Strength and deformability of small-scale rock mass models under large confinements

Triaxial compression tests were performed to determine the strength and deformability of small-scale rock mass models with multiple joint sets and frequencies under confining stresses up to 12 MPa. The cubical sandstone specimens (80 x 80 x 80 mm(3)) with joint sets simulated by saw-cut surfaces were compressed to failure using a true triaxial load frame. The joint frequencies ranged from 26 to 76 joints per meter. The results indicate that the Hoek-Brown criterion with two material parameters (m and s) can describe rock mass strengths as well as the three parameter criteria of Sheorey, Yudhbir and Ramamurthy-Arora. The parameter s notably decreases with increasing joint frequency, while parameter m is less sensitive to joint frequency. The confining stresses tend to enhance the effects of joint frequencies on rock mass compressive strengths. The deformation moduli in the direction normal to the joints tend to be lower than those parallel to the joints. They decrease with increasing joint frequency. Goodman's equation was modified here to allow calculation of the deformation moduli of the rock mass along the three principal directions. The modified equation can sufficiently describe the deformation moduli normal and parallel to the joints for one-joint set and three-joint set specimens under all confining stresses.