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. It is intended to be used the same method for some of the stations from the second metro diameter – Central Railway Station, Lavov most, NDK and Sv. Naum.

 

 

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 it is intended to be used for the tunnels in the section “Road junction Nadezhda – Han Asparuh str.” before reaching Patriarh Evtimiy blvd. Due to the deployment of the tunnels under the level of ground waters, it is envisaged for both of the sections to be 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. 

Fig. Construction of tunnels with mechanized shield

d/ New Austrian tunneling method. This method is applied effectively to sections with appropriate deployment of the terrain and limited water flow. It involves working on small sections of the face chamber stage by stage, then a primary construction /facing with reinforced cement solution/ is deployed above it and after that at certain distance a secondary construction /paneling/ of the tunnel is being reinforced and covered with concrete by 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 Cherny vrah blvd. between Hemus hotel and James Bourchier blvd., the new Austrian method will be applied to their construction. In the section after the metro station under Cherny vrah blvd., due to the significant width of the facilities, shallow deployment, high level of ground water, and the presence of ventilation system facilities above it, the application of this method is not appropriate.