عنوان مقاله [English]
A major concern about construction of tunnels is accurate prediction of ground displacement, which distorts and, in severe cases, damages surface structures. Empirical Peck’s method is commonly used in many tunneling projects to predict surface settlements. Reasonable prediction by this method requires an appropriate estimation of settlement 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 those obtained from the empirical settlement method. Consequently, the maximum settlement and i are predicted for a part of 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 has been varied out by systematic approach, in 35 phases. The empirical settlement method suggested by Peck has been used to predict the transverse settlement trough. The longitudinal and transverse settlements predicted by numerical modeling are also obtained during tunnel excavation. The development of 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 from empirical and numerical approaches.
Results and Conclusions
The numerical results have shown that the surface settlement directly above the tunnel face is around 45% of the maximum surface settlement. Small surface settlement has been induced after installation of the tunnel lining. The development of surface settlement profiles has 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 of 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.