New Approaches for New Missions
Okhta Center is apparently the most ambitious project for Saint Petersburg during the last century. It seemed that the designers were facing a kind of insolvable problem – to erect a 400 meter high skyscraper under the most complex geological engineering conditions. It is obvious that there’s no any experience of constructing such facilities upon earth soils in Saint Petersburg, and even throughout domestic industrial practices. The task set before the geologists was to investigate the entire depth of potentially active soils down to 130 m.
The site is located within Saint Petersburg but somehow out of its historical core at the confluence of the Okhta River and Neva. Geological engineering searches for high-rise structures have some peculiarities:
- poor knowledge about soils lying at depths below 40 m;
- high soil testing loads;
- field testing at great depths;
- lack of relevant regulations.
Actually, there was just few archive research data regarding the depths to be investigated. Addressing to the underground railroad experiences proved to be almost useless due to difference of research tasks. The archaeological excavations of Nyenskans fortress relics discovered at the construction site added complexity to work execution. Because of these issues the prospectors were limited both by time-frame and selection of drilling points.
The importance of the structure and complexity of problems urged drawing quite a number of organizations. The geological engineering searches were commissioned to СУ-299. Laboratory tests were being conducted by Vedeneev VNIIG and Lomonosov MGU, pressuremetric tests - Geodynamic Studies Centre, and plate load tests - Pylon. Later on the Gersevanov NIIOSP was also invited to join the works. The surface geological structure of the site is represented by soft overburden water-saturated sandy-clay bed of various geneses.
The ice-borne sediments, which are represented by morainal semisolid loams from tight- to soft-plastic consistency, lie lower. The upper part of underlying layer consists of Vendian sediments (with faulted structure) represented by semisolid and solid loams. This article is mainly dedicated to these soils, because these are the potential bed for the most loaded central core of the Okhta Center. The Vendian clays relate to Upper Proterozoic deposits of the Vendian system of the Kotlin horizon (Vkt2).
They are bedded at depth of 40 m and further throughout entire thickness of investigated soils. The age of Vendian clays may be up to 500,000,000 years, which makes these soils some of the most ancient deposits all over the planet. Actually, these are not clays, but loams, in essence of solid consistency. The fissile almost horizontal structure with cemented sandstone inclusions makes these soils rather heterogeneous in its physical and mechanical characteristics.
According to the R. E. Dashko’s et alias data, these deposits have almost vertical fractures, which form block structure. According to the results of analysis of physical characteristics of the data obtained at the fundamental phase of geological engineering searches, in the thickness of Vendian deposits the zones of loose soils were singled out.
The analysis of results of the fundamental phase, carried out by NIIOSP specialists proved their insufficiency. Quite a number of questions remained unresolved. The research was expected:
- to confirm vertical fracture of Vendian deposits within the site at the investigated depths and to estimate its influence on bearing capacity of the bed;
- to confirm if there are loose ground zones within Vendian deposits by field and laboratory tests proved by mechanical characteristics of soils;
- to estimate the influence of sampling on consistency of ground monoliths;
- to determine additional characteristics of ground, which are necessary, if contemporary methods of foundation calculation are used;
- to increase representativeness of field methods in terms of number and insight of studies.
Program development, scientific and technical tracking of additional geological engineering searches was commissioned to NIIOSP. This stage of studies encompassed: seismic microzoning of ground at the site, determination of additional characteristics of soils, conducting of plate load tests at maximally possible depths and pressuremetric tests down to 130 m, simulation of sampling process, reconsolidation laboratory tests using the method of phase composition restoring.
Seismic microzoning was executed by VNIIG on the basis of geophysical research data of the main body of the searches. As a result of analysis the presence of loose zones within the Vendian deposits was confirmed, which should be considered in the course of determining of foundation embedding depth. Vertical fracture and block structure of Vendian deposits was not detected by geophysical methods. However, as to macro-unit type structure of Vendian deposits, it may well be so.
At the examined depths the cracks are subject to high pressure, so the ground may seem to be continuous, which makes the real ground structure unidentified by geophysical methods. In this case the Vendian clays can be misconsidered as continuous (not unit type!) medium.
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Text by Alexander TRUFANOV, Cand. of Tech. SC.,
Oleg SHULYATEV, Cand. of Tech. SC .