The paper shows that all brittle material and rocks, irrespective of the body shape and its structure, feature a common intrinsic regularity of shear failure - formation of the tension cracks at the initial phase and the zones of compression and crushing in the principal stress surfaces at the final phase when attaining the ultimate bearing capacity. In the long run, the peak bearing capacity of the rock mass is conditioned by the resistance to compression, the magnitude and the pattern of the stressed state on the specified plane of shear.
Le rapport fait voir que tous les materiaux fragiles et les roches, independamment de la forme du corps et de sa structure, sont soumis à la même loi interne de la rupture au cisaillement: formation des fissures d'extension au stade initial et des zones de compression et de fragmentation des materiaux, suivant les aires principales, au stade final quand la limite de la capacite portante est atteinte. Finallement, la capacite portante limite du massif rocheux est determinee par la resistance des roches à la compression, par la valeur et la nature de l'etat de contrainte dans le plan de cisaillement envisage.
Es ist im Vortrag aufgezeigt worden, daβ allen spröden Materialien und Felsgesteinen unabhangig von der Form und dem Gefuege des Körpers die gleiche innere Gesetzmaβigkeit des Scherbruchs eigen ist und zwar die Bildung der Zugrisse am Anfangsstadium sowie die der Druck- und Ruschelzonen den Hauptflaachen entlang am Abschluβstadium zum Zeitpunkt· wo die Bruchfestigkeit erreicht worden ist. Letzlich wird die Bruchfestigkeit des Felsgesteins durch die Druckfestigkeit sowie den Spannungswert und das Spannungsbild in der jeweiligen Scherebene bestimmt.
Papers presented at the IV and V ISRM Congresses (Fishman 1979; Fishman, Ukhov & Fadeev 1983) and some other publications indicate that failure of the rock foundations under concrete structures differs in some cases from the conventional shear mechanism which is usually presented as the sliding of the structure on the foundation. If there are no shear-prone surfaces of weakening in the foundation (large fractures, interbeds), the typical pattern of failure consists in the formation of a main tension crack on the upstream side of the structure and a compression zone in which crushing of the rock mass coincides with attainment by the system of the peak bearing capacity on the downstream side. And as indicated (Fishman, Panfilov & Sarabeev 1976) principal minimum and maximum stresses are respectively responsible for the failure. The subsequent investigations have established that the described mechanism of shear failure is an intrinsic regularity of the brittle materials and rocks. But its outward manifestation could vary depending on the scale of the unit in question and extent of the material continuity. It is known that a typical feature of the brittle material failure is the formation of tension cracks which are most distinct in uniaxial compression and tension tests. At tangential stresses causing shear strain one or several echelon-like tension cracks are formed which are oriented at an angle to the shear direction (Brace & Byerle 1961). Under the "pure shear" conditions, i.e. when only tangential stresses act, increase in the latter causes the growth of tension cracks and subsequent splitting of the body or the sample. In case of a complex stressed state when in addition to the tangential stresses, the normal stresses are involved, the formation of the tension cracks is not necessarily to lead to the ultimate failure of the material and it constitutes only an initial phase of the failure process. Experiments conducted by many authors show that the complete shear failure of brittle bodies takes place through the joining of the tension cracks and formation of the main shear surface (Obert 1972; Lajtai 1967; etc.). But the mechanics of material failure between tension cracks has not been well studied while its identification is of fundamental importance because it determines the culmination of the failure process, i.e. attainment by the system of the limit bearing capacity. We have experimentally and theoretically found that the failure of undisturbed material between the tension cracks results from their compression in the surfaces of Principal stresses. Hence the pattern of brittle material failure at the microlevel (on the scale of one tension crack and one compression zone) resembles the failure of the rock foundation of the concrete structures at the macrolevel.
