Face Stability Evaluation of a TBM-Driven Tunnel in Heterogeneous Soil Using a Probabilistic Approach

The stability condition of the tunnel face is of great importance in both conventional and modern [tunnel-boring machine (TBM)] methods of construction. Inadequate and excessive amounts of support pressure toward the face lead to surface settlement and blowout, respectively, whereas blowout occurs only in cases of pressurized shield implementation. In this research, the stability condition of a tunnel face excavated by an earth pressure balanced shield (EPBS) in Line 3 of the Tehran subway in Iran was investigated. Considering the sensitivity of urban facilities to ground movement, and to reduce uncertainty, data simulation by Monte Carlo technique was applied. Random data generated through a simulation process were used in an analytical method to calculate the factor of safety and its probability of being less than a predetermined value for a certain amount of applied support pressure. The soil was modeled by finite-element software, and the amount of face pressure at which the face collapses was calculated. These procedures were followed for face stability evaluation in nine design sections along the tunnel path. Comparisons of the results are discussed. (C) 2014 American Society of Civil Engineers.