Professor Igor Sakharov: “Underground construction in Saint Petersburg is a unique experience”

Igor Sakharov, Professor of the Department of Geotechnics at SPbGASU, has repeatedly served as an expert in the most complicated cases related to the construction of underground structures. He agreed to answer a few questions about the unique characteristics of local soils, the specifics of design and construction of underground structures in the Northern capital’s historic centre, and the challenges faced by Saint Petersburg Metro builders.

– Professor, do the soils in Saint Petersburg allow to design and build underground spaces?

 – It’s hard to name another city in the world with such a difficult soil situation as in St. Petersburg. In the center of the city, the thickness of weak soils is more than 30 meters, so the St. Petersburg Metro is one of the deepest. In the Soviet times, civil engineers practically didn’t work with underground structures. Though, we can remember erecting some communal facilities of this kind – the two huge pumping stations in Olgino and on the Bely Island, with a depth of more than 60 meters. But since the 1990s, the city became overburdened with car traffic, so today, as demanded by the authorities, a one- or two-storey underground parking must be constructed with each new building.

– What are the peculiarities of designing underground structures in the context of preserving historic buildings in the city centre?

 – The challenges associated with arrangement of underground spaces in our city are incomparable with those in Moscow. Moscow has excellent soils, while in St. Petersburg builders and designers have to think also about reducing the impact of constructing a new underground structure on the existing buildings. There are lots of architectural monuments in the city centre. To build an underground structure, it is necessary to apply technical solutions and technology that would not have a significant negative impact on these monuments and would not damage them. These conditions naturally lead to increasing construction costs and timelines. 

✔ The calculations for designing underground structures have become very complex, and now they are mainly computerized. Along with foreign software, there is a number of good programmes for such calculations developed in Russia. Previously, we used to say to customers: "This can be easily done, this is rather difficult, and this is impossible." And today everything is possible. If necessary, you can build a basement under a three-storey building to a depth of 20 meters. Present-day technologies and calculations enable us to do this.

 – What problems did the designers and builders of the new stage of the Mariinsky Theatre encounter? We remember well it was necessary to design deep underground structures there.

 – I am familiar with this project. Some of its stages are reflected in the textbook "Bases and Foundations", which we published together with Professor Rashid Mangushev and for which we received an award from the Government of the Russian Federation. The main problem with the construction of this facility was using the "top-down" technology. Under this technology, walls and columns are erected first from the surface, and then the construction of floors begins, and the builders go under the ground. It took a long time to find a contractor to install a horizontal soil-concrete curtain for the bottom slab of this underground structure, and finally they found a company from Perm.

Of course, failures also happen sometimes. For instance, the collapse of a building on Dvinskaya Street in 2002 comes to mind. I was the chief expert in the investigation of that criminal case, as people died in that accident. Since then, there have been no collapses of this kind, since the qualifications and knowledge of geotechnicians in St. Petersburg significantly exceed the average level across the country.

 – What are the difficulties associated with the design and construction of the St. Petersburg Metro?

 – Metro builders have to make driving tunnels at great depths in dense layers of Cambrian clay. The most important problem is the construction of inclined escalator passages. Our stations and inter-station tunnels are located at great depths, while lobbies are on the surface, and the escalator passage, which runs at an angle of 30 degrees, has a length of more than 100 meters, crossing weak water-saturated soils.

✔ In the centre of the city, all inclined passages were made by the freezing method. This means that an ice-rock “shirt” was first created around the future inclined course, then, under protection of this “shirt”, an escalator tunnel was made. After it was put into service, the “shirt” thawed involving the surface subsidence. The trough had enormous dimensions: 150 meters across and 100 meters along the length of the tunnel. This is a gigantic space, the surface of which was subject to settlement affecting the condition of buildings. Therefore, some buildings on the Perinnaya Line, in Shcherbakov Lane and other places had to be strengthened or even dismantled, many cracks appeared in other houses. For example, old buildings located near Admiralteyskaya metro station were covered with cracks. That situation caused public protests, so new methods of engineering calculations had to be employed. In the early 90s, together with Professor A. Fadeev, we developed the first computer programme that allowed estimating the settlement of buildings around an inclined passage.

Currently, new metro stations are usually combined with buildings. For example, Bukharestskaya metro station is combined with a building where a shopping mall is located. The mutual influence of such buildings and metro stations is another interesting problem.

Today, metro builders and geotechnical engineers work together, and this collaboration has proved to be successful. Underground construction in Saint Petersburg is a unique experience.

I have spoken several times about the new project of the metro circle line, which is now being planned. I have criticized the city government for not providing money for the construction of inclined passages and strengthening nearby buildings. The exit from Teatralnaya metro station, which is now under construction, is a good example. It is necessary to strengthen the foundations of many houses located next to this station exit. All this requires huge expenditure, which is simply not available within the city budget.

– What difficulties were typical in the design and construction of the Lakhta Center tower?

 – The underground space of the skyscraper occupies three floors and has a pentagonal shape. Underneath there are piles 65 meters long and 2 meters in diameter. A wall is made along the perimeter of the underground space protecting it from water. The Gulf is nearby, so the water is very close. The piles, which are the longest in St. Petersburg, were installed from the surface.

 – Does the proximity to the Gulf of Finland mean having difficult soils under the Lakhta Center?

 – No, the most difficult soils are on Vasilievsky Island, on the left bank of the Smolenka river. Builders of the Lakhta Center piled up to hard clay, so the building's settlement is not significant.

 – And what about “Zenit Arena”? Is there an underground space near the stadium?

 – In terms of soils and foundations, there is nothing special about this structure. I am an old-school man and I think that the Kirov Stadium created by architect Nikolsky, our university’s graduate, which had been destroyed for building the new arena, was an important landmark. Certainly, Zenit Arena is also an attraction. There is one problem with the slit in the reinforced concrete where the football field rolls in and out. Moscow firms have installed a sensor system in this place, but there are constant problems. There were also problems with the roll-out field itself, which is too resilient. But it has nothing to do with soil bases or foundations.

 – What difficulties arose during the construction of bridges at the Western High-Speed Diameter?

– This is a subject I know well enough. The motorway was built by Turkish companies using round piles of large diameters – from 1.2 to 1.5 meters. They applied the following technology: the casing pipe is pressed in, then the soil is excavated with an auger, a reinforcement cage is inserted, and when the casing is removed, the pile shaft is concreted. And here, during the auger operation, certain problems occurred with absorbing the surrounding soil, which led to the subsidence of neighbouring structures.
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Bored piles used at Zenit Arena, or for installing supports of the Western High-Speed Diameter, are the technologies that came to us from the West in the 1990s. In the Soviet Union, mostly prefabricated foundations were erected; prefabricated piles were driven or pressed in. One of the few structures built on cast-in-place piles in the Soviet era is the KAMAZ plant in Naberezhnye Chelny. But since the mid-90s, bored piles, including large-diameter ones, have been widely used in Russia. 

Text: Elena Shulgina

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