Shield kinematics and its influence on ground settlement in ultra-soft soil: a case study in Suzhou

Construction of tunnels using a shield in very soft soil may result in serious shield alignment problems and ground settle-ment. This paper presents a detailed field instrumentation program that was conducted for the construction of the Suzhou Metro Tunnel-Line S1 in China. The characteristics of the shield kinematics in ultra-soft soil and ground responses were exam-ined, and the mechanisms of ground movements and pore water pressure corresponding to shield motions and construction parameters were interpreted and discussed. In addition, a calculation method for ground loss corrected by shield posture was established, based on which a corresponding volume of the grout required was recommended. The results indicated that the shield typically tunnels in very soft soil with increasing inclination mode characterized by a rising tail owing to the floating segment and with a changing inclination mode characterized by a sudden downward movement and subsequent steering of the shield upon encountering hard soil. Consequently, with the uplifting of the shield tail, it was found that the ground heaves and settles with a change in the shield posture. Moreover, a large longitudinal uneven deformation of the lining was found to occur in most of the above situations, while the pore water pressure was minimally affected by the shield posture.