St. Petersburg is the most northerly city with more than a million inhabitants in the world. Located at the eastern end of the Baltic Sea, on the mouth of the River Neva, its 18 city districts cover an area of 1,431sqkm. It has 300 bridges, 80 museums and 3,500 listed buildings and monuments—enough to earn it a place on the UNESCO list of world heritage sites. But the city is not just one big museum; it is home to over 4.5M people and every day more than half of them use the subway system.
The metro itself is a sight to be seen. Its 63 stations, designed in a wide range of styles, are considered architecturally to be among the most beautiful in the world. The metro is also the world’s deepest subway system, lying more than 100m beneath the surface in parts. This is because Russia’s second largest city, established by Tsar Peter the Great in 1703, was built on a swamp.
Tsar Peter’s motivation was geopolitical—he wanted to give Russia access to the Baltic Sea. The ground beneath the 42 islands on which St. Petersburg now stands is still challenging for construction projects. Its unstable nature means subway constructors have to build tunnels on the solid mudstone bedrock beneath the city. This is why the five metro lines are located at an average depth of 50 to 75m. This situation not only poses challenges for tunnel construction but also presents unusual problems for the shafts connecting stations to the surface. Three escalators next to each other also make considerable shaft diameters necessary. However the biggest challenge is to install a straight tube with a gradient of 30 degrees in the mainly soft clay ground of the swamp area beneath St. Petersburg.
The fact that some of St. Petersburg’s metro stations remained without a connection to the surface for years after they had been completed shows the extent of this construction challenge. Conventional construction techniques such as gradual excavation by temporarily freezing the soft ground and the subsequent concrete lining of the tunnel tubes are time consuming and therefore expensive. Such freezing involves considerable risks. One major problem in densely built-up urban areas is that the ground can be subject to heave when it is frozen and subsidence when it thaws. Before now, TBMs able to cope with these very particular conditions were not available.
The new inclined shaft machine, the Herrenknecht-supplied S-441, took just over two months to work its way along a 105m stretch down to the Obvodny Kanal station at a depth of 65m. This station on Line 5 is one of the ‘sleeping’ stations; people in St. Petersburg have been waiting for it to open for some time. The new canal-side station now finally connects the city’s bus station to the metro system. Until now, passengers have had to walk for 20 minutes through the Obvodny Kanal district to reach the Ligovsky Prospekt metro station, some 1.5km away. This journey time will be considerably reduced following the project.
From the front, this machine looks deceptively like a normal EPB Shield with a diameter of 10.7m. EPB Shields are in their element in cohesive and incohesive soils with high clay and silt content and low water permeability.
However, this new and very specific job required an innovative design engineering approach. The first issue was the machine’s length. The comparatively short tunnel route meant the TBM had to be shortened to just 35m, less than a third of the usual length of an EPB Shield with this kind of diameter.
First and foremost, the back-up housing all the internal logistics for everything from the removal of excavated material to the lining of the tunnel had to be completely redesigned. The gradient meant stairs and platforms had to be added for the use of the operating personnel. Plus, the 30-degree gradient caused a particular problem.
Since the centre of gravity of the machine with cutting wheel and drive unit is located towards the front, great downward forces result. To ward off an uncontrolled sinking of the machine or a deviation of its route, a system of traction ropes and hydraulic cylinders was developed to keep the machine on course. Also, the usual conveyor systems for the removal of the excavated material—belts and spoil cars—were completely insufficient. This was solved using winches to raise two rail-bound cars along controlled routes. On the downward journey, these cars transported the lining segments to the lining segment erector. The lining segments were produced using mould systems supplied by Herrenknecht Formwork.
Starting the machine on a gradient of 30 degrees also required a new approach.
A shield cradle was used to assemble the TBM at the jobsite in a horizontal position at first. Four hydraulic cylinders were then used to tilt the machine into its 30-degree gradient position. However, since the machine was still ‘up in the air’ in this position, it needed a number of blind ring supports to push off from until the first lining segments had been installed.
Tunnelling went smoothly with the innovative Schwanau-built machine in the hands of the Metrostroy specialists, achieving rates of up to 4m a day. The machine drove the 105m long tunnel route at an average speed of 1.5 lining rings of 1m each per day. This eclipses the performance of all previously employed procedures and machines.
The final rings were installed before Christmas 2009, after more than two months of tunnelling. The machine had then been dismantled so that it could be reassembled at a new jobsite in St. Petersburg. Only the S-441’s shield, with a length of around 8m, remained underground as a final element of the tunnel construction.
A replacement for a new ‘sloping’ mission has already been delivered and installed. Its next jobsite is in a prominent location—not far from the western end of St. Petersburg’s famous shopping street, Nevsky Prospekt. There, below the ground, the completed Admiralteyskaya metro station has been waiting to go into operation for several years. This is where the machine started drilling a further project for the St. Petersburg metro in early 2011, its longest access shaft.
At a depth of 102m below the surface, Admiralteyskaya is the deepest station of the deepest subway system in the world. The access shaft will have a drilling length of 160m.
This explains why the station has been spending so long in hibernation. Until now, no technologically and financially viable solutions could be found to make the connection. The Obvodny Kanal job was the first time that engineers were able to open up a viable possibility for such a mission.
There is also a third job in the planning phase, another sloping shaft in St. Petersburg similar to the previous two.
The two shaft locations in St Petersburg A new escalator shaft in the heart of St. Petersburg The pulling forces had to be calculated precisely and then dealt with: the TBM was pulled downwards by a total of 900 tonnes Herrenknecht EPB Shield S-441 constructed for the sloping shaft project in St Petersburg
