Evaluation of Urmia Tunnel Stability using 3D Finite Element and Finite Difference Modeling

Summary Urmia tunnel is being constructed to convey water to the Naghadeh and Urmia agricultural plains. About 1800 m of excavations are located in the alluvial ground. In this paper, a part of Urmia tunnel that is located in the alluvial ground was modeled using the numerical modeling FLAC3D and PLAXIS3D TUNNEL software packages. For this, first, the new Austrian tunneling method (NATM) was modeled. The NATM modeling results showed high displacement and instability of the face of the tunnel. Then, the earth pressure balance (EPB) machine was modeled and the face pressure was calculated. The modeling results showed suitable conformation between the two software packages. Moreover, a short-length part of the tunnel was excavated in the alluvial structure using the NATM, however, according to the modeling results, the face of the tunnel was unstable, and thus, the excavation was stopped. The face pressure, in the tunnel boring machine(TBM) was also calculated.   Introduction Urmia tunnel is a challenging project because of its especial ground and excavation conditions. The main focus of this research is on tunneling in hard conditions like high groundwater level, high depth of tunnel, too much overburden, etc. An example of tunneling in hard conditions is the Urmia tunnel that is studied in this paper. The main objective of this paper is to select a suitable method of excavation in the tunnel.   Methodology and Approaches In this study, the NATM and its combination with soil improvement methods, and also, the EPB machine were modeled using the numerical modeling FLAC3D and PLAXIS 3D TUNNEL software packages. A short-length part of the tunnel, which was excavated in the alluvial ground using the NATM, was considered in this study.   Results and Conclusions The obtained modeling results in this study showed high displacement of both face and crown and instability of the face in the NATM modeling and combination of the NATM and soil improvement methods like jet-grouting and forepoling. Thus, the EPB machine was modeled and the face pressure was calculated.