Determination of the Racking Reduction Factor for an Incomplete Ellipse Shaped Tunnel Considering the Soil-Structure Interaction

In this paper، a new method is presented to find tunnel target racking displacement in the earthquake considering nonlinear behavior of lining and soil and also soil- structure interaction. In the next step tunnel racking reduction factor has been evaluated for an incomplete ellipse tunnel located in two soil types، A and B، with overburden depth of 5 and 20 m by performing nonlinear static analysis. Finally، to verify the values of racking reduction factors، tunnel responses obtained by linear static analyses reduced by the calculated values of racking reduction factor are compared with responses obtained by nonlinear dynamic analyses. Design and analysis of the underground structures such as tunnels are performed on the basis of structure and ground deformations because the seismic response of such structures is very sensitive to the imposed ground deformations. In recent years، Several researches have indicated the importance of the harmful effects of earthquakes on tunnels and underground structures. Methodology and Approaches: In this paper، by combining Wang (1993) and Hashash (2000) methods in determination of tunnel target racking displacement and using the trial and error method to correct tunnel structure stiffness، possibility of redistribution of the forces and deformations became possible and it was tried to present a comprehensive method to determine the tunnel target racking displacement. In the next step، by using the force-racking displacement curve and applying the existing equations of reduction factors of the structures and their generalization for tunnels and underground structures and considering the soil-structure interaction، tunnel racking reduction factor is estimated. Then، using ABAQUS software، a comparison has been performed between dynamic and static forces in the tunnel lining. Results and Comparison of lining force (bending moment and axial force) between linear static analyses modified by tunnel racking reduction factors and nonlinear time history analyses shows that the accuracy of achieved tunnel racking reduction factors is acceptable. The studies also show that implementing linear static analyses without tunnel racking reduction factor leads to very conservative results of lining force in tunnel. Nonlinear time history dynamic analyses are time consuming and complicated in modeling procedure and analysis duration. Thus، By using the proposed racking reduction factors، Linear static analyses could be an alternative for nonlinear dynamic analyses with a suitable accuracy.