Face instability during the advance of an EPB in heterogeneous ground conditions: a case-history

Istanbul is a growing city characterized by large infrastructural projects under planning and construction to develop the city transportation system. The Gayrettepe – Istanbul 3rd Airport Rail Metro Line (or “Gayrettepe – Istanbul Yeni 3rd Havalimani”) is part of this development: the new line will allow integration of city’s major rebroadcasting centers of mass transportation with the urban rail system lines, in order to provide quick access to the new Istanbul Airport, foreseen to be the largest and busiest airport in the World. Unfortunately, a collapse occurred along the alignment during an excavation carried out by an EPB shield machine in a critical area, where there is a sudden change in the geology from good rock to soil. Therefore, a study of the phenomenon has been performed to assess the trigger events which might have induced the failure. However, since several data and parameters were uncertain or unknown, a sensitivity analysis has been implemented. The study started considering the provided information for the case history, particularly the pressure applied at tunnel face which was registered by the EPB sensors, which have been considered constant during all the analyses. The main purpose has been to investigate the applicability and the reliability of analytical and numerical methods for evaluating the safety factor for face pressure in mechanized excavation. Particularly, it showed the impact of the chosen variables on the definition of the safety factor for a tunnel in soil by making a comparison between analytical and numerical methods. Two analytical methods have been considered to investigate the stability of the tunnel: the Anagnostou and Kovàri method and the Carranza – Torres (integrated with Caquot) method. Finally the results provided by these models have been compared and discussed with the results obtained by a 3D-FEM models obtained varying the unknown parameters.