Geotechnic Aspects of the Okhta Tower Design n

The upward tendency was always present in the world architecture. The earliest high-rise building, extant so far, is the 139-meter Pyramid of Khufu erected more than 4,5 thousand years ago. Egyptian pyramids had been unsurpassed until medieval European cathedrals with towers of up to 160 meters outdid them. In those almost all were pieces of ecclesiastical architecture symbolizing divine might and celestial nature.

Passenger elevator invented in 1857 rearranged the concept of multistory buildings making them far more convenient and so allowing multifunctionality. Contemporary skyscrapers, besides its basic function of office or, less frequently, residential building, became the icons of economic strength and technological advance. Presence of a skyscraper a kind of label of economically successful cities. In Soviet Russia the most known high-rise buildings were the so-called “Stalinist high-rises” (up to 240 m), built in Moscow during the 50s of the 20th century.

In 50-90s several buildings of about 150 m were also constructed. In 1996 The Tower 2000 launched the downtown called the Moscow City, including 9 buildings 250-350 m tall, among which is the highest in Europe 509-meter Federation complex. The Yekaterinburg downtown had also been about to start, but the world economic crisis put the project on hold. Today the works are renewed and completion of 215-meter Iset’ tower is scheduled for 2014.

Saint Petersburg is justly called the other capital of Russia both in historical and economic sense. The idea of business district with the high-rise building using a model applied in some major metropolises took shape several years ago. The design competition for the Okhta public-business center held at the end of 2006 was won by British practice RMJM. The project supposes erection of multifunctional complex featuring 396-meter tall tower in rather depressive urban area next to the mouth the Okhta River.

The site is located beyond the historical city core across the Neva River. The adjacent blocks are built up with residential and industrial structures. To be precise, before the Great October Socialist Revolution erection of structures taller than 23,5 meters (the height of upper cornice of the Winter Palace) in the Russian capital of the period was prohibited by His Majesty’s edict. At present, the height of existing high-rises or those underway in the city is 90-120 meters.

Such a tall building as the Okhta Tower has never been erected in Petersburg, that’s why the leading domestic and foreign specialists were invited to elaborate the design. The geotechnic section of the design and scientific and technical tracking at all stages are to be performed by professionals of Gersevanov Research Institute of Foundations and Underground Structures. The 80-year old Institute participated in all significant projects of the USSR and the RF.

Among them the most renowned are those Stalinist high-rises, Ostankino Tower, Moscow City buildings, and also multiple industrial structures. The institute developed practically all normative documents on foundation arrangement, underground building also the content of geotechnic sections for high-rise designing. The working draft was performed in collaboration with Infors Project. The contractor design of barette base was developed by SPI HydroSpetzProject.

The represents the wista image of the Okhta Center, The tower is rendered at the. The basic pentagonal footprint of the tower is generally superposed over the outline of the Nyenskans Fortress destroyed in the 17th Century. Five pointed sections of the structure twist by one degree at each level getting less over the elevation. The planned height of the building is 396 m; number of floors – 67; the basic load is 330 thousand tons, diameter of the tower’s footprint is approximately 50 meters.

Underground part is also pentagonal and each facet is 56 m long; foundation area is 5700 sq. m. The structural scheme with central core bearing most of vertical and horizontal loads surrounded by perimeter columns and few outriggers, which is applied here, is getting even more popular within tall industry. The outriggers are the extra-stiff beams with 2-floor high section, placed at several levels of the elevation to resist horizontal loads. General stability is ensured by the joint three-dimensional work of strong central stem, perimeter columns, floor slabs and outrigger beams.

The central stem is the ferroconcrete pipe with 2 m thick walls at the base, which are gradually getting the higher, the thinner. The central is partitioned inside with walls and slabs of elevator and stairway shafts, which reinforce structural; strength. The core is surrounded with 15 perimeter steel columns of 850×850 mm box-section.

The scheme is peculiar for concentration of great vertical load from the central stem onto rather small of the foundation. Fig. 6 shows the pressure distribution under the foundation (without regard to redistribution due to yielding of foundation), which is 4.5 fold more directly under the core than average all over the base, and by 30% less over the peripheral zone. Most every contemporary high-rise buildings are built on deeply embedded supports, such as piles or barretts (pile of rectangular cross section).

It is conditioned by the fact that plate foundation for a high-rise structure can be applied only if natural-ground horizon is not much deep from the surface (down to 30- 40 m) and arrangement of deep foundation area is economically feasible. At this particular site the rock natural-ground lies at the depth of more than 200 m, that, whilst the surface layers consist of soft soils, which leaves no solution but arrangement of pile foundation.

Still in the 1st Century A.D. the Roman architect Vitruvius in his treatise “De architectura libri decem” formulated one of the foundation design principles, which is followed until now: “for the foundations of these buildings one should dig down to the naturalground, if it is possible to reach it, and even the natural-ground itself should be excavated down to the depth matching the scale of the structure to be erected…” In other words, the weight of extracted ground should be equal to the weight of building.

It’s impossible to use this principle in full under these particular conditions, since arrangement of so deep a foundation in soft soils is not economically feasible. The second principle, for the first time used for Stalinist high-rises in Moscow by the group of design engineers leaded by V. Nikitin (author of the unique Ostankino Television Tower structural design), consists in arrangement of the box foundations, which are more extensive than the footprint of tall component of a structure, to reduce loads onto the foundation plate.

Both these principles were partially realized in the design of foundation and underground part of the Okhta Tower. In terms of tall practice Petersburg is characterized by extremely complex geological engineering conditions, because the surface layers consist of soft soils down to the depth of 30-40 m. However, the rock ground, which are the most desirable bed for high-rise buildings, are located at the depth of more than 200 m, making it unreachable as the base, since capabilities and experience of contemporary industrial technologies is limited by embedding piles down to the depth of 100-120 m, let alone cost inefficiency of such a foundation.

Each additional meter of pile embedding deeper than 100 m becomes literally “gold”. Taking into account the above-listed factors, the solution was made to use the Vendian clays (lying 40-50 m deep) as the base of pile foundation. This study is examining the two layers of Vendian clays - geological engineering elements 12 and 13. Vendian clays are relatively solid, being the clay and semirocky soil at the same time. In the countries, where this kind of soils is widespread, there is a special term – “mudstone”.

The basic issue for supporting the piles by Vendian clays is lack of experience of using this kind of ground for high-rise buildings. There’s a certain controversy about the properties of Vendian clays among the Petersburg’s professional community. Some point out that the clays possess block- fissured structure; others assert that this kind of soils has clearly expressed stratification. The only matter of consensus is that Vendian clays should be thoroughly analyzed.

Previously, before the high-rise projects were developed in the city, the only people, who dealt with the Vendian clays, were the builders of underground railroad; however, their studies had been generally limited study with unloading of the ground massif. Fig. 8 renders the working face of the metro tunnel underway. The Vendian massif is being held by horizontal sheeting.

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Text by Valery PETRUKHIN, doctor of tech. sciences, director,
Oleg SHULYATEV, Cand. of tech. sciences,
Igor BOKOV, engineer, junior research fellow,
Stanislav SHULYATEV, engineer (all of Gersevanov NIIOSP )