The following methods of construction are being applied in the construction of the metro in Sofia:
a/ Cut and cover method in reinforced trenches and in trenches with slopes respectively, depending on the presence of buildings nearby the route, or not. The construction of the metro stations or the tunnels is being implemented in these trenches. After that they are being backfilled and the surface above is being restored. In the so built constructions installing of the equipment and architectural outlining is being done. Massive reinforcements with great extent of hardness are being mainly applied, thus ensuring the construction works in close proximity to the existing buildings and facilities without compromising their integrity. In some cases these reinforcements are parts of the supporting structure of the stations and the tunnels. The first stations from the section Sv. Nedelya sq. – Lyulin RD and three of the stations from the section Sv. Nedelya sq. – Mladost RD, were built in a similar way. Same method is applied on some of the stations from the second metro diameter –Lavov most, NDK and European Union, Druzhba, etc.
b/ Milanese method It is applied when a rapid restoration of the surface above the metro facilities is necessary. The sequence of construction stages is as follows: Along the future walls of the construction of the stations and the tunnels, diaphragm walls, without digging a common trench are being built. The diaphragm walls carry the roof plates of the tunnels or the stations. After the restoration of the traffic above the stations or the tunnels, excavation works are being executed under the protection of the roof plates and the rest of the facility constructions are being implemented. This method has been applied partly in the construction of Joliot Curie and G. M. Dimitrov stations, as well as in some parts of the tunnels in proximity to these stations. In order to minimize the time for stopping or limiting the traffic on main roads, this method is applied at stations: Knyaginya Maria Luisa, Central Railway Station, James Bourchier under Cherni Vrah blvd. in Lozenets district, as well as for the facilities for the change of the direction of the movement after it and Vitosha station. Due to the traffic in Sofia being heavier and heavier – large scale of the stations are being envisaged and constructed exactly under Milanese method.
c/ Shield method. Mechanized shield method is applied for the construction of the tunnels in the central part of the city, where the tunnels are of considerable length. Its usage is imposed also by the availability of archeological remnants, and in order to avoid excavation works along the major boulevards of considerable length. The method involves mechanized excavation of soils in front of a steel cylinder, called “shield” and assembling of the tunnel construction under the protection of this cylinder in its back part. Тhe machine is pushed forward by a system of hydraulic jacks. Because of the considerable depth of the tunnels, this method was applied to the section “Sv. Nedelya Sq. - V. Levski Stadium” and also to the section “Road junction Nadezhda – Han Asparuh str.” (just before reaching Patriarh Evtimiy blvd.). Due to the deployment of the tunnels under the level of ground waters for both of the sections are used shields with hydraulic counter pressure in the face chamber, thus restricting the deformations of the soil and the negative effect on the buildings above. At the moment this method is applied in the construction of Line 3 – the section from Little City Theatre “Off the Channel” to Krasno Selo (intersection of Zhitnitsa str. and Tsar Boris III blvd.).
d/ New Austrian tunneling method (NATM). This method is applied effectively to sections with considerable depth from the ground level and limited water flow. It involves working on small sections of the face chamber stage by stage, then a primary structure (lining of reinforced shotcrete) which is reinforced as a secondary construction /paneling/ of the tunnel is being reinforced at certain distance behind it and then it is covered by shotcrete or by the means of a special movable formwork.
This method was applied to the link between V. Levski Stadium and the tunnel under Dragan Tsankov blvd. in front of the National Radio. Due to the specificity of the geologic conditions, deep ground waters and significant depth of deployment of the tunnels from the second metro diameter in the section under Cherni Vrah blvd. between Hemus hotel and James Bourchier blvd., the new Austrian method was applied to their construction. Later the method is used for the tunnel leading to the next station on Line 2 – MS Vitosha, as well as sections from Line 3 currently under construction: the tunnel between Little City Theatre “Off the Channel” and Hadzhi Dimitar district and part of the tunnel in Ovcha Kupel district.
e/ Modified NATM. Soils excavation and the construction of the stations by this method are carried out in 5-6 phases. It is characteristic for more complex geological and hydrogeological conditions and more significant depth (about 22 - 24 m), as well as the need for a larger cross-section (21 m).
This method is applied to one of the most difficult sites of the completed sections - Serdica metro station from the second line. In view of preserving the archaeological finds discovered during the excavation works on the station, its project was updated and the platform was built according to the modified version of the New Austrian Tunneling Method. The cross-section of the station comes to 258m2 and is one of the largest in Europe. It is of arch type with a width of 21m and height of 16 m. Due to the large size of the cross section, the adverse geological conditions and ample amount of groundwater, its construction is one of the most complex construction sites applied in our practice.
Metro Station of Line 3 which is situated at Orlov Most (Eagle’s Bridge) is also very complex and is being constructed under this method – a central shaft by a size of 30 x 20 m. South of the station the tunnels of the Line 3 pass under the existing tunnels of Line 1 which is why the bottom of the metro station is located at a considerable depth of 26 m. The station will be connected to the existing metro station of SU St. Kliment Ohridski.