Employing frictional parameters in estimating the Hoek-Brown constant m_i for intact rocks

An important parameter of the Hoek-Brown failure criterion for intact rock materials is m_i, a dimensionless material constant that depends on the frictional characteristics of the component minerals. In this study a laboratory testing program was carried out in order to experimentally study the correlations between m_i and two frictional parameters: (a) the sliding friction angle which is determined from direct shear tests on pre-fractured rock specimens and (b) the Mohr-Coulomb internal friction angle which is determined from triaxial compression strength tests conducted at low confining stresses. We carried out direct shear tests on rough tension joints of ten fresh, low porosity (< 5%) natural rocks, including two igneous, three sedimentary and five metamorphic under normal stresses ranged from 0 to 2 MPa and determined the sliding friction angle φ_nd using peak shear stress and dilation measurements. An independent series of triaxial compression tests was conducted on intact cylindrical specimens of the same rocks at various confining pressures up to 70 MPa and the values of the internal friction angle φ_i0 at low confining stresses were determined as well as the values of m_i, σ_ci and the principal stresses at the brittle-ductile transition. Our experimental results show that, within the used range of values of the parameters m_i, φ_nd and φ_i0, m_i increases linearly with decreasing sliding friction angle and increasing internal friction angle. Both correlations were found to be statistically significant at a significance level of 0.001.