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Despite the fact that the underground tunnel boring methods, either using the TBM or conventional mechanical means (NATM), are preferably used in central areas of the city, as we move away from the said areas we turn to the cut & cover method for the construction of both tunnels and Metro stations. This method is also used in case there is available area even if we are in the city centre. This happens because the cut & cover method is cost effective and more simple, safe and easy to control in its implementation. The disadvantages of the method as regards its implementation are as follows: a) all PUO pipes located in the area where excavation works are to be executed should be removed, b) an archaeological investigation should precede in order to identify any antiquities – which is very important for Athens, and c) all required traffic diversions should be effected. These interventions are time consuming, increase the cost, while at the same time the archaeological investigations involve great uncertainty as to the duration and their final cost.

Although the method is simply called “open cut”, in fact it is a “cut & cover” method, since the structures upon their completion are backfilled and they finally become underground, as the case is when construction is made using the underground boring method is used.

Construction methology

The methodology of the cut & cover is simple in terms of conception. At first, the trench is excavated and its slopes are appropriately retained – as to the Metro works, the slopes are always vertical. Then, the permanent bearing structure of the station or the tunnel “is built”, starting from the foundation upwards, i.e. as this is the case for an ordinary structure. Finally, the structure is backfilled up to the surface of the soil and the area is reinstated. In particular, the phases are as follows:

	A geotechnical/geological investigation and tests (on site and laboratory ones) are executed in view of identifying the soil characteristics in the area where our structure is to be constructed.
	A design is prepared (calculations and drawings) related to the excavation and the temporary retaining, based on the geotechnical characteristics of the soil which resulted from the previous stage. Moreover, the design of the permanent bearing structure is carried out.
	Prior to the commencement of the main works, the required archaeological excavations are carried out, all the PUO pipes (related to water supply, power supply, telephone connection etc.) and the eventual traffic diversions are executed.
	The temporary retaining of the excavation usually consists of circular concreting piles, whose diameter is in the order of 0, 80 – 1, 00 m., spaced at 1,50,-2,50m along the perimeter of the anticipated excavation prior to its commencement. The pile row is connected at its pile cap by means of a strong concreting beam. The excavation is carried out using conventional mechanical means (excavators, hammers etc.) up to a fixed depth, e.g. 3,5m and then anchors are placed at holes, which are drilled at the soil through piles. These anchors are long enough (in the order of 15-25m) and they are prestressed using the force provided for by the design. Then a wire mesh is applied along the perimeter of the trench and a shotcrete is placed. Subsequently, the excavation continues up to the next level and another series of anchors is placed and prestressed. This cycle continues up to the final level of the excavation, where the structure will be founded. If there is ground water at the surface of the trench, then the said water is discharged through systematic holes/piping at a depth of 3-4m. on the retaining structure/excavation and they are pumped using the appropriate drainage system.
	The water proofing system of the structure, as the case is for the entire new Metro network, is placed at the invert and the peripheral surfaces at the perimeter of the trench and it consists of geotextile, waterproofing membrane and water stops.
	The construction of the bearing structure is carried out in phases starting from the foundation, and then follow the walls, the roof slab in case of a tunnel. As to the stations, the construction of intermediate flat slabs and walls. The construction commences with the installation of the steel reinforcement of the foundation slab (or general lean concrete slab), as provided for by the design. Subsequently, class C25/30 concrete is injected, in phases along the entire length of the construction with the provision of appropriate joints. The construction of the remaining elements of the permanent structure is made in a similar way.

With regard to the retaining works, it is clarified that the retaining of the excavations at the Athens Metro was executed exclusively with the use of drilling piles made of reinforced concrete (shaft piles) and prestressed anchors. At the first sections, the “Berlin method” was used; based on this method steel piles are placed up to a depth retained squarely with the use of steel struts, while at layers of the subsoil which are at a greater depth, a lighter retaining is used with reinforced shotcrete and passive ground bolts. This methodology was used at Larissa Station and at a large section of Attiki – Larissa tunnel section.