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Athens Metro - design and construction of shallow tunnels to control settlements of surface structures

Abstract:

The paper describes the basic design considerations and the construction of two short shallow double-track tunnesl of the Athens metro. In the first tunnel the gournd consists of highly fractured peridotites. The minimum distance between the tunnel crown and the foundation level of the building is 2.50 m. The second tunnel crosses decomposed argillaceous rocks (engineering soil) at a depth of 5 m beneath multi-storeyed buildings. In order to avoid damage to the overlying structures, the excavation method and the temporary support must ensure that the settlements will be within tolerable limits. Under the specific project conditions, an approach based upon the observational method is not possible. The performance of the ground i not important because arching cannot be relied upon. Since failure could not be allowed to occur, and tolerable deformations during construction would be low prior to the onset of failure, the design has to be less concerned about the variation in rock mass properties and more concerned about the robust structural analysis and detailling of the elements of the support system. In both tunnels, the primary support consists of a reinforced sprayed concrete lining and tunnel excavation takes place under the protection of an umbrella made by bored steel tubes (forepoling). Besides the short excavation stages and the rapid closure of the ring by a reinforced invert lining, a main feature of the applied method is the systematic reinforcement of the face by fibreglass nails and, in the second tunnel, the full face excavation over a height of 8 m. Furthermore, subvertical micropiles at the footings of the shotcrete shell have been additionally used in the second tunnel. Tunnelling caused in both cases settlements of less than 10 mm. In the first tunnel, the average gross advance rate was 0.80 m/calendar day. Due to the higher amount of protective measures (forepoling, face reinforcement, micropiles), construction progress was considerably lower in the second tunnel (0.35 m/calendar day). So the applied construction methods are very time-consuming. Neverteless, they ensure safe execution of the works, lead to minimisation of settlement and are, therefore, appropriate for weak ground tunnelling beneath buildings.

Authors:

Anagnostou, Georgios and Kostikas, Ch. and Iakovidis, G. and Vasilakopoulou, G.

Index Terms:

rock; TunnelingGroup; Anagnostou, Georg; Iakovidis, G.; Kostikas, Ch.; Vasilakopoulou, G.

Further Information:

Date published: 1997