FROM THE GRAVITATIONAL OVER TO TECTONIC HYPOTHESIS

Since the latter half of the outgoing century there has been a turbulent growth in studies related to rock mechanics. Such studies deal with the strained (deformed) state of massifs in the outer crust of the earth where geological prospecting and mining is carried out. For decades rock mechanics as a discipline in its own right has been the domain of the Mining Institute of the Siberian Branch of the Russian Academy of Sciences. Dr. Kulakov, a laboratory head of this Institute, elaborates:

Rock mechanics is a theoretical basis of rock pressure which-as scientists both here and abroad had thought right until the 1960s-was caused by gravitational forces of the earth crust, i.e. by a mass of rock in the gravitational field of the earth. Way back in 1920s Academician Alexander Dinnik formulated a corresponding hypothesis: that vertical stresses exceeded horizontal ones in magnitude, and the latter were equipollent in all directions.

Proceeding from this assumption, geomechanics students ultimately came to the conclusion that the earth crust persisted in an equilibrium state, with gravitational forces being the sole cause of rock stresses. Even if stresses changed under the effect of some factors, they relaxed and leveled off with time. As a consequence, in the opinion of Academician Lev Shevyakov (1889-1963), "the area of stresses in an intact rock massif turns out to be simple enough". For years theoretical and practical studies had been based on such assumptions that were making their way into manuals and the literature.

Yet experimental studies of native rock massifs (studies that were gaining force in the 1960s) led to surprising discoveries: the horizontal components of stress proved as a rule higher than the vertical components and, in addition, not equipollent in various directions. High horizontal stresses, exceeding five- to tenfold the vertical ones, were registered in apatite mines on Kola Peninsula, and in the Tashtagol iron-ore deposit in the Mountain Shoriya. Then such phenomena were detected within the Kursk Magnetic Anomaly, at Norilsk and in the Trans-Baikal area, and also abroad: in South Africa, Scandinavia, the USA, Canada, Japan and China. The experimental evidence was thus at variance with the gravitational theory.

The new discoveries testified to a diversified pattern of stresses in native rock. The point is that the earth crust is always subject to shifts, or displacements owing to tidal waves in the lithosphere, tectonic movements of some of its areas, seismic processes, magmatic mass movements and changing temperature patterns. In fact, the earth crust operates

page 10

as a mechanical-dynamic system and, according to Arkady Leontyev, an authority in this field, is acted upon by different forces engendered by heterodromous (not unidirectional) global processes. All that leaves traces in the earth crust and its rock masses.

Forces accounting for a rise in horizontal stresses are brought to life by tectonic forces (Mikhail Gzovsky was the first to predict such a possibility back in 1954; however, thirteen years were to pass before the first experimental evidence could be obtained).

The detection of tectonic forces operating in certain sections of the earth crust has brought about a cardinal revision of the fundamentals of contemporary geomechanics and enabled a new approach to the problem of shocks. Yet it became clear: stresses caused by such forces cannot be computed, they have to be measured. While within the framework of the gravitational hypothesis the computation of starting (initial) stresses posed problems because of the heterogeneous properties of rock massifs, now yet another random factor has been added-that of tectonic stresses (thus far little is known about their nature).

The newly discovered natural geomechanical factor should be taken into account in mining works carried out within some active deposits, for tectonic stresses may be fairly high there. Clearly, the same is true of newly explored deposits-one cannot design a network of exploring openings or proving holes there without making a thorough study of local fields of stresses. That is why it is essential to develop other methods of prospecting and new equipment alike. Ever since the mid-1960s (when the first instrumental data were obtained on enhanced horizontal stresses) all these problems have been attacked by many R&D bodies of the former Soviet Union, including those in Kazakhstan and Georgia. In turn, the Research Institute of Mining (Siberian Branch of the USSR Academy of Sciences) back in the 1970s set up a laboratory of rock mechanics under Academician Mikhail Kurlenei, and in addition to the then operating laboratory of rock pressure headed by Corresponding Member of the USSR Academy of Sciences Timofei Gorbachev. This work is carried on: both old and new techniques are being developed for measuring stresses, deformations, and displacements, and for studying the physicochemical properties of rock.