Effect of intermediate principal stress on strength of anisotropic rock mass

The Mohr-Coulomb criterion needs to be modified for highly anisotropic rock material and jointed rock masses. Taking sigma(2) into account, a new strength criterion is suggested because both sigma(2) and sigma(3) would contribute to the normal stress on the existing plane of weakness. This criterion explains the enhancement of strength (sigma(2)-sigma(3)) in the underground openings because sigma(2), along the tunnel axis is not related significantly. Another cause of strength enhancement is less reduction in the mass modulus in tunnels due to constrained dilatancy. Empirical correlations obtained from data from block shear tests and uniaxial jacking tests have been suggested to estimate new strength parameters. A correlation for the tensile strength of the rock mass is presented. Finally, Hoek and Brown theory is extended to account for sigma(2). A common strength criterion for both supported underground openings and rock slopes is suggested. (C) 1998 Elsevier Science Ltd.