Paper Title
Effect of Intermediate Principal Stress on Biaxial Compressive Strength of Rocks

Biaxial and uniaxial tests have been performed on cubic specimens of nine rock types, using a biaxial loading device. The objective is to determine the effect of intermediate principal stress (s2) on rock strength. The results indicate that for all tested rocks s2 affects the major principal stress at failure. The maximum strengths are reached at Lode parameter (L) of about -0.5, below and above L =-0.5, the major principal stress at failure decreases. Post-test specimens under high s2 show multiple extensile fractures parallel to s1-s2plane, particularly for strong rocks. Combination of extensile and shear fractures are observed under low s2. The modified Wiebols and Cook criterion is adequate to describe the biaxial compressive strengths under all Lode parameters. For all rock types, the elastic moduli and Poisson’s ratios tend to be independent of the intermediate principal stress. Keywords - Modified Wiebols and Cook, Elastic Modulus, Poisson’s Ratios, Rock Strengths