The dynamic behaviour of a railway station of the Underground line of Milan is analysed under the effects of an earthquake defined according to Eurocode 8, with peak ground acceleration of 0.07g.
The analysis requires the preliminary knowledge of the state of stress and strain in the soil surrounding the tunnel. It depends on the geometry and on the mechanical properties of the soil layers, which are generally non-homogeneous and non-isotropic. Besides, the state depends on excavation and construction procedures. The computer code FLAC, 2004 version, is proved to be capable of representing this state fairly, the main limitation resting on the knowledge of the constitutive law of the soil and on the representation of excavation and construction procedures. In this paper this state of stress and strain is briefly reviewed. It will be referred to as the relaxation state of stress. If the ground is exposed to seismic excitation, the resulting state of stress in the surrounding soil and the sheet-piling is computed by integrating the equilibrium equations during the earthquake duration, with reference to the non-linear behaviour of the soil. Starting from the relaxation state of stress, the program executes the integration step by step, updating the soil properties according to the total strain amplitude. The sheet-piling and the vertical wall of the station are not linked by steel bars and are kept in contact by the earth pressure. The gap between the two structures may be open during the seismic event, depending on the conditions of the tendons. In the model this element is represented by apposite gap elements, able to follow the contact condition. The reinforced concrete structures remain in the linear elastic realm, in practice. Severe damage to this type of structures, although small in number, was caused in Kobe, Japan, by the disastrous Hygo-Ken Nanbu (Kobe) earthquake of 1995. The worst damage to the large-scale underground facilities was the collapse of the Dakai Underground Station in Kobe, pertaining to the underground railway (Kobe Rapid Transit). Other damages have been reported for railways station in occasion of the Athens Earthquake of 1999.
