The Sofia Metropolitan is the rapid transit network servicing the Bulgarian capital city Sofia which began operation in 1998.  Construction of the Metro was not launched until the late 1990s mainly because the city did not experience an urgent need for an underground system. Another factor was the depth, as being one of the oldest cities in Europe, Sofia contains many historical layers underneath its central areas. From 2015, the Sofia Metro consists of two interconnected lines, serving 34 stations, with a total route length of 38.6 km. The Metro links the densely populated districts of Lyulin – Mladost (Line 1 – Red) and Nadezhda – Lozenets (Line 2 – Blue). Also, line 1 has been servicing the Sofia Airport. The general scheme for the development of the lines should consist of three lines with extensions in the periphery, with a total length of 62 km, 63 metro stations, and a 1.1 million daily passenger capacity at the final stage of implementation.

Fig. 1 Red (1), blue (2) and new green line (3) in Sofia Metro

Extension of the line
The route of Metro Line 3 was approved after public hearings and expert council meetings during the second half of 2011. as well as a partial update of the Expert Council in 2012. The 16 km long third line is planned to connect Ovcha Kupel neighborhood (in southwest Sofia) and Vasil Levski neighborhood (in northeast Sofia). With 8 over ground and 11 underground stations, the line will be put into service during 2018. The project design contract was awarded to the Czech company Metroprojekt Praha a.s. A tender for construction of the central section of the third line was announced in 2015. The tunnel of the central section shall be excavated by a TBM, while the construction of stations shall be awarded to other companies. And this is why a new invitation for the project and construction for the extension of metro line 3 is open.

The new section in Sofia metro from km 11 + 941.33 to 15+ 749.00 will include 4 stations 15, 16, 17 and 18, and duration of the contract will be 36 months. The contract is divided into 2 lots. Lot 1, from km11+ 941.33 to km 14+277.56, with 2 underground stations, value 47.03 million euros. Lot 2 from km 14+ 277.56 to km 15+ 749.00 m with 2 underground stations and a railway stop has a value of 43.97 million euros. This contract covers detailed design in all project areas, construction, supply of necessary equipment and commissioning of a section of metro 3 as required by the Contracting Authority.

The total length of the lot is 3805 m. In the section of track between stations 15 and 16 a tunnel is designed, with a length of 1184.98 meters, including ventilation system.  Some of tasks which are required by design are: twin metro route, performed in an open way, in unfortified or fortified pit depending on the type of construction, station 15 will have 3 approaches equipped with stairways, lifts and escalators etc.  This is a project of a European Regional Development Fund (ERDF) and Operational Programme of Transport Infrastructure in Sofia for a period 2014-2020.