عنوان مقاله [English]
Circular Tunnels are used in urban transportation nowadays. One of their major applications is metro tunnel which is built by TBM. An investigation of these structures during an earthquake should be considered in design of these structures.
For studying dynamic behavior of tunnel under earthquake motion, PLAXIS software, which is based on finite element code, is used. In this study, the method of seismic analysis of tunnel with PLAXIS is explained. For more accuracy, the data are used based on Tabriz tunnel properties. The dynamic properties of soil, and also, the method of iterative process for finding acceptable shear strain are obtained from previous study and the result of some dynamic tests. Numerical analyses are performed in both linear and Mohr-Coulumb model for soil.
In order to investigate the effect of tunnel on earthquake motion, the diametric deflection of final lining is calculated on each analysis. The major forces such as bending moments and axial forces occur in maximum deflection in diameter. For finding the effective parameter on dynamic behavior of tunnel, sensitive analyses are performed. In these analyses, variations of dynamic parameters such as maximum acceleration, damping and shear modulus of soil are examined. Moreover, other mechanical properties of medium like friction angel of soil are selected for sensitive analysis. Geometrical properties of final lining such as thickness and diameter of lining are also used to calculate the flexibility ratio. Allowable stress and strain are also calculated and compared with the existence results.
The results of this study show that stress and strain are not different on acceleration below 0.5 g but the differentresults are more evident for acceleration more than 0.5 g. In addition on sensitive analysis in friction angle of soil, the rate of dynamic forces decreases to 30 degrees and then, the results show no variations. These effects are more significant in deeper tunnels.
Investigation on variations of dynamic results versus fallibility ratio show that with increasing this ratio, the rate of degradation of dynamic forces reduces and in F=25 and for more, these variations disappear, and thus, the results are independent of the diameter of the tunnel under study.