Hayato ITO

The Assessment of the Segment Behavior in Sharp Curved Excavation Induced by Shield Execution Loads

Mitsutaka SUGIMOTO

Today, shield tunnels have been constructed in deeper underground with sharp curve, as underground space in urban area became congested. Furthermore, segments become thinner and wider for saving its cost. Consequently, the stress in segment during construction tends to increase and then the segment damage during construction was increased. But these mechanisms have not yet been made clear quantitatively.
In addition, to study segment behavior, the simplified FE model was developed, which can evaluate the segment deformation in the transverse direction of the segment due to tail force, F2, and the segment rigid displacement in the transverse direction of the tunnel due to jack thrust, F3, separately. However, this developed FE model makes the segment deformation in the transverse direction of the segment smooth, since this model simplifies intersegment joints with equivalent bending stiffness. Therefore, in this study, the previous model was modified representing the intersegment joint with circumferential rotation springs to model segments more closer to actual structures. The procedures of the analysis are as follows.
1) The execution loads are estimated through the simulation of the shield behavior using the kinematic shield model.
2) The 3DFEM analysis on shield tunnel displacement is carried out using the calculated execution loads, and the obtained segment displacement is compared with the field measured data.
3) The influence of the execution loads to the segment behavior in sharp curve is examined.
As a result, the followings were concluded:
1) Comparing with the measured data, this analysis results show that the segments deform more elliptical shape in vertical direction and move more to the convex side of the tunnel. In addition, the intersegment joints influence to the segment shape. This indicates that the developed model can represent the segment behavior more closer to the actual behavior, representing the intersegment joints with rotation springs.
2) The measured segment displacement in the transverse direction of the segment can be simulated, setting the stiffness of the shell elements equivalent to the circumferential stiffness and the circumferential bending stiffness of the segments for simulating the segment deformation in the transverse direction of the segment, and the spring constant of the ring joint springs in the axial direction of the tunnel equivalent to the axial bending stiffness of the whole tunnel for simulating the rigid displacement in the transverse direction of the tunnel.
3) The calculated stress at the shield tail was 6122kN/m2, which is about 34 times as much as the vertical earth pressure at spring line. Therefore, it is considered that the phase analysis model which can evaluate the segment rigid displacement, the segment deformation, and the solidification of backfill grouting at each ring excavation is necessary, to evaluate the acting force on shield tail adequately.

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