Structural design of Piraeus port metro station

This case study refers to the construction method and the structural design of a new Metro Station at Piraeus Port within the Athens Metro Line 3 Extension. The Station is fully underground and located near the sea docks, under the main traffic artery, close to a landmark building housing Metro Company offices and an existing modern footbridge. Strict requirements such as the Station's large dimensions, preservation of groundwater level and maintenance of the port traffic as well as the difficult hydrogeological conditions led originally to adopting the top-down method with permanent diaphragm walls. Due to technical reasons, it was later decided to change the method to a classic cut & cover structure with temporary retaining diaphragm walls and prestressed steel struts. Various design issues had to be resolved by the Owner ATTIKO METRO SA and the Contractor/designer. The decision to change the construction method to cut & cover instead of top down offered better solutions, nevertheless disclosed controversies in the design and the construction phases. As large ground and water pressures acted on the walls, the 30m long steel struts, which also suffered temperature axial loads, had to be huge square sections of welded thick plates specially fabricated. The structural design of the Station permanent lining had to consider the consecutive construction stages of rising up the concrete building frame while removing struts plus installing the waterproofing membrane. Another major design issue was the large uplift forces on the Station structure from high groundwater. The Station entrances were also constructed as cut & cover using retaining secant piles or short diaphragms with steel struts. A sensitive monitoring system was installed to record deformations and stresses of the retaining structure itself, as well as of the adjacent structures. Finally, proximity to seawater brought about a carefully chosen waterproofing system that covered the whole of the Station and entrances. Construction is now completed and the Station is fully operational.