Optimum length determination of plastic concrete for further excavation by TBM

U7 station as a backup and service center has been situated in the middle of northern-southern Tehran’s metro line 7 project, in which is being excavated using an Earth Pressure Balance (EPB) machine. In this paper, the most appropriate stabilization method for the exterior portal has been nominated, and afterward the influence of tunnel excavation with TBM upon the stability of the above-mentioned portal has also been explored in the excavating procedure onset using numerical method. As a result, soil substitution with plastic concrete which can also be categorized as one of the ground improvement methods has been selected to mitigate difficulties more effectively. In addition, optimized length of plastic concrete used to minimize tunnel face displacement and to restrict prospect instability has numerically been yielded.
In today’s world, development in infrastructure facility systems such as subways is in the public eyes more than ever. These underground openings are situated in urban and/or residential areas equated with soft soils and alluviums. Furthermore, providing a stable span for the TBM operations in station would be of great importance to avoid encountering any potential hazard like settlement, economic limitation, and even human casualties. Consequently, in EPB mechanized tunneling, fairly proper stabilization method of any station portals is found to be influential.
Methodology and Approaches: In this study, the FLAC3D software has been utilized to carry out 3D numerical modeling to investigate the influence of plastic concrete on the face stability. In addition, the Mohr-Coulomb criterion has been taken into account for geo-mechanical behavior of soil material surrounding the tunnel. Since EPB machine is not capable of producing fairly enough pressure against the tunnel face to maintain stability at the beginning of the excavation up to 1.5 meter, hence plastic concrete length required for face stability is found to be 1.5 meter. Accordingly, five plastic concrete of 2, 3, 4, 5, and 10 meter long has been inserted to a numerical model. Finally, optimized length and lateral expansion of plastic concrete have been designed generally based on result driven from numerical analysis such as tunnel crown and face displacement, raptured zones around the tunnel face and wall.
Results and Three dimensional modeling has revealed that required length to fulfill tunnel face stability is 4 meter long with respect to face displacement magnitudes and also raptured zones. Lateral expansion of plastic concrete, however, should not exceed 1 meter. Results have demonstrated that plastic concrete plays a significant role in preventing tunnel crown from enormous displacement.