The drifting sands or a reason of two incidents at St. Petersburg Metro
This October, Saint-Petersburg saw opening of the three new subway stations. Actually, they should have been opened a month earlier, but in the end it was decided to postpone the event. As Alexander Beglov, the Governor of Saint-Petersburg, explained, ensuring passenger safety was much more important than matching the opening with certain dates. The official opening ceremony of new stations (Dunayskaya, Shushary and Prospect Slavy), however, happened a month prior to the commissioning date and was shortly followed by numerous sarcastic remarks in the press and social media. Admittedly, projects of this scale should never be rushed, and the majority of city inhabitants agreed on that.
There is a version that in 1974 the need to speed up construction works led to technology violations when building interstation tunnels between Lesnaya and Ploschad Muzhestva metro stations. Back in those days, circumventing the drifting sand, through which main line tunnels were supposed to be laid, was considered impossible. Therefore, it was decided to brine-freeze the soil with a solution of calcium chloride, resulting in the temperature of minus 32 degrees Celsius at the mine face.
St. Petersburg is located at the junction of two large tectonic structures - the Baltic Shield and the Russian tectonic plate - the latter one of which consists of malmrocks, limestones and clay layers. Consecutively, underground works are complicated. The most challenging environment to work in are the so-called paleovalleys - old riverbeds. They are filled with fine-grained soil, the particles of which are covered with thin-film water and remain in suspended state, called drifting sands or flow rocks. If mechanically affected, drifting sand is able to flow and permeate the quere.Anatoliy Protosenya, Dean of the Faculty of Construction at St. Petersburg Mining University
Traditional technology of artificial freezing failed, however. The solution went into the soil on several route sections, and huge masses of drifting sand started filling the tunnels, leading to the first accident. The tunnels got flooded, several buildings on the surface appeared to have cracked, and the roads became covered with pit holes.
To eliminate erosion, the most advanced technology of that time was used - freezing with liquid nitrogen. As a result, temperature of the layer was lowered to -70 degrees Celsius, and at the end of 1975 maintenance works at the line extending from Ploschad Lenina to Akademicheskaya were completed. Nevertheless, 20 years later a second erosion happened.
The tunnels were located just one above the other, and water kept soaking into them - about 60 cubic meters per day. Though it was being constantly pumped out, small particles of soil, of which drifting sands are composed of, were seeping out together with water. In the end, soil found its way out of the tunnels and they began sagging, which further caused their depressurisation. The volumes of inflowing liquid were so high that it was decided to stop using the tunnel.
At that time, Anatoliy was a member of the commission established under Saint Petersburg City Administration, which was facing a difficult task - to find technical solutions that would eliminate the consequences of the second erosion. The сommission was comprised of the city’s best engineers, including scientists and researchers from the Mining University who offered a few innovative solutions - in particular, methods of tamponage and tunnel lining construction.
In order to provide a solution for tunnel construction in difficult geological conditions, members of the commission proposed to use slurry shields, which allowed to ensure work safety and exclude the possibility of significant soil compaction at surface level and thus guarantee preservation of buildings and other facilities constructed above the flow rock.
The tunnelling shield Victoria was put into action in February, 2002. It was supposed that negative consequences of the second erosion would have been liquidated by the 300th anniversary of Saint-Petersburg, but repairing works had to be suspended several times, resulting in moving the final date of construction activities. Uninterrupted passenger services between the stations Lesnaya and Ploschad Muzhestva became available only on June 26, 2004.
Over 15 years have passed since that date, but experience gained in dealing with incidents of such scale is still in demand. Scientists at Saint-Petersburg Mining University are pushing through their academic pursuits and continuing research in the field of underground space development, whereas their method of using slurry shields for tunnel driving was acknowledged as an advanced technology and is still actively applied in practice. In particular, it was used for constructing inclined tunnel at the Obvodny Kanal metro station and double-track tunnel for the new section of metro line, extending from Prospect Slavy to Shushary.