Cracking Problems in the Segments of Tabriz Metro Tunnel: A 3D Computational Study

This paper presents numerical models of tunnel segments with respect to the steel rebars using a finite element method under TBM thrust jack referring to the Tabriz metro line 2. The analyses have been carried out in five different steps. Numerical modellings have been done from the lowest ratio of reinforcement (the weakest case) to the highest ratio of it (the strongest case), which means that the curved longitudinal rebars were only placed within the concrete in the first step of the analysis, then other types of rebars are added gradually. The results indicated that stress contours in the z-direction (S33) of the model was approximately equal in all the steps. While distributed stress indicated more different behavior than the S33, which the maximum compressive and tensile distributed stresses are just applied on the reinforcement. The higher the reinforcement ratio, distributed stresses are increased step by step. So, it may be noticed that the reinforcement ratio was so crucial to the stresses which are needed to bear. A cracking form of the segment has been analyzed using different types of strains. The curved longitudinal rebars have a remarkable impact on cracking form in the z-direction in step 1, so that the magnitude of plastic strain in the z-direction (PE33) was remarkably decreased when they were placed within the concrete. Although adding the other main rebars has a negligible impact on the cracking form in z-direction whereas they can decline the plastic strain in the x-direction (PE11). The rebars which were added to the model in step 3, decrease the cracks around the shoulders of the segment. Nonetheless, all the rebars have not affected the segment behavior. Finally, in step 5, the cracking mechanism between the loading surfaces has completely changed. Also, displacement contours showed that the magnitude is decreased when the rebars are added to the reinforcement in every step.