Behavioral Study of Site Effect on the Kermanshah Subway

Overpopulation in metropolises has led to a space reduction in the cities and a tendency to use underground spaces. Different solutions have been proposed for traffic problems; there are many cases in science research in recent years that show the significance of earthquake destruction effects on these structures. Regarding seismicity of the Kermanshah city, we have tried to study the effects of earthquake on the site effect of Kermanshah subway tunnel by using ABAQUS software and finite element method. Each one of BH-7, BH-8 and BH-9 bore holes have been analyzed in three different steps: first, frequency analysis, then free field analysis (without tunnel), and finally the main model. The results derived from the time historical analysis of the three BH-7, BH-8 and BH-9 bore holes show that the maximum amplification occurs in the BH-9 borehole, which is the most critical borehole in terms of amplification received waveforms on the earth surface. Although the maximum amplification occurs in the BH-9 borehole, the highest maximum stress occurs in the tunnel cover at the site of the BH-8 borehole due to the location of this borehole, which is near the bedrock. According to this study results, the type of damage in tunnels cover depends on the geotechnical characteristics of the layers, the content and intensity of the earthquake record, the amplification that occurs in the soil profile, the amount of tunnel overhead load and the strength of materials that covers tunnel with concrete.
 1. Introduction
Nowadays, underground structures such as subway tunnels, water and sewage transfer tunnels, utility tunnels, subway stations and underground parking are among the vital infrastructures of the new urbanization. This kind of structure especially in crowded cities is built to resolve different needs. The study of destructive earthquakes that have occurred in recent decades has clearly shown that geological conditions and site effects play an important role in amplification of the strong movement of the earthquake. Soil type and local geology can play an important role in seismic movement’s amplification and the type of damage caused by strong earthquakes.
 2. Specifications of Materials Used in Numerical Analysis
2.1. Geometry and Tunnels Cover
One of the effective parameters in studying the seismic responses of underground tunnels is the geometric dimensions of the tunnel section. The cross section of Kermanshah City subway tunnel is horseshoe type with an average radius of 4.3 meters. In this case study, two-dimension modeling has been used for modeling with plan strain conditions.
The thickness of the concrete cover of the tunnel is 30 cm. Behavioral model of damaged concrete has been used to define the nonlinear characteristics of concrete tunnel cover. This behavioral model is the most comprehensive and widely used model for concrete in Abacus software. In the damage mechanism, it is assumed that the decrease in stiffness is due to the creation and expansion of small cracks in order that the decrease in stiffness can be measured and determined with a parameter called damage.
 Conclusion
       In this study, we have tried to study the effect of earthquake on the site effect of Kermanshah subway tunnel by using ABAQUS software and finite element method. According to study results, the type of damage in tunnels cover depends on the geotechnical characteristics of the soil layers, the content and intensity of the earthquake record, the amplification that occurs in the soil profile, the amount of tunnel overhead load and the strength of materials which covers tunnel with concrete.