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The studies on the scale effect on the behavior of non-persistent rock joints are limited and the nature of scale dependency is still not well understood. In this study, the scale effect on failure mechanisms and compressive strength of rock blocks tested previously by the authors was investigated numerically. In the experimental study; two different block sizes, having dimensions of (63.5 x 28 x 20.3) cm and (30.5 x 15.24 x 10) cm, were tested. Samples of rock with non-persistent offset joints were subjected to uniaxial loading. The joint inclination angle was maintained at 22.5° in both cases. Also, degree of persistence was kept constant at 0.3 for all tested blocks. However, the offset angle which connects the inner tips of the joints was changed from 30°-90° with an increment of 15°. In the current study, finite element analyses were performed on the arrangements that were studied experimentally. ANSYS 19 Mechanical APDL was utilized to perform static structural analysis. A linear elastic material was assumed for these analyses, mainly because there were no signs of major material damage prior to fracture. Modulus of elasticity of approximately 10510 MPa and a Poisson’s ratio of 0.25 were assumed. The boundary conditions applied to the finite element model were zero vertical displacement along the bottom edge, and a uniform distribution load on the top surface, the magnitude of which was approximately the measured coalescence load (in MPa) for the joint geometry analyzed. The stress distribution within the block and near the tips of joints in particular was analyzed in terms of maximum principal stresses, shear stresses and factor of safety resulted from maximum tensile stress theory. The FEM results were compared with the experimental results. The results of FEM emphasized the observed experimentally results regarding the decrease in strength as the size of block sample increased and no effect of block size on mode of failure.

Numerical Modelling of Scale Effect on Mode of Failure and Strength of Offset Rock Joints