A Numerical Modeling of Longitudinal and Transverse Surface Settlement Induced by EPB Tunneling- A Case Study: Shiraz Subway Tunnels

A major concern of tunnels construction is accurate prediction of ground displacement، which distort and، in severe cases، damage surface structures. Empirical Peck’s method is used commonly in many tunneling projects to predict surface settlements. Reasonable prediction by this method requires an appropriate estimation of trough width (i). This parameter can be obtained from previous case histories with similar ground conditions and excavations. A part of Shiraz subway system is constructed by twin tunnels using EPB machine. Due to the lack of relevant case histories، numerical simulations are used to predict i for the Shiraz tunnels. The longitudinal and transverse surface settlements obtained by numerical analyses are compared with empirical method. Consequently، the maximum settlement and i are predicted for a part Shiraz subway tunnels. The development of settlement profiles obtained by numerical modeling is also investigated in order to achieve the steady state condition behind the tunnel face. Methodology and Approaches: The numerical simulations are performed using PLAXIS 3D Tunnel. The tunnel construction process was done by systematic approach، in 35 phases. The empirical method suggested by Peck used to predict the transverse settlement trough. The longitudinal and transverse settlements predicted by numerical modeling are obtained during the excavation of tunnel. The development of surface settlements is studied to establish the steady state condition. The initial estimation of i is obtained by the current empirical equations. To achieve the appropriate trough width، the initial i is modified by comparison of settlement troughs obtained by empirical and numerical approaches. Results and The numerical results showed that the surface settlement directly above the tunnel face is around 45% of the maximum. Small surface settlement was induced after installation of the tunnel lining. The development of surface settlement profiles indicated that the steady state condition is reached at three times of tunnel diameter beyond the tunnel face. Comparison of the transverse surface settlement troughs indicated that trough width parameter is 1. 8 times the tunnel diameter. The current value can be used for prediction of surface settlement trough induced by tunneling in the similar ground and excavation conditions as Shiraz subway tunnels.