Prague subway's defensive role

Skanska has been contracted to extend the subway system in the Czech city of Prague by 4.6km for US$133M. The four year extension to the C Line is being carried out by a consortium comprising Czech companies Metrostav (36.6%) and Subterra (26.7%), together with Skanska CZ (36.7%), the local arm of contractor Skanska and the largest construction company in the Czech Republic.

The Municipal Transport Company of [the] Capital City of Prague awarded the contract to the joint venture on 16 April. Initial construction activities started in early June.

The tunnels have a dual purpose as both an underground transport system and a civil defence shelter. Driven as twin tubes, they converge to a single tube at the stations. The bored tunnels comprise of 42% of the underground length with 58% being in cut and cover.

The tunnels will be built using NATM techniques, to an 8.9m diameter profile with a primary shotcrete lining. A secondary concrete lining will be poured using hydraulic formwork supplied by Ostu-Stettin.

The geology varies from made ground to residual loam, marlstone, rock clay, glauconic sandstone with quartzite and carbonic rock clay. The contractors intend to use Atlas Copco drill rigs, Atlas Copco access platforms, Meyco shotcrete machines and Volvo 120D loaders.

Currently the execution drawings and method statements are being finalised while the worksites are being prepared. The joint venture is also completing technical negotiations with relevant authorities and utility companies about its methodology.

The tunnelling equates to approximately 60% of the complete project cost. The scheme is being financed by the Municipal Transport Company of Capital City Prague, the City of Prague, Czech Republic state funding, the European Investment Bank and also NATO, which perhaps explains the dual civil defence role.

The extension is being built to allow the subway to pass underneath an area of dense infrastructure. It is scheduled to be completed by the end of September 2007, but there could be challenges ahead for the tunnelling teams if they are to meet this date, due to the history of mining activity in the area from the Middle Ages.

There are no complete maps of the old galleries, drives and exploration drifts in existence and if the obstructions are found to be excessive, then an alternative programme that includes an extra access point will have to be implemented.