavating the lower tunnel first) tunneling procedures. On the base of the Mohr-coulomb index,, combining the ground settlement and the mechanical behavior, author analyzed the effect of overlapped tunnels excavation sequence and summarized the plastic distribution zone around the tunnels as well as its variation regularity. The numerical simulation shows that excavating the lower tunnel first has more advantages than the other way around. Especially, the results show that theplastic zone corresponding to top-down tunneling is 56% higher than the other excavation procedure. KEYWORDS: sandy cobble stratum, overlapped tunnels, plastic zone, excavation sequence. Fund Projects: 1 INTRODUCTION There are a lot of overlapped tunnels in subways as a result of the limited space between adjacent buildings aboveground and underground. A large number of studies have been done on the mechanical behavior analysis of the overlapped tunnel (Liu et al. 2009; Fang and He, 2007; Wang et al. 2012; Addenbrooke and Potts, 2001; Lee and Ng, 2005;. Soliman et al. 1993). However, most of the previous work focused on simple cases like twin tunnels, or the interactions between the new tunnel and adjacent structures which are quite different from overlapped tunnels with complex geological situation. Chengdu metro mainly passes through sandy cobble stratum which has a high content of gravel, large grain size, high strata permeability, high strength of pebbles and boulders that are different form normal soft soil ground. This stratum is rare in the domestic or aboard and has its uniqueness. In addition, due to its poor stability and lag settlement characteristics, sand gravel layer has influence on the construction of the overlapped tunnels(). Based on the west extension line project of the Chengdu metro line 2, the paper studied the ground surface settlement and the structure stress of the tunnel, summarized the plastic distribution zone around overlapped tunnels as well as its variation regularity in different excavation sequence. 2 ENGINEERING SITUATION AND NUMERICAL SIMULATION The earth pressure balance shield with an outer diameter 6m and inner diameter 5.4m was used in the Chengdu Metro Line 2 West Extension Line. The thickness of segment is 300mm and the width is 1.5m. The two driving lines of Chengdu Metro Line 2 West Extension Line

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