Seismic performance of a major twin tunnel with unreinforced concrete lining: Expert witness study for the needs of arbitration

The paper studies the seismic performance of a major twin tunnel, with unreinforced concrete (UC) lining. The work is part of an expert witness study for an arbitration. For confidentiality reasons, the key conclusions are presented, without revealing sensitive information. Situated in Greece, the tunnel is part of a major motorway, under construction at the time of the dispute. With a total length of almost 6 km, and overburden ranging from 30 to 300 m, the tunnel was constructed by conventional means of drill and blast. After its completion, the seismic safety of the UC lining was disputed by the owner. An expert witness of the owner assessed the adequacy of the UC lining through pseudo-static "pushover" analysis. Assuming elastic response, the earthquake-induced tensile stresses were found to exceed the tensile strength of concrete, concluding that the UC lining is insufficient. We were subsequently appointed by the contactor to conduct a more sophisticated expert witness study. The problem was analyzed with the finite element method, employing a nonlinear model for the rock mass, and the concrete damaged plasticity (CDP) model for the UC lining. An initial elastic pushover analysis, using the parameters as the owner's expert witness, gave compatible results. Then, nonlinear pushover analysis was conducted, with and without the surrounding rock mass. Without the beneficial compression offered by the rock mass, the UC lining fails at drift delta = 1.7 mm. Confined by the surrounding rock mass, it is capable of sustaining much larger deformation without collapsing. Finally, nonlinear dynamic time history analyses revealed that tunnel damage is a function of shaking intensity and initial loading. Even with conservative assumptions, no collapse mechanism would develop and the residual drift is negligible.