Brittle-ductile transition stress of different rock types and its relationship with uniaxial compressive strength and Hoek-Brown material constant (mi)

Abstract

Rocks deformed at low confining pressure are brittle, which means that after peak stress, the strength declines to a residual value established by sliding friction. The stress drop is the variation between peak and residual values. But no tension reduction takes place at high confining pressure. A proposed definition of the brittle-ductile transition is the transition pressure at which no loss in strength takes place. However, studies that consider information about the brittle-ductile transition, the criterion's range of applicability, how to determine mi, and how confining pressures affect m_i's values are scarce. This paper aims to investigate the link between brittle-ductile transition stress, uniaxial compressive strength and Hoek-Brown material constant (m_i) for different kinds of rock. It is essential to accurately determine the brittle-ductile transition stress to derive reliable values for m_i. To achieve this purpose, a large amount of data from the literature was chosen, regression analysis was carried out, and brittle-ductile transition stress (σ_TR) was determined based on the combination of Hoek-Brown failure criteria and the recently used brittle-ductile transition stress limit of Mogi. Moreover, new nonlinear correlations were established between uniaxial compressive strength and Hoek-Brown material constant (m_i) for different igneous, sedimentary and metamorphic rock types. Regression analyses show that the determination coefficient between σ_TR and UCS for gneiss is 0.9, sandstone is 0.8, and shale is 0.74. Similarly, the determination coefficient between σ_TR and m_i for gneiss is 0.88. The correlation between Hoek-Brown material constant (m_i) and σ_TR was not notable for sedimentary and metamorphic rocks, probably due to sedimentary rocks' stratification and metamorphic ones' foliation.