Lakhta Center is a multifunctional complex in St. Petersburg with the Gazprom headquarter and public spaces occupying a third of the area. Construction was completed in October 2018, now start work on the arrangement of business and public areas. The opening will take place in about a year, when the complex will be fully prepared to meet guests.
A Skyscraper on a “box” foundation
One of the main objections expressed by the opponents of the construction of a skyscraper which is the tallest in Russia and Europe in the Northern Capital is weak Petersburg soil. According to opponents, swampy soil is not suitable for the construction of a 462m tower because it may glide. Social and Business Center Okhta CJSC Chief Engineer Sergey Nikandrov told Expert S-Z about the technologies providing the stability of the high-rise building.
Drilling to get to Vendian clay
– How do you assess the quality of Petersburg soil in relation to suitability for high-rise construction?
– Despite soil stratification is horizontal in Saint Petersburg, i.e. without any considerable differentials or voids, the areas in the city vary much in relation to the upper layers, in particular. The site in Okhta differs from that in Lakhta. In Okhta it was much more complicated and comprised smaller soft soil lenses at the depth of 80m-110m. We are very lucky with this new site, hard rock occurs fairly close, approximately at 20m from day surface (as compared to Okhta site, at about 40m).
According to an investigation program developed by NIIOSP named after N.M.Gersevanov a detailed investigation of the bearing soil of the high-rise building foundations, Vendian clay of Proterozoic age was carried out. The soil was investigated at the depth of 150m. It turned out that the upper 15m-20m is really very soft, stratified soil on which nothing can be built. Underneath occurs a layer of quite hard clays which are used for piling. If water ingress to this soil is prevented, then drilling is quite comfortable in them, no additional strengthening of the well walls is required. Vendian clays are similar to hard rock in relation to their properties, which may be mounted on during the construction of a high-rise building occur at 30m from the surface. In relation to overall parameters the site is suitable for high-rise construction. In Europe and Asia there are examples of the construction of skyscrapers on soils where hard rock occurs deeper.
– What foundation construction technologies are optimal for soft soil?
– The selection is not so wide: in any case a building weighing 500k tons should be mounted on piling resting upon hard layers. We go through soft soil including sandy loam, loam and sands and mount our building on stronger Vendian clay deposit. 264 piles will serve as a support. They mostly will be deepened into the Vendian clay and taking into account friction occurring on the shaft and the toe which the piles are based on the high-rise building will be absolutely stable. The technology of the construction of heavy-weight buildings on soft soils is the same worldwide. Engineering art implies the selection of an optimal number of piles, their diameter and depth so that a cost efficient and buildable project is designed. This issue is resolved for each project on an individual basis.
– How long are the piles in Lakhta Center?
– Our bored reinforced concrete piles will be of two types: 65m in the center of the building where the loads are the highest. Along the perimeter where the loads are lower they are 55m. Since the settlement in the center of the building will be higher, and along the perimeter it will be lower, in order to mitigate the formation of a bowl, piles of various lengths are designed. It is a standard practice in high-rise construction on a pile base.
– How is the required number and thickness of piles determined?
– High-rise construction is the sphere where the final result is quite difficult to predict based on estimates and drawings only. Thus any engineer’s proposal should be double-checked. The need of testing in practice is connected not only with soil properties but also with construction technologies to be applied, specific contractors and equipment. Each of these elements brings differences in the soil bearing capacity. Therefore we carried out tests by bringing closer the conditions of construction and future operation loading of the building as much as possible. A few piles were selected and based on these the correctness of the estimations was verified.
– How were the tests carried out?
– It is very difficult to test a pile bearing the loading of about 5000 tons and in Russia there is no equipment which would enable to carry it out cost-effectively and locally. That’s why foreign equipment and technology were used, the so-called O-cells. An O-cell is a system of calibrated hydraulic jacks united in a unit. It is installed in the pile shaft directly and divides its upper and lower parts. Then a hydraulic pump installed on the ground surface and connected with hoses to jacks starts delivering pressure. As the loading increases the cell is opened and testing elements displace vertically. Special instruments measure critical loadings.
The tests verified the correctness of engineering estimations carried out by NIIOSP. For many years the specialists of the institute deal with pile foundation design for high-rise buildings, they were involved in the construction of many high-rise buildings in Russia.
– In our country there is no experience in building skyscrapers like Lakhta Center. Were you tempted to invite foreigners to conduct estimates?
