Abstract: Effective stresses should be used instead of normal-stresses in the analysis of rock strength. Terzaghi effective stress is the approximation of structural effective stress. Terzaghi equation is only applicable to porous media like soil. Tight media like rock must adopt the structural effective stress,otherwise error will inevitably occur. Normal-stress analysis overestimates the strength of rock, and Terzaghi effective stress underestimates the strength of rock. Only the structural effective stress can provide the correct answer.

Key words: Mohr strength theory, rock (porous media), effective stress