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Tunneling requires intensive studies to evaluate rheological parameters of the ground, especially under squeezing conditions. In the current study, the evaluation of the required thrust force of the shielded tunnel boring machines (TBMs) under squeezing conditions is made and Beheshtabad water conveyance tunnel is chosen as the case study. The parameters of the Burger's viscoplastic creep model (CVISC) model obtained from laboratory creep tests are verified using the finite difference method. The required thrust force is calculated using a new approach involving the extraction of the normal forces acting on a TBM shield based on different advance rates. The effect of the tunnel diameter is also investigated to evaluate the feasibility of excavating a twin tunnel of a smaller diameter. The effects of over-boring and the standstill time of a machine on thrust reduction are also investigated. The results show that standstill time in smaller over-borings affects the thrust increment more strongly than that in bigger over-borings. Increasing the over-boring from 3 to 10 cm makes thrust decrease by 9.46 and 35.12%, respectively, while advance rate (AR) increases from 1 to 4 m/h. According to the sensitivity analyses done on the effect of the tunnel diameter on thrust reduction, thrust forces reduce by 53.79% in the 5 m-diameter tunnel, while they decrease by 35.13 and 15.05% in the 7- and 9 m-diameter tunnels, respectively, when over-boring is 10 cm. In single-shield machine, an AR of 2 m/h is defined as the substitution AR since the values of thrust are 28.2, 33.01 and 35.1% lower than those in AR of 4 m/h in double-shield TBM with over-borings of 3, 6 and 10 cm, respectively.

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