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  Twin Tunnels Critical Underpass during Sofia Metro Line 3 Extension

  Project Extension of Sofia metro, Line 3, phase I, aims to build almost 8 km of a metro single tube excavated by a 9.4 m EPB Machine and seven new stations. TBM drive challenged the underpass of two existing 5.2 m internal diameter tunnels with tight overburden of only 1.4 m; few meters prior TBM breakthrough into Metro Station 8, which increased the complexity of the operations. Ground improvements execution was not feasible. Thus, it was concluded to calibrate EPB parameters based on a vast analysis of the already excavated stretch. Historical TBM and ground parameters within the Project were collected and processed to estimate forthcoming behaviors. Determined TBM drive parameters and adjusted pressures were controlled by an extensive monitoring plan, both in surface and inside existing tunnels, to determine volume loss and confirm expected ground displacements before, during and after TBM underpass. Finally, boring below existing L1 tunnels and breakthrough into MS8 were successfully performed through an unexpected uniform ground behavior.

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