The tunnel boring machine, which is building the extension of the metro through "Slatina", has reached the "Sitnyakovo" metro station, which is located on the boulevard of the same name and "Geo Milev" Str. This is an important stage in the construction of the underground section and means that the machine has already passed through most of the planned route, announced by "Metropolitan" EAD.
The station on "Sitnyakovo" Blvd. is the last in the six-kilometer section that the machine must reach before continuing to the exit shaft and completing its work.
The metro through „Slatina“ is one of the 6-km branches of line 3 and uses the specially manufactured for Sofia machine „Vitosha“. The section connects several areas – „Oborishte“, „Slatina“ (the „Geo Milev“ and „Hristo Smirnenski“ neighborhoods) and reaches „Tsarigradsko Shose“ Blvd. in the area of the „St. Anna“ Hospital in „Mladost“.
„Zavod pod ziemata“ with laser precision
So far, the tunnel boring machine has built approximately 3,750 meters, or 3.7 km of tunnel. This represents about 80% of the total length of the tunnel. After reaching the “Sitnyakovo“ metro station, the machine faces the final stage - the section that connects the station with the exit shaft of the route.
The cylindrical tunnel boring machine, also called a shield because of the rotating working mechanism in its front part, weighs about 1,600 tons. It is 86 meters long and its head has a diameter of 9.4 meters. In operating mode, the machine advances an average of between 10 and 15 meters per day. The shield method is part of the specifics of the construction of the Sofia metro tunnels and is among the most modern technologies used in underground construction.
Because of the way it works underground, it is also colloquially called the “mole“. But for specialists, its scale, precision and engineering complexity have been compared to an “underground factory“. The shield is controlled by a special computer center located in the machine itself. The direction is monitored with extreme precision by a laser, which controls compliance with the plan and profile of the route. Physical guidance is carried out by supplying pressure to the hydraulic jacks. The tunnel is built from prefabricated concrete segments. For the entire tunnel route through “Slatina“ over 22 thousand segments will be used.
Tunnel with a diameter of 9.05 meters
The construction of the section began at the end of 2023. The route is entirely underground, with a two-lane tunnel with an external diameter of 9.05 meters being built. The tunnel boring machine began work on February 6, 2024 from the starting shaft at Tsarigradsko Shose Blvd. in Mladost. From here, the underground facility began construction of the tunnel. The future Trakia station will be located at this location, located entirely under the local roadway of the entrance-exit artery of Sofia. On its way, the machine passes through the “Arena Sofia“ station, located under the eastern roadway of “Assen Yordanov“ Blvd., which will have one central vestibule. Next is the section to the “SK CSKA“ station, located across “Shipchenski prohod“ Blvd.. The machine builds the tunnel, sequentially connecting the stations.
The next section reaches the “Slatina“ station, located under the roadway of “Geo Milev“ St., west of the Slatinska River, and after it - to the “Geo Milev“ station at the intersection of “Nikolay Kopernik“ and “Geo Milev“ streets. The machine just passes through the stations, as their structures are pre-built.
The first use of a tunnel boring machine in the Sofia metro
For the first time, a tunnel boring machine was used in Sofia when digging a section of the first metro diameter in the period 2006-2008. Then, two single-track tunnels with a diameter of 5.8 meters were built - from “St. Nedelya“ to “Vasil Levski Stadium“ and from “Independence“ Square to “St. Kliment Ohridski“ Sofia University. Later, in 2010-2011, the same technology was used in the construction of a section of the second metro diameter - from the “Nadezhda“ overpass to the intersection of “Patriarh Evtimiy“ Blvd. and “Vitosha“ Blvd.. Then a two-lane tunnel with an external diameter of 9.05 meters and a length of 3.8 km was built.
One of the most serious challenges in the construction of the tunnel in “Slatina“ is the need for the route to also comply with a large number of existing engineering networks and underground infrastructure.
In the various sections, the machine passes near water pipes, sewage networks, electrical infrastructure and other communications. This requires precise planning and constant control, since the tunnel is being built in the conditions of an already existing and actively used urban infrastructure. In such a large-scale infrastructure project, each part of the route has its own specifics and requires careful organization and coordination, also indicated by „Metropolitan“ EAD.
Another 900 meters to the finish line
Approximately 900 meters remain until the completion of the construction of the underground route through „Slatina“. This is the last section between „Sitnyakovo“ and the exit shaft, located in the yard of the „G. S. Rakovski“ Military Academy. The final point of the facility's work is planned there. This is expected to happen by the end of the year. This will complete the main stage of the mechanized excavation of the underground route, and the underground plant will be dismantled into its component parts.
Until then, the “mole“ continues its journey under Sofia. Above it, the city lives in its usual rhythm, and underground, the new connection is already taking shape, along which the metro trains will pass in time.