EVALUATION OF THE SEISMIC BEHAVIOR OF A CIRCULAR TUNNEL IN HETEROGENEOUS SOIL BY USING DIFFERENT APPROACHES

From the past to today, Underground structures constitute a crucial element of modern life, serving diverse purposes such as transportation, storing materials, and managing sewage and water transportation. The most widely used type of these underground structures are tunnels used for transportation in subways, railways, and highways. Unlike different fields in the construction sector, the tunnel industry has followed the age of mechanization. Tunnel Boring Machines (TBM) machines are used for faster and safer excavation. These machines enable tunnels to be excavated in a circular and pre-casted tunnel permanent lining is installed during excavation. The segmental lining must withstand both seismic and static loadings in the active seismic zone to give serviceability to the post-earthquake. The assessment of seismic performance in underground structures is currently a highly relevant research area within geotechnical earthquake and structural engineering. This thesis explains and compares several approaches used by design engineers in quantifying the seismic effect on the tunnel with a real case study in heterogeneous formation. While analytical solution or free-field simplified analysis is generally used in homogeneous soil formation and the preliminary design phase, seismic response analysis or dynamic analysis is used in the soil heterogeneity and detail design phase. The thesis is structured into five chapters, the first two of which are introductory and descriptive, in which the approaches used, based on existing literature, are explained, while the other chapters present the studies about the selected case project. In the first chapter, which has an introductory purpose, the reasons why it was considered interesting to develop a thesis on this topic are explained, found in the literature, that shows earthquake damage in underground works. The second chapter aims to describe the different procedures and methodologies used to date for the seismic design of tunnels, going on to highlight what are the substantial particularities between the different types of analysis. In particular, it focused on the differences between simplified analyses, sometimes even analytical, and somewhat more complex analyses, carried out through numerical modeling software. In the third chapter, the general project properties are described, site investigations and laboratory test results are listed and the critical analysis section is determined. The critical section's hydrogeological, geological, and geotechnical parameters are presented. In the fourth chapter, seismic hazard analyses and site response analyses are carried out for the project location. Determined ground motion parameters and selected earthquake records are presented. The final section presents and compares the results obtained using the methods described in the second section. The applicability of the simplified method in complex geological and geotechnical environments has been evaluated and it has been stated that it may lead to overdesign. A proportional relationship was established between the methods under the conditions of the case study.