The following methods are used in tunnel construction:
а/ Cut and cover method in reinforced trenches and in trenches with slopes respectively, depending on the presence of buildings nearby the route, or the presence of free space for digging trenches with slopes. The construction of the metro stations or the tunnels is implemented in these trenches. After that they are backfilled and the surface above is restored. In the so built constructions installing of the equipment and architectural outlining is done. Massive reinforcements with great extent of hardness are mainly applied, thus allowing for 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 St. Nedelja Sq. – housing complex Ljulin and three of the stations from the section St. Nedelja Sq. – Mladost, were built in a similar way. The same method is planned to be used for some of the stations from the second metro-diameter – Central Railway Station, Lavov most, National Palace of Culture and St. Naum.
b/ Milan method is applied when a rapid restoration of the surface above the metro facilities is necessary. The Milan method allows for the simultaneous building of the underground and overground parts of the facilities. This approach is described as “cover and cut/dig”, which means parallel slit walls, or pilots /sheet pile walls/ built and then connected to a monolith plate over them, after which the soil is excavated.
The Milan method comprises both building slit walls and pilots under the base and the fundamental system, prior to launching the tunnel excavation. Concrete slit walls, built with the help of bentonite solution, are used for enclosing walls. They have a double function: side support during the construction process and construction walls of the facility.
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 close to these stations. In the Second metro-diameter, in order to minimize the time for stopping or to limit the traffic on the main streets, this method is envisaged to be applied at MS-8-II in front of TSUM, and MS 11 – II under Cherny vrah Blvd., in Lozenets, as well as for the facilities to change the direction of movement after it.
c/ Shield method. The mechanized shield method is applied in the construction of tunnels in the central part of the city, where the tunnels are of considerable length; where archaeological sites are present, as well as to avoid open excavation works of considerable length along the major boulevards. The method involves mechanized excavation of soil in the front part of a steel cylinder, called “shield”, and assembling the tunnel construction under the protection of the back part of this cylinder. The machine is pushed forward by a system of hydraulic jacks, stuck in the tunnel construction in the back part of the shield.
Because of the considerable depth of tunnels, this method was applied to the section St. Nedelja 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 Patriarch Evtimij Blvd. Due to the location of the tunnels under the level of underground waters, shields with hydraulic counter pressure in the face chamber will be used for both sections. This will prevent sinking of the ground over the tunnels and the negative effect on the building above. The shield machine, which is 85 m long and 1 600 tons heavy, will dig 9 meters daily and will transport about 1 000 cubic meters or 50 trucks of excavated material.
d/ New Austrian tunnelling 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 /lining/ with reinforced cement solution is deployed above it and after that at certain distance a secondary construction /panelling/ of the tunnel is being reinforced and covered with concrete by means of a special movable formwork.
The essence of the “New Austrian tunnelling method” implies transforming rock masses into support elements. It relies on the inherent strength of the surrounding rock mass being conserved as the main component of tunnel support.
The tunnel is usually dug in a full profile; however it can be excavated in parts. The main idea is through embedding deformable supportive elements – anchors, steel arc frames and shotcrete – to form an alleviating arc to turn it into a support element, substituting the lining. The new stable equilibrium is determined by fading of the radial deformations of the excavation, measured on the spot.
This method is applied in masses with random strength with sufficient tunnel covering to form the alleviating arc, and with tunnels with random size of the cross section.
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 specifics 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.
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