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The underground tunnel boring method using conventional means (known as NATM method or New Austrian Tunneling Method) is the second (in terms of preference) construction method applied internationally for the construction of tunnels using the underground boring method. The Tunnel Boring Machine (ΤΒΜ) is the method, which is preferably used for the construction of tunnels.

In urban areas where Metropolitan Railways (Metro) are constructed, it is important not to disturb the functions of the city even if this implies increase in the financial cost of the projects. Using the underground construction methods for stations and tunnels, the occupation of areas at the surface (squares, streets, private plots, etc), the relocations of PUO pipes (water, power, telephone supply, etc) traffic diversions and archaeological excavations are avoided.

As to the Athens Metro, the NATM method was widely used both for the construction of tunnel sections and some of the Stations at the center of Athens. In particular, it was used for the construction of PANEPISTIMIO, AKROPOLI, AMBELOKIPI, MONASTIRAKI, OMONIA and SYNTAGMA Stations. Moreover, the said method was applied at large sections of the network extensions to Doukissis Plakentias, Aghios Dimitrios, Aghios Antonios and Egaleo.

Construction Methology

The basic principle of this method is to maintain the strength of the environment at the surface surrounding the tunnel and fully utilize it. Controllable soil deformation with the use of flexible retaining – contrary to previous views concerning “heavy” retaining-has positive effect and has as a result the safe development of the soil strength. The methodology of the project design/construction is the following:

	Geotechnical/geological investigations and tests are executed (on site and laboratory) for the identification of soil characteristics in the area where the tunnel has been planned to be bored.
	The design (calculations and drawings) of the excavation and the temporary retaining of the tunnel is under way based on the geotechnical characteristics of the soil, which resulted during the previous phase. Moreover, the design of the permanent (final) lining of the tunnel is prepared.
	The excavation is executed using conventional mechanical means (road header, conventional excavator, etc) and sometimes the excavation front is directly retained at several phases, depending of the quality of the soil.
	Upon completion of the excavation, which is gradually performed depending on the characteristics of the rocks and the project, there follows a system of temporary retaining consisting of shotcrete lining (gunite), rockbolts, steel frames etc. In case of soil with poor characteristics, prior to the excavation, forepoling beams are installed in the entire area over the tunnel vault in the form of an umbrella providing protection to the excavation front. Frequently, excavation is performed in two phases, the upper semi-section (vault) and the lower semi-section (invert). Depending on the subsoil and the geometry of the tunnel the excavation can be performed in more than one phases. The time of installation of the initial retaining, as well as the completion of the full ring of the lining are important for the monitoring of deformations. The system of direct support, along with the soil surrounding the tunnel constitute the bearing structure of the tunnel at this phase. Ground water can be often encountered at the Athenian subsoil; in this case, systematic pumping is performed during the construction.
	Throughout the construction, the behavior of the subsoil and the temporary retaining are monitored on a systematic basis, i.e. the settlements at the soil surface and the adjacent buildings any convergence within the tunnel, the increase/decrease of ground water level, etc are measured. Safety of the buildings located adjacent to or over the alignment of the tunnel is a particularly crucial issue and it is addressed via continuous monitoring by means of the appropriate instruments and on site visits by ATTIKO METRO engineers. The results of the measurements are compared with the assumptions and the results of the design and, if needed, the necessary modifications to the support system and the time sequence of works are made. In addition, these data are used for the identification and/or the checking of the assumptions of the design of the permanent lining of the tunnel, which will subsequently follow.
	The final (permanent) lining of the tunnel is constructed when the system of the initial support has reached conditions of balance. The permanent lining provides increased safety as to the project lifetime creates a unified interior surface and improves its water tightness.  The permanent tunnel lining is made of in situ cast reinforced concrete. Special segment metal forms, usually self-supporting ones, are used, thus significantly reducing the time and the cost of the project. There are hydraulic levers, which can adjust the desirable thickness of the lining. The overall length of such moulds is in the order of 10-12m. depending on the section. Firstly, the lower part of the tunnel (invert) is constructed and special water stops are placed at the construction joints for waterproofing. At a later stage the vault is concreted with the use of self-supporting segment metal forms. Upon completion of injection, it takes some hours to remove the metal forms. In view of achieving adequate concrete strength rather shortly, its mix is enriched with chemical admixtures. Given that there is a small void between the crown of the concrete and the soil at the tunnel top, there follows cement grout injection for filling the voids.

Water tightness

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In the Technical Specifications of the Project, the required degrees of water tightness for the various parts of the Metro structures are specified. In the Base Project, the stations were specified to be fully watertight, while at the tunnels the existence of restricted areas of humidity at the points of the construction joints was acceptable.

The basis for the sufficient waterproofing of the underground projects is always the design and workmanlike construction. Special attention should be paid to the concrete mix, the compaction and maintenance after the laying procedure as well as the adequate coverage of the reinforcement. As to the Metro tunnels, it was not required to place a waterproofing membrane, while the limited water penetration was acceptable. At the new Metro extensions, the specifications were even stricter and it is required to place a waterproofing system even at the tunnels of the Project.

At the Metro stations, any penetration and surface damp patches are not acceptable since these locations house passengers or personnel rooms, machinery and electrical installations areas, architectural finishes etc. In order to ensure the above, water tightness systems are used with materials and work of appropriate quality. Waterproofing membranes are usually made of PVC or polyethylene and are placed between the temporary and the final lining of the tunnel, protected with geotextile. The parts of the membranes are welded in an appropriate manner, while at the locations of the construction joints (concreting interruption or joint displacement) water stops are placed. All materials are subject to tests placed on site the project and adhere to German specifications DS 853 and DIN 16726.