– Keeping in mind the unique nature of the project it was a reasonable thing to do. And we invited English company Ove Arup to carry out a verification calculation in relation to the main elements, i.e. for the underground part and foundation pile base as well as for the superstructure. An independent audit was completed last year, all the design parameters and results included in the design were confirmed. The estimations of Moskovits and Englishmen coincided.
– Did you have to adjust a conventional technology to meet the needs in the construction of Lakhta Center substructure?
– There is some kind of uniqueness. Originally we drill piles from the surface directly, so they turn out to be 20m longer than required, i.e. 75m and 85m. The upper parts of the piles will be cut and extracted together with soft soil and at this depth the foundation of the building is laid. The value of 85m is quite serious for any construction company, be it a Russian-based or a foreign company, that’s why the contactor was selected very carefully. A German company, BAUER, was selected which is a world lawmaker in this sphere. They delivered their own plant specially designated for the diameter of our piles. Each pile is monitored. A video camera is lowered in a well, each pile shaft is checked and the quality of cleanup at the depth of 85m is monitored. A mini report is made up in relation to each pile and we are well aware of what result we are going to get.
– What is the design of the foundations?
– The design of the foundations is interesting. It is designed in a form of a box. The main foundation slab will be laid at the depth of 17m and be mounted on piles. The upper part of the box is made from a monolith slab, it will serve as a support for the tower. Between these slabs there are two floors with one floor slab. In addition star-like traverse beams are designed providing additional stability to the “box”. It is a very efficient solution when the main load is concentrated in the center where a lighter stylobat part of the building is located along the perimeter. The substructure will be usable, if you enter this foundation “box” you will not find yourself inside the foundations, the plant floor and auxiliary areas will be located there.
The upper part of the foundations will be 2m high, the lower part will be 3,6m high. The foundation structure will be manufactured from monolith reinforced concrete; we would like to carry out a one-time pouring of each slab on a separate basis. It is unique in a way as well. Such a large-scale pouring, about 22k cu.m. for the lower slab is done rarely both in Russia and worldwide. To compare, last year during the construction of a skyscraper in Seoul Korean builders performed a one-time pouring of the reinforced concrete foundation slab 28.000 cu.m. for a skyscraper 555m tall.
– How do you plan to build foundations on running soil?
– In order to build a box-type foundation almost in water, a protected space is to be created. That is why a reinforced concrete diaphragm wall will be installed along the perimeter of the building at the depth of about 30m. It will make it possible to excavate the soil, install bracing elements and pour the foundations and later on it will serve as an extra protection of the substructure from the ingress of underground water. This work is performed by Petersburg company “Geostroy” possessing appropriate experience in dealing with this type of walls, though perhaps not so deep ones.
The soft soil layer poses problems during pile drilling. A casing is used to protect a well from underground water. As soon as clays are reached, we go down at 5m and a lock made from clay and metal is generated through which no water can percolate. Then it is drilled with a screw without casing, a cage is lowered and concrete is poured. Based on the investigations carried out a decision is taken to produce piles without bentonite (a retentive mix) to improve the bearing properties of the pile base.
– What is the work program?
– The work program is very tight. The diaphragm wall is built in parallel to piling, it is quite an interesting solution. The diaphragm wall will be completed in one month and a half, then “Geostroy” will clean up the area for Bauer to follow. Over 20 piles are completed already, the remaining piles will be poured in July 2013. The tower foundations are to be poured in 2014.
– They say it took Okhta Center a year to find Chief Engineer for their project until they recruited you. Tell us about your experience in the construction of skyscrapers.
– Prior to moving to Petersburg I worked for British company, Ove Arup dealing with unique buildings in the UK and Russia. One of the recent projects is “The City of Capitals” carried out in conjunction with Capital Group. It is a multifunctional high-rise complex which is a part of an investment and construction project Moscow City. Then jointly with SiterInvest I designed the central transport terminal comprising three high-rise buildings with six underground levels, plus five metro lines which go through an underground floor as well as the exit to a metro station built for Moscow City. I managed to be involved in the construction of interesting buildings in other cities as well, for example, HSBC bank tower in London, the center for water sports for World Student Games in Kazan, one of the largest hotel complexes for the Olympics in Sochi, an office complex to be the first to be certified under BREAM green standard in Moscow.
– Will Lakhta Center be the most complicated of the projects built by you?
– I do not think so. But I am sure it will be one of the most interesting.