The scale effect of rock joint on anisotropic shear strength behavior is studied by performing direct shear test on cylindrical samples of diameter 90 mm, 180 mm and 270 mm. Total number of asperities varies like 5, 8 and 17 for sample diameter 90 mm, 180 mm and 270 mm respectively. Regular surface morphology of saw tooth shape with asperity angle 20° is kept constant for all samples perpendicular to its strike direction. Strike direction is defined in regular joint along which joint offers least resistance to shear stresses. The direct shear test is conducted under constant normal load (CNL) condition with three normal stresses 150 kPa, 300 kPa and 450 kPa. Because of quarter symmetry of the specimen four direct shear test have been carried out at angle of 0°,30°,60° and 90° and assumed that the same results will be repeated in other quadrants.

Results from direct shear tests are recorded in terms of shear stress vs. shear deformation curve along with normal displacement. The graph is used to determine the peak shear strength (τmax) and corresponding shear displacement (ds), shear stiffness (ks) and dilation (δ).

Result shows significant scale as well as anisotropic shear strength behavior, deformation characteristics, shear stiffness, cohesion (c), angle of internal friction (ϕ)and joint roughness coefficient (JRC). JRC are calculated by fitting the test results in Barton's joint model. The laboratory investigation shows that JRC have significantly affected by shear direction and scale of joint.

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