Employer: Sofia Municipality and Taisei Corporation, Japan

Project execution period: 2007 – 2008

Description of the project:

Geotechmin OOD is designing and constructing a connecting tunnel between the existing tramway tunnel under boulevard “Dragan Tzankov” and Metrostation 9. These works are part of the project for the construction of the first metro diameter of the Sofia Metropolitan.

The tunnel section under construction is 208 metres long and has a longitudinal slope of 0.0375 in the direction of MS-9. The earth cover along the tunnel section varies from 11 m at MS-9 to approximately 7.60 m.

The works along the route of the metro-tunnel include the construction of a straight section, a transitional curve and a circular curve. The contour of the metro-tunnel fits the contour of the existing tramway tunnel gradually leaving the tramway section to enter the new section of the tunnel route towards MS-9.

The designed tunnel route is separated into the following 4 sections (with 7 sub-sections):

Section А
This section length is 96 m, between km10+296.00 and km10+392.00. It is traced in a straight line. The overall dimensions of the new metro-tunnel are the same as those of the existing under boulevard “Dragan Tzankov” tunnel. The slope of the new tunnel is bigger than the slope of the existing one. For that reason, a reconstruction of the bottom of the existing tunnel becomes necessary.

Section В
The section length is 68 m, between km10+228.00 and km10+296.00. It is traced in a straight line, a transitional curve and a circular curve, and it is separated into three sub-sections. This is the first tunnel sector which has the new cross-section adopted for the metro-tunnel. The upper part of the cross-section almost fits the overall dimensions of the old tunnel cross-section. It has become necessary that the ceiling of the old tunnel is demolished at a depth of 25 cm. At the same time, the piedrit galleries, which have been injected with concrete class В15, are partly demolished.

Section С
The section is situated between km10+197.00 and km10+228.00. Its length is 31 m and it is traced in a circular curve. The new tunnel construction is the same as section В. Due to the relatively large dislocation of the new tunnel against the existing one, the technical solution of building a support slab of reinforced concrete over the new tunnel cross-section was adopted. The slab is placed at a minimum distance of 60 cm from the outer contour of the tunnel cross-section.

Section D
This is the last tunnel section before the connection with MS-9. It is situated between km10+189.00 and km10+197.00. The new tunnel cross-section does not affect the existing tunnel, but passes under it. The construction of this sector is the same as that of the other sectors. The planned lining has the same parameters as the one for sections В and С.

The section connecting MS-9 with the newly constructed tunnel is located between km10+178.00 and km10+189.00. Its length is 11 m. The shape and the geometrical dimensions of the cross-section are the same as those of tunnel sections B, C and D. The supporting structure consists of primary and secondary lining with waterproofing between the two structural elements.