Investigation of mechanical behavior in intact rock can help to better understand the stressstrain correlation in accordance with yielding. Attention to the formulation of yield criteria can direct the research into two areas, developing the criteria based on interpolation of experimental data and detailed study of micromechanics. The aim in the mathematical theory of yielding is to describe the theoretical respects in relation to stress and strain and recognition of the limit between elastic and plastic behavior. The authors consider studying of yielding in intact rock at different scales. From Weibull theory it is believed that the increase in scale causes a decrease in the strength. This has been quantified by the Hoek-Brown equation for Uniaxial Compressive Strength (UCS). However, the confinement can change this strength and subsequently yielding. Thus, from experimental observations the change of yielding at different scales is investigated. The experiments are conducted on cylindrical samples of sandstone with diameters of 50 and 96 mm. Every specimen was tested at different confining pressures. The distinctions of the initial yield points at different scales are extracted and the yield surfaces are depicted. Finally a simple mathematical yield criterion is developed.
Over the last few decades a significant amount of research has been conducted in rock engineering, including in the areas of rock mass and rock slopes. The main difficulty in regard to rock mass study is the deviation of laboratorial results in comparison with field scale measurements. This ‘scale effect ’ therefore needs to be analyzed more extensively and comprehensively. The mechanical behavior of intact rock over the last years has been investigated by many researchers (Desai 1980, Desai et al. 1984, Kim and Lade 1984, Kim and Lade 1988, Lade and Duncan 1975, Lade and Kim 1988, Lade and Nelson 1987).