A discrepancy between observed and predicted NATM tunnel behaviors and updating: a case study of the Sabzkuh tunnel

The 11 km long Sabzkuh water conveyance tunnel is under construction using conventional and tunnel boring machine tunneling techniques in the Zagros Mountains of south west Iran. During excavation of the conventional section (i.e., 350 m), three daylighting collapses occurred in the tunnel, and large sinkholes developed at the ground surface, due to both poor ground conditions and inappropriate selection of excavation and support class (ESC). In this study, the influence of ESC on tunnel response and stability was investigated through three dimensional (3D) finite difference and finite element methods. The analysis results of numerical modeling indicated that the tunnel face would be stable if a single stage (full face) excavation and an advance step of 1.5 m (defined as ESC#1) were applied. However, the reality was different, and the tunnel collapsed soon after the face was only 35 m away from the portal. Having changed the ESC by taking into account new geological and geotechnical data obtained from face mapping, a field survey of surface cavity, and a trench study, the tunneling advance rate was satisfactory (1.5 m/day) despite the many challenges encountered. This paper presents a brief review of some of the key geological challenges faced in the tunnel design, including characterization of the ground conditions, selection of appropriate design parameters, and evaluation of the excavation and support installation sequence based on monitoring and analyzing ground behavior during construction. The discrepancy between the predicted and observed behavior of the tunnel, and some practical considerations, are also discussed.