Floating tunnels under the Vltava River

In 2002, the tunnels under the Vltava River between the stations Nádraží Holešovice and Kobylisy were awarded the prestigious prize for outstanding concrete construction by the International Federation for Structural Concrete (fib) at the 1st fib Congress in Osaka, Japan. How did the story unfold that led to the implementation of the tunnels under the Vltava River? Read on.

For the routing of the new section of Line C, the location of the then final station Nádraží Holešovice at the Vltava riverbed and the newly planned Kobylisy station on a steep slope on the opposite bank of the river was decisive. Due to the maximum possible gradient of the track sections of 4%, the option of tunnelling under the riverbed was excluded. The option of bridging the river was also rejected and the metro line was therefore designed on the river bed. The traditional construction procedure of building the tunnel in 3 pits was considered, with the construction time estimated at three years.

During the tender period, Metrostav a.s. was already considering tunnel construction variants, which would accelerate the progress of the works. Initial considerations were based on floating Dutch tunnels or a variant with an analogy to drawbridges, but for various reasons these variants were unsatisfactory.

The chosen option combined the procedure of ejection and flooding. The principle was that the whole tunnel was concreted in a dry dock and then moved as a whole to its final position in the river bed. When the tunnel was moved, it was relatively light due to the buoyancy of the water and could therefore be suspended from a pontoon at the front and allowed to slide along a pre-prepared track at the rear. Pre-tensioning of the cables was provided by an additional rear brake hinge. 

The opening phase of the whole construction procedure consisted of the construction of a dry dock, which is a construction pit secured by underground walls at the point where the tunnels continue. It was secured against the Vltava River by a sheet pile wall to a height of about 13 m. The first tunnel tube was concreted in the dry dock. At the same time, a trench was excavated in the bottom of the Vltava, where the tunnels were finally placed and backfilled. The tunnel was concreted in 12 m long segments, of which there are 14 in total, making the total length of the tube 168 m.

After construction, the ends of the tunnel were closed with steel lids and the dry dock was flooded. The tunnel was lifted by hydraulic cylinders, activating hydrostatic buoyancy. The tunnel was suspended approximately 1/3 of its length from the front end on a pontoon, the rear of the tunnel resting on a sliding device on a track in the dry dock. After the sheet pile wall was opened, the tunnel slid in a prescribed path to slightly above its future position until the tunnel face entered the opening in the sump on the Holešovice side. The tunnel was then finally set on the prepared end supports and gradually concreted and anchored into the river bed. The dry dock on the Troja side was sealed, closed and pumped out. The procedure was repeated for the second tunnel.

The watertightness of the tunnels under the Vltava River is ensured by the use of waterproof concrete, completely without the use of any waterproofing. For this reason, particular attention was paid to the concreting procedure. The 12-metre-long segments were concreted in a single working span to limit the number of working joints as much as possible. The concreting had to be slow enough to allow the concrete to harden sufficiently and fast enough to avoid unwanted working joints due to long breaks between concrete deliveries. The sealing of the working joints was multiple. The main protection was provided by a 240 mm wide rubber sealing strip. Additional sealing consisted of a bentonite strip and grouting tubes, which were used in case of water penetration.

The 2002 floods were a great test of the tunnel construction. The water caused almost no damage to the tunnel; the tube shifted only by about 30 cm. If the tunnel had been constructed using the originally proposed method of pitting, the damage would undoubtedly have been significantly higher.