Employer: Sofia Municipality and Taisei Corporation, Japan

Project execution period: 2005 – 2009

Description of the project:

Metrostation 9 (MS-9) is situated in the immediate proximity of Dragan Tzankov Blvd. and Evlogi Georgiev Blvd. intersection. The reason for this choice of location is the possibility to use the existing tunnel "Dragan Tzankov".

As a part of this project, a start shaft for vertical access of the tunnel-boring machine (TBM) was constructed in the direction of MS-8.

Metrostation 9 is entirely subterranean, at 4 levels comprising of an underground entrance hall, 3 levels of car parks and side station platforms. The access to the station is done via exterior amphitheatrical stairs naturally connected to the surrounding park landscape. The depth of the station is chosen in accordance with the necessity for tunnel crossing under Perlovska River and the possibility to use the existing Perlovski drainage collector. The station’s structure is entirely monolithic and complies with all requirements for subway lines, technological equipment and architectural design. In geological engineering respect, the foundations of the station are built onto alluvial sediments with high subsoil waters – the average water level below the existing terrain being 8.60 m.

Construction technology:

In technological respect, the construction of MS-9 is based on the Milan method of subway construction.

The first stage of excavation works is up to the level of diaphragm walls. At the next stage, a dewatering system was constructed comprising of 3 drill wells with diameter 800 mm. The wells are equipped with an automated pump system which keeps a pre-set water level. At the same time started the installation of 139 parallel diaphragm walls at a depth of 22 m. After their completion the excavation of the trench between the diaphragm walls was commenced up to the level of the bottom edge of the Milan slab. The diaphragm walls were simultaneously strengthened with one line of temporary 12 m long IBO anchors and four lines of permanent 22 m long rope injection anchors. The excavation works to the bottom of the Milan slab were followed by the execution of reinforced concrete cast-in-situ supporting piles in pipes with diameter 325 mm. The very construction of the Milan slab started after the completion of these preparatory works.

In order to speed up the excavation work under the Milan slab, Geotechmin proposed implementing the works from two headings – from joint 1 and from joint 3. Consequently, a technological opening was left in the Milan slab, between the vertical shaft and joint 1. This decision turned out to be extremely effective and at present the technological opening is used both to serve the ТВМ and for inserting the rails for the subway trains.

After reaching the level of the bottom slab, a drainage system was constructed in the direction of connector "Dragan Tzankov". Dual seal waterproofing membrane is used for the waterproofing of the bottom. The building of the bottom slab was followed by the construction of support walls with embedded piles. The used shuttering covers a total area of 800 sq.m. In addition, one-sided and two-sided straight and curvilinear formwork with height of 8 m was designed especially for MS-9. The installation of the formwork and the reinforcement is carried out by two Libher tower cranes. The bigger tower crane has a 45 meter-long boom and is situated between the start shaft and joint 3; the smaller one is used in the area after joint 3.

At MS-9 it was for the first time that dilation joints were executed at the diaphragm walls. Also for the first time openings were made in the diaphragm wall between the start shaft and MS-9 by cutting with a diamond rope.

Beside the above mentioned activities, in the conditions of extremely heavy traffic Geotechmin organizes the transportation to a landfill of over 70,000 cub.m. excavated material from the two tunnels driven by TBM.

Conducted works:

1. Temporary construction

• Mobilization and security
• Temporary traffic arrangement
• Fencing of MS-8 and MS-9
• Construction of car washes – 2 units
• Relocation and/or strengthening of existing engineering infrastructure
• Site power supply and lighting
• Site water supply and sewage

2. Cleaning, leveling and mass excavation on MS-9

• Excavation and transportation of earth material – 108,500 cub.m.
• Cleaning and leveling – 11,025 sq.m.
• Strengthening of the excavation slopes with PVC-mesh, IBO anchors, piles and reinforced sprinkled concrete

3. Structure works on MS-9

• Construction of guide curbs – 333 m
• Diaphragm walls – 139 pcs.
  - excavation works – 6,740 cub.m.
  - reinforcement works – 1,390 tons
  - concrete works – 7,000 cub.m.
• Installation and testing of anchors – 18,400 m
• Excavation, including by Milan Method – 60,400 cub.m.
• Concrete works – 19,800 cub.m.
• Reinforcement works – 2,254 tons
• Formwork – 32,623 sq.m.